Where' the power?

As a curiosity, what slide are you using?

MLW said:

I felt that the JS1 cam would work best with the chosen settup.

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Nothing wrong with that, it’s a good cam, a mild uplift and increase in duration without being too radical. They work well. They also wear better than some higher lift variants. Plus they work well with stock pipes and peashooters (straight through). I’m a FCR fan too, but realistically Amal's can easily flow what your motor needs to comfortably surpass 100mph, so they’re not causing your issue here.

Did you gap the rings? Aluminium barrel with nikasil is smaller diameter than standard bore size. Graham.

MexicoMike said:

Are you sure the carb slides are opening fully? I agree that it sounds like a fueling issue IF the ignition/cam timing is correct. A totally stock 750/850 Commando in proper running order has no trouble at all going easily past 100MPH with no hesitation.

It's quite a list of mods; are they based on reliable information that the specific setup works well on a 750 Commando? I have a LOT of experience with engine mods on cars and have seen everything from high performance cams to high compression pistons to free flow exhausts to larger carbs, etc, REDUCE performance compared to stock engines because the mod(s) did not work well together. TBH, I'm a little surprised to see a "performance cam" and higher compression pistons with stock exhaust/induction but IF that setup is well researched and shown to work well, that's all that matters.

Cam timing could certainly cause the problem as could retarded ignition timing but since they have been checked, one could ASSUME they are OK. Clogged intake/exhaust could also cause it but, again, being new that's extremely unlikely.

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I've been there
Big valve head ,high compression etc
Returned to standard and picked up power !

velo54 said:

Did you gap the rings? Aluminium barrel with nikasil is smaller diameter than standard bore size. Graham.

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Also, verify compression and the possibility of poor valve seating.

- Knut

baz said:

I've been there
Big valve head ,high compression etc
Returned to standard and picked up power !

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Makes you realize that the Commando was a pretty decent hotrod in stock form.
That's what I have found when putting my stock original (except 73 type exhaust) mK3 850 slug ,the slowest Commando ever built according to factory experts, up against a hot 750 built by a Norton guru. His 750 was said to be the hottest Commando he's owned, and he has had many. The bikes were about even.
I have also found that my very lightly tuned Rapide will leave a riding partner's much hotrodded Rapide behind on any hill. It did the same to my other Rapide that had a long list of performance upgrades.
But I can also tell you that when the upgrades are right, it really works ( Terry Prince parts for Vincents, no half measures,the complete works)
It seems that more often than not the hot rodding doesn't all come together as planned ,reduces power or just gives no real boost.
The owner is often thrilled with the bike anyway and will recite it's long list of hot parts to anyone who will listen. I sometimes wonder if that's what those parts are made for!

Glen

MLW said:

Timing was checked with a strobe, the timing marks checked with a degree wheel for accuracy.

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What is the timing set to? Probably not your problem, but higher compression can use more advance. For Tri-Spark and a stock engine you should be at 29 BTDC but you might want to go to 32 BTDC (and lose anti-kickback). Also, higher compression especially at higher RPM can make it much harder to spark. I would go max .025" gap if using standard Lucas 6v coils.

I have always gone the other way, retarding timing when adding compression. The stock low compression Vincent is set at 38 degrees BTDC whereas my 11.5 to 1 engine runs at 26. It has two plug heads that are responsible for about half of that timing retardation, the rest is due to the high compression. That very low ignition Timing number was the result of a conference call that Andy,owner of Pazon, set up with Vincent tuning guru Bob Dunn, Andy, and me. Andy and I were both a bit surprised at how little advance Bob suggested. We went with it and it works very well, no detonation at 11.5 to 1 Cr.

Glen

baz said:

I've been there
Big valve head ,high compression etc
Returned to standard and picked up power !

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YIKES!

It sounds to me like you and @MLW both did the research, went in the right direction with everything, used the correct tools and procedures, but BOTH hit a mysterious wall.

You were satisfied to return to stock, @MLW is not (yet).

