High idle.

[Johnny lee]

I recently drug a 400 out of the woods where it has been for the last 20 years, I cleaned the starter up got it to turn over, then pulled carburetors and cleaned them up and got it to fire. Problem I'm having is when it starts it revs up pretty high, takes a few seconds to come back to idle. Then when I turn the throttle it does the same thing. Very slow to idle down. I pulled carburetors and cleaned a second time. Still doing the same thing. I can't figure it out

 

[CaptChrome]

Welcome to the forum.

Probably an air leak. From the carb holders, vacuum lines or the butterfly shaft seals.

 

[Johnny lee]

I plugged off the vacuum ports on both intake boots, the butterfly seems to be tight and smooth, I can't see anything that looks wrong there. The o rings on intake boots seems ok, maybe a little thin
Only thing I can think of is replacing o rings and maybe putting some rtv on them. But thanks for the reply

 

[JPaganel]

I plugged off the vacuum ports on both intake boots, the butterfly seems to be tight and smooth, I can't see anything that looks wrong there. The o rings on intake boots seems ok, maybe a little thin
Only thing I can think of is replacing o rings and maybe putting some rtv on them. But thanks for the reply

Capping the ports does not guarantee that there isn't a leak around where the tube goes into the rubber, especially if the rubber is old. Hell, I had new boots, and the nipples came loose, had to glue them back in.

Butterfly being tight and smooth comes from the shaft and carb bodies being in good shape. Again, that does not at all guarantee the shaft seals are OK.

Shaft seals are not o-rings. They are an actual seal with a lip. They are also 40 year old rubber. That rubber is probably stiff and doesn't actually start sealing until it warms up. You can't see the gap and you can't tell how stiff they are until you pull them and compare to the new ones.

Replacing the o-rings on the boots is a good idea. Same deal - 40 year old rubber ain't the best.

 

[CaptChrome]

I can't see anything that looks wrong there

Per JPaganel's post above, it might not be obvious to the eye. For example you will never be able to tell the condition of the shaft seals unless you completely tear the carbs apart.

But thanks for the reply

You are welcome.

One thing that you can try to diagnose an air leak is to run the bike and then spray a little WD-40 or similar on potential sources one at a time. The fluid will temporarily seal cracks and gaps that you can't see. If you spray on a leak source you should notice the idle respond. It is not fool proof, but it is something that you can do to move your troubleshooting along.

 

[Johnny lee]

Capping the ports does not guarantee that there isn't a leak around where the tube goes into the rubber, especially if the rubber is old. Hell, I had new boots, and the nipples came loose, had to glue them back in.

Butterfly being tight and smooth comes from the shaft and carb bodies being in good shape. Again, that does not at all guarantee the shaft seals are OK.

Shaft seals are not o-rings. They are an actual seal with a lip. They are also 40 year old rubber. That rubber is probably stiff and doesn't actually start sealing until it warms up. You can't see the gap and you can't tell how stiff they are until you pull them and compare to the new ones.

Replacing the o-rings on the boots is a good idea. Same deal - 40 year old rubber ain't the best.

Thanks I will try to check the butterfly shaft seals

 

[Johnny lee]

Capping the ports does not guarantee that there isn't a leak around where the tube goes into the rubber, especially if the rubber is old. Hell, I had new boots, and the nipples came loose, had to glue them back in.

Butterfly being tight and smooth comes from the shaft and carb bodies being in good shape. Again, that does not at all guarantee the shaft seals are OK.

Shaft seals are not o-rings. They are an actual seal with a lip. They are also 40 year old rubber. That rubber is probably stiff and doesn't actually start sealing until it warms up. You can't see the gap and you can't tell how stiff they are until you pull them and compare to the new ones.

Replacing the o-rings on the boots is a good idea. Same deal - 40 year old rubber ain't the best.

I replaced o rings on boots, finally got it to idle. But when I put it in gear it would die. I bypassed the kick stand safety switch and was able to keep it running. Yesterday morning was able to take it down the road a couple miles and back. Was surprised how much giddy up it has for a 400 .thanks everyone for your help

 

[Robert Silvestro Jr]

Sounds like one or both diaphragms are torn or not sealed also spray some WD40 on the slides lift them with your finger, if they go up easily and slide down it could be a dirty needle or jet.