600 Twin Problems

[kbs1240462]

My 600 twin has been off the road with the previous owner for about 10 years. So as you can imagine I am at present chasing oil leaks – but that is not the problem I could do with some advice on. Last Saturday I was taking a run over to Hereford when the bike started spitting flame back out of the carb – this cleared – it started again and then it stopped dead – what felt like a seize,the bottom end was very hot. After a while the bike started again – fist kick as usual and ticks over a treat. Question is what next? My first thoughts are a 2nd oil change and run some flushing oil – as maybe I have dislodged some “muck” and partially blocked an oil way after it have been "asleep" for a while? I have been out for 3 hour + runs before and never seen this problem in about 2k miles and 9 months – so a bit strange. If that does not sort it it sounds like an engine strip down – which whilst that will be fun it does mean the bike is going nowhere for a while.

So I thought if anyone with knowledge of these engines could offer some advice that would be very useful.

[saltbox alf]

DON'T use flushing oil that will cause all sorts of problems!! These engines are designed to have sludge traps and you don't want the muck charging round the engine!!

You might have a sticky inlet valve or some sort of ignition fault, do you have magneto or coil ignition?

[kbs1240462]

Hi,

Magneto ignition, that the previous owner had rebuilt about 2-3 years ago from memory (without digging all the papers out). It has a big healthy spark and never has a problem starting hot or cold, usually first kick either way. It pulls very well and this is the first real issue I have had with the bike. Oil return to the tank looks OK when viewed with the cap off but I also noticed it had blown oil out the top of the oil tank past the cap on the mentioned ride.

Kevin

[saltbox alf]

mmmm? valve stuck, compression up the valve guide and spit back, might be enough to have presurized the crankcase and blown out some oil.
Or a partial seizure, but it's running fine now.
Realisticly the only answer is a strip down of the top end to see if there was a problem.
Or run it and see if it gets too hot again.
Were you giving it some at all?
Is it a manual advance and was it accidently retarded? Get a bit warm then!!

[kbs1240462]

Alan,

Thanks for the reply,

Yes I was giving it some so maybe that could point me in the right direction? Manual Ignition - but it was advanced - I have made that mistake a few times early on with the bike. This weekend looks like a filter clean and oil change, check the drain plug for "bits" and take if for another (gentle) run. I will also check compression etc, to see if I can find any other "pointers".

Kevin

[Julian_S]

Kbs, I'm new to the marque, but in general terms spitting back through the carb tends to be ignition related. (plugs, leads, condenser, points etc etc) A sticky valve is a possibility but I'd go for the 'low fruit' first and check out the ignition. I tend to think this 'hot bottom end' of yours is probably a red herring and it's not really related to spitting back from the carb so using probabilities as a yardstick I'd not worry about that. (for now )

Julian.

[saltbox alf]

naugty !!! but you got to have some fun! Compression test yep that will tell, hope its not a ring gone! A pal nipped up his other AMC bike (no%&on) it still run ok but he had JP pistons which expand more than OE and he broke a ring and scuffed the pistons ( we were raggin it too) end result was new pistons. However if yours are just lightly scuffed, new rings would get you out of trouble.
See what you find. might be your lucky. could even have been tracking across the slip ring in the mag, worth an inspection/clean.

[kbs1240462]

Hi,

The story continues.

57 Model 30.

Cleaned the filter - a reasonable amount of muck - next to no steel (magnet check and sump plug also checked) but some ally and also - joy of joy some silicon gasket goo!

Oil distributer checked and holes clear but some scoring on the barrel of the distributer. Oil flows trough pipe to pump clear and return from pump clear. Oil seen to return to tank when kicking engine over with plugs out to check circulation

Upon inspection after a short run of the engine oil in filter and distributor cavities.

Bike starts first kick but oil flow back to tank is lower than it was and somewhat intermittent.

Before I pull the engine apart does anyone else have any thoughts??

Thanks in anticipation,

Kevin

[1608]

I personally wouldn't worry about intermitent oil return, as has been said many times before ''they all do that sir''. Oil return flow is usually strong to start with and then gurgles and slutters when warmed up or after running a short while.
If the carb spitting is intermitent or worse on warmed up engine it sounds a bit like sticking inlet valve, or poorly seating inlet valve. The inlet valve stems can become carbon fouled .

[kbs1240462]

Hi,

Finally found some time and have had the top end off the bike.

Not good!!

Several of the head bolts were not very tight and I also have found a blown RHS head gasket - possibly from the head bolts not being adequately tight.

LHS cylinder is fine- no marking on the piston or bore - not too much coke on the piston and most easily removed with petrol and rags.

RHS - different story - the crown of the piston is badly burnt and pitted to a depth of about 5mm - the missing aluminium is spattered around the crown and over the valves (easily scraped off) and the piston has "picked up" at several points on the o/d but no badly scored anywhere - so it looks like the metal off crown of the piston has smeared down the sides of the piston has "smeared" down the side of the piston. No sign of having "blown" past the rings - i.e. piston skirt not showing scorching/burning from ring failure. All rings in good clean condition. The good news is the cylinder bore looks fine.

Looking at the inlet manifold it looks like I may have had a leak on the RHS of the carburettor manifold to the RHS head joint. My hypothesis being I have developed an air leak hence the RHS has run weak and hence burnt the piston crown.

Do others agree?

Bores are 40 thou over - can I get a single piston or can I only get a pair, new rings are also I good idea I guess!!

I am just about to strip the heads and re-furbish those but need a different valve compressor as neither of mine can get in!!

Kevin