carb changes

hello all I think after running my bike a mk111 for a couple test rides i'm gonna have to up the main jets now has 932 's amals with 230 main can't get it to run smooth without closing chokes. air valves closed it runs smooth try to open valves after she warms up and she sputters and bogs. do you think 260 main might improve it . M.C.

MC1011,
You haven't said what modifications, if any, you have made to the machine, or at what throttle position you experience a flat spot. Any advice you get without thoses inputs are speculation.

RS

hi mods to my 75 850 new carbs dunstall type mufflers 1.5 headers no balance tube. tri spark ing. stock motor .Made timing change turned just a hair counter clockwise made big improvement now runs with choke off.Did half ass plug chop they show whiteish gray right side same on left. plugs bpr7es ngk .Seems left plug cleared up was always sooty but alot of idling . Heres what i need to know aint crazy about plug color am i shooting for light tan brownish color. and turning my stator counter clock wise to retard and clockwise to advance is this correct M.c.

Main jets will only affect 7/8 to wide open throttle which most people don't use very much if at all. Did you try dropping the needles a notch yet?

Russ

Mc1011,
You haven't said at what throttle position your problem starts, or if it goes away, however with the Dunstalls I suggest you try 240 mains and have the needle in the mid position, put your timing at 28 degrees at 3500 RPM, the tri spark has a good curve and with a stock CR, 28 degrees should be all you need, unless you ride at high altitudes. If you lived in Denver I'd advise you to bump the timing 10 degrees and or to go to a leaner set of main jets (less oxygen mixes better with less fuel).

If, after doing that, you are showing whiteish grey while at stead state mid throttle position I would raise the needles to increse fuel flow one notch and check your plugs again.

I agree with batrider about the frequency of using full throttle, but you still want to hit readline crisply.

There is one semi-informal check that has worked for me. In third gear accelerate with WOT, starting at say 3000RPM, somehere in mid sprint, say 5500RPM, shut the throttle down abruptly, if the machine surges with a spike of power you are too lean on the mains. This is called a lean surge, it occurs when you have a good volumn of fuel/air going into the cylinders and you shut off the throttle; at that poing the air stops immediately, but the fuel, full of inertia and with more mass, doesn't cease quite so quicky. At some point in this process you have the perfect fuel/air ratio and the engine surges up.
If you are having trouble determining at what throttle setting the problem starts put masking tape on the twist grip body and put white-out on the grip then make a scale on the masking tape in increments of 1/4, impossible to get it wrong after that.

RS

Everthing advised is relevant and worth trying. As a technical specification, 260 main jets are the standard fit for a 850. I have just obtained 2 brand new 932's which will be installed today on a stock 850. I will run this in on a mix of low and high speed with plug chops to check the burn. I'll mention any weird results.

Mick

ML,
The 1975 Nortons that were delivered to the states were equpied with the "blackcap" exhausts and were fitted with 220 main jets in 932 concentric carbs, they also had a highly restrictive air box that would have offered the carbs a stronger signal, on balance.

I do not know what the rest of the world was offered for intake/exhaust, but if MC1011's only mod is the Dunstalls, and his machine is a US spec unit, 240's, maybe 250's should be good to go.

My '75 MkIII has a CR near 9.8:1, a flowed head, balanced rotating and reciprcating parts, a Web Cams 12a spec camshaft and I'll be starting with 260s in new 932 concentrics, I have 250s and 270s on hand as well as .107 and /106 needles. (.106 fitted) I am going to start with the 3 1/2 cutaways but also have 3 cutaways on hand if I need less air. My carbs will breath through a fresh K&N.

RS

hello again before i got to read your replies i changed and up the main to 26 0. also other mod no air cleaner box just k/n air cleaner clamped to carbs. Ran her and still wanted to sputter and bog when open air valve closed air valve and ran good .like i said changed timing just a hair or 2 and shes humming now with air valve open. also readjusted float heights when changing jets . Changed plugs to irdium autolite cross from ngk bpr7eix auto lite was all they had.She runs great now no more .heisatation little heavy mid range plenty of pick up even at 70 lots more throttle might have to lower needle one notch got in it in the middle now might make it more crisp off the line . plenty happy also plug colors improved nicely! thanks M.c.

Get a timing light so you know where you are on timing too. It will come in handy.