I'm posting this for a friend so I don't have all the details, but we just spent 3 hours trying to figure it out and we're out of ideas, any help would be greatly appreciated! It is a "simple" build of a 350 vortec around 1998 with lightly more radical cam than stock, about 300hp was expected of the build and he copied the specs/parts from some legitimate source. It currently starts better than most new cars, idles without any issues and revs perfectly fine past 4500, headers dont glow, it runs at normal heat etc... what it doesn't have is any guts and I mean any at all... past 3500 you could almost pretend it's normal, but it should be spinning tires in 1st gear as well and it's got nothing to offer, in lower rpm acceleration it also has a slight odd shake but it's not very noticeable.

so far we have timing at idle set at 10* with vacuum advance disconnected/plugged. If then we rev the engine to about 3000-3500 the timing goes to approximately 28-32* or so (STILL with the vacuum adv disconnected, more on that below..), we are guessing on the actual number because his balancer only has the line, but we're confident it's around that figure. Timing stops increasing at that point as expected. He has an edelbrock 1406 and we messed with the idle mixture screws per the manual, both screws are about 3 full turns out or so right now where they find the higher rpm at idle, we don't hear any pinging , exhaust smells normal, doesn't seem to be too rich or too lean. Otherwise the carb is stock and brand new as is the distributor which is this one https://www.summitracing.com/parts/sum-850001r

Here are some odd things that maybe will make sense to you?
1) we had two backfires in 3 hours of messing with it, nothing crazy but something isn't quite right (plus again, no real power)
2) plugs were more white than I would expect, I'll post a pic tomorrow, we pulled two and both were somewhat white-ish (so lean?) , they are the iridium kind gapped properly (we checked).
3) we tried using both vacuum ports on his carb for the vacuum advance

• on the MANIFOLD vacuum port (dr. side) the idle speed increases vastly from 800 to like 1500 and we actually can't lower it.
• On the timed vacuum port (pass. side) the idle does not change at all, but AS SOON as we rev it the timing is off the charts like 50*+ (I'm guessing but it goes waaaay off the scale)
• is that normal for both? we ended up testing most without any vacuum advance and everything plugged.

4) We were so out of ideas that we ended up pulling a spark plug cable just to see how badly the motor would run, to my shock it wasn't noticeable AT ALL, we then put the #1 wire back, pulled #2 on other side and same thing, couldn't even tell, fired right up and idled and revved the same... that really confused me, I expected a bucking horse type of result.
5) since this is '98 he had to install an electric fuel pump setup, it's all new, gauge shows 5psi at most times, when revved at idle we can see it drop to about 4psi but mostly 5ish, I assume that's fine and since the issue is with no power at lower rpms we ruled fuel pressure out

Unfortunately at that point we blew the oil sensor line and made a mess so the night ended, but what say you oh wise carbed-350 experts?

as far as the motor build itself, the only thing we question that he did himself was lap the valves, he had some concerns that maybe there's issues with those but it seems odd that it would idle so nicely with such a major physical issue as valves not seating.. we can't rule it out, it just seems unlikely. He assembled the rest as well with crank/cam, but there isn't a whole lot I can think of that he could get wrong that would still allow it to run nicely as-is.

 

Mmmm. How sure are you you got the timing chain on exactly right?

 

Total shots in the dark....

If your concern is poor performance above idle, it's no use messing with the idle mixture. How's the carb float level and drop? Jet/needle sizes?

What idle speed when setting timing at distributor? Could the mechanical advance be fooling your reading?

Who did the rebuild? Mis-matched heads/pistons? Low compression issue? Valve clearance?

Hook up a vacuum gauge. Perhaps the needle behavior will tell you something?

 

Reminds me of my 350 build a while back. What should of been a 400hp motor felt like it had 250. The final nail in that motor’s coffin was the rings not seating. Im guessing I ball honed the cylinders too rough because leak down was around 60% on all cylinders, and it started blowing light blue smoke.

