‘72 combat main jets

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.
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ILLF8ED said:
I have considered new Premiers and may go to that eventually, but not convinced it is the carbs.
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4th gear is top - do you really expect the motor to reach redline. If the bike is geared right, it should never reach redline in top. You need a supercharger. Or only ever ride down hill.
 
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It should be possible to over-rev the motor in every gear except top. Some Harleys have 5 speed gearboxes - top gear is an overdrive. If you want redine in top, you might need to lower the overall gearing, What speed will your bike do in top when you try to get to redline ?
 
The overall gearing and the spacing of the gears affect performance in terms of acceleration rate and top speed. If you have close ratio gears, the bike will accelerate faster and have a higher top speed. Commandos usually have wide ratio gearboxes. When you change from third to top, you lose revs - with wide ratios, it takes longer to get them back. The Commando engine is deceptive because of the heavy crank. Lowering the overall gearing will not usually make the bike accelerate faster. The crank always seems to spin up at the same rate regardless of the gearing.
 
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kommando said:
With the Combat cam being a cam with a pronounced step in power delivery and you getting to 7500 rpm in third this sounds more like a 21T gearbox sprocket issue than a main jet issue.
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That was my thought as well. I believe the Combats all had 19 tooth as stock.
DynoDave tells us that the factory ports on the Combat head are quite inefficient, even considered ruined by modern standards.
With that head and the Combat cam, perhaps it just won't pull thru the dead zone in 4th to get up to the rpm where the power is.
Have others tried a 21 tooth on a Combat and if so did it pull ok in 4th?

Note that DynoDave's Combat that made 48rwhp did not have the Combat head fitted. Iirc it had a 71 750 head.

Glen
 
The target is to reach top speed at peak power in top gear.

In absence of silly faults in your engine:…

If it won’t reach peak power rpm in top, it is geared too high.

If it readily exceeds peak power rpm in top gear, it may benefit from raising the gearing.

I have geared my 650
of another popular marque, for 6,500 rpm at 105 mph. That’s almost 4,000 rpm at 60 mph. It’s a good setup for the variety of situations we find, on public roads.
 
worntorn said:
With that head and the Combat cam, perhaps it just won't pull thru the dead zone in 4th to get up to the rpm where the power is.


Glen
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Right!

My Combats, always a 19T, and 230 Main Jets, work fine. However, when you shift from 3rd to 4th, you don't go WOT and hope. You keep adding throttle. At the time of the shift, you are certainly under 3/4 throttle (and not on the mains). With a 19T if you shift at 6500 RPM, you're at 83 MPH, not WOT, and then you're at about 5200 RPM in 4th and below the max power band. If you really want to go and don't care if you blow the engine, shift at 7500 (95 MPH) which will be about 6200 in 4th and making the power to get you to 105-109.
 
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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As you probably know from past posts, I am having what seems to be the identical issue with my 72 combat. I also have RITA ignition with REX upgrade, but my bike has Amal MK2 carbs with K&N cone filters and the Interstate silencers (unmuted). As have you, I feel like I have looked at everyting deemed to be relevant including valve timing, ignition timing, fuel flow, and jetting. At one point I tried varying the main jet size. Here are my notes from that unscientific testing 2 years ago:

Changed out the main jets from 240 to 230. 11 mile ride. WOT performance was improved. In 4th gear it continued accelerating till traffic required slowing. Probably about 75mph.


Changed out the main jets from 230 to 220. 10 mile ride . At WOT in 4th gear it would not go past 4500rpm. Less than WOT kept climbing above 4500rpm.

Tried to run main jet tests. Made runs with 250, 240, 230 & 220. Tried to compare WOT in 3rd & 4th gears and just under ¾ throttle. Traffic did not allow good apples to apples comparison, but qualitatively 240 “felt” best. Noticed on 230 & 220 there was slight surge when backing off WOT. Did not check this for 240 & 250.
 
