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Bethlehem

Another (Can't start my YZ400F) Thread - Already sick of this bike

7 posts in this topic

'99 YZ400F

A guy sold it to me with gas the tank dry as a bone and I could tell it had been sitting. I put a little gas in it and he was able to start it on first kick! The carb was leaking badly from the overflow and bowl gasket but it ran like a striped-ass ape. I've been able to start it a couple times using this method...

 

1 - Choke

2 - Find top dead center

3 - Two twists of the throttle

4 - Full kick from top to bottom

 

Once it started it ran great. - So I removed the bowl and unstuck the needle valve and replaced the bowl gasket. No more leaks. I haven't been able to start it since. I know it's getting fuel, the choke is working, there are no leaks. I'm positive I got the needle valve and float in correctly. It tries to start on the first kick and periodically thereafter. But I didn't buy this bike so I could fight with it. I just want to ride it.

 

Since I messed with the carb I have noticed the compression seems to be lower. It's harder to find top dead center because it never really gets difficult to push the kicker down. But the only thing I touched was the carb.

 

I really don't care about this bike and really just want something to ride trails. If anyone wants to trade for just about any other bike that's easy to start I might be interested. It's too powerful for my kids anyway. It has a ton of aftermarket stuff on it. I'm in Troy, MO.

 

But let me know if you have starting advice too. :banghead:

Edited by Bethlehem

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'99 YZ400F

A guy sold it to me with gas the tank dry as a bone and I could tell it had been sitting. I put a little gas in it and he was able to start it on first kick! The carb was leaking badly from the overflow and bowl gasket but it ran like a striped-ass ape. I've been able to start it a couple times using this method...

 

1 - Choke

2 - Find top dead center

3 - Two twists of the throttle

4 - Full kick from top to bottom

 

Once it started it ran great. - So I removed the bowl and unstuck the needle valve and replaced the bowl gasket. No more leaks. I haven't been able to start it since. I know it's getting fuel, the choke is working, there are no leaks. I'm positive I got the needle valve and float in correctly. It tries to start on the first kick and periodically thereafter. But I didn't buy this bike so I could fight with it. I just want to ride it.

 

Since I messed with the carb I have noticed the compression seems to be lower. It's harder to find top dead center because it never really gets difficult to push the kicker down. But the only thing I touched was the carb.

 

I really don't care about this bike and really just want something to ride trails. If anyone wants to trade for just about any other bike that's easy to start I might be interested. It's too powerful for my kids anyway. It has a ton of aftermarket stuff on it. I'm in Troy, MO.

 

But let me know if you have starting advice too. :banghead:

Strange about the lower compression - whatever you did should not have changed that. I have no answer for that one.

 

the difference in starting AFTER your carb work could be because now it's getting LESS fuel than before (since it was sticking and getting excess fuel.)

 

Try to get more fuel in there before starting it:

On my '99 400, it was best to go just past TDC (manual said 30 degrees past I think), and give it 5 twists of the throttle to get more fuel in there before kicking.

The last thing that helped starting (and helped it run cooler at lower throttle openings) was installing a bigger pilot jet, since the stock one is on the lean side.

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The part that confuses everyone who actually understands that TDC is top dead center is that you are NOT supposed to go PAST TDC.  You're supposed to stop at the point where the first resistance of the compression stroke is felt, and that occurs about 120 degrees BEFORE TDC.  Once you find that spot, you pull the compression release and move the starter lever about one inch farther down at the end, not 30 degrees.

 

If you go past actual TDC, you'll have to turn the engine through almost two complete revolutions to get back to near the top of the compression stroke, where ignition occurs, and the starter won't drive the crank that far in a single stroke.

 

Watch:

http://api.viglink.com/api/click?format=go&jsonp=vglnk_jsonp_14117495599106&key=fd4d0c8927dbf2331e83bbda32ac7c21&libId=85793341-dc64-4dfd-9bba-7162d2b1bf96&loc=http%3A%2F%2Fwww.thumpertalk.com%2Ftopic%2F378677-common-threads-look-here-1st-mods-maintenance-common-issues-and-faqs%2F&v=1&out=http%3A%2F%2Fwww.yamaha-motor.com%2Fsport%2F4stroke_vid%2F4_stroke_vid_a.mpg&title=Common%20Threads%3A%20Look%20here%201st!%20Mods%2C%20Maintenance%2C%20Common%20Issues%20and%20FAQ%27s%20-%20YZ%20400%2F426%2F450%20-%20ThumperTalk&txt=http%3A%2F%2Fwww.yamaha-mo...troke_vid_a.mpg

 

http://api.viglink.com/api/click?format=go&jsonp=vglnk_jsonp_14117495755437&key=fd4d0c8927dbf2331e83bbda32ac7c21&libId=85793341-dc64-4dfd-9bba-7162d2b1bf96&loc=http%3A%2F%2Fwww.thumpertalk.com%2Ftopic%2F378677-common-threads-look-here-1st-mods-maintenance-common-issues-and-faqs%2F&v=1&out=http%3A%2F%2Fwww.yamaha-motor.com%2Fsport%2F4stroke_vid%2F4_stroke_vid_b.mpg&title=Common%20Threads%3A%20Look%20here%201st!%20Mods%2C%20Maintenance%2C%20Common%20Issues%20and%20FAQ%27s%20-%20YZ%20400%2F426%2F450%20-%20ThumperTalk&txt=http%3A%2F%2Fwww.yamaha-mo...troke_vid_b.mpg

 

The lack of compression on some of the older bikes is not terribly uncommon.  Usually, it's the result of carbon built up on the intake valve stems, or near the edge of the valve face, that is holding the valve just slightly off it's seat.  Sometimes, a couple of good fast kicks will get them to reseat.  Other times, it's just tired rings. 

 

The single most important thing to get them to start well is to have the idle mixture adjustment trimmed out right.  After that, it becomes more a matter of whether you have spark at the plug,  The ignition is a little bit marginal at cranking speeds, and the plugs will become fouled fairly easily if you're in the habit of starting it and then shutting it down before the plug has gotten hot enough to burn itself clean.

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Get some staring fluid and see if it bites, then at least you will know if its just a fuel delivery issue.

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Correct me if im wrong....but doesnt the hot start cable that leans it out.....go into the carb? Anyone?

If so....if he messed that up somehow would it mess things up?

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There is no hot start cable on a 400.  The hot start on that carb was not in fact on the carb at all, but a completely separate sub-system.  The air valve and knob are bolted to a frame bracket,and there is a hose that runs clean air from the air boot to the intake stub on the head.

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