‘72 combat main jets

An old issue for me. I’m going to try 260 main jets in the original 932/19 and 932/20 Amals. 230 mains are facrory delivered, but for US 100 octane leaded gas. With the ethanol 260 mains should only be slightly rich. My carbs have over 40,000 miles but were sleeved by Mike Gaylord 26 years ago. Just wondering what the other combat jockeys are using.
And please don’t reply with tales of any model other than ‘72 750 combat.

Thanks

ILLF8ED said:

An old issue for me. I’m going to try 260 main jets in the original 932/19 and 932/20 Amals. 230 mains are facrory delivered, but for US 100 octane leaded gas. With the ethanol 260 mains should only be slightly rich. My carbs have over 40,000 miles but were sleeved by Mike Gaylord 26 years ago. Just wondering what the other combat jockeys are using.
And please don’t reply with tales of any model other than ‘72 750 combat.

Thanks

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AMAL specifies/delivers 220 for Combat. Norton originally specified 220 and then switched to 230, best I can tell. I've seen no Norton alcohol talk, but AMAL and Triumph state that for every 10% alcohol, you need one size larger main jet. I've built/sold several Combats. I use 230 main jets and have noticed no reason to change when testing from 3/4 to full throttle. All have 2S cams and peashooters, but the peashooters are a little more open than the originals. I also time at 31 degrees with Tri-Spark and tell the buyer to back that off to 29 if not using quality E10.

If built as a stock Combat, I can't imagine a reason to go to 260 main jets. Flowing 30 more cc/minute seem like a lot of gas to add to the mixture (the air doesn't change).

How often does anyone ride using full throttle for long periods ? Needle jet size and needle taper are more important, as long as your main jets are slightly too rich. It is even possible to have the fuel metered by the tip of the needle at full throttle - but not desirable. Quicker taper needles enable the throttle to be opened quicker - they compensate for loss of vacuum, but slower taper needles can give more power when the throttle is used a bit more slowly.

acotrel said:

How often does anyone ride using full throttle for long periods ? Needle jet size and needle taper are more important, as long as your main jets are slightly too rich. It is even possible to have the fuel metered by the tip of the needle at full throttle - but not desirable. Quicker taper needles enable the throttle to be opened quicker - they compensate for loss of vacuum, but slower taper needles can give more power when the throttle is used a bit more slowly.

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Does 50 miles WFO count? Daytona to Orlando.
Maybe the track you race on isn't the most extreme conditions after all.

marshg246 said:

AMAL specifies/delivers 220 for Combat. Norton originally specified 220 and then switched to 230, best I can tell. I've seen no Norton alcohol talk, but AMAL and Triumph state that for every 10% alcohol, you need one size larger main jet. I've built/sold several Combats. I use 230 main jets and have noticed no reason to change when testing from 3/4 to full throttle. All have 2S cams and peashooters, but the peashooters are a little more open than the originals. I also time at 31 degrees with Tri-Spark and tell the buyer to back that off to 29 if not using quality E10.

If built as a stock Combat, I can't imagine a reason to go to 260 main jets. Flowing 30 more cc/minute seem like a lot of gas to add to the mixture (the air doesn't change).

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Hi,
Yes NV supplied 220 mains when using muted peashooters. The machines coming to the US had 230 mains. My old problem is lack of power in top gear pushing past 80mph. Everything in the engine has been checked and recheck so approaching this from carburetion. I have not taken it out yet with these jets….a bit cold out there. A reason I have not really gone after the problem, it runs fine in the first three gears and I don’t really go faster than 80. But it should get to 100 like a scalded monkey. I have a 21T counter sprocket. May also try going back to 19T.

ILLF8ED said:

Hi,
Yes NV supplied 220 mains when using muted peashooters. The machines coming to the US had 230 mains. My old problem is lack of power in top gear pushing past 80mph. Everything in the engine has been checked and recheck so approaching this from carburetion. I have not taken it out yet with these jets….a bit cold out there. A reason I have not really gone after the problem, it runs fine in the first three gears and I don’t really go faster than 80. But it should get to 100 like a scalded monkey. I have a 21T counter sprocket. May also try going back to 19T.

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What ignition system? What ignition timing advance? Have you checked the timing marks? I found the timing marks to be 4 degrees off on occasion (really at 24 when "timed" to 28). With E10 you can go higher than 28, Tri-Spark says 29 but to me that is still conservative. Once the 230's are in and the timing is for sure set to what you choose, you can do this simple test (license loss pending). In 4th, go wide open throttle - if you're not quickly getting to 100mph, slightly close the throttle and see if you have more power - if so, your main jet is too small. If that doesn't work, and the power is not there, then your main jet is probably too large. But, I would advance the timing to 31 degrees and try again before deciding.

If it seems weak with 230 main jets, then try 240 - 260 is a far jump.

