Clinton boat motor

sparkwizard

sparkwizard

#1
#1
I saw this 1973 Clinton 7 horse outboard powerhead on eBay about a month ago. I tried to ignore it, hoping it would go away. It has some issues trying to run it on a mini bike. Most are water cooled. This engine is air cooled. The problem is the boats use a "wet leg" as they pump water up below the crankcase to cool the exhaust housing, or the "leg" between the engine and the lower unit, where the gears and prop are. That water gushes out near the exhaust ports and falls back down inside the leg' along with the hot exhaust and blows out in the water behind the boat. On most boat cylinder blocks, the exhaust ports are open to the inside of that leg. On this one, there is a divider. IMG_20250920_124704.jpg
I can cover that port with a header from another brand of race kart engine from the sixties. I do not know what engines these adapters fit, but they do cover and seal my port. IMG_20250920_124731.jpg IMG_20250920_124719.jpg
 
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sparkwizard

sparkwizard

#2
#2
Next issue was the short crank. No clutch will fit the short crank in this thing. This is a one-way ratcheting thing that I do not understand the purpose of. It has a key in it, but it is aluminum, so no, thank you!
IMG_20250920_125112.jpg One small set screw allows me to remove it and expose the actual PTO on the crank.
It has a wood ruff key and a small flat for a set screw.
IMG_20250920_124827.jpg
I can certainly install a sprocket on that shaft. Note the nice threaded four bolt holes, perfect for bolting to a simple engine mount. Remember, 2 stroke engines do not hold oil, so they can run in any orientation. The PTO does spin ccw.
 
sparkwizard

sparkwizard

#4
#4
Most of my vintage chainsaw Tillotson carbs came from 70-100 cc engines. I THINK this one is about 145 cc. I will need to find a bigger carb. Maybe I can start looking at snow monster carbs. The reed plate on this looks like I could swap on the plate from a Clinton D35 chainsaw to get the throttle shaft horizontal after tilting the engine 90 degrees, but this reed plate has a pyramid shaped four petal reed cage in it. I plan to stick with this better performing plate and beauty be damned!
IMG_20250920_124956.jpg
That hose is pulse for the pump in the carb. Could be handy for a remote pump.
 
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sparkwizard

sparkwizard

#5
#5
Outboard boat motors have adjustable ignition timing. At idle, the timing is at the retarded position. As the throttle is opened, the throttle linkage turns the entire magneto plate inside the flywheel. It moves the points, coil, condenser and all. I have not measured it, but it looks like it moves about 30 degrees. This might be fun to mess around with! IMG_20250920_125014.jpg IMG_20250920_125017.jpg
 
Thepaetsguy

Thepaetsguy

Well-Known Member
#6
#6
sparkwizard said:
Outboard boat motors have adjustable ignition timing. At idle, the timing is at the retarded position. As the throttle is opened, the throttle linkage turns the entire magneto plate inside the flywheel. It moves the points, coil, condenser and all. I have not measured it, but it looks like it moves about 30 degrees. This might be fun to mess around with! View attachment 324634 View attachment 324635
Click to expand...
It moves the whole coil plate? o_O I’m always blown away by engineering. There is mechanics to everything! Cool little engine!
 
sparkwizard

sparkwizard

#7
#7
Yes, the whole plate moves with the throttle; or it did originally. I read an interview with a gentleman about a 7.5 hp Clinton boat motor on a kart, and I remember him stating he locked the timing at full advance. I want to play around with it, though.
@scrambler1
The big Clinton
 
