impossible to start and when it starts back fires like crazy out of the exhaust


13 replies to this topic
  • yfzguru

Posted October 13, 2005 - 06:27 PM

#1

well recently after installing a set of hotcams in my yzf 450 ever since the first time i installed them and tired to start it it just back fires out of the exhaust, choke doesnt help, the only way to make it start is 3-4 shots of throttle and hold it open slightyl and kick it over, but before the cams it started up find no trhottle no choke, just one kick and bang she lit right up, so i thoguth it was something to do with the hot cams so i installed the factory cams and it does the samethign with these cams, so i tripple checked the valve timming, valve clearances, made sure its not flooded, wood ruff key on the flywheel isnt sheared off, still the only way to start it is 3 shots of throttle, then hold the throttle open slightly, till she warms up other wise it just dies, then drive it areound and its perfetc but wont idle cold or start up cold, if any body could please help i need it

  • davekaw804

Posted October 13, 2005 - 06:33 PM

#2

The cam timing is off or there is no valve clearance.Hopefully you did`nt bend the valves with the first kick.I`ve installed 40+ sets of these without a problem.Its easy to not have the timing correct.You MUST follow your manual and turn over slowly by hand without the plug in BEFORE kicking.(Sounds like bent valves)

  • yfzguru

Posted October 13, 2005 - 06:39 PM

#3

cam timming is perfect, .009 though exhaust valve clearance and .006 intake, and after everything was assembled turned over by hnad 5 -6 turns and everythign felt good went to kick it over and just back fired and have these symptoms

  • MNellis

Posted October 13, 2005 - 06:48 PM

#4

If it ran fine before you did the cam swap and now it backfires and is difficult then you need to go back and re-check the cam timing as the man said. If you've checked it 5 times then check it a 6th time and/or ask someone who knows what they are doing but who hasn't been a part of the project, to check it for you.

I suppose it's possible the cams were ground wrong or the timing marks are off but doubtful.

  • slappie

Posted October 13, 2005 - 08:18 PM

#5

When I rode a Raptor I had a Hotcam that I installed in it.

It was machined incorrectly and wouldn't even fit in the journals on the head.

Just to let you know you may not be crazy. . . it could be the cam.

Check and double check everything and if your cam timing and everything else all checks out, get Hotcams to send you some replacements and try those.

  • Yamaguy4Life

Posted October 14, 2005 - 09:35 AM

#6

You're not alone. My 2001 426 is doing the same thing. Except, mine came with the Hot Cam. When I got the bike it was easy to start. Just a couple of kicks. Recently I couldn't get it to start hot at all. I would just give up on kicking and had to bump it. Now, it won't even start cold. It's just dead. I'm hopeful that it is something simple. Unfortunately I'm not a pro-tuner, and am very nervous about letting anyone work on my baby.

Good luck. I'll be reading the responses as well.

  • PumpkinHumper

Posted October 14, 2005 - 12:32 PM

#7

Those hot cam gears are bolted on and are slotted right?? Im bettin that the gear is indexed wrong with the cam. What you describe is exactly what happened to my yz400 when the intake cam gear spun. It would only start with a little gas and then ran horrible. Im bettin the cams are indexed wrong.

Look at your cam lobes. They should be horizontal meaning that you can almost put a straight edge across the tops of them.

They should look like this, at least stock cams should look like this that is.

Posted Image

Not like this

Posted Image

See how the intake cam is pointing straight back in the 2nd pict instead of slightly up in the 1st. they should be slightly up like in the 1st pict.

I think the cam lobes should be pointing 105 beyond max lift. Meaning that if max lift is the cams pointing straight down then they should be 15 deg above horizontal. This is an aproximation mind you. But may be enough to give you an idea of what is going on.

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  • sirthumpalot

Posted October 15, 2005 - 04:17 AM

#8

4Life, your symptoms sound exactly like valve clearance issues. Have you checked the valve clearances? If they've tightend up then you can get exactly what you're seeing.

As for the hot cam issue, going outside the box here, if the timing checks out correctly all the way around, and since I read that the stock cames now have the same symptom, give the carb a quick cleaning. It may be unlikely, but certainly possible that something got into the pilot jet. Best of luck and let us know what you find!

  • Matt96xr6

Posted October 15, 2005 - 07:01 AM

#9

Once you check the cam timing and valve clearances, you need to look other places.

Sounds like the hot start plunger is stuck in the carb bore. Since you were messing with the cams, you may have pinched the cable a bit. Inspect the hotstart cable, remove the plunger from the carb, clean and grease it a bit. Then reinstall everything.

  • mcgvr86

Posted October 15, 2005 - 11:30 AM

#10

I havent read every response, so if I missed this already I apologize, but if I was in that situation and wanted to be sure it was a problem with the cam or the valves after the install... I would put the stock cams back in and see if it runs like it did before. If it does then the new cams were made wrong or installed wrong, and the valves are okay. If it doesnt... then you have a valve problem or you have missed the timing mark. The one on the crank is very confusing. It took me a few tries to get the right one. I know its a pain, but its an easy way to pinpoint the trouble. my 2 cents worth.... good luck. Let us know what you find once you figure it out.

dave

  • woodrowman

Posted October 15, 2005 - 12:45 PM

#11

Quick and easy fix, bump the pilot jet up (example, from a 40 to a 42, or a 42 to a 45). Reason, its getting colder, and you need to jet richer! Thanks :banghead:

  • yfzguru

Posted October 15, 2005 - 07:32 PM

#12

woodrow man the problem is not the pilot jet i used a leak down tester to identify that i had leaking intake valves, so i checked the valve clearances again and found out i dint have any clearance at all, soi reshimmed it put it all back together and half a kick later she lit right up also i noted this is a very hard bike to get a feler guage in so lol im gonna try a dial guage next time and lift up and down on the bucklet to determine valve clearance thanks for all of you who replied to this thread i am very grateful

  • yzf450jack

Posted October 15, 2005 - 08:07 PM

#13

When you did your cam swap did you have the slide out of the carb. sounds like the black plate on the slide is upside down

  • PumpkinHumper

Posted October 16, 2005 - 02:31 AM

#14

woodrow man the problem is not the pilot jet i used a leak down tester to identify that i had leaking intake valves, so i checked the valve clearances again and found out i dint have any clearance at all, soi reshimmed it put it all back together and half a kick later she lit right up also i noted this is a very hard bike to get a feler guage in so lol im gonna try a dial guage next time and lift up and down on the bucklet to determine valve clearance thanks for all of you who replied to this thread i am very grateful


Cool deal :banghead:


Keep an eye on those intake valves. When they go to zero tolerance it could mean a ti valve going out. or it could just be differant clearance needed for the cam swap. But you did say that it was the same with the stock cams in it. Might want to re-check it after a few rides.

I dont know about the dial indicator though. I have had great luck with feeler guages. I dont know anyone that uses a dial indicator. I understand the principal just dont know how well it will work.





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