What RPM should a Husqvarna 555 RXT Strimmer be tuned to at full throttle?

[Muddy42]

As per the title, I bought a new 53cc Husqvarna 555 RXT Strimmer last summer.  The manual says the H is screw is tuned at the factory but it should be adjusted after running in, however it gives no advice on max RPM.  Just for fun, I measured it with the tach yesterday - at full throttle it reached about 8400 rpm, clean air filter and with full lengths of string .

 

Is this about right?  I don't need any more power (it has loads already) I just want it to run at a speed that is best for the engine.

 

Thanks

[openspaceman]

2 hours ago, Muddy42 said:

As per the title, I bought a new 53cc Husqvarna 555 RXT Strimmer last summer.  The manual says the H is screw is tuned at the factory but it should be adjusted after running in, however it gives no advice on max RPM.  Just for fun, I measured it with the tach yesterday - at full throttle it reached about 8400 rpm, clean air filter and with full lengths of string .

 

Is this about right?  I don't need any more power (it has loads already) I just want it to run at a speed that is best for the engine.

 

Thanks

The husky brochure says max power at 9k rpm, the thing is does it have a rev limiting coil?

 

One for @adw perhaps

 

We normally tune 2t motors to rev limit by 4 stroking at the maximum high speed, say about 13k rpm for a 50cc engine but it develops it full power at 8-9k rpm. If rev limited it cannot be tuned this way.

[adw]

Governed ignition unit set to 12000 rpm, max power achieved at 9000 rpm.

[Muddy42]

17 hours ago, openspaceman said:

The husky brochure says max power at 9k rpm, the thing is does it have a rev limiting coil?

 

One for @adw perhaps

 

We normally tune 2t motors to rev limit by 4 stroking at the maximum high speed, say about 13k rpm for a 50cc engine but it develops it full power at 8-9k rpm. If rev limited it cannot be tuned this way.

 

14 hours ago, adw said:

Governed ignition unit set to 12000 rpm, max power achieved at 9000 rpm.

Thanks. Sorry, I’m struggling with the terminology. Does that mean the max safe rpm (before leaning) is 12000 rpm and that I should tune to 9000 rpm ?

[openspaceman]

3 hours ago, Muddy42 said:

 

Thanks. Sorry, I’m struggling with the terminology. Does that mean the max safe rpm (before leaning) is 12000 rpm and that I should tune to 9000 rpm ?

@adw is far more knowledgable than I but my take is the safe upper limit is 12000rpm and the rev limiter will prevent it going more but the best performance is at 9000rpm.

 

As the rev limiter will cut in you will have to richen it so 4 stroking occurs below 12000rpm because if it is weak the rev limiter will prevent the revs going higher.

 

Personally I find it difficult to tune these rev limited engines on things like hedge cutters, pole saws etc., so a few hints from the experts wouldn't be amiss.

[adw]

Brush cutters are very easy to set, fit a trimmer head and extend the cord to the correct length, the engine will now be under full load, so lean the mixture, H screw ,until the engine dies, unscrew the jet until the revs are as high as it they go, now back off 5 minutes on a clock face, it’s now set, usually around 9/10000 rpm

[adw]

Just a p.s. you loose a 1000 rpm for every additional 1”of cord  you pull out.

[Muddy42]

On 12/08/2023 at 19:27, adw said:

Just a p.s. you loose a 1000 rpm for every additional 1”of cord  you pull out.

Thank you adw.  Is the reverse situation also true? What I mean is if the engine is tuned per your method above, can worn short strings then cause the the rpms to rise, overheating and potentially lead to lean engine damage? Or does this tuning method allow for enough margin for this to happen safely?

 

As an explanation, I use a Oregon jet fit head rather than a bump feed, where the temptation is to keep going with shorter strings rather than change them. I'm not looking for max power output just a safe tune where the engine is fine even if the strings snap off. Thanks

[Stere]

RXT is autotune  so does it self adjust?

 

 

[Muddy42]

12 minutes ago, Stere said:

RXT is autotune  so does it self adjust?

 

 

I think some models of the RXTs might be autotune, not my one. Thanks