I find ticket loss speeds easy to reach with a Combat, Shell Premium E10, 19T, 230 main jets, 31 degrees of advance, EMGO Peashooters. I weight too much, am too tall, and too wide to get past 100 mph.

Stock wheels and tires diameter: 19T will max you out much quicker, 21T will be faster when max out. Lets call 7500 maxed out (about all a 19T can do): 19T 109mph, 21T in theory 137 mph but in reality, probably won't make it much past 6500 rpm (110 MPH).

Thanks Greg. RITA ignition with Rex’s new ignition board set 28 btdc at 4,000 rpm. After riding and wrench on these for over 50 years this one has me stumped….so far. The TriSpark before had the same issue.

I don't race on big circuits because the difference in speeds down the straights needs to be compensated for in corners. So if the main jets are too lean, I probably would not know it unless the motor developed a miss. If I raced at Daytona, I would be very nervous. I don't like very big, fast circuits. At Winton or Mount Gambier, my overall gearing is already absurdly high.

concours said:

Does 50 miles count? Daytona to Orlando.
Maybe the track you race on isn't the most extreme conditions after all.

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acotrel said:

I don't race on big circuits because the difference in speeds down the straights needs to be compensated for in corners. So if the main jets are too lean, I probably would not know it unless the motor developed a miss. If I raced at Daytona, I would be very nervous. I don't like very big, fast circuits. At Winton or Mount Gambier, my overall gearing is already absurdly high.

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Highway 4

Gearing my bike properly has been a most difficult thing. I use close ratio gears with high overall gearing on small circuits. It is easy to believe your bike is delivering its best, when it is not. The close ratios keep the revs in the power band, but if I raise the overall gearing, the revs do not drop. The motor still spins up at the same rate. With most bikes, if you lower the overall gearing, you get better acceleration.

concours said:

Highway 4

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Do you mean to say you ride flat strap for 50 miles with the throttle wide open ? I would make sure my main jets were too rich. Do you look at your plugs after doing that ?

ILLF8ED said:

Ignition is Lucas RITA with the new Rex’s board. Ignition set at 28btdc at 4000rpm. Have tried 230 and 240 mains. Mufflers are Toga straight through. All the usual problems have been checked over the last several years. FYI I am not a newbie to Commandos, have been riding and wrenching them for over 50 years. However this problem persists. I joined the NCNOC in 1981. None of the clubmen have been able to identify the issue. When I first restored it in 1998 it ran very well, but as time and ethanol entered the engine was nocking. went Through a lot of checking around 10 years ago. Compression was 183psi both sides. Turned out it had a standard cam. After this I put a 2S combat cam in and after that the power issue started. I have checked the cam profile and it is what it should be. I have used race gas several years ago with the standard cam and it did cure the pinging. Oh and cam timing is right on. Compression at 180psi and leakdown less than 3 percent.

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Not trying to knock your skills. I thought you said you received but have not installed the 230 main jets - maybe I misread.

A stock Combat should not knock on any premium gas at 28 degrees timing including E10 and in fact should be less prone to knocking on E10 and in fact should not knock at 31 degrees on premium, E10. So, that may be telling you something (heavily carbon head?) Are you timing marks correct?

I think you said very old carbs, sleeved long ago. Loose slides will let in extra air and all but nearly WOT are affected by that.

You've mentioned 80 mph but not the throttle opening - I would think that 80mph is well less than WOT. At what throttle opening does the problem start? If in 3rd gear and you go WOT does it go to redline? 3rd gear, 21T, 7500 rpm = 105 mph. How about 2nd gear, 21T, 7500 rpm = 62 mph - can you get there WOT?

Let’s see…..
I checked the timing marks on the primary cover with a degree wheel and travel dial to confirm TDC.
I have been running 230 mains and with those I have this problem.
There is no carbon on the pistons or head….been there many times.
The carbs are original, but sleeved by Mike Gaylord in ‘98. There Is no slop there. Not even any wear marks on the stainless slides.
The bike will hit 80mph in 3rd gear easily and will go to redline. It will not go to redline in 4th. Tells me the engine is not getting to max power.
Gearbox has all new bushes, bearings are in great shape, no unusually wear on the gears. It shifts like butter.

I have considered new Premiers and may go to that eventually, but not convinced it is the carbs.

ILLF8ED said:

Let’s see…..
I checked the timing marks on the primary cover with a degree wheel and travel dial to confirm TDC.
I have been running 230 mains and with those I have this problem.
There is no carbon on the pistons or head….been there many times.
The carbs are original, but sleeved by Mike Gaylord in ‘98. There Is no slop there. Not even any wear marks on the stainless slides.
The bike will hit 80mph in 3rd gear easily and will go to redline. It will not go to redline in 4th. Tells me the engine is not getting to max power.
Gearbox has all new bushes, bearings are in great shape, no unusually wear on the gears. It shifts like butter.

I have considered new Premiers and may go to that eventually, but not convinced it is the carbs.

