what is back drag ???

Joined
Jun 7, 2009
Messages
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I have 3 WMK-18 carbs on my 1978(yr), 1150 tower of power, a merc. It smokes realy heavy at idle to lower midrangre rpm .... screams like a raped ape at Wide Open Throttle. I do not know the rpm's because I don't know how to hook it up, plus, I am not sure if the tach does work.

I also have learned a new term, back drag, but do not what that is. Also, I have learned from reading other posts that a motor in the lake is differ than hooking up a water hose, different pressure (load) in the lake than a hose.

Also, I took off the carbs to clean them. However, I am at a stand still because whoever worked on these carbs last few times dinged up the screws(brass or copper, im not sure which) in the direction you turn to take them off. Plus they all leak and I want to replace and fix leaks with the screws.

Also, the top on the fuel bowl has two holes in it. One goes to the other side of the carb, the other is just a opened threaded hole. I plugged that hole with a 10-32 set screw. Is this hole suppose to be opn or plugged ? Attached is a pic and the number on top of it is 1568-3976. What is it and can I get the part.

I have a Seloc manual but I do not like it because pics are black and white and hard to see what arrows are pointing to in the pics.

I am a newbie to all this and want to understand how all these things work.

I would appreciate any info and explaination of things I am asking about.

Thank You. Mr. M.
 

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Chris1956

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Mar 25, 2004
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Re: what is back drag ???

OK, The threaded opening in the carb top is supposed to have the backdrag jet. Since yours is missing, leave the hole open. The dackdrag capability has been disabled.

The backdrag system is supposed to work like this. At mid range or greater, the carbs venturi pull air from the carb bowl (thru small hose). This air is restricted by back drag jet, such that slight vacuum is created in carb bowl. This vacuum causes the fuel-air mix to be leaned out. The carb gives a standard fuel-air mix when accelerating, but leans it out at speed.
 
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