Old time merc gurus please help!!

[Doug Redinger]

I have an old 1960 merc 300 (35 h.p.) that I rebuilt a few years back, and it runs really well for a motor almost 50 years old. I was thinking of kicking up the horsepower a little if I could. My question for the old time merc guys is this, will a front 1/2 of a crankcase from a merc 400,450,or 500 of that era fit the back half of a 300 crankcase. The 300 crankcase is 40 cubic inches will the others are 44 cubic inches. I already have the larger carbs on it now from a merc 400( had to modify the mounting flange on the carbs to get the larger carbs to fit.) I believe the reed valves are the same although I am not completely sure. Did the merc 400,450,or 500 come with a tuned exhaust baffle. I know the 300 just came with a flat stainless steel plate for the exhaust. If so will a tuned exhaust baffle from the higher H.P. models still fit the smaller 300 block.
Will a 400,450,or 500 powerhead of that era fit onto the 300 midsection without any problems. I am sure I can use my starter, ignition and so on from the 300 on the larger powerheads.

 

[thefairlaneman]

Re: Old time merc gurus please help!!

You will run into big problems trying to switch stuff like this so I wouldnt try. I have the same engine. These engines run very strong when properly adjusted.Have you cleaned or rebuilt the carbs? replaced plugs and wires and also made sure the magneto advance is set in sink with the throttle. ?Because they were made for leaded gasoline maybe a little lead additive will help the power.

 

[Chinewalker]

Re: Old time merc gurus please help!!

I have swapped crankcase faces on one of my racing engines (Mark 55H built from parts) without issue. Just take care to make sure everything lines up properly and remove the alignment pins from the crankcase halves so that it will find it's natural centering point.

Have you set the dwell on the points as per the manual? I have a scan of a dwell meter if you want to make one up. Send me a PM with your e-mail address if interested.

Some of the 44 cube powerheads will work. I know the 400 will. Only the earliest 500s will. If you're looking for a sleeper, it'd be easier to find a 500 and put your 300 face plate on it...

Lead additive will do nothing - not necessary or helpful in a 2-stroke.

- Scott