750 Combat top speed problem (2015)

Rohan said:

illf8ed said:

It's really only necessary to do one lobe as the lobes are fixed pisition in relation to each other.

Click to expand...

We would hope !
Given this particular cam's failure to perform (?), maybe we can't take this for granted.

Where did this cam come from ?

Click to expand...

I don't believe the camshaft is the problem at this point of the investigation. It came from Norton Villiers originally. I'm working the fuel starvation angle at present. Waiting for some new Tygon fuel line to arrive to fire up the bike and see if raising the fuel level in the float bowls cured it. Already eliminated gas cap air vent blockage, it is not plugged.

illf8ed said:

I don't believe the camshaft is the problem at this point of the investigation. It came from Norton Villiers originally. I'm working the fuel starvation angle at present. Waiting for some new Tygon fuel line to arrive to fire up the bike and see if raising the fuel level in the float bowls cured it. Already eliminated gas cap air vent blockage, it is not plugged.

Click to expand...

In my bike's case, it was too big of a main jet, not fuel starvation. I only had one symptom, a wall that I could not get the bike to accellerate past. I had no hint of a misfire or poor running of any sort anywhere else. At 80mph my bike just wouldn't accellerate any more. If I held the throttle wide open, the bike seemed to just bog down, but hold at 80mph. If I rolled off the throttle, I could feel it pull for a second, but getting back on it made it just hit the wall again and flatten out at 80. If your symptoms at 80 do the same thing, you might have the same problem that I had. Roll off the throttle a bit at 80 and see if the bike instantly peps up a bit. If it does, I bet your main is too big... (at least that was what my issue was and it should be worth looking into)

I really didn't need to go any faster than 80mph, but I kept reading peoples comments about going 100mph on their commando, so I was trying to slowly work my way up there, trying to get comfortable gradually going faster and faster when I noticed that I couldn't get past 80mph on the flat bit of hiway that I was using as the speed part of my test loop. Needless to say, I was curious as to why the bike ran fine up to hitting the 80mph wall.

If your symptoms are similar to mine, trying a smaller jet and going for a test ride is worth a try even if it's only to eliminate my scenario. It won't even take that much time to do if you have bowls with drain plug and a jet key.

I hope my posting a third time isn't too obnoxious. I'm just trying to be helpful since I had a very similar problem.

o0norton0o said:

illf8ed said:

I don't believe the camshaft is the problem at this point of the investigation. It came from Norton Villiers originally. I'm working the fuel starvation angle at present. Waiting for some new Tygon fuel line to arrive to fire up the bike and see if raising the fuel level in the float bowls cured it. Already eliminated gas cap air vent blockage, it is not plugged.

Click to expand...

In my bike's case, it was too big of a main jet, not fuel starvation. I only had one symptom, a wall that I could not get the bike to accellerate past. I had no hint of a misfire or poor running of any sort anywhere else. At 80mph my bike just wouldn't accellerate any more. If I held the throttle wide open, the bike seemed to just bog down, but hold at 80mph. If I rolled off the throttle, I could feel it pull for a second, but getting back on it made it just hit the wall again and flatten out at 80. If your symptoms at 80 do the same thing, you might have the same problem that I had. Roll off the throttle a bit at 80 and see if the bike instantly peps up a bit. If it does, I bet your main is too big... (at least that was what my issue was and it should be worth looking into)

I really didn't need to go any faster than 80mph, but I kept reading peoples comments about going 100mph on their commando, so I was trying to slowly work my way up there, trying to get comfortable gradually going faster and faster when I noticed that I couldn't get past 80mph on the flat bit of hiway that I was using as the speed part of my test loop. Needless to say, I was curious as to why the bike ran fine up to hitting the 80mph wall.

If your symptoms are similar to mine, trying a smaller jet and going for a test ride is worth a try even if it's only to eliminate my scenario. It won't even take that much time to do if you have bowls with drain plug and a jet key.

I hope my posting a third time isn't too obnoxious. I'm just trying to be helpful since I had a very similar problem.

Click to expand...

Think I've already done this, but if the fuel level in the bowls change doesn't work will try again.

Thanks

o0norton0o said:

illf8ed said:

I don't believe the camshaft is the problem at this point of the investigation. It came from Norton Villiers originally. I'm working the fuel starvation angle at present. Waiting for some new Tygon fuel line to arrive to fire up the bike and see if raising the fuel level in the float bowls cured it. Already eliminated gas cap air vent blockage, it is not plugged.

