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Discussion Starter · #1 ·
Building a project sled right now and I'm wondering what a good starting point would be for jets.

The engine will be going into a 2007 MXZ 440x Racing Chassis.

The engine is an 800HO, with Cudney Head, Vforce 3 Reeds, Dyno port pipe, and trail can, Dynamo Joe Clutch kit, stock air boxes

I always swap out the pilots on all my 800ho engines fom 17.5 to 20.0. I assume that would be ideal for my setup again.

I'm not really sure where to start for the main jets. Right now I haev 420s in there as the only other jetting I have available in my jetting is was 380 and I figured that was a bit low.

I can grab any size just wanted to know what people would do in my situation.

I'm still putting this sled together so I have time to order jets if people have a better suggestion than going with 420 mains and 20.0 pilots.

Any input would be greatly appreciated.

Engine has no porting done to it (yet)

Thanks guys
 

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400 main are stock, head and reeds should not change that. dyno port says to start with stock jetting to start. check plugs

from dyno-port web site

there is a instruction sheet on the pipe on the web site. I could not get it to cut and paste
 

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Discussion Starter · #4 ·
400 main are stock, head and reeds should not change that. dyno port says to start with stock jetting to start. check plugs

from dyno-port web site

there is a instruction sheet on the pipe on the web site. I could not get it to cut and paste
Thanks

Yes I read their website already. Guess i'll have to find some 400s to start. 420 is probably going to be too rich.
 

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Yea, I can agree they will be. I am ported with V-force reeds, DPM, crank shop head (std 12.1 CR) and stock exhaust. I started at 420 and ended at 390, could go to 380 , just wanted a little safety factor. Do you have DPM ? , if not I would watch close in the - temps
 

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Are you staying with ptek /dpm? I ran similar setup in 04 non dpm, Cudney head, dynoport pipe, boyesens, used 400 mains, plugs were clean and light brown.... Up til that point-. Id say the knock sensor might have helped...i ran a few good new york days this season, but burned mag Piston up in first 5 miles in Maine, but it was pretty cold. Silly me. I will rebuild and Will use a 07 ECM and ptek harness, just have to drill and tap the bolt hole for the knock sensor.
 

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I would just leave the 420's in there to start, better to be a little rich and works your way into ideal.. can't really expect to hit it perfect on the first pitch. 400's were stock unless you have a powertek and the addition of an aftermarket pipe usually adds a size or two which might be why you were at 420
 

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Are you staying with ptek /dpm? I ran similar setup in 04 non dpm, Cudney head, dynoport pipe, boyesens, used 400 mains, plugs were clean and light brown.... Up til that point-. Id say the knock sensor might have helped...i ran a few good new york days this season, but burned mag Piston up in first 5 miles in Maine, but it was pretty cold. Silly me. I will rebuild and Will use a 07 ECM and ptek harness, just have to drill and tap the bolt hole for the knock sensor.
The burnt piston gives good clues to whether it died from detonation, pre-ignition or simply lack of piston clearance. Since you already know what jetting worked with what temp and failed at another temp, you already have a very good baseline that where that risk line is. From there all you need is Ignition Timing Advance and Air/Fuel Ratio to work any system from the one you currently have to the 07 ECM . But don't bother working at it blindly. Get our timing and ratio after various rpm/load ranges.
 

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I think 420s & 20.0 is a good place to start. You may be able to sneak it up to a 400, depending on the quality of fuel you run. I see your down south, you run 93 or 94, or just 91? You run in cold, trails, lake running?
 

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I don't know why everyone changes the pilots to larger ones. Yes you might get a hanging throttle with the stock ones but when you go to 20s your fuel mileage suffers. I always got away with small adjustments on the fuel screws. Go to about 7:20 in this video.

 

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I don't know why everyone changes the pilots to larger ones. Yes you might get a hanging throttle with the stock ones but when you go to 20s your fuel mileage suffers. I always got away with small adjustments on the fuel screws. Go to about 7:20 in this video.

These 800s are know for a lean spot at partial throttle, that's the only reason I went to a 20.0 pilot. My idle still hangs abit even with a 20.0.
 
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