8000 rpm or bust

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graydog8josh

Member
Nov 23, 2012
450
4
18
Starkville,MS
I am trying to tune this rt carb. Im not able to get it to rev up past 6500 rpm.
i went from 68 where it seemed to be running really hot but the plug was tan to a 72 and noticed no difference. i went to a 75 jet and it started 4 stroking at 6200 rpm(32mph) now im going to try a 70 jet. maybe this carb just wont do 8000 rpm but i got my nt to do it.
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
Stick to the speed carb or NT for 'tunability',
I've found .6mm works on my motors with the RT.
Other motors may prefer .65mm.
What plug and head are you running?
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
There's your problem, the slant head is not likely to keep a ported 40mm stroke motor cool, I have two motors like this and they both run hot, The 'Softtail' will be getting a different head, the Phantom, likely detuned, as I can not fit a bigger head in the frame.
The slant head gives to much compression and the RT is a pig on a modified motor.
I advice trying a shaved straight fire head, and speed carb.
 
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graydog8josh

Member
Nov 23, 2012
450
4
18
Starkville,MS
ran 35mph with a 65 jet on the rt carb. I really like this carb for its mid range. I am going to try to clean up my trashed fredhead and reuse it in the pursuit of more speed. Also am going to go 2 base gaskets and one head gasket. after all those changes if i can keep it running smoothly i might try to rebuild my speed carb. never got that to work and not leak or vacuum leak but worth a try.
 
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graydog8josh

Member
Nov 23, 2012
450
4
18
Starkville,MS
still 35mph, rt carb 65 jet, crmachine head and trimmed piston skirt. put some 93 octane gas and tried to go faster, but 35 is about as fast as i can get her. im running a 34t sprocket and have a 24" wheel, 7000 rpm is all im getting.


i can rev it up way high with the clutch pulled but it never gets that high under load, it seems to hit 7000 and thats all it can pull. i can only guess i need to use a smaller jet. Any suggestions?
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
Is it 4 stroking? hows your plug colour?
Get it flat out, hold it there, and hit the kill switch, coast/peddle home, let it cool, then check the plug colour.
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
I could not get my RT to give me the Revs that my speed carb does, Messed around a bit with different Jets on different motors, but kept going back to the speed carb, or 15mm Barrel carb. The RT will go back on a pocket bike where it belongs.
 
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mech_engineer

New Member
Mar 3, 2014
175
0
0
Puerto Rico
Use a 36 o 40 tooth sprocket it will rise the rpm... your engine dont have torque for the 34t your engine will not goes over 7000 rpm because of it. Is like trying to get full rpm in a car at 35 mph in 5 or 6 gear...intead of 1st or 2nd gear...or increase compression try this...1 to 1.5mm clearance squish band
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
I agree the 34t may be a little tall, 36t with 24" wheel is a good combination.
Standard angle fire head with 1mm squish however will probably get your compression a little too high. Best to do a compression test first. Then do a squish test.
But more compression will likely help.
I find 150 Psi is good. but may cause overheating with an angle fire head.
Crank balance can have a big contributing factor also to not letting these rev.
Still Speed carb, has proven much better for me than RT.
Also port matching exhaust manifold, check the inlet gasket is not restricting flow.
Not all of these motors were created equal, some have better port timing, some have better ignition timing from the Mag rotor.
 
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mech_engineer

New Member
Mar 3, 2014
175
0
0
Puerto Rico
For your bicycle 24"
34 t sprocket
@8000rpm v=41.1mph
@9000rpm v=46.2mph

36t sprocket
@8000 v=38.7mph
@9000 v=43.5mph

Make your choice if u increase tooth# you will increase torque but reduce top speed. The good thing is your ride will reach faster the top speed
 
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Theon

New Member
Jan 20, 2014
1,440
6
0
FNQ Australia
Mech's right,
I would be getting your self a Degree wheel/protractor, lifting the head, and checking your port timings.
Would also port match everything.
check compression and squish.
Open up the transfer ports.
Widen the inlet and exhaust ports,
open up muffler.
You may need to lift the barrel to get port timing correct.
You may need to shave the barrel to get compression up.
Then you will get 8000+RPM.
Even with your RT and angle fire head.
 
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graydog8josh

Member
Nov 23, 2012
450
4
18
Starkville,MS
64 jet was too lean. I rode around for a while and on a downhill managed to squeeze out 44 mph but then on the way home after running about 30 minutes had engine failure. Opened it up and some plating had come off my cylinder body and the piston was scraped up and rendered useless. Will be swapping out piston and cylinder body and atempting better port matching. Or i might just buy a new motor, i dont know. Ive got a dax im breaking in on another bike, i might put the reeds on that one.
 
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