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m40c starting and idle issue

M

madjbs

New member
I have a new m40c (still sold new here in Thailand) which has just completed the run in period. Today I could not start the engine and noticed that the choke had been jammed on. After fixing that I managed to start the engine but now I can not run it any less than with a third throttle or it just dies. It is then very difficult to start again and I have to use a combination of choke and a lot of throttle to get it started again. I cleaned the plugs and cleaned the carb but it still is doing the same thing. I could not get the main jet out however as it was in very tight and the brass started stripping. Does anyone know what else I can do to get it running right? I think it has something to do with the pilot jet but I am not sure where it is in the carb. I will be taking it back to the dealer if I can't sort it but I was hoping I would be able to fix it tomorrow as I have come away for the weekend and don't want the trip to be a waste of time.Thanks
 
I had chance to mess around with a engine a bit more today. I cleaned the plugs and carbs again and checked the fuel filter and pump. From cold the engine started easily and ran 100%, after about 45 seconds I could tell that it was starting to die at idle. It gradually gets worse untill after about 2 minutes when you cant run it on anything less than half throttle. Once it dies after it has run for a few minutes it is very hard to start and you have to use half throttle+ to get it to fire, the choke doesn't seem to affect it then.

Any ideas? I thought it was the carb but that doesn't explain why it runs from cold, also as its new it is very clean inside and I didn't find any blockages or gunk. Could it be a faulty ignition coil?
 
Could be a lot of things. Problems that are thermally-related can be tricky to find, and require a lot of patience, as you make a test, a change, and a test run. You could have an ignition problem or a fuel problem. If you get the motor in a condition where you have the problem, then run the ignition component tests with a good analog ohmmeter, and that will help you to rule out the ignition. Likewise, if the carb is way too rich, that would produce results similar to what you describe. Always check the plug condition. Soaking wet is typically an ignition failure. Just black is usually too rich, but if it's horribly rich, and really floods, that will give wet plugs as well.
 
Once I get it started with the throttle half open it seems to let out a load of smoke for about 15 seconds, after that the excess fuel burns off and it runs clean until i let it die again, although that is probably just a side effect of having to start with a lot of throttle and not the root cause of the problem. After the first time I tried to start it when the choke was jammed on there was lots of fuel which had collected in the airbox but again that might just be because the choke had been on for so long. What I am trying to work out is if the choke jamming somehow caused the problem now or was just a coincidence.
 
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I think the over rich theory is a good one and the engine was backfiring when I was trying to start it occasionally. The plugs are not soaking wet but are black and are a little wet when I take them out. It could also maybe be related to the choke jam which would cause a lot of fuel to enter the cylinders at the time. The only thing I can think would cause very rich running now is a faulty float and needle valve? Although it appeared to be ok when I checked it.

EDIT: Although thinking about it more carefully the needle valve definitely stopped the float chamber flooding and the fuel bulb became very firm. The carb really is very simple on the m40c, I find it hard to imagine anything could be wrong with it.
 
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Thanks for your suggestions Paul, I did some more tinkering today with your points in mind and I can say that I know for sure now that it is running far too rich. If I disconnect the fuel line as soon as it starts it runs fine until it starts to run out of fuel, If I then quickly connect the line again, give the bulb a squeeze then disconnect again, it carries on running fine for as long as I keep repeating that process. I am not too sure what this means in terms of what part of the fuel system is playing up.

Thanks again for the pointers
 
What is your float height? When you invert the carb, the float should be parallel to the casting.
 
I just stripped the carb down again to check the float and needle valve (unfortunately before I saw your post), it seemed to stop the fuel coming through fine at a reasonable height (not sure if it was parallel or not). Also I noticed a threaded hole on the outside of the carb directly above where the choke wire/cable connects to the lever on the carb, is this meant to have a screw in it, because mine doesn't?. I started it up with my finger over the hole and voila its working perfectly, furthermore it seems to work whether I have my finger over the hole or not! So either something was up with the needle valve and forcing fuel through it fixed it, or a screw has vibrated loose from the hole, or a combination of the two.
 
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Not sure, but you may be describing the stop screw. That would change the idle speed. Odds are you had (have) a fluky float and needle.
 
No it is definitely not the stop screw. It is cast into the carb and is connected inside to one of the brass jets just inside the air intake I think. Anyway it doesn't seem to make any difference if it was open or closed so the problem was probably due to the needle and float. Thanks for your help, it was greatly appreciated.
 
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