It seems everyone here is asking all the right questions, but some issue is STILL slipping through the cracks!

Sadly, there are inter-relational issues that can crop up, that slip past most people's scrutiny when READING about it, and can only be discovered and remedied with multiple heads and hands on the bike IN PERSON, with all the tools and equipment needed to remedy the offensive detail(s).

Perhaps a well-tuned AI (shudder) "app" might be a wonderful thing. It would take a hack of a lot of programming by those indisputably "in-the-know" to carefully input not only the scientifically known factors, but somehow weigh the PROVEN results from others' forays and factor those in as well, as "magic" sometimes happens with certain builds (the dyno, 1/4 mile, and long, lonesome highways don't lie, when there are multiple witnesses with proper / calibrated measuring equipment).

Personally, I don't think I read anything about @MLW 's regional ALTITUDE, which can have some effect on tuning...

Do a "plug chop" ( look up how to them them) at a steady 4000RPMs with new spark plugs.

If this was a race bike plug, I would use a plug 1 step colder. But this is a normal appearance from a recently rebuilt hopped up engine. The tops of the pistons look fine, 180 Lbs. of Compression. Engine has great power at all RPMs.

MLW said:

This is the best photo I have of the completed bike, will post more in the future...

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It's clean enough to go faster. Nice job

The stock parts making more power than what you have the potential to make with your selection of parts is in my experience incredibly optimistic. Maxed out at 6000 RPM is really unusual, unless it is an elevation issue, which will kill power with regularly aspirated carburetion that is not jetted for the elevation. I have no clue what the JS1 cam is like, but the JS2 will wind until the engine comes apart with the right carburetion. I could go on with more build BS, which you have under control, but the point is don't give up. You have some nice parts and they can work well together.

Good luck with the Amal advice.

MLW said:

That was my first thought, I timed it with a degree wheel as per instructions and checked again a second time, cam timing is spot on.

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But what were the numbers and what was the lift? What is the actual Lbs. of compression ?

Waiting to see what the OP says regarding main jets. I have 2 Commando's, one stock MKIII with a Mikuni and 22 tooth sprocket. Nice, smooth bike. great for touring. Other is a 74 with a Comstock big valve head, 9.5 /1 compression, 32 Amals, 314 WebCam. Built it the 1st time with a JS2 cam, but felt it was a bit too top end heavy. Changed to the Web Cam and now it has a real nice powerband up to 7K. In hindsight, I should have advanced the JS2 cam a few degrees to bring the power in earlier. Running 260 mains with std. peashooters. Thinking the 33mm FCR's would work well.

With any bike motor, there are three things in balance, compression ratio, ignition timing and jetting. A bigger bore increases the compression ratio, and has a similar effect to advancing the ignition timing or leaning-off the jetting. At any throttle opening below two thirds, the main jet does nothing unless it is too lean. So use the largest specified main jet, and worry about it after you have tuned the midrange jetting. The main jets are set by doing plug chops, after you have the mid-range jetting right. I suggest a lot of the stuff about race cams is bullshit. The Commando already has a hot cam, different timings move the power band, and a race cam usually loses some power off the bottom for more at the top of the rev range. With a Commando race bike, competing against 4 cylinder bikes, you will usually be slightly slower in the straights, so you need to be faster in the corners. Midrange power and gearing are more important than top end. If you get the midrange jetting lean enough, the bike will come out of corners faster, and you might reach a faster speed down the straights. Other bikes accelerate like buggery down straights - and can be in the next corner ahead of you, - With my bike, I can ride under them.
If your main jets are too big and you whack the throttle open, the bike will usually be slower. If you have the jetting and ignition timing right, you should be forced to feed the throttle on in a controlled manner.
By moving the timing with the bike on a dyno, you probably have finer adjustment than by changing jets - I use fixed timing and never change it. However with carburetors using petrol, moving the timing might be a better way. The exhaust system affects the jetting and the shape of the power band, so do not change it unless you really need to. Some people believe methanol gives more power due to the latent heat effect - its major effect is that because bigger jets are used, small differences in jet sizes are of much less importance.

