A Weird One

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2door

2door

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Sep 15, 2008
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Littleton, Colorado
This engine has a little over 900 miles on it. dax F80 that has always performed perfectly. Yes, I'm an Opti-2 (100:1) user and have always had good luck with it with this engine and several others. Today during a ride the engine suddenly stalled at a stop sign. Very unusual. Started right up when I pedaled away but stalled again the next time I stopped. I disengage the clutch at stops and it has always idled perfectly. I reached down, screwed in the idle screw a little and then found the idle was way too fast. When I backed out on the idle screw, the engine would die.
I rode home and started checking for the usual problems. Good fuel flow...the plug, an NGK B6 is clean, .025 gap...the choke is open...the air filter is clean...there is no air leaks at the carburetor, intake or anywhere else. I disassembled the carb, its clean. No probelm with the needle valve, jet and the bowl is clean. Fuel filter is full. Float level has not changed. If I screw the idle screw in far enough to get it to idle then the first time I twist the throttle it sits there and screams until I screw the idle screw back out then the only way I can keep it running is to continously blip the throttle. It will not idle on its own. I took the carburetor apart again and checked the slide for binding but it moves freely. There is no rough spots in the slide grooves where the idle screw or slide alignment pin rides.( I had a problem with that on another carb one time) Anything above idle speed it runs as it always has. It is and always has been the most powerful engine I've had and still pulls an easy 35mph...but will not idle. I checked both crankshaft seals, no problems there or evidence of air leaks. I'm running out of ideas here. I tried a new plug...checked the plug wire connections, I run solid copper core wire, and even tried a new CDI but the problem remains unchanged. There is no evidence of a head or cylinder to case gasket leak. Keep in mind this problem came on suddenly. It was not a progressive thing. I rode yesterday and it ran and idled perfectly. What the heck? Anyone have a suggestion?
Tom
 
Goat Herder

Goat Herder

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Apr 28, 2008
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2door said:
This engine has a little over 900 miles on it. dax F80 that has always performed perfectly. Yes, I'm an Opti-2 (100:1) user and have always had good luck with it with this engine and several others. Today during a ride the engine suddenly stalled at a stop sign. Very unusual. Started right up when I pedaled away but stalled again the next time I stopped. I disengage the clutch at stops and it has always idled perfectly. I reached down, screwed in the idle screw a little and then found the idle was way too fast. When I backed out on the idle screw, the engine would die.
I rode home and started checking for the usual problems. Good fuel flow...the plug, an NGK B6 is clean, .025 gap...the choke is open...the air filter is clean...there is no air leaks at the carburetor, intake or anywhere else. I disassembled the carb, its clean. No probelm with the needle valve, jet and the bowl is clean. Fuel filter is full. Float level has not changed. If I screw the idle screw in far enough to get it to idle then the first time I twist the throttle it sits there and screams until I screw the idle screw back out then the only way I can keep it running is to continously blip the throttle. It will not idle on its own. I took the carburetor apart again and checked the slide for binding but it moves freely. There is no rough spots in the slide grooves where the idle screw or slide alignment pin rides.( I had a problem with that on another carb one time) Anything above idle speed it runs as it always has. It is and always has been the most powerful engine I've had and still pulls an easy 35mph...but will not idle. I checked both crankshaft seals, no problems there or evidence of air leaks. I'm running out of ideas here. I tried a new plug...checked the plug wire connections, I run solid copper core wire, and even tried a new CDI but the problem remains unchanged. There is no evidence of a head or cylinder to case gasket leak. Keep in mind this problem came on suddenly. It was not a progressive thing. I rode yesterday and it ran and idled perfectly. What the heck? Anyone have a suggestion?
Tom
Click to expand...
Might be easy enough to just swap your carb and perhaps the throttle at the same time from another motor. Only reason I mention this is some how I think perhaps yours has a bind in it even though you have not found it? I ran into something similar on a old carb once. Only thing I can think of at the moment...
 
Goat Herder

Goat Herder

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professor said:
It acts like a vacuum leak somewhere or an idle system not functioning.
Click to expand...
Your right a vacuum leak will change to a poor idle when its cold but when it warms up it will scream howdy with lots of rpms.

A leaking crank seal will do some weird things too. It on one of mine would try to seal up the motor when it was cold. When that seal warms up air could get right past it.
 
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2door

2door

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All good suggestions so far gents, but I've already checked them all. What is odd is that this came on so suddenly; from one day to the next.
I'll take Goat's advice and switch out the carb from another bike tomorrow and see if anything changes. I'll keep you posted. Thanks for the input.
Tom
 
Goat Herder

Goat Herder

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Apr 28, 2008
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Goat Herder said:
Your right a vacuum leak will change to a poor idle when its cold but when it warms up it will scream howdy with lots of rpms.

