Rough idle, low vacuum reading ?
 

New 59L engine preparing for break in on the road. Started it up recently and it ran rough at idle, enough to make the radiator vibrate a little. I checked for crossed up plug wires. They were ok. The vacuum gauge reading was 10-15, "carb adj., or late timing". I found by turning the idle mixture screws in all the way to lean, the vacuum went up to 18-20 in the green, and she smoothed out.
Have not checked the timing, but will as installed a pointer in the timing cover and a notch in the crankshaft pulley on TDC before I put the heads on. Charlie NY rebuilt the 21A dist. recently so we know that should be ok. He also did the 94 I am running back a few years ago, and it ran fine on the old stock engine.
I haven't pulled the plugs to see if it's running way rich, but will. Where would you start ?

Re: Rough idle, low vacuum reading ?
 

Running too rich. bad power valve? float set too high? just some possibilities Almost certainly a carb. problem.

Re: Rough idle, low vacuum reading ?
 

Running too rich. bad power valve? float set too high? just some possibilities Almost certainly a carb. problem. I would start by checking the plugs.

Re: Rough idle, low vacuum reading ?
 

What is the fuel pump output? A lot of the new/rebuilt pumps output to much pressure.

Re: Rough idle, low vacuum reading ?
 

Russ,
Seat both idle mixt screws then back each one out 1 turn. Fire it up and once warmed up turn one screw in until the motor stumbles, back that screw out 1/2 turn
and go to the other one. Once #2 is set go back and fine tune #1 then #2 again.
I'll keep an eye on this post and we'll all get it done.
Charlie ny

Re: Rough idle, low vacuum reading ?
 

If i lean out the carb, and she smooths out I also thought it had to be way rich. Would a defective power valve overpower the idle circuit to make it too rich. I will try a different carb, and also check the fuel pressure while I am at it. Will report back. Thank you.

Re: Rough idle, low vacuum reading ?
 

Just saw Charlie's response, and I pretty much tried that yesterday, and the only way it ran smooth and got good vacuum was with both idle mixtures screwed all the way in. Will keep you posted.

Re: Rough idle, low vacuum reading ?
 

I have always thought this was a sign of a leaking power valve. is this correct?

Re: Rough idle, low vacuum reading ?
 

That sounds like a classic ruptured power valve to me.

Sal

Re: Rough idle, low vacuum reading ?
 

Not all the time...a vacuum leak could be the guilty party....time to send it back....
Charlie
[email protected]

Re: Rough idle, low vacuum reading ?
 

Charlie, I don't believe it is the carb for several reasons. I checked the fuel pressure as JSeery suggsted, it was about 2.5#s. Swapped out the 94 for a 97, pretty much the same issue.
I failed to note that this engine is running a 1007B cam, and has a pretty high compression ratio.
What get's me leaning towards "Late Timing" is the vacuum readings, at idle, between 14-19. There was an excellent post on the barn back about 4 years ago showing 12 illustrations of vacuum gauge dials with analysis of the source. What I am seeing is "Hand drops regularly several divisions, 18 to 13", "late timing" , and "Hand drops 2 or more divisions, 18 to 15', "late timing" . The exhaust does not smell rich, but I didn't have time to pull some plugs or check the timing. Will update later. I will add, I have never had an issue with any carb Charlie has redone.

Re: Rough idle, low vacuum reading ?
 

With that cam, you will need more than stock advance. I would say 6 to 8 deg advance. Problem is, this may increase the max advance too far.

Re: Rough idle, low vacuum reading ?
 

Maybe Spray a little starter fluid around the carb base and throttle arms and see if it revs up a little. Maybe around the intake too. You have vacuum leak which could be the power valve also. But being you changed out to another known carb...


On my old yblock I would set up the dizzy to the crank dot and light. Then adjust a little to best vacuum, 1-2 inches, you are looking at 8-10". Being you can screw in both carb idle screws seems to be an issue with an open vacuum leak. Could be wrong.

Re: Rough idle, low vacuum reading ?
 

For reference, my 276 8BA idles at 550 and about 13-14 vacuum. The needle flickers just a little.

Re: Rough idle, low vacuum reading ?
 

"40cpe" - Since you have a 276, can we assume that your cam is other than stock?

I don't see where Russ mentions what kind of cam he has. Let's make sure were not comparing apples and oranges.

Re: Rough idle, low vacuum reading ?
 

Exactly how I was taught (on a single carb motor). Seat both idles. Turn out 1 and a 1/4. Start motor. Turn drivers side in till it stumbles then a 1/2 turn out. Then passenger side repeat. Repeat a few times. Should baseline it. Fine tune with a vacuum gauge.

Re: Rough idle, low vacuum reading ?
 

Post #11 says it is an Isky 1007B. I run the idle on my 276 with a Winfield SU1A at 800 rpm. I also have a crab distributor. If I was to do it over, I'd use a Charlie 8BA distributor with full vacuum advance at idle.

Re: Rough idle, low vacuum reading ?
 

Thats what I'm running with an 8BA Max 1 cam single 97 carb.
His distributors with an adjustable vaccum advance are great units to run.
Phil NZ

Re: Rough idle, low vacuum reading ?
 

So a personal question for my benefit. When using a high lift cam does it really effect engine vacuum? Meaning you have lower vacuum because of the cam. I know running dual carbs will but you set them up to work with it. Like changing pv to 3.5 vs 7 on a single carb.

Re: Rough idle, low vacuum reading ?
 

It seems I missed that important detail posted in entry #11.

It's not so much the lift of the cam that effects vacuum, but the duration and increased overlap.