YT6800 backfires through carb

[Eric L]

Has anyone had this problem. When trying to start the tractor it backfire through the carb. It will start out being very reluctant to start . With the choke on, and with repeated tries, it will start backfiring through the carb and pufs of black smoke come out. If the battery holds out long enough, it will eventually start. This doesn't happen all the time, but when it does start, it runs great.

     Thanks

[rattosh51]

Sounds like you may be getting excessive fuel through the carb, choke linkage may be mis-adjusted or possibly your air intake is restricted, but the black smoke & backfire suggests too much fuel. I'd try running
some Seafoam additive through it first to see if anything clears up. May have some dirt in the carb.
If the gas has been in it for awhile, it may have gotten gummed up.

[Clueless In Cleveland]

I occasionally get a very loud backfire especially after running the tractor for a long period of time.  Can I also have a choke problem causing excess fuel building up post ignition?

[rattosh51]

You might. Backfiring is one of those harder things to diagnose, as there can be several causes.
After running for a while, do you idle down the speed and then turn off, or do you just turn it off at wide-open-throttle?

[Clueless In Cleveland]

Funny you should ask that.  I had been shutting down at full throttle and have only recently started to throttle down prior to shutting off the engine, and you know what?  It's not backfiring.  

[rattosh51]

Yeah, throttle down first is best...

[bilgrotie]

I am having an identical  problem with my 6700. When it chooses not to start even starting fluid wont help, next day it might start right up. The fuel pump is delivering plenty of fuel.
Anybody have any ideas? Thanks, Bill.

[rattosh51]

bilgrotie,
 I guess I would take the carb off and give it a good cleaning. You may have a bad float, inlet needle and seat, or just some dirt in there not allowing the seat to close properly. Sounds like it's flooding, and that's why it won't restart. You'd have to pull a plug out when this happens and see if it is wet with gas.

[bilgrotie]

Thanks for the reply. My initial thought was that it was lack of fuel, however the pump is supplying more than adequate fuel. There is no indication of flooding. The strange thing is that it starts and runs and idles well but only after prolonged cranking and a couple of backfires through the carb. Other times it may start right up or it might refuse to start at all. I am 67 years old and have been repairing and rebuilding engines large and small for many years but this one is really strange. I just got a factory service manual so I will do some studying and see what I can learn. I also own 4 Yamaha motorcycles, large and small and they are absolutely dependable.

[rattosh51]

bilgrotie--with that wealth of experience, you would likely know more than I. Please keep us updated with what you find. I run into this question a lot and would like to find a definite answer. I always tend to think carb when things are backfiring, but that is a strange one. Good luck with it.

[Eric L]

I know it's been a while, but did you ever resolve the backfiring isue with your Yamaha tractor?

[Eric L]

"Update"

I seem to have resolved my issue with the backfiring. I took both heads off to check the valves to find they were in good shape. I did remove all the carbon on the top of the piston and valves and on the inside of the head. Removing the plastic cowling to get the exhaust pipe off is a pain in the A$$. I had to cut part of it off or I would have had to remove the engine i believe. I adjusted the valves to .004, they REALLY needed adjusting. the gap was probably twice that. I took the carb off, completely dismantled it and cleaned it extremely well. I also emptied the gas from the tank and the lines changed the fuel filter and put fresh gas in and put in fresh gas. I started it (i had to prime it to get fuel to the carb) and it ran like a sewing machine. I only ran it for a minute because I don't have an air filter for it yet but I think I nailed it.

Hope this helps.

[rattosh51]

Great tip Eric...completely forgot about the old valve adjustment...glad you got 'er up & running again.
Where did you get the .004 measurement from?

[Eric L]

I found the gap online doing a lot of googling. Not sure where but it was for this tractor.