MTD opposed briggs twin makes popping either makes popping sound and runs rough

[Coca Kohler]

I have a 1985 Briggs opposed twin 14hp model 400707 type 0124 code 85082312. It starts very well and was serviced in february. I have had it almost a year and it had a popping sound since I had it. A few months ago it started running rough and it was backfiring sometimes so I got a carb rebuild kit and done the carb. During the fix I damaged the impulse line (by accident) that went from the engine to the fuel pump so i also replaced that. Ever since it I have a choice between running it at about 2/3rds choke and it will run a bit rough sounding at it won't rev properly, it will surge and not run right. But on full choke it sounds very well except for a small popping sound which sometimes can get quite loud. It revs well but the popping is really bad. So I choose 2/3rds choke because when it backfired it worries me that the popping might do become a backfire. Sometimes it does sound like a backfire but it doesn't flame out the exhaust. I have read up about a few things like this but those mowers that do this won't start but mine does so could I have some help on how I could fix this problem. It's not a OHV so I haven't adjusted the valves or checked them.

Just a thought, the cylinder on the left when sitting on the mower has a bit of wet black liquid which im 99% sure is oil, around the head gasket. Would the popping be caused by a vacuum leak on the cylinder head?

 

[Rivets]

I would start by checking the gaskets around the carb and into manifold. At that age they may be leaking. To do this I take a can of spray carb cleaner and spray the areas where there is a gasket, while the engine is running. If the engine speed changes you have found a leak. If that doesn't help, I would replace the head gaskets.

 

[ILENGINE]

The opposed twin is bad for dropping valve seats, which could cause a popping noise if it is moving around in the recess, or dropping out temporarily.

 

[Coca Kohler]

Thanks for your response. I will try spraying the carb manifold gasket like you said. I don't have any carb cleaner but I have wd40, brake cleaner, maintenance spray and some other stuff which I can't think of. Should i also try the gasket which go from the manifold to the engine because I have never tested them.

 

[Rivets]

Yes, anywhere along the fuel flow.

 

[Coca Kohler]

I have just been out there again too the mower and I sprayed the fluid on the manifold-carb gaskets and there was no change and there was no change between the manifold-engine.

If I was to take the valve covers of the engine to look at the valves and the seats would I need to drain the oil or replace the gasket?

One last thing, I noticed that the engine was vibrating a lot. Could that be caused by a misfire?

 

[Rivets]

Yes, you will have to replace the head gaskets. Maybe this manual will help you.

http://dl.dropboxusercontent.com/u/12502267/B&S Service Manuals/02_271172TwinCylinderLHead.pdf