guillaumeber

Hello all!

I must first admit that I'm a chevy Camro owner but I'm still trying to get some help from you because all my other attempts failed...

The message will be a little long, I apologize but I'm giving the most details I can to help you out and to proove that I have done my lessons and I have read A LOT ... my family thinks I'm crazy to always be reading on that topic...

I have been playing with my V8 327 chevy for the last month and now have it running pretty great... acceptable in fact...

I have rebuilt VERY carefully a told to be untouched quadrajet carb, adjusted everything, read a lot on the topic and did everything accordingly.

I have tested everything I could find, including vacuum leaks, compression, etc etc, and everything seems perfect, but still have a minor problem, I am addressing to you in case I'm missing something, nobody local has a clue about quadrajets, timing issues, etc etc...

My problem is the following:
The idling is unstable, it fluctuates once in a while, like a 50 (Sometime maximum 100) RPM variation, each 3-4 seconds... it doesn't sound very stable and healty. The vacuum flickers accordingly. I'm having a 2-3'' Hg variation while the idle fluctuates. The average approximate vacuum is 15-16'' , which is a little low, according to what I read. When put in ''D'' (on a rebuilt powerglide), the idle goes down a lot, it barely idles and sometime the engine dies. On ''D'', the vacuum is varying a LOT from 7 to 10'', very low!

My camshaft is a compcams XE256H, which is supposed to be a pretty mild cam with good Idle. http://www.compcams.com/Company/CC/c...x?csid=84&sb=2

Here are my other specs:

I am using Pertronix ignitor so I have nothing to adjust, no points, dwell to adjust, etc etc.
36 degrees all-in timing (VAC disconnected) (All in between 2800-3000 RPM, with the Mr. Gasket gold springs)
14 degrees base timing (VAC disconnected)
B26 VAC, tested directly within the specs, plugged to full manifold vacuum and it idles better than with ported vacuum.
30 degrees total Idle timing with VAC connected (So VAC pulls about 16 degrees, like it should)

While timing with the lamp, the timing sometime varies 1-2 degrees, I guess this is due do my unstable idle.

I'm unable to adjust the Idle mixture screws using the vacuum method because of the vacuum fluctuation of 2-3'' hg all the time. The screws doesn't seem to make a lot of difference on the vacuum. However, i am able to kill the engine with the screws all-in.

Ignition coil, spark plugs wires and sparkplugs are new and tested. Engine rebuilt. Steady 180 PSI compression cold on all cylindres, WOT.

I isolated all the vacuum circuits and plugged the ports. Then, I checked for vacuum leak everywhere using the ''propane torch'' method and also by spraying water everywhere there could be a leak. I also checked for a possible PCV valve leak, tested good.
I just changed the intake gaskets with no apparent difference.

Idle mixture screws kills the engine, so i'm idling on the idle circuit.
I can Idle anywhere in-between 2 to 6 turns out on the idle mixture screws without a difference.
Carb has oem jets, rods, float (and good float height) ; cleaned the idle circuit, removed the idle tubes to clean, etc etc..

I think I have listed here everything Needed... If any more information is needed I'll give needed info.

I will be posting on the team camaro and chevelle forum too to reach more people, just letting you know!

thanks a lot everyone!!

g.b.

guillaumeber

Edit:

I found that the aftermarket springs did not hold the weights completly retracted at idle, this problem has been resolved.

However, it did not address the main problem, the engine still acts the same.
I have tried 2 different rebuilt carbs with exactly same symptoms.

I have tried advancing and retarding the timing on ''Drive'' without beeing able to get a good idle on ''D'', getting VERY low vacuum (5-10) and the engine runing very irregulary and dies after 4-5 seconds on ''D''.

I was wondering... what would be the symptoms of incorrect valves timing?

Otherwise, I read everywhere that stalling in gear is almost always caused by vacuum leaks...
I have checked every possible vacuum leak source... I might be missing some possible spots for vacuum leak...

thanks a lot!

g.b.

OMF

CAMARO....sorry can't help!

Kidding


Since you have tried two other carbs with the exact same symptoms....I'd look at your valve adjustment on all cylinders. You could have one hanging open slightly giving you the erratic idle and fluctuating vacuum.
How did you adjust them to begin with?

I just re-read the main post and see that you have 180 PSI on all cylinders.....so maybe thats not your problem.
How does your intake manifold and the cylinder head line up? Possibly one has been machined for milled heads, and is leaking at the gasket internally....

What manifold are you using?

