750 Combat top speed problem (2015)

Similar problems on an Airhead is often due to the Float bowl level being set too low.

'In the past my experience with this in ability to pull in top gear was due to no enough compression when using a hotter than normal cam profile. I don't think this is the case here as cam timing is confirmed and leak down is 5%.'

How did you find that out ? Has the 750 Combat got a high comp. motor ? Surely once the cam is working above the cam spot the comp. ratio is irrelevant ? What revs were you doing at 80 MPH ?

How did you find that out ? Has the 750 Combat got a high comp. motor ? Surely once the cam is working above the cam spot the comp. ratio is irrelevant ? What revs were you doing at 80 MPH ?[/quote]

Yes, the combat is Norton's high compression Commando engine. Cams with larger overlap of intake and exhaust bleed more compression i.e. the combat cam which is one reason the combat got 10:1 compression from the factory. The experience I had was with a '68 Bonneville rebuilt with DeLong cams and didn't increase the compression. Ran great in gears 1-3, but couldn't pull 4th gear. I don't believe that's the case with this combat now, but maybe something I've over looked. My compression gauge crapped out last year, so only have the leak down tester. It's Dec 21 the first day of winter. Cold and raining here in the San Francisco Bay Area, so won't be going for a trial run anytime soon. Service manual indicates 80mph with 21 tooth front sprocket is around 4500rpm, that's about what mine is. I'm hoping the addition of the resistor plug caps and advancing the ignition 2 degrees makes a difference. I'm not going to tear the engine down again without good indication it's necessary. I'm not entirely sure of the piston seal although the leak down test was good.

illf8ed said:

My "resistor" setup is NGK BP7ES plugs, non resistor wires and just added NGK 5K ohm resistor caps as recommended with the Tri Spark. The problem existed with my previous Lucas RITA ignition which eventually stopped working hence replacing with the TriSpark recently.

Enriching chokes are set up properly and open fully. Carbs are Mike Gaylord resleeved with the new no sink Amal floats and standard for combat 230 main jets. I have tried up and down on main jet sizes and the 230s are the best.

In the past my experience with this in ability to pull in top gear was due to no enough compression when using a hotter than normal cam profile. I don't think this is the case here as cam timing is confirmed and leak down is 5%.

I know it's very difficult to diagnose without being in front of the machine. Appreciate the feedback, was looking to see if anyone had some insight. I will continue and if I find the offending issue, will report back.

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Many years ago I had a Honda 4 which didn't pull well at high speed. After checking everything I could think of a friend told me that NGK plugs don't come ready gapped from the factory. The plug gaps were too tight. I re gapped the plugs to 25 thou and it made a big difference.

illf8ed said:

How did you find that out ? Has the 750 Combat got a high comp. motor ? Surely once the cam is working above the cam spot the comp. ratio is irrelevant ? What revs were you doing at 80 MPH ?

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Yes, the combat is Norton's high compression Commando engine. Cams with larger overlap of intake and exhaust bleed more compression i.e. the combat cam which is one reason the combat got 10:1 compression from the factory. The experience I had was with a '68 Bonneville rebuilt with DeLong cams and didn't increase the compression. Ran great in gears 1-3, but couldn't pull 4th gear. I don't believe that's the case with this combat now, but maybe something I've over looked. My compression gauge crapped out last year, so only have the leak down tester. It's Dec 21 the first day of winter. Cold and raining here in the San Francisco Bay Area, so won't be going for a trial run anytime soon. Service manual indicates 80mph with 21 tooth front sprocket is around 4500rpm, that's about what mine is. I'm hoping the addition of the resistor plug caps and advancing the ignition 2 degrees makes a difference. I'm not going to tear the engine down again without good indication it's necessary. I'm not entirely sure of the piston seal although the leak down test was good.[/quote]

Checking static compression is quite easy:

1. Get a cylinder at TDC on the compression stroke.
2. Lean the bike over so that the spark plug to that cylinder is as vertical as possible and support it (it'll be over a long way)
3. Using a syringe or similar, carefully measure how much oil it takes to fill the combustion chamber. I was taught to fill up to the first two threads of the plug thread to allow for the hollow part of th plug.
4. The calculation is to determine static compression ratio is: (swept volume of one cylinder + combustion chamber volume) / combustion chamber volume.

If your Combat motor is on a stock bore, and it is supposed to be 10:1 CR then the combustion chamber volume should be around 41 - 42 cc (if my maths is correct).

David, have you noted and tried that bit about the smaller main jets (without wading back though all the preceding pages).

Borrow another complete set of carbs and bolt them on as a test.
Divide and conquer.

Always try the simple things first. I had the EXACT same issue on my stock 850 and it drove me nuts. Turned out that the alcohol in the fuel had degraded my petcocks so that they were restricting flow. A new set of taps and the problem went away. I'd have a lot more hair if I'd checked them first.

I tend toward the last two suggestions that the problem is fuel starvation. I did replace the BAP taps with stock type that don't have any rubber inside, only brass. That didn't solve it. Also ran with both main and reserve on, no difference. Still thinking it may be the gas tank cap and I have another one of these, will trade it and see. I'm fairly confident in the existing carbs, but may have to go back there if these others don't work. As you say pulling my hair out and I don't have any to spare. Winter weather is going to slow me down.

illf8ed said:

I tend toward the last two suggestions that the problem is fuel starvation. .

