Help!! 02 yz426 backfiring when kicking

Hey guys this is my first post on TT, and have seen a lot of good info and advice to this point. I have a 02 yz426 that backfires when I kick it over to start. It was running ok a few weeks ago when I checked the valves and found the intakes to be a little loose. I added thicker shims (between 175-185 mm and the valves are within spec now) but it wont start. I did not touch the exhaust shims and I set the carb jetting back to all stock over the winter. I am fairly sure cam timing is correct (on TDC the intake dots are at 9:00 12:00 3:00 with 12 pins between the exhaust 10:00 1:00 4:00) I am under the impression the spark fires every crank revolution so as long as cams are set off TDC timing can not be 180 degrees off? I check all the electronic ohm tests in the shop manual and found no concerns that way. Any other ideas? thanks

You are correct, the spark occurs on each TDC, meaning that the cams are the only thing that determines which TDC is which, and you can't be "180 out".

When you say, "backfires", do you mean that it pops out of the exhaust after kicking a few times? If so, it's pumping fuel/air mix into the exhaust and then igniting it on the next kick or two when it tries to start. It's probably just being cranky, but you might want to clean or replace the plug. Tweak the fuel screw a 1/4 turn the opposite of what was done to the main jet. If you went leaner, go a quarter out. If you changed the pilot, go a half turn. Be sure the slide isn't in backwards or the plate on the slide in the wrong position if you had that out.

426's can be a pain to start if they're started wrong, but the great thing about them is that they almost all start the same way all the time under any given set of circumstances. Be sure you haven't changed your routine any from what always worked.

my 01 426 has backfired a few times when starting..

I think it has been cos i havent followed the drill sometimes.

I can start it first kick in the garage on a sunday am, but when youre bogged down with mud and knackered, its hard to get the drill right, especially if you cant get a good footing to the left.

Just my two penneth

J :)

The only time I've had a similar problem, a change of the plug sorted it out and it started first kick. Might be worth a try.

It also sounds like the last fouled plug I had too...

Btw, you're not giving it any gas when you're kicking are you?

Thanks for the info guys, I'm heading out to the garage and try some things.

No, I'm not giving it any gas when kicking, I learned that the hard way when I first got the bike. I was limping for the next few days due to a soar leg.....

If it was running fine before you adjusted the valves then I'd have to think it was something you did when adjust the shims. Since you removed the cams then I'd go back and check and recheck the cam timing. Don't forget to make sure the crank mark is in the proper location before setting the cams in place. Also, after setting the cams make sure they don't move when you release the cam chain tensioner. You can ensure this by installing the exhaust cam first and ensuring the chain is tight by rotating the cam counterclockwise (when standing on the left side of the bike. After installing the intake cam you should be able to release the cam chain tensioner and everthing should stay in place.

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