Posts
28
Joined
10/13/2014
Location
US
Edited Date/Time
11/4/2014 1:00pm
Hi All,
I have a 87 KX 250. Just rebuilt the entire motor, and new coil, stator, & plug.
When I pull the choke and try to start it about the 5th -6th kick it will backfire. Eventually it will start and run normal.
When I check the plug before starting, it's wet. I've tried changing pilot jets, adjusting the float, and I still get the same thing when starting. I even swapped out another CDI boxes and both had the same problem.
After it warms up, I usually can't get it started if I stall it or shut it off.
I pulled the float bowl off, held the floats and turned the fuel off and I did not see any fuel coming out. I also submerged the floats in fuel and they did not leak or have any holes.
I'm running 32:1.
Could it still be a leaky needle and seat? And could the fuel mixture cause the starting problems?
I've been at this problem for months and still can't find the solution. I feel it has something to do with excess fuel.
I changed the pilot jet from 45 to 42.5...but noticed that new Mikuni VM38 carbs (I have a VM38SS) come with a 30 pilot jet. Wondering if I should buy a 30 size pilot.
Tomorrow I'll see if I can put a timing light on it just to rule that out....the woodrift key is fine.
dinky,
I have a 87 KX 250. Just rebuilt the entire motor, and new coil, stator, & plug.
When I pull the choke and try to start it about the 5th -6th kick it will backfire. Eventually it will start and run normal.
When I check the plug before starting, it's wet. I've tried changing pilot jets, adjusting the float, and I still get the same thing when starting. I even swapped out another CDI boxes and both had the same problem.
After it warms up, I usually can't get it started if I stall it or shut it off.
I pulled the float bowl off, held the floats and turned the fuel off and I did not see any fuel coming out. I also submerged the floats in fuel and they did not leak or have any holes.
I'm running 32:1.
Could it still be a leaky needle and seat? And could the fuel mixture cause the starting problems?
I've been at this problem for months and still can't find the solution. I feel it has something to do with excess fuel.
I changed the pilot jet from 45 to 42.5...but noticed that new Mikuni VM38 carbs (I have a VM38SS) come with a 30 pilot jet. Wondering if I should buy a 30 size pilot.
Tomorrow I'll see if I can put a timing light on it just to rule that out....the woodrift key is fine.
dinky,
dinky,
With the bowl off, I've held the floats closed and I didn't see any fuel leaking past the needle...it's not same as when everything is assembled, so I'm also pointing my finger as this might be a culprit.
dinky,
The Shop
I had the float set according to the manual, 27mm, which I think is too high since the float bowls will bottom out on the casting if I push the floats up. The floats will contact the needle valve and it I push it to where the spring compresses on the needle valve, the floats will touch the carb housing. The carb model is Mikuni VM38SS.
Needless to say, it did not start like this. So I lowered the float 2mm and she didn't start either. It also didn't backfire like it normally does when I get it started.
Today, I'll lower the float again. I've had the bike running with a float height of 31mm....that seems to be the magic number in getting the thing started, but it takes, about 10 kicks and a backfire to get her running. When it runs, it idles fine and with the limited amount of yard space I have (just 1st gear riding) it has power and runs normal. But once it gets up to temps, I can't start it if it stalls or I shut it off. I do remember I started it once though holding the throttle wide open.
I wish I could borrow a carb, but unfortunately, there's no one around here.
Pisses me off because I put so much work in her...
dinky,
I do wonder if you might have an intermittent stator though that may only keep the spark when you get it cooking? You do have a good spark with the simple check don't you? I had a similar issue on a KDX.
I have 2 CDI's and I had the same starting issues with both. I've heard when CDI's go bad, you either have no spark, or a weak spark.
It has new:
coil
reeds
stator
plug
I know if I keep lower the float, eventually I'll get her running....but I'll still probably have the backfire and hard starting issue.
I also want to check the timing using a timing light.
Oh 1 more blow......I usually get parts from partzilla, well I checked today, and they took 1/2 the 87 models away, including my KX250...so good luck to me getting parts...
dinky,
I kept lowering the float 2mm at a time hoping she would start but didn't.
The interesting thing is, when I started at factory height, 27mm (which I think is too high because the floats will actually hit the carb when I depress the floats) the motor wouldn't do anything, like it didn't have spark. Not even a backfire.
So I lowered it 2mm more and I could hear little backfires in the exhaust but still no start.
Lowered 2mm again (about 31mm float height) and she was now backfiring louder and more times.
So I'm confused...I don't know what this means?????
