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Jetting, where to start? 30 Mar 2019 20:29 #801308

  • jason8887
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I've been fiddling with my KZ750 LTD with Keihin carbs for years and have never quite gotten them right. I've tried a K&N Stage 3 kit with a kerker 4 into 1 and POD filters. I've tried the stock airbox with the Kerker and a stage 1 setup. There's always something not quite right though. My current setup is a stock airbox, UNI filter element Kerker 4 into 1 header going into a cherry-bomb glass-pack (kerker can fell off somewhere along the way :lol: ). Last summer saw me trying larger 'secondary main jets' (the ones that go directly under the needle). I currently have the secondary main jet drilled out to with a #51 drill. It pulls really hard all the way until redline but is tripping over itself while cruising at 65 mph (tach reads 5500 due to sprocket setup). I'm guessing too much fuel at partial throttle openings so I am going to try to drop the needle down a spot and see what happens.

But, this all got me thinking: What is the best way to start jetting these carbs? I have a printout from the K&N website from years ago that says you should get the main jet right first, then the needle position, then the fuel level, then the pilot. This is based on a statement that the 'main jet affects everything'. Looking through my shop manual I see a curve that indicates that the pilot circuit is responsible for performance 'up to 1/4 slide opening, based on engine rpm'. The curve then goes on to show that the pilot and main circuits work in tandem until 3/4 slide opening after which the main circuit is in charge.

with all of that said, it seems like I should be able to get the pilot circuit right (right now my mix screws are out at 2.5 turns so I think my pilots need to be bigger) and my 'secondary main jet' right without affecting each other. After that i'm guessing it is the needle position and the 'primary main jet' (stock size is a #65 if memory serves).

Any thoughts from those who are more experienced than I?

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Jetting, where to start? 30 Mar 2019 21:49 #801313

  • Nessism
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I'd buy a Dynojet jet kit. That should get you very close to the proper jetting.

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Jetting, where to start? 31 Mar 2019 07:10 #801327

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I've already got a Stage three k&n kit in it. I'm not sure that a DynoJet kit is going to do anything different. I'm guessing that my setup has enough differences to the setups that the kits use that they aren't applicable without modification, which is where I already am

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Last edit: by jason8887.

Jetting, where to start? 31 Mar 2019 07:44 #801328

  • Move0ver
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At the risk of sounding like a broken record, the easiest and fastest way to tune your carbs is by booking a few runs on a dyno.
It may cost you $100-$200, but with the AFR graph you get from that, you'll immediately know where your lean or rich spots are across the rev range. Then you can adjust your jets/needles with a lot less guessing

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Jetting, where to start? 31 Mar 2019 10:40 #801334

  • loudhvx
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At least in some cases, the K&N kits are just a re-packaged Dynojet kit. I wonder if K&N bought out Dynojet.

I find, if you are starting from scratch on a new jetting formula, it is recursive... meaning what you do in one trial will affect what you will have to do in another trial. This means there is not one single jet size that will work for the main or pilot. You can get multiple formulas to work with slight compromises from one to the other.

The point is, if you at first find a main that works, but it forces you to use other settings that don't work so well, don't be rigid about not trying a different main. I often find I end up using a different main than what I started with because the needle had to be adjusted etc.

Cruising 65mph is usually a very lean part of the overall jetting. That is usually in the 1/16 to 1/8 throttle range. If it is stumbling there, but the bike rips when cranking the throttle, that means your needle or pilot is just a tad lean. I would raise the needle just a tiny bit. Half a shim if you can find it. Full shims are usually about .4 to .6 mm. If you can't find any really small shims, try to find one closer to .4mm if you can (obviously, you need four of them if we're talking about a four cylinder motor, and not the twin). To get the same effect by changing the pilot, you would have to make a drastic change, so I would avoid that and try to use the needle adjustment.

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Last edit: by loudhvx.

Jetting, where to start? 31 Mar 2019 11:25 #801338

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I have to correct myself, I do have a dyno jet kit. I had to get a new battery so I am down for a bit. I'll update this once I get back into the carbs

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Jetting, where to start? 31 Mar 2019 12:23 #801339

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loudhvx wrote: At least in some cases, the K&N kits are just a re-packaged Dynojet kit. I wonder if K&N bought out Dynojet................


There is a K&N / Dynojet relationship. See link below. Ed

www.roadracingworld.com/news/dynojet-fou...motorcycle-business/
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
The following user(s) said Thank You: loudhvx

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Jetting, where to start? 31 Mar 2019 13:28 #801340

  • hardrockminer
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There's a lot of factory thought goes into putting proper carbs on a bike. Maybe you should think about going back to stock jets, synchronize your carbs with gauges and then go out and see where your flat acceleration spots are on the rpm curve. Keep in mind that your jet needles and needle jets are wearing with every ounce of gas that goes through them....maybe change them out for new ones.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Jetting, where to start? 08 Apr 2019 18:39 #801829

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I've made some progress in my jetting quest. So far I have landed on a keihin 150 'secondary main' (the one that goes in-line with the slide). That's a little bit larger than the DynoJet 136 the kit recommends. The DJ136 measures about 1.4mm and the keihin 150 is measure 1.5mm.

Right now though I noticed that my #1 cylinder (outer left when seated on the bike) appears to to have a vacuum leak. My mix screw is 3 turns out (the max I believe) and that cylinder's pipe is not coloring brown the way the other 3 are. The plug is reading leaner than the others and the other mix screws are around 2 out. I pulled the carb off the rail last night and pulled the throttle butterfly and shaft out. I'm wondering if I am sucking air around a bad shaft seal there? Right now there is a rubber seal there but I am wondering if there is supposed to be a felt washer or anything? If anyone has experiencing replacing these i would love some input (and a link to a source for them )

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Jetting, where to start? 26 Apr 2019 13:44 #802869

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An update:
My bike was suffering from the dreaded 2500 to 3500 rpm stumble. My number 1 cylinder has always seem to run lean at idle too. The mix screw for that carb was turned out almost 3 turns and it seemed to want more while the other carbs were around 1.5 to 2. I decided to dig into that before proceeding with more jetting attempts. I talked to my brother who is a bike mechanic and he pointed me to some carb shaft seals that yamaha sells. The yamaha part number for them is 256-14997-00-00. I ordered 6 of them and installed them. They went right in, fitting perfectly. My stumble is gone and it seems my #1 cylinder mix screw is in-line with the rest of them. I'm going to start over with my jetting attempts by going back to the suggested DynoJet sizes and clip positions. I'll update when I have time

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Jetting, where to start? 26 Apr 2019 18:48 #802886

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If not already done, would perform clear tube tests in order to assure fuel levels are within specs.

Good Fortune! :)
1973 Z1
KZ900 LTD

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