Brain teezer

[PaulD]

Let's see if anyone can figure this one out because I'm stumped?
The bike is a GT550, it was brought to me as it went pop on its last run out and on inspection it had holed the centre piston! This I think was due to running pod filters and higgspeed exhaust's on standard jetting. When I tried to start it it only ran on the right cylinder, obviously it was never going to run on the center with the "modified" piston weirdly it didn't run on the left one but I just put it down to a blocked carb or something.
I've stripped it down and the guy was lucky as the bores are perfect after a light honing. Ive used Crusin image pistons and rings plus genuine small end bearings just to be sure. I flushed the crankcase out with parrafin to get rid of any bits of melted piston. The bike has now been rebuilt using genuine gaskets and I've replaced the standard airbox and filter just to be certain we don't have a repeat performance. So, on the start up it appeared to run ok at first then within ten seconds or so the left cylinder dropped off? There is good compression although I don't know any figures as my gauge is broken but enough to blow my thumb off while kicking the motor over. Turning the motor over on the starter with the carbs removed gives strong and equal suction on all three cylinders via the inlets. The bike runs newtronic ignition and the timing is spot on at 130 thou btdc. I reverted back to points and condensers with no improvement. I've swapped the coils for a known good set and the carbs have been scrupulously cleaned in my ultrasonic cleaner and blown through with an airline. They have been rebuilt with genuine Mikuni jets and gaskets and the float heights checked. All this with no improvement. While on a test run I tried pulling the choke lever on and all this did was drown the two good cylinders. The bike runs cleanly on the center and right but just gives the occasional bang on the left pot. Removing the plugs shows a nice colour on the right and center plugs but the left is wet through with fuel. There is a good spark with the plug resting on the cylinder head. Oh yes and I've fitted new plugs as well?
The owner has told me that he's always had problems with the left cylinder and the previous guy who tried to sort it didn't get anywhere with it either? Come on guys help me out here please!!!

[Alan H]

Have you checked the tyre pressures?
There may be an anomaly in the space/time continuum, caused by an imbalance in the complex speroidal anti-matter contained within the rotating masses.
Or it may be something else entirely of course.

Hope this helps.

[Warehouse1001]

Don't laugh...but....I had a similar situation once where the plug fired outside the engine but wouldn't fire under compression. Swapped out a new plug and all was well. Good luck. Chuck

[PaulD]

Chuck, I've had that same thing happen on a car with all four new plugs failing under compression! Thanks but I have tried swapping the plugs around and I've even bought a new set with no improvement. Alan, I think the space between your ears is getting larger

[Jimroid]

You have checked many things in good order. You can' t swap carbs because I believe these are "rack type" . Maybe focus on fuel flow and that all fuel and air passeges inside the carb are good, starting with left cylinder. Double check float/needle/seat for proper function.

[PaulD]

A good (if slightly barking) friend is going to lend me a set of known good carbs to try, thanks Alan if that doesn't cure it I'll set fire to it!

[Alan H]

I'd like to point out here that Paul is a truck driver.
An old truck driver - and that's not someone who drives old trucks, see.
Nothing wrong with that I suppose ( ), but I thought I should tell.

[Clifford]

Try switching plug caps?

[Vintageman]

{quote="PaulD"]but just gives the occasional bang on the left pot.[/quote].

There is the clue Watson. If you can run it on two cyls and every few seconds you here loud BANG or KA-POW out exhaust for example and you believe carb is good (no leaky float causing heavy flooding) it's ignition.. its stops firing and then it does and goes Bang, Boom, Ka Pow!. Now were in the ignition is the issue , cap, coil, loose wire, high arcing on that set of points? Somehow timing way off... not sure how that latter would happen?. But you said people have modifies ignition so I would check wiring or coil and cap as stated,

[PaulD]

Thanks for the replies (apart from Alan's) yes I also thought it was ignition problems so I swapped the coil pack for the one on my Kettle which I know is good? The bike runs electronic ignition (newtronic) which I also suspected so I removed that and reverted back to points and condensers with no difference at all!
Keep the suggestions coming guys (no not you Alan )

[yeadon_m]

Paul,
Its fiddly but you could try swapping left and centre plug leads and at the same time also swapping L and C points wires.

That would prove the points, coil, plug lead/cap and wiring on the left (if it runs the centre) despite your substitution, as clearly something isn't working as it should. If the problem swaps to centre, its ignition. If it doesn't, it isn't (assuming the plug is good). I think...

Good luck!
Mike

[jabcb]

Check the carb fuel level. We don’t know what the spec is, but it should be the same for the 3 carbs.
I’ve started doing that & found them the same but I wasn’t having your problems.
My topic on it is here: viewtopic.php?f=2&t=12589

Also consider draining the left crankcase & the left exhaust.

[akendall1966]

As JABCB above, a bad float valve on that carb / cyl? Had an issue on my T500 where the RH cyl dropped out after a while on idle it was down to the float valve or float not working, so the carb eventually flooded, and started to fill the crankcase with raw fuel. Once it had time to dry out would run on both for a bit again.

Have you pulled the crankcase drain plug (assume it got one) to see if its wet in the crankcase?

[Alan H]

Remember that the bike actually starts on all 3 from cold, which suggests that the left carb is supplying enough(? or too much) fuel to start, but definitely too much when warm. I've gone through the suggestion that Mike mentioned and if things are swapped about it's always the same issue on the same pot. Paul has swapped jets from left to right and vice versa, but still the same pot causes the issue, so I'll get a spare set of carbs to him hopefully this weekend and see if that makes any difference. if it does, then there's either something missing from the left carb body or it's just a dud casting somehow.
I suggested setting the float level less, but it's still the same symptoms. Bloody weird.

Must admit I'm running out of ideas if it isn't that carb casting.

Of course my earlier suggestion was just a leg-pull, but as Paul is 'only' an old truck driver, he probably didn't understand the technical parts.

I have offered to take the bike away and I won't charge much for the work involved.

[jabcb]

Also make sure the choke is adjusted properly. It might be setup so that the center & right cylinder chokes close but the left one stays open a bit.