XR 80 4 STROKE PROBLEMS

My boys bike lost power on our last ride of the season last fall. It was a good long ride of about 20 miles at pretty good speeds, but at the 1/2 way point when we turned around he was having trouble starting from a stand still. I thought maybe he forgot how to shift and work the clutch, but sure enough no power at low RPM's.

So a friend of mine helped me clean the carb and its in great shape and was hardly any more dirty than when we cleaned it when I bought it a year before.

Anyways, once the RPM's come up there is the regular amount of power, but down low or up a hill at low RPM she just has very little power.

My buddie who is an airline mechanic and can rebuild anything that has an engine is a little stumped on what to look at next.

Any one have an idea of what to look at after the carb, or maybe something we are missing in the carb???

Thanks in advance for any help guys.

I need to get his bike running for next month!!!! Spring is almost here!!!

PS, plug was replaced just to rule it out, but it made no difference.

It seems like a point issue from what I can find, but figured I would ask here as I bet one of you knows this exact issue on this bike.

Does it idle ok? Might be a float or main jet issue? Weird that it would run fine and then not anymore, halfway through a ride. I'd also check the packing on the exhaust or the valves, it could be the back pressure causing it not to create enough of a vacuum at low RPM's. When my valves are too tight and not 'tappy' enough I get low rpm issues across the board.

Idles just fine. When you say check packing on exhaust, where and what exactly am I looking for, a bad gasket or a hole???

Points?!? What year is this bike? If it has points change them as that is simple.

Spark, Fuel, compression.... Check the fuel filter? Always start with the easy stuff.

When you say the carb was only slightly dirty, that scares me as it should not be dirty at all. Bad rings will cause the problems you mention and it only takes a small amount of dirt to destroy the rings.

When he says check the exhaust he means just make sure it is not clogged up anywhere. Sometimes on older bikes the muffler can rot away inside and restrict the exhaust flow which would cause the problems you are seeing. A bad gasket or hole will not cause the problems you are seeing, they usually just make the engine sound a little ratty.

Good luck

Bad rubber on intake?

Jets and all that will be looked at by a mechanic with Knowledge but it still needs the right air fuel mixture for combustion.

A small leak will make it run Idle high and make you thing the idle is off.
But? just a bit more air getting sucked in would do just about what you are talking about.:ne_nau:

Quote:

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Originally Posted by Mtnseeker1

Bad rubber on intake.

Jets and all that will be looked at be a mechanic with Knowledge but it still needs the right air fuel mixture for combustion.

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Bad rubber before the carb will not cause a problem, other then letting dirt in. Bad rubber past the carb can, but that is easy to diagnose. Just spray the rubber with starting fluid with the bike idling. If the engine RPM increases you have an air leak. but air leaks usually get worse with rpm's so I don't see that.

It doesn't sound like a clogged main jet, or bad float level as those issues show up on the top end. A clogged pilot jet or passage would cause the bottom end issues.

Yes sir and raw propane will give you the same results.
It is in the air fuel ratio somehow some way I think?
But it could be valves also with a four -t. AS in timing.

I'd start simple before getting aggressive.

Does this bike have an accelerator plunger like most 4-ts?
I know dumb questions sometimes get people thinking like two throttle cables? on this little 4-t?

Its a 2001 XR 80.

I will delve into the advice given this week. Thanks guys!

Best of luck and I'm sure you'll get her figured out.:cool2:

Have you got this fixed yet?

Could you give a little more detail on what you mean by "dirty" carb?

Oxidation tends to turn the brass parts a greenish color. Varnish is an amber color and can flake off and plug or gum stuff up. There is another deposit that I've seen that can plug up the jets and it's kinda like a calcium like substance that's rock hard and will not come off with carb or brake cleaner. It has to be scraped off with a tool of some kind (I've used a bristle from a wire brush to do this, not very well I might add). And of course, there is just plain old dirt and/or sand. Probably some other conditions, but I haven't had to deal with them yet.

