Can't get it to idle right!

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Valkman

Valkman

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Hey all, I've been trying to get my project car sorted out, one of the problems I'm having is that after I get the engine warmed up the idle will jump all over the place!

Here's what I got:

It's a 340 with J heads, RPM intake, an Edelbrock 750, and stock electronic ignition with a vacuum advance.

I don't know much about the motor. I got a great deal on it, it runs great (when not at idling) and it has good oil pressure and compression.

The previous owner told me it has hydraulic cam (it has a pretty good lick to it and driving down the road it really wakes up at around 3K rpm), and it also has a high stall converter.

I set the timing to 30 total advance @ about 2K rpm. all I could figure is that distributor might have issue, or maybe the vacuum advance may be the culprit?

I have never have dealt with a motor this wild before and I could use some advice.:D
 
You say 30 total advance. What's initial?
What's initial w/o the v-can?
How much does the v-can add?
What's your manifold vacuum reading (does it bounce around a lot)?
How many turns on the idle mix screws?
How open is the idle screw?
Are your transfer slots overly or under exposed?

Lots of things to check on and see what it might be.

As a general rule, more cam = more initial timing (but you want to limit total if you change the initial). Initial timing with a decentish cam can range from 14-28 degrees but it will depend on many factors including ACTUAL compression.
More cam can also take more idle air which can expose the transfer slots in the eddie carb. Some carbs will require holes drilled into the butterflies to get enough idle air while keeping the transfer slots closed down enough - search and you'll find a lot of info.
Easy way to tell if the transfer slot needs attention: close the mixture adjustment screws completely. Don't over torque, just lightly hand tight. If the engine keeps running, you're pulling fuel from somewhere and it isn't the idle circuit.
 
What is the initial? What did you do to get 2k? Too light of springs may not let the advance return. Disconnect and plug the advance to rule it out. Have you checked for vacuum leaks?. By idle jump, what rpm is it jumping too and from and is it in great, park or both?
 
Are you running a PCV valve? Give it as much initial timing as it likes. Then check to see how far open the primaries are to keep it running. You may need to increase idle air bypass by either getting a different PCV valve, cracking the secondaries open a little or drilling small holes in the primary throttle plates. Once you get it to settle down a little you need to see if it has enough vacuum to keep the metering rods pulled down at idle. If not you need to get a spring kit. Definitely check for vacuum leaks. If it smooths out when you choke it a little by hand you are losing vacuum somewhere or clogged idle circuits.
 
Thanks for all the replies.

When I set the total timing I just rev til the timing stops advancing, then set it to 30, at idle (900) it,s 12 with dis. Vac connected. The fluxuation, goes from 15k to stall 500, just sitting at idle. No pvc , just a hose to the air cleaner. I should have thought to check the vacuum, I'll do that tommorrow.
 
The usual culprit is a vacuum leak of some kind. The engine drawing more air but less fuel will cause it to run up and down as it tries to "balance" itself. I would start with that. From there the next choice would be carb. Depending on how often and the last time it was run there could be problems with it. You'd be amazed what gas today looks like in just 6 months of sitting inside a carb.
 
Lustle said:
The usual culprit is a vacuum leak of some kind. The engine drawing more air but less fuel will cause it to run up and down as it tries to "balance" itself. I would start with that. From there the next choice would be carb. Depending on how often and the last time it was run there could be problems with it. You'd be amazed what gas today looks like in just 6 months of sitting inside a carb.
Click to expand...

Exactly, this plus the vacuum advance will cause it to surge. As the engine speed reduces, vacuum goes away and so does advance and the rpm drops more.. Rinse repeat.

You really need to set your initial timing properly and then modify your advance mechanism to limit total. Otherwise you'll have idle but problems at WOT or you'll have WOT but no idle..
 
30 Degrees total timing with j heads... sounds short by quite a bit unless it pings like crazy with any more.

I'd bump it to 36 total and see how the idle reacts.
 
