vm28sc jetting

I've searched and read up on this till I'm blue in the face. Can someone shoot me a quick answer on this if they have a similar set up? 1973 Z1-900 with Dyna S ignition, 4/1 Kerker (w/baffle), no airbox (have velocity stacks and Pods available). Motor is stock top to bottom internally. I have 2 sets of carbs and am taking the better of the two jet arrangements, or a mix.

Set 1: 122.5 mains and #20 pilots
Set 2: 115 mains and 17.5 pilots.

Anyone running a similar motor set up and have some quick feedback?

Thanks.

Nick

From Z1E website:

28mm (VM28SC) Z1 & early Z1-A
Main Jet 112.5
Air Jet 1.0
Needle Jet P-8
Jet Needle 5J9-3 - last number is the clip position where 1 is at the top of the needle (leanest) and 5 is at the bottom (richest)
Throttle valve (slide) 2.5mm cutaway
Pilot Jet 20.0
Air Screw setting 1 1/2 turns out


Good Fortune!

hello, If you want to use the velocity stacks I would start with set #1, but you won't really know one way or the other until you try. Even if someone had a similar set up what they have may not work for you. there's just to many variables(motor/carb wear,altitude,ect). pick the best parts you have and try, that will be your starting point. you'll tweek from there till your face is blue again! it can be very frustrating to tune carbs, but you'll get it. try something and see what it does and get back to us, most of the people here will be more than happy to help. just ignore the rest.

I'm thinking the #20 pilot is a better option than the 17.5 for my set up, but I didn't know if 122.5 would be too rich on the top end or if the 115 would starve the motor on the top end. That's my main question at this point. The stock mains were 112.5 and I don't know if +1 size (115 main) will be enough. The 122.5 is +4 jetting and that seems excessive to me. Am I on point here or am I out in left field? Any input would be greatly appreciated. Thanks.

I think ideally, the 117.5 jet would be the perfect starting point, but 115 is closer to that number than 122.5, so using my "higher math" skills and the law of averages, the 115 is where I'm starting and I can always jet up from there. Make sense to anyone else?

Buckeye73z1 wrote:

I'm thinking the #20 pilot is a better option than the 17.5 for my set up, but I didn't know if 122.5 would be too rich on the top end or if the 115 would starve the motor on the top end. That's my main question at this point. The stock mains were 112.5 and I don't know if +1 size (115 main) will be enough. The 122.5 is +4 jetting and that seems excessive to me. Am I on point here or am I out in left field? Any input would be greatly appreciated. Thanks.

I would concur, #20 pilot and #115 main.

Baffled Kerker with stock air box and filter isn't all that demanding for a larger than stock main jet.

These stock 28mm Z1 carbs were produced before later models suffered gelding by EPA rules and regulations.

Lots of us added Kerker to a stock Z1, which performed just fine with the stock jets.

Imo, the acid test for correct main jet sizing is spark plug reading following high speed throttle chop.

Good Fortune!

I don't have an airbox, though. I'm running velocity stacks or pods (have both and should provide about the same airflow). +1 jetting on the main and stock pilot may be a little lean with my stacks is my worry, but I'm pretty confident the 122.5 will be overly rich. I'm in flat land near sea level in OH.

Ok, forget your higher math skills and reasoning! you can't reason with carbs! velocity stacks will way out flow pods. I would go with pods #20 pilots and 115 mains and see what it does. first you should be trying to get it to start and run then set idle and airscrews to get it to run as good as you can. you won't know if you need bigger mains until you ride it. basiclly the pilots and airscrews control idle, slide cutaways and the needle help cover transisions in rpm, and mains are for wide open throttle. but each circut in the carb will overlap at times. if you need bigger or smaller mains you may be able to raise or lower you needle to fool the carb into thinking it has a different main jet. if you have a flat spot at a certain rpm that would be fixed by different slides or moving the needle(maybe). I know I don't spell very well, and I may not be explaining this all very clearly so please ask questions. but at this point my advice would be to put it together and try what you think will be best, get it running and get back to this forum with how it runs and we'll help from there.

Buckeye73z1 wrote:

I've searched and read up on this till I'm blue in the face. Can someone shoot me a quick answer on this if they have a similar set up? 1973 Z1-900 with Dyna S ignition, 4/1 Kerker (w/baffle), no airbox (have velocity stacks and Pods available). Motor is stock top to bottom internally. I have 2 sets of carbs and am taking the better of the two jet arrangements, or a mix.

Set 1: 122.5 mains and #20 pilots
Set 2: 115 mains and 17.5 pilots.

Anyone running a similar motor set up and have some quick feedback?

Thanks.

Nick

what year carbs?....what size?.....i would nt use velocity stacks on the street if you want the motor to live a while.....if you dont want a stock airbox, use pods...a general guideline for 77/78 would be 20/120 4th clip......good starting place ......a lot of variables depending on the carb.....

Carbs are '73 Z1 carbs (VM28) on my '73 Z1 with velocity stacks, kerker 4/1 (w/baffle), and Dyna S ignition, stock internally.