Author Topic: 1987 Chevy 350 engine dieseling  (Read 643 times)

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Offline Bryan L

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1987 Chevy 350 engine dieseling
« on: June 12, 2022, 02:11 AM »
Hello all,

I have been fighting engine dieseling on my 1987 Chevy 350 (21RB Minnie Winnie) off and on for the past 18 months.

My rig only has 36k miles and I have only put on 3k miles in the past 8 years.

In the summer of 2020 it started dieseling when I would shut off the engine, including times it would backfire and shoot out a nice fire ball out the tail pipe.

In 2020 I put on 6 new spark plugs as I simply could not figure out how to reach the 2 front most plugs. I also attempted pouring seafoam directly into my carb 3 times over a couple week span.

Prior to this week, the last time I was able to take it out was November of 2020.

This past week I was able to take it to a friends shop and do even more work on it.

I took off the Rochester Quadrajet four barrel carburetor and disassembled it, cleaned it and put it back together using a rebuild kit. When I put it back on I was able to adjust both idle settings as well as replace my distributor cap and rotor. After putting it together yesterday and taking it for a test drive, it ran like a dream! I took it on a 12 mile, 20 minute drive that included hills, turns, city and freeway driving. Upon returning to the shop, I shut it off, after waiting 1 minutes, and it stopped normally. However, today I took it on the same test drive and it again ran so well. However, when I got back to the shop, waited 1 minute, and shut it off, I got 33 seconds of run on.

I am at a loss for what to do next.

I have attached 4 images and a link to a youtube video showing the last 3 times I shut it off.

https://www.youtube.com/watch?v=XhoAJALJvAQ

  • The first clip is after reinstalling the carburetor and taking it for a 5 mile drive. It still sputtered and even had flames coming out the tail pipe.
  • The second clip is the next day when I took the carburetor back off, adjusted both idle set screws and took it for an 11 mile drive. Apparent success!
  • The 3rd clip was the next day after 24 hours sitting. I took it on the same 11 mile test drive and I got 33 seconds of run on, but no fire.

The images are of my carburetor before I took it out of the RV, when it was taken apart, and after I put it back together. The last image is a still shot of the fire coming out of the tailpipe.

I appreciate any advice you can throw my way.

Offline eXodus

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Re: 1987 Chevy 350 engine dieseling
« Reply #1 on: June 13, 2022, 04:40 PM »
when you turn of the key, the ignition coil should not get any power anymore and the engine should stop running.
Further the fuel pump stops running.

If it keeps running - it's either hot and igniting on something, or the coil not shutting of (relay) or you got a vacuum leak which.

How are you temperatures looking? 

Offline Bryan L

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Re: 1987 Chevy 350 engine dieseling
« Reply #2 on: June 13, 2022, 11:15 PM »

My temperatures look good. The gauge is does move, but the engine appears to remain quite cool at all times.

I did replace all of the vacuum hoses today, used 18' of new line.

Offline Bryan L

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Re: 1987 Chevy 350 engine dieseling
« Reply #3 on: June 14, 2022, 12:03 AM »
UPDATE

Today I was able to replace all of the 35 year old vacuum tubing with new hoses. I don't think there was a leak, but I figured while I was here may as well. It beats a hose going out in the middle of nowhere on a 90* day.

I next attempted to install an after market tach. Could not get it working, so I moved on.

I then adjusted the (I believe curb) 2nd idle speed a bit lower. I just guessed and backed it out about 2/3 of a turn.

I did my best to adjust the timing to 15 BTDC. The issue I had was that with this being the G30, I can only slightly see the retarded section of the timing teeth. There is no way to see the advanced section. I had a friend move it and we did our best to guess where 14 would be.

I then sprayed a whole can of Seafoam down the carb and did a hot soak.

I then fill it up with Premium (92) gas. $150 for 30 gallons.  :'(

Then I took it for a 20 mile spin, it drove well, except that I had also attempted to snug up a belt that in fact became more loose, so I was getting squealing at 60+ mph.

Other than that, all seemed well.

When I got back to the shop, I waited 1 minute and then turned it off. It stopped pretty well and did not diesel.

I am going to go drive it a few times after I get that belt snug and see if it stays good.

If anyone has anything else that I may have missed, please let me know.

Here is what she did today.  :)ThmbUp ... for now.

https://www.youtube.com/shorts/ET9H3mC5u9Y

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Re: 1987 Chevy 350 engine dieseling
« Reply #4 on: June 14, 2022, 11:12 AM »
I can see your mechanically inclined, and you achieved a good end. I will share a few things

Set your initial timing to 10 degrees.

Lower your idle speed to 650/700 rpm, that gets the quad on the idle circuit a very big deal with quads. There are times that is easier said than done, I get that. Can you get there?

Recurving a engine is a two part process, both the carb and the distributor are involved, getting the carb on the idle circuit is critical.If have done this already just disregard my post.

A small bit on this topic. Since this forum revolves around repair/maintenance

https://youtu.be/YWze5iSmiH8

These older engines have yrs of deposits built up in them, using a Rotella t4 oil would be quite helpful...Yes that's only a opinion, and it is diesel oil. Fully rated for gas engines high detergent oil. NOT advised to use if your engine uses a catalytic converter

Offline Mlw

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Re: 1987 Chevy 350 engine dieseling
« Reply #5 on: June 14, 2022, 11:38 AM »
Well, Eyes open beat me to it.

