Hello I have a 1955 tiger cub and it has ran well until recently where it now idles okay but when throttle is opened quickly, it coughs and dies. If you carefully open throttle gradually it revs up okay. I have cleaned the carb, used ignition/plug leads and coil from a known good bike and no difference made. I have set the carb at factory setting which worked well before and the plug is very lean and it runs hot. Checked for air leaks (spray carb cleaner on joints while running) none found. I will admit that I have got myself chasing my own tail trying to sort it out. So I'm hoping someone can offer some advice and help me out. Thanks in advance.
Rolling open the throttle only to die can be a symptom of lean running. (As evidenced by your plug) An easy test would be to try closing the air screw an 1/8-1/4 turn and see if the problem goes away....
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Okay checked compression and it was 90 psi. Checked valves at TDC and they were correct. So a put a little oil in spark plug hole and checked compression again, 130 psi. Started bike after, started better and ran better but still sputtered and stopped when throttle opened quickly but revved up better then got worse till it struggled to tick over and was very hot. I only last year rebuilt the engine with a newly rebored barrell 0.20 thousand over with new piston and matching rings, so expected it to be okay or is it having oil starvation issues. I checked the oil return in the oil tank when running and it was pumping oil back into the tank.
Did you do the compression test when the bike was hot??? Results look like a cold compression test, if it was cold then I wouldn't be too concerned about the numbers. But hot you should see the dry and wet test results get much closer together, maybe 110-120 psi.
does the carb gave a choke slide? warm it up, drive up to cruising speed, drop the choke slide and open it up. if it runs better, then part or all of the problem is lean running. if it's worse, then the problem is probably not mixture.
but it really sounds like tbere's a restriction in fuel flow. petcock? blocked tank vent? plugged lines? goo in the tank?
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Okay had a few minutes to work on the bike. Removed and clean main jet, all secured and clear. The fuel flow is good from the tap, no restrictions. The distributor does have some play in the shaft, but not a big amount. The cam is solid and has no play on it. I redid the compression tests. Cold test dry = 90 psi, cold test wet = 100 psi. Hot test dry = 110 psi, hot test wet = 110 psi. So as you said readings much closer. The carb is a AMAL 332 and has no choke slide or any choke mechanism, I will try to gp for a ride to see how it's behaves. Thanks for responses.
Sorry for delay in replying, I had to fix the family car (dpf problems). I have managed a ride and it ran rough and stopped after 1/2 mile. Upon inspection at home, I found the spark plug very sooty. Also found the needle clip has split and the needle has moved upwards out of all notches, so that would seem to be the issue there. I also checked the needle jet for wear and there is a slight bit of play between needle and jet, about 0.010 ish play. What should I expect and how much would be too much. I have ordered a new needle jet and clips. Thanks again.
► Before you can start any carb repair it is mandatory that you thoroughly inspect and repair your ignition system, and set the ignition timing. If you don't and later find out that your timing was off, then you have to go though all the steps on the carb again.
► 90 psi is minimal. Poor. Barely adequate. Not good. Bad boogie.
► Carb needles and needle jets are replaced as a pair. They are both brass, so how can one wear without the other wearing an equal amount ? It is also important that the float level is checked.
► Friend, some inkling of which one of the 41,396 carbs fitted to Cubs since 1955 might be nice to share if you really want detailed help.
The carb is an AMAL 332/3 no choke mechanism. The float level has been checked right height and float is not punctured. The ignition has had a contacts cleaned, new condenser, points and timed. The distributor rebuilt. True about the carb needle and jet, will order new carb needle. Will do a leak down test as yes the compression is pants. Cheers.
The carb is an AMAL 332/3 no choke mechanism. The float level has been checked right height and float is not punctured.
Thanks for that info.
Originally Posted by cragrat73
The ignition has had a contacts cleaned, new condenser, points and timed. The distributor rebuilt.
Those items constitute about 1/3 of your ignition system. How old is your plug wire ? Is it metal core ? Do you have a resister plug or resistor plug cap ? What plug are you running ? How old is it ? Are you running ethanol fuel ?
Originally Posted by cragrat73
Will do a leak down test as yes the compression is pants.
Why don't you start by detailing how the compression test was done.