Undersized pipes?

[Up north rider]

My plan was to get it somewhere in the ballpark before I plug chop a main jet setting. We just never got to that point. We can idle it in the shop and rev it in neutral ok, but sputtering crap basically the whole way through the rev range. The reason I was skeptical to go into the 130-140 range is my other brother has a 392 DL motor with 28pwk and pro circuits and he has 152-155 mains. His runs great. Plus I don’t wanna melt down this brand new motor. Haha I guess we could make another jet r us order and go back square one or just shelf these pipes for another build. I appreciate the input. 
 

[Up north rider]

1 hour ago, locogato11283 said:

Driveline really liked Shearers with their 4mm motors.

I have cpi’s on my 421 and I love it. He’s not really into the peaky power band, but I think if shearers match well with this motor he should go with it. Also we’re all running pump gas so that peaky power I think is more apparent when you don’t have as much compression. Does that track with the general consensus? Also his cylinders are as cast

[BANSHEE HP]

Get slp pipes if you are looking for a trail/woods pipe

[locogato11283]

35 minutes ago, Up north rider said:

My plan was to get it somewhere in the ballpark before I plug chop a main jet setting. We just never got to that point. We can idle it in the shop and rev it in neutral ok, but sputtering crap basically the whole way through the rev range. The reason I was skeptical to go into the 130-140 range is my other brother has a 392 DL motor with 28pwk and pro circuits and he has 152-155 mains. His runs great. Plus I don’t wanna melt down this brand new motor. Haha I guess we could make another jet r us order and go back square one or just shelf these pipes for another build. I appreciate the input. 
 

That sounds like a CDI issue. I have seen that exact scenario many times. Bike will start and idle perfect. It will rev out clean in neutral. Then, under a load it runs like shit. Do you have a CDI you can swap it with to test?

[Up north rider]

We’ll definitely try the cdi swap,  
and I think he’ll grab a set of those pipes.
thanks again

[trickedcarbine]

On 2/18/2025 at 2:30 PM, Up north rider said:

My plan was to get it somewhere in the ballpark before I plug chop a main jet setting. We just never got to that point. We can idle it in the shop and rev it in neutral ok, but sputtering crap basically the whole way through the rev range. The reason I was skeptical to go into the 130-140 range is my other brother has a 392 DL motor with 28pwk and pro circuits and he has 152-155 mains. His runs great. Plus I don’t wanna melt down this brand new motor. Haha I guess we could make another jet r us order and go back square one or just shelf these pipes for another build. I appreciate the input. 
 

Jetting on the 28’s will be far different than what the 35’s would. 
 

That 421 should be in the 55/150-155 range on good pipes. Those PT Mids may not be the most adequate, so it’s possible they’ll just never move AFM near the potential of that cylinder. Just thinking about the max power potential of what those pipes are capable of, I would say they may just need something in the 140’s for a main. 

You mentioned you looked over electrical, have you checked the ground? Make sure the coil mount is super clean and plug caps are on the wires tight. If it’s just intermittent, I would look more at the CDI.  Maybe swap it with a known good CDI.