Dieseling

Hivolt5.0

Hivolt5.0

Senior Member
I have a problem with my Y dieseling when I turn it off. The distributor is less than a year old points type. The carb is a 600 Holley. I double checked the timing this weekend; I was running 8 degrees advance and pulled it back to 6. I checked the air/fuel ratio with a vaccum gauge and am running a little more that 15 in, if i remember correctly. I know the timing chain has some slack in it. Tuning these engines is not my forte but I'm trying to learn as much as I can.

The engine runs great and has excellent response but just wants to diesel when turning it off.

thanks in advance for the help!
David
 
Try timing with a vacuum gauge. The light method uses the marks which have been known to move after all these years. If you really like the performance when its timed where it is, try switching to one grade better fuel.
 
Timing with a vacuum gauge...I have not heard of that before. How do you do that?

as for gas, I'm currently running ethanal free 87 octane.
 
billwill said:
Try a slower idle
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Billwill is the man. Dieseling is generally caused by idle speeds that are too high. This was resolved later on by using an electrical solenoid for the throttle to stop and ide on. When the key is turned off, the solenoid releases and the carb goes to lower setting (too low to idle).

You can also shut the engine off (if an automatic trans) in gear to eliminate the dieseling if you can't resolve it by slowing the idle speed down.
 
I had actually bumped the idle speed up just a touch because it was wanting to die when I'd clutch in to slow down and stop. Guess I'll try to adjust somewhere in the middle to see if that will help. However, even before I adjusted the idle up it would want to diesel periodically.
 
The objective is to get the engine to idle with the least throttle opening. Do it by carb idle air adjustment and timing. 15 sounds a little low for that engine. Be sure the carb is adjusted as it can cause the the engine to die when you clutch.

The timing by vacuum is done by adjusting the timing to the highest engine vacuum, then lower the timing one degree. You already know the current setting, so if it doesn't work for you it can be returned to the present setting.

You can experiment with connecting your vacuum can to manifold vacuum. This should increase your idle speed, enabling you to decrease the carb throttle opening farther. This won't have any negative effect on performance since ported vacuum can only deliver what is in the manifold.

Best of all, it can all be reversed if you find no improvement.
 
40cpe said:
The objective is to get the engine to idle with the least throttle opening. Do it by carb idle air adjustment and timing. 15 sounds a little low for that engine. Be sure the carb is adjusted as it can cause the the engine to die when you clutch.

The timing by vacuum is done by adjusting the timing to the highest engine vacuum, then lower the timing one degree. You already know the current setting, so if it doesn't work for you it can be returned to the present setting.

You can experiment with connecting your vacuum can to manifold vacuum. This should increase your idle speed, enabling you to decrease the carb throttle opening farther. This won't have any negative effect on performance since ported vacuum can only deliver what is in the manifold.

Best of all, it can all be reversed if you find no improvement.
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Ok, I'll give it a shot! thanks :D
 
Hivolt5.0 said:
I had actually bumped the idle speed up just a touch because it was wanting to die when I'd clutch in to slow down and stop. Guess I'll try to adjust somewhere in the middle to see if that will help. However, even before I adjusted the idle up it would want to diesel periodically.
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Have you tried to adjust the two idle mixture screws at all ? Especially if you changed the idle speed. Maybe it dies when you put the clutch in because the idle mixture isn't optimum. Maybe that will help get rid of the dieseling too if the mixture is off.
 
Also, check the float level. If its dumping a bit extra when you come to a stop, that could affect idle. Then, check the spark plugs. Check color now, and if the tips look as if they have been hot. With so many computer controlled cars now, its hard to find a wide heat range any more. I like NGK the past few years. They seem to burn anything you throw at them, and they last.
 
Scicala - I did check the idle mixture with a vacuum gauge and it was pretty spot on. I tried turning the mixture screws in and out and the vacuum would drop so I ended up back where I started.

Ole Don - I'll check the float level. Sure wish the carb had a site glass.

billwill - I'm not sure what a dash pot is.
 
Hivolt5.0 said:
Scicala - I did check the idle mixture with a vacuum gauge and it was pretty spot on. I tried turning the mixture screws in and out and the vacuum would drop so I ended up back where I started.

Ole Don - I'll check the float level. Sure wish the carb had a site glass.

billwill - I'm not sure what a dash pot is.
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A dash pot is something they used to put on carbs for cars with an automatic trans. It's a small round (about 1 1/2" dia ) that looks similar to a choke pull off diaphragm. It's designed to slow the throttle from slamming shut all the way during a hard stop to avoid stalling. It could help you're issue if everything else checks out.
 
My '57 312/w automatic with a Holley 4 barrel would die every time I took my foot off of the gas. The carb had a too complete of a cut off, enough to kill a fast running engine. It didn't have a dash pot on it when I bought it. So, I had to go and find a dash pot to put on the throttle so that it would have a slower cut off. It works perfect now.
 

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Ok, I finally had some time to tinker with the Ranchero. I hooked my vacuum gauge, my timing light and my dwell/tach meter. I think the engine feels a bit violated now. haha!

Once the engine warmed up I rechecking the timing with the light and the advance was solid a 6 degrees. The vacuum read 18.5 or so inches of Hg. I tried adjusting the distributor to increase the vacuum and I could increase it but when checking with a timing light it put the advance at 30 degrees! I knew this would be too much so I put it back at 6 degrees with the light. Next I adjusted the idle. Initially my meter read just below 800 rpms so I adjusted it to a 600 rpm idle speed. This brought my vacuum down to 15 inches of Hg. Adjusting my dwell/tach meter to read dwell it looked like it could be a bit high. Honestly I didn't look at it long enough and I should have spent more time on it.

So after adjusting the idle speed down and with the advance set back at 6 degrees I took it for a spin. There is light pinging under hard acceleration but I'm running 87 octane gas. Other than that it ran good. When I got back home and turned it off the engine shut off nicely as it should.

What else should I check or do I leave it alone and put some premium in it?
 
When setting the idle mixture, be sure the trans is in neutral and idle the engine down to about 500 RPM. Then after adjusting the mixture, you can idle it back up so that you get the desired RPM in drive. This assures that the idle circuit holes in the carburetor are in full effect. Otherwise, you will not get the correct mixture at idle.
 
really. That is very interesting. I'm a novice at carburetors so I do appreciate the advice. I'll tweak the mixture as you've instructed.

thank you!
 
For what it is worth, the Motors manual shows 3 degrees BTC for timing and 475 as hot idle speed.
 
Motorhead6 said:
When setting the idle mixture, be sure the trans is in neutral and idle the engine down to about 500 RPM. Then after adjusting the mixture, you can idle it back up so that you get the desired RPM in drive. This assures that the idle circuit holes in the carburetor are in full effect. Otherwise, you will not get the correct mixture at idle.
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I can't say that I agree with this. Idle speed and mixture should be adjusted in gear if it's an auto trans. You don't adjust the mixture, then change the idle RPM again without re-adjusting the mixture screws again.

I think you can get a higher vacuum that 15" if you re-adjust the mixture screws after going back down to 600 RPM. It doesn't sound like you re-adjusted the mixture at 600 RPM. It sounds like your car is running better though, so that's good.
 
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