Hey People,

Need some help. I wanna be more 'hands-on' when it comes to setting up the carby. At the moment mine idles kinda fine, has mean (and i mean mean) bottom end but gets to around midrange and looses all its puff. it sometimes feels well rich or something up high in the revs. Now, wats the best plan of attack. Ive been told by my engine builder that the engine has been extensively ported by the previous owner (which i didnt know about!). Now wat i need is a few pointers on how to set up my jetting for mid to high range revs (as im looking to race flat track speedway). i need procedures for plug checking, wat to look for, what settings for the throttle needle etc etc etc. any help on this would be greatly appreciated. Now i know that i could send this problem to a shop, but if i wanna race, im gona have to learn sometime. Thanx [img]i/expressions/face-icon-small-smile.gif[/img]

 

Well i bought myself a 360 main jet. the stardard one for the bike is a 420 and the bike has a 380 in it before which felt rich. Well i ran the 360 yesterday and it felt alot stronger. it was the first VERY hot day ive ridden the zilla so i was also a bit carefull. It felt like the revs were going higher and it pulled longer. I did a plug chop on a brand new plug, and the inside 'dome' and pin, and the lug were all a grey color. the outside ring of the plug hadnt been discolored yet. Wat i wanted to know was, what colour are you after when doing this test? From wat i understood, you want grey. black means its rich and white means too lean. Im really worried about blowin the engine so any help is greatly appreciated. [img]i/expressions/face-icon-small-smile.gif[/img]

 

Yeah grey is a good color. You might try the next bigger size jet and see if its rich with that.

 

I always thought you wanted a chocolate brown color....better wait for a professional to pop in here....

 

Check out websites for Mikuni and Keihin. Beyond the main jet change you have made, you may need to adjust the needle. The following is a basic approach. First of all, richer is safer. If you had a 420 (main jet)in it, try a 410, or 400, 390, etc. until it runs clean. A few jetting items: The pilot (air) jet is for idle to 1/4 throttle. The needle jet affects 1/4 to 3/4 throttle, and the main is from 3/4 to full. Yes, they bleed over, and the above is a basic guide. The main jet is easier to change, so focus on it. Getting your jetting perfect will take time and jets/neddles/etc. But it's worth it. Changing the clip position for the needle jet may help. To go richer, raise the neddle by lowering the clip. Oposite for leaner.

Focus on the main jet, and the clip position for the needle jet. Working with needle jets that have different lengths, taper sizes that vary in three different areas (top/middle/bottom) can be a hassle.

Enjoy.

 

thanks heaps super. Wat can u tell me about the plug reading? where can i get info on that? Also quick question.... After my top end plug chop, the plug was grey. but not a light grey. if this is in fact a little lean for the main jet, is it possible to counter this by raising the needle to run a richer mix from 1/4 to 3/4 throttle? obviously, i dont hold it wide open for long durations so 'blatting' it every now and then shouldnt hurt should it? Its the first time ive felt like the bike was extending itself past midrange revs. But like u say, richer is always better. I bought the bike with a 380 in it, but its been heavily ported (so ive been told after the last rebuild.) so i went a little leaner. the 360 worked very well, just that grey plug that had me worried. might try a 370. do they do the jets in 5 increments? like a 365? thanks to all you Zuke boys for helpin a fellow Zuker [img]i/expressions/face-icon-small-smile.gif[/img]

 

Plug reading - the darker the better. Yes, raise the needle up (lower the clip) to ad additional fuel in the 1/4 to 3/4 range. Only do one change at a time so you can compare results. Don't raise the neddle and change a main jet at the same time. I've only seen Mikuni jets in increments of 10. My experience with porting had me increasing jet sizes for additional fuel. Safer to start richer and then work to a leaner position. Since it easier to change main jets, go large and work down, then work on the needle jet. Good luck.

 

As far as I know the Mikuni carb jets do only come in increments of 10, but there are even ways around that. I'm running a 360 main, and even that was a little to rich for my altitude, but I didn't dare run a 350, so I used an old trick I learned from an old but very reliable crusty mechanic. I took the 360 main and put a drop or two of solder in the hole and used a very small drill bit to drill what can only be guessed as a 355 or maybe even a 356, not exactly sure, but in either case the beast runs excellent, and the plug turns a dark grey after testing. Where there's a will there's a way.

Good luck!

 

ive heard you can do that. I was told you could get the 360 main jet and drill it to a 365. hmmm might investigate. well ive got a 370 on order, should come in today. but im gona put the 380 back in for the weekend. if it doesnt run well ill put the 370 in. Thanx guys [img]i/expressions/face-icon-small-smile.gif[/img]