Which main jet to use?

I have a 78 kz1000 I no longer have stock carbs, I have a drag v/h exhaust, pod filters and as far as I know the carbs are from an 82 kz1000 (not sure which model) not sure if they have any different size adjustments from po. The bike seems to idle fine and accellorates decent, but as soon as you roll of the throttle it spits sputters and back fires with fire balls. How do I fix that can I just use jets or do I need to do something more complicated?

Fire balls out the exhaust? Thats called afterfire.
Back through the carbs is backfire.

If out the exhaust, your too rich on your pilot circuit.
You likely need to change pilot jets if its that bad. if those carbs still have thier stock jetting, everthing else will likely need changing as well.

WHATS IN THERE NOW?
Get LOTS of MAIN JETS and a few PILOT JETS at least.
Only change 1 circuit at a time per test. (example: the four mains down 1 size...) test...

Keep going leaner (smaller jet size) until you notice power loss, then go back to the previous jet size. If you have immediate power loss first try, then go 1 richer on that circuit... test... test... test... test... test... test... test... test... test... test... test... test...
Its going to take lots of patience!
Don't just go ridin around thinking its ok cause it goes ok, it could be too lean, then bye bye bike.

You test your mains at WOT, your needle position at 1/2 throttle, and your pilots @ idle and curb pull away power.
When buying Main Jets, you can go 2 steps at a time: 130, 135, 140 instead of 130, 132.5, etc.
Good luck, and patience!

Well I have put about 3k on it with the carbs the way they are now and with plug inspections it doesnt seem to be running lean... I always assumed that running rich is safer than running lean... Is it more efficiant to purchase like a stage 3 kit from like dyno or simalar or will it be more cost effective to buy a bulk set of mains? Also is ebay really a reliable source to get those or should I use another source? Is there any visual inspection I can do to find out if everything in the carb is stock (any markings or measuring?)

jayw704 wrote:

Well I have put about 3k on it with the carbs the way they are now and with plug inspections it doesnt seem to be running lean... I always assumed that running rich is safer than running lean... Is it more efficiant to purchase like a stage 3 kit from like dyno or simalar or will it be more cost effective to buy a bulk set of mains? Also is ebay really a reliable source to get those or should I use another source? Is there any visual inspection I can do to find out if everything in the carb is stock (any markings or measuring?)

Running lean at wot, or at idle, or cruise? Plugs don't tell the whole story with todays pump gas. Its about power delivery, and engine condition.

Running that rich is a waste of a good fast bike :laugh:

The dyno kits are designed for the carbs origionally installed on the bike, they will not work well in anything else.

I would go with jetting what you have, and buying new jets.

GL