underpowered Mk3 update

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brf1957

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May 15, 2009
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Hi all, had a good check round everything as suggested and still bike wont go over 80, when revving at a stand still it will rev to 7 but when under load wont go past 5, tried 280 then 290 main jet but no improvement so back to 270 tried raising needle a notch just ran worse about 60 and rich, doing plug chops at the 80 things look a bit lean which is worrying for obvious reasons checked slide its going all the way up size 3 and 1/2 by the way gearing is standard, timing rechecked with strobe, choke slide fine, running with both taps on,bit of a mystery ,does anyone think boyer ignition or power box could be to blame, anymore suggestions welcome Brian
 
How confident are you that your camshaft (lobes) are in good condition, and your cam timing is correct i.e. 10 chain roller links apart between the markings?
Have you ever had the bike performing well whilst in your possession?
 
Not sure about cam lobes or valve timing but bike seems to be so healthy otherwise, it goes really well up to 80, it has always been like this since i got it not so long ago ,
 
hi brf,whats happening at 80 is it miss firing or does it seem to hit a brick wall ,does it rev to 7 through the gears,if its looking lean on the plugs at 80 then go bigger main jets ,as reggie says check valve timing that could be the problem
 
Hi chris no misfiring just cant get any more after 80 just the same through the gears cant get above 5000 revs have tried up to 290 main jet do you think i should go even bigger on the single carb
 
If somebody on the board is running a single Amal mk1 carb with satisfactory performance, maybe they could post the setting that they have set the carb to so that you could copy and /or compare?

The other information that would be needed to compare like for like would be what exhaust system do you have, blackcaps, peashooters or aftermarket, and what type of air cleaner that you are using.....presumably not the black box as you have a single carb.

Afterbrf1957 wrote
does anyone think boyer ignition or power box could be to blame
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When you strobe the engine, do the timing marks only line up when you get to 4500 to 5000 rpm as this is the engine speed that you require when setting a Boyer. If this is the case it will suggest that the advance curve is probably OK as well. I cannot see how the powerbox could affect the engine power output, and so would discount that.
 
brf1957 wrote;
cant get above 5000 revs
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This suggests that there is something significantly wrong somewhere to me. My Commando running on worn bores, will rev straight up to 7000rpm in first and second gear, and a quite a bit more slowly get there in third, and will pull over 6000 in top.

For me, if you can get some generic carb settings for a single carb Amal Mk1 that work, and confirm that the Boyer is advancing and is timed properly, if compression is OK and plugs, assuming that your exhaust is of a standard type, I would be looking at the camshaft/timing or camshaft wear as I said in my earlier post.

Obviously this is just guess work as I cannot physically see, hear or ride your bike to get a pants in the seat feel as it were, but this is roughly the order I would try and get to the cause of the problem
 
hi brf,i would start at the bottom with the cam timing , not only having the dots and dashes and the 10 chain pins lining up ,but check with a degree wheel on the crank and see what you,ve got,regarding the jetting while ever its showing weak keep going up in size , 850,s are low compresion so need bigger main jets
 
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850,s are low compresion so need bigger main jets[/quote]
what the *&^^E$,careful next you will be replacing the bigend bearings. :|
 
When was the last time the valve clearance was checked?

I also think a compression check should be made.
 
Now maybe I'm way off here but what about chain and sprocket ratio or something amiss in the primary drive if the bike runs fine but won't go over 80 I'm thinking like with a car {rear wheel drive} if you have say 411 gears you can go like scalded cat from 0 to 90-100 and there is no more. lower gear ratio like 322-25 you can do 150 but from light to light Street drag the 411 is faster to get to 100 just a thought. ...Doxford
 
Hello Brian, I've seen and read some scary stuff about camshaft lobes wearing down to almost nothing.
It wouldn't be too difficult to remove the plugs and rocker covers and measure or see how for the valves are opening when you turn the engine over. At least that will rule the cam wear out, or confirm it's demise.
(please keep in mind I'm a novice when it comes to Nortons)
Regards Graeme.
 
Not getting above 5000 rpm in any gear means something is very wrong. A helathy 850 pulls like a freight train to 6200 and with a 20 tooth sprocket will hit the ton in fourth. The basics are correct timing, clean air filter, adequate fuel flow, and free flowing exhaust. Beyond that, you need to check the cam lobes... this is what can happen to them:

underpowered Mk3 update
 
ok thinking about this again and even with chain and gears different you should still reach 7000 rpm then speed would change not the rpm. Ok so if you have a good engine from 1000 rpm to 5000 rpm I can't see the cam being the problem. If worn cam you would have a problem with all rev range. So what about your throttle cable does it lift the slide or slides all the way open this could cause your problem if your not getting full throttle. If the cam is badly worn you would have smoke very poor performance and difficult to start. ... Doxford
 
I have seen a worn cam that ran great. I suppose it could depend on how worn it was. This one had just worn down to be a very mild cam when my friend rebuilt his engine.
Seems like I had one once myself that was in a V8 and couple of cylinders wore causing loss of power and odd idling.
Wouldn't you have low compression if the cam was worn? It would seem that it could not get as much air in to compress.
Unfortunately most of the truck engines we found with worn cams on rebuild I never saw run (or paid attention to them), and since they had hydraulic lifters you only would probably notice less power anyway.
 
Cookie said:
Wouldn't you have low compression if the cam was worn? It would seem that it could not get as much air in to compress.
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As we've seen in the photo on the previous page, the serious wear always seems to occur on the peak of the cam, so I would think the valve timing would be mostly unaffected? The amount of valve lift would be reduced, but I doubt that would affect compression measurement at the comparatively slow cranking speeds those measurements are normally taken at?
 
It would be interesting to do a compression test on a worn one to see how that would work. A leak down test would probably show nothing at all if everything else was good.
 
This is probably going to sound stupid but it happened on my MKIII interstate all of a sudden the bike wouldn't pull past 75mph on a dual carriage way and seemed sluggish through the 30mph limits the rest of the way home.Assuming your not trying to do 80mph outside you front door and you may have to ride a way before you can do 80mph . After you've ridden 5-6 miles get off the bike try pushing it or put it on the centre stand and rotate the wheels if it is really hard work check either of your brakes aren't binding, My rear brake was binding due to the rubber hose de-laminating and acting like a one way valve.
 
Thanks for all replies , i will be checking things out soon , will let you know the outcome Brian
 
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