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Old 10-26-2010, 03:54 AM
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Slight pulsation at idle?

Hi, i have a 350 small block, mildly built, quadrajet carb. and i notice a slight pulsation at idle. Can anyone clue me in as to what might cause this and if it is a problem? I was thinking it might be caused by a slight vacuum leak somewhere on the carb. or could it maybe have something to do with the ignition?

thanks

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Old 10-26-2010, 06:49 AM
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Check your timing and rpm, you might be at the beginning of your advance curve

what's your vacuum reading at idle?
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Old 10-26-2010, 06:49 AM
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Maybe a lope due to non stock cam?

Idle will typically smooth out w/ increased rpm.

Tune idle by adjusting rich/lean screw (if equipped) and idle speed screw.

I rebuild a Q-Jet a few years ago and can't recall a rich/lean screw?

Can adjust to lowest smooth rpm while tuning rich/lean screw to 1/2 turn CCW past too lean (rough idle) setting.

Some mild engines can be tuned to max idle vacuum.

Verify adequate oil PSI when setting idle at low rpm.
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Old 10-26-2010, 06:57 AM
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One other possibility is to check your throttle plate to see if it is properly adjusted. If not, this can result in surging at idol (exposes too much or too little of the vac slit at the base of the throttle bore).

If so equipped, I'd also suggest checking for a faulty ERG valve. This can also result in surging.
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Old 10-26-2010, 11:22 AM
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Quote:
Originally Posted by bubbahotep
Check your timing and rpm, you might be at the beginning of your advance curve

what's your vacuum reading at idle?
hey bubbahotep, sweet car!

as far as timing goes, I'm not sure where to set the initial timing. I've read different things that say 8 deg btdc, 10 deg, 12 deg...? now it's somewhere between 10 and 12 deg btdc. and that's with the vac adv disconnected. i had the vac adv hooked to ported vac on the carb but recently changed it to full vac to see what that would do, and of course it's much more advanced at idle, it's maybe somewhere around 20 deg, can't tell exactly cuz dont have fully degreed balancer. but it does seem to run better at idle with the full manifold vacuum hooked up.

so my vacuum reading at idle was zero when the distributor was hooked to ported vac, and around 15 psi when hooked to full vac.

also, what do you mean by "beginning of advance curve"? I'm not sure what that is.

thanks
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Old 10-26-2010, 11:27 AM
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Quote:
Originally Posted by 001mustang
Maybe a lope due to non stock cam?

Idle will typically smooth out w/ increased rpm.

Tune idle by adjusting rich/lean screw (if equipped) and idle speed screw.

I rebuild a Q-Jet a few years ago and can't recall a rich/lean screw?

Can adjust to lowest smooth rpm while tuning rich/lean screw to 1/2 turn CCW past too lean (rough idle) setting.

Some mild engines can be tuned to max idle vacuum.

Verify adequate oil PSI when setting idle at low rpm.
I was thinking the cam might be the culprit, but it seems more like a slight surging than a lope.

I tried tuning the idle adjust screws on the bottom front of the carb with a vacuum gage hooked up, but the vacuum gage is surging right along with the idle so it makes it a bit difficult, it pulses between about 14 and 15 psi and if it would stay steady it would be easier to tune those adjusting screws.

so yes, max idle vacuum was what I was trying to achieve.

and yes, there is adequate oil pressure at idle.

thanks
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Old 10-26-2010, 11:29 AM
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Quote:
Originally Posted by cboy
One other possibility is to check your throttle plate to see if it is properly adjusted. If not, this can result in surging at idol (exposes too much or too little of the vac slit at the base of the throttle bore).

If so equipped, I'd also suggest checking for a faulty ERG valve. This can also result in surging.
it's a newly remanufactured q-jet i got from Summit so i doubt there's a problem with the throttle plate, but if it comes down to it I guess I can check that.

and there is not an EGR valve.

thanks.
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Old 10-26-2010, 12:26 PM
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I adjust idle by ear when cammed out due to erratic readings.

A heavy flywheel masks idle surging. Don't suppose you have a HP light flywheel.

If auto, how does idle compare from N to D?

What RPM clears up the surging?

If idle passages are not smooth and clean it could lead to erratic fuel flow in idle circuit. Not likely.

