6hp compression

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Robert Hodges

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Dear gents, and ladies,
I have a 72 6hp that I recently rebuilt. No pryer experiance! This motor was frozen everywhere and with some luck and help I have gotten her to run again. The problem is it doesn't go to fast. I can go 1 mile in 5.5 min. I have a 4hp that does the same. I believe the bigger motor should be faster. The compression on the 4 is 120ish per cylinder. On the 6 it's around 85ish both cylinders. If its leaking air should'nt the gauge go down? New head gasket? Iput block on glass and it was flat but didn't check head cover.
 
jbjennings

jbjennings

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Re: 6hp compression

If you haven't checked for spark with an air gap tester, making sure that spark will cross a 1/4" air gap on both wires. It may be running on 1 cylinder. Or get it running and pull the plug wire off one cylinder and see if it makes a difference. If there is none, then that cylinder is dead.

85psi on a 6hp is not bad at all. It should run nicely with that compression. A 6hp outboard is not a speedster. If it's planing the boat out, it's probably going about as fast as it ever will.
Good luck,
JBJ
 
nwcove

nwcove

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Re: 6hp compression

math isnt my strong suit ...but 11mph with a six hp, isnt to bad?
 
R

Robert Hodges

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Re: 6hp compression

I was hoping for 110 to 120. Is that not reasonable? Just think it should go faster then the 4?
 
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JDusza

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Apr 21, 2009
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Re: 6hp compression

85 psi should run and should run well.
If the cylinder is leaking, yes, the pressure will drop.

Sounds like you're running on one cylinder as per jbjennings.
Be careful pulling the plug wires when running. Use a tool. Weak insulation, you'll be shocked. If you are, you will NEVER touch those wires again.

If you have spark in both cylinders, then your main carburetor jet is dirty or blocked.

I can also tell you that on a 12-foot MirroCraft, the 6 will definitely outpace the 4; no question; even at 85 psi compression.
J
 
nwcove

nwcove

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Re: 6hp compression

"I was hoping for 110 to 120. Is that not reasonable? Just think it should go faster then the 4?"

110 to 120 mph? :eek: ( kidding) how far did you go with the rebuild? is the carb dialed in? points set with an ohm meter? you may just not be getting to the maximum rpm for the motor??
 
JB

JB

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Re: 6hp compression

Don't get hypochondria about 65psi. That is fine for a rope start outboard. The compression is even, which is what really matters.

If you have a problem, and I am not convinced of that yet, it is not compression.
 
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Robert Hodges

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Re: 6hp compression

I did pull the sparkplug wires and found that the top cylinder was not working. It had good spark. So I figured it was the points. I don't know the correct terms but after adjustingthe points for the 4rth or 5th time, I got both cylinders working. Earlier I could only go one mile in 8min. With both cylinders I got down to 5.5 min. Still think I can get get better performance, just not sure where from.
 
J

JDusza

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Re: 6hp compression

Robert Hodges said:
I did pull the sparkplug wires and found that the top cylinder was not working. It had good spark. So I figured it was the points.
Click to expand...
If you had good spark, the problem ain't the points...they're already working!
Check your main jet....
 
T

tx1961whaler

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May 31, 2008
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Re: 6hp compression

What kind of boat? Does either motor get the boat on plane? If you are not getting on plane, then you have discovered that the displacement speed of your hull is 5.5 mph.
 
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Robert Hodges

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Re: 6hp compression

It's an 8 ft. porta boat I use for duck hunting in the marshes of lake Okeechobee. It weighs 50 lbs. Both motors get the boat on plane but with the 4hp if theres another guy in the boat it can't handle the extra weight, thats why I need a slightly bigger motor. Does more com. mean more horsepower?
 
jbjennings

jbjennings

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Re: 6hp compression

The problem with more compression is that 85psi which you already have on the 6hp is about all you're going to get.
Even if it had 95 psi you wouldn't notice much difference. SHort boats do not handle weight very well. If your 6hp is getting the boat on plane I think it's probably running fine and you won't ever see more speed. If you want to plane out 2 guys with duck hunting gear, you need at least a 15hp and a 14foot boat. Is there any way you could post a pic of your 8 foot porta boat with 2 guys, decoys, guns, and shells in it with your 4 or 6hp on the back?
JMO,
JBJ
 
Last edited:
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Robert Hodges

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Re: 6hp compression

Sorry but not till the mini Teal season , around Sep. 20th or so. I did pull the head cover and am sanding it down. It looked pretty good but a little elbow grease won't hurt. I did order a new gasket also. Thank you all for your input, and will respond with comp. test after all is put together!
 
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boobie

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Re: 6hp compression

From reading this post, I think the mtr is running good for a 6 hp. If you need more speed, put a bigger mtr on the boat.
 
nwcove

nwcove

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Re: 6hp compression

tx1961whaler said:
What kind of boat? Does either motor get the boat on plane? If you are not getting on plane, then you have discovered that the displacement speed of your hull is 5.5 mph.
Click to expand...

hes making 11mph, which isnt to bad. id compare the leg length on the motors....the 6 could be a bit longer. but i do believe you arent gunna get much more out of it. ( unless someone re-propped during its 40 years)
 
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Robert Hodges

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Re: 6hp compression

The prop on the motor has 3 blades. I bought a 72 repair manual and the prop in there has 2 blades. Problem?
 
JB

JB

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Re: 6hp compression

The 3 blade props were for heavy duty, low speed operation, like a kicker or troller, Robert. The 2 blade props were for higher speeds on lighter vessels. Start a thread in the Props Forum to narrow down to what is best for your application.

Good luck. :)
 
SeaKaye12

SeaKaye12

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Re: 6hp compression

I think that to really know if there's a problem it would be good to hook up a tachometer. Maybe one of the Tiny Tachs?

Chuck
 
nwcove

nwcove

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Re: 6hp compression

the only prop i can find that the 1972 6hp came with was a two blade 8x7.25
 
R

Robert Hodges

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Re: 6hp compression

The motor came with the three blades but I have a 66 6hp that has two blades so I will change out the propellers. In the meantime I sanded the headcover, and am waiting on a new head gasket. If the compression comes up I hope to get more power. The motor doesn't seem to be maxing out on rpm's. ( not screaming just sort of cruising )
 
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