Low Compression - Cause of high fuel consumption?

atengnr

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Sep 18, 2006
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Hey all. Have this 95 force 70 that I bought this spring as a project/fun motor. Cold starts, idles and runs pretty darn well, except perhaps hot restarts, where its a little finicky. Had it out several times this year, and found that it devours gas. If I ran at 3/4 throttle, it used about 6 gallons of fuel in under 1 hour! Ive read on here about the 7gph rule of thumb, but I think this motor would chew 7 gallons very quickly at WOT. I did recheck compression just now and is 90-100-100. Do you think that this motors near worn out state is the reason that it burns extra fuel?

Im running it on a 16 ft four winns (1100 lbs), and sadly I have no tach yet. It runs about 31mph at wot. Running 115/8 x 11 prop.

I am sort of looking for an excuse to tear into this powerhead, but I suppose I probably wont if you dont think the low compression is the most likely contributor.

Thanks alot.
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Heres what these cylinders looked like back in the spring....
 

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Scott Danforth

Grumpy old guy who plays with boats
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assuming you are commanding 50hp worth of power

your motor should have a BSFC of about 0.55#/HP/hr. at higher RPM's

so 0.55*50*1 = 25# of fuel, so you should have burned about 4 gallons

if your burning more, my guess is you are simply washing fuel down the cylinders and that may be exasperating the low compression. start with the fuel system

your compression numbers are also low, not sure how much that will push the BSFC up.
+------------------------------------------+----------+----------------+-----------+ | Engine | @ 1K RPM | @ Peak Torque | @ Peak HP | +------------------------------------------+----------+----------------+-----------+ | 4-stroke (low compression, carburetor) | 0.62 | 0.47 | 0.52 | | 4-stroke (high compression, carburetor) | 0.60 | 0.42 | 0.47 | | 4-stroke (high compression, closed loop) | 0.52 | 0.35 | 0.42 | | 4-stroke (supercharged, carburetor) | 0.75 | 0.50 | 0.55 | | 4-stroke (turbocharged, closed loop) | 0.57 | 0.45 | 0.50 | | 2-stroke (low compression, carburetor) | 0.85 | 0.55 | 0.60 | | 2-stroke (high compression, carburetor) | 0.80 | 0.50 | 0.55 | | Diesel 4-stroke | 0.35 | 0.25 | 0.30 | | Diesel 2-stroke | 0.40 | 0.29 | 0.34 | +------------------------------------------+----------+----------------+-----------+
 

jerryjerry05

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May 7, 2008
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The comp at 90-100 is low but could be the way it's taken or the equipment used??
Harbor Freight tools= crappy results.
With a good test/tester it should be 150# or close.

The "good" the comp is close all within 10#
The low on #1 is inherent to that motor/carb setup.
That cyl. is usually the first to go bad if the motor dies.

Any idea how many miles to the gallon?
My twin 88/85hp got 3.2(?) MPG

What do the plugs look like when you pull them.
Pay close attention to the #2 plug as it operates the fuel pump.
Possible the excessive fuel gets pulled through the pump and right into the cyl.

BSFC ??????
I didn't understand anything Scott said. :)
 

racerone

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Dec 28, 2013
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The carburetors respond to air flowing through them.----What happens to that air afterwards , compression in the crankcase and then compression in the cylinder does not affect fuel amount used.
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Used my now old craftsman thread on comp gauge. Has been accurate on all past use (building harley motors with new parts and got expected compr readings). Is it worth doing a decarbon to see if this brings up comp?

wow mpg cant be good. Im guessing that its less than 3 mpg at 25 mph cruise (3/4 throttle ish).
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Looks like the route i took that day was about 12 miles round trip in 5+ gallons of fuel. 3 mpg at very best.
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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Can anyone comment on this prop for this setup? I have a mini tach ordered.
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Thanks. It appears that i am way under propped if im reading this chart correctly.
 

jerryjerry05

Supreme Mariner
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3 mpg is good.
Some boats get 1 if lucky.

My 88 85's came with 17p props.about 32mph.
I wanted more speed so I changed to a 19p.
I got 2-3 mph more and the fuel usage went up 15-20% with the change.
 

atengnr

Chief Petty Officer
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Can you really rebuild this motor with crankcase on? I am itching to rering and hone this motor, so i pulled off reed block, and darn looks like re assembly would be tough!

thank
 

racerone

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How would you make sure that rod caps are properly lined up ?-----And putting new rings in possibly worn ring grooves is not a good idea.----How would you clean up the residue ( abrasive ) after honing.
 

jerryjerry05

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May 7, 2008
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It can be done without removing the block.
BUT!! it's way more difficult doing it that way.

One of the past contributors, FrankA (rip) did the rebuild on one cyl. using that method.
Myself I'd be afraid of loosing 1 bearing and ruining the block.

When removing the block, accessing the parts and pieces is way easier and
honing the block or reboring can be done right.
 

racerone

Supreme Mariner
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Dec 28, 2013
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If rings are worn to the point of causing low compression then the ring grooves will also have significant wear !!
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Well, I pulled the powerhead apart today, and cylinder 2 and 3 had broken top ring, heavily scored region on piston in this area. The infamous top cylinder was the best of the 3. Now, what to do?
 

atengnr

Chief Petty Officer
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Sep 18, 2006
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482
Heres piston 3. Piston 2 is similar. So what is the story with the wiseco replacement pistons?
 

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jerryjerry05

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I buy Wisco
You'll need the wristpin kit and their rings.

First take the block to a shop and figure out the size they'll need to bore it out.
If possible go with the .010 over.
That way if it happens again you'll have room to bore again.
Once you get to .030/.040 you might need to re-jet the carb.
 
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