I just went back to bed and thought about something which might interest you guys. Some years ago, my mate had been racing his Triton 650, and showed me the plugs out of it. One side had been running slightly richer than the other. At the time I had forgotten that if a carburetor touches the frame fuel frothing can cause it to deliver more fuel. The motor of my Seeley 850 is rigidly mounted and runs dead smooth between 5,500 RPM and 7300 RPM - its usable rev range. Norton Commandos have isolastics and rigidly mounted carburetors and low balance factors. Rich mixture slows motors. What is the motor doing at high speed ?

I brought up the FCR carburetors because the title of this thread and the issue brought up is about lack of power. I forget that people want to make what they have work and don't give a rat's arse about making more power with different parts. It's just irrelevant food for thought. And clearly not everyone is a tuner willing to put in the time it takes to make an engine perform optimally. For example, once a person figures out how to tune FCR carburetors so that the accelerator pump fully functional is not an issue that little blast if fuel makes a significant difference in producing power quickly and sustaining it. Lack of power feel is the same reason I brought up the JS2 cam. The JS2 is a smooth soft ramp version of something very close to the SS or 2S cam. It too has a noticeable bump in power delivery and a healthy top end. Advancing the cam timing makes the SS and JS2 street friendly. Not as friendly as a stock cam for a lot of people, but very usable for people that have owned race bikes with street license plates on them.

Anywho, just to keep throwing out more of the irrelevant tuning crap, Jim Schmidt sells an offset cam key that can be used to advance or retard the cam timing 2 degrees. It might be fun to try 2 degrees of advance with the JS1 cam to see if it increases the feeling of power.

I suggest that speedway and MX bikes need quicker taper needles in their carburetors, because the throttle response needs to be quicker, the pump serves the same purpose. If you whack the throttle open on a well-tuned road race bike, you would usually get a gasp, due to loss of vacuum. It causes leaness. I have been watching the documentary about John Surtees. He used to work in the Norton factory with Phil Irving and the race team. He got a Vincent Grey Flash going and won a lot of races with Reg Geeson's REG 250 - then got a works ride with Norton. Sammy Miller mentioned riding John Surtees' Manx, and said it was so well tuned that it was very easy to ride. There was a statement by Phil Irving in Tuning For Speed, that 'if methanol fuel is run rich, it still gives good power' - what was he saying about petrol ? My bike runs on methanol - the difference between fast and slow is half of a thousandth of an inch in the needle jet, and I use 6D Mikuni needles - the slowest taper. When you race, you do not usually get to ride the fast guy's bike, and most will not tell you what makes a difference. Reg Geeson's REG 250 provided information for Honda, and Mike Hailwood's father bought a world championship winning Mondial 250 for Bob to learn on. I was actually pleasantly surprised when I got the jetting on my 850 motor lean enough. The 5 notches on the needles actually make the bike faster or slower. With petrol as fuel, I would be pretty-much stuffed. I would need luck to get it right.
With the Isolastic frame, the crankshaft is balanced to run smooth at low revs - at high speed, it probably shakes the shit out of the carburetors. I have not seen many Japanese bikes which did not have rubber-mounted carburetors.

Schwany said:

I brought up the FCR carburetors because the title of this thread and the issue brought up is about lack of power. I forget that people want to make what they have work and don't give a rat's arse about making more power with different parts. It's just irrelevant food for thought. And clearly not everyone is a tuner willing to put in the time it takes to make an engine perform optimally. For example, once a person figures out how to tune FCR carburetors so that the accelerator pump fully functional is not an issue that little blast if fuel makes a significant difference in producing power quickly and sustaining it. Lack of power feel is the same reason I brought up the JS2 cam. The JS2 is a smooth soft ramp version of something very close to the SS or 2S cam. It too has a noticeable bump in power delivery and a healthy top end. Advancing the cam timing makes the SS and JS2 street friendly. Not as friendly as a stock cam for a lot of people, but very usable for people that have owned race bikes with street license plates on them.

