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  #16 (permalink)  
Old 01-27-2013, 02:50 PM
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Originally Posted by vinniekq2 View Post
you should(if needed) build a 360 horse marine engine.
360 x .7 = 250 ish continuous


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  #17 (permalink)  
Old 01-28-2013, 12:34 PM
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You state your are turning 5200 RPM and only going 30 MPH. Might look at the prop pitch.
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  #18 (permalink)  
Old 01-28-2013, 01:17 PM
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Quote:
Originally Posted by gfackrell View Post
Long story short, I bought a long block for my 20 ft boat. Put it in and it was very weak. It was supposed to be around 275 hp, but obviously wasn't.
I did a compression test with it still in the boat, and got very poor compression numbers. I then pulled it out and checked to see if the cam timing was on, at the engine builders recommendation. It was. I then did a leakdown test and these are the results I would like help interpreting.

cyl 1 - 11%, just a tiny bit past the exhaust valve, the rest past the rings
cyl 2 - 15%, a little past the exhaust valve, the rest past the rings
cyl 3 - 25%, a lot past the exhaust valve, the rest past the rings
cyl 4 - 18%, a little past the exhaust valve, the rest past the rings
cyl 5 - 23%, a lot past the exhaust valve, the rest past the rings
cyl 6 - 26%, a lot past the exhaust valve, the rest past the rings
cyl 7 - 15%, a little past the exhaust valve, the rest past the rings
cyl 8 - 11 %, a little past the exhaust valve, the rest past the rings

I could not find any leakage past intake valves on any cylinder, but every single exhaust valve was leaking, some pretty bad.
My main questions is this:
Is ANY leakage permissible past the exhaust valves?


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Way too much loss and too much difference between cylinders. But first how was this tested?

The engine should be brought up to operating temperature before testing. This closes the ring gaps to operating clearance. If you don't do this then a thick engine oil should be introduced before testing a 30, 40 or 50 weight oil so it can seal up the rings without being blown out by the test pressure. Keep in mind that an operating engine is also using cylinder pressure to push the rings outward against the cylinder walls which is what actually makes the seal. Cranking or static testing cannot duplicate this exactly, so the pressure loses appear higher in these tests than they really are. If these things aren't done the leakage rate will look excessive. Yours is high on some cylinders and certainly the differential between cylinders is way out of bed. I look for no more than a 5 percent differential between cylinders. The test should be done with each cylinder at TDC and the crankshaft locked so the pressure cannot rotate the crankshaft, this is both from a safety and unified test process standpoint.

However, I start getting concerned when the loss exceeds 10 percent when following the process I just outlined and the differential between the high and low cylinders exceeds 5 percent.

I'm be less than pleased with leakage past the exhaust valves that you're seeing. This can indicate poor seat conformability between the valve and seat or it shows weak springs. Intakes are more flexible than exhausts simply due to increased size they conform to the seat better. Exhausts need better attention paid to them when doing the seats and may need a bit stouter spring than the intake.

Boat motors work a lot harder than road vehicle motors so extra cautions are in order when it comes to cooling and lubrication as well as timing and mixture. It is mighty easy to toast the exhaust valves and certainly piston and bore wear are greater issues but that said at 20 hours unless the timing and or mixtures are way off you shouldn't see valve damage. Since you are; my list of suspicious charters would immediately turn toward ignition timing against RPM and mixture ratio's. If these things get out of line; the motor becomes toast pretty quickly. Detonation is much harder to hear from a boat motor because of all the other noise, if you're running into this problem it can do a lot of damage before your aware that things are amiss. Sparkplug color is not always a good indicator on boats as they tend to run quite cold when fired up which may carbon the plugs making them appear that the mixture is richer or the timing less advanced than it actually is. Boat motors are lot trickier to diagnose than car/truck motors in these instances.

Bogie
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  #19 (permalink)  
Old 01-28-2013, 04:12 PM
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Your compression numbers are low for a rebuilt engine. Rough calculation would show you at about 7.8 to 1. Not the recipe for a 300+ hp engine. I would take a hard look at the cam timing before you go crazy. The timing chain being off a tooth can affect compression. I would expect to see at least 135 compression in each cylinder. This is assuming the combo is setup to produce around 9:1 compression ratio.
If the exhaust valves are a little tight or the valve job was sub par this could also be part of the problem.
Dont assume you only have 1 issue.
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Old 01-28-2013, 04:32 PM
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I checked the cam timing, and it's indeed on. I'm not assuming i have only one issue, which is why I'm going to tear it all down and work with my local machine shop to get it back to where it should be. Thanks for all the help guys. I learned a valuable lesson. Don't buy a cheap long block off the internet....
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  #21 (permalink)  
Old 01-28-2013, 05:28 PM
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What's the casting number on the heads?
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Old 01-28-2013, 07:34 PM
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good luck

Quote:
Originally Posted by gfackrell View Post
I checked the cam timing, and it's indeed on. I'm not assuming i have only one issue, which is why I'm going to tear it all down and work with my local machine shop to get it back to where it should be. Thanks for all the help guys. I learned a valuable lesson. Don't buy a cheap long block off the internet....
I think "dont buy cheap" says it all

hope the machine shop can do a simple fix
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Old 01-29-2013, 11:55 AM
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What's the casting number on the heads?
333882 casting number
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  #24 (permalink)  
Old 01-29-2013, 02:18 PM
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Quote:
Originally Posted by gfackrell View Post
I followed the rebuilders recommendation and put a zinc additive in the oil for initial startup. I also ran the rpm to 2000 immediately on startup. Ran it like that for 5 or so minutes, then let it come down to idle and varied the rpm for the next 5 minues. Then to the lake, where I drove it at varying speeds for about an hour. For the next 5 or so hours I never pushed it hard, and tried to vary speeds.

It was supposed to be a stock replacement, and yes 275hp isn't very impresive, but I was/am on a budget. No other complaints really, it's just weak. It can only push the boat to 30 mph at 5200 rpm. The boat is empty, and light. I am hoping for 40 mph plus, and thought that a good 275 hp engine would do it.
The valve leakage needs to be addressed. But if you haven't set the timing up correctly, that alone can make a decent engine seem weak.

So do you happen to know what the initial timing and the total timing is, and at what rpm the mechanical timing is all in by? The distributor shouldn't have a vacuum advance, or if it's there it shouldn't be hooked to a vacuum source.
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  #25 (permalink)  
Old 01-29-2013, 02:29 PM
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Quote:
Originally Posted by gfackrell View Post
333882 casting number
This is a low compression emissions head found on 350s and 400s. The piston it's covering will do a lot to set the complete compression ratio with a circular dish which can cover a range of sizes from 10 to 20 something CCs.

Compression needs to walk with the camshaft this gets into the subject of Dynamic Compression Ratio (DCR) which is a measure of stroke lost up to the point of intake valve closure. Under any circumstance low compression costs power while increasing fuel consumption but especially when combined with any amount of aggressive cam (say over 200 degrees of duration measured at .050 inch lift it really cripples engine power and increases fuel consumption.

For a boat I'd really recommend an aftermarket head, these come with a lot more physical beef which takes the punishment that running with high loadings for long periods of time that a boat requires. From that you taylor the chamber size for the needed compression ratio.

Bogie
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