My 426 has me confused!! Help !!

[yz426em]

I had a post on my 426 awhile ago, this is what is goin on... I rebuilt the top end on my bike and installed hot cams intake and exhaust with auto decomp. I have about 15 to 20 hours on the bike since the rebuild and i had no problems with it at all until about the 12 to 15 hour mark. The machine started up within 2 kicks after the rebuild no problems ran fine. Now if u let the bike set it will not start for anything, after about 20 kicks it will run like crap until its warm, once its warmed up it will start right up time after time, but as soon as it cools down it will not start back up!

I have cleaned the carb over and over it is spotless, i also checked the valve clearances, and put in a new spark plug. Any help on this would be great my head is spinning and im tired of working on it ?

[grayracer513]

Now if u let the bike set it will not start for anything, after about 20 kicks it will run like crap until its warm..... as soon as it cools down it will not start back up!

When it's cold, does it act lean? That is, does it not put up with opening the throttle quickly, that sort of thing?

Check the air jets and the air feed to the choke at the mouth of the carb for obstructions, and check your hot start valve for seating fully closed.

[yz426em]

i kicked it like 15 times i thought it was flooded so i took the plug out and it was still dry. I did check the air jets at the mouth of the carb and they are clean. How do u check the hot start valve? Could the timing be off?

[grayracer513]

Once it's running, raise the rpms just above idle and open the hot start. Then close it. There should be a noticeable difference.

When you cold start, do you give it a couple of twists on the throttle to prime it? Use the choke? Has the bike generally been easy to start, or have you always had trouble with it?

The timing is fixed, and can't be changed. Cam timing can change, but you say it runs good when warmed up. Ignition timing can drift out if the CDI unit partially fails, but there again, it runs good hot.

[yz426em]

it used to start by pulling the choke and kickin it 2 times, now u can prime it with 2 twist of the throttle and sometimes it helps it fire, but like i said i have rode this bike quite a few hours and never had a problem starting it.

[Batman]

Those are classic symptoms of tight valves/no clearance but since you already checked that I would double check the clearance and if you find that it's normal do a leak down or compression test . You can check valve sealing with a compression tester by squirting some motor oil into the cylinder to seal up the ring end gaps . If your still low on compression (assuming that's problem) then you've got valve sealing issues . Even if you sent your head to a machine shop for a full valve job dont assume that isn't the prob . Remember that the valve clearance will change with new cams and more importantly the clearance will be tighter after a valve job especially if it's a shitty one . I've also noticed that some automotive machine shops will grind the valves at an interference angle so that they peen into the valve seat and that too will tighten up the clearance over time . It couldn't hurt to fine tooth comb inspect that auto-decompression mechanism either .

Hope that helps .

[yz426em]

I'm gonna do a leak down test tommorow on the motor and see if i can gain any ground their, if not i will probably re-check the valves again, Thanks for the help!!

[Batman]

I'm gonna do a leak down test tommorow on the motor and see if i can gain any ground their, if not i will probably re-check the valves again, Thanks for the help!!

If the valves are no clearance it'll show up when you do your leak down ....

[slowrider]

Let me know if you find a fix, i'm in the same exact boat. I did find that if i twist the throttle 2 times that it fired up on the second kick. I was having the same exact frustrations you were and i dumped like 2400$ into a new hooped up engine rebuild and it went from easy starting to kicking a hole in my boots starting.. good luck

[yz426em]

I will for sure post it when i find something out, hopefully today the leak down will tell me some info or at least set me in the right direction. I also put alot of money into mine and this is not making me happy to say the least!

[yz426em]

Does anyone know off hand what % the leak down should be at?

[Batman]

Less than 5% is good . I've typically seen around 3% on new from the factory engines . If your motor is so leaky that it's hard to start it'll be obvious , you'll hear air hissing out of the pipe or carb or crank case or all 3 .

BTW , make sure you're at TDC on compression stroke and be careful with whatever tool you're using to turn the crank . I've heard of arms being broken when the compressed air in the cylinder blows the piston back down , I've busted a knuckle or doing leak downs ........ I wouldn't be too nit picky on leakage % , you're trying to find out if you HAVE a motor not whether or not it's tight ass racing motor !

[grayracer513]

Unless the engine feels like it has no compression, I think you're barking up the wrong tree looking at reduced compression causing a no start situation. I say that because YZ450's and 426's upgraded to auto decomp start just fine on roughly 40-50 pounds cranking compression, or less.

If you're convinced it's not the carb or a bad spark plug, and the compression feels at all solid, maybe do a strobe test on the timing to check the CDI unit's operation.

[Ga426owner]

Let me know if you find a fix, i'm in the same exact boat. I did find that if i twist the throttle 2 times that it fired up on the second kick. I was having the same exact frustrations you were and i dumped like 2400$ into a new hooped up engine rebuild and it went from easy starting to kicking a hole in my boots starting.. good luck

I agree - you should always blip the throttle a couple of times to prime it ....

You said the plug was dry - indicates no fuel .... do you drain the carb after each ride as I do? This does make starting harder...but keeps buildup in the carb a lot less - as I use U4 fuel

[Batman]

It'll start w/40-50 lbs compression because one good fire in the cylinder will rotate the cam fast enough to let the exhaust valve close back up . This wont be the case if the valves are being held open by the cams when they're supposed to be closed . Plus one of my troubleshooting maxims is ;"if it worked normally before I messed with it then I'm probably the one that broke it!".........Dont ya' just love armchair troubleshooting !

[grayracer513]

It'll start w/40-50 lbs compression because one good fire in the cylinder will rotate the cam fast enough to let the exhaust valve close back up . This wont be the case if the valves are being held open by the cams when they're supposed to be closed .

But when it starts first kick on 40 psi every time, that should tell you something. But you're right about a valve held all the way off the seat. That's why I said, "Unless the engine feels like it has no compression, I think you're barking up the wrong tree looking at reduced compression causing a no start situation".

...one of my troubleshooting maxims is ;"if it worked normally before I messed with it then I'm probably the one that broke it!"....[/quOTE] Amen to that! That's the first place I look. ?

[Batman]

Ahh ! You are worthy opponent Greyracer-san !......we're probably both wrong and he's just leaving the choke on !

[yz426em]

I think i know about the choke and how to ride the bike, so thats out of the question also not very funny! Thanks for the help and the machine usually gets ran out of gas or the bowl drained before storing it after a a ride

[Batman]

I think i know about the choke and how to ride the bike, so thats out of the question also not very funny! Thanks for the help and the machine usually gets ran out of gas or the bowl drained before storing it after a a ride

C'mon it was a little funny !.......Keep us posted on your progress , there should be enough combined brain power here to straighten this out in short order .