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Discussion Starter #1
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/cam-specs/Details.aspx?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 chevelle forum too t reach more people, just letting you know!

thanks a lot everyone!!

g.b.
 

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Well most older engines have a little rpm creep by a slight amount, more with high mileage blocks. Mine wonders back and forth about 30 rpm. This can be caused by non mechanical issues such valve issues or vacuum or bad plugs or wires.

If its got a rhythm to it then its based on a particular cylinder or valve. Mine is tied to the rings on number one. It is slightly lower in compression then the other seven. If you are hitting a difference of 100 rpm then I would think you have more then a creep going on.
 

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Did you install new throttle shaft bushings? Quite often, the bushings will wear out of round causing inconsistent idling. That's where I would start!
 

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Take another look at your timing fluctuations at idle. Sometimes aftermarket recurve kit springs can't pull the centrifugal advance all the way back to full retard at idle speed. In other words, the timing starts advancing at something less than idle speed. This makes for an unstable idle that is difficult to tune. You generally don't want any centrifugal advance until 200-300 rpm above your desired idle speed. Your listed advance numbers are good and the cam you are running should give a smooth idle.

Something to try is to check your timing at idle while a helper puts the car into gear. Do this with the vacuum advance disconnected. The timing should stay the same as when in park. If the timing retards, try some stronger springs or whatever it takes to get the timing stable at idle.
 

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Discussion Starter #6
Hello all!

Red67Camaro, you were right, 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.
 

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Hello all!

Red67Camaro, you were right, 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.
You've eliminated the carb obviously...maybe reaching for straws here, but, have you checked the transmission vacuum modulator and hose since it seems to adversely affect engine operation when shifting to "D"? Is it possible your losing considerable vacuum in that area?
 

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Discussion Starter #8
I have tried with the modulator connected to direct vacuum and also tried not connected at all. No difference.
I tought this modulator needed vacuum signal to know when to shift... not actually for idle?
Am I wrong?
Would a bad modulator cause my symptoms?

Thanks!
G.b.
 

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This is info I have to offer.
Good troubleshooting, you isolated the intake system by plugging all ports.
Leave PCV connected with a breather cap on the other valve cover.
If using alum manifold, eliminate the end rubber seals and use RTV for sealant in these two places. Torque manifold into place with either OE design gaskets or Mr. Gasket paper gaskets dry - as you know, get a good seal.

Vac modulator does control upshift point - more throttle, later upshift.

You are not going to get as much full vacuum with this XE cam verses stock and short stroke of the 327 engine. As I remember, OE 327 pulled 17-18 inches .
XE cam has 110° LSA with ICL of 106°.
Compared to OE cam, 3896929 – 195°/202°@0.050" – .390"/.410" – 112° LSA, therefore, the XE valves are open longer than OE and with short stroke, will not create as much vacuum. So, you have to fudge and move up the idle RPM - ity won't idle at 500-600 like stock.

Crank up rpm and you will see vacuum go up, this is the vac value you would like to get close to at idle. Your idle may need to be around 900 or so.
One method is set initial timing without vac cannister hooked up, to an acceptable value for hot start - if your present 14° is good, cool, go for it. The next you might do is hook vac cannister to full vacuum as this will add timing - most aftermarket cams run better this way.
Then turn down idle screw and place in Drive and see what happens - you should lose about 200 rpms with stock convertor - this doesn't help either, but doable.
You may need to open secondaries for alittle bypass air to help. Good luck with a Q-jet.

Good luck - trial & error.
 

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I have tried with the modulator connected to direct vacuum and also tried not connected at all. No difference.
I tought this modulator needed vacuum signal to know when to shift... not actually for idle?
Am I wrong?
Would a bad modulator cause my symptoms?

Thanks!
G.b.
Guill...you're right about the modulator and shift points, however, a bad modulator will allow tranny fluid to be sucked to the carb and burned, thus, fouling the plugs. It was just a thought as your efforts have been pretty thorough.
 

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Eventually the Pertronix will get affected by heat from the engine and will give you carb problem symptoms then eventually it'll die weeks after. Ask me how I know. Remove the Pertronix and put in points/condensor and see how it runs.
 

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Discussion Starter #12
Thanks for taking the time to answer!

