Engine Run-on help. (dieseling) I promised I searched first.

[jorgensensc]

I promise I searched first, and read a bunch of responses, but they don't quite fix the problem for me.
Background: I just installed an engine for a friend. I didn't build the engine, just installed it. It is a fresh rebuilt 350 .60 over, with fresh rebuilt 305 heads (I didn't build it), and an RV cam. Cast iron intake, fresh rebuilt (by me 2 days ago) Edelbrock 600 electric choke carb, Skip White HEI (pulled off the former 350). 4 speed manual (granny low).

The old engine burned alot of oil, but ran okay when I pulled it. It did not run-on at all. Drove it around quite a bit, and never had an issue. It sat for about 2 weeks while engine was being pulled and new one was put in.
Engine runs fine, doesn't overheat, no misfires, nice and smooth with no stumbling.
However, when ignition turned off it continues to run-on.
I have adjusted the air/fuel mixture to where it is good (3 turns out both sides)
I have adjusted the timing, from when it won't start (retarded) all the way to past hard start (advanced) in 2 degree increments all day. (Vacuum advance plugged)
The idle screw is turned all the way out to where it doesn't have any affect on idle.
Even with the timing almost fully retarded (lowest idle possible) it diesels. as I advance the timing to where I think it should be (between 8-14), it idles way to high. I cannot adjust the idle screw anymore and hav it have any affect on the idle speed.

Do y'all have any suggestions? Would the engine specs have anything to do with it. Is it possible that the low grade (87 octane) that is in the tank won't work on this engine and she will have to go to the good 93 octane stuff? Is the compression really that high?
Any help would be appreciated. I'm ready to get this thing out of hear so I can work on the Burb again!
Thanks!
Shawn

[68gmsee]

I would remove the carburetor and check that the butterflies are closing properly.

The way you describe the idling problem something may be keeping them open and engine vacuum just keeps sucking in more fuel.

[thirdstreettito]

Dieseling? What does that mean?

[67ChevyRedneck]

Originally Posted by thirdstreettito

Dieseling? What does that mean?

The engine keeps running after the key is shut off.


Do you have a temp gauge on the engine? What is the temp when it is shut off? If it's overheating that could cause it too.

What RPM's is the engine spinning when it is shut off?

On my edelbrock I have my air/fuel adjusted to 1-1/2 turns from "closed" and my timing is at 12* but I have a "stock" engine.

[69halfton]

dieseling means running on with out ignition..

a diesel has no spark plugs, and relies on the heat from compression to make the fuel burn.

by saying its dieseling, he is saying the motor is running on, and the ignition is off..

my motor (brand new 383 stroker with new carb etc.) runs on also. i have tried everything, idle low, timing all over the map.
it only does it hot, and i think its just to do with the high compression motor.

does the 87 octane ping or detonate while you drive it/whail it?
if it does, you definatly need the 93 octane.
my solution to my problem for now...shut it off in gear... (mines an automatic)

or just flip the key off, and let the clutch out slowly at the same time..with your foot on the break of course

[jorgensensc]

Originally Posted by 68gmsee

I would remove the carburetor and check that the butterflies are closing properly.

I checked them by looking down the carb They are closed. It did this when I first threw Q-jet on the motor last week (ended up using the edelbrock carb due to strange intake manifold/choke issue with the Q-jet). I figured then it was just because I didn't have the carb adjusted in well. But it ran great after it was warmed up. But when you shut it off it ran on bad. I just figured it was the carb not being adjusted. But it still does it with the Edelbrock and I know I adjusted it correctly.

Originally Posted by 67ChevyRedneck

The engine keeps running after the key is shut off.


Do you have a temp gauge on the engine? What is the temp when it is shut off? If it's overheating that could cause it too.
The engine isn't getting hot at all. I would actually say it is remarkably cool. But yes the gauge is working correctly.


What RPM's is the engine spinning when it is shut off?
I don't really have a way of checking that, however, when I retard the timing so that the idle seems at a reasonably slower speed and shut it off it still runs on. As you advance the timing the RPMs go up, (to what to my ear sounds really high) with no way to lower them to what sounds better since the carb idle adjustment is backed all the way out.
On my edelbrock I have my air/fuel adjusted to 1-1/2 turns from "closed" and my timing is at 12* but I have a "stock" engine.

I have rebuilt a few of these edelbrocks since they are so easy. I turn both of the mixture screws in all the way first (one almost all the way so that it will stay running). Then I count the number of turns until the RPMs plateau, then put it back in and do the other side. When it plateaus I turn the first one back out the number of turns I counted before. THen I turn them both out 1/2 turn more. I generally end up about 2 1/2 to 3 full turns.

