1973 50hp lark Compression Question

[FillupD]

I have a motor that is in the marina for repair. It will not reach WOT of 5500rpm. Compression is 130 each, drop down test of 100 PSI. is OK. Ran 2500 with 19" pitch prop. 4800 with 15" pitch prop and then 3500 with 13" pitch prop. Seems kinda weird?

This motor has been put on a Dyno and has 800 PSI at the prop and the first Marina told me to put a 13" pitch on it and it would run fine. I did and it didn't.They thought that the low rpm's was the 15" pitch prop.
I got 4800 rpm's alone in the boat with the 15" pitch, and when 250 lbs was added, it went down to around 3500 rpm. The 13"pitch was even worse.

I took it to a marina that has a service dept with a test lake. They can't get it to run WOT either. They have looked at everything and now think it may be the reed valve. I'm sure there is a direct correlation between "load and power" concerning the valves.

My question is, wouldn't the compressions tests fail if the reed valves were bad? Does this motor have the reed box style or the fan type? Does this sound like a problem that be fixed by replacing the reed valves?

They have taken out on their lake and get the same results I did. The motor runs great but there is just more there and you can tell it wants to go but just won't. I think we have pretty much taken changing props back and forth out of the picture now. The entire ignition system is mostly new or tested out good.

 

[jbjennings]

Re: 1973 50hp lark Compression Question

I'm not a qualified mechanic but maybe you'll find this info. useful, anyway. Here's a link to a diagram for the intake. It's a reed block style intake. I believe from what I've learned from others is that you could take the carb and reed blocks out of the motor and still get the same compression results. 130 lbs. on both cylinders sounds like nice compression to me. http://www.crowleymarine.com/brp_parts/diagrams/31580.cfm

I am thinking that reeds don't do much past a certain rpm and therefore I wouldn't think they'd affect your wide open throttle problem. If you are getting 4800 with a 15 inch prop and 3500 with a 13 inch prop, I'd think you either dropped a cylinder or the 13 inch wasn't really a 13 inch. Just because the ignition parts are new doesn't mean that something can't be bad, also.
However, if I checked everything else on the motor, my last resort would be to check the reeds. The only motor I've had bad reeds on wouldn't start under any circumstance, and I'd think if your reeds were bad you would have an idle problem, not a WOT problem. But I could be wrong. I hope you get a bite from a real mechanic soon. I'm interested in seeing what the problem is.
Good luck,
JBJ

 

[jay_merrill]

Re: 1973 50hp lark Compression Question

I can't say that I am an expert on symptoms of bad leaf plates but, I can't imagine low compression being one of them.

Compression values in a 2-stroke engine are a function of ring/cylinder wall condition, head/head gasket condition, and spark plug seal. If the motor were a 4-stroke (which it isn't), you could add intake and exhaust valve condition to that. In those motors, and burnt valves and/or valve seats contribute to loss of compression.

If you are getting compression values of 130 and the leakdown test is good, you should be able to get to the high end of the WOT range with the right prop. Having the WOT rpm drop as you decrease pitch, makes no sense - it should be the opposite by about 200 rpm an inch.

Aside from compression issues, there are a bunch of things that will cause an engine not to deliver full power. They can be things like the carbs and ignition not being sychronized, air leaks at the manifold, broken leaf valves, incorrect carburetor mixture, automatic choke engaging when it shouldn't, clogged fuel filter, bad fuel pump, exhaust gases leaking into engine cowl, engine overheating, gearbox problems, etc., etc.

If you do have a manifold leak, broken leaf plates, or your carbs are too lean, the motor will often backfire, so you might have a symptom as a clue.

I would say that you need to get an OEM manual, if you don't have one, and go through the systems one at a time, to try to diagnose the problem.

 

[F_R]

Re: 1973 50hp lark Compression Question

Fillup, be sure to let un know the outcome of this. As you say, it sounds weird. Too weird. In fact it makes no sense at all. I'm thinking pre-ignition, but that is just a wild guess. But if it is, you will soon find out as the pistons bust.

 

[FillupD]

Re: 1973 50hp lark Compression Question

Tanks for the reply's. It will be interesting to find out what this is. I'll let you know what was found. With six certified mechanics working at the marina it is at now, the problem should be found. But I've thought that a couple of times before. At least this one has a test lake.

There was one comment that struck home. The idle needs to be set close to 1000 for it go into gear with out stalling. That is a little high, but maybe that should be an indicator of a problem. I guess I never thought that was that high to be considered a real problem though. It sure starts good.

