# Afterfire and surging at full throttle.



## kernel_panik (Jan 3, 2013)

New to the boards here, hello to all! 

8 hp Craftsman C950-52678-7
(dates from 198?)









I installed:
-a new carb (the carb is bolted solid to the intake and the gasket is new)
-New spark plug

It has:
-Fresh oil (5w 30)
-Fresh gas treated with stabiliser

Runs smooth at idle and at midrange but backfires (afterfires) and surges at full throttle.

I tried adjusting the high speed jet screw and that somewhat helped but it still backfires every once in awhile and stills seems to surge (more like the speed oscillates a little). When I went too rich it would back fire constantly and flames would come out the exhaust (a good hint I went too far counter clockwise ). 

Under load the issue seems to go away mostly but it has stalled on me a few times during my trial runs.

My google research led me to the theory my exhaust valve might be gummed up causing it to intermittently stick partially open during the combustion cycle. This sounds likely since it's an old machine, however does not explain the strange surging/oscillations the engine has at full throttle.

I was thinking of trying some Seafoam in the oil and gas to see it that has any effect on the backfiring (afterfiring), but don't know what else to do asides from trying to tune the carb again to solve the surging.

Does anyone here have some ideas suggestions I could try?

Thanks in advance for all/any help you may offer!


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## HCBPH (Mar 8, 2011)

*Welcome*

First off, welcome to the forum.

Sure sounds like it's possible, though it could still easily be in the carb.

If you want to try cleaning up your valves etc, here's a thread I put together on things I do when going over an engine in a machine I buy to refurbish. Includes decarboning the valves, head and pistion:
Evaluating a used engine

Good luck. Let us know what you find.


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## detdrbuzzard (Jan 20, 2012)

hello kernel panik, welcome to SBF
if you put seafoam in your oil use it as an engine flush and drain it back out right away. seafoam will thin the oil and clean the internals of your engine


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## kernel_panik (Jan 3, 2013)

Thanks for the useful replies!

As it turns out the minus 20 celcius temp that I was running my tests under was the issue it seems.

It snowed last Sunday here in Ottawa and the temp was mild. The snowblower ran perfect after some minor tweaking of the carb.

I did run some Seafoam in the gas so Im sure that could of helped but from what my neighbour (ex mechanic) tells me the very cold weather is what was causing the afterfiring.


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## detdrbuzzard (Jan 20, 2012)

if there is to much seafoam in the gas it will run with a slight hiccup


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