Jets!? AAARGH!!!

[tommystinson]

OK, I thought the Falcon was finally going to be "dialed in" this weekend, but it appears unlikely now. Pod filters, 4 into 1, and accel coils are the relevant details. The PO had installed the 4 into 1, and 140 jets along with it. I bought a set of 147.5's to try after fitting the pods and new coils. Ran like crap, regardless of needle setting. I put the 140's back in and started experimenting with needle height. 4th notch seemed to work out the best. Still too lean, but rideable to be sure. So I ordered 142.5's and 145's from MikesXS. I put the 142.5's in two hours ago and have tried a couple of clip settings on the needle. All ran like crap. Since the 140's were on my bench next to the new, unused 145's, I compared the two jets. I believe the 140's are genuine Mikuni's, while the others I have bought are not. Regardless of the orifice size, the jets are DIFFERENT! The mikuni jets have a larger taper or chamfer leading to the metering orifice, and the bore is much shallower when viewed from the threaded end. I pulled out the 137's from a rebuild kit that I never used and they appeared the same as the generic jets, I.E. NOT like the Mikuni jets. The close proximity of my neighbors prevents me from doing any more swapping tonight, but I think that when I put the 140's back in, things will improve. I'll be searching the local shops for larger MIKUNI jets tomorrow. While I'm at it, has anyone else inspected the plastic needle retainers on their carbs? (78-79's) They're numbered, and the rotation of a tiny little plastic nub is different relative to the two off-center locating nubs. You know, I could have bought a CB750...

[pggg]

Tom that TINY plastic nub skews the needles - still dunno what the hell for. Haven't noticed any numbers on any of mine though.

[strom]

Question

How much PITA is involved in raising the jet needles for a notch or two? Didn't do it yet, but I have to.
I asked a guy who was just doing a service on a friends bike and he says 2h minimum per carb. I kinda dont believe him.
Can it be done without removing the carbs?

LP

[pggg]

I reckon it's more preferable to jet the carbs for the middle notch rather than the other way round and if its tending all lean at mid/low then tweaking the floats is a better choice. Raising those needles even one notch is a coarse adjustment i.e. - too lean to too rich - compared to a couple of jet changes.

[strom]

I have a stock (ok, home made exhaust system, very restrictive) setup... original airbox and filter. Spark plugs say lean. Gas consumtion is bout 8-9 liters per 100km.

Previously the bike had 4in1 marwing, wide open, and a very open air filter (steel mesh) in the original airbox. Fuel was bout 6 l/100km.

Since that setup I only leveled all the floats and synced the carbs.
OK, and put on other exhaust system and original air filter. I think it has way less torque and HP now.

The plan is.. open the mufflers (easy) and put the old air filter in.
What to do with the carbs? I guess its gonna be way too lean then.

LP

[tommystinson]

If you don't have to mess with the stock airbox and have a good pair of snap ring pliers, you can change the needle height in half an hour on all four. If you're taking the time to inspect the slides and diaphragms and such, figure an hour or so. I agree with conventional carb wisdom: Get the main jets right first. Problem is/was the "aftermarket" mains in any size. I took the Mikuni mains to the local Suzuki dealer and he used his drill set to drill them out for me. Apparently the set is sized just for that purpose. No power needed, he just spun them through with his fingers, and didn't charge me a dime. I'll be buying him a beer or two for his efforts! Anyway, a definite improvement from the very lean condition I had, but I still could go up one more "size". The magnanimous Suzuki dealer has a smoking closeout deal on some Shoei helmets that I will be taking advantage of this week. I figure he may drill them out for me one more time, and I won't feel like a slug if I buy something from his shop. $400 helmet for $99! Like I care if it has this years current graphics! Back to the subject: The special has two types of needle jets (emulsion tubes) as TC and others have detailed elsewhere. I may be mistaken, but those little plastic nubs appear to be in different orientation on the needle retainers, which angles the needle in a slightly different direction. If anyone has theirs apart, look in the bottom bore of the plastic, where the notched end of the jet needle sits. There is a number there. mine go 1-2-3...5. I'm sure there is a Mikuni engineer still laughing somewhere!
I wonder if all the trouble some people appear to have jetting for pod filters and such is actually due to generic jets? Has anyone else had a poor experience with them?

[strom]

OK, I know it has been asked before... and explained... but still.
What RPM range do jet needles control? I figure high-WOT? Or all except tickover?
What bout float height?

LP

PS: I got a stock 78E.. with stock stuff in the carbs.

[pggg]

Strom needles affect low and midrange after that the gas is full - on flowing anyway. they'd have to be as low as to stifle full throttle top end. Floats basically cover the the whole range.