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Old 06-11-2010, 02:26 PM
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timing issues...please help

okay...so i drive 1975 custom deluxe 20 with a GM 350 small block that has been freshly rebuilt about a year ago. hasent been driven much until about 6 or 7 months ago when my dad finally let me get licence. so heres the problem. before it would diesel about every time i truned it off...and not to long ago we timed it and it quit dieseling, that was about a 2 months ago...and now in the past couple of days it has been starting to diesel again. not every time but every now then...and it bugs the crap out of me!when we timed it we tested the vacuum and set the idle...and after that it ran great...until like i said a few days ago.
any thoughts or suggestions are greatly apreciated...thanks!
bryce

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Old 06-11-2010, 04:42 PM
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An anti-dieseling solenoid would stop it. What carburetor is on the motor?
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Old 06-11-2010, 07:55 PM
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Quote:
Originally Posted by theV8man
okay...so i drive 1975 custom deluxe 20 with a GM 350 small block that has been freshly rebuilt about a year ago. hasent been driven much until about 6 or 7 months ago when my dad finally let me get licence. so heres the problem. before it would diesel about every time i truned it off...and not to long ago we timed it and it quit dieseling, that was about a 2 months ago...and now in the past couple of days it has been starting to diesel again. not every time but every now then...and it bugs the crap out of me!when we timed it we tested the vacuum and set the idle...and after that it ran great...until like i said a few days ago.
any thoughts or suggestions are greatly apreciated...thanks!
bryce
If the vacuum advance is hooked up to ported vacuum, you could try using manifold vacuum for it.

This will increase the idle speed, which will need to be lowered by using the curb idle screw. You will also want to check the idle mixture screws after doing this just to be sure the vacuum is still at the highest.

By changing the vacuum source then lowering the idle speed, you will have caused the engine to get less air past the throttle butterflies at idle- so when you shut the engine off, there will be less chance of it dieseling.

Shutting the engine off in "Drive" is a pain, but will also usually work, but doesn't really address the problem.

I'll leave the solenoid explanation to techinspector1.

Good luck.
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Old 06-12-2010, 09:04 PM
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Quote:
Originally Posted by cobalt327
I'll leave the solenoid explanation to techinspector1.
Pretty simple really. It's how General Motors corrected the problem back in the early/mid 70's. The fact is that the motor cannot run on (diesel) with the throttle blades completely closed. They attached a solenoid, called a dashpot, to the primary throttle shaft of the carb. When energized with 12v when the key is turned to the ignition-on position, the dashpot moved the shaft to the idle position. When power was removed with the key turned off, the dashpot pulled the throttle shaft to the completely closed position (throttle shaft butterflies completely sealed to the throttle bores) so that no fuel/air mixture could enter the motor.
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Old 06-12-2010, 11:54 PM
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As mentioned, sometimes the issue is the timing is too low or the vacuum advance isn`t connected to a full time source. With the engine at idle and the vacuum advance disconnected and plugged, set the timing 12 degrees before top dead center. Reconnect vac advance and make sure it`s connected to a full time source or one that pulls vacuum at idle. Recheck the timing, it should be around 20-24 degrees advanced. This will raise the idle considerable if the timing was off prior, reset the idle down to around 750 RPM`s. this should solve your dieseling issues. If not, then it`s possible you have a vacuum leak or a fuel issue. If it doesn`t solve the issue, with the truck at idle and the air cleaner removed, shine a light down in the carbs throat, See if you see fuel coming out of the boosters, this will cause it to diesel. If you see fuel dripping either the float is set too high, the carbs needle is stuck open or the fuel pressure is too high.
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Old 06-13-2010, 02:31 AM
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I believe "dashpot" to be a misnomer in this instance, more on the difference between a carburetor dashpot and a solenoid is HERE.

An anti dieseling solenoid is usually mounted to the carb (at least in the case of the Rochester OEM applications) but could be mounted on the intake itself- the requirement being that the plunger has to be able to bear on the linkage in such a way as to allow the throttle to be opened when the solenoid is energized.

The plunger is activated by turning the ignition switch to "ON", and it then extends to a predetermined (and adjustable) position to hold the throttle at the desired idle speed. The plunger's length is adjusted by turning the hex head on the end of the plunger.

Conversely, when the ignition switch is turned "OFF", the plunger retracts, allowing the throttle to completely shut- thus denying the engine any air/fuel that is needed to diesel.



