HELP! Stihl HS80 engine rebuild. NEED HELP (advice)

[WDC1]

I have a number of Stihl equipment that I have successfully rebuilt the engines on. The problem I'm having is a HS80 Hedger, previously rebuilt with no problems. I've recently put on a AM cylinder kit, all new gaskets, crank seals, bearings, carb.,etc. but I cannot get the engine to build over 65-70 PSI. Out of frustration I've torn the engine back down to splitting the case and inspecting every detail rebuilding with a second 34mm cylinder kit just to be cautious. The result is the same 65-70PSI, not the 110-20 that I'm accustomed to.

I should note I have used this AM cylinder kit in the past without any problems. After installing two cylinder kits on this engine, both developing only 65-70PSI, so I suspect the problem is elsewhere. Anyone with Stihl experience have any suggestions of what may cause this?

 

[Mammyrat]

I am a beginner myself, but if it were me I would check the squish and if the piston is directional, check to make sure it's not installed backwards.

 

[Shane¹]

I would verify my compression testers reliability if the Schrader valve is leaking can give really funny numbers. Have you checked a motor that has known good compression

 

[WDC1]

I am a beginner myself, but if it were me I would check the squish and if the piston is directional, check to make sure it's not installed backwards.

Thanks for that I have not done the squish test that was recommended by someone else as well. I have to do a bit of research is I'm not sure how to do this test, e.g. piece of clay on top of the piston, etc., etc..

I have installed the piston the correct direction, error towards the exhaust port.

Thanks

 

[WDC1]

I would verify my compression testers reliability if the Schrader valve is leaking can give really funny numbers. Have you checked a motor that has known good compression

I've tested it using my compressor and very accurate in-line pressure gauge but I have not tested it on another engine and will. I have a chainsaw models 029 upgraded to 039 with a fresh rebuild, I'll test it.
Thanks for your input

 

[Shane¹]

Compressor will show correct but if Schrader valve leaks won't read proper compression because when piston comes back down it pulls the other way and takes compression away

 

[WDC1]

Thanks for that I have not done the squish test that was recommended by someone else as well. I have to do a bit of research is I'm not sure how to do this test, e.g. piece of clay on top of the piston, etc., etc..

I have installed the piston the correct direction, error towards the exhaust port.

Thanks

okay I have found the straightforward procedure for squish test using solder rod in the spark plug hole 180° out on each side of the piston parallel with the piston pin.

.

Does anyone know what the squish dimension should be? Somewhere I heard eighteen thousand but it may be just a matter of reducing the internal space until psi comes up to the proper 110-120 PSI.

I should also note that I have not researched Shane¹ Schrader valve, but will be for making any adjustments.

Thanks gentlemen

 

[Shane¹]

I would guess the squish should be somewhere under .030 but I would check the compression tester on something else first then go from there

 

[WDC1]

I would verify my compression testers reliability if the Schrader valve is leaking can give really funny numbers. Have you checked a motor that has known good compression

Embarrassingly enough, I'm thinking whenever this Schrader valve (SV (not the first time it came up)) is in the engine somewhere, but I now understand the referenced SV in the compression test. I have previously tested this by charging the compression test assembly with a digital (in line) compressor gauge off of my compressor. Both digital and compression test gauges are match & hold, eliminating the SV as the problem. Btw , I have tested both gauge/hose and gauge/hard line, same results.

This takes me back to piston throw, so I was going to remove the cylinder-based gasket and test. If it's not enough, I will shave the cyl. base with my lathe.

Of course any other suggestions are appreciated.

Will

 

[lehman live edge slab]

Embarrassingly enough, I'm thinking whenever this Schrader valve (SV (not the first time it came up)) is in the engine somewhere, but I now understand the referenced SV in the compression test. I have previously tested this by charging the compression test assembly with a digital (in line) compressor gauge off of my compressor. Both digital and compression test gauges are match & hold, eliminating the SV as the problem. Btw , I have tested both gauge/hose and gauge/hard line, same results.

This takes me back to piston throw, so I was going to remove the cylinder-based gasket and test. If it's not enough, I will shave the cyl. base with my lathe.

Of course any other suggestions are appreciated.

Will

You tried 2 china kits on this same machine? If they came from the same run they could both be bad. Lots of the aftermarket companies get runs of parts from the same company or may purchase through different ones and the parts you got may be a different manufacturer than before and of very poor quality.

 

[WDC1]

You tried 2 china kits on this same machine?

Yes, the China Cyl kits on the same engine and each producing 65-70 psi. And a very clean rebuild, nothing dodgy.

I purchased the kits from different suppliers, but that doesn't always mean anything. I think at this point I'm just going to remove the cylinder gasket and see where that takes me. If necessary, chuck the cylinder up and shave the base.