 

[FillupD]

Re: 1973 50hp lark Compression Question

Still waiting for reed valve check. They said tomorrow would be the day that the mech that was working on it will be there.
The service manager said if it wasn't the reeds that I should probably look for a different motor. They run across one that can't be fixed once in a great while.
I think he meant it wasn't going to worth them to tear into it.
The only thing it could be is the lower unit. There may be a bad bearing or something that puts a load on the motor when it is under power. Does this sound like a possibility?

I'll find out in the morning if the reed valves are OK.

 

[FillupD]

Re: 1973 50hp lark Compression Question

I got my motor back with the message to get a new one, that one isn't worth the effort. I took it out today to check it out and the starter was out of it so I couldn't check to see what the boat shop did. I'll have to make sure to thank them for that.
I did talk to a guy there that thought them saying a cylinder or piston was bad was not correct. With a compression of 130 in each cylinder and 100psi drop test it shouldn't be the piston.
He was looking at the motor and was asking about the reed check and commented on how hard it would be to check the reed valves with out taking out any bolts. The bolts on the front of the motor were all painted original and it was evident that they had not been removed. So, I guess I'll start there.

Another suggestion was to change the adjustment on the carb floats a little because it wouldn't idle down real low. Maybe too much gas or pressure, maybe the motor is flooding out at high rpm?
I'm going to try to work on the nightmare motor a little more before I give up. I have given up on boat shops though.
I got fire, compression, and only one thing left, right? Any suggestions on fuel delivery would be appreciated.

 

[R.Johnson]

Re: 1973 50hp lark Compression Question

I'm confused about the leak-down test. 130# compression, but drops to 100. If they are applying 130# to the tester, and it drops to 100, you have found the problem.Did they think you should get rid of your twin 50?

 

[FillupD]

Re: 1973 50hp lark Compression Question

The compression is 130 and they said the cylinders held 100psi without dropping. It put out 800 psi on the dyno, I'm not sure what that means.
I had it in a shop with 3 certified mechanics? and the advice was, the problem is too deep to mess around with. Probably a bad piston. Sometimes you just run into a motor that won't rev up at wot rpm's under a load and this is one of them.
I just got my starter cleaned up and working and I am going out for a test before I take the carbs back off and look at the reed valves myself.
There just seems to be absolutely no reason for this motor to not run right. I might add the idle adjustment screw is all the way in, bottomed out? I don't think that is right either.

 

[tx1961whaler]

Re: 1973 50hp lark Compression Question

Wow! Three 50 hp twins down in one week....

http://forums.iboats.com/showthread.php?t=272927
http://forums.iboats.com/showthread.php?t=273856
http://forums.iboats.com/showthread.php?t=273995

 

[FillupD]

Re: 1973 50hp lark Compression Question

After reading the other threads I think I am going to pop the head off and take a look. RJ sounds pretty convincing that something may be discovered there. I am just going to have to do it.

Its like I was under hypnotism reading his responses, "Pull the head and take a look, what have you got to lose." "Pull the head and take a look, what have you got to lose." "Pull the head and take a look, what have you got to lose." "Pull the head and take a look, what have you got to lose."

Ok enough of that.

I've got the new head gasket sitting in my box of parts. I'll let you know what I see.

It seems like it shakes a little too much when no wake running. Maybe because it will not idle down? (It idles high because it won't idle >1050 rpm.)

 

[freddyray21]

Re: 1973 50hp lark Compression Question

low compression can be the cause of poor idle, but it will usually run out okay. I guess you have nothing to lose, but pulling the head, but I would not do it. If it has that good a compression and that good a leak down test I would look elsewhere. Leak at intake or as you said bad reed valves.

 

[FillupD]

Re: 1973 50hp lark Compression Question

I sprayed grease from an aerosol can around the carburetor shafts and misc. gaskets. There was no difference in running and no leaks around gaskets, (that were obvious anyway.)

If the reed valves are OK, I'm going to move the float 1/32" off of level so the gas shuts off a little sooner. I talked to a guy at the lake this afternoon and he had done that with his 150 hp and gained around 500 rpm's. He also unhooked one of his fuel pumps. He thought the carbs were getting to much fuel and it helped him out. It was on a little speed boat, I think this guy tinkers around quite a bit.
I guess I'm not sure why it helped, but it won't take long to test out that theory. So, the reed valves first, the carb float test, (is this a waste of time) and then take a look at the cylinders and pistons.

 

[Benny1963]

Re: 1973 50hp lark Compression Question

what is the cranking compression.how far down is the cav plate on the boat

 

[FillupD]

Re: 1973 50hp lark Compression Question

Cranking compression is 130 each and the cavitation plate is even with the bottom of the hull.
One other thing, the air fuel mixture screw is only about 1/2 turn out. I tried to set it like the manual sates but it was way to rich. This motor seems to like it really lean. I don't know if that means anything or not.