Anyway what I’m asking is did he do anything with the rings? A compression test on at least a cylinder or two would verify some compression. Also what balancer are you using? Is it new? And from ebay? I used a brand new sbc ebay balancer on that same build and it turned out to be 6-8 degrees off! I verified this with a piston stop to find true tdc. Getting the timing where it was suppose to be woke it up alottt, but it still didn’t have the power it should have. Once it started smoking I did the leakdown test and found my other problem...

I would verify the balancer is accurate and maybe do a compression test.

 

Sounds to me like the cam isn't degreed right. The behavior for the ported vacuum is normal, however on manifold vacuum you should be able to get it to idle down. Something isn't right on the carb, rear throttle plates open too far or the front plates not seating. You need to be able to use manifold vacuum. Vortec heads only need about 32 degrees maximum advance.

 

exactly what cam did he use?

 

Have you looked at your complete Exhaust system to make sure there is not a back pressure problem??
Also I have seen bad Coils do this.

 

If you're gonna post fer a friend, get ALL the details. Armchairing that stuff will never be exact, but you can get opinions based on other peoples experience with a similar problem. The more information the A Q's (armchair QB's) have to work with, the higher quality will be the responses.
If it's making expected power to 3500 then suddenly flattening out, timing and/or airflow. Has it been doing this since it was built? that's 22 years ago, sounds like, or have there been recent changes to the motor?

 

tough to guarantee anything now, but it runs very smooth, starts right up, is mostly driveable on the street just with that low end shake a bit and no power. Any way to tell now that it's assembled?

 

it's the opposite, it's making no low rpm power and then sort of wakes up a little, but still IMHO not enough. The motor is freshly built, he did the rings, dingle honed it, stock bottom end and pistons, stock vortec heads, just cam is little more than stock , he used the LT4 hotcam

 

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LT4 Hot Cam - Hydraulic Roller, 218/228, Lift .492/.492 @ .050. 279 intake duration, 287 exhaust duration, 112 LSAStock roller lifters & push rods

 

balancer is stock, we will do a leakdown test I guess, can't hurt. He dingle honed it at home and did replace the rings with the right gap etc.. At least nothing is smoking so far

 

here's most of the build sheet:

Bottom End

• Stock ’99 Suburban Vortec 5.7L block
  
• Cylinder bores dingle-ball honed
  
• Stock pistons & rods - new rings, proper gap
  
• Stock crank
  
• New crank & rod bearings – nominal clearances
  
• 9.4:1 compression ratio
  

Heads

• Stock Vortec heads off a ’99 Suburban
  
• Machined to accept 3/8” threaded rocker studs – stock rockers
  
• Machined to accept LS6 beehive springs & Comp Cam retainers
  
• Clearanced push rod holes to accept more stroke
  
• Stock valves – hand lapped with compound
  

Cam

• LT4 Hot Cam - Hydraulic Roller, 218/228, Lift .492/.492 @ .050. 279 intake duration, 287 exhaust duration, 112 LSAStock roller lifters & push rods
  
• New cam bearings – nominal clearances
  

Carb

• Edelbrock 1406 600 CFM – New
  
• Electric choke not yet wired
  
• 1” phenolic spacer
  

Intake

• Summit Racing stage 3 dual plane
  

Ignition

• Summit Racing HEI vacuum advance
  
• MSD Street Fire 8mm wires
  
• New battery
  
• AutoLite double platinum spark plugs APP605 0.060” gap
  

 

Sounds like the valve timing is retarded. Gonna be tough to check it now though.

 

Yup, sounds alot like what happens when the cam is retarded (i.e. timing set incorrectly installed).

Do a compression test. Ideally a GOOD combination of cam and static compression should give you CONSISTENT numbers in all 8 cyls of around 180 - 190 psi cold with a GOOD HOT battery that spins the motor fast. The combo you have should be in or near that range, I'd expect maybe 170 - 175 psi, if what you THINK it is, is truly accurate. That's not a "necessity" as such, just, a helpful rule of thumb. Depending on details it could run just fine with as low as 150s or as high as 220, again depending on the cam, its timing, and the static compression, but that's far less likely, and particularly so with your very common, ordinary, familiar, and well-characterized combo.

