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Author Topic:   Reed Valves: Effect On Compression
mantec posted 04-09-2007 04:33 PM ET (US)   Profile for mantec   Send Email to mantec  
Hello. I have a c.1975 4-HP Johnson Seahorse with low compression. I know to check head gasket, rings and bore. The following questions are assumed in the absence of fuel and spark, bench testing compression.

Does the reed valve system affect compression? In other words, if the reed plate is pitted and corroded, does this affect compression?

Thanks for the help.

Royboy posted 04-09-2007 04:40 PM ET (US)     Profile for Royboy  Send Email to Royboy     
No, compression happens after the intake port is closed by the piston. Make sure there is some lubrication (i.e. fule/oil) in the cylinder during testing, or the results will be artificially low. In other words, if the engine has been sitting for years without fuel in it, and you attempt to measure compression without putting gas in it, you will likely get a low reading.


Brys13s posted 04-09-2007 07:12 PM ET (US)     Profile for Brys13s  Send Email to Brys13s     
You might have a stuck ring.Try placing the motor on the ground so that the spark plug is facing up.Remove the spark plug and spray some penetrating lubricant like PB Blaster, Liquid Wrench, or something similar into the cylinder.Let it sit for a day or two.Then do a compression test.If this does not improve compression,then dissassemble the engine and see what's going on.
JBCornwell posted 04-09-2007 07:20 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
What, exactly, is the compression, mantec? How did you measure it?

Differences in gauges and technique can make as much as 50-PSI difference in readings, and the probability of both cylinders going bad at the same time for anything other than a gasket blown between them is near zero.

The intake system, including reeds, does not affect compression readings. Different gauges, engine warm/cold, other plug in/out, rate of turnover while measuring, number of strokes measured all affect readings.

On that engine if the cylinders measure within 15% of one another and exceed 60-PSI when recoil operated I would not be concerned.

If you give us the model number we can tell you what year it is. You will need to know that if you need to buy parts. A 4hp is more likely to be 1980s.

Red sky at night. . .

jimh posted 04-09-2007 08:30 PM ET (US)     Profile for jimh  Send Email to jimh     
A technique for making compression measurements is presented in

and is taken from an OMC manual. The usual technique is to make the measurements when the cylinders are wet with fuel. This probably influences the measurement of cylinder PSI.

mantec posted 04-09-2007 11:48 PM ET (US)     Profile for mantec  Send Email to mantec     
Thanks for all the input so far. A little background: [The c.1975 4-HP Johnson Seahorse] runs, just not very well, does not seem to reach high revs, idles poorly. I cleaned and put in a carb kit and an impeller, even though it was spitting water as it should. I also cleaned the plugs; they were blacker than I would like. No change. I took [the c.1975 4-HP Johnson Seahorse] to a local shop. I will get the [compression readings] in a few days when I go pick it up, unfixed.

The guy is giving up, said it's not worth his time, but added it might just be a head gasket. He said it has low compression. Not sure how he checked it yet or what his reading was, but I'll ask and compare to your notes. He also said [the c.1975 4-HP Johnson Seahorse] is getting hot, but I put an impeller in it before I left it with him and it was spitting water out the tube. He used no-touch temp gun and said the head was at 200-degrees-F and thought that was too hot. Is it? I'm used to much higher temperatures on my bikes, and 200 is about right on my V8 Marinette

[Seeks a recommendation for] a decent mechanic in the Dayton or Cincinnati area? Nobody I can find seems interested if it's not a Mercrusier. Hoping to find a privateer who just likes to tinker and make a few bucks at the same time. Thanks again--Mike

an86carrera posted 04-10-2007 08:37 AM ET (US)     Profile for an86carrera  Send Email to an86carrera     
I do not believe that reed valves make no difference in compression in an engine that is a reed design.

During a two stroke cycle the fuel is forced into the combustion chamber by the compression of the fuel air mixture drawn into the crankcase during the next compression stroke. Then the precharge fuel is pressurized during part of the exhust stroke.

If the reeds where not there you would not pressurize the mixture in the crankcase. Why then, would the mixture go thru the port rather than back out the carb/intake system except for some scavaging effect from the exhaust system.

You would still have compression, but not as high a pressure as you get with the slight superchaging effect of the primary compression in the crankcase.