This is a strange one

 

[lehman live edge slab]

Yes, the China Cyl kits on the same engine and each producing 65-70 psi. And a very clean rebuild, nothing dodgy.

I purchased the kits from different suppliers, but that doesn't always mean anything. I think at this point I'm just going to remove the cylinder gasket and see where that takes me. If necessary, chuck the cylinder up and shave the base.

This is a strange one

Can’t just shave the base, need to know squish and keep at least 20 thousandths clearance at tdc. Also the kits could be very well the same if they’re white box kits with no label, also could be out of round, loose squish, bad rings on piston. These for you or a customer? Personally won’t use aftermarket parts on anything especially for someone else. At very least if they insist and I did I’d also let them know the longevity may be an unknown thing and depending on cost may be better buying a new replacement.

 

[lwn9186]

I am showing the Stihl HS80 piston and cylinder 4137 020 1201 is only 30.00. I would go that route. Forget the Chinese crap.

 

[WDC1]

The squish dimension came out 3X what I generally see quoted "20-30.00" which is suspect. I did not reinstall the first AM kit, but the PSI was consistent. I understand that white box kits could be the same error, but unlikely. Any other ideas? It's pretty straightforward, case bearing, con-rod bearing, piston bearings.
It is a mystery. I have three times the head space.

Thanks

 

[Wonkydonkey]

I was following this thread, I thought I had some good input about the compression. But I thought you were getting there and my input wasn’t really needed.

However … I just came back to this thread. Then it dawned upon me why you maybe getting the low readings !’!?

I ve gone down the path with compression testing, but that’s a part of fault finding now and I don’t get to blinkered on just the readings.

Anyhow I had a 200t that ran very poorly with an AM cylinder. I thought fûck just buy an oem and be done with it….. it was about twice the price of the AM cylinders but when I had both cylinders side by side !
Take a close look at the combustion chambers. They are not the same…. I know yours is a different machine but there the same chimps in the factories…so I’m thinking this is your problem as well.
So if you have an OEM and the AM to compare. It just may be your answer

Btw now I’ve said what I’ve said. I bet you can tell me why my 200t had poor running and bogged easily

which is the am and oem ..

Edit I tried a bit of porting and thought it may have been the problem. But it wasn’t

 

[WDC1]

Thanks Wonkydonkey I appreciate you taking the time to respond , everything is helpful.

Also, I've got you on the AM. I've never had one not run, but some are more fecal than others.

 

[WDC1]

I would guess the squish should be somewhere under .030 but I would check the compression tester on something else first then go from there

Has anyone performed a squish test on a HS80 (or equivalent) having good compression 110-125? .025-.030 is often cited but I am a long way from that on a new (two separate cylinder kits) AM cylinder , e.g. cylinder, piston, wrist pin, bearings. I have performed compression test using both analog and digital gauge 60-65PSI. The headroom space (in line with the wrist pin, Left – Right) with cly gasket .0754 - .0775, without gasket .0605 - .0660 , compression around the same.

The AM squish photo is shown above (#14), new Cyl and 34mm piston. Any thoughts or ideas on where I may have made a mistake?

 

[Wonkydonkey]

Normall most guys would use thinner solder 30thou as the thicker can give false readings (it don’t compress easily) your pic makes it look like the thicker stuff.

If your having difficulty then twist two bits of the 30 thou

I don’t want to say your squish reading is wrong. I know it’s a long way off , but it will help If it is an accurate reading

 

[WDC1]

Normall most guys would use thinner solder 30thou as the thicker can give false readings (it don’t compress easily) your pic makes it look like the thicker stuff.

If your having difficulty then twist two bits of the 30 thou

I don’t want to say your squish reading is wrong. I know it’s a long way off , but it will help If it is an accurate reading

Thanks for your input but in my case, the headspace exceeded the largest available solder .635. I did take X3 sets of readings (left, right), and they were all consistent, including with and without cylinder gasket. Attached are two of the three test strips. Without the cylinder gasket the dimension reduced by the thickness of the gasket, as it should.

I have two new cylinder kits so my thought is to machine the base of one of them until I get .030-.025 headspace unless anyone has a better idea.

Thanks again

PS twisting the solder wire created its own problems with inconsistency.

 

[Wonkydonkey]

I don’t mean to be a pain in the armpit and you think I know it all, cos I will happily say I don’t.

But…… if you’re wanting compression to increase ? Which is what you think is your problem… ? And I agree with you..

I think it was tree monkey that did some porting vids about compression increasing and got some blu-tac / plasticine stuff and some how worked it backwards. ( ie the volume of the plasticine took up to get the compression he wanted ) then took the amount off the base.
Well that’s how I remember it . I could be wrong ..

But then also taking what ever off the base will change the exhaust timing etc…

Anyway I will be interested to know how everything works out… as I’m sure others will

It still sucks testicles that you have to do what you have to get it to run ‍