1977 KZ1000 VM26 questions

I am trying to get a couple of things straightened out on my recently acquired bike. I have the benefit of having a pile of receipts for all the work that was done to it in the past. The KZ is now a 1075 and has had some other mods like a port and polish job to the head. It still has the VM26's, but they have been fitted with a Dynojet kit. Supposedly, the bike was jetted using a dyno, but some things seem goofy. The bike had NO low end at all. I could tell it was way out of sync. SO I just did a carb sync, but first I opened the carbs up to see what jets I'm running. It has a 120 main and a 17.5 pilot. The sync took care of the utter lack of bottom end that I was experiencing, but NOW I get popping on decel. The carb holders are good, and The K&N's are clean, properly seated, and oiled. No evidence of exhaust leaks either. Is it out of the question to need a 20 pilot jet? Just seems large to me from past experience. It seems that the pilot fuel screws were turned out pretty far, I reset them to 1 and 1/2 turns for now. The second thing is that the air vent ports on the carbs had vacuum caps on them. This puzzles me the most. My other '77 has short open tubes attached to these ports, so this is what I went ahead and did on this bike. It seems as though it might have worsened my decel popping, but not sure. Why would someone have capped off the air vent ports?

Someone capped the vents because they didn't know what they were doing. Originally they had hoses that went over the top of the airbox, thru a little holder tab on the filter lid, and down the right side a little ways.

Is this on your 1000LTD or A model, and what exhaust are you running?

Stock jetting on 1000LTD is 105 main, 15 pilot, 1 turn out on mix screw. 1000A model is 107.5 main, 17.5 pilot and 1.25 turns on mix screw.

With PODS I wouldn't think a 20 pilot would be out of the question. Set your mix screws using the highest idle method and see where they end up. If they end up being out further than 2 turns then you probably need bigger pilots. (2 turns is what I have read/heard anyways)

As for popping on decel.....you know what....mine started doing that after a good vacuum sync too. Didn't do it before. Highest idle method after sync put me at 1.5, 1.5, 1.5 and 1.75 turns out on mix screws on my 1000LTD. Mine pops on decel after hard acceleration and also at slow steady speeds (20-25mph) and RPM's around 3K. I'm thinking of trying 17.5 pilots even though I am running stock airbox and stock exhaust and stock pilot is 15. The popping and mix screws being over book stated settings makes me think its still lean. Or pilot circuit could be dirty but I really think it's not.

But then again.....a friend of mine had a Gallardo and now has a Murcielago and guess what.....they both have popping on decel after hard acceleration. They are both fuel injection of course so not a good comparison....but watch this vid of an F40 (also injected) and see if this is kinda what you get.....



Second one from inside.....first 40 seconds give you a better idea of what it probably really sounds like.....This is very similar to what I hear through my helmet.

More of a gurgling sound than a POP - POP - POP huh.....that's what I get.....yours the same?

Thanks for the info! I never knew they had different jetting between the A and LTD, that's interesting. This one is the A model. It has K&N pod filters, and a Vance & Hines 4-1. (the fiberglass packing is gone from the baffle, so I am going to repack it and see if that helps a little) The characteristic of the popping is not too far from the sound in the videos you posted, yes I would say more of a gurgling sound. :lol:

I'll have to try and see if it gets any better adjusting the pilot screws. I went ahead and ordered a set of #20 pilot jets so that I have those on hand if need be. The bike has been running really well, I am just very paranoid about the possibility of running it too lean..

Yup, different jetting and different needle and needle jet too. Has to be because of the different exhaust they had. I don't know of any other differences between the two that would cause the need for different jetting.

I've been trying to find more info on what causes this gurgling on decel. Remember this is all just stuff I have found online so as always.....grain of salt.

Seems it's completely normal on high performance engines and race cars. Excess fuel in the exhaust igniting on decel causes the pop and flames. When the throttle snaps shut there is still a momentary burst of fuel that gets pulled into the combustion chamber....but because there isn't enough air to burn it all in the combustion chamber it passes thru and ignites in the exhaust. So that's actually a rich condition.

Popping on decel also is common in single overhead high performance engines with valve overlap. Again caused by unspent fuel getting into the exhaust. This condition is further exaggerated with open or large exhaust pipes.

