Well for the last week or so, I have been installing my BB 351 kit.

Last weekend, got it fired up, so I must have done something right. Install was a little more tricky than I thought it would be. Getting the new piston and cylinder together in the bike is a bit awkward, and getting the timing and cams back in took 5-6 attempts. Fighting the timing chain through the whole process got old too.

When I first fired it up, it got hot and I mean real hot. This was the first time I ever saw my fan come on. I let it idle for about 20 minutes and it would not hold a steady idle. Also I had to crank the idle way up so it would idle.

I then took it out for its maiden voyage, ran great but at idle it would clatter like a diesel. Maybe it is fighting the stiff new piston? When I got it back home I re-installed the cam chain tensioner and I think it helped a little. No clatter anymore but I think the new setup is noisier. I think there is a little piston slap going on. It has a pretty loud tick noise. Anybody notice this?

So now I have about 30 miles on it. It is running okay, I think it is a little rich on the pilot but I will adjust that later. Idle is still a little funny with it, when it is cold it will idle one speed, but when warm the idle will speed up. Maybe it just needs some more break in time?

I don't know if I would ever do this again, my bike ran great with just a 250 all opened up. I do like the extra torque so far, but this was a lot of money and I don't like the idea of tearing a good running engine apart. In hindsight I probably should have just left the bike alone.

All in all it took me about 10 hours to do the install. I ended up going with a 128 main jet and a 38 pilot jet. It does have a little flat spot right at 5000 rpm, don't know what is causing this but will try and adjust it out.

Just thought I would share my thoughts/opinions, I am not 100% sold on the big bore yet. Hope things smooth out the more I break it in.

 

Take it all back apart and ill buy it off you. lol

 

I hope it's just 'new install' woes and it will get better for you.

Especially since I just returned from shipping out my cylinder to Bill for the 351 kit.

 

The idle will smooth out. You should not have let it idle sitting but I don't think you hurt it.

New piston rings and freshly honed cylinder mean you WILL HAVE TO turn the idle up until it breaks in a little and smooths out the cylinder. This is normal. The freshly honed cylinder and new rings create friction and heat that is not usually there. As it smooths out, it will run better and you can slow down the idle.


Carb will need tweaking as you ride and get to know your new bike.

In time you will love the big bore. Torque is amazing and the power band is in a more usable area.

Adjust your idle mixture for the fastest smoothest idle, then adjust your idle speed. This may need to be repeated as it breaks in. I ended up at 1500 rpm for idle speed.

Enjoy!

David

 

Hmm, sounds like something may be wrong. I remember when I put on my 350 kit it pretty much idled perfectly like a stock 250 right from the first start. I idled it for about 5 mins, then started riding it. It was and still is perfect. Idle speed is about 1,000rpms or so.

Are you sure you have the rings on correctly? Sure the timing is correct?

 

Your feelings mirror my own, my 250 runs about as good as it possibly could, no it ain't a powerhouse but with it opened up it does great and has no issues. It has around 4K miles and will probably go 6 times or so that far with only minor maintance. I'm sure I could do the big bore in a weekend but why? And what if it isn't as impressive for me as others seem to find it to be. When I bought this bike I knew it was a 250cc and that was what I wanted, I'm staying where I'm at for a while unless it decides to puke a piston on it's own and speed up the change.

 

Sounds like this might be the case. I have only ridden it about 3 times so far and it seems with each time, I have to bump the idle down a little bit.

 

Lets hope so, I checked the timing a few times after I put it back together and each time I got it to top dead center, the timing marks were lined up with the head and there were 32 pins in the chain. I am not hitting any valves or anything.

The rings were already on the piston when it came from BB. I just positioned the gaps to where the manual says to. Hope this was the right method.

 

Because you CAN! More power is always better! And because it is the single best mod you can do for the money hands down! hahahaaa

The 351 kit is the bomb-diggity any way you look at it. If I had to go back to the KLX250 I'd sell the bike.

 

All the minor things you are experiencing happened to me too.. Idle had to be cranked up to 1500, head is noisier, etc etc.. but I have put 1500 miles on the big bore, and no problems.. its just a little louder, and more vibration and more power