‘72 combat Amal 932 main jet question

I’m interested in what size main jets you combat owners are using. Original Amal 932 please. Of course using ethanol/gas. Also has anyone changed the jet needle size?

ILLF8ED said:

I’m interested in what size main jets you combat owners are using. Original Amal 932 please. Of course using ethanol/gas. Also has anyone changed the jet needle size?

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ILLF8ED; My 1975 Chilton’s lists these Combat Specs.: 932 Carb. 220 Mains
.106 Needle Jet Needle clip Top Notch Slide 3 Pilot 25. Personally I run a different setting to accommodate the occasional trip to the track that works well for me on the street. If you are interested please e-mail me at northernnortonnut@gmail.com. Stock is a pretty good place to start the tuning dance. Best of luck Mark

Email sent

I run 240 mains, all other settings the same as NNN stated. I do try to avoid ethanol when possible.

ILLF8ED said:

I’m interested in what size main jets you combat owners are using. Original Amal 932 please. Of course using ethanol/gas. Also has anyone changed the jet needle size?

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For new Premier carbs, Amal says the set is PACK 140:
220 MJ
106 NJ
3 Cutaway Slide
#17 Pilot Jet (I provide #19 when I sell a set)
Needle Position 1 (Clip in Top Slot) (I start in the middle slot - would rather be a little rich)
Standard Spray Bar
Standard Needle

If you were going to ride a lot at 3/4 throttle or more open and use E10, I would use a 230 MJ. If rarely 3/4 throttle or more it doesn't really matter.

marshg246 said:

For new Premier carbs, Amal says the set is PACK 140:
220 MJ
106 NJ
3 Cutaway Slide
#17 Pilot Jet (I provide #19 when I sell a set)
Needle Position 1 (Clip in Top Slot) (I start in the middle slot - would rather be a little rich)
Standard Spray Bar
Standard Needle

If you were going to ride a lot at 3/4 throttle or more open and use E10, I would use a 230 MJ. If rarely 3/4 throttle or more it doesn't really matter.

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Greg,
”original Amal, please”. I have 230 mains in my 932 now which is factory spec for combat without muffler mutes. Wondering if others have rejetted for this gasohol. My top end in 4th gear seems to be down on power. All other things are in order…180psi cranking pressure and less than 5% leakdown.

ILLF8ED said:

Greg,
”original Amal, please”. I have 230 mains in my 932 now which is factory spec for combat without muffler mutes. Wondering if others have rejetted for this gasohol. My top end in 4th gear seems to be down on power. All other things are in order…180psi cranking pressure and less than 5% leakdown.

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The Amal spec for Combats has never changed. Amal only has specification for bikes as they left the factory, and those specs were (mostly) what Norton ordered. The Combats I've built all run fine on the Amal specs but with E10 I, like Amal's guide for tuning with alcohol recommend a larger main jet. To run straight alcohol, they say to increase the main jet 150%. In one old Triumph document that I can't find now, they recommend one size larger for each 10% alcohol.

Of course, many things other than just the percentage of alcohol matter. Altitude, humidity, gas quality, mufflers, and have the actual factory spec carbs.

You've mention 932, but have not said what carbs. The Combat carbs were not the same as later 932 carbs used on 850s.

If your carbs are marked R932/19 and L932/20 they are original or original replacements and the Amal part number for the pair is ACK 140 (Premier PACK 140)

If your carbs are marked R932/26 and L932/27 they are equivalent but had a 230 main jet and were for the 73 higher compression 750s. The Amal part number of the pair is ACK 112 (Premier Edit: PACK 112)

The 72 parts manual does not specify the main jet size. The 70-73 Workshop manual does list 230 no mutes and 220 mutes, but that was written long after no Combats were being built.

All other Norton 932 carbs used the Norton-specific spray bar and Norton-specific Needle and a 3-1/2 cutaway slide.

So, try 260 jets (widely available) and see if you do better or worse at WOT. Then a 220 jet and try again. My guess is that with straight through peashooters, a clean hamcan air filter, running fresh E10 from a reputable station, near sea level on a nice day, your best performance will be somewhere in the 230-260 range.

BTW, if original carbs, the worn slides will throw everything but WOT off.

