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07 KXF 250 starting Issues after rebuild, Backfiring

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Hi - first time posting here, my name is Matt.  I have built and raced 2 stroke karts and bikes for many, many years, this is my first time with a "modern" 4 stroke. I'm getting back into this after 10 years away from it as my son wants to try racing.  I searched the forum for similar problems and have followed all the advice I found, but I hope I don't have a CDI box issue, so I thought I would run my scenario by the collective before I go that route.

 

 I have a 2007 KXF 250 that i just put a crank into, it started up 3rd kick after the rebuild, but wouldn't idle on its own - seemed Ok when I held it open. I adjusted the idle and it fired first kick, but still wouldn't idle - throttle response seemed weird - almost delayed a bit if that makes sense, but honestly I wasn't revving a new motor on its first run, thats not correct. I let the throttle go and it shut off, it never fired again, I even push started it up and down the street with not even a hint of ignition.

 

Right now, when its kick-started, I get backfires.

 

details:

 -Cam timing OK, Intake might be .25 - .5 degree advanced, EX spot on.

- Valve lash .004 - .005 IN .006 - .007 EX

-Fuel, 91 Oct Shell, no Ethanol, tank cleaned

 - Plug - New from Kawi

-Oil/FIlter -10w40 SB4 Kawi non-synthetic

 

today I took carb apart, cleaned it out. I replaced with NEW parts:

 

- Main jet 

- Pilot jet (it was plugged)

-Pro Circuit air/fuel screw (poor location - sheesh!) factory manual says 2 1/8 turns, it was at 4.25, I set it to 2 1/2 turns

 

other:

 

-TPS is in good shape, not moved

-All elec connections are good

-Air filter is good

- Float level good

 

 

All I get is the same back firing - but I did get a small 'trailing' ignition - thats where it kinds fires a fades out

Well, sorry for th elong post - i appreciate any insight, thanks!

-MWC

 

 

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Hi Matt, Time should be adjusted perfectly at TDC verified with a screwdriver with exhaust cam dots in the 9:00 and 12:00 position relative to the top of the head and the intake dots at 12:00 and 3:00 relative to the head.  Count 12 pins from 12:00 dot to 12:00 dot on exhaust and intake, including the pins that land on the dots.  There is no way intake can be advanced this way.  Count those pins and confirm you count 12.  Just mentioning that because you indicated intake may be advanced while exhaust was on.  Should not happen.

 

Next, the fuel screw is a joke.  Replace it with an R&D remote fuel screw if you value your time and fingers.  Make sure the O-ring goes into the carb first, then a washer, then the spring.  When that O-ring is missing or messed up, you aren't going anywhere.  Same with the O-ring on the hot start.  Make sure the hot start plunger is working properly and that the O-ring on the front of it is in good shape.  To start the thing, you should need the choke pulled out.  

 

If it started once and won't start again, make sure you have gas (sounds crazy to say that) and make sure fuel is flowing to the carb.  Just pull the line from the carb and let it drain into a suitable container.  Should be a good flow.  Also, drain the float bowl from the bottom.  Maybe some debris got caught in there.  If nothing comes out of that drain, that could be the problem.

 

Others will surely jump in.  Tough to diagnose from a distance, but maybe something above will resonate.  It does sound like a fuel flow issue, though.

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Ran in to the same thing today after a total rebuilt, new head, new cylinder, and new cases. Ran fine while on choke but as soon as I turned off the choke it would backfire like hell on decel, hard to start, and wouldn't idle.

 

I have built several of the 2009 KX250F and have always had trouble with the timing. I had set the timing on the bench and installed the motor. Checked the timing again and it was way off. Reset the timing and fired up first kick and still had backfiring and not idle.

 

It is due to a lean condition, I had the Pilot Screw at factory recommended 2 1/8. I cranked that baby up while it was running to around 3 to 3 1/2 and the baby sang like like the winner of American Idol but still has a tiny bog at the low end.

 

I am going to try a 42 on the slow speed jet to see if that helps, if not then up size the leak jet, from my research, low end bog is due to the accelator pump issues. 

 

Try checking your timing, at TDC, (remove the small and large cover on the magneto side, view the edge of the magneto while turning the crank counter clockwise through the larger hole with a 14 mm socket there should be three marks, you are looking for two marks that are right next to each other, once the second mark aligns with the notches in the smaller window, you should be at TDC) with the left pin at the center line of the top of the head and the right exact opposite, start counting pins on the chain tiil you reach 28, they should be exactly across.

 

As for the Pilot Screw, go buy a adjustable one, Rocky Mountain has a Tusk, it is a time and finger saver.

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Hi - thanks for the replies. I believe the timing is spot on, I counted the 12 pins no problem, I might've thought the one cam was advanced because of the perspective when looking straight at the split line on the head in relation to the one cam versus the other.

There is plenty of fuel flow, the bowl was full. I had my son try starting it for about 15 minutes and it just would backfire again. when I pulled the plug it was wet. The primary jet I put in yesterday was a 35, it had a 40.

My son also told me that the bike would not run off the choke when the prev owner had it - nice of him to share that tidbit of information with me after I put the motor back in...lol - kids!

The pic of the mag cover hole shows the 2nd line centered on the slot

Edit: sorry pics are sideways, no idea how that happened either, geez, I'm batting 1000 right now -lol

image.jpg

image.jpg

image.jpg

Edited by MWC

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i took the carb right out and it's apart - I had the O ring on air screw on the wrong side of the washer. But I don't know if that fixed it because I'm soaking the carb right now but I have another question:

When I crank it over I hear compression leaking into the crank case, a lot more than I would expect to hear - and that's with the spark plug out. Could I have installed the compression ring upside down? There was no marking dot but there was a tiny bevel, I thought I got it right, but now who knows? I don't have an adapter for that small thread plug or I'd do a comp check

Any ideas?

