Cdi performance

Joshbus

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Nov 12, 2016
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I just built a bike to sell. It's running pretty good , 47 mm piston type A. Dellorto clone sha carb running a .66 jet. Gasbike thruster exhaust, same as bicycle motorworks Bulldog. I'm getting a slight crackle/skip at wide open throttle. It doesn't go faster when I let off , but the crackle goes away. It was alot worse before upgrading the plug wire. I'm wondering if I should go up to a .68 jet ( plug is dry after a ride , but not black , or tan more like a gray tone ) could a performance coil be the solution , or could I just need a tad more fuel ? Thanks , Josh
 
Gray is lean or too hot, that crackle is sounding like detonation (bad).

What octane? If it's too low it can detonate under all the heat, try for 90 or more to help, I find in the winter I can use 87 pretty reliably but in the summer I need to use 93 or it just gets too hot.

66 jet might just be too lean, or you have a tiny air leak possibly, and that will also lead to similar issues.
 
I was thinking gray was lean. I have another bike running great with the same carb , a Fred Head, and a performance cdi with a dirt bike style coil , and a .64 jet . Everyone claims
.70 jets are too rich , but my piston has slight bevels to open up transfer ports , hence why I think I need a bigger jet. No air leaks it stalls when I choke it. Been through the air leak nightmare lol... Thanks for the info.
 
I was thinking gray was lean. I have another bike running great with the same carb , a Fred Head, and a performance cdi with a dirt bike style coil , and a .64 jet . Everyone claims
.70 jets are too rich , but my piston has slight bevels to open up transfer ports , hence why I think I need a bigger jet. No air leaks it stalls when I choke it. Been through the air leak nightmare lol... Thanks for the info.
70 is not too rich, it's all in the motor, some do 70 out of the box, others do 75, some 65.

In the winter, with a very big ported motor and Reed valves I can end up with an 85 jet.

I noticed a 75 in a Dellorto can been just right for a motor, but the same 75 in a different carb type makes the engine run too rich or even too lean.

The best thing to do is always tune into to the motor's need, since really almost no 2 are the same.
 
I've figured that out. Reason I never throw away parts unless they are broken. I'll try the 70 I took out which is the biggest I have right now. I can order larger sets of 5 from MZ for $8 It's good to have options. I'll try the 70 tomorrow.
 
Yes they are. He's fast too. I buy from him alot. I like his square cylinder heads.
 
I was thinking gray was lean. I have another bike running great with the same carb , a Fred Head, and a performance cdi with a dirt bike style coil , and a .64 jet . Everyone claims
.70 jets are too rich , but my piston has slight bevels to open up transfer ports , hence why I think I need a bigger jet. No air leaks it stalls when I choke it. Been through the air leak nightmare lol... Thanks for the info.
Just some info for those who have done the ramping of piston at the transfers,I know jag wrote that slight ramping of the piston aids in incresing flow witch it would somewhat but he also contradicts himself because if you cut the ramps it's the same as raising the port witch reduces blow down duration in the op's case with an A type engine the stock port timing yeilds a blow down of 13.79 not very good but it works and if you cut even just 1mm ramps that drops to 10.15 and that sucks!Idealy we want more blow down duration to insure that the exhaust charge isnt pulling to much of the intake charge out with it and if pipe timing is right then recover some of that loss,The best we can get it is around 20 and thats only achivable by raising the exhaust port timing and sacraficing some bottom end!
 
Just some info for those who have done the ramping of piston at the transfers,I know jag wrote that slight ramping of the piston aids in incresing flow witch it would somewhat but he also contradicts himself because if you cut the ramps it's the same as raising the port witch reduces blow down duration in the op's case with an A type engine the stock port timing yeilds a blow down of 13.79 not very good but it works and if you cut even just 1mm ramps that drops to 10.15 and that sucks!Idealy we want more blow down duration to insure that the exhaust charge isnt pulling to much of the intake charge out with it and if pipe timing is right then recover some of that loss,The best we can get it is around 20 and thats only achivable by raising the exhaust port timing and sacraficing some bottom end!
So ramping the piston at exhaust as well as the transfers maintains the stock blow down and aids smoother flow at all three ports. You loose some duration of the comp. and comb. portion of the stroke. I think that is where low end torque is lost, but you can regain some of the bottom end power loss with modifications increasing the compression.So with the smoother flow, advanced port timing with 3 ramps and comp. mods. you can widen the potential range. Is my thinking right on this one?

So would ramping only at the exhaust 1mm, and leaving transfers stock increase blow down to 17.43?
 
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