richiev88

Hi All,

I finally got my 427 up and going ....
I have done some tuning as follows.
Engine is a 427cu, appox 10.7 comp static, running 250@50 duration cam,LS6 GM manifold, MSD pro billet dizzy. Carb is a old 800cfm double pumper holley, Brodix RR alloy heads
I have set up timing curve all in at 2500rpm, since i trying to run pump gas, limited advance to 35 deg total.
On initial start up car was running on race gaswith 38 total timing, was going really hard, but running on on shut down.
Since I am planning on running premium pump gas (98oct here, think equilivant to your 93) i knocked some timing out of it. I was fine for a bit but now has started to run on again on shut down.
On plug checks, mixture looks to be bang on
Is my idle fuel mixture too lean ? any ideas appreciated

Cheers

elle88

my same situation. never get rid of the run on.
427,ZL1 cam , 11:1 CR , idle is 1250 rpm , 98 Ron gas.
i always turn off the engine with gear in and i release the clutch while engine dies.no run on in this way

richiev88

Hi, yes thats what I have been doing, just had a thought, perhaps its too much initial for the high idle speed?
Could the mechanical advance curve be modified to eliminate this ?

You have more cam than me, slightly more compression, mine will idle ( sort of) around 900- 1000 rpm.
How much total advance are you running ? mine did go much better with 38 deg total

Richie

elle88

i run 38.5 total adv and no vacuum adv . But i plan on adding a B28 vac can and reduce the total timing to 35 or 36 deg to possibly go with lower than 98 ron octane gas
my cam requires lot of initial adv to keep the engine idling

roscobbc

My set-up is not too far away from what you have, but with the exception of a milder cam. I had similar 'running-on' issues with mine when it was first built even with a 22 degree ignition advance and 10.5 c/r!. Yes super unleaded fuel helped (I always use it). I wonder if the problem is not a weak idle mixture, but an idle mixture too rich! - my logic being that with the 'stiffer' cam and the resulting poorer vacumn at idle it is all too easy to richen-up the mixture to get it to idle in a satisfactory manner. The risk of having an over-rich idle mixture is then having un-burnt carbon 'hot-spots' in the combustion chamber creating pre-ignition and 'running-on'. That is my theory, feel free to shoot it down!

richiev88

No shooting down done here!!!
Yep thats a thought too, the cam does run rich at idle, and it does seem to be a problem after long periods of slow running .....will explore

Thanks

bashcraft

The cause of run-on is a high idle (because of the cam) and the throttle plates being open too far to achieve this high idle. You need to install a solenoid that opens the throttle plates to the desired idle speed and then closes the throttle plates when the ignition is turned off.

damoroso

Run on is usually caused by either to much initial timing (this also can cause detonation or pinging are you getting any of that?) or to high an idle or a rich condition at idle. With large cams, sometimes it's difficult to get the idle down to what you typically like in a street car, mine idles about 850-900, but that's as low as it can go and be stable. I had to change my vacuum can to one that pulls in at about 7-8 inches of vacuum; otherwise I had to have the idle so high I had the same problems you guys are having. I'd go after the timing and idle mixture settings first.

The solenoid bashcraft is talking about can fix the problem, you simply use a solenoid that's typically used for the idle kick up with an A/C system. Wire it to a switched hot lead, set the idle with it, and there you go.

bashcraft

[QUOTE=damoroso;1580336614]Run on is usually caused by either to much initial timing (this also can cause detonation or pinging are you getting any of that?[)/QUOTE]

How can initial timing cause run-on? The ignition is turned off.

damoroso

My understanding is because it causes higher than normal temps in the heads...another term for this is "dieseling"...

Les

My combo isn't too much different from yours, but I did mill the pistons to get 9.5 to 1 comp. ratio because gas in Calif. is crap- probably way worse than yours which likely puts us at a similar point regarding detonation and run on problems. I struggled to find timing that worked for all circumstances until a local Chevy big block expert told me to disconnect the vacuum from the distributor and work from there. I now run about 16 initial with a total of about 33-34 all in by something like 2500 RPMs IIRC and it runs like a very angry motor with no issues at all. I do have a carb with 4 corner idle adjustments and that may help me idle at 850 RPMs.

damoroso

Les, did the same thing, but I'm still able to run 10.5:1. 16 initial degrees, 34 total, all in by 2500 with a Q-jet.

ajrothm

+1 on idle speed.

Mine will run on a bit as well...I have to keep the idle in Park at around 1100 rpms to give me around 850rpm in gear, if its hot out, mine will ever so slightly run on a couple sputters.... Not much that can be done about it unless you setup an "anti dieseling solenoid" like they had from the factory.... Unfortunately I cant do that because I already have an idle solenoid to increase idle speed for AC.

I have learned to just work around it... I will either kill the engine with the AC on (the load from the compressor will prevent run on) or I will kill the engine with it in DRIVE(in gear) preventing run on.... But most of the time I just kill it and let it cough...

Less timing advance does improve run on but you have to pull so much timing out of it to make a difference, it is not good for the tune when its running.

96 lt-4

Cut it off in gear and then put it in park.The lower idle speed in gear will prevent run on.Not a perfect solution but it should work.

richiev88

No sign of detonation or pinking, plug checks look fine, MSD pro billet dizzy has no vac advance, just mechanical. No A/C either, will pull the carb and check over

Thanks

TheSkunkWorks

One thing I'd try would be slowing down the advance, and perhaps 2-4* less initial; all in ASAP doesn't always work. As for the carb, I'm no guru on the subject by any means, but IMHO if it won't idle with the throttle plates oriented properly at idle, you've got more tuning to do. In any event, completely dial in the ignition system before working on the carb.

damoroso

DRIVESHAFT

I haven't seen you list what your initial timing is. What is it?
Your running a big cam and your engine wants quite a bit of initial timing to idle well.
A lot of guys will actually lock out the timing on an engine like that and run 36°-40° all the time.
If you will add more initial timing your idle speed will rise.
Then you can close your throttle blades slightly to put your idle where you want it.
The throttle being open too far is the #1 reason for engine run on. Closing the throttle some will probably cure your run-on issues.
You will of course have to slightly limit the amount of mechanical advance in the distributor afterwards to keep from having too much total timing, but thats not too much trouble to do.

69427

I think we found the problem.

I run 38* at WOT with my 427. The vacuum advance improves the idle quality significantly, and the improved thermal efficiency allows a smaller throttle opening at idle, preventing run-on/dieseling at shutoff.

69427

[QUOTE=bashcraft;1580336688]If the timing is too retarded, the throttle plates have to be opened up further to keep the engine running due to the lower combustion efficiency (heating things up), and this increased throttle opening allows a bunch of air and fuel in at shutdown that autoignites (ie: diesels) near TDC when the engine coasts down.