engine with big bore kit won't start(well)

Spaten Optimator.

 

Double up on spark plug washers to drop compression.

 

...and increase spark shrouding. Not a great solution in my opinion, but a cheap way to assess the situation.
Nevertheless, problem was solved by replacing the old, tired starter.

 

As for jetting, there are many discussions on the web on which direction to go after a big bore. Many convinced you will need to go down a size with the main jets. Yamaha throws cold water on that argument, look at the 750 and 650 jets. They both use the same carbs and the 750 does have larger jets. I'm not sure if they have same size air box, can't think of anything else which might play a part?
Based on my own experience I would see how it runs with the 750 jets and take it from there. Professional help is highly recommended as running it lean could quickly melt pistons. Dyno runs with exhaust gas analyzer will tell you instantly which way you need to go. You haven't stated but for your own sanity I hope you're starting with stock jetting and air box.

 

General rule of thumb: More bigger = sucks in more air = more fuel required.

The 650 and 750 airboxes are almost exactly the same size (back to that packaging issue being a limiting factor for airbox design).

 

The following question was sent in to Kevin Cameron at Cycle World. It is opposite of what Yamaha did with their XJ650/750.
I kept my jetting the same when I took my Maxim-X motor from 750 to 911 and it ran great but it was slightly rich prior to the big bore.
Bottom line - don't change anything until you put it on a Dyno with O2 measurement, or at least do a plug chop.

Question: Recently, I came across information which implied that when increasing displacement—such as installing a big-bore kit on a bike engine—if the carburetors stay the same throat diameter, the jetting might need to be leaner than that used with the previous smaller displacement. This seems counterintuitive. A larger displacement cylinder would require both more air and more fuel, so why would leaner jetting be needed?


Robb Millet

Milford, MA

Answer: I learned about this effect from the different cylinder sizes on three different Kawasaki two-stroke racers I worked with, all of which used the same-size 35mm carburetor. The 167cc cylinder of the H1-R 500 triple needed a 310-330 main jet, yet on the 250cc cylinder of the H2-R 750 triple, the same carb needed a 260-270 main jet. And on the 350cc cylinder of the F5 race engine, the main jet size went down slightly again, to 260.

Why? On a constant-size carburetor, the smaller the cylinder, the weaker the intake suction signal, and the smaller the fuel flow. Therefore, to give the 167cc cylinder the fuel it needs, a larger main jet must be used than for the more strongly pulsing 250cc cylinder. Flow through the carburetor is not steady and continuous, but is a series of pulses.

Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.

 

Like I said; general rule of thumb. There are exceptions.

Consider that the intake pulses of a 2-stroke engine happen twice as often, and are of a different duration and strength. Additionally there is more unburned fuel present in the blowback from the intake ports closing (as well as in the exhaust), so scavenging that unburnt fuel (desirable) plays a role in jetting choices (more fuel to scavenge = smaller jets needed to get the same AFR).
Also consider that the jetting needed for maximum power production in a race setting (midrange to max RMP (rideability at off idle can be sacrificed for more top-end), no concern for emissions or engine longevity beyond the racing season (if you have the money)) is going to be different that what is typically needed for a street setting (Off idle to midrange (typically), emissions of great concern, and longvity of at least a day past the end of the warranty period (ideally far longer in order to build and maintain a reputation)).

No matter what, the AFR needs to be measured (plug chops, colortune, gas analyzer) and the jetting changed accordingly (and not just at idle).

Engines loooove to run lean. More complete combustion occurs on the lean side, which means more heat, which means a healthy-sounding engine...until it's lean enough that the pistons get holes in them, (you'll hear a lean backfire (popping) on deceleration long before you get to that point...if you listen).