Engine Trouble Losing rpm once engine gets hot

CedarHillbiker

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Aug 3, 2015
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Ive finally built a screamer (almost). When I first top out my 66cc motor from the start, I'm hitting about 8500 rpm(36mph). I have a high compression head, I've raised the exhaust and redirected the transfer ports and installed an expansion chamber. I'm running a B7HS plug. Here's the problem: once the motor gets good and hot, I lose about 1000 rpm (32mph). Is this because it's running too hot and I need to go to a cooler plug?.....or a hotter plug? NGK makes a Range from B5HS-B10HS. What else could it possibly be? Anyone have any insight?
 


Frankfort MB's

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You could be running rich. When a motor first starts it uses more fuel (so it will lean the motor out, the reason why you have to choke) after a motor warms up it requires less fuel, so you could get too rich after warm up
 

Frankfort MB's

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Apr 3, 2016
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Ive finally built a screamer (almost). When I first top out my 66cc motor from the start, I'm hitting about 8500 rpm(36mph). I have a high compression head, I've raised the exhaust and redirected the transfer ports and installed an expansion chamber. I'm running a B7HS plug. Here's the problem: once the motor gets good and hot, I lose about 1000 rpm (32mph). Is this because it's running too hot and I need to go to a cooler plug?.....or a hotter plug? NGK makes a Range from B5HS-B10HS. What else could it possibly be? Anyone have any insight?
Do you happen to hear any pinging in the engine at speed? If you don't then your plug should be alright.
 

CedarHillbiker

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Aug 3, 2015
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You could be running rich. When a motor first starts it uses more fuel (so it will lean the motor out, the reason why you have to choke) after a motor warms up it requires less fuel, so you could get too rich after warm up
The plug is bone dry and I am already running a 60 main jet. I'm at about 600 ft above sea level. If it is pinging, does that mean I need a hotter plug? Any other thoughts?
 

Frankfort MB's

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If it's pinging you need to run higher octane fuel and maybe a colder plug. I seriously doubt you would need a colder plug since your using a b7hs.....

A full synthetic oil at about a 32:1 ratio is always a good go to in case your not using enough oil....
That'd be about all the things I could think of unless your head gasket is heating up causing it too loose a good seal.
But you would have compression issues if something like that was occurring.
 
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