Help with 1973 7.5 HP Carburetor problems?

[Cornell]

My '73 Merc was built as a 10 Hp but Merc needed to compete in the 7 HP models so they took the 10 HP and de-horse powered it to fit into that market. The report said that the carb and reed valve are the only differences. This is per Consumer Reports Annual report recommending this motor as being the most powerful in it's class with about 8.5 HP. Specks confirm it as it is the same weight, looks etc. I bought it new. Now it doesn't start cold unless I prime the carb. Once it starts it runs pretty good. I took the carb apart and took the jets out and cleaned them along with all passages with carb cleaner and compressed air. I didn't take apart the diaphram section as I haven't been able to get a gasket kit yet. But it all went together clean with same gaskets. But still the same problem.
My question: Should I try to get a 10 HP carb. and will it help increase my performance if I don't change the reed valves? I'm not sure I know what the reed valves are as they have got to be in the intake manifold??
Or has anyone had this problem before and know a quick fix for my original carb.? thecruiseguy@comcast.net

 

[achris]

Re: Help with 1973 7.5 HP Carburetor problems?

Now it doesn't start cold unless I prime the carb. Once it starts it runs pretty good.

That sounds quite normal to me.

My question: Should I try to get a 10 HP carb. and will it help increase my performance if I don't change the reed valves? I'm not sure I know what the reed valves are as they have got to be in the intake manifold??
Or has anyone had this problem before and know a quick fix for my original carb.?

It's usually cheaper (and a lot less headaches) to sell an engine and buy the power you want than try and alter an existing engine. To change reeds means removing the powerhead and stripping the crankcase cover off it to get at the reedblock. It's common for manufacturers to use the same basic block (pistons, crankshaft, rods etc) and use different configurations of timing and carbs to create a range of horsepower options. for example, Merc used the same block for all the in-line sixes, from 90hp to 150hp and the small engines are the same. The only difference between the 20 and the 25 (2 stroke) is the carb. As it is with the 9.9/15 and the 30/40 and the 75/90 and the 100/115/125. Johnrude did it with the crossflow V4s, 85/90/100/110/115/135/140.

Chris..........

 

[Laddies]

Re: Help with 1973 7.5 HP Carburetor problems?

According to the parts manual both engines use the same reeds and KB carbs meaning the venturis are the same size with different jetting, the reed stops are set the same height on both the 7,5 and 9.8 engines so I don't see where they got the story thats how Mercury dropped the HP to 7.5 we use to install a kit from merc and all it was was a plate that when between the carb and block with a small venturi hole in it and new decals most people installed the decals and thru the plate away

 

[Cornell]

Re: Help with 1973 7.5 HP Carburetor problems?

Thanks Laddies. My carb isn't a KB it is BB2A then a space followed by 5. The parts store would have to order the gasket kit for me showing only BB2A. But I have been waiting to figure out if I should order the float and needle which are both separate as they don't come with gaskets. My float seems to be OK and the needle looks good as well.
RE: "kit from merc and all it was was a plate that when between the carb and block with a small venturi hole"
Would this kit still be available and would it help improve performance of my 7.5 HP? Sounds like the spacer is some type of "high rise manifold" feature.?? Does the venturi hole suck air?
Could you also shed any light on my problem with carb not starting unless I spray fuel down carb?? Do you think the entry area where there seems to be diaphrams are the problem? I didn't take them apart but sprayed cleaner and air in there twice.

 

[Laddies]

Re: Help with 1973 7.5 HP Carburetor problems?

The kit was to reduce a 9.8 to 7.5HP, check on the side of the engine block if it is cast with the word COMET it's a 7.5 and I think that the 9.8 was marked Rocket but I am niot sure and don't have one in the shop to check