Lean 276 carb

[Clive54bsa]

I have a 276GG on my '54 Golden Flash, and ever since I got it running from a basket case, I've had a problem with the motor spitting back thru the carb off idle, I've cleaned the carb thoroughly several times, checked the idle circuit in particular, I went to a 3 1/2 slide instead of a 4. I went to a 190 main jet instead of a 170 and I raised the needle, I tried someone's suggestion of sealing any possible air leaks with hi-temp silicone, but the only way it will run without spitting back is with the choke 2/3 on, When I take it for a run it runs Ok except when I throttle down to approach a stop sign or light, then it spits back thru the carb again. I have the original air filter on it, (clean of course). After a run the plugs are a light grey, I've tried high octane gasoline, 100 unleaded and 112 leaded (pricey) no real change though. The timing appears to be OK, static 3/8" BTDC. There does seem to be a 5 thou variation in the cam ring, i.e. 12 thou on one lobe and 17 on another.My idle mixture screw is about 3/4 of a turn open. Friends from my local BSA club tell me to put a 375, or 389 monobloc on it but I like the look of the 276. Any ideas anyone?

[Ted_Flash]

Possibly the float bowl height is too low, or float needle cutting off flow at too low a level.

Check for too many washers at the float bowl mounting.  Is the correct float bowl fitted, there are different types.  The clip that connects the float bowl to the needle must be in the groove, and float not sinking due to leaking and filling up with petrol.

Check the level by removing the float chamber top and transfer the level to the main body, I'm not sure of the exact level (someone on here will know!)

Hope this helps,

[muskrat]

 G'day Clive,
                 5 thou is a big difference on the cam ring. I think that's about 10 degrees timing wise. So one might be set at 34 the other would be 24 or 44 degrees. Are the plugs the same colour?
Most carb problems are electrical, but it does sound a bit lean. Closing the choke and richening the mix can compensate for too far advanced timing.
I'd sort the magie first.
Cheers
               

[cotterpinkid]

I've posted about this several times. I had a problem with hotter running on the drive side cylinder which ended up being due to a dodgy new cam ring which was about 5 degrees out. My initial thoughts were induction bias, but I checked the timing out with a strobe and found a difference in timing between the cylinders. I've been lent an original cam ring and the timing is spot on with even temperature and ignition on both cylinders.

Brian

[wilko]

Have you taken the jetblock out and made absolutely sure the pinhole for the idle crcuit is clear?

[Clive54bsa]

Thanks Ted_Flash, It is the correct float bowl and the float clip is in the groove, everything on the float bowl side looks good.
Regarding Wilko's advice, I've had the jet block out several times looking for that pin hole I might have missed, but I'm confident that all the orifices are clear. The magneto cam ring difference is only after the cam is well and truly on the lobe like when you gap your points. The initial opening, where you would normally set your static timing, are very close to being the same. When I set my timing I use a 2 thou shim place in the points and "jog" the motor over by hand until the point just starts to let go of the shim. It seem consistent on both cylinders. So it would seem that the 5 thou difference would be on what the Americans call "Dwell". But I don't know whether  that difference would cause it to be lean off idle. Magnetos are "voodoo" to me. I appreciate the responses I've had on this issue.
Clive

[Clive54bsa]

I removed the cam ring to see if I could "dress" the cam lobe with a dremel to try to attain a points gap that was the same on both lobes. and noticed some corrosion on the out side of the cam ring and the mag housing. So I wire wheeled the back of the cam ring and sanded the corrosion off in the housing, then oiled the back of the camring and that little felt strip at the bottom of the housing and replaced the cam ring and the 5 thou difference disappeared, and the timing seems pretty close. So I fired it up and after a minute, it ran without choke, which is better, but there is still a spitback thru the carb off idle, maybe I'll try a Number 3 slide which I found on ebay some time ago.

[duTch]

I noticed a variation about the same in mine, but put it in the 'hard-bucket' for then,having said that I need to strip the whole show anyway so may as well check it.
 Cheers