Which carbs came on a 750 four cyl

I am having some exhausting "cold start" issues. The bike has a .738 liter engine, not the original motor on this '78 KZ 650 B2 frame. I have pored through the archives and read quite a few threads on bikes having a hard time starting. Mine has typical symptoms. It won't start without starting fluid when it has been left for a few days. I have gone through everything; spark, compression; timing; enriching circuit. After it gets hot, it runs great. It starts fine when hot or warm and usually the next day, although this is hit or miss.
The thing is, it has the original mechanical VM24SS carbs, with the original throttle control and vacuum petcock, etc. I have a set of CV carbs, although I would have to alter the throttle hook up, since it is a sigle cable type.
My question is what carbs generally came on a .738 liter 4-cyl engine? I believe I traced this motor to 1980 E model or LTD perhaps. Anyway, The starting circuit for the VM24SS carbs is built for a 652cc (KZ650) engine and would really supply a fixed starting solution when the "choke" lever is fully egaged. Since this engine is larger by 86cc would this cause a starting issue?
I've hammered away at all the other possibilities. Do you think CV carbs that were originally stock with the 750-four cyl would supply a greater amount of fuel/air in its starting circuit, enough to make the difference.
I'm looking for confirmation here, if you follow my trian of thought.
i157.photobucket.com/albums/t43/benfs/78rightside.jpg

My bike came with Keihin CV34s, but I switched to Mikuni BS34s and am quite happy with them...

Cold start, no choke:



Post edited by: guitargeek, at: 2008/03/06 18:38

Am thinking the problem is not size of the carbs and is not the 86cc larger engine displacement.

Will presume the enrichener linkage to all four carbs is being correctly operated by the choke lever, meaning the starter plungers are all being lifted when the choke lever is lifted.

But does seem like a problem may exist in the enrichener circuit (choke circuit) inside the carbs.

Could be clogged starter pipe air bleed holes or maybe clogged starter pipe fuel pickup jet in floatbowl or even clog in passage from fuel pipe to plunger chamber.

The enrichener circuit requires correct amount of both air and fuel in the proper mixture in the plunger chamber (when the starter plunger is raised by pulling up the choke lever) to be drawn into the carb bore, as this design is supposed to provide the proper amount of fuel mixture to crank up a cold engine.

All this is designed to function with the throttle left alone and in the completely closed position.

Just keep hand off the throttle when using the enrichener (choke) to crank up a cold engine, and don't be rotating the throttle (i.e., don't be opening and closing the throttle) while the starter motor (or kick starter) is being operated.

First test would be attempting to crank when cold by using the choke lever in the normal way, and without touching the throttle so that it remains completely closed.

Good luck with the diagnosis and correction!

I don't rotate the throttle at all. I believe that affects the normal operation of the starting circuit.

On my other kaw (link picture in my profile), Which starts immediately no matter how cold, all I do is move the choke lever back off slowly as RPM's increase. I sort of nurse the choke off until it holds at idle and let it warm up so it will take off in first.

Carbs on the bike we're talking about were done by WG and they are in great shape. That's (the enriching circuit) not it, spark is strong; compression is good; timing is good. It's just not getting enough fuel in the start.

Just thought I'd see if anyone else has tried the VM24SS carbs on the .738 liter four cyl motor.

I'm going to try my CV's, but I need to identify them to make sure they are the correct carbs for this engine.

The only other possability I can come up with affecting the starting circuit, besides these carbs not being designed for this size displacement, is that I have a 4 into 1 Kerker exhaust on it. This exhaust should flow more air (correct me if I'm wrong) than a stock 4 into 2. Perhaps during start up only, the extra airflow is leaning out the starting circuit a bit too much. add this to the bigger dispalcement and it could exaggerate the problem. . . just a thought.

I have an extra 4 into 2 exhaust I could swap out and see, although it is nice looking and sounding with the Kerker . Of course then I might have to put smaller main jets in (correct me if I'm wrong).

