Clone Gearing & RPM Issues

D

D&M Racing

Member
We just purchased what we consider to be a brand new very nice open clone engine (built specifically for my son's QRC wing kart). The engine builder indicated that the target RPMs for this motor was 6800-7000. We have not even gotten the engine close to the target RPMs in the 2 outings we have had it. The track is 1/5th of a mile with slight banks in each corner but fairly long straights. We are running brand new Hoosier treaded tires on his kart and he is staying hooked up and a bully clutch with the stall speed set per the motor builder. We have tried every gearing combination we can think of and the closest we have gotten is 6000 rpms (Mychron 4) with at 13/64 (4.92) but still got beat by half a lap. Other clone racers (different engine builders) say they are running 15/57 (3.80) on this track but when we tried that combination on this motor 4200 RPMs was all we could get out of it. Below is a list of what we have tried and what we got out of the motor. FYI the motor builder has been extremely responsive and trying to help too. Any suggestions would greatly be appreciated.

15/67 - 3400 RPMs
15/57 - 4200 RPMs
14/65 - 4600 RPMs
13/57 - 5400 RPMs
13/64 - 6000 RPMs
12/60 - 5500 RPMs
 
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is the choke on? is the throttle able to pull to the throttle stop? check main jet for obstructions and see if any issues with butterfly etc.
builder may have been a little too heavy on the valve preload and your valves may not be sealing. Best to take to builder if possible.
 
Something is off, possibly valve springs. We've had that issue with a brand new motor, that was a dog.
Has it been back on the dyno since the 2 outings ?
 
J'-remy said:
is the choke on? is the throttle able to pull to the throttle stop? check main jet for obstructions and see if any issues with butterfly etc.
builder may have been a little too heavy on the valve preload and your valves may not be sealing. Best to take to builder if possible.
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No, the choke is off. Yes the throttle is fully engaging. Builder isn't local but if it needs to be returned to be checked then it can be shipped.
 
Kj26 said:
Something is off, possibly valve springs. We've had that issue with a brand new motor, that was a dog.
Has it been back on the dyno since the 2 outings ?
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No it has not, builder is along way off but can ship it back if necessary. Going to try it again tomorrow and if we don't get any better results it will be back in the crate come Monday morning.
 
Check to make sure bearings are good and chain is straight. We dropped rpms on our Jr sportsman to find the rear bearings locked up.
 
D&M Racing said:
Bearings were just replaced and chain is good and tight.
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By looking at the rpms on your gearing tells me something is in a bind. Is your mount an adjustable top plate? A 15/67 at 3400 rpms is a dead giveaway. Whatever is in a bind is overworking the engine.
 
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The chain will always be tight on top under load, but too loose on bottom will promote chain loss. I usually do a bit more than this diagram, around 3/4" or so. It always changes when you tighten the motor mount down, so be sure you check it the same way EVERY time.

Based on your description, I'm wondering if the kart is bound up by not taking weight off the LR. If the chassis is fighting the turn, the RPMs go way down...
 
D&M Racing said:
We just purchased what we consider to be a brand new very nice open clone engine (built specifically for my son's QRC wing kart). The engine builder indicated that the target RPMs for this motor was 6800-7000. We have not even gotten the engine close to the target RPMs in the 2 outings we have had it. The track is 1/5th of a mile with slight banks in each corner but fairly long straights. We are running brand new Hoosier treaded tires on his kart and he is staying hooked up and a bully clutch with the stall speed set per the motor builder. We have tried every gearing combination we can think of and the closest we have gotten is 6000 rpms (Mychron 4) with at 13/64 (4.92) but still got beat by half a lap. Other clone racers (different engine builders) say they are running 15/57 (3.80) on this track but when we tried that combination on this motor 4200 RPMs was all we could get out of it. Below is a list of what we have tried and what we got out of the motor. FYI the motor builder has been extremely responsive and trying to help too. Any suggestions would greatly be appreciated.

15/67 - 3400 RPMs
15/57 - 4200 RPMs
14/65 - 4600 RPMs
13/57 - 5400 RPMs
13/64 - 6000 RPMs
12/60 - 5500 RPMs
Click to expand...
If I read correctly your first post and your answers to questions posed. then ....

You have setup or driver issues to fix before you fix what your reporting as an ok engine.
 
Is it solid rear axle or hollow rear axle ? If it’s a solid qrc axle it will pull an engine down pretty good over a hollow. What do we know about the engine other than someone saying it’s an open? To some a builders prepared style cam and a bigger bore carb is an “open”
 
D&M Racing said:
We just purchased what we consider to be a brand new very nice open clone engine (built specifically for my son's QRC wing kart). The engine builder indicated that the target RPMs for this motor was 6800-7000. We have not even gotten the engine close to the target RPMs in the 2 outings we have had it. The track is 1/5th of a mile with slight banks in each corner but fairly long straights. We are running brand new Hoosier treaded tires on his kart and he is staying hooked up and a bully clutch with the stall speed set per the motor builder. We have tried every gearing combination we can think of and the closest we have gotten is 6000 rpms (Mychron 4) with at 13/64 (4.92) but still got beat by half a lap. Other clone racers (different engine builders) say they are running 15/57 (3.80) on this track but when we tried that combination on this motor 4200 RPMs was all we could get out of it. Below is a list of what we have tried and what we got out of the motor. FYI the motor builder has been extremely responsive and trying to help too. Any suggestions would greatly be appreciated.

15/67 - 3400 RPMs
15/57 - 4200 RPMs
14/65 - 4600 RPMs
13/57 - 5400 RPMs
13/64 - 6000 RPMs
12/60 - 5500 RPMs
Click to expand...
Check your fuel pump, fuel lines, and fuel filter. A cracked fuel filter will suck air in the line. Likewise fuel lines not fastened tight each step of the way will suck air.
 
last saturday i ran a flat kart with unrestricted clone on a big1/6 mile with 16/70 and turned 6850 rpm if that helps any.
 
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