G50B Runs bad after 30 minutes. Ignition?

[Panella]

I was looking for how it might run without valves (per Don), and I came up with this by chance.

"if you have a tach in the machine try disconnecting it i had same trouble a few years ago and that was the problem."

Source: http://www.6x6world.com/forums/showt...3692#post13692

[ARGOJIM]

I'd check the crank seal on the PTO side as well. This engine has a reputation for blowing that side out occasionally. (no valves in the G50B).

Don, I was wondering if you'd chime in, my thoughts as well.
However the quick recoup and immediate return does make one wonder if the fuel cap is not acting up. Have you tried to open the cap when it happens and try to restart? Also 50mm carb seems big for a stock engine. One also might wonder about the fuel line and pump, I recall a few issues of vapor locking if the fuel pump is mounted to close to the engine. Are all the cooling shrouds in place?
Just some thoughts.

[Don]

I'm still guessing it's an ignition problem, just trying to diagnose over the net. It's so common for these to lose spark when they're hot, then recover when they cool off. The point ignition on all the G series engines seems to do that eventually.

[Superchief500]

In fact, if one cylinder is hotter than the other, one side might be leaner. This might suggest a leak at the cylinder head on that side. Check your intake manifold gasket and bolts on that side.

Panella, thank you for the huge list of things to look for! The carb is new, but I agree that 50mm seems too big. I am hoping someone can give me the stock carb info and pic. The rubber motor mounts are new and tight. All of the carb vents are open. The gas cap doesn't make any difference in the engine performance when it starts running bad. I did find a broken stator wire that I need to fix yet. I will check all of the intake bolts, gaskets, ect. I will have to thumb through your posts to see what I have missed lol! Thanks!

[Superchief500]

The temp you mention certainly sounds like it is running lean. as soon as it starts running bad, reach up and unscrew the gas cap. Vent may be plugged. I would also replace all the fuel line, it could be sucking shut somewhere and starving the engine for fuel.

That is a good idea. I should replace the fuel line. I am guessing the line has never been changed. I have tried the gas cap trick with no luck. However the engine doesn't die, it just loses power badly. It will sit there and idle just fine. Just cant go anywhere lol

[Superchief500]

Don, I was wondering if you'd chime in, my thoughts as well.
However the quick recoup and immediate return does make one wonder if the fuel cap is not acting up. Have you tried to open the cap when it happens and try to restart? Also 50mm carb seems big for a stock engine. One also might wonder about the fuel line and pump, I recall a few issues of vapor locking if the fuel pump is mounted to close to the engine. Are all the cooling shrouds in place?
Just some thoughts.

Thanks for the reply! I have tried the gas cap trick with no luck. When the engine acts up, it loses power severely, it will idle fine and rev up some. As soon as I try to move, it bogs down immediately. The fuel pump is mounted right beside the crankcase on the motor mount. Maybe I need to relocate it. I dont know where the pump was mounted originally. I agree that the 50mm carb is too big. I am hoping someone can help me out with information on an original carb and a pic. Thanks!

[Don]

Stock Super chief came with a Mikuni VM30. Lots of us run VM 36 or 38, but I don't remember ever seeing bigger than that.

[Superchief500]

I'm still guessing it's an ignition problem, just trying to diagnose over the net. It's so common for these to lose spark when they're hot, then recover when they cool off. The point ignition on all the G series engines seems to do that eventually.

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!

[dirtdobber]

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.

[ARGOJIM]

Don,

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!

My g44bs both came with Walbro carbs, mine are wd6 but it is possible it has been changed or has a Tillotson on there. Both of those carbs have a external high and low adjustable jet on them.