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Old 05-23-2021, 07:56 AM   #1
russcc
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Default 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 ?
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Old 05-23-2021, 08:11 AM   #2
fordwife
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Default Re: Rough idle, low vacuum reading ?

Running too rich. bad power valve? float set too high? just some possibilities Almost certainly a carb. problem.
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Old 05-23-2021, 08:13 AM   #3
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Default 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.
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Old 05-23-2021, 08:19 AM   #4
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Default Re: Rough idle, low vacuum reading ?

What is the fuel pump output? A lot of the new/rebuilt pumps output to much pressure.
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Old 05-23-2021, 09:17 AM   #5
Charlie ny
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Default 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.
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Old 05-23-2021, 09:18 AM   #6
russcc
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Default Re: Rough idle, low vacuum reading ?

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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.
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Old 05-23-2021, 09:23 AM   #7
russcc
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Default 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.
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Old 05-23-2021, 09:51 AM   #8
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Default Re: Rough idle, low vacuum reading ?

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Originally Posted by russcc View Post
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.
I have always thought this was a sign of a leaking power valve. is this correct?
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Old 05-23-2021, 11:37 AM   #9
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Default Re: Rough idle, low vacuum reading ?

That sounds like a classic ruptured power valve to me.

Sal
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Old 05-23-2021, 02:52 PM   #10
Charlie ny
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Default Re: Rough idle, low vacuum reading ?

Not all the time...a vacuum leak could be the guilty party....time to send it back....
Charlie
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Old 05-23-2021, 06:52 PM   #11
russcc
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Default 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.
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Old 05-23-2021, 07:18 PM   #12
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Default 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.
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Old 05-23-2021, 10:14 PM   #13
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Default 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.

Last edited by Tinker; 05-23-2021 at 11:04 PM.
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Old 05-24-2021, 06:22 PM   #14
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Default Re: Rough idle, low vacuum reading ?

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Originally Posted by russcc View Post
the vacuum readings, at idle, between 14-19. .
For reference, my 276 8BA idles at 550 and about 13-14 vacuum. The needle flickers just a little.
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Old 05-24-2021, 06:36 PM   #15
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Default 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.
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Old 05-24-2021, 08:59 PM   #16
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Default Re: Rough idle, low vacuum reading ?

Quote:
Originally Posted by Charlie ny View Post
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 turnand go to the other one. Once #2 is set go back and fine tune #1 then #2 again.
Charlie ny

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.
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Old 05-24-2021, 10:18 PM   #17
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Default Re: Rough idle, low vacuum reading ?

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"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.
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.
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Old 05-24-2021, 10:48 PM   #18
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Default Re: Rough idle, low vacuum reading ?

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Originally Posted by flatjack9 View Post
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.
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
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Old 05-24-2021, 11:09 PM   #19
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Default 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.
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Old 05-24-2021, 11:39 PM   #20
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Default 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.
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