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Discussion Starter #1
Picked up an 01 Trailblazer to fix up for the kids. Oil injection is blocked off so I have to premix and the carb was crap so I bought a 34mm Mikuni to replace the stock 30mm. Needs a slight bore, stock piston is right at service level with 115lb compression so I'm putting in a new 73mm piston. No other mods done. What are the benefits to shaving the head to up the compression? Is it worth it on an otherwise stock bike with a little bigger carb? And last, any suggestions on where to start jetting as far as premix ratio and primary/main jet size, clip position, etc.? I have a 45 pilot and 220 main in the carb now but that's quite the step up from the 30mm Mikuni with a 40 primary and 140 main. Thanks for any help guys!
 

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Shaving the head .040 will up the compression for a noticeable boost in low end performance - go to a colder heat range spark plug and use 93 or 94 octane fuel.

Going to a larger carb necessitates jetting smaller than the what was in the original carb. I found this out back in the 70's when I was racing a Kawasaki F7 175 - I had the original carb bored 1 mm and found it ran rich with the original jets. The larger venturi resulted in lowering the velocity of the air passing through the venturi - the lower velocity meant the air passing over the needle jet had more time to pick up fuel and subsequently ran too rich with the stock jets. I spent the better part of an entire day riding, changing jets and riding again until I had it running right. When I was done with the porting, tuned exhaust and larger carb, my 175 would keep up with the 250's of the day.

Going up 4 mm, I'm going to guess you will need about a 30 pilot jet and a 90 to 100 main jet

As to pre-mix - start at 20:1 with mineral oil or 24:1 with synthetic oil
 

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No compression release unless someone installed one - the manufacturer did not provide a place for one, but if the crankcase drain plug is missing, it would affect compression

About 120 psi is standard compression
 

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I appreciate the info! Just got the head studs out. That was a good time. Lol. No oil injection lines were on the carb which but the pump is still there and I believe works since it was leaking past the plug they had in it and there was still oil in the tank. The inlet on the intake was connected to the oil pump with a small hose and that inlet is completely plugged up. Is that where the oil injects into the mix and is that the only two lines involved, from the tank to the pump and from the pump to the intake port? If so, I could clear the port and use the factory oil injector. This bike is for my 11 year old daughter so easy and dependable is more important than maximum performance. The carb, compression bump and brown clutch spring is my attempt at more bottom end since that's where she'll be most of the time.
 

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I do appreciate the input guys and have another question about the head. The place I'm taking this to for the machine work said dont go over .0025 shaving the head or I could run into detonation/pre-ignition issues. Latebird I know u said cut .0040. Have you had issues before cutting it down that much?
 

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Only oil lines are from tank to pump and pump to port on intake - should be a cable connected to the pump via the throttle cable via a splitter
 

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Yes sir it is. Should I be able to blow through the port on the intake? I can spray carb cleaner through it and it sprays through but it's hard to blow through it. Should it be open or is it metered?
 

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I do appreciate the input guys and have another question about the head. The place I'm taking this to for the machine work said dont go over .0025 shaving the head or I could run into detonation/pre-ignition issues. Latebird I know u said cut .0040. Have you had issues before cutting it down that much?
That was .0400 (40 thousandths or 1 mm) - I also said that you would have to run 93-94 octcne fuel and use a colder heat range spark plug, but you could take off .020 and go another .020 later depending on if you think you need to.

I have taken over .060 off heads and had to replace them - I have welded up the combustion chamber and reshaped them looking for more power - some things worked and many things didn't.

You can raise the power band (start later and end later) by raising the ports - you can raise the ports by about .020 just by installing 2 base gaskets, but that increases the combustion chamber (raises the head) by about .020..........

It's fun to look for the changes that improve performance
 
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