Where' the power?

In the 1950s, Manx Nortons were single cylinder and running on petrol. Petrol is twice as critical in respect of tuning errors when compared with methanol. When you set the needle height in an Amal carb, the usual method is to lower the needle until you get the cough then raise it one notch. If you change the needle jets by one thou steps in internal diameter, there are 5 notches, so the one thou steps are divided by 5. Those size steps are too big, even with methanol. I suggest in the 1950s, when Manx Nortons ran on petrol, the British might have filed the needles to remove the miss from the motor. It can be done with a single carburetor. Even the taper on the needle makes a difference, as the throttle is fed on slowly. Best power is very close to the miss. If the motor misses, that is when damage is usually done. What happens between one third and two thirds throttle is most important. When it is right, the acceleration rate is much faster. Two strokes should not be tuned in this way.

marshg246 said:

What is your highest RPM in 4th gear and what throttle opening at the highest RPM?

Click to expand...

6000 rpm @ wot, 5000rpm in 4th is achieved with less than 1/4 throttle.

hhh said:

Agreed, but sometimes you have to compromise a bit to save having to replace kick start components at an accelerated rate.

Here's what AI (not Al) has to say: "No, octane rating and flame speed are largely unrelated properties of fuel, as octane measures knock resistance while flame speed is determined by the fuel's chemical composition and operating conditions, not its octane level. High-octane fuels are more resistant to pre-ignition, but this property does not mean they burn faster or slower; some high-octane fuels burn quickly, while others burn slowly. Using higher octane fuel than necessary for an engine can actually reduce performance and slow the burn rate."

Click to expand...

COOL! Fresh ammo for future discussions.

This really is interesting.

MLW said:

6000 rpm @ wot, 5000rpm in 4th is achieved with less than 1/4 throttle.

Click to expand...

OK, now I'm really confused. 6000rpm is 102 MPH, IMHO 110mph is about the max. 5000 rpm is achieved on the slide cutaway, not even on the needle/needle jet yet so you're going 85 mph at less than 1/4 throttle. What happens at 1/2 throttle? At 3/4 throttle?

Are you positive your clutch is not slipping? Are you big? Are you getting out of the wind? Besides being very heavy, I am very wide - getting a Norton to 110mph would be really hard for me unless I were laying on the tank and even then it probably would not get there.

Maybe @jseng1 can state what top RPM you can expect in 4th with a 21T front sprocket. 7500 is 127mph and I've not heard of a street Norton getting there.

MLW said:

Well this thead has certainly taken on a life of it's own! The selection of main jets I ordered will arrive next week. I will follow Jim's suggestions and jump to 280 main jets which incidentally is also what is recommended in the Dunstall race manual. I will report on the results.
I will also comment on this, the symptoms are only apparent in 4th gear and somewhat in 3rd. Otherwise, it starts first kick, hot or cold every time, Idles consistenty and has not changed once set, (yes with Amals!). Throttle response from idle is instant with no hint of lag or hesitaion as is often typical of slide valve carbs without accelerator pumps. The motor revs up smoothly, quickly and freely up to and beyond 7000rpm, (if I would let it) with a noticable surge of acceleration as the tach swings past 5000 in both first and second gear. To be honest, it pulls harder in first and second gear than any classic Amal Brit bike has the right to!

Click to expand...

Sounds like there probably isn't much of a problem and you might be shifting too soon for 4th. It should do close the same in every gear if there is enough road out in front of you. The Molnar lighter weight flywheel is different. Your engine spins up quickly, but has less inertia from the crank so gotta stay on the throttle and keep the revs up. May sound like jibberish to those that have never used the parts you are using, but I have the same parts inside my engine. Only difference is I have more cam and less torque down low. My head is different, but flows really well. Good luck with the bigger mains.

The BSA owners handbook's advice on obtaining top speed is to use the revs in every gear. On a 4 speed box getting to 7000 in each gear gets you back into the torque band when you change up into the next gear. However you do need to have the right main jet for 4th gears top speed and the lower gears will hide a wrong sized main jet.

Be interesting to see how this plays out.
Please keep us informed.
We all learn.