Edited by MWC

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That noise is normal.  I had the same problem with the adapter and rigged one up.  Went to NAPA and got a brake line fitting that fits perfectly in the spark plug hole, then went to Home Depot to get brass fittings that bridged the gap between what I got from NAPA and the other end of the extension.  The Home Depot guys know when I am rigging something and they always try to help.  I suppose you could just order one.  Some rings have orientation, some don't.  I believe the Wiseco instructions said "if there is a bevel . . . etc, then orient this way", but I never remember having to worry about it.  If that o-ring was below the washer, it makes a bigger difference than you think.  That fuel screw is very sensitive based on my experience.  What are you soaking the carb in?  I ask only because the AP diaphragm and the gasket on the plate (the one where the hole goes on the bottom) are very sensitive to carb cleaner.  I disintegrated mine.  It was a bad learning day.  Also, there are tiny o-rings in the AP cover that need to be there and intact for the bike to run right.  Make sure they are.  Hope all that helps and doesn't distract you too much.  R&D remote fuel screw is your friend.  I bought a knock off variety of the one you did for my 06 and it did not work.  Maybe yours will.  Better than what you had in there.  Keep us posted.

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Well, i cleaned the carb up and I put it together - the R and D flexi fuel screw was not in my city, so the pro circuit unit - now installed correctly - will have to do. I wanted to do a comp test before I put it back together, but that 8mm plug made it a little tricky. I finally found a compression tester with an 8mm adapter and tested it...0 compression! I sprayed a little light oil down there and did it again - 0 again.

Doh.

Top end coming off now...the first 4 stroke is one to remember!

Edit: I never soak rubber parts - I know that one too! I was using LPS heavy duty degreaser, it's as close to 111 Trichloroethane as I can find nowadays

Edited by MWC

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You should still have some compression even with the ring installed incorrectly. My guess is a valve is not seating right and most likely the exhaust side.

before taking it apart, use your compression gauge hose and put compressed air into the cylinder at top dead and listen , make sure your compression release is locked out.

I do this now before I button up any engine. You will get some blowby into the case but none should be coming out of the exhaust or intake.

Edited by Altune

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I checked the valve seats before i put it back together, there was no problem. It is unusual, but I'm getting zero compression. It's a brand-new gauge

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Easy way to check for valve seating is to install the valves, flip the head over and put some liquid in the recess area , use a rag and blow gun, inject air into the exhaust port, intake port and look for bubbles at the valves. You should not use full compressor pressure 30 psi should be ok, too much pressure will unseat the valves.

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I just took it apart and it appears that one exhaust valve was actually kissing the piston - I doesn't look as if anything really happened. It appears to be seated correctly and there's no leaks at the valve/seat that was touching the Piston. But how did the happen in the first place?

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Hate to say but your valve timing must be off. Sometimes the sprocket spins on the cam , ( sheered dowel )

take a closer look. here is the correct cam lobe position in relation to the timing marks.

post-36861-0-31940400-1368505894.jpg

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As you see the exhaust lobe should be centered on the sprocket hole.

I can't see any other reason the exhaust valve would come close to the piston.

 

Al

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Thanks for the info and the diagram is great - I know for the Piston to contact the valve, there has to be a misalignment with the exhaust cam timing - I just don't understand how it got that way when I was very very careful when aligning the cam timing marks. I guess It's just how it is sometimes! I'm gonna head out to the shop and pull the valves out, make triple sure nothing is bent - thanks again!

I'll update again after I go get the new parts...again lol!

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one last thing, use a long screw driver or welding rod, insert it into the spark plug hole and feel the rise and drop at top dead center to confirm the marks on the flywheel are correct, sometimes the keyway sheers off. There should be a "dead spot" where the piston does not rise or drop and that should get you very close.

Al

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The strangest thing just occurred. I was pulling off the buckets, and when I took off the bucket for the suspect exhaust valve what was staring at me but one of the valve locks, just floating on top! how bizarre is that - I have no idea how that happened I checked the valve lash on that very same exhaust valve, got .007".

I took the rest of it out and the stem is chewed up where the lock was, so I need new a valve and new locks now as well

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Update: went to kawi dealer for new parts and that ex valve is the most expensive valve in the world at $91.22!! So after all is said and done I'm out another $150 - but at least it should work now

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I had that same thing happen to me. I had basically set up the head, did the shimming and had it ready to be installed. A "Friend" of mine came by to visit and he did say he saw the head as I had done some minor port and polishing.

Installed the head and found one of the shims had mysteriously moved out of the retainer pocket and caused the valve to be left open all the time. I had a stern conversation with him and he admitted he had turned the head over and one of the buckets fell out, he put it back in wrong... oh boy so much for friends.

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Update: I got it mostly back together again. Hopefully someone can tell me why the exhaust cam gear/decompression mechanism is contacting the chain guide?

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Update: I got it mostly back together again. Hopefully someone can tell me why the exhaust cam gear/decompression mechanism is contacting the chain guide?

 

Did you put the guide in before putting the head on?  You have to, because there is a pin on the guide that slides into a notch on the cylinder and it is locked in by the head.  If you put the guide in after, you need to pull the head off and seat that pin first.  If you already did that take a pic.  You reset the tensioner, right?

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