Saw cuts out full throttle???

[Wildman1024]

OK here it goes. I have a saw that runs basically perfect. Starts right up, Idles nicely and revs easily and nicely until just before full throttle. Once you hold it full throttle for about 3 seconds it cuts out and almost stalls but if you let off the throttle it will recover and go back to idle just fine and run great again.

It just had the carb rebuilt, new fuel line from the tank to the carb and new reeds installed. All work was done by someone who knows what they are doing. i got a tip that maybe it was the filter in the tank because that line wasn't replaced and when I pulled the tank it was so dirty and nasty I don't know how the saw ran at all. I took a new piece of line and ran it from a bottle to the carb (simulating a clean tank) and it still does the same thing

Anyone have any ideas as to what is going on? Could the tank have been so dirty that it needs the carb cleaned again???

 

[spudulike]

Sounds like fuel starvation at high revs, is your "H" carb adjuster around 1 turn out from fully screwed in? is your tank breather functioning ok - may be causing a vacuum within the tank! Possibly muck in your carb - the dirty filter in theory should filter more effectively to the point it will cause fuel starvation but worth checking your carbs internal filters if they havent been done - am assuming your air filter is in good condition.

Sounds like the carb has enough fuel for a short high speed run and then runs out of fuel and can only support idle speed.

Check petrol and impulse lines.

Worth checking your coil and HT leads - check the coil to flywheel distance - I use two layer strips of A4 paper. Would think spark/HT probs would cause rough running though - favourite is fuel starvation - check the carb/inlet manifold for leaks - ending with crank seals if all else fails:-(

Spud

 

[Fish]

Your first post seems kind of contradictary.

Did the guy that rebuilt the carb know what he was doing?

Sounds like the passage under the needle is partially clogged.

 

[Wildman1024]

I'm leaning tward the fuel starvation to. I ran a fresh line from the carb to a bottle of fuel and it was still doing it. So all I can think of is maybe the carb is getting plugged again from the crap that was in the tank.

I did notice some bubbles in the fuel line. I blew threw the vent on the cap and it was clear and easy to do so i think thats ok. I really do not think it is an ignition issue because it starts right up everytime and runs great except WOT.

 

[Wildman1024]

Your first post seems kind of contradictary.

Did the guy that rebuilt the carb know what he was doing?

Sounds like the passage under the needle is partially clogged.

Yes this person defiantly know what they are doing. Maybe it got clogged again from the crap that was in the tank

 

[SawTroll]

It doesn't sound as a typical tank vent issue, but I would take a look at it anyway.......opcorn:

 

[Fish]

Yes this person defiantly know what they are doing. Maybe it got clogged again from the crap that was in the tank

Well, that is kind of what I mean..........

So someone put old crappy fuel and a fouled fuel filter on/in the saw after this
guy worked on it?

When rebuilding a carb, you definitely take care of those areas as well.

 

[Wildman1024]

I understand what you mean and I know the person meant well and knew what they were doing. I don't think they realized the tank was that bad

 

[Fish]

Not trying to bust your balls, but I think your problem is still in the carb.

And don't keep running the saw until you get it fixed.....

 

[Wildman1024]

A little ball busting is ok! I'll admit I'm still young and learning. I pulled the carb today and it was pretty clean. There was some stuff in the filter but no wear near enough to cause any kind of issue. I pulled the entire carb apart and sprayed it down with carb cleaner and made sure everything was good to go and reassembled.

I gotta grab a new piece of fuel line and filter for the tank before I put that back together and attempt to get it running again. If that does not solve the problem I'm stumped.

 

[Scooterbum]

I had a Mac610 that did exactly that.Turned out it wasn't fuel related at all.
The spark plug wire was shorting out under the cover.

Just something to check........

 

[Wildman1024]

I had a Mac610 that did exactly that.Turned out it wasn't fuel related at all.
The spark plug wire was shorting out under the cover.

Just something to check........

I'll have to give that a try. If it isn't fuel or that then I'm really lost.

 

[pgg]

If it was a grunged fuel filter, there'll be debris clogged on the small round mesh filter in the carb. Also the saw won't run properly without a filter on the fuelhose end

 

[Fish]

You need to tear the carb down again and remove the needle/spring
and hold the carb up to a light, look through the hole up to the screen on the other side, that passage is likely choked up with gunk, it lets the saw get enough fuel until you get to high rpms, then starves the saw.
Blow out that passage and hold up to a light again. Visually verify that it is clear.

 

[Wildman1024]

I put the new fuel line and filter on it today and ran it for a few minutes. Still a no go. It still runs great until you hold WOT for 2-3 seconds then it still cuts out.

I was watching the fuel line going to the carb while I was running it today though. The line was nice and solid/full of fuel until I held it WOT. Once I did that I got alot of bubbles in the line then it started to stall. once it goes to stall and you let off the throttle it will recover and idle but if you try and rev it again it will bog out and stall. Once it stalls its very very hard to get running again

I did take the carb apart again and the filter was clean and the rest of the carb was clean. Fish do you still think I gotta check the needle?

 

[Trigger Man]

I had a maruyama trimmer that was doing the samething you describe and the carb was very dirty, pump diaphram and metering diaphram where shot make sure there's no dirt under the little tab that pushes on the metering lever. you may have another problem not sure. Like fish suggested check for dirt arond the H needle.

 

[KRP]

It doesn't sound as a typical tank vent issue, but I would take a look at it anyway.......opcorn:

Fuel demand at WOT is asking for more, but getting air. A fuel system vacuum is being created at WOT. The vacuum is released after fuel demand drops. It's most likely in the carb, check the needle. Are your fuel lines tight?, where is the air coming from?

 

[Wildman1024]

Fuel demand at WOT is asking for more, but getting air. A fuel system vacuum is being created at WOT. The vacuum is released after fuel demand drops. It's most likely in the carb, check the needle. Are your fuel lines tight?, where is the air coming from?

It looks like the air comes from the carb and goes back to the tank but usually gets trapped in the line between carb and tank. It looks like groups of very small bubbles.

 

[Fish]

You need to tear the carb down again and remove the needle/spring
and hold the carb up to a light, look through the hole up to the screen on the other side, that passage is likely choked up with gunk, it lets the saw get enough fuel until you get to high rpms, then starves the saw.
Blow out that passage and hold up to a light again. Visually verify that it is clear.

Yes, you need to do this. At this point, you might want to take another look at the piston through the exhaust.

 

[edisto]

Yes, you need to do this. At this point, you might want to take another look at the piston through the exhaust.

I'm starting to feel your pain Fish.