MLW said:

Well this thead has certainly taken on a life of it's own! The selection of main jets I ordered will arrive next week. I will follow Jim's suggestions and jump to 280 main jets which incidentally is also what is recommended in the Dunstall race manual. I will report on the results.
I will also comment on this, the symptoms are only apparent in 4th gear and somewhat in 3rd. Otherwise, it starts first kick, hot or cold every time, Idles consistenty and has not changed once set, (yes with Amals!). Throttle response from idle is instant with no hint of lag or hesitaion as is often typical of slide valve carbs without accelerator pumps. The motor revs up smoothly, quickly and freely up to and beyond 7000rpm, (if I would let it) with a noticable surge of acceleration as the tach swings past 5000 in both first and second gear. To be honest, it pulls harder in first and second gear than any classic Amal Brit bike has the right to!

Click to expand...

If you had given all that info in your first few letters most would point the problem to main jets or fuel level or fuel starvation. I will wait for a few Pug chops at different throttle positions.

marshg246 said:

OK, now I'm really confused. 6000rpm is 102 MPH, IMHO 110mph is about the max. 5000 rpm is achieved on the slide cutaway, not even on the needle/needle jet yet so you're going 85 mph at less than 1/4 throttle. What happens at 1/2 throttle? At 3/4 throttle?

Are you positive your clutch is not slipping? Are you big? Are you getting out of the wind? Besides being very heavy, I am very wide - getting a Norton to 110mph would be really hard for me unless I were laying on the tank and even then it probably would not get there.

Maybe @jseng1 can state what top RPM you can expect in 4th with a 21T front sprocket. 7500 is 127mph and I've not heard of a street Norton getting there.

Click to expand...

The problem is poor acceleration above 1/2 throttle in to gear.

MichaelB said:

Be interesting to see how this plays out.
Please keep us informed.
We all learn.

Click to expand...

I originally posted the question to see if others had similar experiences and to get any ideas on something I may not have already checked. I will certainly keep you posted on future developments.

MLW said:

The problem is poor acceleration above 1/2 throttle in to gear.

Click to expand...

Still confusing. At less than 1/4 throttle you're at 5000 RPM, at WOT you're at 6000. At 1/2 throttle your acceleration is not what you expect. IMHO, quit thinking about main jets until you get your needles and/or carb sync, and/or fuel level, and/or air leaks right. Once you are at about 6000 RPM at 3/4 throttle, then if it won't accelerate more, think about main jets. I think you said you had the needles at full rich - I would try again in the middle slot.

If you're riding at less than 1/4 throttle and all the sudden go to WOT, don't expect much - you're way past the engine and dumping in a lot of extra air, not fuel/air as there's not enough vacuum to pull the fuel. Somewhat the same at 1/2 throttle all of the sudden going WOT. Would be different if the bike had fuel injection or the carbs had accelerator pumps. Rolling on the power will get you going faster - faster than WOT all at once.

I'm probably not going to have an opportunity to give my new to me Commando a good thrapping for a while but I distinctly remember that back in the day, my 850 would struggle past 100mph if rolled on in top gear, but if redlined in 3rd, it would pull up to redline in top gear...120mph indicated. Maybe the OPs expectations are optimistic.

Well in my country going flat out in top gear just gets you in trouble with the law when riding on the road, so how many times you want to go flat out in top gear, if the bike is revving fully through the gears then all good as winding it flat out in top gear you need a lot of straight road, better if you have a big hill to run down to the long straight will help you to get there but as a few say get the revs in the high revs through the gears (rev it right out) before putting it into top gear.
In my younger days it was normal to find out how fast they do, my stock 850 Commando would only rev to 6500 RPM and just run out of puff after that, when I built my motor for the Featherbed frame with the cam, pistons, port work, carbs jetted right, open exhaust system and a K&N air filter my motor now revs very freely way past 7500+ RPMs but how far do you want to push it past that rev range, things tend to fly apart if pushed too far, there is a big gap from the end of 3rd to the end of 4th and seems like its not getting the fuel there, when trying do you turn on both fuel taps as to get the fuel you need to run flat out in top gear with the normal fuel tap and reserve fuel tap open, I found that out a very long time ago to open both fuel taps when doing high speed runs, if you just use one tap to feed both carbies the fuel be used up before you get there, you need to have both fuel taps open for high speed runs.
So does the OP run with both fuel taps on when he trying to run flat out in top gear as well revving it to the max in 3rd gear before shifting into top gear.

Ashley

I started typing but nothing but gibberish appeared on the screen.

