71 400 idle prblems

snowcrazy · Post by **snowcrazy** » Sun Jan 30, 2011 4:17 pm

I have a 71 400. It has a major problem of running smoothly. Idle is not stable, and it stummbles all thru the range of rpms. I'm thinking of changing the carb to a mukini vm 34-80. I would some relief (thoughts) in what is going on with the situation & what jets I need to transform the mukini.

The idle stays roughly @ 2600 until I ride it for 30 mins, then soars to 3000. The idle screw has no effect on it.

Post by **Matt** » Sun Jan 30, 2011 6:39 pm

First year 400 is '72. A sled manufactured in fall of '71 is a '72. Have you made any attempts to service the original carb (Walbro?)

JDT · Post by **JDT** » Sun Jan 30, 2011 6:47 pm

Could be a leak in the crank seals.

snowcrazy · Post by **snowcrazy** » Sun Jan 30, 2011 7:17 pm

The walbro has been serviced 4 times in the last 5 yrs.. The pto seal has been replaced, mag side has not. Will try that. Any info for the mukini? The serial is #10265, WI DNR wants to call it a 71, I realize its a 72, lol.

JDT · Post by **JDT** » Sun Jan 30, 2011 7:27 pm

The idle screw as it is called is actually a mixture adjustment.

Yes it does have some effect on idle but that is only a consquince of getting the mixture corerect that the idle increases.

If you want to adjust the idle speed RPM's then that is done with the cable and the bracket screw.

There is a write up on Walbro to Mikuni converstion in an old post.
It does involve more than just a carb swap but can and has been done with sucess.

Any Mikuni carb from a similar sized 340 twin should work for you.

snowcrazy · Post by **snowcrazy** » Fri Feb 04, 2011 9:38 pm

Is there an easy way to replace the fan side seal, other than tearing the engine apart?

400brian · Post by **400brian** » Sat Feb 05, 2011 3:09 am

My understanding is that the '72s and probably the '73s, require you to split the case to replace the seals.
Those engines use a washer-like seal retainer, whereas the newer engines did not.
I don't think you have to completely tear the engine down though, you just have to pull the bottom half of the case off.
Replace the seals, re-seal the case, and re-assemble.

The on-line service manual will give you the procedure.

snowcrazy · Post by **snowcrazy** » Sat Feb 05, 2011 6:03 pm

Thanks for the info on seal replacement. Where do I find the online manual? Today my son & I did a compression check while hot. The compression was 145. Then after the engine was cold, the pressure was back to its original 165. What's up with that? While running on the trail it did fine for awhile, then started its mild back firing, cutting out a little. So we returned and did the compression check.

Sprailfire340 · Post by **Sprailfire340** » Sat Feb 05, 2011 6:10 pm

Most of the manuals are on Webman's website (member here). Here is the direct link for your sled.
http://www.vintagesnow.com/John_Deere_files/sm2100.pdf

400brian · Post by **400brian** » Sat Feb 05, 2011 8:33 pm

What color are your plugs?

snowcrazy · Post by **snowcrazy** » Sat Feb 05, 2011 9:28 pm

The plugs are medium to dark brown

snowcrazy · Post by **snowcrazy** » Tue Oct 16, 2012 6:08 am

Well gang I found the problem to my sled. One of the points contact had worked loose from the rivet. Now I'm having a problem understanding why do you time the engine the way they describe it in the book. First the points, then the mag.help!

71 400 idle prblems

71 400 idle prblems

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