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Old 11-17-2017, 02:53 PM   #1
bman1985
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Smoke from exhaust under load

I have an 84 GMC C1500 with a carbed 350 goodwrench motor on it. No idea how many miles it was a junkyard pull. It has headers, a performer rpm intake manifold and a summit 600 cfm carb. I set the idle speed with a tach and vacuum gauge and adjusted the floats buy jetting is still factory. It runs pretty well. It doesn't use excessive oil. My problem is that when the truck is under excessive load, like pulling my 6000 lb boat and trailer out of the water, it starts to blow dark smoke, more from one exhaust pipe than the other (it has straight pipes) It will smoke for a minute or so after I put the truck in park. Any ideas what might be causing it and how to diagnose and fix? My thoughts are rich carb, worn rings or maybe valve seals?
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Old 11-17-2017, 03:44 PM   #2
b454rat
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Re: Smoke from exhaust under load

Dark sounds like it’s rich....
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Old 11-17-2017, 06:33 PM   #3
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Re: Smoke from exhaust under load

Exhaust smoke can be tricky to diagnose sometimes. What may look like dark smoke can be either carb running rich or oil. Does the smoke have a bluish tint or is it a dark gray to black color? A bluish tint indicates oil consumption and dark gray is carb too rich.

First thing you need to see is if it uses oil more than normal. One quart in 2 to 3K is considered okay. If it burns more than that it could be worn rings, valve stem seals or valve guides.

If you determine it's oil burning, before doing any major work, check the valve stem seals. I've seen them so bad they just crumbled from being old and dried up.
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Old 11-17-2017, 07:05 PM   #4
bman1985
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Re: Smoke from exhaust under load

It's more black/gray than it is blue..sometimes it smokes on a cold start too. It's definitely not the familiar blue cloud that my parent's '89 conversion van used to cloud the driveway with after it sat a few days. It had leaky valve seals for 10 years.

If the compression checks out ok is it most likely not rings? As far as the carb do you think maybe the jets need to be resized in I don't know anything about properly sizing jets
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Old 11-17-2017, 07:38 PM   #5
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Re: Smoke from exhaust under load

Also I only drive the truck about 500-1000 miles a year but I never need to add oil.
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Old 11-17-2017, 07:48 PM   #6
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Re: Smoke from exhaust under load

That definitely sounds like carb running rich. Maybe check the choke adjustment. Manually activate the throttle lever when the choke is closed and see if it opens up when throttle lever is actuated to fully open the butterfly valves.
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Old 11-18-2017, 11:26 PM   #7
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Re: Smoke from exhaust under load

Maybe a pic of the carb or model number. Summit sells a lot of different carbs.

Ethanol gas? Infrequently driven vehicles tend to gum up pretty fast.
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Old 11-18-2017, 11:30 PM   #8
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Re: Smoke from exhaust under load

Tested today, the choke does fully open. Also it did make some blue oil smoke when it first started from the driver's side exhaust.

Last edited by bman1985; 11-18-2017 at 11:36 PM.
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Old 11-19-2017, 12:08 AM   #9
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Re: Smoke from exhaust under load

Quote:
Originally Posted by bman1985 View Post
Tested today, the choke does fully open. Also it did make some blue oil smoke when it first started from the driver's side exhaust.
What year engine do you have?

If it smokes when first started it may be valve guides or valve stem seals. The seals can get dry and brittle allowing oil down the valve guides. I'd check that first and maybe replace them just to be sure.



Before doing any major work, remove the valve covers and check the
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Old 11-19-2017, 12:30 AM   #10
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Re: Smoke from exhaust under load

Summit SUM-M08600VS carb

Not sure what year the motor is. I've owned it 6 years, the previous owner put in a motor from the junk yard so it's a mystery
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Old 11-19-2017, 08:11 AM   #11
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Re: Smoke from exhaust under load

It sounds rich to me too. Maybe it's the incline messing with the carb. At 1000 miles per year, around here we would say "drive 'er Mcgyver".
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Old 11-19-2017, 02:26 PM   #12
bman1985
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Re: Smoke from exhaust under load

You might be onto something with the incline messing with it. It smells like it runs rich, maybe I need to re tune the mixture screw, I guess they did back out on their own sometimes. I haven't adjusted it in a few years. I'll give the valve seals a check too.
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