Odd noises

[trem1964]

Good afternoon everybody, I wonder if anyone can help ?
I have a 1958 Matchless G9, not had it that long ( im used to velocettes and bsas) and firstly had a problem with the oil pump, but have replaced that and shes pumping well. Now the bike starts on first or second kick and ticks over nicely, but when opening up the throttle, im getting what can only be described as back firing from the offside barrel between head and barrel. Not much better with the advance retard moved, any thoughts ? Iv checked the tappets by the way and now getting fed up.

[SPRIDDLER]

but when opening up the throttle, im getting what can only be described as back firing from the offside barrel between head and barrel.

Welcome to the Forums, Martin.

Hmm, that's not much to go on.
Head gasket blowing? Any sign of leakage at the head to barrel interface?
Poss air leak at offside carb manifold to head joint...........?
I don't have any rational reason to suggest it but just for the purpose of elimination I'd try with new offside sparkling plug.

Other off-the-wall thoughts....
Exhaust valve not sealing well (lower compression that side)?
Glowing carbon deposit(s) in the cyl head?

[trem1964]

Yes its a tricky one, I whipped the offside head off when checking the oil ways re lack of oil, I have had the exhaust and carb off several times when taking the head off etc.

[Groily]

If it's really 'between head and barrel', probably the head gasket that side? They can seem OK under no load, but as soon as you give her some revs or put her under any load - Wham! Misfires, loud noises and onto one cylinder would be typical . . . a barrel base gasket leak that side would not be untypical either, if the head & barrel through-studs aren't torqued down properly.

The good news is that they're nice to work on and the bits can be had easily, mostly.

[Groily]

Yes its a tricky one, I whipped the offside head off when checking the oil ways re lack of oil, I have had the exhaust and carb off several times when taking the head off etc.

Aah. Always revisit what you did last first! Too much of a coincidence for the issue not to be there somewhere.
There can be some 'funnies' when refitting heads on these, to do with gasket thickness and the spigot on the cylinder and recess in the head. The gasket must be pinched before metal meets metal. Also, the inner edge of the gasket, between the cylinders, MUST be trimmed if necessary to get it to lie flat and square - just as the holes for the pushrods should be made to match perfectly with the hooles in the metal bits, or they'll chafe to 'extra-thin-walled' tubes quite quickly.

[trem1964]

Thats interesting, and im really greatfull for this advice, i will have a look on the spares bit and order one up, it looked fine when i popped it back on, and last night I checked the torque, but all seemed ok and dontt want to over tighten, nobody think its magneto based ? as I said, it does tick over and start fine,

[shaunstaples]

I had the condenser open circuit, when I bought my m20 and the symptoms sound similar, it would start and tick over fine but misfire and backfire as soon as you increased revs.



I fitted an easycap condenser that was easily fitted without fully stripping the magneto, because the old was open circuit and will dig the old faulty one out this winter.

I’m Just staying local this year, getting to know it and assessing what needs to be done.
I’d be interested to know what problem was with the oil pump, as my oil pressure seems a bit suspect to me, I’m hitting the pressure relief valve at 120psi when cold but only to drop to needle barely moving after about 2 or 3 miles. Just seems vast difference. I can see wear in endplate/shaft of pump so that won’t be helping.
Cheers Shaun

[Groily]

it looked fine when i popped it back on

If you're talking the gasket, probably not a good plan to reuse a composite one if it is such (although it has been known!) - and if copper, it would have needed annealing. Composite ones need at least one check-tighten after running the engine to operating temp and allowing to cool right down - I think most people do it twice, once immediately, then after a few hundred miles maybe. Is what I do anyway. Copper, I check once - and they are usually OK I have found.
Not ruling out the mag of course - but you said 'between head and barrel' on one side rather than a general misfire which is what the condenser would cause probably. A dodgy HT pickup or lead could cause a one-side problem though, as could a very shrunken points gap on one side if the mag's a bit tired.

[Duncan]

Additionally to the head gasket reuse question, are you lightly assembling the inlet manifold on the head before/while you are torqueing it down? As the heads are separate air leaks can occur around the manifold/head interface if they do not go on square to each other

[trem1964]

Fitted a new club supplied (super quick delivery) head gasket and hay presto its sorted, although on a ride out tonight theirs a knocking under load, and thinking it may be in the chain case side.