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OK, redline (I'm using 7500) is 105mph in third gear - it will get there without problem? How about 6000 rpm which gives you 83mph with a 21T in 3rd gear - you go there easily and then shift and struggle to go faster?

IMHO, 7500 is out of reach with a 21T. 7500rpm = 127 mph, 7000 rpm = 119 mph, 6500 rpm = 110 mph and I've never read a report of any stock Commando making 110 mph.

I bet when you got to 260 main jets, you will be even slower but the good news is that your engine will run cooler

What Altitude .

What Humidity .

What else .

Id start at 270 ! .

The correct proceedure , at Operational Temperature , with the air cleaners removed , is toaccelerate to full speed . ( after having ensured the other parameters are correctly adjusted )

If , when you pull on some choke , it accelerates ; Your TO LEAN . on the other hand , if you pull on the chokers & it decelerates , its to rich . The HS2 S.U. jet being manually adjustable in situ.

ROVER P4 Workshop Manual : ' Tuning . Carburators . '

A similar proceedure should hold true here .
A intresting thing , a F I expert sprayed CRC into the intake at part throttle , the increase in revs being indicitive of inadequte fuel pressure . You could try this at full speed !
A tube into the filter & a ( small ) bottle of petrol should ensure the fires controlable : Unpinch Metered feed at full wellie , if it throws you off the back , its picked up a bit .

marshg246 said:

OK, redline (I'm using 7500) is 105mph in third gear - it will get there without problem? How about 6000 rpm which gives you 83mph with a 21T in 3rd gear - you go there easily and then shift and struggle to go faster?

IMHO, 7500 is out of reach with a 21T. 7500rpm = 127 mph, 7000 rpm = 119 mph, 6500 rpm = 110 mph and I've never read a report of any stock Commando making 110 mph.

I bet when you got to 260 main jets, you will be even slower but the good news is that your engine will run cooler

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On the first question…yep struggle to go faster after third.
Interesting on stock Commando speed. I don’t know about 110, but my first ‘72 combat completely stock and very low mileage was run at Ontario Speedway in ‘73. It managed a quarter mile end speed of 103 and it was still pulling after 12 point something seconds. I have no idea what the speedometer said as I was not watching that. Oh and at 20 years old and not knowing not to do it I regularly put that first Commando into redline and beyond. What a rush of power when it got to that point.

Actually something just came to mind and a little of what Matt said above. I should remove the K&N air filter and see if that makes a difference. Think I will go back to 230 mains and try that.

ILLF8ED said:

On the first question…yep struggle to go faster after third.
Interesting on stock Commando speed. I don’t know about 110, but my first ‘72 combat completely stock and very low mileage was run at Ontario Speedway in ‘73. It managed a quarter mile end speed of 103 and it was still pulling after 12 point something seconds. I have no idea what the speedometer said as I was not watching that. Oh and at 20 years old and not knowing not to do it I regularly put that first Commando into redline and beyond. What a rush of power when it got to that point.

Actually something just came to mind and a little of what Matt said above. I should remove the K&N air filter and see if that makes a difference. Think I will go back to 230 mains and try that.

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So, I see no reason to consider jets. If you can go WOT in 3rd and shifting to 4th won't pull then IMHO, it's clearly not a main jet issue as you would not be at WOT (or should not be) right after you shift.

If you really need bigger jets, you have too much air - not too little. If removing the air cleaner improves things, then 220 or 230 should work. Lowering the choke or as I said in post #6...

From an old post here:

September 1972 Cycle Guide Commando 750 COMBAT Roadster 13.09 at 103.68 top speed tested 109.5

Anyway, I'm not helping so I'm out.

What type of silencers does the bike have fitted?

Glen

ILLF8ED said:

On the first question…yep struggle to go faster after third.
Interesting on stock Commando speed. I don’t know about 110, but my first ‘72 combat completely stock and very low mileage was run at Ontario Speedway in ‘73. It managed a quarter mile end speed of 103 and it was still pulling after 12 point something seconds. I have no idea what the speedometer said as I was not watching that. Oh and at 20 years old and not knowing not to do it I regularly put that first Commando into redline and beyond. What a rush of power when it got to that point.

Actually something just came to mind and a little of what Matt said above. I should remove the K&N air filter and see if that makes a difference. Think I will go back to 230 mains and try that.

Click to expand...

Pictures of the air filter please.

acotrel said:

Do you mean to say you ride flat strap for 50 miles with the throttle wide open ? I would make sure my main jets were too rich. Do you look at your plugs after doing that ?

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That is correct.
A friend I've known 35 years, we seem to race anything & everything anytime we get together. From golf carts to garbage trucks, and everything in between. Not planned, it just happens.
Anyway, 20 mile WFO stretches on the Commando happen semi-regularly.
I have posted the spark plug images before.
Not that it matters, unleaded pump gas.