Z28HO

I am beginning to regret the whole project I started. I had a good running comp Q-Jet car but wanted more. So I pull the comp, carb,& dist. In favor of a Reman. GM Hei Vaccum Advance, And a Carb Shop Qjet built to my engine specs non comp controlled. I have posted my problems before and have had plenty of good suggestions from a few of you thanks. I am still stumped, I have sent the carb back to Carb shop for a 2nd rebuild after I got a hold of its internals and screwed it up. I it back after the rebuilt it and ran it on there dyno engine which they where proud to say it made 496 flywheel hp. Thats great I said is this too much carb for my 305 is that the problem he said no. And I have posted that the primary jet #73 and primary rods 50m and the secondary rods are DP. Which I have been told aren't to extreme.

First let me tell you what it is doing. The car starts and runs. It idles smooth minus the choppy cam. I am running 37 degrees total timing. Which I believe is 12-14 degrees advanced. I have my fuel pressure set around 5 psi. Now with these in order I try to free rev the car in park and it does fine from idle to 1500 then it feels and sounds like the engine is loading up until it hits 2800 then it wraps fine to 6000. I can do this multiple times with no change. I check the exhaust and don't get any smoke or rich running signs. I put the car in gear and try to drive and it idles away fine go to put a load on it to accelerate and it coughs and sputers. I hold it in first and floor it and she just coughs til 2500-3000 and then comes alive and pulls strong but not 100%. I let it shift and try accelerating and it just loads up to the point it almost stalls. I have to rev it in neutral to kind of unload it and put it in gear, and then the only way it will drive is manually puting it in 1st and flooring it at this point. But once I stop and park it idles fine and free revs like I stated earlier. What gives? I have pulled the plugs to check them for fouling and they are not black but I don't think they have been under any sufficient load to foul them. I say this because they are a clean wet looking and they do smell like gas but they are not black

So at this point I am not sure what to do. 1.is it a vaccum problem, 2.is it an ignition problem, 3. is it a carb/fuel problem, 4. is it a engine problem.

I have done some research on these carbs and read up on some of the theory on how they work so let me know what you think I should look for and if I am even going in the right direction.

1a. Vaccum & idle speed, idle speed needs to be high enough RPM to pull decen tvaccum this prevents carb from loading up. Which can be caused by a weak vaccum signal to the power piston. My question what is decent vaccum?

1b. What if vaccum is too low? Well the engine thinks you are accelerating under a load so the power system cuts in and richens the mixture. If this is true that could be my plug fouling problem that I experienced earlier when the car would drive now I just have wet looking gas smelling plugs

1c. All of the following are problems I excperience hesitation, plug, fouling, and performance loss off idle. These were all listed in an article I read. With possible cause being 1. Vaccum leaks from carb, 2. Too big a cam, 3. Malfunctionin Dist. Vac.Advance

1d. What I have done in refrence to these possible problems. I have replace every possible vaccum line, sprayed carb cleaner every where while engine is running to find RPM change in engine. I have a healthy cam in the engine but with the comp controled q-jet I had none of these problems so I rule out the cam. The distributor is a brand newly reman unit from the parts store which I have exchanged twice so I don't think this is my problem coil,cap, & rotor also new. I have checked vaccum draw with the engine running at idle I am reading between 7 & 8 in. hg.. holding the RPM steady at 2500 I read around 15-19 with it fluctating.

1e. Vaccum Power enrichment circuit. This function of the q-jet makes or break off idle acceleration. In this circuit The primary jets, along with corresponding metering rods or power piston spring tension all work together. Under operating conditions the power piston meters fuel by raising & lowering the metereing rods in and out of the primary jet. The motive force for this movment is engine vaccum and its resisting force is the tension of the spring underneath the piston. So if vaccum is too low for the spring tension then the power piston will be pushed upwards, which raises the rods out of the jets and richens the fuel mixture. This is normally what happens at wide open throttle. However if the spring tension is mismatched to the engine vaccum(camshaft) or if the metering are mismatched to the jet sizes, then performance will suffer. Basically if the springtension is not right for my low vaccum producing cam this could be causing part of my problem.

