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TOPIC: Choke Air mod

Choke Air mod 4 months 6 days ago #23540

  • Kawboy
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What size did you bore your pressed in jets too Russ?
The air supply orifice is a cast finish in the carb body ( good cast) but definitely not a bored finish. I ended up predrilling the bores before tapping even though the bores were only smaller by .004" than the tap drill. It did make a difference.

I do like the idea of being able to remove the restriction and changing if the mod ends up being too lean or too rich.

Mikuni on the VM series carbs and whatever series the KZ1300 carbs are, have used an enrichening idle circuit to effectively create a "choke circuit" On most carburetors, the "choke circuit " mechanicals open the main butterfly a certain amount to achieve a reasonable fast idle and at the same time choke the inlet air to pull additional fuel through the main jets to supplement the fuel entering the engine. With these Mikuni carbs, they created a mini carb circuit within the carb that when open by what we call the "choke plunger" actually opens up a mini carb circuit that allows air intake with an enriched fuel amount to achieve a fast idle with enriched fuel. On the old Kawasaki triple 2 stokes we had a "start lever" beside the throttle. You didn't touch the throttle, only pressed down on the start lever and then kick started. You held that start lever down until the bike started to what we called "4 stroked" which was when the enriched circuit was too rich to let the engine run properly, then released the start lever and blipped the throttle to keep it running until the idle circuit would keep it running. Then you could stop attending to the throttle and put on your helmet and off you went.
Our "choke plungers" is the same as the start lever I just described just operated through a lever on a rod instead of the cables pulling up directly on the plungers.

I think the real issue is that intake vacuum at idle has been reported between 7-10 inches of mercury which by most common standards is really low. Usually we see between 14-18 inches of mercury at idel rising to 24-30 inches at a fast idle. The cams in the KZ1300 are advertised at 270 deg. and 280 degrees duration with 50 degrees of overlap. Add to that stretch in the cam timing chain will delay the intake ( and exhaust) timing which will also have an adverse effect on intake vacuum and the Mikuni enrichened idle circuit does not compensate for nor can be adjusted to compensate for this mechanical "wear". This might explain why some of the members which have replaced their camshaft timing chains and primary/ secondary chains find that this carb mod is running a little on the rich side and some fiddling of the choke lever is required.

(Some of my engineering background from Nuclear is showing.) Engineering Change Control @!#$@#$ Retired 10 years ago and I still have it inbred in me.

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Choke Air mod 4 months 6 days ago #23541

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The OD on each jet was different because, as you correctly said, the hole in the carb is a cast finish. Just made the jet a slight interference fit on each one. They had a 2.5mm hole drilled in them, I can pull them out if needed and change the hole size if needed although after reading through the posts and seeing how well the bike starts I think it's about right.

On another note, now I've posted pictures I can't see them - just says "you don't have permissions to access this page" where the pictures are supposed to be so I don't know if they are on the post?
Russ

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Last edit: by Russ.

Choke Air mod 4 months 6 days ago #23542

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Russ wrote:
On another note, now I've posted pictures I can't see them, I don't know if they are on the post?
Russ

I'll take a stab at this Russ. New Members are required to make 2 posts which have to be "approved " by either the Administrator (KZQ) or a moderator (which I am). Once we've determined that you are in fact interested in discussing all things KZ1300 and Voyager and not signed up to advertise your "other" personal business, then you're free to post as you wish keeping on topic of course. I think what may have happened here is a temporary ban on posting pics until you've been approved for the first 2 posts.
You can now go back in and "edit your posts and at that point "attach" pics which are less than 2 mg in size each. Once you've attached the pics to the post, you can place your cursor in the post where you want the pic to show up and then "insert pic" or "insert all". A text will appear showing that the "attachment" is in the post. Once you "submit " the post, the system will open the attachment and show it as a pic in the post.
Hope that helps.
KB

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Choke Air mod 4 months 5 days ago #23547

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Thanks for the heads up Kawboy, all sorted now.
Russ

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Choke Air mod 4 months 5 days ago #23548

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kawboy wrote: "I think the real issue is that intake vacuum at idle has been reported between 7-10 inches of mercury which by most common standards is really low. "

The best vacuum reading I can achieve on mine is 8.5"hg but this I have found is very close to Binding the linkage, as the adjusters are screwed in all the way. So I've learned that 8"hg is my max. Reports I've seen here (a few over the years) in the range of 18" to 20"hg can not be true. As I've pointed out several times, the enriching diaphragm starts to move (closed) at 10"hg and is fully closed to the set-point at 22"hg ! I know because I've tested this. My conclusion is that the "User" has gauges or "sticks" that are calibrated in cm/hg. 8"hg = 20.32 cm/hg. Seems fairly obvious the user is misinterpreting the readings.
While on the topic: The manual indicates the tiny set-screw in the richening diaphragm cover is to adjust the spring pressure. This isn't correct although it makes the explanation a lot simpler because everybody can understand "spring-pressure". The set-screw is adjusted at the factory and in fact is a travel-limiter for the piston that closes off the air in this part of the circuit, which the manual claims prevents back-firing under hard deceleration. When fully closed the piston is actually several thou. from completely cutting off the air. Technically speaking, the pistons, springs, covers and diaphragms must be kept with their original carb body to maintain this setting. The variances are slight but this is why the three (individual) set-screws may appear to be at different heights in their covers. I can guarantee that over the years of various P.O.'s and innumerable disassembles, all of the above mentioned components have been swapped around unwittingly. And this would include me. This critical factory adjustment was likely to achieve a very specific and required "reading" on some piece of equipment, at the time. It's never created a back-firing problem for myself and I'm not aware of any topic being started on the subject. So the manual is clearly over-stating what appears to be a moot point.
"And that's all I have to say about that".;)
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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