NickZ said:
As you probably know from past posts, I am having what seems to be the identical issue with my 72 combat. I also have RITA ignition with REX upgrade, but my bike has Amal MK2 carbs with K&N cone filters and the Interstate silencers (unmuted). As have you, I feel like I have looked at everyting deemed to be relevant including valve timing, ignition timing, fuel flow, and jetting. At one point I tried varying the main jet size. Here are my notes from that unscientific testing 2 years ago:

Changed out the main jets from 240 to 230. 11 mile ride. WOT performance was improved. In 4th gear it continued accelerating till traffic required slowing. Probably about 75mph.


Changed out the main jets from 230 to 220. 10 mile ride . At WOT in 4th gear it would not go past 4500rpm. Less than WOT kept climbing above 4500rpm.

Tried to run main jet tests. Made runs with 250, 240, 230 & 220. Tried to compare WOT in 3rd & 4th gears and just under ¾ throttle. Traffic did not allow good apples to apples comparison, but qualitatively 240 “felt” best. Noticed on 230 & 220 there was slight surge when backing off WOT. Did not check this for 240 & 250.
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When you back-off , fuel metering is done by the needles, which are usually leaner than the mains. So if the mains are too rich, you should get more power when you back off. The problem with running very lean needles, is the fuel can be metered off the tips of the needles instead of the mains. With my bike, I use the mains to give the motor a reprieve. I run the needles as lean as possible. But the circumstances in racing are different from riding on public roads. I don't worry if I go a bit slower towards the ends of the straights.
 
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I learned a long time ago to never trust the timing marks set on the rota, for the last 40 + odd years since building my hot motor for the Featherbed and running EI for over 32 years I set my timing fully advance till I get kick back then ever so slightly adjust till no kick back at all, with the 2S cam and port work and balanced crank, the Amals for the 850 260 main jets for the stock motor I just increased the main jets to 270s and have no problems at all with running issues, if I get a slight hesitation opening the throttle I lift the needles up usually solves that problem.
Another thing I replace my jets and needles every 3 years or so depends on how many miles I clock up so around 30k miles I go through the carbs, but I also run short straight through mufflers with ever so slight baffles inside, putting around the streets my Norton is quiet but open it up it gets rid of the exhaust very quickly and loud, my Norton has no problems from 3rd to 4th it will just keep pulling but I just run with the stock 19 tooth front sprocket and rear sprocket.
But I have done a few up grades in the last 15 years with my hotrod and now running the Joe Hunt maggie set the same way as I have always done it and the JH is set at full advance and with that big spark it produces my hotrod Norton is running the best it has ever run from low revs to full revs and my motor runs so freely you got to watch in how far you take the revs to and know when to stop reving before it goes bang, I never trust tacos or speedos, but after riding my Norton since new I just know how far I can go.
As to everyone who thinks you need to retard the timing to start our Nortons, well I can sure you my Norton it fires up every time on the first kick with that JH maggie without kick back, the first kick for the day is the biggest kick but once is has fire for the day the rest of the time it will fire up on 1/2 through the kick every time and have no problems kicking it to life while sitting on the seat, that big spark from the JH is so good, was never like that with the EI and I gone through 3 of them as well standard points in the 47+ years of ownership.
I built my motor but the machining work was done by others, I am no carb expert but I do know my own bike and it has no hesitation running through the revs even from going from 3rd to 4th it just keeps pulling.
If your Norton is not pulling once it gets into 4th then something is not right, timing/ignition/ carbs or electrical and when my Norton was new and stock I had my share of problems from stock points/advance units as well bad coils and faulty condensers so went to EI but also had a Lucas Rita fail withing 12 months and a Boyar fail from 100 miles from home but that was caused by a minor fire, well a big fire lol, the replacement Boyar was still going when I replaced it with the Joe Hunt so 32 years and many bad batteries in that time lol.
My Norton was an everyday rider except when doing work/upgrades to it and has now been semi retired since 2013 when I brought a new Thruxton, but I still take it out regularly and it still runs great as its such a fun bike to ride.
Remember a lot of carb problems could be electrical, you just got to go through one thing at a time to sort it out and if you get frustrated walk away and think about it first before going back to it, mistakes get made when angry and learnt that lession a long time ago.
Anyway I hope you get to the bottom of it and sort it out and if you want to go flat out in 4th gear then a 21 tooth front sprocket is not the way to go.

