Reducing Squish on a Poulan Clamshell

[redfin]

Thanks for posting this Chris. I'm eager to see the results.

 

[VinceGU05]

i recently done up a MS390. being a clamshell. i went the easiest option to boost the comp.



tried this but fouled.. so got a stock piston and with the aid of Plasticine, got am imprint from the combustion chamber along with the plug in!


ended up with something similar to this . and ended up with 185 psi. all standard port timing as nothing else was changed.

 

[Moparmyway]

The 3D printer is about the only "high tech" equipment I do have access to. Other than that it an old worn out lathe driven by a leather belt, some drill presses and belt sanders. The CNC stuff is all set up for production work and I don't know how to use it anyway (although I have been considering learning to program one). And a Dremel.

Handicapped or blessed ......... even a combination of them, you did a fantastic job. What I am really looking forward to is the fact that you could easily cut another cap and swap out the stuffed one to see what difference those 3D inventions make. I love the timing charts you do !!

Great job !!

 

[Chris-PA]

OK, Chris, I'll double check all of that. But, I think that's what I already did in November. Compression still seems nil compared to any Stihl that I own, and that's usually the bottom line. I was very careful when rebuilding the top end and saw no difference in rope pull resistance after completing it. Not much is more depressing than that. I'll report back what I find and show you a Pic of the piston before I started.

Do you know what the squish is? These engines are easy to pull over stock, so maybe it is not a sign of a problem with the piston/cylinder.

 

[angelo c]

OK, I'm all ears. After installing a new piston and rings (saw looked like it was straight-gassed), it still seems to have insufficient compression to start, even with a good spark and carb settings as recommended. Saw has very few hours on it and looks brand new. My Stihl dealer told me to stop wasting my time on junk. What's next for me to try?

Doc,
will it fire if you force feed a few drops down the exhaust or intake ? should still pop and fire somewhat uncontrolled.

 

[auskip07]

Doc,
will it fire if you force feed a few drops down the exhaust or intake ? should still pop and fire somewhat uncontrolled.

Not if he isnt getting spark

 

[Chris-PA]

Sitting here at work wishing I was home and the ground & wood were not covered with snow so I could try that saw out for real.

 

[angelo c]

Not if he isnt getting spark

Absotively , my assumption was he confirmed spark and was concerned for compression. If it won't fire with known spark well then it can only be "timing" or compression.

 

[angelo c]

Sitting here at work wishing I was home and the ground & wood were not covered with snow so I could try that saw out for real.

Soon we'll be complaining about the heat and ticks.... !!!

 

[Chris-PA]

Soon we'll be complaining about the heat and ticks.... !!!

And mud.

 

[angelo c]

Sitting here at work wishing I was home and the ground & wood were not covered with snow so I could try that saw out for real.

I hear ya...

 

[Chris-PA]

heat ticks and mud all sound wonderful this weather is really getting me down...........
View attachment 406418

What is that that "-" symbol in front of the number 18? That's a bad thing, right?

View attachment 406419

I hear ya...

Hey, I see some wood sticking out there - enough to run a saw in for a few minutes anyway!

 

[angelo c]

What is that that "-" symbol in front of the number 18? That's a bad thing, right?


Hey, I see some wood sticking out there - enough to run a saw in for a few minutes anyway!

Sure thing...come and join me. Bring some saws. I got a few we can run too.
Id say we'd last a good 15-20 minutes at least....shop is in the basement. ...plenty heat there.

 

[Wood Doctor]

Doc,
will it fire if you force feed a few drops down the exhaust or intake ? should still pop and fire somewhat uncontrolled.

Model is 18/42. Saw measures 25 psi compression, which is nothing. All cylinder mounting bolts are tight. Original was straight gassed. I installed a new piston and rings. Spark is good. Engine hardly has any hours on it.

Is it worth removing the cylinder again? How could there be practically no compression at all? I've never seen anything like this.

 

[auskip07]

Model is 18/42. Saw measures 25 psi compression, which is nothing. All cylinder mounting bolts are tight. Original was straight gassed. I installed a new piston and rings. Spark is good. Engine hardly has any hours on it.

Is it worth removing the cylinder again? How could there be practically no compression at all? I've never seen anything like this.

I would say kep trying. There has to be something you missed.

 

[angelo c]

Model is 18/42. Saw measures 25 psi compression, which is nothing. All cylinder mounting bolts are tight. Original was straight gassed. I installed a new piston and rings. Spark is good. Engine hardly has any hours on it.

Is it worth removing the cylinder again? How could there be practically no compression at all? I've never seen anything like this.

Doc,
Its got to be leaking somewhere. Spark plug, case fittings, seals ect. can you add some pressure and hunt down the leak.
maybe start another thread and keep Chris's focused on hotrodding a clam. lets get yers runnin then have Chris turn up the volume !!!

 

[drf255]

i recently done up a MS390. being a clamshell. i went the easiest option to boost the comp.



tried this but fouled.. so got a stock piston and with the aid of Plasticine, got am imprint from the combustion chamber along with the plug in!


ended up with something similar to this . and ended up with 185 psi. all standard port timing as nothing else was changed.

Great job. Did you use 4047 filler rod?

It seems like whatever size dome your minds eye predicts for these should be halved. I'm surprised you got only 185 psi with that dome. The chamber space much be rather large on a 390.

Did you relieve the piston under the dome at all? What weight difference did you come up with?

 

[Wood Doctor]

Doc,
It's got to be leaking somewhere. Spark plug, case fittings, seals ect. can you add some pressure and hunt down the leak. Maybe start another thread and keep Chris's focused on hotrodding a clam. lets get yers runnin then have Chris turn up the volume !!!

OK, I'll back out. I took it apart again and found the problem. A chunk of the lower right rear (carb side) corner of the cylinder broke off. It must have had a hairline crack in it that I didn't see when I installed the new piston and ring. Funny thing also, the piston ring was broken, even though the saw never started and ran after I installed it. Anyway, at least we now know why there was no compression.

Are you guys sure that these saws are built well enough and tough enough to hot rod them? I have to wonder after seeing this.

 

[Chris-PA]

Are you guys sure that these saws are built well enough and tough enough to hot rod them?

The castings are quite nice on these - remember, the only machining that gets done is the bore and the spark plug hole. The rest must be accurate enough to use without machining, so it can't be junk. I'm not sure what happened on yours, but I've never had one break - just the typical scoring and stuff.

 

[auskip07]

Oh yes. I think the guys had a wild thing build off a couple years ago. The ones that lost the competition were amazing. The winner was unbelievable in the power it put out. This is Masterminds saw. Seems to run pretty well even with the air leak.

Where is that thread? i would love to read it. Im sure i could get my hands on a wildthing for around 30 dollars