A similar solenoid is used to raise the idle speed of an AC-equipped vehicle to keep the idle speed from dropping too low when the compressor activates at an idle. It is wired to apply as the compressor clutch is engaged, and to retract when the compressor clutch is off.
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Old 06-13-2010, 08:42 AM
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classic sign the throttle blades are open to much at idle.

more ignition timing will fix that.

plugging the vacuum advance into the manifold source is the easiest fix as it raises the idle speed with less throttle opening. don't be alarmed with very high timing at idle with this setup. 10 mechanical plus another 15 to 20 from the vacuum advance will have 25 to 30 degrees at idle.

do a web search for " ignition timing 101".
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Old 06-13-2010, 10:05 AM
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"and not to long ago we timed it and it quit dieseling, that was about a 2 months ago...and now in the past couple of days it has been starting to diesel again"





Just a thought but there is always a chance your timing is not where you had it when the dieseling stopped. The distributor may have moved and thus thrown your timing off. I would start by pulling out the timing light again and taking a look.

Beagle
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Old 06-13-2010, 07:30 PM
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Diesel?

How long do you run it for before it does this? If you start and run it for 5 minutes and shut it off does it do this? Generally the engine needs to be pretty hot to have it diesel. Also look to be sure all the vacuum lines are plugged. It definitely sounds like a fuel air combo in a hot cylinder issue.

Not sure why timing matters... key off is no spark no matter where the timing is set. Would have perhaps run cooler with correct timing. Try varying the timing 2 degrees advance past where you have it and see if it helps.

I remember having that problem on a station wagon.... yup, just would put it in drive.
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Old 06-14-2010, 02:34 PM
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A hot spot in the combustion chamber or even a single plug that's damaged or accidentally the wrong (extra hot) heat range can cause dieseling. A set of the wrong heat range spark plugs will diesel like a freaking Peterbilt.

A hot spot acts just like a glow plug in a diesel or model airplane engine- it provides a point of ignition. For the hot area to become hot enough to act as a glow plug doesn't necessarily take long at all, depending on what the hot area is; a small edge of the chamber or a damaged ground strap on a spark plug will glow almost immediately.

You'd expect there to be preignition associated w/this- and there may well be, but it can be largely unheard and/or unfelt, if it's isolated to just one cylinder.

On the other hand, an incorrect heat range set of spark plugs may take longer to become fully hot- but once there, can cause hellacious dieseling.
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Old 06-14-2010, 02:46 PM
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?

please excuse my ignorance i have never heard of diesel-ing what exactly would this sound/feel like and could damage come from this?

thanks
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Old 06-14-2010, 04:55 PM
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Quote:
Originally Posted by velle307
please excuse my ignorance i have never heard of diesel-ing what exactly would this sound/feel like and could damage come from this?

thanks
When the engine continues to "run" (term used loosely- the engine is turning over on its own but that's about it) w/the key turned to "OFF", this is considered to be dieseling, aka "run on".

It sounds like a combination of spark knock and what an engine sounds like running on one or two cylinders- which is to say BADLY! There can be a sneeze up through the carb, too. Dieseling is almost always accompanied by a foul smell that I do not know the cause of.

Damage certainly CAN occur: the timing set is stressed because in the final throes of dieseling, the engine very often runs backwards! This isn't good for the timing set as you may well realize and could be the straw that broke the camel's back for a worn or marginal timing set.

There's spark knock caused by preignition that will always accompany the dieseling, and this alone is hard on the pistons and bearings. But in the bigger scheme of things, dieseling won't in and of itself cause a catastrophic failure or melt the engine down- it's more of an embarrassment/nuisance, IMO.
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Old 06-14-2010, 05:44 PM
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dieseling

Diesel motors do not have spark plugs and hence no ignition. They are controlled for the most part by the fuel entering the combustion chamber and the heat of compression to ignite it. If a gas motor runs with no spark, never a desired thing, then it is said to be dieseling.
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Old 06-15-2010, 06:33 AM
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well it has a holly carb on it.

and the vacuum is already a manifold vacuum

and we did time 2 degress advance


Thanks
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Old 06-15-2010, 07:23 AM
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if you can measure any vacuum on the ported vacuum source then the throttle blades are open too far at idle (past the transfer slots). This makes the engine idle very rich and will cause it to run on.

if this is the case then you need to let in more air with less throttle blade opening. This can be done by opening the secondary throttle blades or by drilling one 0.070" hole in each front throttle blade.

the secondary blades have an adjustment screw on the bottom side of the carb (need to pull the carb and flip it over to see it). one turn on the rear blades is equal to one turn on the fronts.

Last edited by 454C10; 06-15-2010 at 12:17 PM.
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