1987 Yamaha Enticer - Jets - Snowmobile World : Your #1 Snowmobile Forum
 
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post #1 of 6 (permalink) Old 01-02-2015, 10:27 AM Thread Starter
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1987 Yamaha Enticer - Jets

Originally posted this in the Yamaha forum but no responses yet.

Hi Folks,

I had to replace the original Mikuni butterfly style carburetor on my 87 Enticer 340 - had some plugged internal passages that there was no way to get cleared (tried everything many times). I found a Mikuni VM34 round slide carburetor, off a 377 Bombardier. Took it apart, cleaned it up and everything looks as new. Also added a primer so I could get the machine started more easily.

The main issue remaining is the top end lack of power (from about 3/4 throttle to full). Starts & idles perfect, mid range okay, upper mid-range screams, but bogs out past 3/4 throttle. Sometimes has a bit of a hard time finding the "sweet spot" but when it does it goes like a scalded cat. It also seems to run better when cold.

After playing with the air screw, I have gotten it to accelerate from idle smoothly, without hesitation - seems to like to be around 1 turn out, or less. Too much and it bogs hard from an idle.

I have had the jet needle up and down through all the notches on the stem. It works best with the c-clip on the top notch (needle at lowest position, leanest).

I also lowered the float a bit (less fuel in the bowl). That seemed to help somewhat, but I am not sure.

I am wondering about the size of the main jet. The jet that was in the VM34 was a #320. I took that out and put in the jet from the original Mikuni butterfly carb. I could not find a marking on it but it appeared to be about the same size.

Could the main jet be too large and causing too rich a mixture above 3/4 throttle? Any suggestions on what size jet may be appropriate (looking for a good starting point)? I am at sea level (Nova Scotia) and it is usually fairly mild.

Any help is most appreciated.
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post #2 of 6 (permalink) Old 01-02-2015, 10:55 AM
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I would take a wide open pass about 1/2 mile at wide open. While holding the throttle to the pin, hit the kill switch and let the sled stop. Pull the plugs and check the plug color.

If the plugs are white or grey, the main is too small, you are lean. Go bigger on the main.

If the plugs are black, you are rich and can safely jet smaller on the main.

You are looking for a cardboard brown color on the plugs.

You can also look at the piston crown for piston wash. Piston wash is the area on the piston crown that is cleaned off by the fresh fuel/air as it enters the cylinder.

Put the piston at bottom dead center, use a small light (bend-a-light, or small LED's work great). You are looking at the piston crown where the fuel/air enters at the intake ports. Good piston wash will be about the size of a thumbnail. If you are rich, a bigger area of the crown will be washed. If the mix is lean, there will be no piston wash.

It's better to be too rich than too lean. Too lean will increase cylinder temperatures to the point that you can melt a hole in the piston.

Once you have the main selected, go back and do the same ride test at mid-range. That will set the needle height.

The 1/4 throttle to 3/4 throttle fuel/air is metered by the needle jet/jet needle as it comes from the main. At about 3/4 throttle, the needle is clear of the jet and the main is metering. If you lean out the main, you also lean out the needle jet/jet needle and may have to lower the clip (raise the needle) a notch to compensate. Testing will tell.

Good luck!

2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah
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post #3 of 6 (permalink) Old 01-03-2015, 06:38 AM Thread Starter
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Thanks BC Dan for this excellent guidance. That seems like a very prudent approach to avoid any possibility of going too lean and burning a piston, or worse.

Right now, after running in general, I am getting the cardboard brown color on the plugs - but that is overall, not necessarily at the mid and full throttle conditions.

I will do the checks you suggest and report back. Thanks!
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post #4 of 6 (permalink) Old 01-03-2015, 11:44 AM
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You're welcome! I keep a piston on my desk with a hole in it to remind me not to go too lean Spendy day, that was! They all run perfect just before they don't.

2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah
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post #5 of 6 (permalink) Old 01-03-2015, 02:30 PM Thread Starter
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Okay - made the two runs today to check plug condition. Plug conditions after the mid-throttle run - nice tan-brown color. Machine ran excellent.

Plug condition after full throttle run - Plugs still looked nice and brown; one was a bit wetter than during mid range run. Neither looked obviously fouled or black. I did notice the clutch side was hotter to the touch than the pull-cord side. Machine was boggy.

Not exactly what I expected to find. Was not able to see onto piston tops yet. Will try again with a better LED.

Advice?
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post #6 of 6 (permalink) Old 01-03-2015, 03:02 PM
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What's your compression?

Plugs should be dry when you pull 'em. I don't know if I would go too much leaner if you are cardboard brown. Dark brown, maybe go a half size leaner (if you can find a jet that size!) You can also stagger jetting, if one side is perfect for color and wash and the other tests rich, go smaller on the rich side only. Just be careful getting to the edge, it's easy to fall over and burn holes in the pistons.

How are the clutches? How new is the belt? Is the belt adjusted so it's not sitting down in the secondary? When were the springs changed last? Weak springs can cause the primary to shift too fast, and the engine can't keep up. That will also cause a bog. That's a few more things to check and to rule out.

Fun, huh!

2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah
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