Engine noise is back!

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Ozmadman
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Engine noise is back!

Post by Ozmadman »

This is starting to bug me now!! After thinking that I had solved it by making sure both stands were padded out and that any loose cables etc were secured and that there was nothing that could be causing this noise externally the dreaded engine noise/knock is back!!! it only comes on during acceleration and when holding a speed around 45mph and above, it isn't there when you run and rev the engine stationary and goes away when the throttle is closed. As most will know I did an engine rebuild a short while ago so I am pretty confident the engine is good but this is starting to get to me now!! and to me, had I not known the engine had been rebuilt, I would have said it sounds like a small end knock?? One suggestion I have had is spark plug knock/pre-ignition...so does anyone think this is a possibility? I have done some research on this and the culprits could be...

1) too lean mixture causing it to run hot (I have the standard settings, the carb has been rebuilt by Martyn Bratby the exhaust is dry and sooty and i am running an air cleaner as well, so unlikely)
2) too hot a spark plug ( I have the standard NGKB7ES) so maybe a cooler plug will help?
3) Timing too far advanced ( timing is spot on and it starts fine with no kickback) maybe retarding it a tad will help?
4) rubbish fuel (I put the best fuel in it)
5)too much carbon build up (there is no carbon build up)

I did read that using a piston with valve cutouts could cause problems with the sharp cut out edges heating up and igniting the fuel before proper ignition? I have a +0.40" JP piston fitted with such cutouts and which states the compression ration is 7.4:1 whereas the standard 350 is only 6.9:1 could that be causing a problem?
Finally, I have a spark plug thread insert fitted which has a flange on the outer edge about the thickness of a squashed spark plug washer so when the plug is screwed fully home it will not protrude into the combustion chamber quite as far as it should could this be an issue.
Maybe it's none of these things but it's making me not to want to ride the bike with this noise going on under the tank, can anyone else post a video of their model 8/G5 on a run so I have a comparison.

Here is a link to the youtube clip, listen carefully beyond the engine noise with headphones or decent speakers and let me know what you think



thanks..
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
JohnHewes
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Re: Engine noise is back!

Post by JohnHewes »

Maybe with a higher compression than standard you do need to retard it a smidgen? But you would recognise pinking, I guess?
(There you are, two questions to answer your questions!)
John
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Ozmadman
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Re: Engine noise is back!

Post by Ozmadman »

Thanks, yes in days gone past I had cars that "pinked" but that was more of a higher pitched rattle and only lasted a few moments until the revs picked up, this is there all the time?
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
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Janet
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Re: Engine noise is back!

Post by Janet »

A similar noise on my LE turned out to be caused by worn big end bushes. The low pitched knock only came on under load, so high revs going nowhere didn't bring it on and neither did gentle riding.
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Ozmadman
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Re: Engine noise is back!

Post by Ozmadman »

Found this statement on a spark plug article

"Thread inserts in aluminum heads – this past year we ran across an interesting situation. A racer had stripped out the threads in one of the plug holes of his aluminum heads, so he fixed his problem with a steel thread insert. The next time he ran the car the plug in this hole melted the tip off. What happened is the heat dissipating ability of the head, and thus the plug, had been changed rendering that heat range much too hot. In this case the steel insert certainly had an effect but just as importantly was the Loctite used to hold it in places as the bonding agent created a great thermal barrier."

This is the web page, interesting stuff about spark plugs, all you need to know

http://vdlfuelsystems.com/?p=273
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
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clanger9
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Re: Engine noise is back!

Post by clanger9 »

That is an odd noise. It sounds too light for a big end, too heavy for pre ignition.

Is the valve timing OK? Does it vary at all with revs or load? Is the exhaust pipe seated properly in the head? Is the clutch fastened on properly? (!) Can you feel it as a vibration or is it just a noise?

Strange.
1989 Moto Morini Dart 350
1993 Ducati 750SS
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Ozmadman
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Re: Engine noise is back!

Post by Ozmadman »

clanger9 wrote:That is an odd noise. It sounds too light for a big end, too heavy for pre ignition.
Is the valve timing OK? Does it vary at all with revs or load? Is the exhaust pipe seated properly in the head? Is the clutch fastened on properly? (!) Can you feel it as a vibration or is it just a noise?
Strange.
1) valve timing is fine, can't really get it wrong when you assemble the engine just a mark on two gears to line up...2) no real variation with load or revs. Once it starts it is there all the time unless you shut off the throttle, it is often an erratic sound and not in time with the engine if that makes any sense....3) the exhaust pipe is secure, no leaks or rattling...4) I fastened the clutch main nut with loctite so I presume it is still tight but I will check it and the primary chain as I have to remove the cover to tighten the clutch springs a tad... 5) just a noise, the bike is surprisingly almost vibration free!! well for a Brit bike that is.... thanks for your suggestions
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
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robcurrie
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Re: Engine noise is back!

Post by robcurrie »

Paul, could it be the carb slide?

Rob C
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Ozmadman
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Re: Engine noise is back!

Post by Ozmadman »

robcurrie wrote:Paul, could it be the carb slide?

Rob C
Hi Rob, not likely as the carb was reamed and an oversize slide fitted last year and there is no play there at all now. The bike starts, runs and ticks over perfectly. thanks for the suggestion though. I have got a few hours spare today so will have another look at the obvious things already mentioned
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
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clanger9
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Re: Engine noise is back!

Post by clanger9 »

"Not in time with the engine" and "no real variation with load or revs" - both significant, I think. That would rule out big end/piston/ignition/carb in my view. If it doesn't vary with road speed then that sort of rules out the wheels & final drive as well.

The only thing I can think of that is out of time with the engine is the primary drive: clutch, gearbox input shaft etc. I'd have a look in the primary drive case to see if anything is catching in there...

Does it go away if you pull the clutch in (while keeping the revs constant)?
1989 Moto Morini Dart 350
1993 Ducati 750SS
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