I need a little help dialing this carb in or deciding if I have other issues going on with the motor. I currently need to tackle the idle before I can progress to any other tuning so here it is.


Car idles at 7" of vacuum and this fluctuates rapidly +/- about 1".
Idles extremely rich.
Cam is .540/.562 lift (plus 1.6 rr's)
242/248 duration @ .050 on a 110 LSA

Had a Speed Demon on it (that idled very rich as well) which is not recommended for such high duration. Took it off and have a Summit branded 750 DP.

Does anyone have experience with larger duration cams? Should I drill these butterflys to help clean up the idle? I can play with the air bleeds on this unit if need be.


Is this low vacuum and slight fluctuation inherent with a cam of this size or one one suspect something else might be going on?


Thanks in advance....I can almost taste driving this car on a regular basis.

 

do you have a part number for the carb? if it has a power valve you should try a 35. i use a 25. my cam is 252/258 duration @ .050

 

Power valve is covered. Running a 4.5

 

Getting some additional air flow thru holes in th butterflies should help... about 1/16" is a good size.

I have to ask, why so much cam???? Do you have the big heads, high compression, big tube headers, and hot spark ignition to go with it??? Hopefully this is a manual trans car, with an automatic it would take about 3,500 stall converter and some really tall rear end gears to make the cam work.

 

First, lets talk about timing. That is a pretty big cam. You are probably going to need to be around 18-22* base with vacuum advance on top of that at idle. What is your timing setup now? Don't even bother messing with the carb until the timing is addressed. You'll just be chasing your tail. Also, what is your compression?

 

Those 1.6 rockers will add .030 lift making almost .600 lift on the eshaust. They also add a few degrees to your duration. Just something to think about.

 

This is weekend fun car only so I can cope with a few big cam gremlins. I wanted to go as big as I thought my combo would work with because of that. Car is a T-5, 383 with Trickflow 195cc heads, right now only 1 5/8" shorties which I know is a big hindrance to the power. Crappy ProComp large cap HEI powers everything. I have my MSD box waiting to be hooked up but didn't want to tackle that until I had it running halfway decent.


Compression should be a max of approx 10.2:1, timing is set at 18* initial and max at 35* I have left the vac advance unhooked until I smoothed things out a bit.


I know, fairly large cammed. That's why I'm thinking drilling might give me the airflow I need.

 

Get the vacuum advance hooked up and see where your timing sits at idle and what your vacuum reading is. You will need to be around 45* advance at idle and probably 50* at part throttle cruise. After this, we mess with the throttle blade settings and idle mixture screws. Drilling holes in the blades is an option...but save that until last.

 

Ok, you're not going to get much vacuum at all with all the duration and overlap. 8 or 9" would probably be a good day! Start with the basics, set the timing at about 10 degrees advanced on the light with vacuum advance unhooked. Then check it at about 2500 rpm, all the advance should be in by then, total advance of around 34 to 37 degrees should be ok with good fuel. Need a lot of voltage at the coil to fire the thing! Air bleeds in the throttle blades should help some, start with the idle screws out about 1 1/2 turns. You'll also have to have good plugs and wires. Check for vacuum leaks around the carb and intake.

But that cam, I don't run that much duration (though it's a hydraulic roller) in a 406 Brand X engine with stepped headers, MSD ignition, 200 heads, and 11.5 CR!!!

 

10* advance isn't nearly enough. I have a 236/242 @ .050 roller cam in my 350. Base timing is set at 20*, total is at 36* and the vacuum advance can add another 18*. I have the idle set at 850RPM (it pulls about 12" of vacuum) and the timing is at 44* at idle. This is with only 10.2:1 compression, and seems about right. With 11:1, you probably can't get away with this type of curve. I have actually debated locking out my mechanical and keeping the vacuum advance. That is a little hard on the starter though.

 

I'm running a very similiar cam to that the Comp Cams Duration @ 050 inch Lift 242 int. 248 exh. with 0.570 int./0.576 exh. lift (I have 1.6 RR also). I can 13-15 inches at slower idle rpm in gear and I have 210cc heads with 2.08 valve even. I read it using the cheap gauge from AutoZone.

The only time I've ever seen 8-9 inches at idle was here lately when my return spring wasn't pulling the carb all the way back shut again. I changed out springs and that fixed that issue.

I'm currently using 16* base and I changed the bushing to the 19* in the MSD distributor. I'm also running vacuum advance.

I'd say make sure you're butterflies are shut far enough. Like I said the only time I've seen that was when they got stuck open a little and I had a pretty fast idle of around 1100 RPM or so in gear.

 

Vacuum advance shouldn't even be a factor until you open the throttle blades, right? Unless you hook it up to a full vacuum source but most of the time vac advance should be hooked to the ported source?


Hmmmmm...this makes me wonder. I should be pulling more vacuum from what it sounds like. My throttle blades are closing fully.
What is your vacuum reading looking like? Steady, erratic, fluctuating?

Lets assume the valve springs I got are the correct ones and my valve guides are not totally shot since they have had new seals installed. The only thing I can really think of is a vacuum leak from the lifter valley somehow but there is no way to test that. The block has been decked slightly and the heads milled slightly, I just don't want to go chasing my tail anymore.

