Main jets,needle position and carb synching ??

Ok guys, If I change my main jet from the current 102.5 to the 105, will I need to re-synch the carbs ? Will I need to change the needle setting ? If I change the needle setting, will I need to re-synch the carbs ? What changes if any make it to where the carbs will need re-synched , after being synched with other settings ? Thanks Wayne

If it is on the 78-C2, you shouldn't have to resync the carbs. When you change the jets you won't be doing anything with the sync adjustment.
After you have the jets in, see how it runs. If you have to adjust the needles then you will have to resync because you will be tearing the top end apart.
Les

Before it comes back to bite me. I mean the top end of the carbs.....:laugh: :laugh:

Post edited by: the milkman, at: 2006/03/14 16:02

Went for several rides today and it was sputtering at low speed in third gear at about 3,000 Rpm again. It also has a bogg from idle to 2,000 Rpm range when pulling away from a stop sign. I checked the sync and one carb was off a little. Re-synched the carb to the others and it is better at low speed in third gear just puttering around at a constant 3,000 Rpm, but still has a bogg in the idle to 2,000 Rpm range pulling away from a stop sign. Bike idles great at about 1,000 to 1,200 Rpm with no noticable miss. Could the needle setting be causing this bogg or the jetting ? Thanks guys.. Wayne

(going by the VM24SS theory in my manual)

From the theory of operation that I've read (here, manuals, and elsewhere) the main jet only has full effect from about 3/4 to full throttle. The needle controls the flow through the main jet from about 1/4 to 3/4 throttle or thereabouts. At idle and just a bit above to around 1/4 throttle the pilot jet supplies the fuel.

So, I would suspect the pilots first as below 2000 RPM seems to low for the needle to have much effect (this isn't an exact science). The next question to answer is whether the mixture is too rich or too lean when it bogs.

WRT to synchronizing. All a sync does is match the slide openings so the same amount of air is being drawn into each cylinder. It does not affect the fuel delivery per se. That said, proper sync results in a smooth running engine. Unless the adjustment screws are bumped loose, you shouldn't need to resync every time you take the carbs apart. It would be wise to check it once your problem is solved.

HTH,

- Nate >>

I've uploaded some more KZ650 or general carb tuning files, primarily from Murphyaus old 650 site, to the not model specific section of the file base. I've downloaded everything that I could beg/borrow/steal in the last few years, not always sure of the source.

Wayne,
When I hear the word 'bogging' I think of too much air from opening the throttle. The idle jets and jet needle handle this low rpm range. I remember from your first threads about the carb issues you had, and I think you had reset the needle clip and upped the main jet size. But I don't remember if you changed the idle jets. You may need to go up to the next size on idle jets.

I just remembered something, after all the previous adjustments and jet changes you made, did you adjust the air screws again? This helps in that rpm range also.

Post edited by: OKC_Kent, at: 2006/03/16 15:03

I didnt change the pilot jet and I did adjust the air screw to two turns out. It seemed to idle good at two turns out.

Factory Service Manual for KZ650's
Poor Running at Low Speed:
Spark Wear
Battery voltage low
Spark plug dirty, defective, or maladjusted
Spark plug cap or high tension wiring defective
Spark Plug Cap not in good contact
Spark plug incorrect
Contact breaker points dirty or damaged
Contact breaker point gap maladjusted
Capacitor defective
Ignition coil defective
Fuel/air mixture incorrect
Pilot screw(s) maladjusted
Pilot jet, or air passage clogged
Air bleed pipe bleed holes clogged
Air cleaner clogged, poorly sealed , or missing
Air cleaner duct poorly sealed
Starter plunger stuck open
Fuel level too high or too low
Fuel tank air vent obstructed
Carburetor holders loose
Compression Low
Spark plug loose
Cylinder head not sufficiently tightened down
No valve clearance
Cylinder, piston worn
Piston rings bad(worn, weak, broken or sticking)
Piston ring/land clearance excessive
Cylinder head gasket damaged
Cylinder head warped
Valve spring broken or weak
Valve not seating properly (valve bent, worn, or carbon accumulation on the seating surface)
Other
Ignition timing maladjusted
Timing not advancing (spring broken or stretched)
Carburetors not synchronizing
Throttle valves don't slide smoothly
Engine oil viscosity too high
Brakes dragging

Post edited by: gas, at: 2006/03/21 11:26