1f. Power Piston Springs. Camshafts with low vaccum characteristics need a matching spring with weak tension. This is because if a strong tension spring is used, the spring will overcome the weak vaccum signal and cause the power system to cut in prematurely. Which is leting in more fuel that richens the mixture. So what I am guessing might be a problem is that I have to tight a spring installed the is leting the power system cut in to early since I idle at 7 in. hg. I believe it to be kicking in instantly. If I had a weaker spring I believe there are some made weaker to delay the power sytem to activate later like around 3in. hg.

2a. Ignition Problem. I don't believe I have an ignition problem in general I do know this thought that when I had my MSD Digital 6 ignition hooked up the car would drive longer with fouled plug s than it would without it. If anyone thinks I need to check ignition please let me know?

3a. Carb/Fuel problem. I believe I covered mos tof the possible carb problems above except fuel. I am running a 150 gph electric pump with a regulator set to 5 psi. I know volume is not a problem. It is not starving for fuel. Actually Fuel prseeure doesn't play a major role on a q-jet like I thought. Basically what I have read is that a q-jet with enough fuel volume can run of 2 psi as long as it is adecuate enough to keep the bowl full. But I believe an excessive amount of furl pressure could hurt performance especially without a return line on my system.

4a. Is it an engine problem? I don't know if I might have an internal problem. I wouldn't guess so but I do plan on a compression test. I have been able to idle the car for as long as I want and free rev it and nothing sounds off like missfires or dead cylinders. I also plan on resealing my intake to possibly increase engine vaccum.

As you can see I have put a lot of time in to diagnosing this problem with no luck as of yet. Sorry about the length but, If you are still with me and you have any good sugestions or anything in the that matter that I should check please let me know

Thank you

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Damon

Mark, first off- calm down. Take a few deep breaths and relax. It's going to work out OK, I promise. Listen to the sound of my voice..... you feel... SLEEPY....very sleepy.....

Seriously- You are on the right track here- elimiate one thing at a time. Here's a few things that are NOT the problem:

1. Your cam. Unless the thing is very very lumpy you are OK. A Comp Cams Magnum 280 will work OK with a near-stock QJet.

2. Weak vacuum signal. If you are over 12" vacuum idling at 900 in park don't worry about it- you are fine. Also, the power valve pring in your QJet is liklely to be the typical hi-perf "6-3" spring most rebuilders use. Starts enrichement at 6" vacuum and goes full-rich at 3". Nothing to worrry about.

3. Your distibutor. Stock HEIs have about 20* of centrifugal advance in them. So if you are at 12* initial your total timing would be 12 + 20 = 32 total timing. Make sure your distributor is at least "in the ballpark" with these specs. To get to 37* total that would imply a 17* initial setting. Disconnect the vacuum advance for all future testing- it only complicates things. In any case this is 99% likely NOT your problem.

4. It's not your engine. If it was mechanical it would do it all the time. Period end of sentence- don't even worry about this.

5. Vacuum leak- possibly- do you run an EGR valve or charcoal canister or any other emissions device? Could it be hooked to the wrong vacuum port (keeping in mind that it may be hooked to the SAME vacuum port as on the e-QJet but the port may do something different on your "old school" QJet). An EGR that is opening at the wrong time could really screw things up.

What's left? The carb.

Before we go any further let me telly you about my recent carb problem. Ran fine until the FIRST time I got into the throttle after a restart. Then it wouldn't idle right and had terrible part throttle dirvability and a nasty miss to it until a shut-down and restart . Plugs were coal-black. Problem ultimately turned out to be that my secondary throttle plates had worked loose and were giving funky a/f mixture becuase the secondary throttle plates wouldn't close at idle like they were supposed to- but only after th first time you hit the throttle. So I KNOW about wierd problems!