Patton, I definately have been focused on the carbs for quite a while, but I have come up with nothing there that is out of sorts. Carbs have been the usual culprit in most of my ill running conditions. I have other carbs, both VM's and CV's I can monkey around (I'm real good at monkeying around) with and swap to show same symtoms with different carbs . . . Thanks, Patton.

Do you turn the petcock to the "Prime" position for a few seconds prior to starting it? If not, then give that a try.
It may be time to rebuild the vacuum petcock.

The enrichening circuit on the VM24 has small passageways and if some of them are plugged, the circuit doesn't work as well as it should. You also failed to mention which airbox/filter set up is being used and what jetting is being used. If the bike has a somewhat lean pilot circuit, it can start fairly hard and warm slowly. The 750 came with both the CV34 and BS34 assemblies (depending on model) but the VM series (VM24) work fine if the enrichener circuit is clean and the jetting is correct. The starting issue is really not dependent on the carb type as much as the other factors I noted plus the general state of tune of the bike.

I use steel's technique on my kz400 with CV34 carbs. When I pull the cover off in the morning, the first thing I do is switch the petcock to prime while I'm pushing it around to the front (about 30 seconds). Switch it back to On and start'r up.

See if that helps.

Thanks, WG, if I understand you correctly these carbs (VM24SS) should work just fine on the .738 liter engine. The bike has a stock airbox, paper filter, and the main jets are 102.5's, Kerker 4 into 1 exhaust. I'm not sure of the pilot needle size for the fuel screws ('78 so fuel screws point straight down center, front, middle of each carb) and will check them. How will I know size (I thought the fuel/pilot screws were of one generic size)? I'll check plugs to get an idea of weather its running lean or rich soon, 102.5's were acouple sizes up from what you set it at when it had pods. It seems to run like a chmap once it's hot.

Steel: Excellent thought, I will try. It is clearly an old petcock, vacuum style, and could be part of the issue. But would not the amount of fuel still in the float bowls be enough for the first blast? Thanks, great idea!

Steel, I tried the petcock to prime and still same thing. I have an extra manual petcock and could change the vacuum petcock out to a manual and cap the vacuum port on the '78 VM24SS carb and see if that helps.

Question: Is there any variables as far as changing the enrichener circuit so it will put more fuel in the fuel to air ratio (when choke lever is on full)? I was thinking that the enriching circuit is sort of a fixed deal, not adjustable. Obviously, the carb is adjustable with jet sizes and needle size and placement, etc. but this occurs after the choke lever is pushed down and the enriching circuit is out of the picture.

I am thinking maybe the 4 into 1 Kerker is leting more air pass through and leaning out the starting circuit.

If I changed the exhaust out to a 4 into 2, might that help? (I like the sound of the Kerker, though)

It won't start without starting fluid when it has been left for a few days. I have gone through everything; spark, compression; timing; enriching circuit. After it gets hot, it runs great. It starts fine when hot or warm and usually the next day, although this is hit or miss.

Here's my reasoning:

If it's the enrichener circuit, then it's going to behave the same way when it's cold, whether it's been four hours or four days, it's not going to be "hit or miss".

So instead of being temp dependent, the problem appears to be time dependent, in other words it happens after enough time has passed.

When you shut down the motor there is no more airflow cooling the motor, the heat from the motor heats the fuel bowls causing the gasoline to evaporate rather quickly.
When you try and start the motor with the petcock in the "on" position, no fuel flows until there is enough vacuum to open the petcock. If there is a vacuum leak, or a hole in the diaphragm, then it's going to take more vacuum than is available to open the petcock. So it won't start without starting fluid.

If you start the bike, get it warm, then let it sit for 5-6 hours (or put a fan on it if you are in a hurry), then it restarts just fine, then it can't be enrichener circuit at fault.

Seems logical to me anyway