QuestionQUESTION: hi, i'm restoring a 1979 gs425 twin cafe racer style. After painstakingly fabricating a seat/rear unit from fibre glass, costumising the tank, de-lugging and removing paint from the frame, restoring all the parts(brakes, speedo and revcounter, controls....etc) all on my own by hand then painting the whole lot with virtually no money but a lot of effort i get to the engine. Now i know the motor was ok when i left it 18months ago so i've done the obvious, change plugs, oil, cleaned carbs and then gave the whole lump a good clean. I'm now gutted to find that although it will still start it just dies straight away and cuts out when the slightist throttle is applied although sometimes it will kick in and rev for a few seconds. the exhaust from the right hand cylinder gets hot as soon as the bike turns over, it seems to have unburnt petrol in it and obviously backfires. The plug also gets well sooted yet the left hand side being the opposite, does't get hot and seems very wet on the plug. when i can get revs the motor seems to start leveling out but this only lasts a few seconds. I now have weakend the mix and it seems to help a little. There is definetly petrol goin to both cylinders because when i remove the carbs the is residue in both inlets i have checked over and over, set ignition timing, both plugs give good sparks and there seems to be good compression. I've stripped the carbs a couple of times but can't find anything. my rectifier is a bit suspect along with the alternator but i don't think that this would be the problem. after all my hard work i'm now kicking myself in the head.
any ideas?
ANSWER: If the motor was in good working order 18 months ago and you are certain that the ignition system is working properly, then the answer most likely lies somewhere within the fuel system. I am assuming that you are using fresh gas in the tank and that the float bowls were drained prior to use, so all my suggestions are based upon that premise.
You cleaned the carbs, but did not detail how this was done. If you did a full disassemble, clean and rebuild then one of the first things to check is that they were reassembled correctly and that all gaskets and jets are in good shape. If you get a lot of fuel sitting in the intake runners when the carbs are removed, it is possible that one of the floats is stuck in the open position and flooding the cylinder, resulting in a wet spark plug and no combustion. A carb synchronisation is required every time you remove them from the motor, as the throttle cable settings are disturbed. It is possible that they are not set up properly and need tweaking (there's more to it than just adjusting the mixture screws). Unbalanced carbs can cause poor performance and power loss.
Another area to check is the air intake path. Is the air filter clean? Are there any obstructions that would limit air flow to the carbs? Are you certain the chokes on the carbs are operating properly? Too little air flow will result in an overly rich mixture and cause symptoms very similar to what you have detailed.
There is a remote possibility that the problem lies within the ignition system, too. After sitting for an extended period of time the counterweights on the points could be seized, not allowing the ignition to advance under throttle like it normally should. Check the points gap and operation, preferably using a timing light if you have one. Sometimes a carb synch and a points setting session is all a bike needs to be happy again.
A long shot, last ditch, Hail Mary attempt at salvation is to check the charging system. If the battery is almost fully discharged, it may not provide enough voltage under load to operate the ignition system. A blown regulator/rectifier unit definitely won't help matters any, but a fully charged battery should at least be able to power the bike for a couple of hours before fading away.
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QUESTION: hi again,thanks for the imput. i've pretty much tried all this though, very fustrated.. managed to get a few revs today and noticed that if i obstrust the right air filter it will suddenly pick up then die. i noticed this because i was thinking the rh rings might be shot or valves are out and with nothing to test compression properly i put my hand across the air intakes to see if they are any differances and the right hand side definetly is pulling in less air but differs from time to time, the left has a very hard pull. i have now managed to get a nice dry plug on the left with hours of tinkering and another carb rebuild. the right still has a sooted plug and overheated exhaust when i can get it to fire, really wet if not. what do you think? i'm lost. valves/pistons? i hope not i wouldn't know were to start. any imput would be cool. thanks!
ANSWER: I don't think the problem is related to worn valve guides or piston rings, as you would have no combustion at all and the spark plug would be wet with oil on that cylinder. However, if the intake valve isn't seating properly it would cause a loss of compression and blow back some of the fuel mixture into the carb intake runner. But if that were the case you would be getting some really wicked backfires through that carb. At any rate, it might be worth a look to pop off the rocker cover and check the valve adjustment and spring condition on the offending cylinder.
With regard to the motor speeding up when you restrict the right side intake, that has me curious. Check the rubber intake runner for leaks where the carb bolts up to the cylinder head, it's possible you have an air leak that is leaning out the mixture.
Try swapping the spark plug wires and the spark plugs themselves to see if the problem migrates with it. I'm not totally convinced the ignition system isn't at fault, but it's hard to diagnose over the internet so I'm just guessing here.
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QUESTION: oh i forgot to mention i have put new contact beakers on, new plugs, checked the leads and tided electrics (better earths, checked/renewed connections)swaped coils and condensors. i have found a very small hole in rh exhaust no bigger the a pencil tip (less than 2m) and it has kicked back through the rh carb a couple of times while trying to turn it over. i also forgot to mention that when i set the ingnition timing realised the ATU left side has no "F", just "L"and "T", so i put it back were it was and the points start to open on the "L". the right hand side has all marks "R" "F" and "T". it is set on the "F" like it was before. strange though? the left seems to be the good side to. i've now removed the rocker cover to check the valves but i find this very confusing and the manual hard to follow. i also noticed the cam chain was loose because the tensioner was pretty stuck. i've took the tensioner off freed it up and replaced it. (would this be a problem?). hope you have some imput. anything will help i've stretched past everything i know now. thanks
AnswerYou're killing me here, mate.
The loose cam chain is probably not an issue unless it has jumped a tooth on the sprocket, in which case it would be pretty much impossible to start. If you have a service manual, it will detail how to check the cam chain timing. Valve adjustments are best performed with the sparkplugs removed and the motor turned over by hand - shine a flashlight into the cylinder hole and watch for the piston to come up in the cylinder as you watch the valve movements. When it stops at the top make sure both valves are closed. If not, rotate the engine until they are when the piston is at the top. This is the TDC (top dead center) position where you use the feeler gauge to check/adjust valve clearance. As for the points settings, again the service manual will detail how to set those.
If you've followed all the above procedures and the bike still refuses to run properly, I'm at a loss as to what to check for next and cannot be of any more help to you.
