Merc manual wrong on 2bbl Mercarb float drop?

[natemoore]

Happy Thanksgiving, everyone. Just killing some time before I start the turkey on my new Charbroil Big Easy "oil-less" fryer, so I'm piddling on my Mercarb 2bbl for my 3.7L.

I'm checking the float drop before reassembling it. The Merc #8 page 5C-6 shows how to measure the float drop: from the bottom of the air horn gasket to the "toe" of the float, which would be the top dot when oriented upright.

The specs on page 5C-2 specify a float drop of 1-3/4". Here's the problem: the float bowl (chamber..whatever it's called) is only 1-7/8" deep. Therefore, the float will bottom out long before toes falls 1-3/4" below the gasket.

Doesn't make sense to me. Is this a typo?

Incidentally, the remanufactured 5.7L 2bbl I bought to cannibalize the choke/choke plate off of specifies a float drop of 1-3/32", which is in the realm of "reasonable" to me. In the 3.7L manual the "Pump Rod" heading lists 1-3/32." Hmmmm.

I'd appreciate some help.

 

[mslater]

Re: Merc manual wrong on 2bbl Mercarb float drop?

What are you using to measure it? I recently rebuilt a mercarb, the measurements had me scratching my head. Untill I noticed the little L measuring stick that came with the kit didnt start at 0 at the edge but in the bend of the L. Which was causing me to set the float to low, like low enough that it would hit the bottom of the bowl when empty. Once I figured the ruler out it was a snap. Whats weird is there are two dots on the float, why??? Just to confuse people???
I set mine up with the supplied specs on the carb rebuild sheet, your specs are the same as mine. 1 3/32 float drop. Spring needle height is 9/16, solid is 3/8.
After i set it with these heights my flooding issue was fixed. Well at least the carb issue, after unclogging the fuel sight tube I was getting a steady raw fuel flow into the carb. New pump on order. More money into the pit!

 

[natemoore]

Re: Merc manual wrong on 2bbl Mercarb float drop?

What are you using to measure it?

...your specs are the same as mine. 1 3/32 float drop. Spring needle height is 9/16, solid is 3/8.

I've got a small metal ruler that measures in 64th.

My listed spec for float drop is 1-3/4", not 1-3/32", although the latter seems more reasonable.

Are you saying that the sheet that came with your carb kit lists a float drop of 1-3/32"?

 

[mslater]

Re: Merc manual wrong on 2bbl Mercarb float drop?

Yes, the tech sheet has a ton of engines listed, the 3.7 is the same as the 4.3. 1 3/32 drop. Depends on the float needle as to the upper float level. Hope this helps!

 

[natemoore]

Re: Merc manual wrong on 2bbl Mercarb float drop?

Yes, the tech sheet has a ton of engines listed, the 3.7 is the same as the 4.3. 1 3/32 drop. Depends on the float needle as to the upper float level. Hope this helps!

Thanks. That makes more sense. A typo.

 

[Don S]

Re: Merc manual wrong on 2bbl Mercarb float drop?

My guess is the pictures showing how to measure it in the Merc manual is wrong for the dimension they are using.
What you are really looking for with the float drop is to keep the float OFF the floor of the float well it sits in, yet open the needle up to allow full flow around it. Therefore, if you use 1-3/4" dimension on the lowest part of the float, it will be fine for a 1 7/8 well yet still open the needle far enough.

 

[EddiePetty]

Re: Merc manual wrong on 2bbl Mercarb float drop?

....and to continue with Don S's explanation:

Float 'drop' provides for full opening of the needle without allowing the float to bottom-out in the well, causing the float to wear, sink and flood the carb.
I had similar float problems with my Rochester 2 barrel. I found that the replacement spring-loaded needle assembly in my NAPA kit was 1/32" shorter than the original needle. The only recourse I had to obtain proper float settings was to 'shim' the needle seat with an additional gasket. Two seasons later, my 3.7 is still ticking along quite economically.
FWIW....Ed in 'ol Virginny

 

[natemoore]

Re: Merc manual wrong on 2bbl Mercarb float drop?

....and to continue with Don S's explanation:

Float 'drop' provides for full opening of the needle without allowing the float to bottom-out in the well, causing the float to wear, sink and flood the carb.
I had similar float problems with my Rochester 2 barrel. I found that the replacement spring-loaded needle assembly in my NAPA kit was 1/32" shorter than the original needle. The only recourse I had to obtain proper float settings was to 'shim' the needle seat with an additional gasket. Two seasons later, my 3.7 is still ticking along quite economically.
FWIW....Ed in 'ol Virginny

My only question would be (not because I wish to argue, but only increase my knowledge) why not the same spec for the 5.7L? I bought the reman 5.7L carb for the choke assembly, but I also used the float because I found a patched hole in my original one. As far as I could tell, the float, needle valve, and well/float chamber are all identical. Seems like you'd want a longer drop on the engine that really sucks up the fuel, i.e., the 5.7L.

But then again, the fuel entering the chamber is pressurized and the maximum flow rate is probably achieved shortly after the needle is unseated, making the last "three turns of the faucet" useless....maybe?

Teach me o' wise ones.

 

[natemoore]

Re: Merc manual wrong on 2bbl Mercarb float drop?

Also, regarding the two piece needle versus the spring loaded needle, is one more desirable than the other?

Nevermind. I'll just use the original setup.

 

[mslater]

Re: Merc manual wrong on 2bbl Mercarb float drop?

My rebuild kit said to use the spring loaded needle if you are having flooding problems. I dont know what mine had originally as it had been rebuilt before I got it.