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sparkwizard

sparkwizard

#9
#9
I believe I will be sticking the original reed plate. This is an eBay picture of a stock one.https://www.ebay.com/itm/145842934511?_skw=Clinton+K700&itmmeta=01K5P9XCX9WB05GCVTDVAPX567&hash=item21f4ea92ef:g:KcMAAOSwN-Bix~i3&itmprp=enc:AQAKAAAA0FkggFvd1GGDu0w3yXCmi1f/xG1l1obxxMU7dz4erPB5CZN2+Mkqu3Y/sV819JSFIIQ1Epv28b7Ke3bpzGr7DgikeyncjOhKOpa19+KrX1KdwR2PlrRWmesxNTBl1cuKlrPewHU2a1XNAAte3InrQS5tPEk3apmRvJ0xJweVvyjQk1dnyGbihheFOSarSiT+9H5FTdHZlaFyVBOmxyhiShMJ+swMi3MUBF5uequsXHE0+afD9qgdrKiNA8mw2lIVGJ+op9RIwKNOAoebKwVj1ZI=|tkp:Bk9SR-bO9cmtZg
Note all of the reed petals around the circle. I stated earlier that it has a "pyramid reed cage". I was wrong, but I will show a pyramid cage compared to a flat plate as soon as my NOS Homelite saw reeds arrive.;)
In a 2 cycle engine, the entire crankcase is essentially the intake manifold, and the reeds are kind of like the intake valve. They are a check valve, with the carb on one side with atmospheric pressure, and the crankcase on the other side. When the piston goes up, pressure in the case drops, the reeds allow some fuel and air into the case. That mixture contains the oil to lube the crank, rod, piston and rings. When the piston moves down, pressure in the case rises, closing the reeds, trapping the mixture. The mixture above the piston is burning, forcing the piston down. Roughly halfway down, the piston uncovers the exhaust port in the cylinder wall, allowing the high pressure exhaust out to the muffler. After opening the exhaust port, the piston continues down, compressing the case a little more, then opens the intake ports in the cylinder wall. The intake charge rushes into the cylinder, pushing the exhaust out. Hopefully, the piston rises up in time to close both ports before the fresh fuel/air charge escapes out the exhaust port. Piston near TDC, plug fires, and the cycle continues. Since there is no camshaft or valves, you can see why more reed petal surface is important to filling the case. Ill get a good picture of a small reed plate from a "universal engine": a governed Clinton A400 Panther for comparison.
 
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sparkwizard

sparkwizard

#10
#10
Stock Panther reed plate. Note the two tiny reed petals under the "stops" . IMG_20250921_092320.jpg
The curved steel plates are there to keep the reeds from flexing too much and breaking off. Remember, the reeds are inside the crankcase with the spinning parts. Reed failures can be catastrophic.
 
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Minimichael

Minimichael

Well-Known Member
#14
#14
This explanation helps a lot. Thank you again. So it seems reed design would be a sweet spot for performance tweaking?

Swapping out that 3hp for the 7hp on your terra two-piece bike is going to be interesting. Will you have to modify your custom jackshaft train?
 
sparkwizard

sparkwizard

#15
#15
Minimichael said:
This explanation helps a lot. Thank you again. So it seems reed design would be a sweet spot for performance tweaking?

Swapping out that 3hp for the 7hp on your terra two-piece bike is going to be interesting. Will you have to modify your custom jackshaft train?
Click to expand...
No modification necessary. The engines have a sprocket, and a chain runs to the TC driver shaft. After that, nothing needs to be changed.
I really need to find the time to finish that little bike.
 
sparkwizard

sparkwizard

#17
#17
I think the black 7 horse will probably be staying on that frame. It should have pretty good torque and decent RPM, so with the torque converter, it should be fun. I plan to start making a mount for it today. I might angle the cylinder up a little bit when I drill the holes. I think it would look good in there with a little bit of rake to it.
 
sparkwizard

sparkwizard

#19
#19
Thank you. That is the article/interview I was referring to earlier. Reading that was part of the reason I could not pass on this engine for $35.00.
I can build the mount, flip it on its side, find a carb for it and build an exhaust for it. This one got me off my butt and back in the shop!
I hope that owner will offer his wisdom here.
 
sparkwizard

sparkwizard

#20
#20
I made some progress today. IMG_20250922_122107.jpg
I may not be able to use this on my Terra project because the shaft is too short. Maybe I can rearrange the sprockets and bearings on the jack shaft or (shudder) extend the 5/8 crankshaft.
 
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