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650 unit tuning

1292 Views 5 Replies 4 Participants Last post by  scraper
im about halfway through the break in period on my freshly rebuilt 650 unit, single carb motor. im starting to look for some tuning tips. the motor is .040 over, milled head & has megacycle cams. Im running a brand new amal 930 with a .240 main jet (.01 over stock). it has a short velo stack with a homemade air filter i sewed onto the screen (temporary filter 'til i can get a K & N) Im also running open stock bonnie header pipes (1.5 dia.?).

The bike starts on the 1st kick. It has a nice idle (sounds like an HD with the big cams) & it runs really smooth. It seems a bit flat on the bottom end of the rpms but the midrange & top end pull hard enough to slide me off the back of the seat. i spent some time on the highway with it yesterday for the 1st time (55-65 mph) & it ran smooth. The pipes never turned brown or blue. i pulled the plugs the other day (after a slow cruise around town & some idling) & they were pretty black, maybe a tad dark brown in the center.

Since Ive never owned or ridden an old trump until i built this one for myself, im not sure what the motor should feel like with the mods it has. does anyone else have a similar setup to compare it to? does anyone suggest a dyno tune & who would do one to my bike around south jersey? should i be running mufflers or should i put a resistor in the exhaust for some back pressure? i think that would give me more bottom end but im not sure how the motor would respond since its got big cams.
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thats the scary part. i got the motor with the oversized pistons & aftermarket cams already installed. the motor was never rebuilt correctly until recently so it must have never run properly. my triumph motor guru who rebuilt it said they werent actually megacycle cams but they were regrinds that were copies of megacycle's. he said there were numbers on the cams but he couldnt come up with any info on them. i dont know much about 650 triumph cams but i looked at them compared to stock ones & they didnt seem to have crazy duration, just more lift. how many different cams are available for the 650?
Any time you have a bike that starts within the first three kicks, your doing great. The timing sounds spot on. Your plug reading sounds like you may be a little rich at idle. I have a 1965 TR6rs moderately built with twin Mukuni carbs. Hotter cams usualy give up a little bottom end for more power on the top end. I also think that the lack of an accelerator pump adds to the low end flat spot. Mine is a little flat untill mid range and then it's like someone kicked it in the ass and it rocketts.
What kind of gearing are you running?

I put a bigger front sproket on... Makes a big difference on the high end, because the engine isn't winding out as hard at 50 or 60. I'm also bored 40 over. Hard to tell how much of a difference it made on the lower end, cuz it was old and crusty before the re-build... but may be something to check.

sideshow01 said:
What kind of gearing are you running?
im running a 19T up front right now.
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