Top speed reduced

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
56G80S
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Top speed reduced

Post by 56G80S »

Since all the work on the Matchless - new carb from Club, re-sleeve and standard piston, re-worked head (latter two from T&L) I ran in fairly carefully.

Went out this afternoon and find top speed not what it was.

Pulls lovely but flattens out at an indicated 65ish. I now know that the carb spacer is slightly smaller than the bore in the head and carb but it always was when the old concentric was on (Monobloc now). I still run without an air filter. Now completed a bit short of 800 miles since the rebuild.

I'll check that I don't have the primary chain too tight but confident that's not likely. I am inclined to think that the main jet maybe too small or I need to raise the needle a notch?

Or wait until there are more miles on the clock. When I last looked the plug wasn't quite the straw colour, a tad darker but I had run with the pilot air screw over rich early doors as I was using very small throttle openings. Tuned that up a bit to get the current colour at the end of a run (no, I've not done a plug chop).

At the throttle openings I'm talking about pilot air irrelevant; more about needle height, cutaway and main jet in that order?

Maybe it's a hint to slow down.

Johnny B
SPRIDDLER
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Re: Top speed reduced

Post by SPRIDDLER »

John. Do you realise you've posted this in the 'Buyers Guide'?
Re the main jet - try closing the choke a bit at full throttle and see if she will go a bit quicker.
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raffles
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Re: Top speed reduced

Post by raffles »

Very basic I know But check all chains also for tight spots. Could you have brakes adjusted too tight or could it be valve timing
a tooth out? All things to check after a rebuild.
Good luck
Tony
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Eamonn
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Re: Top speed reduced

Post by Eamonn »

SPRIDDLER wrote:John. Do you realise you've posted this in the 'Buyers Guide'?
Now moved to correct forum area.
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david pollard
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Re: Top speed reduced

Post by david pollard »

sounds like valve timing, what settings did you use? david
Mick D
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Re: Top speed reduced

Post by Mick D »

Hi

Could the ignition be a little too retarded, have you tried advancing it?

Regards Mick
56G80S
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Re: Top speed reduced

Post by 56G80S »

First - Thanks Eamonn, I was in too much of a rush.

Second - Thanks for those responses. Valve timing unchanged only activity was on the top end.

Third - Should've thought of the choke but I never use it and the bolt's done up tight fully off - will try slackening that and actually using the choke.

Fourth - Think I mentioned the primary chain but I think they are all fine but an easy check.

Fifth - 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).

I'll try out the points you've suggested. I also maybe should've mentioned that (apparently not unusually) the JPM gudgeon pin was a tad too large for the little end and I had to ream it it ever so slightly. It's unlikely to be that as I reamed it to be an ever, ever, ever, ever so slightly slack sliding fit. At the time I was planning a full strip down this winter and have a spare little end bush but that didn't happen.

I have adjusted the tappets during the run in and compression is good enough but I'll take another look in case anything has tightened up.

Will post pic of spark plug.
smaller v2.png
Cheers

Johnny B
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1608
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Re: Top speed reduced

Post by 1608 »

Plug looks a good colour.
m knight.
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Re: Top speed reduced

Post by m knight. »

I have just built a G80cs replica using a long stroke road motor alloy trials barrel and std pre 56head and it was very slow! I removed the spacer under the barrel and that helped but still not as quick as I expected. I brought a slightly domed piston from a bloke in California and again a little quicker but not enough, it still struggled to top 50MPH! In desperation a friend welded up the combustion chamber in the head a little and again it improved, up to 60MPH now! Perhaps in re-sleeving and fitting a new piston you have inadvertently reduced your compression ratio?
56G80S
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Re: Top speed reduced

Post by 56G80S »

Michael

Thanks for the suggestion and I bet you'll get there. The JPM piston profile appeared to match the piston that came out. I think it's the change in carb but the old concentric was very worn and don't really want to swap that back on.

When it stops snowing I am going to go through the points raised earlier and also drop the Monobloc off and open out the carb spacer to properly match the carb and head inlet bore.

If all else fails I might take the tip from another thread and change the valve timing to match the 350 timing marks after successfully finding the SH cam and getting it fixed and fitted and go back to the standard inlet cam and SH exhaust which I found worked well previously several years ago. Although that's something I'll hold over until next winter!

Johnny B
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