Ignition problem with ‘73 400

[73-400]

The problem i’m having is that after I installed new end seals the motor started just fine, drove it around the yard put it back into my garage and turned it off. Next time I tried to start it, it wouldn’t even fire with starting fluid, acts like it is flooding with fuel spitting out the front of the carb. I have put a carb kit into it and adjusted the metering rod to account for the aftermarket diaphragm thickness. It has strong spark. If I remove the flywheel and put it back on, it’ll start and run until I turn it off, then same thing, won’t even fire. I have followed the timing setting procedure using a timing light made with alligator clips and a flashlight. I am at a loss. Any ideas and help would be greatly appreciated.

[400brian]

Well I certainly can't explain all that. But if you think it is over fueling, with fuel running out the throat of the carb, I would suggest a few things.
1) the needle and seat could be damaged or worn. The seat is not replaceable.
2)you could have the gasket on the main body upside down, which means the hole for the fuel return is blocked off, causing fuel to flow out the throat.
3)lower the metering lever slightly below the main body and see if that helps.

[73-400]

Thanks, I’ll try going through the carb again to see if that’s it.

[Steven Rempel]

try lowering the meter arm even more. I have found that some carbs and new carb gasket set need to be much lower that you would think.

[Steven Rempel]

what does the return line look like when you do get it running? lots of fuel with no air or like a 50/50 air fuel mix in the return line back to the tank.

If you pull the plugs I assume they are wet. if so pull the fuel line and burn off the plugs to dry them and the extra fuel in the cylinders (spark plug connected to the plug wire and placed in the spark plug hole but NOT screwed in, and try to start the motor a few times, this will burn off the extra fuel from the plugs and the cylinders and what might be extra in the crank case) and see if you can get it running on spray. this might help verify that it is a fuel over delv issue.

[73-400]

The return line has a mix of fuel and air, but mostly fuel. I will take the carb off this weekend and open it up again to lower the metering rod and check to make sure the gaskets are correct. It’s been -20’s for daytime high here this week so I have to wait until this weekend when it supposed to be above zero again. Thanks for the suggestions.

[73-400]

Could anybody let me know if the fuel tank on a ‘73 400 is supposed to be vented, it seems like it is keeping a siphon from the tank to the carb after I turn it off so it just loads the crankcase.

[400brian]

The steel tanks were vented through the gauge. The other thing you have related to the tank is the dual fuel line fitting. One supply, the other return. On the supply side you have a hose that drops down to the bottom of the tank, and on the end of the line you have a screened fitting with a check ball in it that prevents the fuel from draining back to the tank.

[73-400]

If the pickup screen with check valve is broken off in the tank and fuel is draining back into the tank, could that cause a siphon to pull fuel and backfeed through the return line and fill the crank case through the impulse line? How far into the tank should the return line drop, all the way to the bottom? Thanks.

[400brian]

The pick up needs to go to the bottom of the tank. As for fuel going into the case, the most likely way would be a bad diaphragm in the carb.

[73-400]

Thanks again for the info.
I put a new kit from Winderosa into the carb at the start of all of this, maybe when I get the tank, fuel lines, and carb back on and if it still does it, I’ll maybe have to order another kit.

[400brian]

Can you see fuel flowing through the impulse line? I don't mean a hint of fuel residue, but actual flowing liquid?

[73-400]

Yes, I have fuel flowing through the return line. It does mix with some air. How much is actually supposed to flow through the line, all fuel or like a 50/50 mix? I am not familiar with walbro carbs, this is my first one.

[400brian]

On the return line back to the tank, it is normal to see a mix of fuel and vapor. On the impulse line between the carb and the engine case, there should be no flow, perhaps just a little residue.

[73-400]

I think I have now, thanks for everybody’s help. It is much appreciated.