Posts
3310
Joined
5/25/2014
Location
San Diego, CA
US
Edited Date/Time
12/10/2020 7:47pm
Short version: Can low compression mimic some symptoms of a bad float valve?
Long version: helping work on a friend's 2005 KTM 200. It has the symptoms of a bad float valve. It does not overflow gas when sitting on the stand. However when you fire it up and have the petcock in the off position, it idles and revs just fine. But when you turn the petcock on, it will load up, clearly gurgling too much gas. Turn the petcock back off and after you clean it out, it will run clean again. Trying to ride it with the gas on is exciting as it will surge and accelerate unpredictably.
Took the carburetor apart and cleaned everything out. (It was clean to begin with, though.) Put in a new float needle valve and set float height properly. Lapped the float seat with a wood dowl and some compound. Pulled the reeds while I was at it and they looked primo. Put everything back together and it still does the same thing.
Since I have a bitchin 2004 KTM 200 that runs like a top, I took the carburetor off of my bike and put it on our buddies bike. Fired it up and it still does the same thing! Much head-scratching ensued.
Started browsing around on the forums and it was all the usual stuff for these sort of symptoms that you would normally think- the float needle, float height, etc. However. I came across one thread, and God damned if I can't find it again, but a guy was having a similar problem, was going back and forth. And then for whatever reason, did a top end on his bike. Suddenly the problem went away. There was no conclusive answer in the thread. He just kind of said "did a top-end. Runs great. Adios."
So today in the shop I slapped a compression tester in the spark plug hole and gave the few kicks. It only pumped out 90 PSI! I have no idea how it even starts first kick and runs as well as it does!
Obviously, this is highly coincidental. But has anyone here heard of something like low compression causing this sort of weirdness? I mean, if it was my bike and I only pumping out 90 PSI, I would immediately do a top-end no matter what. But it's somebody else's bike and it's going to be hard for me to tell him to do a top-end to fix the problem as we are just not seeing a correlation.
Thoughts?
and can't recall. That was many top ends ago. As a matter of fact, mine is currently apart for a little cylinder head work.
how to connect those dots.
I'm super curious now, as well, and want to take apart the motor myself to see what it might be.
The Shop
Paw Paw
When engine compression is low, the crankcase/piston has a difficult time creating enough "vacuum" to properly pull the fuel into the motor which causes very poor atomization of the fuel inside the combustion chamber. Thus the bike appears to run rich or loaded-up.
The reason why it runs better with the fuel shutoff is because you are 1) decreasing the height of the fuel in the emulsion tube which gives it a longer distance to travel before it reaches the combustion chamber. & 2) it is pulling in less overall volume of fuel. In this state, the bike will run a little bit better because the fuel atomization is increased.
I'm pretty confident there is nothing wrong with the machine other than it needing a top end rebuild.
Finally got around to getting a chance to work on this again and so I have some info!
Yes, as some had mentioned, I was curious about the accuracy of the HF tester myself as that just seems WAY too low. Plugged the HF tester into the nephew's 144, which runs like a demon, and it also showed 90psi. Checked both bikes with a different guage, Autometer, I believe, and both bikes read more what I would think at right about 185-190psi.
Result: A cheer from my nephew as the the HF gauge made a graceful arc into the dumpster about 30 feet away. hahha
Now what? Crap.
Others had mentioned the powervalve and doing some digging, I saw on TT I think it was, someone saying they had a similar condition and it turned out to be the PV. What exactly, I'm not sure. But I have heard that the little actuating arm in the KTM can wear at the point where it slides in the governor and can cause weirdness.
First, took off the inspection plate on the left side of the cylinder. The timing marks looked correct and when revving the bike, the PV did move though my amateur eye did sorta think it seemed to lag behind the revs. Not really sure how it is supposed to look so I shrugged and moved on.
Off came the right side case to get a look at the actuator. Removed that giant nut from the cover and ran the adjuster in and out several times. Sprayed things out. Spun things around. Looked mint. Damn. No obvious signs of something out of the ordinary.
Put everything back together. Set the timing marks back to their proper position. Fired up the bike to verify movement with motor running . . . . Holy Shit! It's not doing the weird running thing!!! WHAT?
Put the inspection plate back on the cylinder. Started it up. Petcock on, petcock off petcock on and no loading up.
Zip up and down the street and it seems to run smooth.
It seems like the only thing I did was mess around with the powervalve, that must have been what was causing the issue, no? It would certainly explain "Trying to ride it with the gas on is exciting as it will surge and accelerate unpredictably." I guess maybe it was sticking somehow? Though since I didn't take off the cylinder, it is inconclusive whether the issue is in the governor or in my goofing the the system, I possibly knocked something free in the PV itself.
Anyway, I just thought I would include this latest development, as vague as it is, for us to draw from next time someone has a similar, strange issue.
I have messed with mine to a crazy degree for tuning and felt what you felt in my journey.
Post a reply to: Can low compression mimic some symptoms of a bad float valve? Update!!