Jump to content
Sign in to follow this  

08 kx450f cold start problem

Recommended Posts

I have an 08 kx450f with Pro Circuit exhaust and Boyesen QS3. The bike doesn't want to start when cold. I have to adjust the idle screw way far in to get the bike to start, then adjust it back out. Once the bike is warm, it starts right up. But let it cool down again, and I'm back where i started.. It does also seems lean as it pops on deceleration.

Is this a valve problem or jetting? Anyone have any ideas where to start since i really don't want to pay a ton of money to have someone else fix it.

A MC mechanic i know told me to take the exhaust off and put everything back to stock and see if it still acts the same. Then go from there. From what I've been reading it seems more like a stuck valve or something than a jetting problem. Plus Pro Circuit recommends changing only the leak jet and the position of the fuel mixture screw with the exhaust.

The only other engine mod is a boyesen water pump.

The hot start nipple is a brand new Pro Circuit aluminum one since I had problems with the plastic one previously. Also have a brand new cable on it and it operates as it's suppose to.

Any help is greatly appreciated. Tired of messing with the thing to no avail.

Thanks

Share this post


Link to post
Share on other sites

The Boyesen QS3 is a source of problems for many riders.

Just put on a 45 leak jet and be done with it, or, get a better quality version from R&D.

My '08 is cold blooded too. I have to turn the idle in 3/4 turn, choke on, no throttle, two slow kicks and one fast one, and it starts every time. But, you can't let it die before it's warm, or it's a bitch to re-start. Got to keep it running for at least 2 minutes using gradual throttle, then blip the throttle for the first time and the rpms will rise, and stay running till you decide to drop the idle back down.

I was told by Kawi that the carb on that bike is just not modern enough to keep up with the wide powerband and still do everything right.

The R&D power bowl II made a huge run-ability difference on my bike,

Share this post


Link to post
Share on other sites

thanks for the reply. I have heard that about the qs3 but tried it anyway after having no luck with just swapping out the leak jet to a 55. Of course I'm thinking jetting Is'nt why it wont start cold.

Got home from work tonight and checked the valve clearances...

The exhaust valves seem way too tight. .13mm and .10mm

suppose to be between .17-.22mm

The intake are closer to spec at .08 and .10mm, but still tighter than they should be(.10-.15mm)

From what I've read (https://thumpertalk.com/forum/archive/index.php/t-598866.html) it seems that this may be the biggest reason for the problems I'm having.. Sound accurate at all?

If all i need to do is shim the valves, I'll ditch the qs3 and go back to the 55 leak jet..or possibly the r&d

Share this post


Link to post
Share on other sites
thanks for the reply. I have heard that about the qs3 but tried it anyway after having no luck with just swapping out the leak jet to a 55. Of course I'm thinking jetting Is'nt why it wont start cold.

Got home from work tonight and checked the valve clearances...

The exhaust valves seem way too tight. .13mm and .10mm

suppose to be between .17-.22mm

The intake are closer to spec at .08 and .10mm, but still tighter than they should be(.10-.15mm)

From what I've read (https://thumpertalk.com/forum/archive/index.php/t-598866.html) it seems that this may be the biggest reason for the problems I'm having.. Sound accurate at all?

If all i need to do is shim the valves, I'll ditch the qs3 and go back to the 55 leak jet..or possibly the r&d

Yeah, those valves are tight.

The R&D or Merge Racing apump linkage spring upgrade is THE mod to make for better throttle response. It comes with the R&D powerbowl II kit, or separately. It is a MUST HAVE MOD for any FCR carb. That, and the 50 leak jet. You still may need to adjust your apump linkage, cause about 20% of the bikes get carbs where the linkage was never adjusted by Keihin.

The Honda CRF's use a specially modified Kehin carb after 2007, that accomplishes the same thing.

Share this post


Link to post
Share on other sites

I would start getting the valves back into spec and then going from there. I wouldn't try changing alot of things if you already know where 1 problem is at. Fix 1 problem, if that doesn't fix it. start with tuning the carb

Share this post


Link to post
Share on other sites

If you are desperate, and assuming your carb is all stock and normal, then try a new float valve. Or turn it around 180 deg for a fresh seal. Or if you have the patience to remove the carb, then check the float level.

For 4 years, my 06 started cold on the 1st or second kick. Cold here is about 50 or 60F so not real cold but cold enough. I always gently and slowly kick it over a few times with the kill button pressed to prime up some oil pressure, get it to TDC, then it was usually 1 kick to start it. And the 06 has more power everywhere than the 08 so the cams are not mild.

Then a few months ago, it started being difficult to start cold. I took the bowl off to check for dirt or whatever but found nothing. Then by chance I noticed that the fuel overflow was pouring out on the ground when the bike leaned just a little to either side.

It turned out that my float valve wasn't sealing correctly, so the float level was too high. So I turned the valve around 180 deg and it's been fine since. Until now I forgot I needed to buy a new one.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...