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Author Topic:   150-PSI Engine Cylinder Compression: Too Much?
blueboat posted 04-28-2003 11:13 AM ET (US)   Profile for blueboat   Send Email to blueboat  
[This thread is going to read somewhat awkwardly because the authors of most of it kept starting new threads instead of replying to the original--jimh, Moderator]

1974 70 hp Johnson. I just put the head back on and did a comp test. I know it should be warm when done but I just wanted to see if it had the same compression on all 3. It did. 150 psi seems like to much to me but maybe once it's warm that will go down a bit. Any info on this? Thanks for all the great info I get from you guys. Happy boating.

where2 posted 04-28-2003 05:42 PM ET (US)     Profile for where2  Send Email to where2     
150 in all 3 cylinders! Drop some high-test in it and go have some fun... Sounds fine to me... never read a specification on "too much" compression. Other than the people trying to convert a Diesel block to run gasoline...
brisboats posted 04-28-2003 09:23 PM ET (US)     Profile for brisboats  Send Email to brisboats     
150psi sounds higher than normal for that motor, is this a fresh rebuild? It may just be your guage reading a bit on the high side seems that even the high quality ones rarely read the same.

It is possible to have too much compression, the racer crowd swaps heads for less cc's in the combustion chamber, planes the surface, and richens the jets and runs on the edge of meltdown.

Run high octane and listen carefully for any detonation next time you run it.


Clark Roberts posted 04-29-2003 08:24 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Carbon build-up behind rings can cause higher than normal compression and this "ring jacking" can cause higher heat and eventually lead to scored cyl walls and/or seize-up! A good de-carboning is most likely in order. You can use the Merc "Power Tune" or Yamaha "Combustion Deposit Cleaner" or OMC's cleaner (follow directions on can) or put a bottle of Chevron's "Techron" in about 12 gals of gas. Happy Whalin'.. Clark.. Spruce Creek Navy
Clark Roberts posted 04-29-2003 08:32 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
PS> if fresh rebuild above doesn't apply....Clark
Steve Leone posted 04-30-2003 01:29 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
Compression readings will increase, not decrease when the engine is warmed up. Wet cylinder walls will show higher than dry. Cold compression readings are more accurate for diagnosis. I beleieve that model should be 125lbs psi factory. You may have carbon or your guage is telling you lies. I used to buy outboards from a guy with a bad compression guage..... Steve
blueboat posted 04-30-2003 10:46 AM ET (US)     Profile for blueboat  Send Email to blueboat     
1974 john 70 on a 17ft whaler: Took the boat on the bay, 1st test run after self installed head gasket. Boat picked up 7-8 mph. 33-34mph. ran much better but stil had an issue. At 75throttle it ran the same speed as 100%, switched from the 14X17 to the 13 3/4 X 13 did the same thing. engine sounded like it was starving for gas. This is a new boat and engine to me. I have heard carbs are a common issue. any tips? Thanks a bunch!
JBCornwell posted 04-30-2003 12:50 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hmmm. Something isn't right, BB.

Middle 30s is a reasonable top speed for a 17' with 70HP. The prop of choice is usually a 17" pitch. Any diameter 17" pitch prop that fits and doesn't cavitate will do about the same. How did you measure the speed?

What is missing is RPM. It should have been very close to 5500. What was it?

Now, something that is wrong. Are you telling us that with a 13" pitch prop you got 33-34mph? What was the RPM? About 6300-6400 sounds right, but is way too fast to turn that engine.

Throttle control could very well be misadjusted to open throttles 100% at 75% setting. Did RPM change between 75% and 100% on the control?

Let us know.

Red sky at night. . .

blueboat posted 04-30-2003 01:14 PM ET (US)     Profile for blueboat  Send Email to blueboat     
13 3/4 X 13 got in the mid 20's 17 got me in the mid 30's. maybe i should proof read my stuff or people will think i am nuts :)- no rpm change between 75 and 100% throttle just changed the sound a little, my service manual that came with it says 5000rpm is top, the 1975 on had a list of 5200-5500. i am getting a tach but but dont have it yet. I have been boating all my life so have a good idea of what "sounds about right". Could high altitude jets do this?
Dr T posted 04-30-2003 01:25 PM ET (US)     Profile for Dr T  Send Email to Dr T     
High altitude jets will make the engine run leaner. Are you running this at altitude or down on the ocean?
Dr T posted 04-30-2003 01:29 PM ET (US)     Profile for Dr T  Send Email to Dr T     
I should add that if you are at altitude (>5000 ft) the torque curve will drop rapidly at higher RPMs. For me, this just limits the engine speed. For example, my near-antique '82 35 hp Johnson needs a 13" pitch prop at sea level, 11" pitch at 5000 ft, and 9" pitch at 7000 ft.
jimh posted 04-30-2003 01:40 PM ET (US)     Profile for jimh  Send Email to jimh     
It sounds to me like you have some problems with your Johnson outboard.

