Stihl HT75 frustration!

[Overlooker]

I have two HT75s, both cut well. One, however, bogs when I throttle it up from an idle. It has a Zama with only the L mixture screw. I have ultrasonically cleaned and rebuilt the carb, and it pressure-tests just fine. I have pressure-tested the engine, checking for air leaks, and found none. It starts easily and runs and cuts great, but I have to slowly bring it up from an idle to full throttle. What am missing? Any ideas? Thanks, T-A

 

[tomalophicon]

I just acquired a husky 365 that bogged from idle and it had a scored piston on both intake and exhaust sides (presumably from piss revving it from cold). I could coax it up to revs but would just die once it came back down.
Have you checked piston?

edit: or maybe your L mixture screw needs attention.

 

[cscltd]

New latest version Zara carb with high and low mixture screws is really only cure. Single screw versions are great when they work- when give troubles-replace them

 

[Overlooker]

New latest version Zara carb with high and low mixture screws is really only cure. Single screw versions are great when they work- when give troubles-replace them

That's the most reasonable solution I've come up with so far, but I hate to admit defeat. T-A

 

[tomalophicon]

A blocked spark arrestor sounds like a reasonable possibility too if it has one.

 

[Overlooker]

A blocked spark arrestor sounds like a reasonable possibility too if it has one.

I will check to see if I might have overlooked that. Tnx, T-A

 

[Overlooker]

edit: or maybe your L mixture screw needs attention.

It might still need further adjustment, but not because I haven't tried. I have concluded it's fuel starved, but backing the L screw out even more doesn't fix the problem and makes the saw less likely to idle at all. It doesn't add up, in mind. T-A

 

[lone wolf]

That's the most reasonable solution I've come up with so far, but I hate to admit defeat. T-A

It aint defeat once its fixed.

 

[HarleyT]

Swap carbs to make sure that the carb is the problem.

 

[Overlooker]

Swap carbs to make sure that the carb is the problem.

That thought actually crossed my mind, but, until you brought it up, I didn't realize how brilliant that idea is. Tnx! T-A

 

[L34]

Had nothing but trouble with carbs with no H screw.
Fit a carb with both screws.

 

[Overlooker]

I plan to swap carbs on my two HT75s to help diagnose whether or not the carb is the problem. But, in the meantime, my friendly local Stihl mechanic suggested that I check the impulse line. On an HT75, there is no impulse hose, as far as I can tell, so it's less likely to have an impulse problem. But, besides that, would an impulse problem be suspected on a saw that idles okay and cuts well at high rpm, but bogs when you ask it to go from idle to full throttle?? T-A

 

[HarleyT]

Look closely at the intake block/manifold. It can develop cracks.

 

[67L36Driver]

There is kits on evilbay, carb plus fuel lines plus grommets etc. Twelve bucks.

https://www.ebay.com/itm/163984384060
Cured my HS75 & HT75. [emoji106]

 

[Overlooker]

Look closely at the intake block/manifold. It can develop cracks.

When I pressure tested the engine, it held 10 psi very well, so I think I have ruled out a cracked intake manifold, bad oil seals, and a leaky cylinder gasket. I still need to do the carb swap to answer more of my questions. Thanks for the input. T-A

 

[HarleyT]

I still need to do the carb swap to answer more of my questions.

I mentioned the intake block, because you mentioned the impulse line. Swapping the carb is a great idea, as it isolates your problem area either toward the carb, or away from it. Whoever suggested that is a genius.

 

[Overlooker]

I mentioned the intake block, because you mentioned the impulse line. Swapping the carb is a great idea, as it isolates your problem area either toward the carb, or away from it. Whoever suggested that is a genius.

I appreciate any input, because, even if it's not exactly the answer to my questions, it might lead me to something else that I hadn't considered. Thanks, T-A

 

[Overlooker]

Swap carbs to make sure that the carb is the problem.

News flash: IT WAS THE CARB THAT DID IT! I finally found time to swap carbs, and, by doing so, I proved that the carb was the problem. Which ever saw has the questionable carb is the one that I can't get to run right. My local Stihl dlr will sell me a 2 screw appropriate replacement carb for $42. Then I just need the wherewithal to throw the problem carb far our into the Mississippi, so I'm not tempted to waste any more time trying to fix it! Thanks to all who shared their thoughts with me. T-A

 

[67L36Driver]

Read post #14.

Carb plus extras is $12 on evilbay.

 

[HarleyT]

Yeah, I gave up a long time ago trying to rebuild the carbs usually, with the high costs of OEM carb kits. And I generally try to avoid the Chinese junk, as the quality is very unpredictable. And as always I opt for pinpointing the problem part before spending any money, or suggesting such on internet discussion boards. I would feel guilty if I recommended a replacement part, and it didn't fix the problem. But these forums are great though, as it ensures that the market for selling flywheels and coils and clutches will be good. Except when someone gets a coil or carb, and it doesn't fix their saw, they tend to say that somehow the new part is defective, etc...
So having two of the same model makes it all pretty easy to isolate a problem area. Then you can save the old carb and futz with it for the sake of learning, etc...