Good luck fixing your engine.


jimh posted 04-10-2007 09:03 AM ET (US)     Profile for jimh  Send Email to jimh     
I think Len is correct. Certainly an engine with reed valves will not operate or run properly if there is a problem with the reed valves. However, it is hard to predict exactly what effect they will have on a reading of cylinder PSI.
Royboy posted 04-10-2007 03:17 PM ET (US)     Profile for Royboy  Send Email to Royboy     
Re-reading my post above, I can see that I may have inadvertantly given the wrong impression about the compression stroke. I will try to clarify it here.

On the compression stroke, the piston is rising in the cylinder. The exaust port is the first to close (as the psiton passes it by), allowing the piston to pick up fuel as it passes the intake port. I don't think it makes any difference what sort of valve is allowing the fuel in. Once the piston travels up in the cylinder far enough, the intake port is closed and compression begins. At the top of the stroke, maximum compression will be realized, and the piston will then begin its downward travel (aided by the explosion of the fuel/air mixture).

In other words, a two stroke engine will achieve compression without a carburator even installed, although the cylinder walls need to be wet with fuel/oil for the rings to function properly (form a seal between the piston and the cylnder walls).


an86carrera posted 04-10-2007 05:13 PM ET (US)     Profile for an86carrera  Send Email to an86carrera     
Four stroke engines combustion chamber are only filled by atmospheric pressure, except for supercharged and turbo charged engines.

Two stroke engines recieve a pressurized charge from the crankcase. In the case of a reed valve design, if they seal well, it is more pressurized going in, so...they are compressing pre-pressurized gases thus slightly higher final compression numbers.

Could be proven wrong I'm sure by some internal combustion scientist. But this is my understanding of the system.


Peter posted 04-10-2007 07:12 PM ET (US)     Profile for Peter  Send Email to Peter     
The reed valve is merely a check valve that helps direct the fuel/air flow within the engine. On the up stroke, the valve is open letting a new charge of the air/fuel mixture into the crankcase. On the down stroke, the valve is closed and the air/fuel charge in the crankcase is pushed into the intake port and combustion chamber. See for an animated illustration of a 2-stroke that shows the operation of the intake (reed) valve. I would bet that the reed valve function has very little, if any, impact on the compression ratio of the cylinder.

The 4-HP probably could use a decarboning. Have you adjusted the high and low speed jets? Also, the WOT operating range for that 4 HP is not a high reving motor. WOT operating range is 4000 to 5000 RPM with 4 HP rated at 4500 RPM. That 4 HP is one of the longest lived engine lines OMC ever made. Probably had a 40+ year run starting in the mid 50s. Darn near bulletproof given the torture I put one through as a kid.

Brys13s posted 04-10-2007 08:02 PM ET (US)     Profile for Brys13s  Send Email to Brys13s     
mantec,When you say you cleaned the carb,what exactly did you do in the cleaning process? Can you give us an engine serial#,numbers off the carb,engine model#?Was the low compression diagnosis by feel or did you remove the spark plug and screw in a compression guage? If you used a compression guage,was the engine cold or hot? What were the readings?
an86carrera posted 04-10-2007 08:46 PM ET (US)     Profile for an86carrera  Send Email to an86carrera     
You really do need to do a compression check with a guage. If the two cyl. are the same or close and over 110 psi you should have a good chance in getting it running well.

Let us know what they are. If the reeds are pitted it will not keep this thing from running except in a very serious state of corrosion. They are usually made of a good quality material that I have never seen corroded. Broken, bent yes but, they will mostly affect idle.

I think that year still may have points if so, check those too. That could cause the black plugs.

Keep us posted.


mantec posted 04-17-2007 03:33 PM ET (US)     Profile for mantec  Send Email to mantec     
Hey, Just got the motor back, not fixed, from the shop,
I was taking his word on compression being low,
I will test it myself, I have a gage.

I will follow up soon

Thanks for all the good input, I have picked up
on several things I hadn't thought of.

Still looking for a good small outboard mechanic
in the Dayton, Cincinnati area if any one knows

Thanks Mike

sternorama posted 04-19-2007 03:57 PM ET (US)     Profile for sternorama  Send Email to sternorama     
I had a couple of overheating incidents with outboards. One was due to corrosion of the engine block in the area of the water passages. A new block corrected this.

Another instance was constriction of flow between the pump and the powerhead in the area of the grommet or seal that goes beteewn the powerhead and the tube that brings the cooling water to the power head.

My point is that a hot engine (if it feels hot to your touch, then is it running hot!, i.e. 200 is too hot) can be caused by a number of things....

Good Luck!

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