TOPIC: Safe RPMs 1967 650 4cyl

I'm about to re-prop my 1967 650 4cyl that is on my 62 Shell Lake Tempest. It now has a 48 77340 A4 13P on it and is very slow out of the hole. It seems to run fairly well once on plane but the motor does not sound like it's wound up much.
I now have a tach for it and will GPS for speed. I'm not looking to make a race boat out of it but I would like to get on plane quicker.
I do not mind running this sweet old Merc at max RPMs at WOT once in a while but not for long runs. I just don't want to blow it up!!!
It has T&T as well, Gene

Gene - My Merc Manual says for motor years 64, 65, 66, and 67 - that full throttle range is 4,800 - 5,200 rpm (and in the foot notes it says if your motor is for commercial use, to stay closer to 4,800rpm.

The 1968, and 69 model shows a range of 4,800 - 5300rpm.

Your '67 and the 68, and 69 models show a 62.4cu. in.

The 64, 65, and 66 show 60cu. in. motor size

The book lists 2 models under 1967:
1. 650SS which uses a L19Vspark plug and says (under Distributor or Magneto Type) Kiekhaefer Dist.

2. 650S which uses a L4J spark plug gapped at .30" and Kiekhafer Mag.

Hope this helps. Also - I can't remember where I have seen them, but perhaps those old Mercury performance test sheets they put out for lots of boats might show some performance test numbers with prop sizes. Good luck and enjoy that motor...my Dad had the 1966 models.

Additional Note: Since everyone knows I am not a real Dr., I certainly recommend getting a second opinion from a Real "Doc" - like Doc FrankenMerc or other real "Motor Dr.s". I just read my information out of my Merc Manual, but I would feel more comfortable if others also found the same thing and could back up this information.

Gene - I just don't want to have read something wrong and have it lead to motor problems for you...we have to treat these classic motors with respect so they continue to last

Take care,
Mark (Dr.Go!)

lean toward 5300.

Hi,
I'm not much of a fan of utilizing the superseded propellers like the 48-77340 on a 1963-69 Merc 650 4-cylinder (they are fine on a 1970-71 due to a gear ratio change). There were smaller diameter (thus smaller blade area) propellers specially for them. I would use a vintage 650 propeller, a cut-down blade conversion by a good propeller shop (likely pretty costly to have done), or a small blade area modern aftermarket propeller (very few to choose from). Write to me if you like once you know what RPMs you have with the 48-77340, and possibly I can be of some help.

Regards,
Joe
www.fergusonpoolemarine.com
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If you dont have a set of trim tabs/plates I'd invest if you want to get on plane quickly. After installing them i'd never not have them.

Thanks for all the reply's,
@ Dr.Go! good info.
@ Joe, I'll let you know what I find in RPMs. I have spoke to a prop guy in Manitowish Wi. that has been in business many years and he talks the same as you. Most marinas up north here have a pile of old props so I might just pick up a couple and let him grind and shape for me, besides I can spend some time with him (it's been a while) and play on one of my favorite chains of lakes.
@g3bill Wow I hadn't thought of trim tabs. We have a Lill Champion bass boat we are fixing up for musky league this year and it has trim tabs good idea.
Thanks, Gene
P.S. I grew up drag racing with my dad and 2 brothers so I am not afraid to push the limits in a short race. Can I run 5500 or 6000 RPMs short burst? Man I here some of these old Mercs screaming at at what must be 10 or 11,000.

Since I am offering my opinion, I do not favor trim tab much of the time. Certainly if the outboard (or inboard) size is on the smaller side for the hull in question, then they work well (if placed correctly) without loss of performance. But if the outboard size is relative near or at the maximum for the hull, very great care must be employed to place trim tabs such that they still help take-off, while not noticeably slowing top speed. Fortunately, as the Merc 650 has "jet prop" exhaust, vent or acceleration holes can be drilled in the propeller outer tube to obtain similar take-off enhancement to installing tabs. Note that some performance stainless propellers have these holes from the factory.
Yes, 5500 would be absolutely no issue, and although 6000 would seemingly be less than the best set-up, neither would this be a problem.

Regards,
Joe
www.fergusonpoolemarine.com
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Thanks again Joe

Another possibility is a gear swap for a lower ratio in the lower unit. I have done this with the 650 triples, swapping in a set of 50 hp gears with spectacular results. A later lower unit would provide the gears, but there are differences between the years in the shift shaft and reverse lock hook cams, so you have to do a little homework to see if a whole gearcase would swap onto the leg..