When I 1st started working on it there was a lot of popping back in the carb and wouldn't run off choke,
I have cleaned the carb 3 times. Every passage is clear all just clean, new fuel cleaned tank and petcock. Timing is dead on. Adjusted the valves and sprayed everything with carb cleaner and wd 40 searching for air leaks. replaced all O rings also.
From cold starts on full choke, then go to half choke then when good and warm take it off. I drove it 10 miles and it ran perfect until the last 1/2 mile. The 1st 1/4 throttle felt like it was a huge flat spot but fine when I passed it. I pulled to park it and it died. Only way to restart was with 1/2 choke. It idles real high with the choke on.Starts off normal then revs up. when is cools off I can do it all over again. There is some signs of old grease buildup at the head gasket. I spray the area with carb or wd no change. I thought when the head expanded it sucked air. compression is great also.
Any ideas would be much appreciated.

 

<div class="FTQUOTE"><begin quote>Originally posted by: terryw35

When I 1st started working on it there was a lot of popping back in the carb and wouldn't run off choke,

I have cleaned the carb 3 times. Every passage is clear all just clean, new fuel cleaned tank and petcock. Timing is dead on. Adjusted the valves and sprayed everything with carb cleaner and wd 40 searching for air leaks. replaced all O rings also.

From cold starts on full choke, then go to half choke then when good and warm take it off. I drove it 10 miles and it ran perfect until the last 1/2 mile. The 1st 1/4 throttle felt like it was a huge flat spot but fine when I passed it. I pulled to park it and it died. Only way to restart was with 1/2 choke. It idles real high with the choke on.Starts off normal then revs up. when is cools off I can do it all over again. There is some signs of old grease buildup at the head gasket. I spray the area with carb or wd no change. I thought when the head expanded it sucked air. compression is great also.

Any ideas would be much appreciated.</end quote></div>

You're getting too much air when warm. Then when you choke, It's running lean(and I bet smoother than normal too) and that's gonna make something real hot. then you're gonna buy a piston.....or something. Could be a worn out throttle shaft, a hole that grows when warm on the intake side somewhere..... Check intake boots and such with carb cleaner, not the head.

It's definitely mixture related from the description given. Since it changes when warm, you gotta find where it's leaking while it's running and warm.

Good luck, and let us know what you find. We all can learn from that issue.

 

terryw35 the carb fuel mixture could be off , but it sound's like either the carb jetting is off or like Terrelster said about an air leak between the carb and intake/carb and air filter ; but i've seen everything from motorcycle's/atv's and lawn and garden equipment with overhead valve's off spec !

You can be off one tooth on the timing chain and cam shaft timing mark's cause this condition , you should check the timing mark's again then when adjusting the valve's making sure you're on the compression stroke when adjusting the valve's ; the best way to tell when you're on the compression stroke is to remove the spark plug and stick your finger in the spark plug hole while you turn the crank shaft you'll feel pressure pushing againt your finger on the compression stroke stop then turn the crankshaft untill you line up the timing mark's at TDC /Top Dead Center on the compression Stroke !

In over 20 yrs. in the Small Engine Serive and Repair Industry i've seen 4 stroke engine's brought into my shop with this problem !

Good luck on getting it going if the timing and valve's aren't right on spec's you can warp valves and knock hole's in the top of the piston's , i hope this isn't the problem in your case !.......

 

Did you remove the pilot jet (idle jet), and look thru it when you cleaned the carb? Im betting its plugged...youve got all the classic symptoms of a plugged pilot jet.
If the jets arent removed from the carb body...then they cant be cleaned properly.

 

sounds like a leaky crank seal getting warm and leaking

 

<div class="FTQUOTE"><begin quote>Originally posted by: itch

sounds like a leaky crank seal getting warm and leaking</end quote></div>

Its a 4 stroke

 

Thanks for the help all.
I am sure the timing, valve adjustments, carb was completely disassembled cleaned and rechecked after still having trouble.
It sure seems like an air leak but I have sprayed everything with carb cleaner and wd.
Does anyone know the starting adjustment for idle and fuel mixture screw? The fuel mixture screw was 3 1/2 turns out when I started on it, that's after being at the local Yamaha dealer for this prob.
Is out rich?
I'm trying to help a neighbor kid and his mom on this and they don't have a book.

 

Yep, out is richer... and 3 turns is ok, if thats what it takes for it to run correctly. It will vary by temperature and altitude, and degree of crud in the pilot jet. The mixture screw is the fine adjustment of the pilot jet.
The choke is actually an enrichener, and when you pull it on, it adds gas....rather than 'choke' off air. The not wanting to run with out the choke points to the pilot jet. Seems to me, that carb has the smallest pilot jet made..11.5 or so, and thats about .008" on the center hole. It doesnt take much of a piece of crud to partially or fully block it.
Im still thinking the pilot jet is the problem. Remove it completely from the carb body, and look thru it, just like looking thru a telescope. You should see the light from the far side, coming thru the center of the jet. If you cant....dont waste your time trying to clean it, you wont find a wire small enough to push out the crud. Just go to the yami dealer, and bring the old jet with you, and get the next size bigger. The number is stamped on the side of the jet. If you get the next size bigger, itll start and run alot better when cold.
After you install the bigger jet, readjust the mixture screw.
Here a procedure for adjusting that screw.
<a target=_blank class=ftalternatingbarlinklarge href="http://www.motorcyclecruiser.com/tech/motorcycle_jet_kits_carburetion/index.html
">http://www.motorcyclecruiser.c...tion/index.html
</a>
If it has an inline filter, make sure its the brass screen type. Believe it or not, the paper element type filters, can resist flow enough to starve the carb of gas. Theres no fuel pump, and it acts only by gravity.
Change out the pilot jet.

 

Again thanks for the help. I have pulled the bowl off and pulled the idle jet twice. When I cleaned it both times I completly pulled it apart every piece, replaced the o-rings too. It doesn't rev up on choke when cool. Only after it warms up and stalls out. Because its adding fuel I can see why. I don't understand why I can play with the choke after it starts to die and get it back to the point of running withoutit. It will not start withatleast pulling half way out on the choke.

I always look at a light to see through jets. I use a piece of wire from a wire brush in the jet. I am sure of 1 thing. Every passage and jet is clean. I have also replaced the filter and ran it with no filter just 1 piece line. Same result.

Also when it's warm and running right it doesn't pop it the carb, only when it starts acting up. when it's off choke but still warming up it has a just off idle flat spot that goes away when warm. The plug was new when I rode it for 10 minutes the last time and it hasa light tan color. It looks great.
I am in East Texas, 72 degres today about the same when I last rode it.

 

The rubber hose, which is on the bottom of the float bowl..is it clean? Thats a vent for the float bowl, as well as the overflow, and if its plugged, the carb wont refill.
Also check the vent at the gas cap.
If the air filter, or lid is off, the engine will run real lean.