Originally Posted by 69halfton

my motor (brand new 383 stroker with new carb etc.) runs on also. i have tried everything, idle low, timing all over the map.
it only does it hot, and i think its just to do with the high compression motor.

does the 87 octane ping or detonate while you drive it/whail it?
if it does, you definatly need the 93 octane.
I haven't driven it yet since putting the edelbrock on. I was trying to get the timing adjusted all in first.
my solution to my problem for now...shut it off in gear... (mines an automatic)

or just flip the key off, and let the clutch out slowly at the same time..with your foot on the break of course

If it was mine it wouldn't be a problem. Unfortunately this is for a Female friend and she doesn't need the hassle of doing all that.

Will Octane booster in a can work for the moment? She has over half a tank of gas and I don't want to fill it with premium and it still be so diluted it won't work.
Thanks for all of your help so far guys!
Shawn

[bigguy69]

i have a motor that does the same thing. mine is built the exact same way except it has an alunium intake. I have read that with a .060 over motor and steel shim head(which mine has) gasket it has right at 10.1 comp. I put 93 octane gas in mine and that seems to make it happy. i am pretty sure it is the comp. ratio on mine as i have also tried timing and other stuff that you did.

[69halfton]

octane booster may work, not sure, i have never tried it..

how old is the gas in the tank?...that maybe having some effect as well..

how do the plugs look?

i cant remember weather its lean or rich...i think its rich that tend to make them run-on..

[Arad68]

What spark plugs are in it? A plug that is old or too hot will make it run on. The rpm being too high at idle is most likely the cause.

What is the cam timing set at?

[67_C-30]

The problem is compression ratio. As I mentioned in another thread, 305 heads range between 58cc - 60cc, with most being 58. Your .060 over 350 is actually 360 cubic inches. I'm assuming you probably used flattops which are -5 to -7cc for the valve reliefs for the rebuild. Composite head gaskets compressed thickness are usually around .038" making your static compression ratio between roughly 10.4 - 10.7 : 1 depending on which 305 head you have. The small shrouded design of the 305 head creates a lot of excessive heat coupled with the CR being at least a point over the general "safe" 9.5 : 1 limit for a cast iron headed engine. Just being a std. bore 350 would drop it back to the 10.2 - 10.4 range, so you have several things working against you.

93 octane will help, but may still may not be enough. I would actually put a light on it and get the true timing and try to set it at around 2 degrees BTDC initial and see how it acts. There are some other tricks that would help if the engine weren't already assembled. Polishing the combustion chambers works wonders for getting away with higher combustion with pump gas, but unfortunately that's not an easy option at thing for you at this point.

[67_C-30]

BTW, I just reread your post, and i would determine why the idle adjustment stops first. You should be able to back it off until it dies. If not there's a vacuum leak either around the intake or in the carb. It has to be getting air somewhere to idle. If you get that fixed, you may be get away with it, but the CR of that engine is still really pushing it for pump gas!

[Arad68]

Originally Posted by 67_C-30

The problem is compression ratio. As I mentioned in another thread, 305 heads range between 58cc - 60cc, with most being 58. Your .060 350 is actually 360 cubic inches. I'm assuming you probably used flattops which are -5 to -7cc for the valve reliefs for the rebuild. Composite head gaskets compressed thickness are usually around .038" making your static compression ratio between roughly 10.4 - 10.7 : 1 depending on which 305 head you have. The small shrouded design of the 305 head creates a lot of excessive heat coupled with the CR being at least a point over the general "safe" 9.5 : 1 limit for a cast iron headed engine. Just being a std. bore 350 would drop it back to the 10.2 - 10.4 range, so you have several things working against you.

93 octane will help, but may still may not be enough. I would actually put a light on it and get the true timing and try to set it at around 2 degrees BTDC initial and see how it acts. There are some other tricks that would help if the engine weren't already assembled. Polishing the combustion chambers works wonders for getting away with higher combustion with pump gas, but unfortunately that's not an easy option at thing for you at this point.

Thanks for shedding some light on this subject. I am sure this is why my 383 runs on. I have these heads on it right now. http://www.jegs.com/i/Edelbrock/350/60859/10002/-1 If the idle is any higher than 750 rpms, it runs on. I have tried cooler plugs and severely retarded timing as well to no avail.

[67_C-30]

Originally Posted by Arad68

Thanks for shedding some light on this subject. I am sure this is why my 383 runs on. I have these heads on it right now. http://www.jegs.com/i/Edelbrock/350/60859/10002/-1 If the idle is any higher than 750 rpms, it runs on. I have tried cooler plugs and severely retarded timing as well to no avail.

