how much jetting change should a big bore upgrade require?

I understand all things are not equal, but just in general, if you have a bike with pods, a 4-1, and it is jetted out right, how much of a jetting change generally does there need to be?

I can't say my bike was jetted out right to begin with but thats not the point :pinch:

There's another guy doing a 550 to 615 kit on is 85 gpz, who had it running right with pods and a 4-1 using the stage 3 dynojet kit. He wants to think that the jetting should be OK as well once the big bore kit is installed. I really don't know, but I can't imagine it would hold true.

If he reads this, I don't want him to get the idea that I'm faulting his logic or anything, just trying to figure out where to start with my bike.

Anybody think it would at least be close enough to ride it safely for those critical first minutes and miles where the engine really breaks in? I'm worried about taht

(((((moderator moves thread to carb section)))))

Post edited by: wiredgeorge, at: 2006/06/07 15:39

Why would simply increasing the displacement a little over 10% require a jetting change? A jet simply allows the air flowing across it to pick up fuel (that's the simple version), more air flowing across it means more fuel is picked up. If you were to increase the displacement to the point that the maximum flow capacity of the jet was reached (that would be max rpm at WOT), then you would need to increase the size of the jet.

Increasing VE is a different matter, but increasing displacement does not increase VE.

My $0.00002 anyway

From my experience with KZ1000 and 900' if you put the carbs (29mm smooth bores) fromt the 900 to 1000 you have to go up one jet size 117.5 to 120.5. . The big question is how worn out is the 550. The 615 will be fresh and have better cyclinder pressure and may need a jet increase. I would say that it will probably be on the border line of needing a jet change.

I've got a 78 KZ650 with a 4 into 1 pipe and pod filters. I'm running the stock carbs, VM24.
Main jet was 118, pilot jet was 15, the needle jet clip was in the middle slot. Mixtrue screw turned out 1 1/2 turns. This gave me perfet jetting witht he mods I had.
Last year I installed a Wiseco 720cc big bore kit.
Here are my new carb changes to get it spot on with the kit.
Main jet remained the same 118
pilot jet I went to a 17.5
Needle clip moved down one slot, making it slightly richer in the mid range.
Mixture screw turned out 1 1/2 turns.
As you can see the changes were not as dramatic as I thought they would be.

Alright, so I should probably be safe with the 136's from the dynojet kit, at least as a baseline.

Basically at this point, the only difference between my bike and a gpz with the 615 kit would be intake valve size.

Sorry, forgot there was a carb forum!

To steell: Isn't that the idea of a main jet though? Shouldn't whatever main that is in there, be flowing as much fuel as it possibly can at WOT and max rpm?

In the case that the jets would be the same and work equally well on both modified and stock engine, that would mean that at WOT and max rpm, the 136 main jet is not flowing as much fuel as it can if installed on the stock engine. Either that, or the modified engine does not require any more fuel than the stock engine, which doesn't make sense.

What I am saying is, if only as much fuel as is needed is pulled through a jet, why do we have to fool with jetting changes?

Increasing VE is a different matter, but increasing displacement does not increase VE.

VE=Engine efficiency

Each jet number is a compromise to cover a range of operating conditions, from a 100 degree ambient air temp and low barometric pressure, to a 30 degree day with high barometric pressure, in addition to altitude varying from below sea level to 3000-4000 feet above sea level. There is only one specific set of conditions where a particular jet size will be perfect.

Shouldn't whatever main that is in there, be flowing as much fuel as it possibly can at WOT and max rpm?

If it did that on a bad air day, then it would lean out on a good air day wouldn't it? Since motorcycle manufacturers would get really bummed out if a bunch of their bikes holed their pistons or seized they make sure the stock main jet will pass enough fuel for the most optimum conditions they can imagine.
Keep in mind that at WOT the needle is pretty much out of the way. Part throttle fueling is highly dependent on the pilot/idle jet, and is often set a little lean, so like another poster said, you may have to adjust that for power and drivabilty.


Generally, if all you do is increase the displacement by 10% and everything else stays the same, the stock main jet will still be within it operating envelope.

Do a little porting and add a free flowing exhaust and intake and you will exceed the stock jets capabilty though.

That's why I am impressed with EFI, it's capable of providing exactly the right amount of fuel under most operating conditions (all if it's done right)

I'm not sure I clarified anything

yea, actaully that helps.

Pods and an exhaust help the engine breath better, which makes a bigger difference.. basically.

You are comparing the change in displacement with atmospheric changes and such, which I get. I guess my lack of understanding was just how in drasticaly it would affect the engine.

Granted, it is not a stock jet, but a dynojet kit that works like stock with pods and a 4-1 (on a gpz550 anyway) So saying, it should not make a huge amount of difference, except maybe in the pilot and needle area.

However, given there is that large range of operational conditions, wouldn't it be possible that the bike could be fringe jetted? IE on a high density day at sealevel, it could run lean enough to be noticable?

Also, 2 somewhat unrelated questions.

No one has been able to answer for me, why an 83 KZ550 with 26mm carbs would have a 120 main jet, while 84/85 GPz440 would have 114 mains in 27mm (but otherwise basically identical) carbs?

Also, intake valve size effects efficiency, right? How much difference would the be between bikes that are otherwise basically identical?

The different size main is used because the carbs have smaller/larger holes in the needle jets. Fuel requirements increase when compression ratio changes.