Jetting, please help

[xj1514]

Ive got two blasters Im rejetting. First one has FMF Fatty pipe, no airbox lid, boyesen reeds and a 66.5mm bore. It came set up like this, but has never run right. Lots of blue smoke, wet plugs, bogging from 3/4 to top end. Idles ok. Current main is a 270 which seems a bit on the lean side, but produces rich symptoms. I checked the needle it was on the middle clip so I moved it up one clip to lean it out a bit. Was this the correct thing to do, or should I have done a leaner main like a 260? I did not do a plug chop, just pulled the plugged and had a look.

Second blaster. Fresh rebuild on top with a 66.5mm bore. I pulled the airbox lid and jetted up from a 230 to a 250, ran about perfect, great plug color, stock pipe. Now, I purchased a brand new FMF Fatty setup, Boyesen reeds, same as the other bike. Question is, what main should i start with? I would think a whole new pipe and reed would warrant something close to the 300 range, but with the other bike running rich on a 270, I opted for a 280 main on the second bike, obviously aiming for rich to start, and go lean from there. Question is, is 280 rich enough for this setup? I didnt check the clip position yet, figured it was running good, so I left it alone.

Both carbs are stock.

Can anyone show me my errors in my jetting procedure? Im just a little baffled because of the first bike running rich on a 270. I was expecting to see a 280-300 main in there, and then start jetting the second bike real close to it since they are about identical except for bore sizes.

 

[Luni]

What elevation do you live at? Ive noticed a LOT of the guys on here who talk about jetting mikunis are at a pretty low elevation. Elevation plays quite a bit of a role on your jets.

Heres the basic rule of thumb with jetting:

To start, get the pilot dialed. 1.5-2 turns out on the airscrew when warm to idle. You SHOULD have to choke it if the engine is cold to get it to start. If you have to turn it all the way in to get it to idle, youre too lean and need to go up a pilot size. If you have to turn it all the way out to get it to idle, youre too rich and need to go down a pilot size. **NOTE: Note, that most of the vets on here who have been doing this for awhile will say you should never have to change the pilot out on the stock carb. So, I think the pilot on a stock Mikuni is like a 32.5 or something really close to that. So start with that. For the needle, set it in the middle to start.**

Get it idling nicely and then set the main. Put in a main that bogs when you go WOT, (keep going up in main till it does this), then start pulling back size at a time until you can go WOT without it bogging. Now youre in plug chop territory. Do some WOT plug chops and make sure your main is where you want it.

Once youve set the main, then you figure out driveability. Thats done with the needle. When you rev it, the engine sould pick up crisply and not rev too high too fast, just a nice steady wind up and a good healthy exhaust note. If it revs up too fast, youre lean and your needle needs to go down on the clip (which pulls the needle farther away from the jet so more fuel flows through the main on transition), if it picks up slowly and it bogs, go up on the clip which will put your needle farther down into the main restricting the flow of fuel to lean it out so youre not too rich on transition.

Hope this helps.

 

[Larry's Shee]

Only thing I can add is maybe do leak down on #1 to rule out seal sucking trans oil .

Excellant explination !! rep given I:I

 

[Blaner]

good info there, if you still not clear on the procedure, search for the jetting stickies in the engine section. your motors are not ported i assume, so i dont think you will need to be as rich as a 300.

By the way, elevation is a big factor. Look forward to a fully explained elevation effect on jetting write up COMING SOON!

 

[TechBlaster]

Agree, great jetting explanation, for sure...
As for the setups, needle on middle clip, and a 260 is gonna do just fine with the FMF/Lid off, ALL DAY. You are tooooo rich on a 280 and a 270 might be a bit too much as well(but as said depends on specific elevation). FMF's do NOT call for much over jetting, that is what confuses people around here. They figure cause it is an aftermarket pipe JET UP UP UP, not the case with the FMF on stocker VM26. I ran a 240(stock main) with just changing needle position with my FMF, chops were dead on, moved to a 260 after reeds and lid removed/UNI filter, perfect, ALL DAY. Toomey was a whole DIFFERENT story...

Carry on...

 

[xj1514]

Sorry for not listing the elevation, its 3000 feet. I have read all the stickies, and done alot of searching, but I believe my error was in comparing other pipes to my FMF setup which I now believe from your intel doesnt need as big of a main.

The first bike, it was bogging with a 270 main, needle in center clip position. Since I moved it one clip leaner and from what you guys said it sounds like I should try the 260 main and put the clip back in the middle?

Second bike, Ill put in a 270, go center clip, and go down from there.

So I should adjust the needle clip position after I jet up, get boggy at WOT, jet one down, clears up a bit, not as bogging at WOT, then change clip position to make fine adjustments, then do plug chops.

Also, not sure on the leak down test mentioned. Guess Ill search that out too.

 

[Blaner]

yes, you must get yourself a leaktester, its a very important tool in your inventry of diagnosis things...http://www.blasterforum.com/do-yourself-20/build-use-leakdown-tester-10663/ i made my own, but Slickerthanyou is selling them too, PM him if you want.

 

[1995blaster]

do plug chops also to tell jetting dont just look at the end the end of the plug.
Also im in the same boat just run a rich jet tell you can jet it right in one day. Also a huge bog could possibly be reeds. Usually if your runnin rich it just cuts out in the top end and isnt runnin 100%