Stihl HT 101 pole saw.... WTF?

[Arrowhead]

I had a guy bring in a Stihl HT101 pole saw today. Says it cuts out and runs like crap. After he left, I pulled the muffler after noticing the strange design. It has valves. Are these things a hybrid 2/4 stroke? Just wanted to know a little about them, this is a first for me.
I ordered a carb kit, the metering diaphragm was hard... most likely the whole issue.

 

[naturelover]

Tis the 4-mix engine, a two stroke with valves...

There was a cut-a-way of one at my local Stihl dealer, interesting design for sure.

 

[Arrowhead]

Tis the 4-mix engine, a two stroke with valves...

There was a cut-a-way of one at my local Stihl dealer, interesting design for sure.

Thanks... I've heard the term "4-mix" before, just never had my hands on one.
I'd love to see a cut-a-way engine.

 

[naturelover]

Thanks... I've heard the term "4-mix" before, just never had my hands on one.
I'd love to see a cut-a-way engine.

I'll see if its still there, maybe they'll let me snap a couple pics... :msp_biggrin:

I guess the idea was to get all the mix above the piston, and at the right time, thus ensuring a more complete burn? Someone may hafta chime in on that one.

 

[ZeroJunk]

They need to have the valves adjusted ever so often and it is almost imperative to run a good synthetic like Stihl Ultra to keep the valves from carboning up.

 

[barneyrb]

Yep, valves adjusted every 50 hours or so and plastic camshafts......welcome to the world of Stihl quality.

 

[nmurph]

They are a 4 stroke that uses pre-mix.

 

[NORMZILLA44]

They need to have the valves adjusted ever so often and it is almost imperative to run a good synthetic like Stihl Ultra to keep the valves from carboning up.

REP my friend:msp_thumbsup: You took the words right out of my mouth, great heads up!

 

[Jacob J.]

Ed- you're just now working on a four-mix engine for the first time? Slacker.

 

[Arrowhead]

Ed- you're just now working on a four-mix engine for the first time? Slacker.

Slacker?.... or lucky?

Thanks to everybody for the info/help. I'm sure the diaphragm is the problem, but if not.... I'll be pickin your brains in about 5 days once the carb kit comes in.

 

[Andyshine77]

Slacker?.... or lucky?

Thanks to everybody for the info/help. I'm sure the diaphragm is the problem, but if not.... I'll be pickin your brains in about 5 days once the carb kit comes in.

I think a whole new carb is like 20 bucks, I normally just replace the carbs on most Stihl trimmers and blowers. The valves are super easy to adjust as well. With proper care the 4-mix engines seem to do OK.

 

[c5rulz]

My neighbors Stihl pole saw has that motor and I use it often. He has the smallest motor and you really have to take your time, but it idles great.:msp_smile:

 

[Arrowhead]

I think a whole new carb is like 20 bucks, I normally just replace the carbs on most Stihl trimmers and blowers. The valves are super easy to adjust as well. With proper care the 4-mix engines seem to do OK.

Thanks for the info on the valves Andy.
When I ordered the kit earlier today, the cheapest complete new carb I was able to find on ebay was $40. However, I only did a quicky search....

 

[Andyshine77]

Thanks for the info on the valves Andy.
When I ordered the kit earlier today, the cheapest complete new carb I was able to find on ebay was $40. However, I only did a quicky search....

Some models have more expensive carbs, sounds like you have one.

 

[naturelover]

They are a 4 stroke that uses pre-mix.

Oh? So they fire as a four stroke?

Well, now that I think about it, makes sense. Thanks for the clarification nmurph.


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[galde]

One thing to be aware of with the early HT-101's (and possibly others) is a problem with the ignition modules. The modules automatically advance the spark timing at higher RPM's. They had a tendency to fail to return to retarded timing for the next starting, and they will not fire in spite of good spark and good compression. I think it was Fish who clued me onto this when my 101 wouldn't start. A new $50 module fixed me up.

 

[naturelover]

Well, now that you said that......

A couple of years ago, guy brought me a four mix trimmer that wouldn't start after it had fell to the floor from its storage rack.

No visible damage, good compression and spark, but I never did get that thing to start, not even pop.

The only thing I could come up with was it was somehow out of time, but the flywheel wasn't sheared. I took the valve cover off and all looked well, but didn't go any further.

Wonder now if that was what was wrong with it....


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[Rein]

Well, now that you said that......

A couple of years ago, guy brought me a four mix trimmer that wouldn't start after it had fell to the floor from its storage rack.

No visible damage, good compression and spark, but I never did get that thing to start, not even pop.

The only thing I could come up with was it was somehow out of time, but the flywheel wasn't sheared. I took the valve cover off and all looked well, but didn't go any further.

Wonder now if that was what was wrong with it....


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I know this is years later and no one will see it, but I did see Themowermedic1 do a video about a problem that sounds very similar. A 4-mix polesaw was being started (or maybe I misremember and it was idling) in a 90 degree rotated position (i.e. not upright as usual) and something happened and it just stopped running. No amount of fiddling and pulling could get it to pop after that. Turns out one of the valve tappets had slid off the top of its valve stem, causing it to have zero compression (hence no fire). Something like - the valve spring wasn't fast enough to return the valve all the way before gravity shifted the tappet over. It was just a strange coincidence of circumstances and very rare, but it does apparently happen. A really easy fix - just a tough diagnosis because it's not something most mechanics ever hear about. Hope this helps someone.