Reply To: 7-1/2 Merc75 Idle Problem

[David Stanton]

Ok – so back to the motor problem. Since I typed that long post yesterday, I took another look at the 9.8 carb vs the 7.5 carb and found that in addition to the high speed jet being different size, now upon taking out the brass “nozzle” that goes from the center of the float bowl to the plastic venturi in the carb throat, that nozzle was a lot different! The hole up the middle was same size, but the 7.5 nozzle is much shorter with fewer side holes, so that explains why it ran bad. All other carb parts except the nozzle and the jet appear to be identical. I expected the carb throat to be different diameter somewhere along its length – but no it’s not. Extracting the nozzle from the 7.5 carb was a challenge because it hung up on a small burr where the threaded hole for the high speed jet comes through, and I couldn’t get any tools in there to pull it out. I didn’t want to file the burr with a small round file because I was afraid of damaging the threads for the center bolt that fastens the bowl on. I finally managed to thread a very small sheet metal screw part way into the nozzle and pry on the screw head to pull it out.

I’ll try out the motor tomorrow with the 7.5 nozzle and jet in the 9.8 carb. Still a mystery why the 7.5 motor ran too rich to idle good with it’s original carb, but I don’t care any more – if it runs OK now, I’ll have it out of my workshop! I’ve learned a lot about these dark side carbs! I’ll let you know how it runs.
Dave