help!

[jerseyrider]

i have a 421 cub motor and stock gearing and duncan mid range pipes, 1st,2nd and 3rd gear are revving out fast, just ran 2 tanks so far do i still need some break in time or some cpis or change my geAring??? any suggestions from exp members?? thanks

[SLORYDER]

Yeah, check the torque on your base and head nuts and you should be good as far as break-in.

 

You need to gear up. I'd go to a 16t front sprocket and see how you like that.

You could get 10-15 hp more out of some CPI pipes. The combination you have is bad. Pipes come on too soon for the engine and sign off too soon as well.

 

 

Where are you at as far as compresion/ timing/ rider weight?

[jerseyrider]

Yeah, check the torque on your base and head nuts and you should be good as far as break-in.

 

You need to gear up. I'd go to a 16t front sprocket and see how you like that.

You could get 10-15 hp more out of some CPI pipes. The combination you have is bad. Pipes come on too soon for the engine and sign off too soon as well.

 

 

Where are you at as far as compresion/ timing/ rider weight?

 

 

how do i check the compression and timing? im 215 weight, and the pipes are a bad combo for the motor?

[SLORYDER]

how do i check the compression and timing? im 215 weight, and the pipes are a bad combo for the motor?

 

I assume you run pump fuel from your reply. Stock timing as well?

 

You need to buy a compression gauge to check the compression.

 

I suggest milling the head (assuming stock head) to run race fuel- shaving approx. .035" to leave .035" or more squish. This will help you pull taller gearing without bogging off the line.

 

I would also highly recommend adding an adjustable timing plate and advancing your ignition timing. I'm not too keen on cub timing on race fuel. If you take off your flywheel cover and look behind the flywheel you should see elongated bolt holes on the timing plate and incremental lines for measuring the rotation of the plate. Google timing plate banshee.

 

 

 

 

But yes, you will benifit greatly from running taller gearing-acceleration wise. To do this you need more compression and timing because with the cub engine you have very little bottom end to begin with.

 

And yes, the cub engines like a "high rev" pipe. If you put a midrange pipe on them the port will still be open after the pipe returns its supercharging pulse, and the pressure will bleed back out the port at higher rpm's.

 

Soooooo

Shave stock head .035" after verifying .070" or more squish, or if your squish is less than .070" cut enough to end up with .035" squish. Or you could mill .030" and run 50/50 race fuel/ premium.

Increase timing advance

Gear up to 16t front

CPI pipes

Edited July 5, 2010 by SLORYDER

[SLORYDER]

And yes, the cub engines like a "high rev" pipe. If you put a midrange pipe on them the port will close before the pulse returns at higher rpms and you won't get anywhere near maximum power. Also the returned pulse is too weak at lower RPM's because the pipe is trying to make the powerband wide when the short power stroke makes the engine unwilling

 

Cubs are not designed to have a wide powerband. Therefore you should have a pipe that is tuned to "crunch" the powerband and raise the peak HP in cohorts with that the engine wants

 

Here I fixed my previous post

 

You will be very pleased with the pipes and gearing change. Don't forget to re-jet.

Edited July 6, 2010 by SLORYDER

[firebanshee]

i have a 421 cub motor and stock gearing and duncan mid range pipes, 1st,2nd and 3rd gear are revving out fast, just ran 2 tanks so far do i still need some break in time or some cpis or change my geAring??? any suggestions from exp members?? thanks

You should go to a cpi,shearer,rocket,vito's fat bastard or the new sniper pipe,that will let your motor rev out farther and match your carbs better.I run a 15-41 gearing on my cub on alky with 22 inch tall tires.I would change the pipes and then figure out your gearing,what kind of riding do you do with the bike?

[jerseyrider]

we ride in open pits,straight runs, and some trails just to get from one pit to another!!! but if i know i will be with trail riders on that day ,i ride my trx450r !!!