 

Last time I checked the vacuum readings was last fall when I tuned the carb up to get it on the road. It jumped a little bit at idle between 13 to 15 or so. I haven't gotten around to putting the O2 sensor bung back on the exhaust so I might also be running rich at idle.

I recently just brought the fuel pressure down cause I found out my cheap Summit gauge was off by 1.5 PSI when I hooked up an Autometer I found in storage last weekend. So adjusted it down to 6.5 PSI and reset the floats. I blew the Fuel pump fuse so I couldn't get my idle screws reset, but it was pulling around 13 inches or so again anyways before that happened.

Oh I forgot to mention I did put around 100 street/highway miles on it and maintained a 15 MPG average.

 

Wow, impressive man.

I do have a leaky exhaust where the header meets the ypipe but that cannot be the reason this thing is so far off. I threw a little vid of the car running...see what you think?

http://www.youtube.com/watch?v=pH83rYnYFJ8

 

Vacuum advance to a ported source was a smog era thing. My experience has been that most cars like it to manifold vacuum. However, every combination is a little different, so you should try both. I don't think the leaky exhaust is having much impact here. It is possible that the valves are adjusted with too much preload (this is a hydraulic cam, right?). It could be the manifold gaskets. If the heads are shaved and the block decked, your "V" between the cylinder banks is smaller. The manifold doesn't want to sit down all the way. From the outside, it would seal. The problem would be in the lifter valley. If it is minor decking, I doubt that is an issue.

Check the carb baseplate gasket (between carb and manifold). Make sure the manifold bolts are tight. Check vacuum lines. Spray carb cleaner around the manifold gaskets and see if the idle pick up. Last on my list would be to pull the manifold and check the gaskets. That's all I can think of. Maybe someone else will have some better ideas.

 

I appreciate it man. I can pretty much rule out the baseplate gasket and vacuum lines.


The thing is, the car idles so inconsistent and rough right now you can pull a vacuum line off and not notice much, if any, of a difference. This makes me believe the manifold gakets may be leaking quite a bit but I can't rule out ignition or other valvetrain issues.

I'm thinking if I can get additional air to lean it out a bit it will clean up the idle and give some higher and more consistent vacuum readings.

Is there anything else that would be a prominent cause of that very erratic idle besides a vacuum leak?

 

Does it have more vacuum while in gear? I've noticed mine pulls much more vacuum while in gear due to the idle RPM coming down.

 

Dang I wish I had 15" vacuum at idle LOL. I have 10 @900+/- rpm
8.5:1comp. 274 blower cam 10* idle timing 25-28*total.

 

How do you guy's get the power brakes to work with that low vacuum ??

Auggie

 

Have manual trans I get 20"+ when I down shift. I don't put much effort on the brake pedal. Lol

 

You could insert some fine speaker wire into the idle feed restrictor to reduce the amount of gas entering the idle circuit. This is pretty easy to try just to see if it makes a difference (do a google search and you will find pictures showing what I mean). You have to remove the floats bowels and metering blocks. The IFR is at the top of the main wells (I think that is what they are called). I put the wire in and then up through the metering block gasket (so it stays in place when the bowel screws are tightened).

Auggie - I use a vacuum pump to get enough vacuum for my brakes. It works good, but I would love to go manual brakes and lighten up the front end!

 

Manual trans car. I don't have a gauge inside the cab so I can't get any readings under load.

I tried that trick on my Demon but, unfortunately, did not notice much of a difference. This carb is designed a little different but I do have the capability to tune both of those items.

The PV restrictions are merely pressed in, however.

 

Sounds like you are looking in the right places. If all else fails, you could try drilling the throttle blades. I have never personally had to do this, but I know people that have. I believe you are supposed to drill the hole in some sort of relationship to the transfer slots, but I am not sure what that relationship is.

That low vacuum still has me concerned. If there is a leaking intake gasket, you will never tune it away with changes to the carb.

 

Thanks for all the help, I need support. I just want to cover all my bases before I make permenant changes and/or a cam swap.

I need to take a step back and rethink everything but there are just so many possibilities for it running so badly.

Weak/MIssing Ignition...
Vacuum leak...
Cam out of time...


Anything else?

 

Valves adjusted incorrectly (or a bent push rod) is another possibility.

You could borrow a carb off someone just to see if it makes any difference at all.

 

Hey CamaroRUS,
Looks like you have lots of opinions and info there, I didn't have a chance to read it all.. Just to tell you what I have;


350 CID, 240/240 on 108 LSA solid flat tappet cam
10.3:1 CR, quadrajet carb. 4000' elevation. Ported 416 (305) heads.

I run timing locked out at 36*. It was a bear to get it to idle with less timing. I have a toggle switch in my ashtray to turn off the ignition for hot starting.
I have drilled my idle mixture screw holes open to 3/32"
I also have ~1/16" holes in my butterflies.
I have a 3-7" power piston spring in my Q-jet (irrelevant to you, but worth mentioning).
I have about 10-11" of vacuum at idle. Brakes work fine with 10", but as mentioned above, a stickshift downshifted will give you 20+" which is LOADS.