Your main jet size does not really matter as long as it is slightly too big. The needle jet size and taper on the needles is of critical importance. If you lower the needles until your motor misses when you ride the bike, rhen raise them one notch, the miss should disappear. With quick taper needles, the mixture richens quicker as you open the throttle - that is what you need when you open the throttle fast. For better acceleration , a slow taper needle is better, but you need to open the throttle a bit slower.
You might think opening the throttle faster with a richer mixture would give better results - but it doesn't, because slower opening gives the heavy ctank more time to respond ro the throttle. It actully spins-up faster with a leaner mixture.
If your motor misses when you ride the bike, stop and raise the neegles.
With the heavy crank and even quick taper needles, you can whack the throttle wide open and still have to wait for the response.
What I am talking about is the rate of increase in torque, not it's actual size. With a close ratio gearbox, the crank does not need to spin-up so far in the vrev range.

Greg,

yes 932/19 and 932/20 original carbs that were sleeved by Mike Gaylord in 1998. By the way almost no wear after these years and over 40,000 miles. Gearbox sprocket is 21T. Engine runs very strong in 1-3 gears but lacks power in 4th to pull to 6,000 RPM. Tank vent is clear. Combat cam has been measure for lift and cam timing verified by dial indicator and degree wheel. 180psi compression on both cylinders and less than 5% leakdown. I have tried 240 mains without Improvement. I may give 250 or 260 a try.
Thanks

ILLF8ED said:

Greg,

yes 932/19 and 932/20 original carbs that were sleeved by Mike Gaylord in 1998. By the way almost no wear after these years and over 40,000 miles. Gearbox sprocket is 21T. Engine runs very strong in 1-3 gears but lacks power in 4th to pull to 6,000 RPM. Tank vent is clear. Combat cam has been measure for lift and cam timing verified by dial indicator and degree wheel. 180psi compression on both cylinders and less than 5% leakdown. I have tried 240 mains without Improvement. I may give 250 or 260 a try.
Thanks

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Also, if you haven't, try with both petcocks open and verify that your fuel level in the bowls is correct - shouldn't matter much at WOT but worth checking.

I have no experience with a 21T front sprocket on a Combat - I always install 19T which restores the bottom end lost by the cam (at least that's what I think). I was looking for the dyno tests I think I've seen here but didn't find them. The attachment might be of some help.

marshg246 said:

Also, if you haven't, try with both petcocks open and verify that your fuel level in the bowls is correct - shouldn't matter much at WOT but worth checking.

I have no experience with a 21T front sprocket on a Combat - I always install 19T which restores the bottom end lost by the cam (at least that's what I think). I was looking for the dyno tests I think I've seen here but didn't find them. The attachment might be of some help.

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For background this problem has persisted for about 8 years. I don’t really need 100mph so it hasn’t been critical….just bugs me. I have done both petcocks open as well as gas cap open, nope. Have considered dropping back to a 19T counter sprocket. I don’t like taking it out at freeway speeds anyway. Checked my bin of Amal main jets and see 240s and 260s, so I’m sure I have tried those.

ILLF8ED said:

For background this problem has persisted for about 8 years. I don’t really need 100mph so it hasn’t been critical….just bugs me. I have done both petcocks open as well as gas cap open, nope. Have considered dropping back to a 19T counter sprocket. I don’t like taking it out at freeway speeds anyway. Checked my bin of Amal main jets and see 240s and 260s, so I’m sure I have tried those.

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Then go counter intuitive and try 220. That's what I use with new Combat builds and the Peashooters I use are straight through (loud). Tri-Spark ignition, Tri-Spark MOSFET regulator, PACK 140 carbs with #19 pilot jets - everything else is stock Combat. All gas here is E10 and I run Shell Premium mostly. My elevation is about the same as yours. I may be the heaviest rider on the forum and am 6'1" and I sit up so the load on the bike is high. 6000 RPM in the first three gears is no issue and 5500 for short bursts in 4th (85 mph) is all I dare - don't like the thought of jail.

I will on the 220. I don’t recall in the years if I have. Mine will hit 5500 rpm in 4th, but not much more. My first combat bought in 1973 would get to 100 very easily. It had only 300 miles on it when I bought it one year old. The Fontana Yamaha dealer took it on trade for an XS650. Dealer told me the original owner was a local cop and the Norton scared the H*** out of him. Of course at my 20 years age that was all I needed to buy it immediately. I never bought another Japanese street bike to this day.