Yz426f not starting and just backfiring

[brandi696]

:banghead:Hey guys I am new here, and have few questions for someone.

I have a 2001 yz426f and it has been completely gone through, changed to the decomp cam to hot cams,(no more decomp lever) so here we go, after we shimmed it and put it back togther it started and ran great, so we shut it off, then we were able to restart it 6 times after that, then this morning I went to start it so I could break it in today and all it is doing is kicking over and backfiring, so we tore it back down to check the clerence on the vavles there just fine nothnig worng with them. What in the heck is wrong with this stupid a$$ bike? Runs great lastnight, then nothing today. As of right now I am ready to throw the "B" out and run it over with my truck.

Thanks guys....?

[wanawakeboard]

mine does that sometimes, a couple of slow rolls on the throttle before I kick and it will usually go, maybe confirm the plug gap if it still wont start. mines weird sometimes one kick other times pop pop out the exhaust..

[grayracer513]

Fouled plug is my first guess.

462's are notorious for that when you start them up and then don't actually take them out and run them long enough for the deposits to burn off of the spark plug. The 400/426 has fairly weak spark at cranking speeds, and it's easy to slop a plug up to the point where it effectively won't work. Try a fresh new plug in it. Then be sure you adjust your pilot screw for the new cam.

Another point that you may be overlooking with the new cam. When you had the manual lever the routine was to:

1. bring up against compression,
   
2. pull the lever,
   
3. advance the kick starter about one more inch,
   
4. then draw back and kick.
   

What the auto decomp does is to take care of steps 2 and 3 when you do steps 1 and 4. In other words, what you should be doing with the AD setup is to push the engine up against compression and then draw the lever back up and kick it right from there. Don't over think it and try to put the engine somewhere else in the cycle.

[brandi696]

Hello again,

So today we changed the plug while in the shop and also put the stock cam back in and it wont even fire.? Have no clue what the heck is wrong with this dang bike. Yesterday it would kick over and try but back fire and today nothing, and yes we checked the spark and it is awesome. What could possible bve wrong with it? Please help if you can anything at this point is worth trying.

What we have treid,

put oem valve cam back in,

changed the plug 8 times,

shimmed and checked timing

cleaned carb (very well)

put fresh gad in it.

checked the spark again still good strong spark.

What is the heck could it be?

[wanawakeboard]

sometimes its hard to figure out when our bikes act up, when I am frustrated I try to go back to basics, what does that engine need to run?

fuel (air mix at proper ratio)

Spark (at the right time)

and compression. (plenty of it)

If you have verified each of these then it is likely it is a combination of problems or even a random issue.

I wish I could think of specifics to try but sometimes the advise to just go back and think about the basics can be helpful. Frustration clouds judgment.

[brandi696]

I have to say at this point I am giving the bike a week to run. We went back over the basics started at point 1 and still nothing, we have the stuff for it to run but it just refeuses to actually run, saturday we were able to start it and let it run for 10 mintues, and after that it started just fine. But then sunday nothing but a backfire, today nothing. This even has the mechanic in aww. He can not figure it.... Just tired of it all ready.

[brandi696]

I do have a question though do you have to do anything special to the nuetral switch after you put the comp cam in? Cause we took it off and put a block off plate on it, but it did not say if there was anything special we had to do after we took it off. Does anyone know about this? Hopeing maybe this is the problem?!?!??!

[brandi696]

Well we went back at again and started from point A. And still nothing, just can not figure it out. The beast has a week the ole man says and its gone. I just can nt understand why all of sudden this happened. Compression is awesome, spark is great, fule air miture is right on and still nothing. What I am wondering is; is it going to matter that we put a neutral switch block off plate on? Everyone says that that wont cause a starting issue. We have even took it a shop and it has the mechanics stumped also. THey said there was no logical reason for it to be doing this. Any new idea of what this could be?

[grayracer513]

If you suspect the neutral switch, you can duplicate the condition that is created in neutral by grounding the light blue lead that went to the switch. That makes a slight difference in ignition timing and resets the rev limit down to 7000.

One thing I wonder is whether you are strictly paying attention to the closed throttle rule when starting. If you are, that might be the problem. Try cracking the throttle open just above idle to get it going.

[dalvaradosa]

Did somebody ever figure this problem out, because I am having exactly the same problem except I did not do anything with the neutral, new shims, new hot cams, new clutch basket, etc and motorcycle backfires trying to start it and eventually I get to start it but with the choke and after 50 kicks maybe and it runs great but starting it again is always a battle. Any ideas?

[dirtpaw12]

Did somebody ever figure this problem out, because I am having exactly the same problem except I did not do anything with the neutral, new shims, new hot cams, new clutch basket, etc and motorcycle backfires trying to start it and eventually I get to start it but with the choke and after 50 kicks maybe and it runs great but starting it again is always a battle. Any ideas?

Had the same problem. Problem? I wasn't advancing the kick starter all the way down, and the choke problem is probably a clogged pilot jet or fuel set screw that rattled out. Look up how to start a YZ426F on Youtube and make sure you kick it all the way.

