|
Sponsored Links (Register now to hide all advertisements) |
|
|
Thread Tools | Display Modes |
05-23-2021, 07:56 AM | #1 |
Senior Member
Join Date: May 2010
Posts: 2,871
|
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 ? |
05-23-2021, 08:11 AM | #2 |
Senior Member
Join Date: Dec 2015
Posts: 287
|
Re: Rough idle, low vacuum reading ?
Running too rich. bad power valve? float set too high? just some possibilities Almost certainly a carb. problem.
|
Sponsored Links (Register now to hide all advertisements) |
|
05-23-2021, 08:13 AM | #3 |
Senior Member
Join Date: Dec 2015
Posts: 287
|
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.
|
05-23-2021, 08:19 AM | #4 |
Member Emeritus
Join Date: Nov 2012
Location: Wichita KS
Posts: 16,132
|
Re: Rough idle, low vacuum reading ?
What is the fuel pump output? A lot of the new/rebuilt pumps output to much pressure.
|
05-23-2021, 09:17 AM | #5 |
BANNED
Join Date: May 2010
Posts: 3,019
|
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 |
05-23-2021, 09:18 AM | #6 |
Senior Member
Join Date: May 2010
Posts: 2,871
|
Re: Rough idle, low vacuum reading ?
Sponsored Links (Register now to hide all advertisements)
|
05-23-2021, 09:23 AM | #7 |
Senior Member
Join Date: May 2010
Posts: 2,871
|
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.
|
05-23-2021, 09:51 AM | #8 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,260
|
Re: Rough idle, low vacuum reading ?
I have always thought this was a sign of a leaking power valve. is this correct?
|
05-23-2021, 11:37 AM | #9 |
Senior Member
Join Date: Jun 2010
Location: Detroit suburb, MI
Posts: 3,697
|
Re: Rough idle, low vacuum reading ?
That sounds like a classic ruptured power valve to me.
Sal |
05-23-2021, 02:52 PM | #10 |
BANNED
Join Date: May 2010
Posts: 3,019
|
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] |
05-23-2021, 06:52 PM | #11 |
Senior Member
Join Date: May 2010
Posts: 2,871
|
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. |
05-23-2021, 07:18 PM | #12 |
Senior Member
Join Date: May 2010
Location: Oshkosh, Wi
Posts: 4,516
|
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.
|
05-23-2021, 10:14 PM | #13 |
Senior Member
Join Date: May 2010
Location: MN
Posts: 7,053
|
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. |
05-24-2021, 06:22 PM | #14 |
Senior Member
Join Date: May 2010
Location: Star, MS
Posts: 4,022
|
Re: Rough idle, low vacuum reading ?
|
05-24-2021, 06:36 PM | #15 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,260
|
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. |
05-24-2021, 08:59 PM | #16 | |
Senior Member
Join Date: May 2010
Location: MN
Posts: 7,053
|
Re: Rough idle, low vacuum reading ?
Quote:
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. |
|
05-24-2021, 10:18 PM | #17 |
Senior Member
Join Date: May 2010
Location: Oshkosh, Wi
Posts: 4,516
|
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.
|
05-24-2021, 10:48 PM | #18 | |
Senior Member
Join Date: Sep 2011
Location: Christchurch New Zealand
Posts: 1,603
|
Re: Rough idle, low vacuum reading ?
Quote:
His distributors with an adjustable vaccum advance are great units to run. Phil NZ |
|
05-24-2021, 11:09 PM | #19 |
Senior Member
Join Date: May 2010
Location: MN
Posts: 7,053
|
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.
|
05-24-2021, 11:39 PM | #20 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,260
|
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. |
Thread Tools | |
Display Modes | |
|
|
Sponsored Links (Register now to hide all advertisements) |
|