Purpose of Nipples on Top of 6 HP p/n 3R4-03200-0 Carb?

I am attempting to increase the HP on my 1999 MFS5A from 5Hp to 6HP. The replacement carb from Amazon, 3R4-03200-0 has 2 additional small nipples on the top, not present on the stock OEM carb. Does anyone know what to do with these two nipples. Do I, 1) leave them open, 2) connect them to something, or 3 block them off?

I can return the carb if this is not the right one and order the right one.

Thanks

pvanv said:

Part 3R4-03200-0 was superceded to 3R4032001M
Those are vents. On the newer motors, they have 1/8" (3mm) fuel lines that go out through the lower cowl. It will work fine as is.

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Thanks Paul

Thanks Paul

The carb swap discussed above went well and this originally 5 HP motor has now been converted to a 6HP Tohatsu MFS 6A2S. We took our 20 foot Grumann cargo canoe, loaded fairly heavy with two guys (equalling 500 lbs total — fairly heavy boat for this small motor) out on a local lake for speed trials. We found that the top speed is now increased from 8 to 10 mph, using the stock 7.9” inch diameter X 7.9” pitch prop.

At full throttle with this prop, the RPMs are approx. 4750 rpm. Since the spec’d max rpms for the 6 HP motor are 5000 to 6000 rpm, I am thinking that this prop is too coarse a pitch, and that I should try the 7.9” X 7.0 inch pitch prop, to get the rpm’s up into the 5000 to 6000rpm range, and so tweak the top speed up maybe a mile or two. But I do not have access to technical data for this motor, or relevant experience really know

The reason this is critical is that our fishing and hunting occurring here in Alaska is on the fairly fast flowing (about 5mph) Tanana River. So, when going up-river, the 10 mph speed that we have now achieved is half used up by the River’s 5mph speed - meaning that our actual speed up the River is only about 5mph! So, you can see that a 1 or 2 mph increase cuts our transit time and round trip fuel burn greatly.

A couple of factors to consider are 1) We can’t just use a larger motor on this boat because of canoe weight limitations, and 2) this is a strong motor and we are running synthetic oil for extra protection.

Can anyone comment, either from experience and/or access to technical data (for instance a power curve)?

Thank you.

Don't bother with synthetic oil; regular mineral-based 10w-30 is fine, especially in cold weather. The current setup is lugging a bit. You want to get as close to 6000 RPM as possible. I would try the mouse ear prop:

Thanks again,Paul. The finer pitch saildrive is on order. If I get even 2mph increase in speed, that would be huge. Will report.

NorthPilot said:

Thanks again,Paul. The finer pitch saildrive is on order. If I get even 2mph increase in speed, that would be huge. Will report.

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I received the finer pitch OEM prop that Tohatsu specifies for the 6HP saildrive engine, fitted it and field tested it. It bumped the rpm’s up from 4750 to 5200 and increased the WOT speed from 10mph to 11 1/2mph.

This has increased our upriver speed on the Tanana River to 7-8 mph, permitting practical access to our favorite Pike fishing slough.

Thanks again Paul for spot-on advice.

My pleasure.