Click to expand...

In my bike's case, it was too big of a main jet, not fuel starvation. I only had one symptom, a wall that I could not get the bike to accellerate past. I had no hint of a misfire or poor running of any sort anywhere else. At 80mph my bike just wouldn't accellerate any more. If I held the throttle wide open, the bike seemed to just bog down, but hold at 80mph. If I rolled off the throttle, I could feel it pull for a second, but getting back on it made it just hit the wall again and flatten out at 80. If your symptoms at 80 do the same thing, you might have the same problem that I had. Roll off the throttle a bit at 80 and see if the bike instantly peps up a bit. If it does, I bet your main is too big... (at least that was what my issue was and it should be worth looking into)

I really didn't need to go any faster than 80mph, but I kept reading peoples comments about going 100mph on their commando, so I was trying to slowly work my way up there, trying to get comfortable gradually going faster and faster when I noticed that I couldn't get past 80mph on the flat bit of hiway that I was using as the speed part of my test loop. Needless to say, I was curious as to why the bike ran fine up to hitting the 80mph wall.

If your symptoms are similar to mine, trying a smaller jet and going for a test ride is worth a try even if it's only to eliminate my scenario. It won't even take that much time to do if you have bowls with drain plug and a jet key.

I hope my posting a third time isn't too obnoxious. I'm just trying to be helpful since I had a very similar problem.

Click to expand...

Go to page 3: http://www.hitchcocksmotorcycles.com/pi ... ettors.pdf and read what is in RED.
Tom
CNN

CanukNortonNut said:

Go to page 3: http://www.hitchcocksmotorcycles.com/pi ... ettors.pdf and read what is in RED.
Tom
CNN

Click to expand...

Well,... if you think about how amals work, An incorrect main jet size issue may not show up until 3/4 throttle so whether the main jet is too large OR too small, What do you think the bike would do differently when you reach 3/4 throttle and the mixture is now either too rich or too lean?? Would too lean or too rich of a main jet do something so vastly different at 3/4 throttle that you would instantly know the difference?? pinging maybe??

In my bike's case, the fact that my issue was isolated to the full throttle range meant that the main jet size was suspect. If you go back and read what I wrote, you'll see I was ready to try both a larger and a smaller main jet. I tried the smaller jet first because I was already running a larger than stock main jet (I have dunstall exhaust pipes) and my bike zoomed right past the wall that previously contained it at 80mph with the smaller main jet installed, so I had no need to try the larger jet.

No doubt david will try both larger and smaller main jets, IF the first jet change he tries doesn't fix his issue... since the range where both our similar problems occured points to main jetting.

Not a Norton but.....Years ago when I was racing I set my bike up for ice racing (from MX) and first time out as I opened the throttle the bike died, just as if the ignition was turned off. Rolling back to less than 3/4 throttle she started running strong again. Opened the throttle and died again. I had to go up many sizes on the main jet to get it to run right. Obviously the huge difference from summer riding to extreme cold winter riding was the cause but I didn't think it would be that much that the bike completely shut off!

Rohan said:

illf8ed said:

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

Click to expand...

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

Click to expand...

Raising the fuel level in the carb bowels improved the situation. Took it out in 48 degree weather this morning. It pulls to 90mph now. At that point I opened the throttle all the way with no more speed. Just to see if a lean condition I closed the choke and it made no difference (will mention again Amal 932 with 230 mains for reference). So Rohan the next step is drop to 220 mains as you suggest. I don't have this size in my collection and will have to buy some. By the way in gears 1 to 3 very strong with the power surge at around 4500 rpm as should be for a combat. I'm getting there and am stubborn about making original parts work until it's obviously not possible.

Happy New Year everyone.

If giving it some choke didn't take speed OFF, I'm back to thinking I don't like that particular cam....
??

Its not impossible to solder up jets, and redrill the holes out to suit.
Amal probably don't recommend this, but since thats about what they do anyway to get different size jets...

Maybe you should try bigger jets first ?

If your chokes are working properly (ie going all the way down) I just don't see how they can have no effect on an engine being run at WOT?

The choke on mk1 concentrics is basically akin to a flat slide and thus it is essentially the same overall effect as (almost) closing the throttle!

I imagine your chokes can't be working correctly...

If they are, something else has to be quite wrong for this situation to be possible.

I recall you are running re-sleeved carbs. I don't know who did this for you, or how much you have looked at them since, but an error in this process, either the machining or re-assembly, would be my first port of call here.

