Stihl 039 issues

[Outdoorguy1979]

So I came across a stihl 039 for 20 bucks, so I got it. Found out it needed a new piston and cylinder. I found a cross performance piston and cylinder combo for a good price.

I have rebuilt the saw 2 times already and am only getting about 130 psi, I think I should be about 150 psi. The problem I'm running into is this the carb.
I lost the original carb so I went and bought 2 new ones. One carb i have no low side on the other carb i have no high side. Both carbs and spitting gas out of the carb am at a loss as i need to get my saw running, any advice is appreciated and thanks in advance

 

[Old2stroke]

Need some clarification. By "low" side and "high" side, do you mean the L and H adjustment screws? I have never seen a carb with a H adjustment screw but no L screw, many carbs have a low speed mixture screw (L) and and a fixed high speed jet (no H screw) but any correct carb for that saw should have both L and H screws as well as a third screw that adjusts the throttle opening for idle speed. Do you get spit back from the carb at cranking speed or does it actually fire and run and then give spit back while running? In either case something is wrong if that is actually happening, I don't believe that saw has reed valve induction so if you are getting serious spit back from the carb if usually means the piston is the wrong one for that cylinder.

 

[Outdoorguy1979]

Yes your right it has a h and a l side with idle screw. I get spit back at full throttle enough to drench my hand

 

[Vintage Engine Repairs]

I’m really interested to learn what causes it to spit fuel back into the carb so I’ll watch this post carefully.

 

[Yukon Stihl]

Just curious if you are going to address the low compression...
you should try another guage,if it is the actual reading measure your squish.
AM piston and jug is a good candidate for low compression

 

[spudulike]

I would expect the blow back through the carb to be caused by blow by past the piston skirt (inlet side) through wear on the piston or bore, incorrect piston so the skirt doesn't seal the inlet port or someone may have widened the inlet port without understanding that the skirt must cover each end of the port by 2mm or more.
It is usually down to poor air filter maintenance and the ingress of fine wood chip wearing the piston and bore.

 

[Old2stroke]

As you have rebuilt with new P/C, I don't think wear or funny porting can be a problem. If the piston is the right one for the cylinder, the intake port should be closed by the skirt soon enough in the downstroke to prevent spitback. Either the piston is the wrong one or the P/C is meant for a saw with a slightly longer stroke. Hopefully someone will join in with part numbers that can shed some light.

 

[spudulike]

It is possible the piston fit isn't that great. I have noticed that cheap AM cylinder kits tend to make less compression and the pistons will have a fair bit of slop in the bore and this may be one reason why the carb is ejecting fuel vapour.
If the carb is set far too rich, this could also cause the issue. Try going "off piste" with the carb settings if they are non original AM ones and see if it helps. Try 1/2 turn on the H screw but if it screams....do NOT put it under load or it may pop. A tach would be a good tool if you have it, circa 12,500rpm is about right! Sounds like the non standard carbs may just be flaky on their settings.
130psi is low on a 039, they never make top compression but 150 is more like it!

 

[toadman]

subbed. I don't have an answer, but I'm curious to the final outcome.

 

[Maintenance supervisor]

Could the piston be installed" back to front " backwards?
Like oops, exhaust side facing the intake.

 

[Pioneer]

This may sound like an odd question, but what is your muffler like? Is it really choked like the 390's? I had horrendous spitback problems on a couple of ms250's that had the stock restrictive mufflers, very similar to the stock 390. The extremely small outlet holes would not allow proper flow and as a result there was a lot of spitback which caused a wet air filter and fuel pooling in the filter assembly. After muffler mods, the problem disappeared, and the saws ran properly.
As for compression, the 390 I worked on was nothing to brag about, Even when broken in I was only seeing 150 psi.
Excessive piston to bore clearance will not help with both problems either.

 

[grizz55chev]

Just a thought, check the flywheel key for damage, could be a timing issue.

 

[toadman]

This may sound like an odd question, but what is your muffler like? Is it really choked like the 390's? I had horrendous spitback problems on a couple of ms250's that had the stock restrictive mufflers, very similar to the stock 390. The extremely small outlet holes would not allow proper flow and as a result there was a lot of spitback which caused a wet air filter and fuel pooling in the filter assembly. After muffler mods, the problem disappeared, and the saws ran properly.
As for compression, the 390 I worked on was nothing to brag about, Even when broken in I was only seeing 150 psi.
Excessive piston to bore clearance will not help with both problems either.

I had a ms 250 that would flood on every single person who started it, except my grandpa. He had the secret recipe perfect on that saw from day one.
I muffler modded it, with what I thought was an excessive sized hole one day. I was pissed after my guys struggling and complaining about it all day, & I didn't really care if it blew up.
best mod I ever made.
It no longer flooded for anyone & made a noticeable amount more power in the wood -after retuning it
I also noticed the filter would be wet with fuel at the end of the day when I cleaned it...
Never saw the filter get wet again after the MM, but I attributed this fact to the retuning I did to the carb at the same time.

 

[motorhead99999]

I also vote muffler mod it. All the clamshells are turds till you open them up.