[hillclimb426]

Sounds odd but make sure you are pulling the choke out and not the hot start have seen the knobs mixed on more than one 426 from when guys clean the carb then can't figure out why it won't start

[dalvaradosa]

I already look up in you tube and everything I can imagine, this morning I take the carburetor down and check two things I was not sure about one was the accelerator pump diaphragm and also the position of the throttle valve plate. I have the manual for the bike and also follow everything step by step. Only thing I found very interesting is that for some reason the valve clearance was intake 0.50 mm, 0.20 mm and 0.50 mm and 0.38 mm exhaust and it was starting perfectly fine, but with really bad final speed I will say no more than 40 MPH in 5 gear and full throttle with OEM sprockets, so after I install a new hotcams intake camshaft (bike already comes with hotcams autodecompressor) adjusted the valves to specs how the manual suggest 0.15 mm for intake and 0.20 mm exhaust and there was where my nightmare starts to the point I already had the bike loaded to take it with me to Costa Rica where I know a place where they will machine both faces cylinder and head and adjust the valves too to the new measurement due to not find a place here in Ohio where they do that and me leaving home for the winter. Next year I will come and look at some other bike with the engine seized to fix it myself again for a reasonable price. So I am ruling my problem has uneven faces/timing cam chain. I did start it this morning but again after 12 right way kicks and even when the final speed is much much better the bike backfires a lot, position sensor is bitwen 0.560 v and 0.700v something like the manual says, also manual rules backfiring like lean gas so I installed a 170 main jet instead the 162 but nothing seems to help out a lot.     

[NWsurfkid]

I already look up in you tube and everything I can imagine, this morning I take the carburetor down and check two things I was not sure about one was the accelerator pump diaphragm and also the position of the throttle valve plate. I have the manual for the bike and also follow everything step by step. Only thing I found very interesting is that for some reason the valve clearance was intake 0.50 mm, 0.20 mm and 0.50 mm and 0.38 mm exhaust and it was starting perfectly fine, but with really bad final speed I will say no more than 40 MPH in 5 gear and full throttle with OEM sprockets, so after I install a new hotcams intake camshaft (bike already comes with hotcams autodecompressor) adjusted the valves to specs how the manual suggest 0.15 mm for intake and 0.20 mm exhaust and there was where my nightmare starts to the point I already had the bike loaded to take it with me to Costa Rica where I know a place where they will machine both faces cylinder and head and adjust the valves too to the new measurement due to not find a place here in Ohio where they do that and me leaving home for the winter. Next year I will come and look at some other bike with the engine seized to fix it myself again for a reasonable price. So I am ruling my problem has uneven faces/timing cam chain. I did start it this morning but again after 12 right way kicks and even when the final speed is much much better the bike backfires a lot, position sensor is bitwen 0.560 v and 0.700v something like the manual says, also manual rules backfiring like lean gas so I installed a 170 main jet instead the 162 but nothing seems to help out a lot.

Holy run on sentence batman...

Seems like the hotcam timing is off. If it is a diff stage cam it should be shimmed as per Hotcam notes.

Also, I believe they recommend new springs...or you broke one...so maybe you decompression is stuck open.

[dalvaradosa]

Huuu!!! That makes a lot of sense to me now and change my diagnosis, I did not have an idea that hotcams parts spec where different than OEM I have to look at the box and hotcams info but I taught the new intake camshaft says stage D and the shaft was a little bit wider but after carefully measurements I notice it will fit in the space without hitting anywhere and just replace it (bearing in the old one was rough) and in the exhaust one with the autodecomp I did not touch anything. Thanks!!! I will put the old shims back and give it a try, only problem bike is already on his way to Costa Rica by boat. Hopefully I can make it work so it would be a $6000 bike to sell before I came back next year.

[NWsurfkid]

I'm not sure what a "stage D" cam is...

The only cams that I know fit the 426 are from certain year YZ's with autodecomp (Cam Mod), or Hotcam Stage 1/Stage 2 (Aftermarket upgrade).

I don't recall Stage D stamped on my OEM '01 426 cam...?

[dalvaradosa]

Sorry my mistake Hot Cams 4002-1IN Camshaft new from amazon and I did find some information in the box which was very interesting 0.45mm for intake instead the 0.10mm to 0.15mm the manual suggest and 0.60mm for the exhaust instead the 0.20mm to 0.25mm hopefully that will solve my nightmare but I will not know until a get to put my hands on the bike again. Thank you NWsurfkid I never will think of that difference due to follow the manual specs as the bible.

[grayracer513]

0.60mm is way too loose.  Where did that number come from?

 

Are you certain that both the cams are for a 426 and not a 450?  Can't use the timing marks in a normal manner when using the exhaust cam (or the intake) for a 450 in an earlier head.

[dalvaradosa]

It came from the only paper I found inside the original box of the hotcam intake cam I installed on my YZ426F and I believe it is for this bike at least that says in Amazon where I bought it from. It textually says " We recommend .045" clearance on the intake side and .060" on the exhaust side"

[dalvaradosa]

If those are not the right clearances then no idea which is the better one of this hotcams cams for the YZ426F, the exhaust cam was already installed in the bike when I bought it.