1986 Marathon. No power unless muffler removed

[Justinh722]

I’m fairly good with small engines in general but I’m new to carts. I’ve been working on a 1986 marathon with the robin 244cc engine for an older woman who paid $2700 and was only able to drive it a week before it died on her.

I’ve tuned it up and checked all the basics. There were signs of leaking at both crankshaft seals so those have been replaced. I did replace the carburetor with an aftermarket but it runs the same. Reason for replacement is the Mikuni has loose throttle linkage and looks like someone tried fixing it and ruined it. Cart is getting fuel. Previous owner had bypassed oil injection so I emptied the tank and put some non-ethanol in and mixed it 128-1.

I’ve had this here for about a month now researching to my wits end. Belt drags a little at idle but nothing major. Does make the cart hard to push. Verified the pulsar coil has the correct resistance as well as the primary and secondary windings in the ignition coil. Pulsar is adjusted right at the middle. I marked the slots when removing during seal replacement. It runs okay and builds RPMs in both forward and reverse but only with the muffler removed. But when running in forward it makes a clattering noise. Muffler was barbecued in a hot fire for 3-4 hours. Seems to have more air flow when blowing through it from the tail pipe to the flange. I’ve yet to check the exhaust port on the engine for any kind of carbon blockage. I’m currently testing the cart with the rear up in the air and the drive belt removed while making sure I don’t over-rev it.

I thought I would give this a shot to get some other suggestions while I’m out checking on that. Engine has 125 psi of compression. Air filter is completely plugged so I have one on order and I’m currently just running it without for testing purposes. Spraying carb cleaner in the intake kills the engine so the issue is definitely air flow. Does she need to spend the money on a new muffler or has anyone else not had luck with the barbecue method for the muffler? I’ve yet to run into a small engine repair that has stumped me like this aside from completely disassembling the engine and checking everything. The noise in forward has me concerned a bit. Any guru’s out there familiar with this issue?