Car idles at 900 RPM or so. I could lope it lower if I wanted, but there's more chance of stalling it when taking off, so I leave it a smidge higher.

For your cam, I would think you'd need to lock out full timing. Fluctuating vacuum levels however usually means a vacuum leak or mis-set valvetrain.

I'd check:

- valve lash
- vacuum leaks
- lock out timing at 36*, put *vacuum* advance to 14* or so, and put it on ported
- set idle speed to keep it running, then adjust mixture along with speed for max vacuum at 800-900RPM. Drive it around and see if you like it?

 

Tried my 750 Speed Demon, this Summit brand double pumper, and a vac secondary Holley. Not sure which model but it is 2 corner idle; I'm thinking 4160?

Its a hyd roller and, if anything, the valve lash might be a bit on the loose side (its pretty much on though). I would rule out a bent pushrod just because I bought them new recently.



Thanks man, that helps a lot...maybe all isn't lost!

How's your idle....still rich? How is the vac readings at idle...do they fluctuate at all? If you looked at the video I posted the vacuum is all over the charts.

I will lock out the timing and try that along with opening up the air bleeds if I can. I just wish there was an easier way to tell if I have a vacuum leak from the lifter valley.

Keep it coming

 

Hey CamaroRUS;
Yea, my idle is rich. That's NEVER going to change with a big cam though. There's just too much overlap. You get so much exhaust reversion at low RPM that your intake charge is contaminated (think 2 stroke eh?), so in order to keep it running, you need to dump in more fuel. A big cam = rich exhaust. No way around it.

Idle readings are dead on smooth at 11" or whatever it was. I haven't driven it in a little while, as i'm restoring a monte carlo now for a buddy.

Check for vacuum leaks and reset your lash. Pull valve covers while it's idling and see what's up.

 

Do these pics give any indication if the manifold wouldn't be sealing up with the heads properly? There are the #2 and #1 spark plugs as well if that helps determine anything...

 

The manifold gap looks pretty consistent from top to bottom. I don't think the angles are off. That doesn't mean it isn't leaking. Also, the plugs look a little rich...but not horrible. I've seen much worse. I also watched your video. In the beginning, your vacuum reading looks like it is at about 11-12". Then, it drops and the car eventually dies. What changes between the beginning of the video and the end? Is the idle speed the same? Have you kept an eye on the voltage reading?

By the way, nice cars!!

 

Thanks man, those plugs only have about 20 minutes run time but look better than when the Demon was on it!

The beginning of the vid showed 8" with a little fluctuation (I would be happy with that if it was consistent!)...this was about 1000RPM.

When my helper opened the throttle a few times the idle did not settle back down but stayed around 1250RPM and I was at approx 10" vacuum.

You can see how sometimes the vacuum reading would float up or down over an inch randomly (not referring to the slight oscillation of the needle at all times) telling me something is wrong. The only time the throttle was opened is the 3 blips you can clearly make out.

At 1:00 I lowered the idle back down to 1000RPM and it then only showed 6" vacuum!! Before, you remember, it was at 8" at 1000RPM and, if you noticed, the vac fluctuated to almost 8" before it dropped to 5" then died...

Any ideas

 

You could experiment with the locked out timing by just increasing your initial (with a dial back timing light to check). Crank it up until you read about 40* at idle. Just don't rev the engine or your overall advance will go through the roof!

If that doesn't make a difference, I would be pulling the intake just to make sure there isn't a leak. The way the vacuum starts out good and then slowly drops makes me think that there is a vacuum leak OR the carb is leaking gas into the intake from somewhere and it is loading up and dying.

 

I would lean towards the carb loading up before an intake leak. with the vacuum readings as low as they are your fuel is not staying in suspension but rather regrouping into dropplettes thus dieing do to poor fuel atomization. your cam sounds very similar to our racecar cam and needs to idle at 1250- 1500 RPM's to keep the velocity high in the runners. otherwise the fuel deatomizes and the plugs look fouled from a rich syndrome.

 

That would be nice. I first suspected an intake leak from the lifter valley but when I pulled the plugs for the pictures i sis not look like any oil was present on them.

Actually, in the video when the car stalled out is the first time that has happened really

 

Something I just noticed with my setup since I redid my carb tune (FP, Floats, set screw, idle mixtures) I got it to idle in the 700-900 range, instead of the 1200 or so it was before. It now surges and sees the same vacuum conditions you describe it bounces from 6 to 10 inches. As soon as I increase RPMs it pulls up and steadies again. Maybe like Duckmanquacker said it just wants more idle speed?

 

You may be right, I can't see it wanting more than 1100 but I all for giving it what it wants. Problem is doing so gets my into the transfer slots and therefore pulling fuel from the main jets and running rich as snot!

I have to get it to idle around 1100 while keeping the .020" showing on the transfer slot.

 

one way to reduce the throttleplate angle is to drill a small hole in the primary side near the idle discharge port. start with about .060 and no larger than .125 . of course this means the only way to reverse the effects is to replace the plates.