Everett: I am using stock intake. I have used fel-pro intake gaskets with RTV instead of the end rubber seals. I have used a small amount of rtv on all the ports, including the intake ports just to be 100% sure of no leak in that area.
I just remembered that, when rebuilding the engine, the shop put a 350 crank in the 327 block, thus giving a longer stroke, almost transforming this block into a 350 one. Would the vacuum be significantly higher comparing a 327 and a 350?

Actually, to get a good idle, I can't go anywhere under 900 RPM, this is when I am getting 14-15 ''hg (tried 2 different vacuum gauges)

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)

I have been able to get a perfectly stable idle (900rpm) with the second carb I tried and also using stiffer distributor springs to be sure there was NO mechanical advance at all at itle. However, this new stable idle did not solve my problem of stalling in gear.

Vettefreak: Thanks for the input, I didn't know oil could be sucked from that modulator.
However, for the purpose of diagnosis, this system is and will stay disconnected.

Copo: I am thinking of trying a stock points setup. I am doubtfull as I get very stong spark when unplugging each wires (at least 3/4 inch of blue spark) and I have read very good comments on the pertronix system. I will have to learn how to setup a standard points distributor, the dwell, etc etc... :(

Thanks again all, still looking!

g.b.
 

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Apparently, I misread on stroke. But, with stock 350, they should obtain 18"-20" of vacuum.
Your 15" at idle is good.
 

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Discussion Starter #14
I'm confused... not sure why my 15'' at idle is good when you say a stock 350 should get near 20''... my 327 has a 350 crank, thus giving a 350 stroke lenght. I understand the cam gives a little less vacuum but the cam is pretty mild

thanks
g.b.
 

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Check out this website, it explains how to use and interpet a vacuum gauge. Click on each scenerio trying to find yours. It is nothing wrong with 15 hg. of vacuum. The more duration you have the less vacuum you will have. The lift doesn't affect the vacuum that much. Engine timing effects vacuum too. Also a lower LSA will will give a lower vacuum reading than a 112 or higher LSA of a camshaft with the same duration.
 

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Discussion Starter #16
Humm, which site? No link?
I can live with the 15'' vacuum at idle... the problem is the 5-10'' when put in ''D''

thanks
g.b.
 

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With my low vacuum at idle (10") my stock 201 15 VA can was not working. So I bought a B28 but it gave me 34* timing at idle and the engine was surging up and down. So I made a limiter plate to give me 24* at idle with full manifold vacuum on my 70 Z28 with a roller cam.
It's all in this section on my site http://www.nastyz28.com/~copo/distributor.htm



 

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Discussion Starter #18
Hello COPO!

I will surely need to do something like this when my engine runs right.

For the moment, I tried all the possible timing setting at idle (without using the vac canister, I went from 10 to 35 degrees) and nothing made my engine idle in ''D'' or 'Reverse''

Thanks for the info and pictures!

g.b.
 

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Pull a couple spark plugs and check to see if they are black with soot.
I suspect they are - too rich.
I believe the reason being, when in gear and vacuum falls - 15" to 5" - the primary metering rods are pulled up out of their jets by the spring as less vacuum on the piston to pull them down.

Get lighter spring allowing the piston to do its job, or get/beg/borrow/cumshaw a Holley carb.

Think about the stroke between the two engines.
The shorter stroke has less volume for creating vacuum compared to the longer stroke due to piston speed on the intake stroke is faster on the long stroke than short stroke, thus more vacuum is created. Stock 327's generally had 16"-17" Hg, 350's have 19"-20" Hg.
You have a 350 since you put a 3.48" stroke shaft into a 4" bore block; 327's are the same 4" bore with a 3.25" stroke.

More volume plus faster speed = more vacuum.
 

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Stuff like this is maddening.
A wise man once said: "Most carburetor problems are ignition related."

How about some history. When did it start? Has it ever idled correctly since being built?

Here are my two caveats: 1. I have no experience with that cam. Was it installed straight up?
2. I have never installed a pertronix.

Do you have an extra points dist to stick in? That is where I would start. The pertronix may be superior when working correctly. But, this wouldn't be the first one to have issues. If the carb is the issue, and I am not saying it is, you cannot diagnose it correctly until you have eliminated the electrical.
 
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