Ok I did some checking today. I went ahead and pulled the distributor to see if it might have a broken advance spring or something, but all looked good. the heavy spring has been replaced with a aftermarket light one though. I tried to set the initial timing but I still was having the varying RPM issue. It's not so much as surging, but revs up on it's own to about 2k after a about 10 seconds sitting at idle and it stays there. If I back so much as a 1/2 turn on the throttle screw, it stalls! I did check the vacuum and it reads a solid 12 at 900 rpm, and 22 at higher rpm's. I even spray starter fluid around the intake and the base of the carb with no variances?

:banghead:
 
Where did you set the initial this time? Please list those details or no one can help.

Disconnect the vac can for now - it's not helping isolate the problem.

How many turns out on your idle mix screws are you?
 
Is the plug missing from the back of the carb where the auxiliary vacuum port is? You do need a PCV valve to help keep oil leaks at bay. The last 750 Eddy I had had loose throttle plates. You may want to take the carb off and back out the idle screw and make sure the throttle plates are centered and the screws are tight.
 
If it runs perfect when cold but idles up and down hot, I would suspect fuel percolation.
When I had a see thru filter mounted horizontally above the intake I could watch the filter fill, then empty, then fill again, cycling the engine lean repeatedly. Only after hot though.
 
Do you have headers or cast iron manifolds? Sounds like the heat crossover in your intake is boiling the carb. You may have a stuck EFE valve in the passenger exhaust manifold. Also a thick base gasket under the carb is a must.
 
Thanks, all for input:D

I don't think it's a vacuum leak, because spraying the manifold and carb base with starter fluid didn't effect anything.

Phreakish

I set the initial timing at 12 deg. btc at about 900 rpm vacuum advance blocked, but hard to say because it doesn't stay there long

mguner

Yes a cast iron magnum manifolds so no heat riser valve so I don't think that's it.

RedFish said:
If it runs perfect when cold but idles up and down hot, I would suspect fuel percolation.
When I had a see thru filter mounted horizontally above the intake I could watch the filter fill, then empty, then fill again, cycling the engine lean repeatedly. Only after hot though.
Click to expand...

I think your on to something there redfish. I have a clear filter, and it does look like it's running out of fuel. What did you do to fix this?
 
Valkman said:
Thanks, all for input:D

I don't think it's a vacuum leak, because spraying the manifold and carb base with starter fluid didn't effect anything.

Phreakish

I set the initial timing at 12 deg. btc at about 900 rpm vacuum advance blocked, but hard to say because it doesn't stay there long

mguner

Yes a cast iron magnum manifolds so no heat riser valve so I don't think that's it.



I think your on to something there redfish. I have a clear filter, and it does look like it's running out of fuel. What did you do to fix this?
Click to expand...

That's probably not enough. Most folks would suggest starting at 18-22 with the vac plugged.

Air or lack of fuel at the filter isn't abnormal, having it cycle when hot is though.

Does the idle only fluctuate or stall when warm? Or all the time? Electric or mechanical fuel pump?
 
Yeah when it goes off choke (I have an electronic choke) it seems to start acting up.
 
And I have a stock mechanical fuel pump
 
Valkman said:
Thanks, all for input:D

I think your on to something there redfish. I have a clear filter, and it does look like it's running out of fuel. What did you do to fix this?
Click to expand...

Same things factory did. Reroute fuel line, relocate fuel filter, thicker base gasket under carb. And of course make sure the bowl vent remains open when throttle is on curb idle stop. That adjustment varies with carb type.
When my base timing went up, curb idle went up, backed of idle stop, vent closed.
 
your still running winter blended fuel and the temp jumps to 80 try mixing some racing fuel with the gas if its heat related fuel it should stop surging .
 
does your car have a power brake booster. If so you may have a vacuum leak from a bad booster. if you pinch the vacuum line from the booster to the carb the problem should go away. Then change the booster.
 
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