454 kickback/dieseling after shutdown?

[Slooptin]

So, I just started having this issue today. Engine is a 454 (stock internals/stock q jet) set at 8* BTDC @ idle (~800 rpm). Sitting at ~19 inHg at idle, mixture screws are about 2 turns out from bottom. The engine only does this after running for a couple minutes (I drive it around the block to replicate) and when I **** it down, it sounds like it diesels for a second and then kicks back air/fuel out of the intake, opening the secondaries. I haven't had this issue at all until today, and no matter what I change (idle speed, timing, mixture) it still happens to different degrees. Can anyone point me in the right direction?

 

[Bextreme04]

vacuum leak?

 

[bucket]

Automatic? Try shutting it down while still in gear and see what that does.

 

[Slooptin]

vacuum leak?

Nope

Automatic? Try shutting it down while still in gear and see what that does.

That doesn't actually solve the problem though

 

[Slooptin]

its getting significantly worse. did it for about 30 seconds after shutdown today and blew a huge plume of vapor out the intake. I have a vid and I'll try and upload later

I've been reading around and found some info about the anti-dieseling solenoid on my q jet. i took mine off recently because it wasn't hooked up so I'm thinking that was it. My question is, ow, Mine is vacuum controlled and I don't know which vacuum port to connect it to, so if anyone has any info on that, it would be very helpful

 

[bucket]

That doesn't actually solve the problem though

You're right, it doesn't. But it does help diagnose the problem. Lower RPM fixes it and you are on the right track with the solenoid. Although I prefer the older ones that just get a 12v signal.

What year is the truck? Are the smog pump(s) and such still in place? The reason I ask is because I've really only had problems with run-on in 80's vehicles with all the smog stuff. It must be the focus on lean burn and the valves running hotter, combined with a carbon buildup.

 

[Slooptin]

You're right, it doesn't. But it does help diagnose the problem. Lower RPM fixes it and you are on the right track with the solenoid. Although I prefer the older ones that just get a 12v signal.

What year is the truck? Are the smog pump(s) and such still in place? The reason I ask is because I've really only had problems with run-on in 80's vehicles with all the smog stuff. It must be the focus on lean burn and the valves running hotter, combined with a carbon buildup.

Gotcha. 1985, no smog pumps or egr. Should be no carbon buildup as I just did heads and cleaned ports/valves before reassembly. Maybe I have it too lean (8* before TDC and only 2 turns out on idle mixture screws) so I might try advancing the timing & re-connecting/re-setting the anti-dieseling solenoid

 

[Snoots]

Time to take it out on a long stretch of road and blow the gunk out!

 

[da_raabi]

I had this problem for a while. Just figured it out a couple days ago. Carb was loose. So I had a vacuum leak. Tightened down the carb (summit brand) and no more diesel.

 

[Slooptin]

I've changed the carb gasket, adjusted fuel, timing, reconnected the anti-dieseling solenoid and still no solution. plus i cant get this thing to run down the block, much less drive down the road. I'm, about to toss this quad in the trash and go buy something else. The crazy thing is, I can get this thing to idle just fine and then whenever I throw it into gear, it just turns to crap

 

[Slooptin]

Also it will still diesel/backfire out of the intake when i shut it off in gear

 

[Craig 85]

My truck is a stock 454 with all the smog equipment. My truck has an 11/84 production date so it has a single smog pump emissions system for 85. The OEM timing is 4* @ 700 RPM.

By anti-dieseling valve, to you mean # 1 & 4 (photo 2)? From what I've read, it is actuated by #3 (photo 1) which sends an electrical signal to #4 which opens up the vacuum to #1. If I understand the system correctly, it is designed to work during deceleration between 1500-1800 RPM to prevent backfires through the carb. It should not be operational at idle.

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[bucket]

Also it will still diesel/backfire out of the intake when i shut it off in gear

Maybe the timing is actually way off, due to a slipped balancer ring?

 

[emoze]

does it come down to normal idle when you get off the pedal?

very often - this can be caused by a throttle not closing completely - mine does that sometimes - could be idle arm adjustment; could be cruise arm or some other component is holding it open a bit; i've seen both problems & occasionally the d***mn throttle just sticks a bit open for reasons i've yet to discover - but kicking the pedal a few times with engine off seems to "cure" it

 

[Snoots]

I can't help but lean towards a vacuum leak.