Let's go back to basics- let's figure out if it's on the primary or secondary side of the carb, to narrow things down. Do this by totally disconnecting the secondary throttle linkage- not tough to do- 2 springs hold on the secondary throttle arm (remember how they go!). Take off the throttle arm and disconnect the pushrod that comes from the primary throttle arm, and keep it out of the way so it son't bind as the primary throttle moves). Your secondaries are non-fuctional now. Take'er for a spin. Any difference? If it's "cured" you problem is on the secondary side of the carb. If it's no different then it's on the primary side.

Let's say it's no different (primary probem indicated). What next? Well, I've got some suggestions like temporarily taking out the primary metereing rods and other things to try but let's get the problem narrowed down to either the primary or secondary first, OK?

Damon

Hold on- you're gettting HOW MUCH vacuum at idle? Only 7-8"? What cam do you have in it? What are the specs on it?

THis is a bit low on the vacuum side. This could be the fault of the cam, or the carb itself could be out CAUSING the low vacuum from being out of tune for the engine, basically "bogging" the carb even at idle/no load.

Also, regardless of the cam once you are at 2800 RPMs the engine vacuum should NOT fluctuate as much as 4" from high to low (wandering up and down). It should be pretty well rock solid.

Keep the info coming. I'm getting a few ideas formed on this.


[This message has been edited by Damon (edited October 17, 2000).]

MRZ28HO

Refresh my memory, did you check the condition of the plug wires also? I had a similar problem with my 305 (it also pulls 5"-7" Hg with a 268H ), it would hesitate upon accelaration. It turned out to be the wires were fried. I popped some R44TS and MSD SC 8.8mm wires (routed under headers) and it revved a hell of alot better. Maybe if you give us a list of things you are positive work right (and specs on the engine), then we can eliminate the double troubleshooting. I have one final thing to say, I think that 12' is a bit too much (although my 305 really liked 12', but when the carb was outta tune). Try setting the timing at 8-10 and then adjusting the idle bleed jets. Also make sure the throttle stop screw is just barely keeping the primaries open 1/32" (IIRC). But as Damon said, everything will be alright and we are here to help.

------------------
George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB

Z28HO

Ok i just wanted to start off by thanking you guys for being here for me. This car is kicking my butt lately. Don't get me wrong I love working on it except when I spend hour&hours on it and get nothing done except for a bigger headache. Anyhow I don't have to tell you guys how that is I am sure you've excperienced it at one time or another.

So lets see what we can figure out here are my Cam specs per Comp Cams

COMPETITION CAMS CAMSHAFT SPECIFICATION SHEET

PART # 12-209-2
GRIND NUMBER: CS 275DEH-10
ENGINE: CHEV SML BLK 265-400
INTAKE EXHAUST
VALVE ADJUSTMENT HYD HYD
GROSS VALVE LIFT .462 .482
.006 TAPPET LIFT 275 277
VALVE TIMING OPEN CLOSE
AT .006 INT 32 BTDC 63 ABDC
EXH 73 BBDC 24 ATDC
THESE SPECS ARE FOR CAM INSTALLED
AT 106 INTAKE CENTER LINE
INTAKE EXHAUST
DURATION AT .050 219 229
LOBE LIFT .3077 .3210
LOBE SEPARATION 110
THIS CAM SHOULD USE SPRING # 981-16

Let's see you wanted to see what other mods I have done to the car welll here is a link to a page with my car.
http://www.cfcc-dfw.org/members/140.html

George you know I might just buy another set of plug wires to test your theory and some fresh plugs I am so tired of cleaning plugs witha wire brush. I had my 10.4mm taylors on and I changed them for a stock set to see if the taylors weren't working properly. It did run fine with the taylors before so maybe I have a bad stock wire if that's my problem I going to kick myself well. It is going to be tonight before I can try this but I will let you know.

