'89 Ninja 750 Carb Question

[Glaxtx]

Hey guys, I just picked up an '89 Ninja 750, and had a couple of questions. The place I got it from said they did a full tune up on it back in November, which included going through the carb, and replacing the jets. He said they went with factory specs on it. It has a Muzzy exhaust, and the guy said they put the K&N kit on it, in place of the stock air box. It's got just under 20k on it. I'm getting a dead spot around 4-5k, but below, and over that it's just fine. I can run right through that accelleration and never notice it, but if you hang too long there at cruising, it falls flat until it breaks through 5k. They told me the carb just needs to be readjusted because they didn't take into consideration the exhaust and intake, so it's running lean in the rpm range. My question is, does that sounds correct, and if so, how do I go about making the proper adjustments?

This is my first bike, so I'm still learning the mechanical side of it. I've read a lot on here, and it seems that might be a little big for a beginner, but I'm extremely cautious when it comes to riding, and other people on the road. And yes, I'm already signed up for a course.

Matt

[Green Knight]

Yes, they are right that it needs to be adjusted. I'm not sure why they didn't do it to begin with. You'll need to replace the jets with different sized ones. In the FAQ section Freakinout has a link for rejetting your carbs. It should help you out and explain what is needed.

[Glaxtx]

Knight, I've got a little update. I took it to a shop that Kawasaki recommended to me. The guy told me he wasn't sure if it was the jets or not, but there was another piece inside the carb that is vacuum operated that slides when more throttle is applied, and allows more fuel flow, unfortunately I've slept since I talked to him, and can't remember the name of the part. He said he could re-jet it for me, but thinks this other part might be more of the problem. I have just found a kit through Dynojet that is for my bike, for use with a pipe, and individual filters, which is what my bike has. Looks like this kit is $115, but he's telling me if it needs the other piece he mentioned, they are somewhere around $100 each, and all 4 carbs may need them replaced. Does what he's telling me sound about right? Also, I'm shopping on bikebandit.com, are there other sites you may recommend?
Thanks again!
Matt

[wiredgeorge]

Errrr... the flat spot you describe is in the mid-range when the bike is operating on the jet needle. The jet needle is tapered and fits into a hole that meters gas called the needle jet. If the bike is running out of gas at 4-5K rpm on constant throttle, the problem doesn't require new jets or new parts. Someone is not knowledgeable regarding the operation of a motorcycle carburetor. The solution is to raise the jet needle relative to the needle jet at that throttle position. This can be accomplished by removing the carb tops, pulling the vacuum diaphragms out and then the jet needles. SOME have clip positons. If yours does, move the circlip down one slot to raise the jet needle. This will richen the mixture a tad at the 4-5K rpm range. If your jet needles don't have slots for adjustment, purchase some jet needle shims from www.z1enterprises.com - these are precision cut washers you put under the jet needle to raise it 1/2 clip position. Cheap and effective way to solve that flat spot.

The main cause of the flat spot is the shape and lack of back pressure in the fancy aftermarket exhaust. It doesn't flow air equally through all rpm ranges and leans the mixture in the range you have problems in.... a common occurance. If moving the jet needle doesn't fix the problem but changes the rpm range, you might consider putting some packing in the pipes for adding some back pressure or putting some different pipes on. Putting new jets in for correcting a flat spot doesn't make any sense. Also, I would need to know what this magic part is that the fellow wants to swap out because I can't visualize any part on a carburetor having any effect on a flat spot other than the jet needle position unless someone unhooked your throttle position sensor line from the carburetor OR the switch it actuates on the igniter box. The TPS system provides a source of changing vacuum based on throttle position to the igniter so the igniter can alter the advance curve which essentially tells the ignition when to fire based on throttle position. I doubt this stuff is what the fella was referring to and would be curious to know what he was trying to sell you.