1973 mercury 7.5 hp carburetor questions

[Fishingnut73]

I have recently received a 1973 mercury 7.5 hp outboard. The serial number on the outboard is 3621659. The carb on it needed to be rebuilt, so I began to look into it. I noticed while I was taking it apart that the venturi inside was broke. I have attempted to locate one and can not seem to find one. Does anybody know where I can find one? And if not is there a way to put a new carb on it? I do not have the model number for the carb though so any information on locating that would be greatly appreciated also. Thank You for the information.

 

[racerone]

Looked or parts or a carburetor on E-Bay.----There may be dealers with parts motors as well.-----Your location is ??

 

[Fishingnut73]

I am in Pentwater MI, I have attempted to contact mercury and they don't carry parts for it. I have checked e-bay but have not found any carbs that are for a 7.5 motor. Do they make an aftermarket carb I could put on it that is a newer model?

 

[Mi duckdown]

Got pics of broken part? Pentwater area should have boat motor repair shops, that you might find parts for it. NO aftermarket carbs. PICS.

 

[emckelvy]

Looking up your motor by serial #, it would appear that your carb used the thicker-diameter venturi, P/N 64247, which is indeed NLA from Merc.

I checked the next serial # run of carbs & found that somewhere in-between Merc went to a different-style, smaller-diameter venturi, P/N 69764, which is still available.

The 69764 will fit into the carb the same as the old NLA venturi. However, it will allow more air to flow thru the engine (but only as much as the engine is capable of flowing, due to port size & other restrictions). So, any idea of it turning into a 9.8 hp probably ain't gonna pan out.

But it probably will at least run OK, and if it does tend to run a bit lean on the top end (sound like it's starving for fuel), there are larger main jets available.

I've attached a cropped carb diagram which shows the difference between the 2 venturis.

BTW there is a newer-style venturi listed on eBay:

http://www.ebay.com/itm/321677827763

Hope that helps...........ed

 

[bigtgc6]

I had the same issue with my 1975 - Serial#4255104. The carb is a 1364-5994 model. The jet that was installed in my carb was a .032. I replaced my damaged float with a new plastic float and set to manual specs then replaced all fuel pump gaskets etc. With the stock jet and the new 21b Venturi I was able to get the engine to idle - somewhat rough and not as low and smooth as I've seen others on YouTube, but it would idle. However in my barrel testing it would cut out about the START throttle setting - and the only way to achieve full RPM was the hold the choke between full and half and it would still ultimately die. The theory of having to jet up the fixed main jet is justified. The next serial number run listed on marine outboards shows only one jet for this carb (or extremely similar) set up with the 21b venturi - .045. So I ordered and replaced my 032 jet with the .045 jet and it will idle and rev up in my barrel testing. I am testing the boat out under full load with a 14' aluminum deep v 1971 Mirro Craft fisher tonight and I will post the results to see if it is functioning properly.

In the meantime - has anyone had to do the same thing and what jets worked best for your set up? The only other Jet I can seem to find that is available would be the .034 and it just doesn't seem to me that is a significant enough jump to overcome the increased airflow the new style venturi is allowing for.

 

[bigtgc6]

Ran the motor tonight and she ran well, however got back to late to do a plug check, will have to wait on that. Seems like it has slightly more power than prior with old venturi and smaller jet. No hesitation in the midrange as before. Water temp felt a tad warmer to me than it had been running, but still not warmer than hot tap water?