Ashley
 
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marshg246 said:
Right!

My Combats, always a 19T, and 230 Main Jets, work fine. However, when you shift from 3rd to 4th, you don't go WOT and hope. You keep adding throttle. At the time of the shift, you are certainly under 3/4 throttle (and not on the mains). With a 19T if you shift at 6500 RPM, you're at 83 MPH, not WOT, and then you're at about 5200 RPM in 4th and below the max power band. If you really want to go and don't care if you blow the engine, shift at 7500 (95 MPH) which will be about 6200 in 4th and making the power to get you to 105-109.
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19T coming from CommandoSpecialties end of this week. I put the 230 mains back in. Appreciate your comment that your combats have 19T. It occurs to me I had not ever run a 21T on a combat in the past. I’m not going on any long runs of freeways anymore, so it should be fine.
 
ILLF8ED said:
19T coming from CommandoSpecialties end of this week. I put the 230 mains back in. Appreciate your comment that your combats have 19T. It occurs to me I had not ever run a 21T on a combat in the past. I’m not going on any long runs of freeways anymore, so it should be fine.
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Let it have its head through the gears and see what it does with the 19 t
I'm hoping you'll be surprised 👍👍
 
With tuning each bike is different and one carb, can be completely set different to the other even with needle height, it's also good to have a play with different size jets, always start with stock set up to begin with and always tune when at road running temps, set and take for a run, then just do small adjustments as you go even out riding it's handy to have a small screw driver and stop and have a fiddle, but of course having your timing and valves set right before any carb adjustments is need and plugs play a big part as well tune with new plugs to start off with.
But as I say each bike is different and each carbie is different in settings and the old school way of tuning twin carbs is to adjust the idle up and kill the other side cylinder to tune the other side and do the same again to the other side when you get it adjusted, I also use my ears and a hand over each side of the rear mufflers, you can normally feel if one cylinder is different to the other side as well by listening and the heat from the exhaust on the hand, works for me, but of course the road test will let you know if any final adjustments are needed.
I am no carb expert but I do know how to make my own Norton run sweet and learned a few tricks in the 47+ years and have also made a few mistakes as well, its a good way to learn about your own bike and I didn't have a clue when I brought my Norton new at 17 years old.

Ashley
 
baz said:
Let it have its head through the gears and see what it does with the 19 t
I'm hoping you'll be surprised 👍👍
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Me too….thanks. Last time I road a combat with 19T was 1974. This guy.
‘72 combat main jets
 
When I tune, I usually set the ignition advance to a defined value, then jet the carbs. I do not raise compression ratios. While I am tuning, i also do not usually alter the exhaust system. If you change fuel, comp.ratio, ignition advance or exhaust system or cam timing, you need to check your jetting. Jetting is easy to get correct if you stick to the procedure. In the end, it is trial and error. Most race cams have a power band which begins at about 4000 RPM, the top end for a Commando engine is about 7,300 RPM. There should be no dead zone in the middle of the rev range.
 
ILLF8ED said:
19T coming from CommandoSpecialties end of this week. I put the 230 mains back in. Appreciate your comment that your combats have 19T. It occurs to me I had not ever run a 21T on a combat in the past. I’m not going on any long runs of freeways anymore, so it should be fine.
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Reckon you’re doing the right thing there. Just remember to try and keep the front wheel down !!
 
acotrel said:
When I tune, I usually set the ignition advance to a defined value, then jet the carbs. I do not raise compression ratios. While I am tuning, i also do not usually alter the exhaust system. If you change fuel, comp.ratio, ignition advance or exhaust system or cam timing, you need to check your jetting. Jetting is easy to get correct if you stick to the procedure. In the end, it is trial and error. Most race cams have a power band which begins at about 4000 RPM, the top end for a Commando engine is about 7,300 RPM. There should be no dead zone in the middle of the rev range.
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You talking methanol? Gasoline?
 
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