bore engine out

i have a 1977 kz1000 i was thinking of overboring up to the next size. will vm28's be enough? i do not plan on hot-rodding it. just a day to day rider. thanks for the input. i realize that there is another thread similar to this, but it did not address this issue.

there are plenty of very fast bikes with 28-29mm carbs,ive seen some really fast 1075-1105s running them!

Stock 28's will be just fine.

thanks for the help. all i can get are 28's.. i do have a set of 29's but they will not behave for me, run rather rough. i've did all i know they still won't run smoothly. the reason i asked was i have had alot of success with vm28's.

Arobsum, you're lucky if you have some 29 smoothbores that you don't need. I've kept my eyes on fleaBay to see if it was possible to buy some, but they are all selling for way too much. Back on topic though, if you change to some hotter cams and slightly higher compression, then you will probably not be happy with the 28's as they won't deliver at higher rpm. If you don't use high rpm, then the 28's could be tuned to produce a lot of torque without changing cams. I guess it all depends on how and where you ride; if you ride twisties, then it's always nice to be able to keep the bike in gear and produce power at high rpm's, but if you ride city blocks then you're never going to run out of revsB)

i know of an extremely warmed over 1105 that puts the fear of god into me whenever i see it coming down the road.its got a 900 crank,1105 kit with a milled and ported head with a healthy set of cams .he has a big autometer tach mounted where the stock gauges normally go with the shift indicater set at 10000rpms,and he is not afraid to use it!he has a set of my 29mm smoothbores with pumps that i gave him to try on it,he says the 28s work better!whers wired george?

wireguy wrote:

i know of an extremely warmed over 1105 that puts the fear of god into me whenever i see it coming down the road.its got a 900 crank,1105 kit with a milled and ported head with a healthy set of cams .he has a big autometer tach mounted where the stock gauges normally go with the shift indicater set at 10000rpms,and he is not afraid to use it!he has a set of my 29mm smoothbores with pumps that i gave him to try on it,he says the 28s work better!whers wired george?

thats exactly whats happening to me..the 28's work great and the 29's are rough.. can't figure it out. i'm glad other folks have had this problem..it's not just me. i have heard alot of good things about the 29's, but they sure are finnicky.

Please give me a choice about responding to a specific issue as opposed to calling me into a discussion. I have a carburetor repair/rebuild and selling business and am under constraints as to how and what I respond to... I will try and reply in a manner that doesn't point to my business or services...

The question was are 28s sufficient for a "built" motor? First, it depends on how you planning on riding. By implication, you have built up a motor for increased performance and I guess that is how the answer should be slanted...

Quite honestly, if your motor is built up, the 28s may not provide enough gas. The fact that the venturi is 2mm larger allows more air through them but they certainly won't flow more gas for most of the rpm range. The fact is, the later 28s were set up pretty lean and some internal stuff needs to be modified so that they can flow as much gas as say a VM26 from 76... The early 28s on the Z1s flowed more gas but are a tad harder to tune and keep in tune. The 29s flow more gas because they have larger fuel pipes... In other words, your main jet on a 1979/1980 style VM28 is the fuel inlet opening... same as the VM26. Neither will flow sufficient gas to feed a heavily hot rodded engine so that it will make max horsepower. Both the VM26/28 will work FINE for day to day street riding but if you want your engine work dollars to pay off as well as possible, you need to look at other carburetion solutions.

The VM29 smoothbore has achieved cult status because it is the ONLY reliable VM series carb that flows enough gas to make a hot rodded engine happy at higher rpms. (The VM33 is a tuning nightmare for a lot of folks so I won't mention these)... For all the voodoo talk where folks make claims about the smoothbore carbs, just look at the fuel tubes... they are over twice as large as the 26/28 family. That is their main advantage. They also mix the air/fuel due to an improved venturi/slide design but their main advantage is having the increased fuel to mix... That is why you need to use good sized fuel lines and a good fuel tap so THEY don't become the main jet for the carburetors. BTW: The VM29 is NOT finicky to tune. They just need to be cleaned properly and the person doing the tuning needs to understand what they are doing plus there are some tricks involved, as with any art. I have no problems tuning these carburetors properly for my customers.

If you have a hot rodded engine, switch to CV carburetors if you don't like the bite of the high priced smoothbores. MOST people using smoothbores, don't really need them and the difference they make is ONLY at the upper end of the rpm range. A good and properly set up set of CV carbs will feel better through the entire rpm range and a hot rodded engine won't run out of gas. I enjoy tuning and testing Mikuni CV (BS family) carbs on my stock carb test bikes because of their nice performance through the entire rpm range. I am going to try and fab some adapters so I can use a stock airbox during the coming weeks as the CV carbs now require pod filters since I have no way of adapting them to the factory airboxes I have.

sorry george,didnt want to put you in a spot!but when it comes to carbs,ya da man!

Hello Wiredgeorge, sorry to butt in but I thought 28vms only came on the early Z1, all the others were 26 including the early 1000's. I have smoothbore 29 on my Z1 but prefere early 28's that are bored to 30's. My original Z1 had that and had no problems with carbs. I would turn about 11,500 when racing. They just didn't get good gas milage, of course taking 160 mains and boring them didn't help. tfh

well, you folks have certainly answered my question. i wanted to up one size on my 77 kz1000, but keep it relatively stock, and i was wondering if the 28's would be enough. thank you. but i am learning alot reading the other posts too. i never knew you could "bore" out a carb. thats cool.

A set of 26s can actually be bored out a bit farther than 28s as they actually have greater outside diameter on the venturi that goes into the intake manifold. There are still shops that do this but they charge an arm and a leg and I am not sure any slight performance gain would be worth what they charge. VM28 carbs w/accelerator pumps came on a number of models in 1979/80 and the Z1R had VM28 carbs without the pumps. FWIW: IF you change out internal stuff in the pumpers and get rid of the pump mechanisms, the later VM28s can be made to work real well with a modified engine. The MAIN function of the pump is to squirt some extra gas in one time when the throttle is cracked open to avoid a lean condition set up by the puny jetting of the time. I have seen considerable problems trying to jet pumpers for high performance applications... they work best in stock type set ups. I attached a set of VM28 w/pump from either a 79 or 80 KZ1000. Some KZ1000 models of that vintage also had VM26 assemblies with a two stage accelerator pump where the VM28 had a single stage pump.

Post edited by: wiredgeorge, at: 2005/12/23 08:31