240 Big Bore kit re-jetting

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chrisc713

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Sep 27, 2009
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I just bought a 2000 Yamaha Blaster with a TRUE 240cc Big Bore Kit. It has a stock carb, a DG pipe and an open airbox with a stock filter. I'm not sure if the reeds are stock (I cant find a name on them). I'm located in upstate NY (~1000 ft above sea level). With the choke on, the quad starts no problem but it will not idle.

I don't know what jets are in the carb right now (I'm suspecting too small), but I have a kit and plan on re-jetting it. My question is, what is a good starting point to begin jetting? What sizes should I start with for the main jet, the pilot jet and what position should the needle be in?

I know I wont get optimum performance with the stock carb, but for now, I just want to get it running decently. Please help! Thanks in advance!!
 
might help to get a slightly bigger carb. the stock one wont be able to keep up with that set up at speed. but yeah need to know what size jets are in it now to help more.
 
I bought it blown up, already with the 240 big bore kit installed. I just had it rebuilt and re-bored (i think .060 over the big bore). when I put it all back together, it started no problem, but would not idle. The throttle response was crisp, too, but I did not ride it so it wasn't under load.

I suspected that it was running lean so I just took the carb apart. It looks like the main jet is a 230. I don't know about the pilot jet or the needle at this time though, I didn't touch them.

Just to see what would happen, I just replaced the 230 main jet with a 300. When I put it all together, it idled, but idled very high, and it seems like its now bogging under moderate throttle (keep in mind I haven't ridden the quad yet, this is all just revving the engine in my garage).

I'm suspecting an air leak at the reed gasket is causing the high idle, and I'm working on fixing that now. But I still have no idea where to start with the jets.

Where do you guys think I should start with the main, pilot and needle?

Also, I'm in Oneonta, NY.
 
hmmmm, first thing wrong here is you can only bore a big bore .20 i belive nowheres near .60. revving high usually means you have a airleak someplace. what brand is this big bore supposed to be? with a stock carb i would GUESS a true bbk would need at least a 350 main. but like i said thats a guess, nut im not too sure you have a real bbk..
 
I was guessing on the bore- over, i could very well be wrong on the .060 over, and by what you're saying, it sounds like I actually am wrong. I gotta look at the receipt. Unfortunately, I don't know what brand the big bore was. The guy I bought i from was certain it was a bbk, and he said that he sent out his stock cylinder to obtain this big bore cylinder and kit.

As you could imagine, I was skeptical about it being a true big bore until I took it to the local shop to get rebuilt, they said they had to order the bigger piston because it had the "big bore kit" in it.

Is there any way by looking at the assembled engine to tell and be certain?

Also, thanks for the guess on the 350 main jet, it gives me somewhere to start. Any ideas on the pilot and needle?
 
no clue on the pilot, i would leave the clip in the middle till you figure out the main and pilot. sounds like the PO might have have a big bore sleeve installed which does make it a 72mm bbk but still leaves the crappy yamaha port layout and head.
 
Thanks for your help.

Yeah, I am still trying to figure out how to go about finding the right pilot and main jet. I think I will start near 350 for the main since that is what you suggested.

When I play around with these jets, what am I looking for? If I replace the main jet, what should I look for as I approach the right jet size? What should I look for as I approach the right pilot size? what should I look for as I approach the right needle position? what should I beware of?
 
chrisc713 said:
Thanks for your help.

Yeah, I am still trying to figure out how to go about finding the right pilot and main jet. I think I will start near 350 for the main since that is what you suggested.

When I play around with these jets, what am I looking for? If I replace the main jet, what should I look for as I approach the right jet size? What should I look for as I approach the right pilot size? what should I look for as I approach the right needle position? what should I beware of?
Click to expand...

whoa there fella hahaa, first thing you need to do is figure out why you are revving high, if you have a airleak you could blow the motor again. what does the plug look like now? once you start messing with you jetting you will need to do "plug chops" type it into the search box and go to town haha I:I I:I
 
Hmmm, you might find the 350 a bit large. Even tho you have a 240, the limiting factor is going to be the 26mm carb and DG exhaust. I would think a 35 pilot might be in order tho. The important thing is to get it to idle, at this point.

Do you still have the TORS box on the carb? Has the TORS been disconnected/defeated at all?
 
I think I have eliminated the air leak, I'll confirm that shortly.

Plug chops? yea I need to do some studying on that, never heard of it but it sounds important.

The TORS has been disconnected, and get this, the idle adjustment screw on top is missing- its just gone. theres now just a hole where the screw used to be. This should be fun...

I would really like to just get this thing running decently without dropping more money into it, thats why i havent considered getting a new carb- thanks to this rebuild, I'm broke!
 
I called vitos wheb I got my bbk....they.told me stock pilot.....and needle on the third clip down...however...I found that on the forth down it ran a lot better...and I started with a 340 and got it perfect the first time...I'm at about 800 above sea level.....oh...and check out my 240 break in thread in the vid section...I have a stock carb and mine f*ckin rips
 
Sounds like good info, Yamaturd.

Chris, without that screw you are phucked as far as getting it to idle. Since your TORS is disabled anyway, your cheapest solution is a TORS eliminator cap kit for the carb.

Best solution is a different carb, for more power. You can probably get a used carb, what it will be depends on what you are willing to pay.
 
Thanks for the info guys. That is exactly what I was looking for. I just wanted a baseline to start at. I think I will start with a 340 main jet, keep the pilot stock, and put the needle at the fourth position down. As long as I don't have any other air leaks or anything, I think I can get this thing running decently enough to fine tune it later.

As far as the TORS goes, I will look into getting a TORS eliminator kit and see what I can do with that.

Thanks guys, all the help was much appreciated!

If anyone wants to add some advice or past experiences pertaining to this topic, feel free to post!
 
just remember...the further down you put the clip on the needle, the richer you make it....which is your mid....pilot is your idle...
 
If anyone is still active for this thread I have a similar question on this topic. I just put a 240 on my blaster, won't start by kick start unless motor was just shut down. If I pull/push start it it starts fine and runs like a scolded dog. I'm running stock carb with 240 main jet, stock pilot and factory settings and stock exhaust. (I know it needs to be richer). My question is any thoughts why I can't get it to kick start. Was thinking it's due to small pilot
 
Your pilot jet may be fine at stock. You may have completely different results with a stock exhaust. The stock exhaust restricts soo much. Most problems with it being hard to start are because of low compression. Make sure that new 240 was done right with a compression test. You may need to play with the idle screw and the air screw. BTW pull starting a brand new motor is a terrible idea.
 
The 240 actually isn't brand new, bought it off a friend. It is an actual bbk though, that I do know. Compression is right around 145-150psi
 
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