Hobbyist Forums banner

1 - 9 of 9 Posts

·
Registered
Joined
·
74 Posts
Discussion Starter #1
I have studied the operation of this carb and understand most of the way it works. I have been repairing a line trimmer that is poor starting and the plug is wet when I pull it and it repeats the same thing when I install it. I do not understand how so much gas can get into the cylinder if it only gets there by way of the tiny high and low speed passages drilled in the carb body. Surely it is getting the plug wet by some other route. I suspect the crankcase gets loaded with gas and it is getting to the top of the cylinder and wetting the plug. It has good compression and I have cleaned the carb thoroughly and the ignition does work when I am able to get it started.
Where is all the gas coming from?
This is a Bolens trimmer with the lousy spring assist starter which I think is a terrible mechanism that requires you to pull the rope 3 or 4 feet to get the thing to turn over more than once! Your thoughts?
 

·
Registered
Joined
·
6,115 Posts
A improperly adjusted fulcrum lever or a stiff metering diaphragm can cause a carburetor to flood. If the fuel pump diaphragm is bad or the fuel lines are not hooked up to the carburetor correctly you can get excess fuel into the crankcase.
 

·
Registered
Joined
·
74 Posts
Discussion Starter #3
I am sure the fuel lines are hooked up correctly and the pump diaphragm does not appear to have any breaks. The metering diaphragm could be the culprit. The lever is level with the ledge as it is supposed to be.
Your idea about fuel lines sending gas to the combustion chamber made me stop and thing though. Maybe the check valve on the primer is leaking and sending gas to the crankcase as well as to the fuel tank! I never thought of that. There just seems to be a lot of gas getting on the spark plug considering it normally just comes in through the very small ports on the high and low speed circuits!
 

·
Registered
Joined
·
1,292 Posts
A improperly adjusted fulcrum lever or a stiff metering diaphragm can cause a carburetor to flood. If the fuel pump diaphragm is bad or the fuel lines are not hooked up to the carburetor correctly you can get excess fuel into the crankcase.
What he said. Also, the primer absolutely CAN'T leak fuel to the crankcase. One direction is back to the fuel tank, and the other is to the metering chamber, in which case as 30yr. said you have a diaphragm or inlet needle issue.

In all seriousness, most of the MTD carbs. are so cheap (OEM), it's not worth writing about here. Buy a new one and move on...
 

·
Registered
Joined
·
74 Posts
Discussion Starter #5
Yes, it makes no sense to spend the time I have spent on this and a new carb is the best solution. I am retired and enjoy learning how things work as a retired mechanical engineer. I have studied this carb a lot and still have trouble actually repairing them. I have bought many of these weed whackers, blowers, chain saws at farm sales and repair them and sell them on craigslist but often get one that I simply cannot figure out! Right now I have 4 weed whackers that will try to run but simply cannot be fixed by me. I get the primer bulbs, filters, and fuel lines from a wholesale firm in Indiana cheap enough for me to fix these issues. I try to re use the gaskets or make them and that is likely a source of my problems! I guess you are correct that I have diaphragm or needle seat issues but I keep working at it!
 

·
Registered
Joined
·
74 Posts
Discussion Starter #6
I guess that since I have a problem getting too much fuel in the crankcase, the problem must be in the needle not seating. The diaphragms are doing the job pumping and opening the needle it would seem. The only way gas can get to much into the crankcase is past the needle unless the metering diaphram is holding the needle open. Correct?
 

·
Registered
Joined
·
616 Posts
You are totally missing how critical the metering diaphragm and the little one way valves are to fueling those engines. The rubber or mylar there must be new and live to meter correctly. After that the metering valve must be pretty much dead on, even a few thousandths error there can kill you. Reusing gaskets is death there. And I firmly believe in reusing gaskets everywhere you can. Absolute no-no there though.

The metering diaphragm meters a certain chunk of fuel load that the needles then meter out in finer amount, too much or too little and the needle settings are for nothing. The two have to match up throughout the power range of the device. It helps to think of the diaphragm as performing the float needle task in a normal carb, it provides a certain reservoir of fuel for the needles to draw from.
 

·
Registered
Joined
·
4 Posts
This looks like the same problem I'm having. Wondering if this problem was ever solved? I have exchanged carbs nd spark plugs with a unit that is running properly and the one unit runs flawlessly and the other simply will not run. This is an older thread so I don't know if anyone is still active on it.. Thought I'd try .. I'm new to the forum.
 
1 - 9 of 9 Posts
Top