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Hey guys, Long story short I have a rev 600 with a 440 race engine in it. The engine has vm38 carbs on it which aren't original. With the little knowledge I have of spark plug reading the insulator is dark and seems to be rich?? But the electrode leads me to believe it might e running a little lean with the tip not being a nice brown. Am I over thinking things?? Any help would he greatly appreciated
 

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I would read the top of the pistons. With todays gas you just cant read plugs that good anymore.
 

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Pump gas 91 octane mixed 40:1
Pictures I initially posted were with 520n mains from a 800. Threw them in to try them because
I installed 400 mains yesterday which looked incredibly lean (in my opinion) but I could be wrong.
Just trying to get more input cause I'm pretty much shooting in the dark
 

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First I would check needle clip position. my 440's where always in the middle position. 40:1 was always around 300-340 depending on temperature. pilots were 25's. air screw turned out 1.5 turns.This all based off of memory. plugs were always cardboard brown...…….. if you had 400's and they were white I would be betting on the needle clip position being on the 2 position from the top groove...…….. oh and that last picture you posted seems like it didn't have a lot of time on it. well not enough time to get a good reading on the plug.
 

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Discussion Starter · #6 ·
Thank you for your input.Keep in mind these are not the original tmx34 carbs these are vm38's that are on the snowmobile. Engine was warm and swapped new plugs in and performed plug chop
 

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If you had 400’s and that last picture I would go 420-430. Should be close.......... how did you manage to get 38’s to fit the carb boots ?
 

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Discussion Starter · #11 ·
All photos have been plug chops. Engine warm swap to fresh plugs run hard kill ignition
 

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Do you have a decent set of used plugs with combustion deposits on them? You won’t get a proper reading off new plugs until they have been run for awhile, long enough to do damage from incorrect jetting. The vm carbs are good, but like said re position the needles to a richer position and start your main jets rich as well. Forget about using the N main jets. They have a different flow rate. Suggest you premix at 32/1 with that stressed an engine. Use a plug reading light with a magnifier or similar to read the base of the electrode. It should have a light brown ring extending up about 1 mm. There should be a light brown burn on the side electrode about to the curve on the electrode back from the tip. That tells you your timing is correct. The outer ring should be a dry, darker brown. Centre insulator should be a nice cardboard or lighter colour. I’ll try and bring up my plug reading diagram. I’ve posted it before but lost it with that phone. As far as wash, you won’t get a good carbon build up for a hundred miles or so.
 

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Remember that it isn’t fully on the main until the last 1/8 of throttle travel. When you get the main close do a shorter mid range chop to ensure the needle is rich enough.
 

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Discussion Starter · #16 ·
Ordered some smaller jets threw them in it's currently -15 took the sled for a rip.

1st picture is with 430 main jets
2nd photo is with 460 mains.

460 mains while foil the MAG side cylinder after 5 minutes of riding.

430 mains both plugs fine but MAG side richer then PTO starting to think I might have an issue with PTO cylinder as its always leaner. And advice would be appreciated!
 

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could be the pto seal is bad. quick easy test is to start the sled on the stand with the clutch guard off. spray starting fluid between the clutch and motor down by the crank. if the engine gains rpm seal is bad. also check carb boots and reeds. 440's usually had issues with reeds. they were (light tension) and would chip/break, or the peddles would start to lift off the cages. the plugs should be fairly close in color. In all of the 440's I had the mag side had one jet size bigger because it ran leaner on that size.
 
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