A severely retarded cam (like, one tooth off) will cause low compression test readings, low torque, low vacuum, late shifting, a generally "lazy" feel to the motor, no "leave", but will eventually begin to run somewhat OK but seem to wind out forever although never really "lighting up". You can do a rough test for timing set accuracy by watching the #1 & #6 valves as the engine passes through ACCURATELY KNOWN by some RELIABLE method of finding TDC such as observation with a borescope camera (as opposed to "mark" and "tab") at #1/#6 TDC. Whichever of those 2 cyls is at the firing instance of #1 TDC at that time, the other's valves will be at the EC/IO state, aka "rocking"; exh just closing, and int just opening, with equal amounts of motion on the 2. Retarded cam (valve event) timing will cause the exh to be MUCH farther open than the int at this point in the engine cycle, or perhaps even, the exh still WAAAAAAAAAYYYYY open and the int nowhere near even getting started opening.

Severely retarded ignition timing will cause just altogether lackluster performance at all RPMs, poor gas mileage, low idle speed, and hot running. You can troubleshoot for that by simply adding ignition advance until it starts to ping, then backing it off until it quits, then back off a small amount more for a safety margin. That won't necessarily be "right", probably won't in fact although it might, but as a troubleshooting aid, it will tell you REAL QUICK if the timing "numbers" you're force-feeding the motor, differ by too much from WHAT THE MOTOR WANTS. It's BEYOND INCREDIBLE how many people do that... either they irrationally and without justification, let alone REASON, get in their head that THEY "WANT" some particular reading with light and mark; or their mark system is totally inaccurate. A VERY TYPICAL source of simple inaccurate timing marks is using a stock crank damper intended for use with a timing tab welded to the center of the timing cover (the 12:30 orientation), in conjunction with a BOLT-ON tab at the 1:30 orientation. That combination will lead to timing #s around 35° WRONG, in the retarded direction. Plus of course, since the mark is on a remote part not connected to the crankshaft by anything except a blob of spooge, the mark could be completely wrong all on its own. I would tend to discount any sentiments or assumptions of the type "my ignition timing is 'right' because my light says it is" without verification by the above ping/no-ping test.

 

A couple things jump out at me:

More info needed:
1) Were the pushrod guide holes elongated using the Louis tool, or opened up with a 1/2" drill bit?
2) Did each valve go back to where it originally came from? Were the valve tips chamfered before removing them? In other words, were any burrs removed from the stems before they were forcefully pulled out through the valve guides? So no valve job was done by the machine shop?
3) Was each valve spring height set for a specific measurement, or were the parts just assembled?
4) Is the choke plate wired open?
5) Plug gap is too large. Close them up to .045"

 

I'll find out on the 1-3, but for
4) his carb has an electric choke but it is still disconnected and not wired up, I didn't think that would matter but could it? once the engine warmed up it idled fine but of course without a choke when cold it wanted to stall (and did)
5) we can try that, but I believe the gap for those specific plugs was the .060

speaking of the plugs.. here's a picture of one we pulled.

 

Wire the choke fully open for now. The choke plate will try to stay closed while cold, and without a +12v source, it will not heat up properly. Most likely the plate will be flopping around somewhere between ON and OFF. This is not what you want while troubleshooting problems.

The plugs might have been made with a .060" gap, but unless the ignition system has an added on aftermarket amplifier "box", a stock ignition can have a hard time firing that large gap. GM spec'd that gap in the early days of HEI, and later brought it back down to around .045".
Always use anti-seize on those plug threads.

 

I agree with Sofa, do a compression test on at least a couple cylinders. Cranking PSI at 9.4:1, if that's accurate, should be in the area that he mentioned, 180-190 psi.
I'd want to be in the area of .040" plug gap, I think even .045 is too big.
My best bet is still on the cam being in wrong. Compression test will tell more.

 

I had a similar issue about 28 years ago after a 305 to 350 swap. I tried numerous things and eventually tracked it down to my catalytic convertor which broke apart and lodged pieces in the exhaust restricting flow. Night and day difference after dumping the cat and clearing the exhaust which I later swapped out for straight 3".