I also thought timing an listening to the video's I also thought the idle was really high., Better the second part, but still to high. Also just turning one of the idle screws not using your ear-o-meter of a vacuumgauge is probably not a good idea because you change the ratio which needs to be perfectly balanced. Of course it can be it was off and you improved it.

The fireexplosion (rather then a fireball) out of the tailpipe is easy, Ignition while your exhaust valve is still open = wrong timing.
Fireballs come from fuel in a hot exhaust: https://www.youtube.com/watch?v=v1O_Rb3c0f0

But what Exodus says holds ground too, certainly with Quadrajets as explained by Uncle Tony:

https://www.youtube.com/watch?v=zYO1jrVzsG8

And did you eventually renew the last two sparkplugs?


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Re: 1987 Chevy 350 engine dieseling
« Reply #6 on: June 14, 2022, 11:54 AM »
Yes idle screws play a fundamental role in tuning a engine. I can only assume the poster understands there function. It would be a very bad ideal to recurve a engine if the carb is not functioning properly on it idle circuit. Kaboom comes quickly and we have seen that backfire already...

Offline Mlw

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Re: 1987 Chevy 350 engine dieseling
« Reply #7 on: June 14, 2022, 12:27 PM »
Hmm I was changing the message as you respond. Uncle Tony had a nice addition to you video, which extended Exodus comment.



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Re: 1987 Chevy 350 engine dieseling
« Reply #8 on: June 14, 2022, 01:21 PM »
Yes Rochester are incredible carbs, far beyond there time, there's a old wives tale. They meter fuel based upon the engines needs, in short if your engine needs 750 c.f.m or 600 c.f.m to run properly a 850 will do it automatically and correctly. That happens to be true, and it starts with the idle circuit. It is critical...Lol and now you might understand how they became known as Quadra junk....most don't have a clue to how they operate

Offline Mlw

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Re: 1987 Chevy 350 engine dieseling
« Reply #9 on: June 14, 2022, 02:59 PM »
 :D :D :D

Yep, and that's were it gets interesting for me.

Offline Bryan L

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Re: 1987 Chevy 350 engine dieseling
« Reply #10 on: June 15, 2022, 02:42 AM »
Thanks for all of the replies.

I was not able to get out to the van today, so no updates.

I have not changed the front 2 spark plugs. When I changed the other 6 2  years ago, I simply saw no way to get them out, let alone back in. I guess I can give it another run to see what I can do. The van has ran very well the last few days.

I may not be using the correct terminology. The 2 screws on the front of the carburetor are capped off, and I have not changed them. What I did adjust was what I saw on a YouTube video and the guy called them the initial idle screw, tucked under the vacuum thing on the passenger side, and the curb idle screw on the drivers side near the throttle. I believe I set the first one on the passenger side, and then the 2nd one on the drivers side does not kick in until the choke opens.

The other 2 screws I can not change because they are plugged off from the factory.

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Re: 1987 Chevy 350 engine dieseling
« Reply #11 on: June 15, 2022, 09:19 AM »
Those plugs are called Welch plugs, actually just trivia
 Below is a link that gives some insight for removal, it is also a sight that contains a wealth of information about the Quadra jet. One can get parts there to, by serial number and the parts are the correct design for ones carb, known to function using today's modern fuel. I do not know of any other site/Parts replacement that does the job as correctly as Ruggles site. Perhaps SMI but Ruggles gets it right so why bother.

https://cliffshighperformance.com/simplemachinesforum/index.php?topic=4204.0

Offline Mlw

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Re: 1987 Chevy 350 engine dieseling
« Reply #12 on: June 19, 2022, 12:10 PM »
And a little (entertaining) extra.

easy explenation of your problem from 35:27

https://www.youtube.com/watch?v=uJYeMhat7tA

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Re: 1987 Chevy 350 engine dieseling
« Reply #13 on: July 18, 2022, 07:50 PM »
That actually is a great video he lays out the idle circuit well or transfer slots. If a person sets their base timing to say 8 degrees and cannot get the idle down 700 rpm...Then you know the carb is not running on the transfer slot..or idle circuit.

Offline AOBrodie

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Re: 1987 Chevy 350 engine dieseling
« Reply #14 on: July 30, 2022, 07:57 PM »
Run on, or dieseling is auto ignition. In a gas engine it is caused by something in the combustion chamber that is hot enough to cause ignition after the ignition is turned off. Playing with the ignition timing may help a bit, but it will mess with power/efficiency.

Basically, something is too hot. You have to find out what it is.

(Assuming no vacuum leaks and a good carburetor)

1. Adjust idle speed as low as is reasonable
2. Run the low end of recommended coolant temperature.
3. Make sure your plugs are correct. That means getting those other 2 plugs changed. Don't assume the plugs that are in it are correct. Look it up and see if they are right. Read the plugs. If they look clean, consider installing one range colder (the plug itself may be hot enough to autoignite). If they show carbon build up, consider one range hotter (the carbon may be the hot spot) .
4. There may be carbon build up in the combustion chamber that is hot enough to cause it. In this case, you can try the various chemicals, or do a valve job.
5. If it has egr or air injection, make sure they are functioning properly.