If carb float fuel shutoff valve is leaking it could surge at times when carb is flooding.

Most important timing adjustment is total advance. I run 38* on 351W. Idle advance can be adjusted like you did to achieve your desires. I run full vac to dist (351W) for improved starting and idle.

If you find no joy you could post a video on youtube so others can hear it's sound.
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Old 10-26-2010, 12:45 PM
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What can you tell us about the cam's specs?

Pat
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Old 10-26-2010, 01:08 PM
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cams specs? compression? initial timing? Vacuum at idle? Have you tried using a vacuum canister?
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Old 10-26-2010, 10:27 PM
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Quote:
Originally Posted by 001mustang
I adjust idle by ear when cammed out due to erratic readings.

A heavy flywheel masks idle surging. Don't suppose you have a HP light flywheel.

If auto, how does idle compare from N to D?

What RPM clears up the surging?

If idle passages are not smooth and clean it could lead to erratic fuel flow in idle circuit. Not likely.

If carb float fuel shutoff valve is leaking it could surge at times when carb is flooding.

Most important timing adjustment is total advance. I run 38* on 351W. Idle advance can be adjusted like you did to achieve your desires. I run full vac to dist (351W) for improved starting and idle.

If you find no joy you could post a video on youtube so others can hear it's sound.
it's an auto, i've only checked it in park, i'll have to try N and D and get back to you.

as far as total ignition timing, I really don't even know how to do that. Could someone tell me how to set total ignition timing, or total advance, I only know how to set initial timing at idle. I have a standard GM HEI ignition., thanks.
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Old 10-26-2010, 10:33 PM
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Quote:
Originally Posted by PatM
What can you tell us about the cam's specs?

Pat
cam specs: it's a Crane Energizer Camshaft #100052 Hydraulic flat tappet

Duration at 050 inch Lift 216 int./216 exh.
Advertised Duration 272 int./272 exh.
Valve Lift with Factory Rocker Arm Ratio 0.454 int./0.454 exh.
Lobe Separation (degrees) 110
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Old 10-26-2010, 10:39 PM
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Quote:
Originally Posted by turbolover
cams specs? compression? initial timing? Vacuum at idle? Have you tried using a vacuum canister?
cam specs: listed above

compression: don't know exactly, but I'm pretty sure it's somewhere between 9:1 and 9.5:1, it doesn't knock running 87 octane.

initial timing: I was experimenting with different amounts of initial timing between 8btdc and 12btdc with the vacuum adv disconnected and plugged. I'm not sure what the proper place to set it to is.

the measured manifold vacuum at idle was around 15 psi so I didnt think i needed a vacuum canister. is 15 a good amount?

thanks,
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Old 10-26-2010, 10:45 PM
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With my Q-jet that I rebuilt with parts and info from Cliff Ruggles I had to turn the idle jets out 5-1/2 turn from bottom to get it to idle smooth . I called him back and he said that was not unusual . I have a stock grind camshaft and run 22 in Hg at 800 rpm idle with manifold vacuum advance and 24 deg advance at idle . BTW Vacuum is not measured in PSI . It is inches of mercury (Hg) . I know 15 in. Hg is not a stock cam
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Old 10-26-2010, 11:12 PM
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Quote:
Originally Posted by adantessr
With my Q-jet that I rebuilt with parts and info from Cliff Ruggles I had to turn the idle jets out 5-1/2 turn from bottom to get it to idle smooth . I called him back and he said that was not unusual . I have a stock grind camshaft and run 22 in Hg at 800 rpm idle with manifold vacuum advance and 24 deg advance at idle . BTW Vacuum is not measured in PSI . It is inches of mercury (Hg) . I know 15 in. Hg is not a stock cam
oops, guess i didnt really look at the units on the vacuum gage, thanks,

5-1/2 turns? wow, mine are around 3 turns, i thought that was normal, but i can try adjusting them more and see where it gets me.

i do have the Cliff Ruggles book: "How to Rebuild and Modify Rochester Quadrajet Carburetors", but it seems to be more about rebuilding them than about tuning them. I wish it had more info on tuning.

also, I have my idle set around 600 rpm, per the instructions that came with the carb from Summit. do you think i should try it higher, around 800 rpm as you have it?

thanks.
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