Damon the the vaccum fluctating might have been me trying to hold the throttle and the guage at the same time but I will check this again after I try the wires.

Kevin at the carb shop wants me to send the carb back again so he can do some more runs and tests on there dyno engine. But I think I want to eliminate/rule out any other possibile problems first.

Well wish me luck and I will post my results as soon as I get done.



------------------
Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Z28HO

Hell you know buy the time its all said and done I am going to be a Q-Jet excpert like you guys. Not that's not what I was planning on but I have to say that I have learned a lot from this little project/adventure and from your help

Thanks Again

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Biochem

Can a tranny problem cause this sort of thing too? Have you checked your levels/color lately? I'm willing to bet $$$ that MY rough idle is being caused by the 0.100" longer pushrods then the carb., so it can be a few things that you wouldn't think about.

Would smaller rods (like .045) be better for a smaller engine too?

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock... all the suspension stuff... 9-bolt posi disk is in... K&N filter... 93 octane...

-=ICON Motorsports=-

Z28HO

The tranny is fine. It worked perfect before the swap and it even worked fine a couple of times after the swap when the car was running better. The level is good and the fluid is fresh. But the car has a dead spot in park/neutral when I free rev the engine it hesitates kind of loads up between 1500-3000 rpm almost like its flooded and trying to clear itself out and after that it pegs redline fine.



------------------
Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

FastBroker

That's a pretty big cam for your motor, man. I believe you ARE having some sort of Q-jet fuel delivery/vacuum leak sort of problem(s) going on. IMO, it will be very hard to set your carb up for your motor/cam setup but if you can get someone to dial it in for you, you'll be ok.

Also, CHECK your maximum advance with an advance timing light. Set the max (centrifugal) advance to 34-36 degrees at 3600rpm w/vacuum advance disconnected and don't worry about where your initial advance ends up. Then hook back up your vacuum advance and go.

Also, if you are using stock heads with your induction/exhaust systems and stock compresion ratio, it will NEVER run right with a carb, IMO. You need at least 9.5 CR with that cam or your cylinder pressures will be nill. Did you do a compression test on it to check cyl pressure?

[This message has been edited by FastBroker (edited October 18, 2000).]

Z28HO

I know the cam is big I found an article years ago in a performance mag doing a 305 build up with this cam and my heads and compression ration and they were making in the neighborhood of 300hp. My motor has cast iron 58cc heads with 1.94/1.5 valves it is a factory L69 engine with stock pistons and the factory rated this engine at 9.5to1 compression ratio with the same sized 58cc heads. So I don't believe compression ratio or small cubes is my problem. Now tuning the carb is my problem. To start with I used to run the factory Q-jet that was built by carb shop to run with thte same motor and i had an off road super chip. The car ran great did many high 13 low 14 second passes. Now I wanted to add NOS so I decided to pull the comp Q-jet and dist so I wouldn't have the computer screwing anything up while I was spraying. For timing the motor I used a timing light set to 37 and with the motor reved to 3500 rpm's. I might have had the vaccum advance hooked to the distributor but I can't remember. Actually I think I tried it both ways and didn't see a change. But like I said I can't rember exactly

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

five7kid

iTrader: (14)

I was just going to ask that question: Did you disconnect the vacuum when you set the total timing? If it didn't make a difference, you either have no vacuum to the distributor, or some other big-time problem. It wouldn't (shouldn't) affect the timing at idle if you have it hooked to ported vacuum. But, at 3500 RPM, it had better make a difference.

Another way to set the total timing is to take the advance weight springs off (a little more work, since you have to pop the cap, remove rotor, remove springs, replace rotor, replace cap, start engine, set timing, shut off engine, remove cap, remove rotor, replace springs, replace rotor, replace cap...). That way you don't have to rev the engine to get total advance, since it will advance at idle just like it would at high RPM with the springs. You still should disconnect the vacuum advance, though.

