Top speed reduced

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
m knight.
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Location: CORNWALL UK

Re: Top speed reduced

Post by m knight. »

I have SH cams in my scrambler, we may both have the same problem!
m knight.
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Re: Top speed reduced

Post by m knight. »

Evening all. I measured an old "H" cam that I have and it had aprox 0.300' lift, then i measured the clearance from the top of my rocker to the inside of the rocker box with valve both fully closed and fully open and the difference was also 0.300' when I supposedly had a SH cam fitted! I checked the cam timing when I fitted my cams using a degree disc and they were only a couple of degrees out from the charts I had on AMC cams but I never checked their lift. I have come to the conclusion that the SH cams sold by the club and possibly others may have the correct profile but they do not have enough lift to give our bikes the performance boost we were after. I have now brought a G5 rocker box so that I can pinch the shorter rockers from them to increase the lift on my cams to see if it increases the power of my sluggish G80!
SPRIDDLER
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Re: Top speed reduced

Post by SPRIDDLER »

56G80S wrote:It's an SR1 and hence auto advance & retard but I may have it set a tad too retarded, not quite the full 1/2" in a bid to take it a little easy on the old big end. Previously (before the work on the top half) had a problem with it being too advanced with the auto unit sticking fully advanced and consequent healthy kick back! After thorough cleaning out and plenty of WD40 all good on that front (thanks Rob H).
John, even with standard cams it shouldn't be 'sluggish'. You say you have freed off the A/R but are you sure the ign is advancing fully? Does the zorst have a decent crisp note and not a flat (as in flatulent) sounding note?
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Lionel
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Re: Top speed reduced

Post by Lionel »

Given that your carb spacer is the same and it's just the carb that's changed it points to main jet which is an easy to change and experiment with. Not that long ago I used Bsa A65 single carb bike as a ride to work bike. I knew it had done over 100 ,000 miles with a rebuild at 70,000 is but was not sure if sludge trap had been cleaned so fitted smaller main jet to act as a "governer ". Meant top speed was 75 mph or 4000rpm in any gear but I could thrash it without doing any damage ! I did put proper size main jet in just before a rebuild and top speed was back to over 95 mph on the speedo. As a matter of interest the rebuild at over 120000 miles revealed sludge trap had been cleaned !
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Pharisee
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Re: Top speed reduced

Post by Pharisee »

I've found on all my old bikes that with the modern alcohol loaded fuel, increasing the main jet size by 10% brought a noticeable gain in top end performance. Alcohol has a lower calorific value than petrol and you need to feed more of it to the engine.
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56G80S
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Re: Top speed reduced

Post by 56G80S »

Thanks for the useful thoughts. I am over in Scotland at present (no security worries, the 6' 4" robust eldest son at home) but will be getting work done on the Matchless on return.

Yet to check the chains (and will also check freewheel in case brakes binding but no reason they should be).

Yes, will check A/R unit but sounds fine.

At same time will re-check the timing.

Will open out the spacer as that needs doing anyway.

That's the no-cost stuff.

Then take Pharisee's advice and fit a larger main jet

Cam change a long way off but the damaged exhaust SH cam is an AMC one all to do next winter not now.

Cheers.

Johnny B
m knight.
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Re: Top speed reduced

Post by m knight. »

Just looked through the reciepts from the engine rebuild and found that the cams I brought came from AMC spares not the owners club spares scheme! Sorry if I have given people the wrong impression though it would be helpful if both the club and other suppliers stated lift on cams when listed, as at least then you could compare the difference between the ones you have and the ones on offer just by measuring your old ones with a vernier.
m knight.
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Re: Top speed reduced

Post by m knight. »

PS I have now fitted the shorter G5 rocker arm on the inlet side and the bike is considerably quicker. I will if I get time do the same to the exhaust side this afternoon and inform you all if it makes much difference. Apparently the only problem with this mod is on some bikes with aftermarket push rod tubes the push rods can rub the side of the tubes as the shorter rockers move the rod away from the middle of the tube.
m knight.
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Re: Top speed reduced

Post by m knight. »

I have fitted the shorter rocker on the exhaust side and the bike is quicker again! Bike is now producing the power I thought it should, very happy with the results.
AJSingleDave18
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Re: Top speed reduced

Post by AJSingleDave18 »

1960 AJS model 18 (500) in standard engine tune and fitted with a worn and distorted monobloc 15/32” (29.4mm). It pulls OK up to 60 to 65mph (my GPS gives very similar figures) then you have to wait for any increase after that. Remember that road tests in the day gave the 500 single a top speed of 80 ±1 mph. That was with a small rider wearing snug leathers lying flat on the tank. If you are bigger, sitting upright and wearing baggy textiles you shouldn't expect a true top speed much over 73. As you approach top speed the acceleration will progressively slow down.

I am just about to change to a 30mm concentric because they had a reputation for breathing more freely than a monobloc (i.e. gives a little more power) and they are much cheaper.

As you haven't changed the valve or ignition timing, and for the 500 single there are limited or no options in new piston compression ratios it seems the reduction is due to the change in carburettor.

Make sure the size of your monobloc and its jets are to the original specification. A bigger or smaller choke tube (i.e. 11/8” or 13/16” instead of 15/32”) will have a big affect as well as requiring very different jet sizes.

From my experience of jetting carbs even small variations in the engine spec can cause a noticeable change in the optimum jet size. I have added a screw-on conical air filter to my monobloc and this required a decrease in main jet from 310 to 280.

When cruising at full throttle try closing the throttle slightly. If the engine picks up the main jet is too small. Or as SPRIGGLER advises “try closing the choke a bit at full throttle and see if she will go a bit quicker”. If it does then again the main jet is too small.

If slightly closing the throttle just slows the bike try going back to full throttle and see if the engine feels woolly during picking up, or if slightly closing the choke causes the power to drop then possibly the main jet is too big.

When near full throttle try opening and closing the throttle slightly just makes the engine cleanly speed up and cleanly slow down the main jet is near optimum.

Looking at the colour of your plug it looks good in general but checking plug colour for a specific engine condition is not that straightforward. By the time you kill the engine, stop the bike, then remove and check the hot plug it could have been contaminated. A darker plug is due to miss-fire and this may be due to over or under mixture strength.

As a matter of interest the optimum main jet in a concentric will be much smaller than in a monobloc.
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