Reply To: 7-1/2 Merc75 Idle Problem

[David Stanton]

Update: More frustration and a couple questions.

I took the carb apart again and found there was no spring on the float needle, so I thought – OK ,someone took it apart and didn’t get it together right. . I went to an older 7-1/2 parts motor, but that had the carb with separate fuel pump. Then I went to a couple year older 9.8 HP parts motor and pulled the carb. It had the spring on the float needle. I thought – OK – I’ll swap that float and float needle and spring to the 7-1/2 carb. Well, the 7-1/2 carb had the float’s hinge pin crimped on the ends so it had to be driven out. I put the carb in a vice, grabbing it by the rectangular fuel pump part. I tapped the pin with a small diameter punch and hammer. It was starting to drive out when the casting the hinge pin was pressed into broke off. (I’ve found since looking at Marine Engine .com parts lists that the 7-1/2 could have a longer needle without the spring or the needle/spring combination, but it only showed the spring version for the 9.8 that I raided – read on). OK – So I needed another carb body or complete carb. I got to comparing the 7-1./2 carb with the 9.8 carb and found they appear to be identical except for the main jet size. Comparing parts lists at marineengine, yes from what I could compare, all was same except the 9.8 had a .041 jet and the 7-1/2 had an .045 jet,. and that is what the carbs had (yes marineengine also verified the 7-1/2 had the bigger jet). The carb throats were same diameter too, so I swapped the main jets and put the 9.8 carb on the 7-1/2. It ran great in the tank, and idles good like never before, but I don’t have a test prop, so I put it on a boat, and it bogs and almost quits when I get above about 1/3 throttle. I pumped the gasline, thinking maybe the fuel pump on the carb I swapped in wasn’t keeping up, but that didn’t help (hey it was something was easy to try!). Before swapping carbs, it ran great at high speeds. I did blow carb cleaner through everything on that 9.8 carb before the swap.

OK – so now Have a a 7-1/2 HP motor with a 9.8 carb with the 7-1/2 main jet, a good idle, but runs terrible at higher speeds, acts like it’s running on one cylinder up to 3/4 throttle and almost dies above that. The spark plugs came out wet with gas, so it’s running rich. I put the surface mount plugs back in and that was slightly worse.

So wondering where I go from here? The carb part they call the main nozzle (part #6 at marineengine, part #28 in the Mercury shop manual) looks the same installed, but I haven’t removed it and compared, and marine engine doesn’t show part numbers so I can’t compare. It’s what I’d call the venturi.

Are these carbs identical except for the high speed fixed jet? If not, is there something else I need to swap to make it run on the 7-1/2? Suggestions? In my hurry to swap, I didn’t measure the float drop and float level, so maybe that’s my problem, and I have that procedure in the shop manual.

A secondary frustration: Now that it idles good, it won’t shut off by the tiller throttle and the kill switch is physically broken. I looked at kill switches from my parts motors, but being a couple years older, I found they are not just a pair of contacts that grounds out the ignition, like the 7-1/2s kill switch does, doing an ohmmeter eval;uation, there seems to be a diode and maybe other parts inside them, so those won’t work. To stop it, I’ve been pulling out the choke, and it starts right up again fine, so I think I’ll just tell the owner to stop it that way unless someone here can tell me that’s a bad thing to do?? Stopping that way works on the big old RD Johnsons!

Unless I hear otherwise, my next step will be to go back into the carb and make sure the float adjustments are correct.

I’ve had it with this motor for today so I’m gonna go getta beer now and calm down!
Thanks for listening to my ramblings here!
Dave