Yeah, see your's would be in the same range. If you a running flattops on your 383, your's would be around 10.5 - 10.7 depending on what piston you have (assuming its a flattop). You can get away with more CR with aluminum heads, but there's just a point where today's crappy fuel says enough is enough. With the 10% - 15% ethanol they are mixing in now, its even worse. EFI get away with more because of the computer actively retarding the the timing.

[Longhorn321]

Originally Posted by 67_C-30

You be able able to back it off until it dies. If not there's a vacuum leak either around the intake or in the carb. It has to be getting air somewhere to idle.

I was thinking vacuum leak too. Spray some carb cleaner around the intake and carb and see if the idle changes.

[Arad68]

My pistons are dished -19 ccs, but the heads are milled at least .010. They are forged and it has 6" h-beam rods. I figured the compression ratio at about 9.9-10.0 to 1. It also has this cam in it. http://www.jegs.com/i/Comp+Cams/249/12-242-2/10002/-1

I retarded the cam timing -2 degrees using a cloyes hex -a - just timing chain gears.

[blaserman]

Mt 84 Blazer still does that sometimes. I just keep it in gear and turn it off.

[jorgensensc]

Originally Posted by 67_C-30

The problem is compression ratio. As I mentioned in another thread, 305 heads range between 58cc - 60cc, with most being 58. Your .060 over 350 is actually 360 cubic inches. I'm assuming you probably used flattops which are -5 to -7cc for the valve reliefs for the rebuild. Composite head gaskets compressed thickness are usually around .038" making your static compression ratio between roughly 10.4 - 10.7 : 1 depending on which 305 head you have. The small shrouded design of the 305 head creates a lot of excessive heat coupled with the CR being at least a point over the general "safe" 9.5 : 1 limit for a cast iron headed engine. Just being a std. bore 350 would drop it back to the 10.2 - 10.4 range, so you have several things working against you.

93 octane will help, but may still may not be enough. I would actually put a light on it and get the true timing and try to set it at around 2 degrees BTDC initial and see how it acts. There are some other tricks that would help if the engine weren't already assembled. Polishing the combustion chambers works wonders for getting away with higher combustion with pump gas, but unfortunately that's not an easy option at thing for you at this point.

THanks for the insightful and knowledgeable explanation. It helps quite a bit. Unfortunately I didn't build this engine. SHe had another shadetree mechanic build it and she really has no recourse to have him fix it. I new it had 305 heads on it, and knew it was an old school tick to build compression, but it sounds like he way overdid it. I did try and set the timing as low as I could. I was actually on the other side of 0, So I guess you would say ATDC? I had it around 2 degrees ATDC and the idle was much better, and the run-on was alot less, but still present. Hopefully putting it around 2 degrees BTDC and high octane full will work for her.

Originally Posted by 67_C-30

BTW, I just reread your post, and i would determine why the idle adjustment stops first. You should be able to back it off until it dies. If not there's a vacuum leak either around the intake or in the carb. It has to be getting air somewhere to idle. If you get that fixed, you may be get away with it, but the CR of that engine is still really pushing it for pump gas!

Originally Posted by Longhorn321

I was thinking vacuum leak too. Spray some carb cleaner around the intake and carb and see if the idle changes.

I already did this thinking the same thing. I got no reaction from spraying the cleaner anywhere around the intake or carb. I tried to make sure it is buttoned up well. If I adjust the air/fuel mixture screws I can get it low, but unfortunately when I use the regular Idle screw it just won't back out far enough. I have tried to manually pull back on it, but it is at its mechanical limit of movement. I checked the carb I have on my other 350 (samething edelbrock 600) and that one has what appears to be the same exact mechanical limits. Is it possible that the engine being at 4 or 6 degrees BTDC is considered that far advanced for this particular setup?Like I said, past 0 I can get the idle down to a pretty reasonable level. It will run okay, and throttle wise responds well, but I can't actually start it that low. It has to be a little higher for it to start. Thanks again for all of your help and advice!

[70 Suburban]

how high is your idle set?

[68gmsee]

......Is it possible that the engine being at 4 or 6 degrees BTDC is considered that far advanced for this particular setup?Like I said, past 0 I can get the idle down to a pretty reasonable level. It will run okay, and throttle wise responds well, but I can't actually start it that low. It has to be a little higher for it to start. Thanks again for all of your help and advice!

Man, it looks like the you've tried everything but throw the sink at it.
And it almost looks like you may have more than one problem. Dieseling and idling high. You may have to start from scratch on this one.

Different carb is an option. But another thing you could check if you haven't already, is to make sure the timing chain/distributor are not a tooth or two off.

You'll have to remove the valve cover to make sure the intake/exhaust valves are completely closed and piston is at it's "maximum" upwards travel and balancer at TDC.

[67shortie]

Do Edelbrocks have a secondary throttle shaft adjusting screw like the Holleys do?

