G50B Runs bad after 30 minutes. Ignition?

[ARGOJIM]

I am back. It's been awhile since I had the chance to tinker with the atex. So i took it out and about 30 minutes later, it started losing power. After i notice it starting to lose power, it's only a matter of maybe one minute and then it wont move anymore. The power loss happens very quickly. It will idle and it revs up slowly to about 3/4 of the max RPM. Then as I put a load on the motor, it falls on its face. I pulled the plugs and the left plug is dry and the right plug is wet. I am finding about a 60 degreed difference in exhaust manifold temp from side to side. The wet plug cylinder holds the lower temp. I pulled the plugs right after it acts up and both plugs show spark. Both cylinders show 120psi compression while hot. I cant hold the throttle open and pull the rope at the same time so that reading is not wide open. If I let it set for 15 minutes, it will run fine for another 15 to 20 minutes. I am not sure where to start here. Its dropping the right cylinder believe. It has a 50mm Mikuni bowl type carb on it along with some flex pipe from the manifold to the silencer. Can someone please post a pic of the stock pipe and carb? The PO has it set up this way. Thank you in advance

The pipe is correct, is the carb a BN style flat slide or VM round slide style?

[Don]

Don,

Would I still see spark after pulling the plugs on the hot engine if there was an ignition problem? I can see spark on both plugs when I pull them right after the engine looses power. Or maybe the spark gets weak and I am not able to tell? I have looked around trying to find ohm specs for the ignition system and have come up empty handed. Where would I get the main seals if I need them? When it loses power, there isnt a ping, or a back fire. It will idle fine and rev up some, but falls on its face as soon as I move the sticks. I appreciate all that you and everyone is doing to help me out. Internet diagnostics is a pain lol. Also, do you know what the original carb was? There is a 50mm carb on it now and I think an original carb would be better. A pic would help too. I have a diaphram carb that came with this machine, but I dont know if it's the right carb. Thanks again!

I'm not sure if you'd see a spark on the weak side after you pulled the plug out- not sure I ever tried it when one of the G50Bs was acting that way, but it's possible the plug would show fire under no pressure and then fail under compression. You can still get the seals at Manufacturer's supply as far as I know. I don't have a picture of a stock carb on a Superchief, but I'll send a picture of the McKay racer- I think it's a 38mm (stock was 30mm). This engine has been changed over to CDI ignition and in my experience, doing the change eliminates a lot of problems with these engines. If yours has a 50mm Mikuni VM type carburetor, I think you are overfeeding it by a bunch.
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[Superchief500]

I usually ask the odd ball questions when it come to a mysterious issue like this. So here goes, but keep in mind I know nothing about these engines:

1) have you adjusted the carb with the engine hot and acting up? I have a briggs engine that will run fine hot, or cold depending on the carb adjustment, but not both.

2) Have you done a compression test with it hot and acting up? I just wonder if there is something cracked that doesn't show up until a certain temp.

I need all the help I can get lol so ask away. I have checked the compression while hot ...120-125lbs. I have not played with the carb much. There is only a mixture screw that can be adjusted while the engine is running. I can move the needle clip up or down and see if that changes anything in between runs. I think I need to take coil ohm readings cold and hot to see if there is a big difference side to side. I have no idea what the ohm tolerances are supposed to be though.

[Superchief500]

The pipe is correct, is the carb a BN style flat slide or VM round slide style?

It is a round slide Mikuni carb. I think I should try to find an original carb. I dont see any advantage to put a hog of a carb on it and not increase the flow of air on the exhaust side. It doesnt make sense to me anyway.

[Superchief500]

Don,
I totally agree that the carb is too big. I need to find a stock carb. The cdi is something I have to "build" right? I cant just buy a kit. I will check into Manufacturers Supply. I can find it on the web I assume? Thank you for your help!

[Don]

The CDI ignition from any of the G series twins will work on the G50B, and most of the other models are more common. The G29B, G34B, G40B and G44B were all used in snowmobiles and any with the "SS" in the model number was equipped with CDI. If the sled wasn't equipped with electric start, it may not have a ring gear on the flywheel, but the old one can be pressed off your point type flywheel and onto the new CDI flywheel. The liquid cooled Xenoah engine ignitions can also be adapted for use, but most don't have ring gears and those flywheels will not accept one.

[Superchief500]

The CDI ignition from any of the G series twins will work on the G50B, and most of the other models are more common. The G29B, G34B, G40B and G44B were all used in snowmobiles and any with the "SS" in the model number was equipped with CDI. If the sled wasn't equipped with electric start, it may not have a ring gear on the flywheel, but the old one can be pressed off your point type flywheel and onto the new CDI flywheel. The liquid cooled Xenoah engine ignitions can also be adapted for use, but most don't have ring gears and those flywheels will not accept one.

Thank you Don. I am sure finding these parts is not easy. I will keep my eyes open to hopefully locate them. I will need to call on you again, if I find the parts, to wire it up. Thanks again for sharing your wisdom