rearenginekarts.com

Hey there! I am happy to join the Rear Engine Community. Last year towards the end of spring, my dad and I bought a go-kart. It was an old MC90, with a purple jig, sparkily steering wheel, and an MC90 engine. As you might have imagined, it was a pain to find instructions or even parts, for this engine. Since production only lasted about 8 months it is extremely hard to find manuals, parts, or and info on this engine. We fixed the recoil spring after heating the shell up with a blow torch to take the shell off and then rewinding the spring. We mixed the proper gas-oil "ratio" mixture and put it into the tank. We try to start it and after many attempts it wouldn't budge. My dad stopped not wanting to ruin the engine. The gas would not go up the tube into the carburetor.
To my knowledge, it is supposed to create a vacuum effect, pulling the gas up through the tube. Could this mean the carburetor is not working. The only time this worked was when we poured some gasoline directly into the carburetor. I have done a lot of digging online and now I am turning to the community, does anyone have knowledge of this motor and/or have a clue of what might be going on. We are open to any suggestions you may have. If not, we are just going to take the whole thing apart and try and diagnose the problem ourselves. Thank you for taking the time to read this!

Sounds like you have both compression and spark & only fuel when you pour it in the carb. Does it fire a few times when you pour the gas in? Does it have a flat back carb or a BDC model carb? More than likely after sitting for years the carb diaphragms are hard and not pumping fuel. (won't pull up the line) Start there and get back to us. My email is in the contacts and you can send me a picture of the carb will help. Also be careful about running it by pouring gas in the carb it's dangerous and you might have some other issues you are not aware of in an old engine that has been sitting.

You may want to also check the reeds for cracks and seal at the manifold. If the reeds are not sealing you will be able to see light coming thru under them and sometimes you can flip them over and get them to seal however your best bet is replacement if they are originals.

Where do you live TJ ? Sometimes there is someone close by to you that we can hook you up with that can give you actual hands on help.

R.E.A.R.

Rob, it's a flatback to my knowledge. It is the carb that came with the engine. I will email you a picture.

REAR, I will check the reeds, thank you for the advice of going to someone nearby, but I would much rather fix this with my dad.

Thanks to both of you!

TJ

Here are some pictures:

Yep 2 flatbacks. You probably don't know if they have been modified for alcohol correct? I don't know if anyone has any kits but I'm sure someone out there knows if Vince Hughes has them anymore. His contact info can be found in parts & resources.

If not a good bolt on replacement that you can get parts for and probably a lot less hassle would be a Tillotson.

First things first. I do hope you aren't pouring gas that's not mixed with oil into the carbs? I don't know how much you know about 2 cycle engines, but if this has been sitting for a long time, I wouldn't even THINK about starting it without first replacing the crankshaft seals and all gaskets. 2 cycles are extremely sensitive to air leaks. If there is a leak and you start the engine, it will go "lean" on the air/fuel mixture and may well run away at high speed until either something breaks or the piston seizes. Any Mac that has been sitting must be disassembled, completely cleaned and the seals and gaskets replaced. It is also a VERY good idea to have someone like Terry Ives to replace the needle bearings in the piston wrist pin. They gum up over time. If they are not free, you will certainly ruin a good engine. The Mc90 is fairly rare, though in operation it's like any other Mac engine. It was McCulloch's first boost ported "standard" series engine, and can easily be made to run about as well as a Mc91 series engine. While you look for carb rebuild kits, why not follow Rob's advice and switch to a single Tillotson HL carb until you get used to the motor? A single carb manifold can easily be found on eBay or from another member. Good luck, but get some education into Mac peculiarities. Ted

Simply make a block off plate and you can use the manifold that you have with 1 carb.