 

[MrGED05]

Re: 1973 50hp lark Compression Question

Sir:
If the idle adjustment screw is "all the way in" that means that you have zero fuel supply through the carb at idle. Thus you can only idle at 1000 rpm on the fuel supply through the high speed needle and seat.
Ignition must be decent, otherwise starting would be REALLY difficult with no idle venturi fuel flow.
Rebbuild your carb. Cheap and easy to do, maybe forty bucks if you have to buy carb cleaner. My best guess is the you have a "way off" fuel/air mixture at full throttle, could be caused by any number of carb issues, from varnish buildup to damaged needle valves.
With 130 psi static compression pressure and a "leak-down" to only 100 psi, the engine has sound internals.
You most likely have a fuel supply/mix problem.
Good luck, this one should be cheap to fix.
Gary

 

[R.Johnson]

Re: 1973 50hp lark Compression Question

I would like to know what the leak-down figures truly are on this engine. You can't put in 100#, and the engine will hold 100#. There will always be at least leakage through the ring gaps. A 10% pressure loss is borderline. No engine will ever hold full inlet pressure. Something is wrong, or someone misunderstood the test.

 

[FillupD]

Re: 1973 50hp lark Compression Question

The guys that put the motor on the dyno just said that the motor checked out good. Compression was great and they put 100 psi on the cylinders and it held for over an hour. The test prop showed good pressure at the output shaft.
At the time it had a 19" pitch prop on it and they thought that that was too much so I put the 15" pitch prop on to get the rpm's up to snuff. I left with the boat thinking that the problem would be solved after the mechanic said it checked out good, but he had a problem getting it to idle to spec. He said if I can live with that; it would be OK.
The 15" prop failed to get the rpm's up. Closer to 4800 but when I added a person and my dog, the rpm's went down to around 3500. The load, extra weight, had really made a large difference.
He thought the air/mixture valves were working OK and commented that it was pretty touchy as how far you could turn them. After the carb rebuild, I set the needles to 1 1/2 turns out for initial setting and the motor ran very rich. Then I read in the manual to set them at 5/8 turn out. Still to much, motor was still running rich. I screwed them in and they are aprox. a 1/2 turn out and the motor starts great.
I started at 5/8 and turned them in until the motor was going to die and backed the off a tiny bit and it smoothed out.
It kind of seems that the motor is kind of slow getting up to 4800 rpm's (planes out OK) and when it does, I know it wants to go more but it almost seems like it is flooding out.
Could it be possible that the high speed jets are worn out and letting to much fuel in at WOT? But then you have to wonder why it is running with such a lean mixture. I almost want to mix up some 100:1 and see what it does. But I doubt if I will until the very last resort.
I checked TDC with the mark on the flywheel thinking maybe the timing mark was off, but TDC is TDC on the timing marks.

 

[jbjennings]

Re: 1973 50hp lark Compression Question

FillupD,
This is probably a stupid question but is there any chance you have a fiberglass boat that has water under the floor soaked into the foam? It almost sounds like your boat is too heavy. Particularly since it ran good on the "Dyno". I really don't understand the dyno thing either. I've never heard of an outboard dyno. Is it using a test prop on it to make a load?
Anyway, have you tried adjusting the idle stop? Does it have fixed high speed jets? I find it really odd that it'll plane out but won't open up. Myself, I wouldn't dare run it 100:1. That won't fix your rich problem for sure. I would think reducing the oil would just put that much more fuel through your jets since lower oil would thin the fuel out. Have you checked the reeds yet? How many times have you cleaned your carb? It's possible if you have a fixed jet it's not clean enough and if you have an adjustable high speed that the little gasket deep inside that the tip rides on is bad or missing. I don't have any other ideas.
I hope you get it fixed and if R. Johnson says pull the head, I'd consider it----sounds like he's seen a few of 'em before. You can't beat experience.
JBJ

 

[FillupD]

Re: 1973 50hp lark Compression Question

It is an aluminum boat and I asked the last marina about water logged and they said that they would have propped that problem away by now if that was it, so that was kind of out of the troubleshooting scenario.
The nylon bushings in the fuel mixture screws are there. I removed them when I boiled my carbs out.
Here is a pic of my linkage when I got it back. I am going to check the timing now. I'm not sure if idle timing will be 4? in that position, but it sure starts good. Something just doesn't look right. I'll get to take a look at the reed valves pretty soon and go from there though.