Seems to me both of these causes is actually caused by a rich condition! But not in the pilot system but in the main......but not that its actually running rich in the mains.....just that when the throttle is closed it causes a temp rich condition. So I dunno man.....maybe someone who had one of these bikes off the showroom floor can tell us if it did it when factory fresh.

I'm going to throw my 17.5 pilots in there and see what happens anyways. I already have a set from when bike was on PODS. (Yes it was running lean on PODS, PO did it not me, I took PODS off ASAP) I'll clean the pilot system AGAIN while the carbs are off just to be sure. I'm also going to recheck my valves while I'm at it.

I've always read on this forum popping on decel means pilot system is too lean, either from jetting or dirty. So there ya go. Confused much?

20 pilots. I always called the symptoms you are explaining a lean miss. Sounds funny to call it that since it is not missing on deceleration but all the gear heads around here know what you are talking about when we use that term.

madmatt1 wrote: I am trying to get a couple of things straightened out on my recently acquired bike. I have the benefit of having a pile of receipts for all the work that was done to it in the past. The KZ is now a 1075 and has had some other mods like a port and polish job to the head. It still has the VM26's, but they have been fitted with a Dynojet kit. Supposedly, the bike was jetted using a dyno, but some things seem goofy. The bike had NO low end at all. I could tell it was way out of sync. SO I just did a carb sync, but first I opened the carbs up to see what jets I'm running. It has a 120 main and a 17.5 pilot. The sync took care of the utter lack of bottom end that I was experiencing, but NOW I get popping on decel. The carb holders are good, and The K&N's are clean, properly seated, and oiled. No evidence of exhaust leaks either. Is it out of the question to need a 20 pilot jet? Just seems large to me from past experience. It seems that the pilot fuel screws were turned out pretty far, I reset them to 1 and 1/2 turns for now. The second thing is that the air vent ports on the carbs had vacuum caps on them. This puzzles me the most. My other '77 has short open tubes attached to these ports, so this is what I went ahead and did on this bike. It seems as though it might have worsened my decel popping, but not sure. Why would someone have capped off the air vent ports?

Not trying to hijack your thread here but when I got my bike 77 KZ1000a1 it had a stock airbox. My carbs also had the air vet ports capped off. I have kept them that way and had the bike running quite well before I wrecked it. It did run perfect but I always thought that was due to the excessive oil burn due to hardened oil seals. If I am not running the stock air box what am I supposed to to with these air vent ports? Leave them capped? Attached tubes and let them lay? What are the air vent ports for?

It was always my understanding that the vent tubes were needed to allow the floats to function properly. That's what puzzles me as to how the bike can still run so well with them capped off. Personally, I just make a loop with some hose (no kinks!) and route them pointing down between the carb bodies. As mentioned earlier, they originally ran back to the airbox.

Well today I went and picked up an exhaust repack kit and packed the baffle in my V&H pipe. (It had nothing on it to begin with) I went out for a test ride, and would have to say that my "decel popping" must have been reduced by about 50 percent if not more. Low end power is better than ever at the current time. When I have more time, I will do some pilot mixture adjusting, and perhaps go ahead and give the #20's a try if I get so ambitious...will report back!

kaw-a-holic wrote: ... What are the air vent ports for?

Serves basically the same purpose for gasoline in the float bowl as vent in the fuel tank cap serves for gasoline in the fuel tank.

Good Fortune!

madmatt1 wrote: Well today I went and picked up an exhaust repack kit and packed the baffle in my V&H pipe. (It had nothing on it to begin with) I went out for a test ride, and would have to say that my "decel popping" must have been reduced by about 50 percent if not more.

Do you think it reduced the popping that much or just cant hear it as well now? If it really did reduce it that much than that makes me think I could have a problem with my exhaust. Insufficient back pressure causing it? Just kinda guessing here. But I'm betting my exhaust is leaking at the head, pipes have been off at least a few times I know of without gaskets being replaced.

I have a new set of gaskets from Z1E, just haven't gotten around to installing them yet. Maybe this weekend I'll put 'em in and see what happens.

Slightly enriching pilot circuits will often reduce or eliminate popping on deceleration, say ⅛~¼ turn each.

This presupposes clean pilot circuits, correct fuel levels, decent sync, adequate clean fuel supply, and no air leaks.

Good Fortune!