TOPIC:

Hello all!

I have been leaning on this site the past few months as I restored a 1977 KZ650B1 model with 20,000 original miles. I have kept it as close to stock as possible during the rebuild except for one item: I decided to go with pod filters on the carburetors.

I have the early-style slide carburetors on my bike. They have the air mixture screws on the top left side of the carburetor body.

My question is jet sizes. I have the stock exhaust system on the bike, #15 pilot jets and #105 mains. The needle is still in the stock position. The mixture screws are 2 1/2 turns out from seated. The bike runs OK like this but the idle tends to jump around and it seems to dislike starting in cold (Minnesota) weather. I need some advice from others who have been down this road -- how did you dial in your carbs? I have vacuum gauges, multiple jets and time to monkey with it but I need to know if sticking close to the stock jet sizes and messing with the mixture screws is the right path, or do I need to start swapping out jets and adjusting the needles?

Hey there!

You shouldn't need to be worrying about rejetting on them just from adding pods. Tuning is what you need.

Your pilot and main don't really have anything to do with the cold start problem. That'd have to do with tuning your enrichener (choke). Your carbs sound like they have the enrichener adjustment on them.

Your main definitely doesn't have anything to do with the idle OR the cold start as it's not part of either of those systems.

Idle sounds like it just needs some tuning too.

Does it only jump when cold? When warm?

One thing you will find is that it's a definite plus to have carbs with a completely seperate circuit for things. (Enrichener, Idle, Low/Mid, High).

Post edited by: caffcruiser, at: 2006/11/15 20:27

The starting problem is this: it sits a few days and when it is damp and humid out it will start with the choke on, then the idle starts to drop and it starts to cough and stumble, then die. After that I pull the plugs and they seem wet, almost like moisture gets in the cylinders. This fall when it was warmer out it would run and idle fine but when the temps dropped below the 40s and it got damp, she became hard to start.

Oh I have a new battery, new voltage regulator and voltage rectifier on the bike, the charging system is at full snuff.

I tend to view this issue a bit differently. First thing, if you live where it is cold/damp, read the coil repowering mod write up on my web site and dab some dielectic grease inside your plug caps.

Next, I feel it is necessary to rejet with pods and your jetting isn't far off. At start up, the pilot and choke circuit control starting. Having good gas flow through the hole in the bowl to the choke pickup tube and ensuring that the little hole in the tube tip is clear is very important. Both the check valve in the float bowl and little hole in the tube tip tend to clog up. Spray some aerosol carb cleaner in the hole in the side/bottom of the float bowl where the tube seat and it should go through the check valve and out the top edge. If it doesn't you will have to do some cleaning. Poke a small wire through the hole in the tube tip and the holes in the sides of the tubes on each carb where the choke picks up gas.

OK... once you have that done, spray carb cleaner in the pilot air jet... located on venturi edge. One of those small 3/8" holes is an air jet for the main jet and the other the pilot jet. The pilot air jet is generally on the same side as the pilot jet. You should see carb cleaner come out the small hole on the bottom of the venturi on the engine side of the needle jet. If it doesn't, you are due for a good cleaning; either dip or ultrasonic or both.

If the pilot circuit is clear, you probably could use some #17.5 pilot jets... the #15s are lean and since a cold bike likes a rich mixture, this should help starting and quicker warm ups. If you idle hunts up and down, it isn't a jetting issue... you have a leak somewhere... choke plungers, caps on vacuum ports, carb holders, etc... any rubber part can be leaking. Since you have air screws, it isn't as likely that the orings on the air screws are the culprit but I would also check those. If the idle hunts (varies in rpm), try jiggling your choke arm when it does this. If the idle quits moving around, the choke plungers are the likely culprit.

Hope that helps.

Thank you for the thorough analysis.

I gave the carburetors a complete clean and rebuild when I had the rest of the bike apart. All jets, needles, seats, etc. were pulled and cleaned, the carb passages cleaned and I used 10 psi compressed air to blast out any residue.

The pilot jets on my carbs are the Mikunk VM22/210 size, I have the option of a #15 or a #20 pilot, no #17.5 is listed. I could bump out to a set of #20 jets, but this will require adjusting the air mixture screws. How many turns out from seated is considered too many before I should go up or down in pilot jet size?

I will add some grease to the plug caps per your post as well. Thanks again.

I've got a 77 b1 and before I added my Mac exhaust was running a stock configuration with pod filters, just like you're running now. We went trial and error when jetting the carbs on the rebuild and ended up going one size up on the main jets when I was running just the pod filters. I didn't change pilot jet sizes until I added the 4-1 exhaust a couple months later, and now I'm running 17.5 pilots and she's a first kick starter and runs like a top through the entire rpm range.

I sometimes wish I could just blow 10 psi of compressed air through carbs and get stuff out of the inside that way. When I made my suggestions on what and where to clean, they are based on some first hand trial and error. Anyway, get some #17.5 pilot jets (VM22/210 STYLE). The #20 pilots are too big for your application and will be a bit rich.

Update:

Had some time and cooperative weather today (Sunday, 42 out) so I messed with a few things. I cleaned out the plug caps with some contact cleaner, then added a dab of di-electric grease to each cap. She cranked a few times then fired right up. I adjusted the idle speed after she was warm to about 1100 rpm and everything seemed fine. I also fattened the mixture screws up (2 1/4 turns out now instead of 2 1/2) and she pulled really nice on the throttle and the idle speed didn't bounce around. It seems pretty well sorted... just in time for winter!

I appreciate all the prompt, thoughtful replys to my original question. I'm sure I will be back over the winter as I try to finish off the little bits and pieces on my '77.

The bike runs OK like this but the idle tends to jump around and it seems to dislike starting in cold (Minnesota) weather.

Yeah, I can relate to that problem...I don't like to start up on cold mornings...

Re-jet the carbs, one size for the pods, another if you go to straight pipes, and so forth. On almost all bikes, the carbs, airfilters, and pipes are joined at the hips. Change either the filters or pipes you usually have to re-jet. It happens often enough so I figure it's part of anything I do with either of them.

One more thing--if continued exposure to dampness is part of your picture, you might want to reconsider pods. Get them wet, and you're going nowhere for a while.

If it runs better after messing w/pilot jets-cold,cruising fine-just adding air cleaners alone u will need 10 to 11% more main jet to start with, to be rich enough on the high rpm side! play around a little bit(up in size,fisrt)You`ll be able to tell how it acts.Stumble and such,back it off a size.start two sizes up to start,above your 10%.