Working on a cool old 77 YZ80D for my daughter- sat for a few years as I understand it. Got it running right away, ran good for a while, but then started dying. Took the fuel tank off and completely cleaned it out, as well as the fuel valve. Installed new fuel filter and hoses. Then, took carb all apart, very carefully and completely. Completely cleaned out all orifices and replaced all that I could-- all gaskets, being careful not to change or damage float, needle/seat, etc. Cleaned and blew out jet, everything. Put back together. Now, it will run great for just a few minutes, then will begin to run rough and die. When checking the plug, it looks fuel-fouled. Clean up the plug, and it then starts all over-- runs great for a few minutes, then begins to run rough and dies. Plug out-- it looks very wet. Would an incorrect (not hot enough) plug cause this? It has a main jet size of 170, which if memory serves when sourcing parts the offerings were 160 and 180. Any ideas? When it's running normally for those few minutes, it runs great. Oil mixture I think I have right, but just in case, too much oil-- would that cause this? thx-

  reluctorvalve said:

Working on a cool old 77 YZ80D for my daughter- sat for a few years as I understand it. Got it running right away, ran good for a while, but then started dying. Took the fuel tank off and completely cleaned it out, as well as the fuel valve. Installed new fuel filter and hoses. Then, took carb all apart, very carefully and completely. Completely cleaned out all orifices and replaced all that I could-- all gaskets, being careful not to change or damage float, needle/seat, etc. Cleaned and blew out jet, everything. Put back together. Now, it will run great for just a few minutes, then will begin to run rough and die. When checking the plug, it looks fuel-fouled. Clean up the plug, and it then starts all over-- runs great for a few minutes, then begins to run rough and dies. Plug out-- it looks very wet. Would an incorrect (not hot enough) plug cause this? It has a main jet size of 170, which if memory serves when sourcing parts the offerings were 160 and 180. Any ideas? When it's running normally for those few minutes, it runs great. Oil mixture I think I have right, but just in case, too much oil-- would that cause this? thx-

What plug are you running and what is the gap setting, also tell me your fuel to oil ratio. 170 main jet comes standard in that bike.

Thanks for the reply-

It is an NGK B8ES, newer in good shape. Gap is .032" Oil mixture-- I got that off of a Yamaha Service Data site, which is now now longer up, but I did document it on my computer-- it's 20:1 hope I mixed it correctly. Would too thick cause this?

Thanks for your help-

r.

  Twin Shock said:

What plug are you running and what is the gap setting, also tell me your fuel to oil ratio. 170 main jet comes standard in that bike.

  reluctorvalve said:

Thanks for the reply-

It is an NGK B8ES, newer in good shape. Gap is .032" Oil mixture-- I got that off of a Yamaha Service Data site, which is now now longer up, but I did document it on my computer-- it's 20:1 hope I mixed it correctly. Would too thick cause this?

Thanks for your help-

r.

Your plug gap is too wide, change to .22" and run the oil mixture @ 32:1

  Twin Shock said:

Your plug gap is too wide, change to .22" and run the oil mixture @ 32:1

Thanks for that info. I'll give it a try and report back. By the way, do you know where I could find some good tech info for this bike online?

Thank you-

  Twin Shock said:

Your plug gap is too wide, change to .22" and run the oil mixture @ 32:1

changed the gap-- started right up, sounded better, but still floods out after a minute or two. I think there may be something wrong with the carb-- I took it all apart, did a deep clean, compressed air in all the orifices, etc. Replaced a few gaskets and o rings. shook the float and it sounded good with no audible evidence of gas inside. wondering maybe if the needle is sticking in the seat? something is flooding this thing. It ran good, normally....for hours. Then started to die. Shame on me for not cleaning out the tank, petcock, etc. So then I took it all apart and did a complete cleaning. Now it floods. hm.....got any ideas? should I take the carb apart again and check float height and needle movement? not sure--

thx for your input.

  reluctorvalve said:

changed the gap-- started right up, sounded better, but still floods out after a minute or two. I think there may be something wrong with the carb-- I took it all apart, did a deep clean, compressed air in all the orifices, etc. Replaced a few gaskets and o rings. shook the float and it sounded good with no audible evidence of gas inside. wondering maybe if the needle is sticking in the seat? something is flooding this thing. It ran good, normally....for hours. Then started to die. Shame on me for not cleaning out the tank, petcock, etc. So then I took it all apart and did a complete cleaning. Now it floods. hm.....got any ideas? should I take the carb apart again and check float height and needle movement? not sure--

thx for your input.

Go ahead and change the neddle and seat and if that don't solve your problem you might need to look into your ignition coil. If the bike is flooding ,fuel should be flowing out the overflow pipe. Or it can load up like you are describing.

hello if you cleaned tank and carb your prob alright on that end. oil mix is 32 to 1. Check your reed valves. over time they get worn out and create this situation you have. mostly because they stay open and allow to much fuel then over time it eventually causes the bike to die after warm up

Up for sale! For anyone interested in this highly complete and original little dirt racer, it's for sale up on Ebay, item # 320864825643. Still flooding the plug with gas- took the carb out again, thoroughly cleaned it, verified sharp strong spark, etc.

The kick start assembly is new, used OEM NOS parts, including the inner driven gear and also a new cover oil seal that doesn't leak. I have no time to mess with this little bike anymore, and my child is way too large and old for this now. Hate to sell it, would tinker futher as maybe a Grandpa some day, but I really haven't the time or room now. Check it out if you are interested- questions/pic requests to me directly at reluctorvalve@yahoo.com

Thx for all your suggestions!