Backfiring through the carb after rebuild

[eprovenzano]

I completed a rebuild of our 99 400EX because it smoked like a 2 stroke. The rebuild consisted of a new wiesco piston and rings(std bore), redoing the valves with new seals, and the having the cylinder honed.

I put the engine back together and at 1st it would not run. The valves were way out of adjustment. I adjusted the valves and she started right up.

Now for the problem... after its running usually while idling, , it will backfire through the carb. It did not do this prior to the rebuild. My guess is the valves (exhaust valves) are still not adjusted correctly, but I'm just guessing

Any suggestions?

[UNBROKEN]

does this happen when the motor is hot to and when was the carb last cleaned

[eprovenzano]

It does it when the motor is warm. Its been a while since the carb was cleaned, but it did not do it prior to the rebuild. Something has changed since the rebuild. The only thing I can trace it to is the valves. I've rebuilt many 2 strokes, but this is my 1st stroker. I don't believe the piston and rings would cause the backfiring. I'm leaning towards the valves, (exhaust valves) as the culprit.

[JOHNDOE83]

popping back through the carb means its to lean.....

[eprovenzano]

I had a little time today. I pulled the valve cover off, and rechecked the timing. It was dead on... I then checked the valves and the exhaust were a little too tight. I adjusted them a little. The bike will now idle for several minutes, and then a slight backfire through the carb. Sometimes it’s enough to kill the engine, other times its not. If it does kill the engine, just hit the button and it fires right up.

As my son put it, we are so close. So I think I still need to adjust the valves some more. Boy we are so close.... I just need some more time to dial it in.

[Wheelie]

It sounds like the intake valve clearance is too tight.

[eprovenzano]

I think I will adjust the valves again just to make sure all is well, and as others have suggested, richen it up a bit.

My question is how do I richen the mixture.

Thanks for all the help

[JOHNDOE83]

If its backfiring at idle or decelleration change the pilot jet to a bigger one... stock carb try a 42 pilot jet... 450r carb try 50 pilot. They are located in the float bowl area on the carb, the bigger brass one is the main jet, and the little brass one is the pilot jet.

Also, first try turning your air/fuel mixture screw all the way in, then turn it three turns out... if it still backfires after that, and checking the valves again, get the bigger jets. If you do get the jets, its as easy as pulling the carb off, taking off four screws on the bowl, changing one jet, and putting it back together. Then you done, no more backfiring...

[eprovenzano]

I finally has some time to work on the 400. I re-adjusted the valves, and now it idles pretty well, although it will still backfire once in a while. Low range seems strong, mid is clean, but the upper rev's are a little blubbery. I pulled the plug, and its wet and black... so I think I'm running a little rich.

I'm planning on having my son ride it around the yard to get everything seated (valves and rings) before making any more adjustments. I fell like I'm chasing my tail. Once the valves and rings are seated, I'll have one final adjustment on the valves and slight tweak to the carb and all shoud be well.

No sence in getting into a big hurry, we stil have 25 inches of snow on the ground... :(

[Wheelie]

What are your mods and jetting?

[eprovenzano]

I'm not sure what the current jetting is but it was fine before the rebuild. The rebuild consists of of a Weisco 10:1 compression pistion std bore, with the valve seals replaced. Mods opend air box, and HMF slip on. The jetting was adjusted when the HMF was installed prior to my getting the quad.

Currently it will ilde ok, but not like it should. It will rev, but not clean on top, it gets blubbery. I checked the timing and it is dead on. I keep coming back to valve adjustments. The valves are set according to the specs. I was told to open the intake vlave setting to .006. So this will be my next attempt to clean her up.

[eprovenzano]

I called in a favor and a friend on mine came over to give me a hand. We re-adjusted the valves, to spec as some were a little too tight, while others too loose. The quad will now idle fine, and no more backfiring through the carb. But, it would not rev cleanly, and was backfiring through the exhaust. I also pulled the plug and it was really black. So I now know its running a little rich, amd more adjustment was needed. I rotated the carb so I could access the pilot screw. I counted the turns in to see where it was set (2 and 1/2). I tried it at 2 turns out, it was better but not perfect.... we're so close. I think another 1/4 turn in and we got it. I'll make the adjustment and will try it again.

[JOHNDOE83]

Try www.jetsrus.com they explain why its backfiring....and running the way it is.......

[eprovenzano]

The backfiring was still occurring... GRRR.. So I stepped back, and started to review a few things. I was going to disassemble to recheck the timing... again... when I noticed the smell of gas... hmmm… When I took the carb of, I looked at the bottom of the carb intake boot. It was wet... closer inspection shows a crack.... Me thinks I've found the problem.... The gas I smelled was coming from the crack in the intake boot, and the noise I've been hearing (The best way I could describe it ) was air escaping through the crack... killing the engine

I was ordering the boot on-line when I noticed something strange.... The new one I ordered bolts to the spacer on the head, while mine simply slides onto the spacer. I did not like the way the intake was set up as it wasn’t very secure. There must have been a design change, and they corrected the intake.

I’ll have the part in a few days, and I post the results.

[eprovenzano]

New parts arrived.... :thumbsup: I put the new intake on, and made some final adjustments to the carb... I can finally say, the rebuild is complete!!!


The problem was in the intake boot leaking air.. Also now that I've increased the compression (both with a higher compression piston, and new piston and rings), I had to make some tweaks to the carb. The pilot screw is now at 1 1/4 turns out (stock is 2 and 1/4).

The bike starts easily, idles well and revs out as expected.

Thanks for everyone's help.

[eprovenzano]

I thought I'd update this thread one final time.


The weather is finally cooperating after a long cold wet winter... My son took the quad for its maiden voyage since the rebuild. Up until today, he hasn't been able to ride it at all... school, girls, cars.... seem to get in the way. He went for a nice 3 hour ride. His assessment..... The old girl grew some balls.... the power is stronger everywhere. It runs great, other then the idle being a little too high (easy adjustment). I guess replacement of the 10 year old piston, rings, and valve seals makes a difference in performance.... :ermm:

Thanks to all those you helped, your assistance is appreciated
.

[bigd's ex]

Glad to hear ya got running good. Where do you guys ride? I live just down the road from you, in Warwood.