I'll lower the float again and see what happens.
BTW, this was with a new plug.
dinky,
Left-side main seal leaking? Causing a lean condition?
Pilot jet is clean also...I actually took the carb apart and clean it prior to.
As far as the main seal, well I replaced just about everything internally even the seals. There's no signs of pre mix on the stator side of the case.
I'm thinking about running a pressure test just to see.....
Could a lean condition cause backfire through the exhaust? I'm assuming since the plug is always wet that it's a rich condition.
Plus when she does run an reach operating temp I have a hard time starting her...that is, if she starts.
dinky,
And, I would think a back fire through the exhaust could be an exhaust leak- not sure. Also, does your bike have the old points system or just a regular stator like the newer bikes? Could the timing be too retarded somhow?
But I've been lucky so far having her start at the right float height...I just haven't gotten to the "right" float height for her to start.
The cold weather now might be playing a roll also.
I'm going to try making a float bowl plug with a tube so I can accurately set the float....then she should at least start.
dinky,
Like I said, I think even the smallest amount of gas in the carb should be enough for the pilot/idle jet to suck up the fuel. I think only too high of float level would flood the cylinder. I wonder if the jet is way off size. I assume you already checked for proper jetting. I feel like I might be bringing up old stuff by this point. LOL.
(No reply necessary but keep us posted! I'm curious what you figure out.)
Ryan
Pit Row
I adjusted the float level using a clear tube plugged into the bottom of the float bowl and ran it along side the carb to spec, and let it sit like that for about 15-20mins. to make sure the fuel level didn't rise....
Backfired still and more often...It did run for about 2 sec, but died when I opened the throttle (is was a cold day in the low 40's).
The interesting this is, I took the exhaust off and about 1/2 cup of black oil poured out....I'm assuming it was the unburned, back fired fuel.....
I'm wondering if there's any of the black oil in the crank section? Probably just blew out the pipe instead of going past the rings...
I did a compression check with my new tester when the engine was cold and about 40 deg. outside and got a little over 120psi.
When the weather was warmer, I got 150psi, again not warming up the engine, and a leaky compression tester.
Curious why it's low considering I just rebuilt the entire engine...but when I looked through the intake and exhaust ports the rings, piston and cylinder looked normal so I'm assuming I should get better reading when the engine is warmed up...
Nevertheless, it should at least start.
I'm thinking the cold weather is keeping it from starting besides the carb or whatever issue.
dinky,
The carb is original, and I've cleaned it, changed, pilot and main jets but still the same thing. It would originally backfired before starting when the weather was warm.
Furthermore, when the engine warmed up and it stalled or I shut it off, I couldn't get it started.
Would a faulty carb cause it to backfire and hard starting?
I want to try another carb, but I don't have any to borrow.
dinky,
Here's a link when I started it a 3rd time. You can see that I couldn't get a full kick a couple of times, but it started without backfiring.
Yesterday like I said, I pouring about a 1/2 of black premix out from the exhaust pipe. Today I took the reed cage out, inserted a tube inside the bottom of the crack and blew into the tube. I could tell there was fuel in there since I heard some gurgling.
It didn't sound like much but it shouldn't be in there.
Question is how do you get out? Can I just turn the bike upside down?
Anyway, my theory is, after the rebuild it started fine...then at some point it started to backfire before starting....Now I can get it started but it backfires so I'm putting more fuel in the crank trying to start it.
At some point the carb must have started leaking internally and slowly was putting fuel inside the crank. It would backfire because it's pulling that fuel in the crankcase plus from the carb. Now there's too much and it needs to get removed....I never rebuilt the carb...I just cleaned it..
Seems like it would explain my issues....
dinky,
Make sure the timing is set to standard specs.
Make sure jetting is in the neighborhood of the standard specs.
Make sure the float is shutting off the fuel supply when the bowl is full.
Make sure the flywheel and stator are not loose.
Verify a nice blue spark.
Perform a leak down test and repair any leaks (if found).
Perform a compression test. The service manual shows a usable range of 109psi to 171psi but I prefer to keep things above 150psi.
Make sure the piston is not in backwards. The arrow on top of the piston should point to the exhaust.
Make sure the reeds are in good shape.
If all of these things check out OK there is a very good chance it will run.
I'll go through them again....
Originally I had everything setup according to spec's, but it developed this backfire before it started.
That was during the summer. And I started replacing things but it didn't fix the problem. Only thing I didn't replace was the carb, but I don't have the funds or one I can borrow right now.
I'll be busy in the next few days doing tests.
dinky,
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