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I would agree.
I had wondered if that mention of changing 240 mainjets to 220s had incidentally changed/fixed something else.

Rohan said:

illf8ed said:

I tend toward the last two suggestions that the problem is fuel starvation. .

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I would agree.
I had wondered if that mention of changing 240 mainjets to 220s had incidentally changed/fixed something else.

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Hi Rohan,

I tried 220 mains, didn't make any difference. May try this again later after I find the offending issue.
Just checked the gas cap and it breaths as well as the one on my fiberglass tank hanging on the wall.
Think I will pull the Amal float bowls tomorrow and check the float height again. Supposed to be between 0.17" and 0.24" from top of the float to the top edge of the bowl according to Amal website using the new no sink floats.
I find after owning this particular Commando since 1997, the problems that rise get tougher to solve. 55,486 miles on the odometer. 35,000 miles are mine.

illf8ed said:

Think I will pull the Amal float bowls tomorrow and check the float height again. Supposed to be between 0.17" and 0.24" from top of the float to the top edge of the bowl according to Amal website using the new no sink floats.

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That is the fuel level, not the distance from the top of the float.

http://amalcarb.co.uk/optimising-mark-1 ... uel-levels

The correct fuel level for all Mark 1 Concentric carburetters is 0.21" plus or minus 0.040" below the top edge of the float bowl. Thus when the needle valve is being held shut by the tangs of the float, the level of the fuel will be between 0.17" to 0.24" (4.33mm to 6.35mm) from the top of the bowl.

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L.A.B. said:

http://amalcarb.co.uk/optimising-mark-1 ... uel-levels

The correct fuel level for all Mark 1 Concentric carburetters is 0.21" plus or minus 0.040" below the top edge of the float bowl. Thus when the needle valve is being held shut by the tangs of the float, the level of the fuel will be between 0.17" to 0.24" (4.33mm to 6.35mm) from the top of the bowl.

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0.21" plus 0.040" is 0.25" cf 0.24" quoted. (not that this is a huge difference).

6.35 mm is 0.25" too.

Perhaps calculators should be issued to all persons doing sums and metric conversions....

If it has 6 volt coils , it might do 5000 rpm , downhill. on a good day .

Taping on a pair of a cortina would test this .

Its not well know that Lucas Technitions in the late 60s were related to mickey mouse .

Don't look now Matt, but all Commandos came stock with 6v coils.
Didn't seem to hinder them any.
But you did have to keep those points in fine fettle....

Rohan said:

Don't look now Matt, but all Commandos came stock with 6v coils.

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Not all, (pre-'71 had 12V coils) but nothing wrong with 6V coils. Matt seems to be under the impression that 6V coils give a weaker spark.

Rohan said:

illf8ed said:

I tend toward the last two suggestions that the problem is fuel starvation. .

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I would agree.
I had wondered if that mention of changing 240 mainjets to 220s had incidentally changed/fixed something else.

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Since I mentioned it, I will respond... In my case, my bike is NOT a combat bike and the 220's are the standard main jet sizes for my bike... but since I am into overthinking things.... I thought I could run a richer mixture through a larger main jet to have a little cooler running engine.

Yeah, that's a little bit of Dr. Frankenstein logic, but I tried it anyway. It only was an issue right where you would expect, at nearly wide open throttle where the bike would stop accellerating at 80mph... with still more room to twist the throttle. After doing a lot of reading and scratching my head, I came to the conclusion that for a bike that runs well everywhere else, the most likely issue to effect wide open throttle is the main jet, so I actually had both 260's and 220 main jets ready on the test day to see if that was my problem. I tried the 220's first, which I changed through the float bowl drain plug. I took the bike out and it zoomed past 80mph, so I didn't try the 260's at all.

In my case, I put the stock jets in and got the better performance result... I know that Illf8ted already has his stock main jets in his combat, but I wanted to mention my experience since it mirrored his symptoms closely.... I thought that maybe he could set up the same scenario as I did to test (and eliminate possibly) main jet size as the cause of his issue. That test having jet sizes on both sides of his present jet size and doing test runs from his garage to see if changing jet sizes effects his issue for better or worse....

Of course the other related setting to check and maybe experiment with would be needle heights.

I'm not definatively diagnosing for Illf8ted... I'm just sharing a problem that I had which had a similar symptom as his. Mine was fuel delivery related... so I'd be inclined to think his is also.

David,

Not sure if you mentioned what type of air filter you're running. I've scrolled back and didn't see anything.
Have you tried running it without an air filter just as a test (assuming you don't travel on dirt roads)?

I'd just like to point out that early on Christmas Day morning is great for doing high speed runs and plugs chops on public roads whilst everyone else is preparing the turkey etc !

Fast Eddie said:

I'd just like to point out that early on Christmas Day morning is great for doing high speed runs and plugs chops on public roads whilst everyone else is preparing the turkey etc !

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Just don't do it while still running the bike in, a sticky exhaust valve and a failure to proceed can result in a difficult phone call home, especially if you sneak out without saying ...
2 years ago i think it was.