Jetting help with 1075

I asked for help in my post on the "Engine" forum, but I thought I might get more of a response here.

1977 KZ1000 with MTC 1075cc 10.5:1 piston kit. Head is rebuilt and ported by Pit Stop Performance. Vance and Hine 4-into-1 with competition baffle. VM28 carbs with 20 pilots, 120 mains, and non-adjustable needles. Also have K&N air pods.

Here's the problem. It starts great and idles good, but too high. When cruising down the highway it surges and acts like it's starving for fuel. If I turn on the choke it helps tremendously. I have not done a full throttle run because this is a fresh rebuild.

I think the needles need to be richer. I have a set of shims. Will one shim per needle be enough? I also have the original VM26's that have adjustable needles. Can I use them in the VM28's, or should I just shim the ones in them now?

I run a 1075 with V&H street header, 29mm smoothbores and pods and have the same jetting except I have my needles raised one notch...or two...can't remember as it's been a few years since I've done it. I would say you need to raise the needles for sure and would just use shims/washers to raise it.Have done it that way before and it works fine. Don't know how thick the washers were I used but you wouldn't want anything really thick as it wouldn't take much. Not sure if the needles will interchange between the two style carbs though. Maybe someone else will know.

what is your elevation that is a good starting point on lots of my hot rod stuff i raise the the needles all the way up with 26 mm 28 mm or even 29 smooth bores. that definitly will richen things up in the midrange for you. the rest of your jetting sounds close to me.

I just thought of something that I did wrong that is probably part of my problem. I switched from VM26's to VM28's. I wondered why the 28's had three vents since the 26's only had two. What I thought was an extra vent is a vacuum fitting. I'll cap it tomorrow and see how much that improves it.

DOH! :blink: :laugh: :laugh: :laugh:

Several things ware going to occur here...

1) You will be reaaaaaaaal happy something so minor fixed your issues.

2) You are going to be reaaaaaaaaal bummed of something so stupid and you'll have to hear us bust your balls...

"Wall of Dumbass" shame on this one if that's it...

Hahaha. I figure if that's the dumbest thing that I did on this rebuild, I'm ok with it! I capped the vacuum fitting off tonight and it helped, but it's still running way lean. I took the carbs off and shimmed the needles. I also took the 120 mains out and replaced them with 122.5.

I also took the intakes off and resealed them just to be sure I didn't have a leak. I'll put the carbs back on in the morning and see how it runs.

Well unfortunately you already changed the mains without testing the needle clip change Larry mentioned first...

One tip of advice that I learned off Larry and Plummen for jetting...

Do one jet change at a time otherwise you'll be chasing the issues all over the place...

Start with your mains and get that dialed in.
Then needle/clip positions and get them dialed in.
Then pilots/slow jets.

Your close so keep at it.... It will be worth the learning curve with out a doubt...

OMR

Here are two articles about adjusting VM carbs.


AAAAARRRRRGGGHHHH!!!!

They are pdf. If you would like them, send me a pm and I'll email them.

After putting the carbs back together last night, I got to thinking about how the shims help. The slides in these carbs have a floating needle with a spring on them. The clip on the needle doesn't rest on the slide like my old ones. The holes in these slides are drilled out bigger and the whole needle assembly sits inside this hole. The spring goes in first, followed by the needle. The spring keeps the needle pressed up against the part of the throttle linkage that screws to the slide.

On top of the needle clip is a bushing that prevents the needle from raising up into the recess of the throttle linkage. The only way I see to richen the needles is to either file the bushing thinner or replace it with washers. Before I go modifying parts that I'm sure can't be bought anymore, I'd like to hear from someone else that has run into this problem.

The other thought that I had would be to use the slides out of my VM26's. Is that possible? It would solve the needle issue. The slides in my old VM26's are 1.5. The VM28's are 2.0.

Also I don't see how it would be possible to just swap needles with my old VM26's. Those needles are probably 3/8" longer.

Bigger main jets will solve your problem. Put some in and move on.

I posted on another forum about the surging problem that I was having. I received on suggestion to clean and oil the K&N's. The poster said he had surging problems and did this and it helped. I was very skeptical since my filters looked clean, but at this point I was willing to try anything.

Long story short, I don't think my pods had any oil on them. I bought them used off of Ebay and since they looked clean I didn't do anything to them. After cleaning and oiling them it's like a whole different bike! I'd say the surging is 95% gone! I may still tweak on the air screws a little just to see what affect they have.

I'm thrilled at this point. The bike is running excellent. I fixed a small exhaust leak on the #4 header this morning. This weekend I plan on putting a few miles on it. I can't wait to get it broke in and see how it runs on top end.

Right on ... glad it was an easy fix for ya!