Running retarded timing could give all the symptoms you describe.

[This message has been edited by five7kid (edited October 19, 2000).]

MRZ28HO

Is it just me or does anyone else see this. Your car ran fine with the CC Q-jet, but when the carb shop sent you a non-cc Q-jet you started having problems? Did you suggest to the carb shop to tune the non-cc Q-jet to similar specs as the cc Q-jet. I don't think they did set the non-cc Q-jet to you specs, you mentioned previously that on "their" engine dyno they cranked out 496 fwhp (IIRC)? Sounds like you are over carb'ed and they didn't use your engine specs, but a beefier one. JHO though.

Z28HO

George thats what I have been thinking this entire time but Kevin at the carb shop assures me that it is not over built.

And I copied this from a previous post from Damon

Tuning is in the ballpark. I don't like the lean primary rods at all, though. 50s are WAY WAY WAY lean primaries. My QJet runs a 76 jet with 44 rods on a 400ci motor. That's a little rich and would probably be even richer on a little 305 but I'd knock that primary rod size down to 44-46, using the same 73 jet.
Just make sure you get the RIGHT primary rods- carbs starting with part numbers 170xxxxx use different primary rods than carbs starting with 70xxxxx.
The secondary rods could also use a little fattening up, probably. DPs are a little on the lean side. Stock 305 HO motors use DR rods which are about perfect for 90% of ALL QJet applications.
Neither of these minor tuning isses is causing your immediate problem, however.

As you can see I am confused as hell. The carb shop wants me to send it back to run more test on there engine but I don't know that its doing any good.



------------------
Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Z28HO

Ok I have another idea. I had purchased one of the Mr gasket Hei curve kits. I have not used it yet because I have not been able to drive the car long enough. Do you think that change the springs or weights would help me or should I just wait and try to eliminate my problem with the distributor left stock first. Also would getting an adjustable vaccum advance help me any I don't know to much about them really to know what kind of gains I would recieve.


------------------
Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Z28HO

Damon
You mentioned vaccum port routing could be a possible problem. I know for sure that I have the right port for my dist. vaccum advance. But the other ones I guessed I still have the basic stock vaccum hoses setup minus the ported vaccem switch and egr and diverter valve. I still have a charcol canister and pcv is there anything listed that would help me in dtermining if I have run my vaccum lines to the correct ports on the carb.

------------------
Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

MRZ28HO

I say try it. I also had a similar situation that different (actually the lighest) springs and weights were able to solve. If you feel no difference, you can just put the newer springs that resemble your OEM spring tension.

FastBroker

Have you mapped out your present advance curve, yet? With vac adv off, have abuddy rev up the car, holding rpm levels and write down the advance curve. You shaould start cent advance at like 800-1000 rpm and it should end at like 3200-3700 or so. If it does this in a smooth curve, and you have about 26deg or so of added advance, you are ok.

Like this:
800 1
1000 3
1200 5
1400 7
1600 8
2000 11
2400 14
2800 17
3000 20
3300 23
3600 26

These numbers are, of course, added to your initial advance to give total advance... Just make sure your numbers go up like my chart, not "all of a sudden" up 10 degrees somewhere. You'll be able to se if you have a problem if you take the time to do this.

Also should still do a compression check, man. Don't cost nothing and it tells you a lot.

[This message has been edited by FastBroker (edited October 20, 2000).]

MRZ28HO

Hey Mark, I just thought of something that might be able to help us diagnose your situation. Do you still have an O2 bung on your headers? Have you taken measurements with a digital volt meter from your O2 (see tech article on this site)? I think this will help tells us what the A/F is at idle, cruise (2000-3000 rpms) and WOT. That way we can elliminate the carb as a problem if the A/F ratio is somewhat right on. Or it can also tell us that the carb is too rich or too lean. Let us know.

roadknee

Mark:

I agree with Damon. I am 99.9% sure your primary metering rods are way too large. You stated in the beginning of your post you have 73 jets with 50 metering rods. The rule of thumb for metering rod size is jet size minus 30. Thus, you should try a pair of number 43 metering rods.

