RPM at tickover and at speed

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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RPM at tickover and at speed

Postby Jdraper » Fri Jul 19, 2019 3:21 pm

I have been playing with a discrete electronic tachometer, which uses a signal wire wrapped a few turns around the HT lead. Setting the scaling for 1 pulse for 2 revs, gives an indicated tickover of around 900-1000rpm. This is quite steady and any lower can be unstable - not good at the lights. No idea if this is correct.

I can't seem to find any specs for tickover rpm or a redline (!) for our machines. The bike feels happy at an indicated 60/65mph, but other than doing the calcs with gearbox ratios and sprockets etc I have no idea how close to max rpm that is. I suspect the bike is undergeared at the moment, with a 17 tooth sprocket, but the spares scheme seems to be out of bigger ones at the moment.

The manual talks of fast and slow tickover, but doesn't say what that is.

Any ideas?

Jeremy
PS this will help with checking that the advance is working correctly

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Re: RPM at tickover and at speed

Postby Rob Harknett » Fri Jul 19, 2019 3:50 pm

Assuming your carb and parts are correct and working OK. AR also OK and fully advanced when warm. When the bike cuts at lights due to slow tick over. When you get to the next set, feel down and adjust the air screw a bit.

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Re: RPM at tickover and at speed

Postby SPRIDDLER » Fri Jul 19, 2019 4:04 pm

Jdraper wrote:......gives an indicated tickover of around 900-1000rpm. This is quite steady and any lower can be unstable - not good at the lights.

I've no figures but 900 - 1000 RPM *sounds* fast to me. Once warmed up you should be able to set the tickover to 'thumper-thumper-thumper......' at Newsreader speed ( :roll: ). If it's unstable at slow tickover speeds my first check would be to ensure that the pilot jet and its tiny drillings in the carb body aren't obstructed. At tickover it's the pilot assembly that's supplying the fuel. As a rough guide the pilot screw should be between one and two turns open. Once the throttle is open say more than half way other jets (main, throttle slide cutaway) are involved so it would run happily at 60 'ish.
You shouldn't need to open the throttle to start the engine.
Also, as Rob has suggested check that the A/R returns to retarded when the throttle's closed.
It could also be due to a weak mag or a dodgy spark plug................ :?
I poke badgers with spoons.

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Re: RPM at tickover and at speed

Postby Rob Harknett » Fri Jul 19, 2019 6:30 pm

I should have said Manual A/R should be fully advanced. Retarded will slow down engine revs. Not sure what you have.

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Re: RPM at tickover and at speed

Postby SPRIDDLER » Fri Jul 19, 2019 6:55 pm

Rob Harknett wrote:Not sure what you have.


I took it to have auto A/R............

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Re: RPM at tickover and at speed

Postby Rob Harknett » Fri Jul 19, 2019 9:18 pm

SPRIDDLER wrote:
Rob Harknett wrote:Not sure what you have.


I took it to have auto A/R............

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If year and model is not stated Neville, I do not try and find out. Quite often the person does not state bikes owned, if they do, the bike in question may not be listed. Could even be a mates bike. I learnt the lesson to state year and model when I first starting riding. No good going in a shop asking for a brake cable for my 350 Matchless. The sharp reply was always " what year and model "

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Re: RPM at tickover and at speed

Postby Jdraper » Sat Jul 20, 2019 7:26 am

Apologies for not giving details. The bike is a 55 18S, with auto advance. I have spent some time getting the bike back to as close as I can to 55 spec, which is difficult for a transition year... Carb is clean and jetted to spec, but the exhaust may be a less than perfect fit at the head, which can't help a clean tickover.

Jeremy

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