Anywho, just to keep throwing out more of the irrelevant tuning crap, Jim Schmidt sells an offset cam key that can be used to advance or retard the cam timing 2 degrees. It might be fun to try 2 degrees of advance with the JS1 cam to see if it increases the feeling of power.

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Depends on the exhaust system. If it is restrictive, it will work better, but be louder. I have a 2 into 1 exhaust, and my cam is advanced by 12 degrees - I had two more keyways broached into the sprocket at random positions. At one time, I did a lot of work cutting and welding exhausts, and changing cam timings. Not much makes much difference. My bike is horrendously loud, but the 2 into 1 exhaust really works. I think Paul Dunstall had the right answer - divide the outlet and have two mufflers.

acotrel said:

Depends on the exhaust system. If it is restrictive, it will work better, but be louder. I have a 2 into 1 exhaust, and my cam is advanced by 12 degrees - I had two more keyways broached into the sprocket at random positions. At one time, I did a lot of work cutting and welding exhausts, and changing cam timings. Not much makes much difference. My bike is horrendously loud, but the 2 into 1 exhaust really works. I think Paul Dunstall had the right answer - divide the outlet and have two mufflers.

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My exhaust is a 2 into 1 and is basically open. It is also a long way to the tip at the arse end of the megaphone. It works and is obnoxious on the throttle. Not terrible putting around getting out of the neighborhood, since I made an 18" x 1.75" open baffle for it.

I agree that 2 degrees of cam advance is close to imperceptible. The people that can't tell when a cam lobe is ground down to the base circle sure as heck wouldn't notice.

9 timing chain rollers between dots on the cam and intermediate sprockets is 11 degrees of advance if I am not mistaken. Are you using a .040 or thicker head gasket with a base gasket, or does 12 degrees of cam advance work without the valves hitting the tops of the stock flat top Commando pistons? Just curious. I wouldn't do it myself.

Something not right with the amount of good bits thrown in this motor, it should rev very freely and keep revving way past 7k RPMs but how far do you push it is another thing, if its not revving like it should then ignition or carburation is what is stopping it, Amal carbs work well if set up right with the right jetting for the motor and all the adjustment done right.
In 49+ years of owning my Norton and back in the early 80s building my 850 motor for the featherbed frame I did the 2S cam, balance crank, pistons and head work, open exhaust, I even done other carbies from Delotos pumpers, to Jims PWKs, to EIs to now the Joe Hunt magneto, but found my Amals and JH work the best if set up right.
In all these years of ownership and working on my Norton I have only ever used a timing light once when tuning, my cam is set as the bible said (workshop manual) but with timing of the ignition no matter what ignition system I am running I set the timing statically then advance the timing till it get some kick back when kicking then retard ever so slightly till no kick back, every so slightly on the adjustments, once set right and easy kick starting (first kick every time) then setting the carbs, might take a few time to get right and lots of mucking around changing settings, but if jets are right, needle in the right position, float levels right, slides lifting close enough to both carbs, then carry a small screw driver in your pocket and take it out on the road and do final adjustments when out and about, once home do the hands test at the back of the mufflers to feel if both pipes have that thump evenly and set your idle if not.
Going back to my old Amals and with the Joe Hunt my Norton has been running the best it ever has in all the years I have owned it, my Norton has no restrictions at all in how things have been built and with the work I have done my Norton runs very freely you just got to know how far you can take it as it don't hesitate at all from slow revs to opening it right up and with the work done to the OP bike it should be the same without any hesitation at all if things are set right, but in his case something is not.
Sometimes doing things by the book don't work and doing a few changes that the book say to do it, look outside the box, it might turn out just something simple might be the problem, but don't give up and don't waste your money by changing things that might not be the cause of the problem.
Could be something as simple as a valve adjustment to an air leak and it can get very frustrating but do change one at a time and you will win, but don't give up, I am not tune up expert but my hotrod Norton is going so good as well very reliable from my hands.

Ashley