Your engine works good to 1,500 RPM because you are basically using the idle circuit at this engine speed. Above this engine speed, the main circuit is way too lean to provide the required fuel. The Carb Shop most likely also installed the 3" to 6" power piston spring, so you really have to open the throttle blades to lower manifold vacuum enough to meter any fuel.

Quit fooling with everything else and install number 43 primary metering rods. Again, I am 99.9% sure this will SOLVE your problem. If you want to calibrate the carburetor any closer than this, install an oxygen sensor and shoot for 0.5 volts at part throttle and 0.77-0.78 volts at wide open throttle.

Please let us know if this works.

Ron.

Damon

I can't think of any other way of figuring out your vacuum lines other than to either look it up on the hood tag (no longer valid since you are using a different QJet with different vacuum ports) or to understand the function of every vacuum line- which ones are supposed to receive full manifold vacuum, off-idle manifold vacuum or one of the the low-vacuum emissions ports (charcoal canister).

My suggestion would be, in general, to disconnect the EGR, charcoal canister, distributor vac advance and any other non-necessary things that have vacuum lines going to them TEMPORARILY FOR TESTING. If that just by luck happens to FIX your problem then you KNOW you had one of them hooked up wrong and it's a simple matter of reconnecting them one at a time until the problem returns and then- bingo, you are on your way to figuring it out.

I'll stand by my tuning suggestion with the primary rods. They are WAY too lean. And just becuase the carb made 460+ HP on their dyno means diddly squat when you are having part throttle drivability problems. Totally different situations- and you said yourself that your car runs like a scalded cat at WOT.

One other thing you could be experiencing is a problem with the distributor module. SOmetimes in rare instances the module will go bad and cuase the engine to break-up real bad only in a certain RPM range. I doubt this is the problem here but it's one to put in the "maybe" bucket to check later if the carb is totally eliminated as the culprit in the future.

Z28HO

Its been a while since I have posted but I just wanted to let you guys know that the car is running great now. I had to send the Q-jet back again and during that time some jackass hit my dakota r/t. I now needed my car so I borrowed a friends modified holley 750 vaccum secondary. Before I nstalled it I went ahead and resealed my intake partly because I droped a bolt down the intake so I had to remove it to find it. Anyway through the holley on and my friend jetted it. Wow thats all I could say at first. My car screamed I could not beleive it must have picked up 20hp. I almost didn't care to put the qjet back on but after about a month and a half went by the holley screwed up on me partly my fault for not running an air cleaner the needle and seat were dirty and the float stuck nothing major. But I decided it was time to put the qjet back on. I was not totally impressed with the qjet at first but I realized that the plugs where probably fouled from the holley running rich when the float stuck. So i decided to throw in some new rapidfire plugs and my digital 6 MSD back on. I was very impressed other than a slight hickup at first press of the throttle the power was equal if not a little better than the holley some due to the fact the ignition was much better but none the less the qj-et works great. I have to give Kevin at the carb shop a hand he did a great job on this carb to bad I was so far from them it meant sending the carb back twice for rewroking to get it dialed in but it was worth it. Now on to my next project. I just aquired a worked over ZZ3 crate motor. This motor had been punched out to a 355 and forged pistons are installed along witha very healthy crane cam. Trick Flow gen 2 aluminum heads with 2.02 1.60 valves replace the corvette aluminum heads and the vlavetrain has been hopped up with a AFR rev kit and Crane 1.6 roller rockers. This is a very strong engine in my setup I am hoping for for mid 12's on motor and 11's on the bottle. I will keep you posted on how it turns out.


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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's