 

When I lined mine up a month or so ago, I had to be very careful. It was pretty easy to get the cam off one tooth without noticing. I used a straight edge and checked it like three times.

 

For all that don't already know, for SBC, aligning the timing marks dot-to-dot is actually #6 firing.
Which is also the reason that so many can't understand how they got the distributor 180 degrees off when they first fire the engine and "something is wrong".
If you align with the cam gear dot at the top of the gear (#1 firing) and double check with a straight edge, it is easier to see that you could be 1 tooth off.
If you don't like this method, you could always align the conventional way, then rotate the crank exactly 1 turn. Then do the double check to verify.

 

Interesting. I did not know about the third mark. That makes it much more clear then. I'll bet a cam off 180, the engine won't even run, but one tooth, its can act weird.

 

There is not a 3rd mark. It's the same dot, either on the bottom (#6 firing), or at the top (#1 firing) for the cam gear.
The dot on the crank gear is in the same spot for both conditions.

 

Just an update..

• My friend took pics when building the motor, he is quite sure the cam/crank timing marks aligned properly
• He is also quite sure the new timing cover timing tab matches the original

with that in mind we think the next logical step is a compression test, leakdown test, double check TDC/position of balancer/distributor rotor position. Thank you for all your help so far, I'll try to keep this thread going whenever we make some progress.

 

He will need a piston stop and degree wheel to verify TDC, and the zero mark on the balancer aligning with the pointer on the timing tab.

 

That's what everybody says.

CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK

Believe NOTHING anybody says, no matter how vigorously they protest.

CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK CHECK

 

Post the pics; they may shed some light on the problem.

 

Ah. Too bad. That would be excellent instead of finding a third point on the block. One thing I was worried about was that my straight line between the two marks would not be perfectly vertical to the block axis and off one cam tooth and I wouldn't notice easily.

 

Yup. With the 2 dots so close together when they are pointing at each other, it's easy to be off 1 tooth, even though it looks correct.
By checking alignment at #1 firing, your straight edge needs to be going right through the center of the crankshaft and camshaft (and the 2 dots).

 

those plug straps look good to me... if it idles that well I think you have a different issue:

You changed the cam, did you install a looser torque converter? a cam change even what you consider "slightly more than stock" will need a torque converter with a "slightly more than stock stall speed"

try shifting manually and see if the car wakes up when you hold it in first gear then shift to 2nd at 5500... if you get better performance your problem is you need a looser converter.

 

I am the friend zracTA is helping. Thank you for welcoming me in and for your time/input to date. Also, thanks for your patience while he and I pull together the data you're requesting and for your future patience while we investigate and test amidst our crazy daily schedules. I am confident in my abilities, but making mistakes is certainly within the realm of my abilities as well! If something wasn't wrong, this thing would be breathing fire.

Next courses of action: Compression test on a minimum of 3 cylinders, balancer "0" mark verification (#1 piston stop method), int/exh valve action verification on #1 & #6, dizzy rotor positioning at #1 TDC

I've tried a few times to post a pic of the timing set installed. Will keep trying.

 

I think this worked ...

 

Hi 13lifecrisis,
OK, I will buy that. The timing gears look to be aligned correctly.

 

I think you might be off half a tooth. In the pics, the gear teeth tips are together, but the index marks are not on the tooth-tips. I tried to envision moving the cam gear one tooth clockwise and, from the pic, I can see the index marks lining up better than in the pic.

 

I think because the dots are stamped into "raised" material on the gears and my picture is taken at an angle, the dots don't appear to line up with teeth. However, I've confirmed on the Cloyes site that the dots on my 9-1157 set do line up with teeth. If compression testing and valve positioning at TDC reviews tell me I have an issue with timing, I'll tear this thing down.

 

Fair enough. Make sure that choke is wired open as '84 mentioned. I would check the exhaust as well. Air in and air out. Have you taken a close look at the distributor?

