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Old 01-27-2013, 11:44 AM
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Need help diagnosing really weak rebuilt 350

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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Old 01-27-2013, 11:57 AM
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Its getting tired. some old cars continue for awhile in that shape.boats need to use more power than a car.
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Old 01-27-2013, 12:03 PM
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How many hr.s on this engine? If its a fresh eng. how many hr.s do you have on the engine break in to seat ring's? Did you set valves when warmed up after cam break in ? did you do a dry and then a wet comp. check to determine loss of comp. on valves or rings?

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Old 01-27-2013, 12:36 PM
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He's right, boat take more power. Boat motors are a different breed. They are just cammed different. lThey usualy built a lilttle dilfferent. rebuild it and study up on the right build. You could get by, by just rebuilding the heads, valve job, but that may not be enough. If your boat is realy good, i'd do a complete rebuild.
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Old 01-27-2013, 01:08 PM
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The engine only has 20 hours on it. Sorry forgot to ad that tidbit.
Yes I adjusted the valves after it was broke in and warm. Would a good rebuilt engine leak at all out the valves?
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Old 01-27-2013, 01:23 PM
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If the engine is new,after 20 hours it should either improve or deteriorate quickly. I hope it was broken in properly?after 20 hours should be good to go and then some. 275 horses is not really a very high out put.Other than power,what complaints do you have?
I should have reread your first post,,,,what do you expect your boat to do with 170 ish horse power.A lot of boat v-8s run at 70% and its 20 feet long.
re leak down,it would be nicer if all the values were equal.
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Old 01-27-2013, 01:29 PM
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A rebuilt head should have zero leakage past the valves.
Piston ring leakage of 1-10% would be acceptable for a stock rebuild.
Did you check the cranking and running compression?
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Old 01-27-2013, 01:40 PM
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Quote:
Originally Posted by lmsport View Post
A rebuilt head should have zero leakage past the valves.
Piston ring leakage of 1-10% would be acceptable for a stock rebuild.
Did you check the cranking and running compression?
That's what I figured. Based on that I would say that the valves were very poorly rebuilt, and that the rings could be fine. Is that a decent assumption?

Yes I did a compression test prior to the leakdown. I was getting 115 psi on my best cylinders, and 89 on my worst. that's what clued me into a problem in the first place.
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Old 01-27-2013, 01:41 PM
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Quote:
Originally Posted by gfackrell View Post
The engine only has 20 hours on it. Sorry forgot to ad that tidbit.
Yes I adjusted the valves after it was broke in and warm. Would a good rebuilt engine leak at all out the valves?
Sure, you'll get SOME leakage from all components. Take into consideration though, that under actual operation, the valves and rings have very little TIME to leak, unlike a leakdown test, where the components have plenty of TIME to leak down and things look worse than they actually are.

I know it's very frustrating to lay down your hard-earned money to someone and then find that what you bought does not lend itself to the work at hand.

As a young man, I went through this a couple of times until I finally realized that there are few really honest people on this rock and I never again bought anything "running". I read all I could and talked to people who knew what they were doing until I had the skills to choose the proper parts and machine shop operations to arrive at a pre-determined goal. I've been doin' this stuff since the 50's and I still learn something new every day.

I know I'm not helping you much, but if I can get to a few younguns who are reading this thread and educate them about how to do this stuff without gettin' "had", then it will be worth it.
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Old 01-27-2013, 01:46 PM
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Quote:
Originally Posted by vinniekq2 View Post
If the engine is new,after 20 hours it should either improve or deteriorate quickly. I hope it was broken in properly?after 20 hours should be good to go and then some. 275 horses is not really a very high out put.Other than power,what complaints do you have?
I should have reread your first post,,,,what do you expect your boat to do with 170 ish horse power.A lot of boat v-8s run at 70% and its 20 feet long.
re leak down,it would be nicer if all the values were equal.
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.
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Old 01-27-2013, 01:50 PM
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you stated you adjusted the valves! and only the exhausts are leaking, try backing off the exhaust valves a little. to give them a little more clearance and they might seat better! The intake valves run cooler and the exhaust valve stems expand a little with the combustion temps and might be rising just a hair off the seats! Do the exhaust valves have rotators installed ? Is it a solid cam or hyd.? What did you set the valve clearances at? Is this a Chevy, Volvo, Ford, ETC? Reverse rotation? What r.p.m. did your old engine turn at 30?


Jester

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Old 01-27-2013, 01:54 PM
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sounds like the rings arent seated or the valve springs are weak
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Old 01-27-2013, 01:57 PM
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If you are reaching 5200 RPM,thats probably all you will get.You might be running out of prop? and what painted jester posted?
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Old 01-27-2013, 02:11 PM
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Quote:
Originally Posted by painted jester View Post
you stated you adjusted the valves! and only the exhausts are leaking, try backing off the exhaust valves a little. to give them a little more clearance and they might seat better! The intake valves run cooler and the exhaust valve stems expand a little with the combustion temps and might be rising just a hair off the seats! Do the exhaust valves have rotators installed ? Is it a solid cam or hyd.? What did you set the valve clearances at? Is this a Chevy, Volvo, Ford, ETC? Reverse rotation? What r.p.m. did your old engine turn at 30?


Jester
'79 chevy 350, regular Rotation, The old engine didn't run when I got the boat. I re-adjusted the valves according to a write up by a very knowledgeable guy on another forurm, (honestly can't remember the exact details of how he had me do it. Was months ago) and it made no difference from where the rebuilder had adjusted them.
I don't know if the exhaust valves have rotators. Sorry for not having much info.

The engine is on the stand at this point, and honestly I'm just looking to see which parts of it aren't completly crap. I am ready to put together an engine myself that doesn't suck. What I'm looking for is ways to quickly diagnose if the bottom end is any good. I'm pretty much planning on new heads, intake, carb, etc. If I need to have the bottom end gone thru, I have a great machine shop that can do it. I would be happy for suggestions of what heads intake, etc for a good 300 hp budget build.
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Old 01-27-2013, 02:21 PM
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you should(if needed) build a 360 horse marine engine.
360 x .7 = 250 ish continuous
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