YZ450f 2005 won't kick start after rebuild

Hi all bran new here so excuse me if I do it all wrong but I have well loved YZ450 I have installed new steel valves / springs cylinder & piston every year since 05 and always had good results so you would assume I know how to do it but, just this week I did the new rebuild as above except I opted for the 479 cylinder and piston kit steel valves and springs from Pro X I get nothing but the odd back fire when trying to kick start but it will start and run & idle well if I go to the effort to push start it ?? Anyone have any thoughts on what THE! i'm stumped I have un installed all parts and re installed to double check everything HELP!

Assuming your timing is good (may want to double check the indexing of the flywheel on the crank), the most likely cause is a pilot circuit that isn't properly tuned.

Could you expand on tuning pilot circuit tuning - Fly wheel indexing are you meaning it is showing I mark at tdc ? sorry very interested in your comment but not sure what you mean regards J

If the flywheel key is sheared or out of position, the TDC mark on the flywheel will line up when the crank is actually somewhere else. Check by probing for real TDC through the spark plug hole (with something that won't fall through the hole) and see where the marks on the flywheel are.

The pilot (idle) circuit must be set right or the engine will start poorly. Read:



Thanks mate - I always check true TDC by probing then confirm TDC by the I mark on fly wheel so that is all good - I will go have a good read now of the links you sent on Pilot circuit tuning that interests me a lot as never heard of it regards J

Ok read the links thanks for that turns out I was aware of the pilot circuit tuning just never heard it called that my beast has always been set at 2 to 2 & 1/4 turns out so I am probably at the later end of scale and could look at smaller pilot but still is just with in the suggested range - these setting above would they all completely change with this big bore kit on it as I must confess I have not started the bike more than 5 minutes as I was afraid of I done something wrong in rebuild to result in no kick start ability so spent all my time double checking what I had done as bike was running not to bad before the new parts installed ?

The increase in displacement will affect the pilot, yes. And then there is the possibility that the pilot jet may have a film of dried varnish in it from having sat through the rebuild. Given that the jet orifice is something close to 0.40mm, you can see what an .05mm film would do to it.

OK will change to smaller pilot get it started and go through pilot circuit tuning procedure - definitely not dirty jets as i change them all to new ones any time the carbi comes off - I did however keep the same sizes on all cheers for your thoughts and help

Wouldn't you think you'd need a larger pilot jet, J?

If you've richened (turned) your fuel screw out to the point it's out more than 2 & 3/4 turns or so, you likely need a richer pilot jet, I'm thinkin'......


to be honest I completely stumped never had a problem until this big bore kit and new valves it has 45 jet in it now brand new I am fouling plug when kick starting but will push start very easily (apart from the pushing bit of course) runs well once started and appears 2 & 1 quarter turns out on fuel screw is where it likes to be you are correct I am close to the may as well increase pilot size and try to keep it closer to 1 & half turns out on fuel screw but be honest it just doesn't want to kick start no matter what I do - I really stumped

does any one have any thought towards a rule of thumb - Do big bore kits generally require bigger or smaller jets I know all bikes different but being displacement change would that naturally point towards - now need bigger pilots or now need smaller pilots and or main jet regards J

I don't know if there's a firm rule on what to do when you've put in a big bore kit, but here's something Grayracer513 said earlier:

The increase in displacement will affect the pilot, yes.

I kinda understood (maybe misunderstood?) it to mean you may need a bigger pilot jet, due to the increase in displacement. And normally, most jetting gurus will say that if you've gone out more than about 2 & 5/8 turns on the fuel screw ("Pilot Screw", as Yamaha calls it....), you need a bigger pilot jet, and begin tuning again about 1 & 1/2 turns out of the screw......

Now, this may have no bearing at all on your situation, but when I've read about big bore kits for the little thumpers (the 250fs; makes them anywhere from 265s to 300s), those guys generally seem to have to go bigger on their pilot jets, and a step smaller on thier mains. Generally speaking, more or less........


Edited by Diesel Goober
Added a thought......

An increase in displacement with the original carb usually results in increased intake vacuum and jets that behave bigger than they originally did.

Fouling plugs makes me think you have an ignition problem. If it's fouling new plugs, I would be suspicious of the stator. Run some tests.

OK well I open to any suggestions at the moment I had 48 pilot on first try of start up once completed install - got no where kicking but fired instantly on push start. checked plug after 10 minute ride was black and wet. I then took the assumption drop jet sizes so I put the the pilot and main back to standard sizes 45 & 165 got no improvement but again fired straight away on push start then I figured must have done wrong in the rebuild so un installed everything paying even more attention to valve seating, valve clearance and probing the piston to find true TDC then confirming TDC by I mark on fly wheel everything check out OK I have no answers except revert back to standard cylinder etc at this stage ...

Thanks GrayRacer I was heading that way my self it has to be fuel or spark - I am yet to exhaust going down the change jet theory but the next step is has to be confirm stator in good order. only confirmation on the stator so far is 1) it was working fine before rebuild and 2) with plug out it sparks nicely by turning the motor over by hand but I realize this is no real test and the stator is 5 years old ? regards J

i hope your valve seats cut with those new valves.....

Thanks for reply Howard 113 I was hoping some one would mention valves I have installed pro x steel & springs also new stem seals do you feel this could be an area to explore and can you not install new valves with out re cutting seats ... any light or tech advice on valve be well appreciated regards J

Ps I did leak test with fuel and sealed all good straight out the box - so I only smooth compound lapped them in could all my starting problems be with in this step??

Post up a fairly close-up picture of your cams from both the left side and right side of the engine with the piston at TDC.

after you verify timing on the cam gears, check your valve clearances, same thing happened when I did work on my sons friends' CR250, valves where to tight.

It would run when push started, but wouldn't fire up kicking it..

Hey, may as well go the easy route first since you'll have the cam cover off...

good luck

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