I'll close as I opened... I just cannot see how it is possible to close Amal chokes on a large capacity engine that is on WOT and sucking all that air in and it make "no difference" !?!

Unless we have the "opposite" problem...

Meaning, have you confirmed your slides are actually fully open when you are (intending to be) at WOT ?

Maybe the carb sleeves are a loose press fit and slide down when hot w.o.t. Then when throttle off they go back into place as the slide hits the bottom ?

one of the issues of using a jet key to remove the main jet through the threaded drain plug in the bottom of each float bowl is that the jet holder needs to be locktited into the body of the carb so when you try to remove the main jet with the jet key, the whole jet holder doesn't spin off. If the jet holder spins instead of just the jet, then you have to remove the entire float bowl (which is a pain in the ass) to have access to the jet holder to change the main jet.

IF the main jet turns out of the jet holder, it will pass right out of the float bowl drain plug and can be changed in a few minutes, making it easy to test different main jet sizes if problems arise that point to issues at nearly full throttle operation. Like I said, putting blue locktite on the threads of the "jet holder" when you thread it into the carb body, insures that the jet will spin off and on easily and make jet changes quick and simple without disturbing the seal on the float bowls, nor the floats, float needles, and bowl gaskets...

Anyway, if you can do main jet quick changes like I detailed above, you can quickly either fix, or eliminate main jetting as the issue, but trying 240's and 220's and taking a quick test rides with both... In my case, I got it right on the first jet change and was pretty happy that it was fixed without a whole lot of searching for invisible gremlins like a bad ground wire or a dead battery cell, or some other hard to find issue.

The other thing worth mentioning which could also come into play near WOT, would be the choke lever, If the choke bodies of the carbs were not retracting all the way, maybe they would block the airflow slightly when the throttle bodies were near WOT... (another thing to look at....)

Heck I would get rid of the choke assemblies altogether and plug the cable entries with the Amal bolts for the job.

Since this has gone on for 11 pages, I'd be desperate to find an answer once and for all. It may have been said already (there are far too many posts to check), but the first thing I'd do is borrow a set of carbs off a bike that runs nicely, and see what that does. If there's no change, I'd then replace the entire ignition system in the same manner. If neither fix the issue, at least you'll have narrowed things down massively.

Paddy_SP said:

Since this has gone on for 11 pages, I'd be desperate to find an answer once and for all. It may have been said already (there are far too many posts to check), but the first thing I'd do is borrow a set of carbs off a bike that runs nicely, and see what that does. If there's no change, I'd then replace the entire ignition system in the same manner. If neither fix the issue, at least you'll have narrowed things down massively.

Click to expand...

I agree with you, this string has been going on long enough. Since I started the string, will stop posting on this subject. Appreciate the feedback to now. I'm pursuing 220 main jets (in the mail).

illf8ed said:

Paddy_SP said:

Since this has gone on for 11 pages, I'd be desperate to find an answer once and for all. It may have been said already (there are far too many posts to check), but the first thing I'd do is borrow a set of carbs off a bike that runs nicely, and see what that does. If there's no change, I'd then replace the entire ignition system in the same manner. If neither fix the issue, at least you'll have narrowed things down massively.

Click to expand...

I agree with you, this string has been going on long enough. Since I started the string, will stop posting on this subject. Appreciate the feedback to now. I'm pursuing 220 main jets (in the mail).

Click to expand...

I can understand your frustration David. But please tell us you'll at least keep us informed as to your findings... we're involved now... !

illf8ed said:

I agree with you, this string has been going on long enough. Since I started the string, will stop posting on this subject. Appreciate the feedback to now. I'm pursuing 220 main jets (in the mail).

Click to expand...

Please don't get me wrong - I want you to get the bike sorted, and I'd be upset if you thought I was fed up of hearing about it. I just think that a radical 'start from scratch' approach would help!

David, did you find a solution to the problem?

Glen

Well somewhat yes. I believe a lot of the problem was the RITA ignition was went bad and I put in a TriSpark. The smaller 220 main jets also help a bit. It still is not pulling easily to 100mph and it should. It runs very well below 85mph, so leaving it alone for the season.

Back at it now at the end of 2018. In the garage today checking intake open and close timing. Looks like the cam is retarded 15 degrees. Will pull the timing cover and recheck the alignment of crankshaft gear and intermediate gear. 15 degrees is one tooth mis-aligned. Maybe get a look tomorrow.