I just bought a '02 Scrambler 400. The thing hadn't been run in three years so I am currently going through everything.

List of previous mods:

- Hot Seat pipe and silencer
- Trail ported cyl
- V-force reeds and spacer
- 38 mm mikuni carb

I tested compression at 125psi with the carb off.

I cleaned out the tank, cleaned the carb, and installed a fuel filter. It seems like I am having a fuel delivery problem. The thing runs great then will all of a sudden quit like it is starving for fuel. After a while when you choke it and restart it will run great then do the same thing again. I've taken apart the carb a couple times and the floats don't seem to be sticking but the only thing I can think of is a carb problem. I was thinking of getting a new carb but I want to be able to use the stock airbox. Would a PWK 39 be the best choice?

 

Float might be set to low..does it happen right away, only happen on fast runs??. Also are u using oil injection or premix.

 

I'm still using the oil injection. It does not do it right away. I can make a lap around the yard then it quits. It runs great for a while.

I'm still trying to figure out what the setup needs to be with these mods. I think the original carb was supposed to be a 34mm but the previous owner cobbed together an intake to use a 38mm round slide.

I read somewhere that a Keichin PWK 39 is supposed to be the way to go but I have no experience with this.

Compression seems good and the reeds look good.

 

Recheck compression after it gets warm and dies. Could be loosing compression (too much piston to cylinder clearance) when cylinder heats up and rings may not be sealing well against cylinder wall? OPT

 

OK I got it good and warm and rechecked compression and about the best I could get was 105 with the throttle open. Seems a bit low to me.

Maybe I'll pull the cyl and get it bored with a new piston and then try again.

 

Usually what happens when they get warm and die on you and will crank up after they cool down! Most of the time its the top end needs to be looked into! Be sure there is NO up and down slack on the rod when you tear it down. Side to side play acceptable.Also if you have it bored out to next oversize,DON"T forget to bleed the 10mm bolt on the head by the sensor to remove any air pockets! This is critical to do to keep the head or cylinder from warping and overheating! This is overlooked a lot! OPT

 

Thank you. Good advice.

I just pulled the cyl and it is scored pretty bad. I'm not going to be able to get away with honing this. I'm actually surprised it ran as good as it did.

I don't feel any play on the rod which is good. I didn't feel like pulling the whole motor.

I did notice that one of the mounting flanges on the cyl was welded in the past. It looks good but still makes me nervous. I'll have the machine shop take a look at it.

Any piston recommendations? I did the top end on a trailblazer 250 in the past and used a Numura piston and didn't have any problems.

Well I guess I now realize why I got a good deal on this thing.

 

Wiseco!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Opt