If so, this might be worth looking at next, it could be cracking the rear throttle blades open to much letting in too much idle air causing the run on.

You should be able to kill the engine (or darn near) by backing the main idle adjustment screw all the way out. I know I can on all of my engines.

just a thought

[70rs/ss]

Well, something is up with the idle, no doubt about it, but I think to high of an idle isn't so much vacuum leak, as when I have forgotten a plug or the like I can't get it to idle low, it dies, so maybe this is more a fuel problem. Edelbrocks do not like more than 3-4.5 lbs of fuel pressure, so it could be pushing fuel into the carb, but not positive on that either. I have never not been able to get the idle down, maybe the throttle linkage is too short?? The Edelbrock for sure should be able to be set down low enough to kill it for sure, so something isn't right, make sure the linkages aren't bent, try adjusting idle with the throttle linkage to the truck unhooked.


As for the heads and the dieseling, too high of a heat range plug, along with the compression and a near stock cam will cause all kinds of issues, a cam swap might help, but it would have to be a choppy cam and in a daily driver work truck, not good. Fix the carb thing for sure, shoot for at least one, but probably 2 heat ranges lower than you are now and dump 93 into it, octane booster works, but you'd need a lot to bring up the 87, so use maybe 2 cans and top off with 93, and you should be good to go! Heat range pict below shows what is what and why.

[jorgensensc]

Update!
Thanks for all the advice guys. I ended up topping it off with 93 octane, throwing a couple of cans of booster in the tank too. Even if I remove the linkage it still won't get any further back on the arm. It is mechanically stopped (closed butterflies too.) Moved the plunger rod up to the top notch on the plunger (it was in the middle, one down from the top) so it wouldn't dump so much fuel when you apply the pedal, backed the timing back down to about 2 deg. BTDC. I checked all the butterflies and they are closing completely. Sprayed carb cleaner everywhere with no change, so no leaks. Now it does pretty good. Occasionally there will be a little shudder when you turn it off, but that is really only when you pull in after driving it and immediately shut it off. If you pull in and let it idle for 15-30 seconds or so it seems to do better. I did notice that the little clear plastic fuel filter she has inline isn't filling up like it was before. It doesn't seem to be pumping near the volume it was before, but maybe that has something to do with the idle being lower. The fuel pump is fairly new, so I don't think it is going bad. I drove it all over the place for about 2 hours today and it did good. I probably stopped 6 or 7 times to adjust things. I got one shudder at the end and on good run-on before I adjusted the plunger on the carb. It will have to do. I really do think it is the nature of the engine he built for her. THanks for all of y'alls help on this. I might go back and get colder plugs for it too. It never appeared to get hot either. The temp always stayed in the cool range.
SHawn

[70 Suburban]

What do you have your Idle at? what carb are you running again. does it still have the manual choke .(clock spring)

[jorgensensc]

Okay guys, I went back and checked for a 3rd time, but only because you guys are really knowledgeable. I did find one more problem, and I think I fixed the issue. It was the secondaries. as I watched it and I blipped the throttle alot, it would idle low and shut off fine. But the secondaries weren't opening up because I didn't blip it hard enough. So I blipped it hard enough to open the secondaries, but when i let off, the idle was higher than it was before the secondaries would open. So I got to looking at it after I played with the secondary linkage. The primary linkage wasn't forcing the secondary linkage all the way back and completely closing the butterflies. THe engine vacuum was keeping them cracked open. Their was a 1/32 gap between the two linkages. So I bent the primary linkage out a little bit so when it closed it actually closed the secondary linkage like it should. Now it idled back to low even after opening it up all the way causing the secondaries to open and close. That is why when I looked inside the butterflies appeared closed. With the engine running after the secondaries would open, the vacuum wouldn't allow them to close completely (very small gap). I figured this out by playing with the top butterflies on the secondaries. They are weighted and closed, but if I manually opened them, then the engine speed changed. Tat meant the secondaries weren't closed completely, so I looked inside and sure enough they were barely cracked. I pulled the linkage to close them and the truck idled back down. THat is when I noticed the primary linkage wasn't forcing close the secondary linkage. But after you would turn it off and the engine had no vacuum, the secondaries would completely close, then I would look inside and see them closed, start up the truck and it would idle low again. As long as I didn't open up the secondaries while driving the truck would turn off fine. If I really got on it and opened them up it would run-on, but upon restart, idle low. I know it is a long explanation, and I'm sure the engine build still has something to do with the run-on (I got the advance up to about 4 now), but hopefully all of this exercise in checking and rechecking will help somebody else out.
So the moral of the story is this. Make sure your primaries are completely shutting down your secondaries.
Thanks again for all of your help!
SHawn

[70 Suburban]

Hmmm that where i was going with it....