- Joined
- Jul 1, 2019
- Messages
- 113
If your carb is flooding, you will never fix the problem by changing jets. However, if it is not flooding - when you fit an air-cleaner, you might need to go down a size in needle jet. If you lower the needle, you should be able to force the motor to cough when you ride the bike, then raise the needle at least one notch and get good performance.
The first things to do when tuning a bike, is to get the float levels right and replace the needle jets. With old bikes, the needle jets wear. With petrol as fuel that is bad.
I have a VM36 on my 74/850, this is the jetting I have, my plugs look perfect.
2,150 feet above sea level.
260 Main Jet
30 Pilot Jet
6DH3 Needle, e-clip on number 3
159 P0 Needle Jet
2.5 Throttle valve slide
2.0 Air Jet (BS30/97)
Air screw: 1-1/2 turns out
Personally, I think a VM36 is too big of a carb, a VM34 is better suited for a Norton in my opinion.
We all know there is more to a nice running ride and good-looking plugs than just carb settings.
Your carb should not be flushing fuel out of the overflow tube, somethings not right there.
Measurement is from top of the rib to top of the float arm. Lift it another mm?
-correct valve adjustment
-finding true TDC with a degree wheel (only have to do this once) and setting correct timing.
Nortons will start and run with the timing way out of whack, make sure its set correctly and retard it just a tiny bit if its giving you kickback.
-good coils and correct plug leads and plugs.
Electrics count big time.
I use a RM23 single-phase Lucas stator with a Podtronics single-phase regulator/rectifier with my 2MC capacitor still in circuit.
Boyer MK4 analog electronic ignition with 5 ohm Dyna coil, solid copper core plug wires, non-resistor plug caps and non-resistor NGK BP7ES plugs at .025-.026” gap.
All that movement I see with the engine is just weirdness with the video on youtube I hope
Luke, I don't think this will affect your low end running much, but note that Chuck mentions the Air Jet. You don't. For some reason a lot of people don't seem to register that the Mikuni has one.
They come from the factory with a 2.0, which Chuck says he has fitted, and I would bet most people have and don't know. This size is generally too large for any 4 stroke, but it mainly only affects higher rpm, where too large an air jet can cause lean out. Some users will never really notice an issue, making adjustments to the main jet because there is interaction. This way you can end up with too large a main jet. And 280 is pretty large.
Overall your jets look large, and that will have some effect on your low end running.
Maybe try Chuck's P0 before changing the slide, one it is cheaper and two a 2.5 slide should be fine. The mixture screw setting should be set at a high rpm for best running, I expect you will end up nearer Chuck's 1 1/2 turns than your 1.
I have a pair of 36mms and have gone down to a 0.7 air jet to improve high end running. On my 34mms I didn't have to reduce it as far as that to get what I needed.
Fullauto on here is probably the only guy I can recall reducing the air jet size and he reported good improvements, I think he went to 1.2 or something like. I recall he was advised on this topic by Comnoz. The Victory Mikuni manual gives an indication of suitable sizes, but I think still errs on the larger end.
I did find with my 34mms that getting the float height right did help stop fuel coming out of the vents. I am not 100% sure why, but fuel venting didn't stop completely until I reduced the air jet on the 34mm to around 1.2, however it only showed after a track session with periods of erratic high end running!
No point using my float height figure (which I don't recall off the top of my head anyway) because my engine mounting and inlet manifolds puts the carbs at a steeper angle. The standard figure should work fine for you, but I agree with you and Chuck by going right to the lower fuel height end of the range to start.
(Chuck should also note from my comment that I won't agree with him that a single 36mm is too large! Really I think a single 34mm is too small. But horses for courses, you won't notice much until high rpms in high gears.)
& I was under the impression the main jet handled the top end range?
Blimey Luke, won't need a carb for tomorrow if it stays like this, just put a sail up and will be there in a flash. Given up in the shed and locked down for today. Missus has gone negative about ride over, so maybe on my own. Whoopee !
LOL, yes a bit of a difference. When we go ripping off into the Alberta/B.C. mountains it can get to 5,500 ft above sea level.Blimey, I'm only 197ft above sea level here, bit of a difference!