8MLHU limited RPM

[pvanv]

A customer brought me this little 100:1 2 cylinder 2 stroke. Complains it doesn't rev fast enough. Hmm. Carb opening fully. Spark advancing fully. Compression great (about 130 pounds). Plugs look to be burning fine. Starts easy on about 2 pulls. Throttles up great until about 2950. Then more throttle does nothing. Odd. Ideas?

 

[99yam40]

is the timing advancing as it should.
timing light should show what the timing is doing

 

[QBhoy]

What boat is it trying to push and what pitch prop is on it. They come with a 7 or 8” prop new, usually.

 

[pvanv]

The timing seems good, as it goes right up to the stop. I am running in my test tank. The motor is to push a small sailboat.

 

[99yam40]

So, you have not used a timing light and are just guessing the timing is good/in spec.
most motors on sail boats are high thrust motors. they cannot get the boat moving fast
they have different gearing to help the motor push high trust props
good chance it is overloaded with the prop you have
but hard to tell from way over here on a computer

 

[QBhoy]

Hi. How are you managing to read the rpm off it ? And is it under load on the sail boat when you read the 2800 rpm ? I’m sure you’ll already know anyway…but these wee things generally need to be a jack of all trades. So one of these on a small inflatable dinghy will see high rpms and plane the boat at 15-20 mph on occasion. Then some of them are fitted as an auxiliary motor on sail boats or other heavier boats as a back up. In this case, they will never see their max rpm and the difference between just over half throttle and full throttle is only that it will lug and use more fuel with next to no gain in speed. Not sure of the size of the sail boat…but if it’s anything like your average small sail boat say 15-20ft. The 8hp will do just as described above. It’s never going to see anything like it’s max rpms specified. That would only be achieved on a planing small dinghy, rib, dory or ally boat or similar.
All that said, perhaps I’m off track and it’s not performing as it should. But in your described application, the owner shouldn’t expect too much from it. What prop is on it now, what size is the boat and what does it achieve in terms of speed ? I’ve got two of these wee things. Great engines. Mine will push a 19ft cuddy at around 6-7mph on a good day and push a wee 2.7m rib to 20 mph. The difference in rpms between the two uses is significant. And like I say…anything over 2/3rd throttle as an auxiliary doesn’t see any more speed gains really. Never gets to anything like it’s max rpms as an auxiliary

 

[saltchuckmatt]

Hi. How are you managing to read the rpm off it ? And is it under load on the sail boat when you read the 2800 rpm ? I’m sure you’ll already know anyway…but these wee things generally need to be a jack of all trades. So one of these on a small inflatable dinghy will see high rpms and plane the boat at 15-20 mph on occasion. Then some of them are fitted as an auxiliary motor on sail boats or other heavier boats as a back up. In this case, they will never see their max rpm and the difference between just over half throttle and full throttle is only that it will lug and use more fuel with next to no gain in speed. Not sure of the size of the sail boat…but if it’s anything like your average small sail boat say 15-20ft. The 8hp will do just as described above. It’s never going to see anything like it’s max rpms specified. That would only be achieved on a planing small dinghy, rib, dory or ally boat or similar.
All that said, perhaps I’m off track and it’s not performing as it should. But in your described application, the owner shouldn’t expect too much from it. What prop is on it now, what size is the boat and what does it achieve in terms of speed ? I’ve got two of these wee things. Great engines. Mine will push a 19ft cuddy at around 6-7mph on a good day and push a wee 2.7m rib to 20 mph. The difference in rpms between the two uses is significant. And like I say…anything over 2/3rd throttle as an auxiliary doesn’t see any more speed gains really. Never gets to anything like it’s max rpms as an auxiliary

Agree with above. And if your using it in a test tank and trying to go WOT then that's not good. You would need a dyno.

 

[Scott Danforth]

if timing is right, my guess is you are running out of fuel.

Have you gone thru the carb and fuel pump?

 

[pvanv]

if timing is right, my guess is you are running out of fuel.

Have you gone thru the carb and fuel pump?

Yeah. That has the pump on the side of the carb. Works really fine up to 2800, but then no more. If you blip in a wee bit of carb cleaner while running (to richen it), it nearly stalls. I had the same thought. And, since the mechanical linkage for spark advance goes all the way to the stop, I am still baffled.

 

[Scott Danforth]

not all carb cleaner is combustible.

does squeezing the bulb help?

are both cylinders operating properly? are both the same temp with an IR gun?

 

[pvanv]

It's acetone spray. No squeezing the bulb does not help. Yes, temp is pretty uniform. Not overheating at all. Both plugs are newish and look fine. Odd.