How about the carb to manifold gasket.....are you using an open one ore the four hole version?

guillaumeber

The engine has been rebuilt by a professional shop about 5-6 years ago. However, this shop doesn't exist anymore. Can't say about the valves adjustment to be honnest but the shop must have adjusted them right... my compression test also prooves it good. (I think I have hydraulic lifters, which, I think, means that I have no valves adjustment? )

I am using the stock intake. Stock heads type too. seems like the head and manifold lines up very good but is there any way to check the aligment? I have installed a new set of intake gaskets last week using a small amount of RTV on the intake ports just to be 100%% sure of no leak... made no difference!

For the carb to intake gasket, I'm using the 4 holes type. The thin metal ''gasket'' sandwiched between 2 normal gaskets.
I have tried to source a leak at that area with the ''propane'' method and also by spraying water all around... with no success..

Thankss
g.b.

MajD

Your engine is behaving a lot like there is a vacuum leak somewhere. I don't know what your accessory set up is, but often there is a vacuum line feed off of the manifold. If there is a leak down stream of that feed, it could cause the symptoms you describe. An easy way to check it is to pinch the feed shut with a needle nose and see if the idle picks up and remains steady.

MajD

I forgot to mention also that as vacuum line ages, it shrinks and stiffens. This causes it to be loose at the connecting points, and sometimes crack, which gives you an overall leaky system that could be tough to diagnose using the pinch method. If you can't find the leak by pinching the lines, or pinching the lines doesn't change the idel much, I would consider replacing the lines if you haven't already. Vacuum line is inexpensive.

Unforgiven1

What year model are the carbs you have used? Later model Qjets can have this issue due to insufficient fuel flow in the idle circuit. Often not an issue until the engine is modified, an aftermarket cam is enough to cause a problem. The circuit can be modified if you work carefully. Run a search on the internet for 'quadrajet idle circuit fix' or 'quadrajet idle circuit too lean' and you should be able to find instructions.

OMF

The correct gasket for your carb is about 1/4" thick with hard inserts where the bolts go through....use the four hole version for the best idle with a stock intake.

What do your plugs look like?
Maybe your having a misfire due to crossed plug wires, spark going astray.....try running the car in the dark and look around for stray sparks....

Have you checked to see if the cam has gone flat on a lobe??

guillaumeber

Hello hello!
Thanks all for your inputs!
Everything I read everywhere seems to lead to vacuum leak... BUT
All the hoses are brand new AND
I also disconnected every vacuum tubes from the intake to the accessories and plugged the intake side with rubber caps I have. Even the power brake tube.

I have tried running 2 different quadrajets from 1969 for 350 vettes.
Will try a later quadrajet from a 1984 caprice 305 tomorrow.

I tried all imaginable timing setups too. I get strong spark when un-plugging the wires... at least 3/4 inch of blue spark at each wire.

Are there any ways to be sureee that the intake gaskets are not leaking internally?

Would bad valves/cam timing be a possible cause of my problem?

Is it possible for my powerglide converter to grab WAY to soon and stall the engine?

I am able to rev the engine without ANY problem and flawlessly up to high rpms. Very responsive.

Ill be able to do a leak down test soon to confirm all my internals, even if the compression seems perfect.

Thanks again
g.b.

guillaumeber

Sstocker:
All wires are new (ac delco) and far from dry. I will however test for spark jumping from a wire to another in the dark but im pretty sure it's not the case.

My plugs looks just as they should, according to the various " charts" i see for correct running engine.

The camshaft is also brand new. Has ran about 3-4 hours total... could it really go flat in such a small running time?

Dont get me wrong but I don't think the correct gasket is not the 1/4 thick one. Ive seen the thin metal sheet like gasket everywhere combined with a thin grey composite gasket. In any manner, I couldn't go with 1/4 thick because I am using the divorced choke setting and the rod from the intake to the carb wouldn't be long enough to reach...

Still thanks!

G.b.

guillaumeber

I should maybe say that I tried(for a test) raising my idle a lot.. around 1100-1200 rpm to see of it would act the same when put on "D"
it did exactly the same... very rough running. .. lowww vacuum and died after some second.

What should be a correct idle vacuum for a pretty much stock engine like mine?
It has a cam but reallyyy nothing excessive. .. you can see the specs of the cam in 1st post.

G.b.

