750 Combat - main jets

Hi Combat owners,

I'm looking for a consensus. For '72 750 combat engines with sleeved Amal concentric 932 carbs, what main jet size are you using these days and how is this size performing. Let me know any other adjustment changes such as timing and spark plug heat range. Stock is 230 main jets for 32mm Amals.

Thanks,

illf8ed said:

Hi Combat owners,

I'm looking for a consensus. For '72 750 combat engines with sleeved Amal concentric 932 carbs, what main jet size are you using these days and how is this size performing. Let me know any other adjustment changes such as timing and spark plug heat range. Stock is 230 main jets for 32mm Amals.

Thanks,

Click to expand...

Why wouldn't you just use the stock size?

I'm trying to zero in on my serious engine pinging problem. Already went up to 240 and tried 260 mains and backed the ignition to 26 degrees TTDC full advance with BP8ES plugs - still pinging. The unknown is what other combat owners are experiencing with current fuels. I may be looking at the wrong area. Before pulling the head - again, trying to eliminate easier things. A concern is getting 55-60 mpg fuel economy - too good, but don't quite understand what would cause this other than running too lean. Please don't suggest an aluminum plate to reduce compression, not interested in lower performance.

Thanks,

I'm not a Combat owner (Mk1A 850), nor do I know what octane fuel you use. But NGK BP8ES seem too cold (BP7ES being the standard). Perhaps there's a specific reason why you chose them? I use BP6ES with 98 octane. I received advice from a source I respect to use a hotter plug with current fuel, no pinking.

I have a '72 Combat currently bored .020" over with old sleeved 932's, old 106 needle jets, 850-style balanced pipes and Dunstall Decibel mufflers--the raggedy E-Bay mufflers alone were worth an extra five mpg, or 10%. Power and mileage improved slightly going from 230's to 220's--I just got 210's and 200's to experiment with further. The needles are as low as they can go. The bike will idle all afternoon at 750-850 rpm, and the rings and valves have over forty thousand miles on them. Since I installed mushroom-headed tappets I often pull the covers to discover that there's nothing to be done.

Despite the conventional wisdom I find that main-jet size seems to affect performance on my Combat through a wide range of throttle openings. I've also found that exhaust layouts and tract length affect power much more than I anticipated, having experimented with SS-style high exhausts, balanced pipes, straight-through peashooters, BUBs and now Dunstalls.

A 60-inch exhaust tract seems to offer a good compromise between low-end torque and top end whether the pipes are high or low, although I plan to revisit the Dunstall layout with the muffler tips past the license plate now that I have balanced pipes to play with--chrome-plated brass plumbing drain pipe is just rigid enough to extend the headers and locate the muffler inlets securely.

Best of luck with sorting out your engine--when Combats work well they are simply delightful.



Tim Kraakevik
kraakevik@voyager.net