Sudden rough running

[JZ05220r]

Oh I totally agree with you on the crank seal, but I’m trying to avoid running into the same issue after I rebuild it you know what I mean because I don’t think this is what is causing my issue. For sure I will need to use some type of sealer on the od of the seal before pressing it in not sure what yet. Something non permanent obviously.

[JZ05220r]

I bought a cheap tusk air filter and popped that in making sure not to over oil. I actually used aerosol to apply the oil vs my typical soak and squeeze. It actually went from a sputter transitioning quickly to a big bog one 2nd gear pull. It was noticeably worse and I’m thinking that that may be because the tusk filter is obstructed compared to the uni. What I mean is the lip that seals against the airbox is sticks further into the intake tract than it needs to be on the tusk filter creating a restriction compared to the uni filter, which is just enough to make the seal nothing extra.

And just to clarify, of course I don’t want to assume that the bike isn’t lean, but most things I have found with my testing shows that it’s running rich.


Another thing that came to mind…. when I emptied my tank the other day and filled the tank up with my tuff jug quick fill can, fuel was leaking out of the overflow almost the entire duration during filling. Bike was on the side stand it stopped leaking once I was basically done filling. Also, my overflow line is wet as if it has been leaking while I ride. Where I parked it, there’s no drips or anything.

I have checked my float height and it is in spec. Picture for example, I must’ve taken this one in the past month trying to solve the issue. I had been through the carburetor and polished the seat with a new needle valve 50 hours ago. But again I just tested another carb and had the same issue.

[JZ05220r]

Here is a video of the bike warming up before I tested it. For what it’s worth. It’s a pretty stable idle. I turned it down toward the end. But it was just how I left it last test. No surprises, choke on first kick and it lights up.. as you can see in the video, the bike is stripped for testing.

[ss109]

That crank seal is extremely bad. It's very hard to solve one problem when you have an existing serious issue. I honestly don't know if it can be done. Just an FYI, you can split the cases and not replace anything inside other than the crank seals. While it is recommended to go ahead and replace crank bearing, and to rebuild the crank, you don't have to. That dramatically reduces the cost and time to replace the seals. You could also do like some members have and modify your cases so you can remove the seals without splitting the cases. I know I am strongly considering do it myself to my 220.

I do believe you are running very rich looking at the piston wash and the plugs. I know most think the plugs show lean but I disagree. I'm looking at the deposits that are actually left on them. Those deposits are very dark to black which is far from that nice chocolate brown color we're always shooting for. You won't get a nice full ring of deposits if they're seeing an excessive amount of fuel (plug stays too cool = less deposits). Considering the info about the overflow you look to be dumping fuel in the cylinder. I know you said you polished the needle seat but that needle could use being replaced to rule it out. Remember, you're dealing with the here and now and what was good then might not be now. Also, the float itself might be sticking. Make sure it isn't binding when it's assembled by moving the carb by turning it over gentle in many various ways. Double check that it is set at 18mm and NOT the factory 16mm. The only time I've ever had any of mine leak while on the kickstand or riding has always been a needle/seat and/or float level issue.

[JZ05220r]

That crank seal is extremely bad. It's very hard to solve one problem when you have an existing serious issue. I honestly don't know if it can be done. Just an FYI, you can split the cases and not replace anything inside other than the crank seals. While it is recommended to go ahead and replace crank bearing, and to rebuild the crank, you don't have to. That dramatically reduces the cost and time to replace the seals. You could also do like some members have and modify your cases so you can remove the seals without splitting the cases. I know I am strongly considering do it myself to my 220.

I do believe you are running very rich looking at the piston wash and the plugs. I know most think the plugs show lean but I disagree. I'm looking at the deposits that are actually left on them. Those deposits are very dark to black which is far from that nice chocolate brown color we're always shooting for. You won't get a nice full ring of deposits if they're seeing an excessive amount of fuel (plug stays too cool = less deposits). Considering the info about the overflow you look to be dumping fuel in the cylinder. I know you said you polished the needle seat but that needle could use being replaced to rule it out. Remember, you're dealing with the here and now and what was good then might not be now. Also, the float itself might be sticking. Make sure it isn't binding when it's assembled by moving the carb by turning it over gentle in many various ways. Double check that it is set at 18mm and NOT the factory 16mm. The only time I've ever had any of mine leak while on the kickstand or riding has always been a needle/seat and/or float level issue.

Ok, so it seems like I have to make a decision. Split cases replace seals and clean up the cylinder with a new piston or sell the bike one way or another.

I will check the float again. Back when I polished the seat I used a new oem needle valve. Tested the suction and there was zero leakage and with the carb upside down there was no pressure loss when blowing into the line. I do not however think the fuel is flooding the engine. I’ll get to this this weekend. Won’t replace the needle valve if it looks good but I’ll give a quick polish and test like I described above. I know the clear tube method but I lost the hose on the end of the adapter cap.

[JZ05220r]

I noticed my 1991 kdx uses full rubber seals without a metal shell. Wondering if something like that might be a good option. They are NOK seals on the 91.

[Chuck78]

I noticed my 1991 kdx uses full rubber seals without a metal shell. Wondering if something like that might be a good option. They are NOK seals on the 91.

That's interesting to know! With the metal shell type, I always degrease them and smear a little bit of RTV silicone on them when I press them in. A metal to metal press fit does not seem to be a guaranteed way to stop an air leak or even a liquids a leak.


On polishing the float needles seat, it's a good possibility that it could be pitted or even polishing it could leave it slightly out of round or uneven. If it's overflowing fuel out the overflow tubes, it's going to be running pretty rich and accumulating excess fuel in the bottom of the crankcase, which you are probably compensating for by dropping main jet sizes down perhaps?

I know you said you tried a different carburetor also, but perhaps it also is overflowing from sitting on the shelf and having a gummed up seat?
It's also quite possible that you have two or three different issues going on simultaneously and compounding together.

[JZ05220r]

Yes just found this kit I’m about to order. If I can keep cost down below, say $400, it’ll be worth it for me to rebuild and hope for the best. That said I will want to buy a new piston and reestablish a good cylinder finish to seat the rings.

.https://www.ebay.com/itm/336315921338?c ... tpEALw_wcB

[ss109]

Beware of counterfeit parts on Ebay! Don't know if those are legit or not. As for the needle, there is no real way to know for sure if it's still good other than the obvious things. I would inspect it very closely with the strongest magnifier you got.

Like I said, you don't have to do all that on the engine. You can literally only replace the crank seals, the cylinder base gasket, some Yamabond sealer, and a quart of transmission oil. Through RMATV would be just under $50 not counting tax/shipping and you can do the trans fluid locally. So probably around $75 total to fix the issue. This way you can keep things very affordable.

[JZ05220r]

Do you see an issue with reusing the rings and piston? I mean, I know the rings have a good seal right now to the cylinder, but reinstallation can sometimes create a nick.

[Chuck78]

Do you see an issue with reusing the rings and piston? I mean, I know the rings have a good seal right now to the cylinder, but reinstallation can sometimes create a nick.

I would have no issue at all reusing the rings. I would clean up the cylinder and lightly deglaze it with s Scotch-Brite pad, clean the power valves, grease power valve parts with molybdenum disulfide grease, and slap it all back together. I think what SS109 was saying is don't even remove the cylinder head. I'm not sure how I feel about reusing head gaskets although people do that often and I haven't heard anyone complaining about them not sealing, but I probably wouldn't.