Unforgiven1

If you're sure that both carbs are from 69, they should have a rich enough idle circuit. The 1984 unit most likely will not. Yes, the cam could have run a lobe or more flat in that short of a time. Try doing a cylinder balance test by pulling one plug wire at a time and watch for the same amount of rpm drop from each cylinder. Did you adjust the valves or did someone else? What method was used? Are you sure the timing reading is correct? Bump the base timing by 2 degrees at a time and see if it makes any difference.

guillaumeber

Hello thanks for your time!

Unforgiven1: How could I test for a flat camshaft lobe? Would the balance test show it?
What would cause a flat lobe on a brand new cam? I have been using break-in oil when engine new.Will do the balance test for sure... Already did but did not check the exact amount of RPM drop.

The valves have been adjusted by the shop that rebuilt my engine some years ago. Don't know about the method. I am using hydraulic lifters which means I have no adjustment on the valves...I think?

I have adjusted base timing (VAC disconnected) to anywhere between 10 and 20 degrees. Then plugging the cannister to full vacuum. I have also tried sucking the cannister diaphragm with a vacuum pump because I know my engine is not able to pull the vacuum cannister completly with it's low vacuum at idle. (I might have to buy a softer VAC canister but I'm simulating higher vac with hand pump to pinpoint where my problem is)

No difference.
I'm thinking now, after all my tests, that the problem is NOT carb and NOT distributor/timing...

This is starting to frustrate me... I am putting so much effort!

thanks all for the help!

gb..

OMF

Yes a cam can go flat in short order....I had one go during breakin 20 minutes tops.
You can check this with a dial gage on the valvspring retainer, Yours should be .447" intake and .454" Exhaust.

You might want to confirm that you have a solid 12v getting to your distributor at idle. I've seen this before where it isn't.

When you look down the carb at idle can you see any fuel dripping out onto the butterfly valves....(there shouldn't be any fuel dripping)

The balance test that Unforgiven1 refers to should show any valves that are mis-adjusted, so do that first to find out what you have....if adjustment is needed we can help with that too.

guillaumeber

Hey!
Am I wrong saying that having hydraulic lifters, my valves shouldn't need any adjustment? Maybe I'm wrong... not sure.
I have checked for constant 12V at my pertronix at idle and it does have the same voltage as direct battery (about 0.05V less)

How in the world could a cam go flat after 20minutes (even after some hours) of running? Poor cam material? wrong adjustments? Comp cams seems to be a respectable company... that surprises me!

I'll take a look at the inside of the carb next time I start it and let it idle... I don't think that the nozzles are dripping and I think that the car idles on it'S idle circuit because screwing the idle mixture screws completly IN kills the engine. however, i'll still take a look!

thanks once again!
g.b.

blue427

just a long shot,but did you check the line (steel and rubber) to the vacuum modulator valve? Maybe a small internal leak in the valve itself? They're cheap enough to change.We used to get cars in with the lines that would rub and chaff(that was in the 60's so it's been a long time since I've given any thought to problems with automatic trans.

OMF

Hydraulic lifters do need to be properly adjusted....there is a zero lash preload that needs to be set. You do this by seting the cam lobe on it's base circle, tightening the rocker arm nut until you can just feel no up and down movement of the pushrod when moving it up and down pinched between your fingers, and then usually 1/4 to 1/2 turn more. If it's tightened too much it can hang a valve open.

Solid lifters should have around a .020" gap between the rocker arm and the valvestem at Top dead centre, or on the base of the lobe as above.

guillaumeber

Great! Will pull the valve covers and check... However, wouldn't my compression test show me a valve stuck open?

Also, where can I get the good spec for the torque, you said ''Usually'' 1/2 to 1/4 ...

g.b.

Patro46

Sounds like a vacuum leak. With the engine running restrict the air somewhat going into the throat of the carburetor. If a partial restriction causes an increase in idle you've got yourself a leak. You can also take something like WE 40 and spray around the intake runners and carb base as well, looking for a change in idle. Any fluctuations and you have a vacuum leak. Also, the idle air bleed circuit on a Quadrajet is tiny. I'd probably pull the adjustment needles and blow the passage out with some Berryman or compressed air. Timing with aftermarket cam. Advance the timing to where it pings under load. Retard timing until it doesn't ping. Leave it and move on. From your description, my vote is vacuum leak.

guillaumeber

Hello!

blue 427: Those hoses are new and, for the purpose of diagnosis, I unplugged it from the intake and plugged with rubber cap!

Patro: Will check again for vacuum leak.. restricting intake did nothing and finally killed the engine last time I tried!


g.b.