Strange

[AGoodfellow]

Hi all,

wondering if someone can help. I recently changed the cylinder head gaskets on my 62' G12. Since then the bike has not run very well. the problem is i now cannot run the engine with the choke off. When i do it miss fires, as if there is a very weak mixture. so in turn i am having to run it with the choke slightly on about 1/4 choke.

before i changed the gaskets the bike ran very well. so I'm puzzled by it.

I have tightened down the head twice now and rechecked the valve clearness they are set to 0.008". checked and replaced the condenser and points set at 0.012" and coil. The carb is standard 389 so a D needle on position no.4 main jet 390, needle Jet .106 and pilot jet 20. I've just cleaned the crab for the 3rd time, and the fuel filter shows no sign of any residual. While running i have sprayed some WD40 around the inlet manifold to check for leaks but none.

Is there something that i might of done while replacing the heads?

Any help is welcome.

thanks Andrew

[JimFitz]

Did you tighten the inlet manifold before tightening the heads? I know you said you checked the manifold joint using WD40 but it is more difficult finding a suction leak than a compression leak.
Can't think what else it could be apart from the carb flange possibly overtightened and distorting causing an air leak and weakened mixture.

Jim

[saltbox alf]

I wonder why you changed the gaskets?

If you are set up as per book and no air leaks as you say, then I would suggest a compression test as first step. Each side should be within 5-10lbs psi of each other and depending on CR , perhaps 115/120 for low compression pistons and 130/150 high compression pistons.
I have read that Trioomph with 10 to 1 gives about 160 psi.

You could of course have some carp in the carb, even if the filter is clear.

NB, Filter is no guarantee of gremlins putting rubbish in a jet

[AGoodfellow]

Many thanks for the replies so far.

The compression is equal across both and they are 115psi. Carbs been stripped and cleaned 3 times. But there is no harm in doing it again.

Re the cylinder head Jim, that was a thought. Could you expand on the inlet manifold and tightening the head. As i may have got it wrong but i'm not sure what is correct, i think i followed it by the manual.


Just changed the plugs encase they were breaking down, no difference.

thanks for the help so far

Andrew

[1608]

As AMC twins have two separate barrels it is important that the inlet manifold is tightened ( just a bit more than finger tight ) before the heads are tightened down. If the barrels are torqued down and then the inlet m'fold is tightened afterwards there is a tendency to be a biased and the mating faces are not square to each other. Symptoms definitly sound like an air leak on the carburation side of things.
ps. why did you change the head gaskets.

[AGoodfellow]

I changed the gaskets as one of the barrels head gasket had blown.

If this is the case and there not square on then what do you recommend i do to resolve it. is it a case of unbolting everything and rebolting it all back up or can i use gasket sealer?

[clive]

As AMC twins have two separate barrels it is important that the inlet manifold is tightened ( just a bit more than finger tight ) before the heads are tightened down. If the barrels are torqued down and then the inlet m'fold is tightened afterwards there is a tendency to be a biased and the mating faces are not square to each other. Symptoms definitly sound like an air leak on the carburation side of things.

The factory manual says " If the nuts retaining the
cylinder heads are finally tightened before the manifold is
fitted, a bias can develop between the manifold to cylinder
head joint, causing a bad air leak. "

I have taken that to mean get the manifold in place but not tightened down

[dave16mct]

The proper way would be to slacken off the head and manifold nuts, then tighten in the correct sequence. If it were me I would be reluctant to disturb the head nuts if they're torqued down right. I'd take the manifold off and use a good bead of silicone gasket. It can't get into oilways in this case and is heat-proof. Just my opinion! Also if you haven't already, use solid copper headgaskets with 'O'rings (they're great!).
Cheers Dave.

[JimFitz]

The way I read it is to very slightly tighten the heads so there is still a little movement on the head gasket and then fully tighten the manifold before fully tightening the heads. If the mating faces between the heads and manifold are slightly off then the ali manifold will distort very slightly when tightening the heads to make up for any slight misalignment. If you do it the other way round then you are unlikely to get enough torque on the manifold to settle in the mating faces.
If it were me, I would slacken the head bolts and manifold and start again. As Dave has said, the quick fix is to use some silicone.

Jim

[AGoodfellow]

thanks for all the advise so far guys. I managed to get back on the bike this afternoon. stripped the carb and inlet manfold off machined the surfaces flat, sealed them with gasket silicone.
Test rode it and its just the same as it was. I still need to run with choke on between a 1/4 and a 1/5 choke. Grr this is doing my head in..

Any other idea please. The neddle is on position 4 and its a D needle. I will try on position 5 and 3 and report back.

thanks again for all the help so far