It is okay to use the work "problem" on this website. I know many people think that current usage requires people to avoid saying "problems" and instead use "issues" but here we are mostly not sensitive new-age guys, and we understand that is something is not working properly there is a problem, not an issue.

Issues are in my mind something that magazine publishers print on a monthly basis, or living beings that come from wombs, as in "I am the issue of the union of my father and mother."

Don't be afraid--use "problem" when you have a problem. You'll feel much better afterwards.

If you have a problem with me saying this, then that would be an issue...

jimh posted 04-30-2003 01:44 PM ET (US)     Profile for jimh  Send Email to jimh     
Also, creating new threads instead of appending follow up questions and commetns to the original one is not recommended. There are now three separate threads on this topic (from what I can tell). It makes following the discussion much harder when comments are spread out across three separate threads.
blueboat posted 04-30-2003 04:16 PM ET (US)     Profile for blueboat  Send Email to blueboat     
Will do. I have boat problems and will reply instead of new threads. Thanks Jim.

P.S is Sal mad? I didnt mean to cause a problem

Sal DiMercurio posted 04-30-2003 12:06 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Why are you running a 14 diameter that belongs on a "V'6, on a 3 cylinder engine ?
Again, why a 13-3/4 diameter that belongs on a v4 or v6 on that 3 cylinder - 70 hp ?
Theres absolutly no need for all that diameter on your 17' boat, unless you feel the engine needs to be stressed & lead to an early grave.
You don't say what your maximum rpms are with either prop.
It's possible your doing damage to that engine doing what your doing.
Sal DiMercurio posted 04-30-2003 12:08 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Don't know how this post got here, as I was answering Blueboats post of 150 psi update, 4/30/03
acassidy posted 04-30-2003 12:23 PM ET (US)     Profile for acassidy  Send Email to acassidy     
Talk about leaving us hanging. Sounds like someone was getting it.
jimh posted 04-30-2003 01:29 PM ET (US)     Profile for jimh  Send Email to jimh     
You hit "NEW THREAD" instead of "REPLY". Either that or a RAM DIMM in the server was struck by a cosmic ray or a neutrino as a result created a new thread, gave it a new topic, and used your name and password to do it.
skred posted 04-30-2003 02:49 PM ET (US)     Profile for skred  Send Email to skred     
Yeah, ya gotta watch those neutrinos: I hear they all use Michigan Wheels....
blueboat posted 04-30-2003 04:04 PM ET (US)     Profile for blueboat  Send Email to blueboat     
Sal, Sorry if it made you mad I was running those props. Seems like your leashing out at me. The reason I am running those props is that I have a manual and that is what it lists. You are the 1st person on several boards that has objected to the prop size. What would you recommend? Have you run a 70 hp 3cyc 1974 model on a 17' to get results from? I am a test engineer and I am always looking at improving things but please donít get hostile or talk down to people. Nobody benefits
Dr T posted 04-30-2003 04:21 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Cosmic rays--I guess our circuit boards are getting a bit dense....How do you distinguish between a random and a RAM DIMM error?
Sal DiMercurio posted 04-30-2003 07:29 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Blue, a 14" dia prop dosen't belong on a 3 cy 70 hp engine, that size is for a v6 engine 150 hp & up.
I'm not getting hostile, just wondering why the huge prop.
The largest dia I recommend for a 70 hp on your boat is 13", even a 12" will give you better performance.
Think of it this way, engineers build 4x4 pickups & figure the correct tire dia, ...then the kids go out & buy rims for an 18 wheeler & put these monster tires on it, the tranny catches hell bigtime & the speedo isn't even close & what used to be first gear, now acts like 3rd gear,.....same pricipal with the huge prop.
Go to a prop shop & borrow a 12 or 13" dia prop with the same pitch your running now, then come back here & let us know if the engine & boat & you are happier.
Sorry if I sounded gruff, didn't mean to, but did raise my eye brows as to why that size.
Try what I said & let us know.
jimh posted 04-30-2003 07:38 PM ET (US)     Profile for jimh  Send Email to jimh     
[Threaded the three threads into one; hope it still makes sense.--jimh]
whalersman posted 04-30-2003 07:51 PM ET (US)     Profile for whalersman  Send Email to whalersman     
I don't know much about props but am learning from Sal and many others here.

