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Author Topic: Hs81 won't rev up once warm  (Read 222 times)

Offline Marks Mowers

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Hs81 won't rev up once warm
« on: November 01, 2018, 06:02:24 PM »
Hi everyone,

Please, please, please help me on this one!

HS81 once warm bogs down under full throttle.

Sounds like a blocked exhaust/ stuck blades. But only does it after about 10-20 minutes of use.
Tick over is unaffected.

Will be fine again once cool.

So far it has had a new fuel pipe from filter to carb as the old one had a split.
A new tank breather. New spark plug.
Pressure tested and found a leak from the crankcase so that has been resealed.
A needle bearing had failed on the lower gearcase.

When apart I blew through the exhaust and it seemed clear, piston and cylinder looked in good nick.

Any thought and suggestions muchly appreciated.



Offline Mick

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Re: Hs81 won't regret once warm
« Reply #1 on: November 01, 2018, 06:29:48 PM »
Hi Mark.

That's a strange one, is it a 81R or 81T just out of interest?

Offline Marks Mowers

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Re: Hs81 won't regret once warm
« Reply #2 on: November 01, 2018, 06:32:12 PM »
Ooh, I'm pretty certain it's an 81r.

Offline Marks Mowers

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Re: Hs81 won't regret once warm
« Reply #3 on: November 01, 2018, 06:32:48 PM »
Also, that was supposed to say rev up. Not regret in the title. Damn autocorrect

Offline Mick

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Re: Hs81 won't regret once warm
« Reply #4 on: November 01, 2018, 06:39:29 PM »
Also, that was supposed to say rev up. Not regret in the title. Damn autocorrect

lol.. I'll change that for you in minute.

81R. Ok so when you say bogs down, do you mean it seems to lean out, or run rich?  The only problems I've had where contractors are using them on a daily basis is the pump diaphragm, not sure if you changed it already, but on one that's had fairly heavy use, if you hold the pump dia up to the light, and look where the two little flaps that act as the one way valves, just check that you can't see through them.  Had a few of these, and all played up after a few minutes running, and were fine when cold.

Offline Marks Mowers

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Re: Hs81 won't regret once warm
« Reply #5 on: November 01, 2018, 06:47:37 PM »
That's a nice trick, thanks I'll check that tomorrow.

To me it sounds like a blocked exhaust, neither lean nor rich. More like the engine is suffocating on its own gases. Or like when somebody crams it full of grease and the blades can't turn.

I only heard it mess about for the first time today as I've not been able to recreate the issue myself until this afternoon.

So carried out the other repairs after being told it won't rev up, after visual inspection and pressure testing found issues. So thought that it was cured until it came back today.