 

This is the second carb I've had on the motor, both with the same symptoms. First carb was manual choke. Regardless, I need to wire the choke anyway. If my mufflers collapsed internally or if mice made a home, that may be an issue. I doubt it, but I'll check. As far as the dizzy goes, no I haven't dug into it. I've had the cap off and everything appear to be in place per other pictures I've referenced. It's brand new, but that doesn't mean there isn't an issue. I have another dizzy that ran great on my old motor, I could always pillage that and transfer it over. She's dirty as hell, could help determine my issues.

Thoughts on the torque converter theory? I'm trying to wrap my head around the manual shift test ... if it doesn't get out of it's own way in auto 1st, not sure it'll act any different if I start in manual 1st.

 

it is a fact not a theory.

since you don't have enough stall speed (the stock converter is too tight for the aftermarket cam) you want to hold it first gear as long as possible to see if the performance improves.

Leaving it "D" even if you floor it will not hold it in first gear long enough
The problem is worse if you installed a lower rear gear along with that cam and did not replace the stock toque converter.

it is simple 1 minute road test.

 

FRMULA88 - Roger, thanks for the explanation. Wasn’t rejecting the idea, just curious to the mechanics behind it. It’ll be the first test after I fix my oil pressure sender mess.

 

The cam isn't THAT big, How tight are the lifters tightened after lash is removed?
Did you put a melonized gear on the distributor?
Have you ran it with manifold vacuum hooked to the distributor?
Bump the timing up to 14-16 degrees initial and hoook the vacuum can to manifold vacuum and see how it runs.

 

thats what she said LOL

Any change from the stock cam or rear gears on a any car with an automatic transmission requires a change in torque converter stall speed. A 500 to 800 rpm increase in stall speed Could make all the difference in the performance

This is basic hot rodding 101 gents.

call any reputable transmission supplier, summit, or jegs tech supprort with your vehicle, engine cam, and rear gear specs and they will get you the right converter. The stock one is made for OEM stock application that you don’t have anymore.

 

Not 100% true.
While optimizing stall speed is crucial to maximum performance, I know that you can still drive a car with the wrong converter in it and not have the problem he's having.
My 87 L98 IROC, I put in a 350HO cam, specs were .480/.480 and 230/230 @ .050 and it drove fine with no tune and the stock converter. Was it optimized? No... but it ran like a raped ape. Up to 4800rpm.. LOL!
I suppose if the car has like 2.41 gear in the rear, nothing is gonna help low end on that setup.

 

Respectfully disagree 100%.

An engine is going to make power at RPM under WOT regardless of what it is connected to.

 

how do you know its even getting enough fuel without a wideband?

 

Please pop the distributor cap off and look under the rotor to make the springs & weights are as they should be.

 

Then you don't know how a torque converter works, and that's OK, you dont' know what you don't know.

OP has some troubleshooting to do and all the suggestions are valid however;
Playing mechanic on the internet is like diagnosing a medical symptom online without actually seeing doctor... sure that lump COULD be cancer but maybe it's just a cyst... unless you actually check it out you are just chasing your tail.

 

Agreed 100% , there is no way the TC stall speed is gonna have this engine hobbled as bad as the OP describes once the engine reaches it's max power RPM . Were this a problem of the engine not reaching it's powerband soon enough for a desired performance level , sure I'd say the TC stall speed needs to be examined , but the complete lack of any real power in any gear at any speed/RPM is enough to say that the TC stall speed is not the problem the OP is looking for .

And see that ? I was able to present my point without the condescending garbage of telling folks "you don't know what you don't know" .......

 

Thank you all for the input ... I have some testing to do and I'll post my findings as I go. No test is a dumb test, so I'll try as much as I can from all your suggestions. Hopefully the data will lead us in the right direction.

 

I had a 1983 Camaro with a 355 th350 and a B&M converter, maybe 2800 or 3400stall or something.
The rear gear was 2.77:1 , you know it sucked,

The car was an absolute slug. The slowest thing I ever drove in my life. The trans and converter were new, shift kit, etc...
I took all that out and put in a 4-speed manual trans and the car ripped, was nice and fast with the same engine.

The combo of th350 and high stall with the shitty rear gear was the reason it was terrible. Woulda been fine with a 3.73 or something but they dont make it easy on those 9-bolt rears.