I thought of this: If you put race parts in a motorcycle engine you might have to ride it like a race bike to get the most benefit from those parts. Nope still crap...

MLW said:

Hi folks, I'm hoping someone can help me. Just completed a full 750 Commando engine build & it's speced out for hot street settup with the following goodies:
Fullauto head
Complete JS1 cam & valvetrain
JS rods & 9.5:1 pistons
32mm Premier carbs
Full stock exhaust from Andover
Stock airbox
Trispark ignition.
Molnar crank & nikasil barrels
21 tooth gearbox sprocket
etc.

After putting in a few break in miles and starting to apply more liberal throttle applications, there no additional power after 1/2 throttle, just more noise in top gear. The bike struggles to achieve 100mph.
I set the valve timing with a degree wheel, the ignition timing and marks have been double checked, I bupmed the main jet size from 220 to 240 with no noticable difference.
I think this setup should work much better than this but I can't seem to figure out what the issue is. Please help!

Click to expand...

Hi, sounds like it should be fabulous, I suspect the problem will be staring you in the face when you find it. Obviously mixture was the correct place to start, more noise and no more power does sound lean to me. Rechecking cam timing will have to occur if all other variables are eliminated. You will get a thrill when it all comes together. Good luck

If cam timing is too advanced engine will be responsive ar low rpm then go flat as rpm increases.
If cam timing is too retarded low rpm response will be poor but when it "comes on cam" it will take off like a two stroke

Andy Higham said:

If cam timing is too advanced engine will be responsive ar low rpm then go flat as rpm increases.
If cam timing is too retarded low rpm response will be poor but when it "comes on cam" it will take off like a two stroke

Click to expand...

Per your suggestions and his observations, his cam is too advanced.

In my engine building days we used to get guys all the time who would come in, tell us what "race-type"mods they wanted for their (street) car. Our response was, "You don't want that for your car; it will be miserable to drive." Usually they went along with our recommendations. On one occasion, we were ignored and the guy insisted on "his" mods. Several months later he came back complaining that the car was "miserable" to drive! He had us change it to what we had recommended initially.

I suspect most of us have been sucked in at one time or another by the allure of the racing machine, ridden by [name your favorite rider/driver] carving corners/blasting down the straights. "I want to be like/look like [whoever]!" But what works on the race track has little in common with what works for operating on public roads.

MLW, there are still many variables that you haven’t clarified, and also your plot / storyline thickens !

You fitted 9.5:1 pistons… but you haven’t told us what your measured static CR is. Why is this important? Because the piston makers figure is only an estimate, the actual figure depends on many things like gaskets used etc. basically it is quite probable that you have less that that, perhaps only around the stock CR.

You have a FA head. Is it one of the original Australian ones, or the newer ones?

You have a JS #1 cam. You checked the timing, but haven’t divulged the numbers.

So… you could potentially have a stock CR and a sporty cam timed too advanced… which would definitely fit your symptoms.

However… assuming that’s not the case, and all was well, if your bike was put on the dyno I use it would be in the 50 to low 50s rwhp range. Now, even though that’s a healthy number for an old bike, it’s still not a lot in absolute terms, and will be an issue if you’re used to riding modern bikes.

With that cam, and the lighter crank, your engine should feel VERY responsive on the throttle, when you blip the throttle it should feel / sound very different to stock. That cam and crank complement each other in this regard. BUT they also compliment each other in being things that reduced torque to the wheel below around 4,500rpm. So, if you’re expecting to out drag a good stock bike when opening the throttle at 3,000rpm, you’re likely gonna be disappointed.

As others have already said, you’ve built a sporty engine, it needs revs. To get max rpm in top gear on a bike like yours means riding it to high rpm in the lower gears to get there.

If you’re a big guy, with a lot of weight and poor aerodynamics, and if you have high-ish western bars, and if you’re sat upright, you can’t just expect to open the throttle at 50mph in top and expect it to romp up to 120 quickly.

So, what I’m saying is, please address / report back on the ideas put forward thus far (carb settings, cam timing, CR numbers, etc) BUT also consider that some of your problem could be down to expectation management.

I'm not sure I could count the number of times I saw "more carburation" reduce engine performance. As has been mentioned many times, all this mod stuff has to work together. Very few mods can actually add useful power on their own. Yeah, I know it's hard for us performance-oriented types to accept but most of the time the motorcycle/car manufacturer knows what they're doing.