c16 ms missing under load

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mavs
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Joined: Mon Apr 19, 2021 3:25 pm
Location: Surrey UK

c16 ms missing under load

Post by mavs »

Hi, my 1952 c16ms misses when pushed in high- ish revs or underload ( up hills, etc ). Runs fine at low to mid revs but the more its pushed, the more it misses. New contacts, new needle and new jet set as per manual, new resistor free spark plug and suppressor cap.
Any help would be greatly appreciated.
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Duncan
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Location: HAMPSHIRE UK

Re: c16 ms missing under load

Post by Duncan »

Hi Philip

Welcome to the forum, to set the scene is the bike new to you/has it only just started doing this?

If you hadn't already worked on the carb I would have suggested water in it as one possible cause, it could the fuel side in the form of fuel starvation with a partial fuel tap/line partial blockage or blocked vent in the petrol tank filler cap, choke/air-slide not fully lifting, alternatively it could be the sparking side, a failing mag, dirty pickup or a hairline crack (rev it up in a dark shed/garage to see if it is earthing), chaffed HT lead, slipped timing or weak points spring. You have already started with the logical bits so keep working through the simple stuff and even try another sparkplug as even new ones can fail.
Andy G
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Re: c16 ms missing under load

Post by Andy G »

What he said:
Duncan wrote: Wed Jul 28, 2021 9:15 pm fuel starvation with a partial fuel tap/line partial blockage or blocked vent in the petrol tank filler cap, choke/air-slide not fully lifting
Then
Try closing the air valve when it happens again, if the situation improves, suspect too lean. Lift the needle one notch and try again.

If air lever doesn't affect the misfire, check timing. May be retarded to the point that the spark doesn't have enough time to develop a flame front before the compression drops on the down stroke.
For same reason check the advance/retard mechanism is moving freely.
Bombling contentedly around rural Aberdeenshire
mavs
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Joined: Mon Apr 19, 2021 3:25 pm
Location: Surrey UK

Re: c16 ms missing under load

Post by mavs »

Thanks for the prompt feedback and ideas to try. I'll feedback soon. Cheers
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REW
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Re: c16 ms missing under load

Post by REW »

My '51 did exactly this until I flattened the carb inlet side flange, fitted new, thicker gasket and avoided over tightening, though tight enough to seal the job. This is with a pre mono block so your set up may vary.
Ron

1951 Matchless G3L thumping round the Durham Dales.
mavs
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Re: c16 ms missing under load

Post by mavs »

Thanks for your replies. I have only had the bike a couple of months and it was flooding badly. Eventually traced that to deposit on the base of the conical needle for the float ( pre-concentric amal carb ).That was masking the current problem. Today double checked the output from the mag, outlet from mag, ht lead and cap. All well and no shorting. Checked the petrol, no water and flow from the tap. Checked the vent in the petrol filler cap. The bike fires up easily cold or hot. However as I was bend over the bike lifting the carb slide manually I noticed a leak from the cylinder head gasket inbetween the pushrod tubes.
I know I have to fix this.
Opinions as to whether this is the cause of the current problem of missing underload please.
Groily
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Re: c16 ms missing under load

Post by Groily »

mavs wrote: Thu Jul 29, 2021 1:04 pm Opinions as to whether this is the cause of the current problem of missing underload please.
More than highly likely I'd say! Would be a pretty typical effect when running under load, yes.
mavs
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Re: c16 ms missing under load

Post by mavs »

Cheers Groily. Another success for the forum
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