A leaking crank seal will do some weird things too. It on one of mine would try to seal up the motor when it was cold. When that seal warms up air could get right past it.
Click to expand...
Got another educated guess here. A leaking seal can give the ghost to a air leak at higher rpm's and load the motor with more air temporarily. Something that is harder to leak check at Idle with carb spray etc. Because not only in this scenario its not leaking enough at Idle a liquid from evaporating carb spray is still thicker than air too.. I had one do exactly this to me..and I was trying to make up my mind where the leak was. [a crank seal on the clutch side was my culprit there]

It might be annoying as all heck when if'n it revs. try bringing the Idle up higher then leak check with spray? I know that could be terrible advice lol. I would used Brake cleaner because it deadens the motor instead of revving it for this trick or simply use carb spray and yeah chlorinated brake cleaner smells horrible lol. Not for inhaling either. I don't recommend it. For somebody reading this do it in a well ventilated area then still leave just go away for awhile. Use ''non chlorinated'' brake cleaner is the right thing and really only suggested in my opinion for this purpose.

The only other option is to pressure test the motor With soap and bubbles.. I like the first idea becuase it can be caught in the act. Which is my favorite way to prove things.

I think 2's is the carby out of idear's at this point. I know I am speaking to my self at the moment but somebody else might get something from it?
 
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2door

2door

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The problem followed the carburetor.
I took another bike out this morning and it ran fine. I came back and switched carbs, not changing anything else and the engine I was having trouble with ran perfect; good idle, good top speed. I put the carb back on the bike I switched from and it behaved exactly as on the other bike...No idle unless you screwed the idle speed screw in then the first time you twist the throttle the idle speed went through the roof. I've had the carburetor apart and examined it thoroughly(three times) and can find no problem with it. There is no bind in the slide (barrel) and absolutely no air leaks. I'll pick up a new carburetor from dax and keep this weird one for a paperweight.
One theory: I know that Norm has found that the taper on the needle is supposedly critical. Could miles of vibration wear the needle and alter its profile enough to cause erratic running conditions? Visually, there is no sign of this but its all I can come up with right now. If I ever discover the reason I'll let you know.
Thanks for the suggestions, guys.
Tom
 
2door

2door

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Littleton, Colorado
You know, in retrospect, the needle theory just doesn't pan out. This problem came up in the middle of a ride. I had ridden, probably two miles from home and made several stops before the no idle problem came up. One stop and everything was normal, the next stop it wouldn't idle anymore. That's what's so darn weird about this. I rode again, twice, before I started looking for the problem. If it was a wear issue on the brass needle then it should have been a progressive thing. Not just immediately from one block to the next. Whatever, Its running good again with a different carburetor so I'm happy. Just wish I could pin-point the problem for the benefit of anyone else who might have the same issue crop up.
Tom
 
breno

breno

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Aug 19, 2010
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I know this sounds stoopid but I'd check all the electrical side check magnetofor ohms check for no continuity over kill switch and check all cables. Only saying this to rule it out.
Get a comp tester and check ur comp. I think anything over 120 is cool but I could b wrong might need to further investigate and then theres fuel/carby.
I'm sure u'd know if u had a seal leak anywhere over it.
Simple stuff I know but I try to always go back to basics and it hasn't failed me just yet.
 
breno

breno

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So sorry read it again my bad pls disregard my posts 2door, etc. on ya mate for findin a carb issue
 
Fossil

Fossil

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Mar 15, 2008
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It definitely sounds like the idle circuit in the carb picked up a small blockage. Blow all the passages out with compressed air preferably from the inside the throat. I set a carb down on a flat surface and just using my hand to cover the top opening I put the air gun nozzel between my fingers and pressurized the carb that way. That pushes air back through all the passages the opposite direction the fuel normally flows. That always works for me. After that a good flush with carb cleaner and you should be good to go.

Jim
 
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2door

2door

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Sep 15, 2008
16,302
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Littleton, Colorado
Fossil said:
It definitely sounds like the idle circuit in the carb picked up a small blockage. Blow all the passages out with compressed air preferably from the inside the throat. I have set a carb down on a flat surface and just using my hand to cover the top opening. I put the air gun nozzel between my fingers and pressurized the carb that way. That pushes air back through all the passages the opposite direction the fuel normally flows. That always works for me. After that a good flush with carb cleaner and you should be good to go.

Jim
Click to expand...
Jim,
Thanks for the advice but I can assure you that the carburetor is clean, inside and out. I had it apart several times examining every aspect and there is simply no reason for the idle problems. Anyway, the subject is moot. That carb is now in a scrap parts bucket and a new one has been installed. No problems now. Thanks again for the concern.
Tom
 
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