Rejet with Kerker 4 into 1

Hey guys, I recently purchased an after market exhaust: 4-into-1 Kerker exhaust, and a stage 3 jet kit for a 79' KZ650B3

The carbs that are on the bike are VM24's with accel pump, stock pilot (#15) stock main (#97.5), needle on the 3rd from the top setting, air screws 1 1/2 turns out.

With the stock exhaust/jetting/airbox, the bike runs like a champ, but the exhaust is falling apart after being neglected for 20 years so it needs replacing, and I think the new exhaust/jet kit will give it a less of a rice burner sound and maybe a little more umph at the top end.

My question is this: With the jet kit and larger headers, do I actually need pods or can I use the original airbox? The jet kit states "for use with stock or slightly modified engine, aftermarket exhaust and individual air filters" but I have heard many horror stories about pods causing a lean condition with cross winds, or water getting sucked in if it rains, and would like to avoid that if possible.

If I have to use the pods, fine, or is there a way to modify the stock airbox for more airflow? I am a little leery about drilling holes in it without some kind of confirmation. Any help would be appreciated, thanks!

Good aftermarket pipes like Kerker will work very well with the original airbox, and probably run better overall than pods IMHO. I think the jet kit is saying it can help to correct the flow mismatch created by using pods, but I think the pipes are tuned to run with a stock airbox so you would not be forced to use pods and screw around with the needles and jets. I would definitely try the airbox first.

You may need to go one size up on the main jet to get full performance from the pipe. That's assuming you are using the street baffle. If you use a comp baffle or open pipe you need to go a lot more up on the jets.

Like said, I would try the setup with the airbox first, and use the smallest main jets included in the jet kit. You will need to be less turns out than is described in the jet kit instructions for sure - closer to 1 3/4, but mix screws require proper tuning anyways - I wont get into it. :laugh:

Then later try the pods and see where it improves/where it lags, and you'll know the only thing you've changed is more air - so it should be a little easier to theorize your next step.

Good luck.

Thanks for the responses, guys. Just to clarify, with the headers being larger than stock, wont that require more air/fuel to get the proper throttle response? IE, something needs to be done on the intake side?

The jet kit (Dynojet kit) I got had two different mains (118, 122) new needles that are slightly more tapered, but no pilots, so I am assuming this will mostly effect the upper RPM range?
I am concerned that if I put a stock airbox on, the bike will bog at high RPM due to lack of air.

And yes bountyhunter, I will be using the street baffle, I'd like to keep my hearing for a while!!

Hi KZconvert, I have a KZ 650 E , with 4 into 1 exhaust and K and N pods. I run 17.5 idle jets and have the 115 mains, which are a tad rich. I'm going to drop to 112.5 mains. The metering rods are on full rich, all the way on the 4th slot. I know the jetting on these bikes were on the lean side from the factory, so I think the 17.5 idle jets will work with no trouble. The jets you have in the kit seem to be excessively too rich for these carbs. If anyone else has any input, I'm sure KZconvert will appreciate it. My bike runs great and as I said, is a tad rich on the high end. Bob

The change on the intake side will be your jet kit, and the adjustment made to your mixture screws (pilot system).

Download the instructions for the stage 1 dynojet kit and see what the difference is between your kit and it.

PS- If you want to prevent some bluing on your fancy new pipes - put a few coats of high temp header paint on the inside of the downtubes - concentrating on the first bend area, helps alot.

ALSO! - Mikuni Jets, Keihin Jets, and Dynojet Jets - all have different flow rates, even with matching numbers!

Here is a nice chart to help you:
...next post - oops...
Cheers.

AHA, thank you Tek9iNe, I had wondered if the flow rates were different for different kits, that chart helps a ton!! Considering these are Mikuni carbs, and I have a Dynojet kit, means the mains won't be as big a jump as I thought.

Did seem a little odd that it stated 118 when stock was 97.5!!!

And thanks for advice about the headers, I had a big concern about blueing my new pipes while trying to tune it, that is a great idea!! I am also going to put the stock airbox back in tonight especially now that i know the jets are not such a big jump.

tinlizzie, where did you get your pilots? I was surprised that my kit did not include them, so the assumption was I wouldn't need them. I'm going to try getting it started and hopefully sync the carbs tonight, so not sure if I need them, but would be good to know where I could them if need be.

You can get jets at Z1Enterprises. Should be less that two bucks each. Roy

kzconvert_86 wrote:

I will be using the street baffle, I'd like to keep my hearing for a while!!

smart move.

kzconvert_86 wrote:

Thanks for the responses, guys. Just to clarify, with the headers being larger than stock, wont that require more air/fuel to get the proper throttle response? IE, something needs to be done on the intake side?

Good headers are tuned to work pretty well with the stock intake system but typically give a power increase if you jet up a bit. I haven't tried Kerkers on mine, but I would wager the pipe with the street baffle will run very well with the stock airbox and main jets clicked up maybe +5 above stock as a starter point. The jets in your kit seem a bit large, maybe intended for use with pods?

EDIT: if the 118 is really equivalent to 105, it might be usable. Maybe a shade rich, but maybe OK.

I just installed a Kerker with street baffle on my KZ900. I've been extremely happy, I noticed an immediate improvement in mid-range torque- 4k-7k. The 4-into-2 system I was running before was jetted much to rich, I was fouling plugs like nobody's business. Since the Kerker went on, I'm still running a tad rich, but it's better than it was before for sure.

Also, I'm running the stock airbox with a K&N airbox filter from Z1. I believe this is what you'd need: www.z1enterprises.com/detail.aspx?ID=1494 . I like my airbox (and the atmospheric stability that is derived from it) and have not made any internal engine mods necessitating pods.

bountyhunter, I think they are the equivalent to 110 based on the chart tek9ine posted (please correct me if I am wrong), so maybe way to rich! The reason I say this (as per my previous post) i re-installed the stock airbox and the new exhaust (which I think I may have drooled on :woohoo: ) and fired her up. Started without barely touching the starter, did a quick adjustment on the air screws and did a carb sync (again adjusting the airscrews).

She purrs along just fine at about 900-1000 RPM, shut the bike down checked the plugs, nice and tan. Started it again, and got on the throttle to about 4k for 30 seconds and hit the kill switch, pulled the plugs and they are a little wet. I know I should be running it down the road for this to get it under load, but still, seems a wee bit rich. Plus I am running a stock air filter, so that might be a problem too. I ordered a K&N and smaller jets just in case the high flow filter doesn't help. If the weather gets better this weekend, I might take her for a quick run down the road and give it a whirl, might make a difference.

Thanks mtbspeedfreak for the tip, I went with that filter so hope it works!!

bounty, yes the jet kits states that "intended for stock or slightly modified engines with aftermarket exhaust and individual air filters" so I am taking your advice with the slightly smaller jets just in case.

Thanks guys, I'll post again when I get the jets and filter let you know what I came up with.