I do know that my 1985 70 HP Evinrude (and Johnson) originally came with a 13 1/4 x 17 prop. My Owners manual and parts catalog also gives different options of props to run for different conditions.

I am running this engine on a 1978 Montauk and it gives me acceptable performance. The prop I am using at the moment is a 13 3/4 x 15 prop. This gives me a little better hole shot and a little less top end speed.

A friend of mine has the basically the same boat (1977 Montauk) with a 1980 70 HP Evinrude, running the original 13 1/4 x 17 prop... I beat him out of the hole but he finally catches me after about a mile running.

My engine at WOT is about 5300 to 5400 RPM. I am not sure what his is running. Both of our engines are mounted in the lowest hole. From what people have recommended here, the engine should be raised up 1 hole for better top end performance.

blueboat posted 04-30-2003 08:20 PM ET (US)     Profile for blueboat  Send Email to blueboat     
I am here to learn!!! I just picked up a tach, The back of the tach has a terminal post labeled "signal". where dos it go to?
whalersman posted 04-30-2003 08:25 PM ET (US)     Profile for whalersman  Send Email to whalersman     

The "Gray" wire is the signal for the Tach....

Sal DiMercurio posted 04-30-2003 10:03 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Blue, Whaler is right [ gray wire ], also, is there a dial on the back of the tach, if so, put it on number #6, thats the # for omc engines.
Is it an omc bomb tach?
If not, it might drive you nuts, as i'v had after market tachs just not listen or learn to read, as I said, #6 is omc, but some after market tachs just will not work on a Johnson no matter what number you dial.
Now that gray wire connects to your rectifier, just follow the directions to the "T".
Let us know how many rpms the engine turns at wot, trimmed out, normal load, not empty, also what the highest rpms are recommended by Johnson for that particular engine.
Salmon Tub posted 05-01-2003 05:30 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Blueboat, not to familiar with OMC but would imagine that they must be pretty standard for carbed 2 stroke outboards. Check the synchronization of the engine. Meaning, make sure that the timing, throttle, and carburators are in sync per the manufacturers specifications.
Sal DiMercurio posted 05-01-2003 06:22 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Salmon, every engine made by different companies have their own number.
Had a teleflex absolutly drive me nuts on an omc, finally just got an omc tach & it was fine.
blueboat posted 05-02-2003 10:29 AM ET (US)     Profile for blueboat  Send Email to blueboat     
I got the jets out of the carb, they are in fact the high alt jets. at least one of them is. The guys at the boat shop said I should have the following two jetsbig one is 61D and the small one 33. I have a 61D and a 30. So, in addition to the tach I will have to do the jets before the next test run. Why arenít weekend 5 days and weekdays 2 ??
Whalerdan posted 05-02-2003 10:51 AM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
IMHO this SHOULD have been three different threads.
Salmon Tub posted 05-02-2003 03:51 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Sal, by synchronization, I mean making sure all the lengths of the linkages between the carb(s), the throttle advance, and the timing (mechanical advance) are as per OMC specs. This also includes the open and closed positions of the butterfly in each of the carb(s). But I think he does need to get a tach first.

By the way, Blueboat, you need to check (with the engine off) what the throttle cable is doing inside the engine those last 25%. It may be that you are maxed out at 75% as far as the cable is concerned, you may be comeing up against a set screw or stop. Do this with another person, have them slowly advance the throttle while you look at what is happening under the cowel. Do this while the engine is not running though.

Salmon Tub posted 05-02-2003 03:54 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
And also, careful with the jets, double and triple check which ones you do need. Too small of a jet and you a running lean, that will litterally melt an outboard from inside out.
blueboat posted 05-05-2003 02:05 PM ET (US)     Profile for blueboat  Send Email to blueboat     
The Tach didnt work, gotta get the right one for the Johnson. I have gotten 3 diff answers from 3 diff boat repair shops. Can anyone tell me the correct Jet size for a 1974 Johnson 70hp Sea level jets? I was told a need a 30, 33 or a 34 for the low speed and a 61D for the high speed. I do have the 61D but I have a 30 in the low speed, do I need a 33 or 34?
Thank you.

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