6 Cylinder Mercury Question

[BJuby]

I am looking for some incite on a Mercury engine I have been struggling with. So I'll start by saying what the engine is and all I know about it with regard to internals etc and how it was when I got it.

- Merc 700 block (1961 serial number), no pads have been welded in
- Opened reed blocks
- Opened intake and exhaust ports: however, looking at the intake ports it does not look like the person who did them was careful to have them uniform across all intake ports as they are different from cylinder to cylinder in some cases.
- 2 out of 3 rings deleted on pistons
- Open exhaust: Pipes are short in comparison to my Bayer stacks on my 55H mod.
- Electrical Ignition Box driven ignition system, i think it's the standard Thunderbolt box
- Engine runs on a Speedmaster lowerunit

Also, the engine originally had KA-2A carbs adapted with alky jets for running the engine on alcohol. I removed them and installed regular KA carbs to run the engine on gas. The engine also has flat gap spark plugs which I am replacing but have not had the chance yet. Engine had 3 fuel pumps installed and i went back to a more standard fuel pump and fuel line setup. I also through Doug Kay (and after 2 years of searching) installed a new refurbished water pump (it was deleted when I received it). Unfortunately the pump would not clear the midsection swivel pin so I had to fabricate and insert 1/2" of aluminum adapter plates between the foot and tower. That problem is thankfully solved.

Initially the engine was not timed properly the first time I ran the engine on my F Sidson hydro. The throttle response was non existent from about 30-40% throttle squeeze. I had a hard time getting up on plane (and was even more worried because of no water pump). I got it up on plane but probably didn't exceed 35 mph. A few months later I retimed the engine. Had it on the garden hose and it was much better. However, (and carefully) I ran it without a prop, then engine still was dead after about 80% throttle squeezing. The engine should have had the potential to scream with no load correct? Or are 6 cylinder engines different from 4 cylinders. Without a prop they should be able to rev to what they would do when cavitating on the water, no? I have not had a chance to run the engine on the boat since that test tank session, i moved on to my stock 55H.

Again, I haven't tried the new standard J6C plugs, i think the flat gaps are definitely too hot. Remember, it appears this is a stock block, so it should run on gas, are the ports altered so badly, is the timing still not right? Looking for some opinions here.

Some additional info. The engine builder suggested this engine "has as much top end or more than a padded six". Also he suggested the "need to use two small batteries wired in series to feed it 24 volts, that fouls the ign. box and lets it rev. over 10,000 rpms". Thoughts on those statements?

Thanks a lot,
Brandon

[Ed Hatch]

Small 6 cyl Mercs had two different crankshafts. The early motor, MK75/78 had a crank based the original prototype tuning, in effect Three 2 cyl cranks. The later Merc 800/850 cranks had a different firing order and were essentially tuned as Two 3 cyl cranks. if you have the later crank in the motor the Siamesed 2 into one elbows probably will not work as intended.

[BJuby]

Thanks Ed, I will keep that in mind

[curbman]

Brandon what did you set the timing to? (at Max advance?)

[BJuby]

It was locked at about 40 degrees. I'm going to pull it back on the timing a tad and richen the carb settings by a significant amount for a start.

[curbman]

bye