Cleaning the near microscopic ports and holes in the carb can be a major pain.



So, let me try to get this straight...

It idles fine, then when you dial on the gas, it has no power until it increases in RPM's and then it's ok. Is that correct?

If correct, that would most likely rule out fuel delivery problems (ie, clogged shutoff valve, filter (probably doesn't have one anyway), or fuel line) as that would manifest itself more at higher RPM. It would most likely rule out any restriction in the exhaust system for the same reason.

It could possibly be a loose electrical connection--check them all, but not likely.

I'm guessing you have some sort of restriction in the carb, maybe caused by some of the "dirt" you possibly missed after your first cleaning, broke loose and clogged something up. If it were me, I'd just replace part #18 JET, SLOW and go from there.

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Mikuni Carburetor.... I can rebuild those at night, drunk, in a snow storm, with my eyes closed.... they are very simple to work on.

What is listed as the "slow jet" is more popularly known as the pilot jet. Remove the jet and you can easily tell if it is clogged just by looking at it. Now with the pilot jet removed take a fine wire and clear the passage the pilot jet screws into. The passage has a habit of becoming clogged if dirt slips past the air cleaner or past the fuel filter.... you are running a fuel filter I assume?

The pilot jet controls the fuel from just off idle to about half throttle....

I'm with Iceaxe. Sounds like the pilot circuit. A clean carb means little if any orifice is clogged. I've purchased a Kawi 110 for cheap because it ran like crap. It was a clogged pilot jet. Needless to say, I got a hell of a deal! :-)

Sent from my HTCONE using Tapatalk

I'm Cusrious?
How does she run?

You my friend, deserve a huge thanks.

We ended up cleaning #18 Slow jet again and found that it was clogged more than we thought. Cleaned it out and put back together and it ran ok, but still not great. So my friend had the smallest drill set I have ever seen. Had 30 or so bits in the set and then went from a human hair size, to a dog hair size. He ran bigger and bigger sizes until just the smallest amount of brass shaving came out, and then put it back together and whalla!!!!!! She runs better than when we bought her! THANKS!!!!!!!!

Really do appreciate all the help guys, but Mr. Sandstone wins big!!!!!!

The one thing my friend noticed is that it ran better on choke, and so he said #18 made alot of sense! I dont have a clue, but it helped that your suspicion led us to look again, and focus on what it most easily could have been.

As a side note, I just picked up a nice little KLX 110 for the 8 year old. He loves it. Should be a fun summer!!!!

Quote:

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Originally Posted by Sandstone Addiction

Have you got this fixed yet?

Could you give a little more detail on what you mean by "dirty" carb?

Oxidation tends to turn the brass parts a greenish color. Varnish is an amber color and can flake off and plug or gum stuff up. There is another deposit that I've seen that can plug up the jets and it's kinda like a calcium like substance that's rock hard and will not come off with carb or brake cleaner. It has to be scraped off with a tool of some kind (I've used a bristle from a wire brush to do this, not very well I might add). And of course, there is just plain old dirt and/or sand. Probably some other conditions, but I haven't had to deal with them yet.

Cleaning the near microscopic ports and holes in the carb can be a major pain.



So, let me try to get this straight...

It idles fine, then when you dial on the gas, it has no power until it increases in RPM's and then it's ok. Is that correct?

If correct, that would most likely rule out fuel delivery problems (ie, clogged shutoff valve, filter (probably doesn't have one anyway), or fuel line) as that would manifest itself more at higher RPM. It would most likely rule out any restriction in the exhaust system for the same reason.

It could possibly be a loose electrical connection--check them all, but not likely.

I'm guessing you have some sort of restriction in the carb, maybe caused by some of the "dirt" you possibly missed after your first cleaning, broke loose and clogged something up. If it were me, I'd just replace part #18 JET, SLOW and go from there.

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