Mercury 7.5 HP Carb

[JJB]

I have a mercury1979 7.5 HP serial#5440232. I have two carbs for it a 7.5 that came with the motor and a 9.8 carb I acquired. they both fit and the motor starts with both. The one 9.8 carb does not have the bleed drain tube connection on it as the 7.5 does. can I plug the drain tube line on the block if I wanted to run the 9.8 carb instead of the 7.5 hp carb?

thanks for the input JJB

 

[jimmbo]

Think you are going to turn your 7.5 in to a 9.8? It might work if there are no other Differences the engine, the Block for example. They may have the same Displacement, but they have a different WOT RPM range. So it might be a case of restricting the Induction side with a smaller carb, or it could also be differences regarding the Intake and Exhaust Ports in the block and/or differences in the Piston.
One would need to go through Parts Books/Lists for both your 7.5 and the 9.8 comparing many parts, looking for differences.

You might gain 1 or 2 Mph, but there might also be weaker hole shot with the 9.8 vs. the 7.5

As for the 'Drain Tube' can you post a pic of it?

 

[racerone]

The 7.5 block has a sticker ( COMET ) and the intake ports are smaller on those !---Other parts ( pistons ) are the same.------It is easy to install a nipple for the recirculation tube on the 9.8 carburetor.

 

[JJB]

Thanks for the input , Racerone can you expand on the how to install the nipple on the recirculation tube on the 9.8 carb and where can I get a tube for it . thanks JJB

 

[racerone]

Just drill a hole in the same spot ( or the carburetor flange ) and install / press in a nipple for the hose.

 

[JJB]

Thanks , any suggestions where to get a nipple to press in ? what does that port do? recycle fuel off the lower end ? Thanks for your input . JJB

 

[green4themoney]

The lower hose goes to a check valve in the bottom of the crankcase, it drains the fuel from building up in the bottom bearing cavity, some of the older models ran the line to the intake transfer ports, which is probably why your 9.8 carb doesnt have it..... and racerone is right, the ports on the lightning (9.8) are bigger, its not just the carb..... the 9.8 has a .39 main jet and the 7.5 has a .40 the difference in the carb wouldnt be that big.....