200HP 1980 Johnson Compression?

[Randy W.]

Well..I think I just recorded some bad news. I picked up a motor that had not been run for a number of years. Before spending much time on a tune-up, etc., I took a compression reading on all six cylinders. The readings varied from a low of 75 to a high of 90 on all six cylinders with most residing in the 80 range. Prior to turning the motor over, I put in a solution of 2 cycle oil and marvel mystery oil and let it sit for a day. The motor seemed totally free but obviously seems to have low readings. Before I jump the gun with assumptions and incorrect conclusions (I do that alot!), is there anything else to check? I'm thinking my next move is to pull both heads and snoop around to look for scoring, etc.? I read JB's post on compression and it seems the bigger concern is the "delta" between the readings. 15lbs. is quite a bit between cylinder readings. I did not have the throttle open all the way. Is that a big deal? Any other thoughts? Thanks for your thoughts and advice.

 

[emdsapmgr]

Re: 200HP 1980 Johnson Compression?

In new condition, that engine should run in the 110 lbs compression range with the stock 456/457 heads. Not only are the numbers not even, they are too low. If your compression is that low, you may have bad head gaskets. In any event you need to pull the heads and check the head gaskets. Replace if bad. If they appear to be intact and functioning, you need to start examining the rings and cylinder walls. If you don't want to pull the whole engine apart, you can need to pull the 6 intake side covers off or the exhaust cover in order to have a look at the ringsets. An overheat tends to crack the rings. You may also find the rings carboned up. Extreme carbon accumulation around the rings can affect compression also. If the rings appear in good condition, but carboned up-run some Bombardier Engine Tuner through it a couple of times and see what happens to the compression numbers.

 

[Randy W.]

Re: 200HP 1980 Johnson Compression?

In new condition, that engine should run in the 110 lbs compression range with the stock 456/457 heads. Not only are the numbers not even, they are too low. If your compression is that low, you may have bad head gaskets. In any event you need to pull the heads and check the head gaskets. Replace if bad. If they appear to be intact and functioning, you need to start examining the rings and cylinder walls. If you don't want to pull the whole engine apart, you can need to pull the 6 intake side covers off or the exhaust cover in order to have a look at the ringsets. An overheat tends to crack the rings. You may also find the rings carboned up. Extreme carbon accumulation around the rings can affect compression also. If the rings appear in good condition, but carboned up-run some Bombardier Engine Tuner through it a couple of times and see what happens to the compression numbers.

Thanks for that advice, emdsapmgr! I'm going to follow those procedures and see what happens! A couple of head gaskets and a good cleaning shouldn't be difficult or expensive. I do have a manual to assist me also. Maybe I could get #'s in the low nineties. Not great, but might be adequate? If it'll run decent and pull a skier, no one's gonna look across the lake and yell, "Hey, your compression's down!" LOL

 

[jonesg]

Re: 200HP 1980 Johnson Compression?

Hasn't been run in yrs, unknown service history..

I'd seafoam it and retest compression. Put some directly in the sparkplug holes and let it soak a while.

 

[Randy W.]

Re: 200HP 1980 Johnson Compression?

Thanks jonesg:

Yup...right on both counts. No service history and at least 5yrs. since it has been started.
I will Seafoam now and let it soak and retest. I can then later pull heads and inspect cylinders/head gaskets and then decarb when/if I get it to the lake. Thanks again for your advice!

 

[JB]

Re: 200HP 1980 Johnson Compression?

Not at all unusual for those big V6s to psi in the 80psi range while still in good shape.

The range from 75 to 90 is too big, though. . .over 15%

Decarb her and retest. My money is on a good outcome.

 

[Randy W.]

Re: 200HP 1980 Johnson Compression?

Not at all unusual for those big V6s to psi in the 80psi range while still in good shape.

The range from 75 to 90 is too big, though. . .over 15%

Decarb her and retest. My money is on a good outcome.

Great info...It may be awhile for me to test/decarb since I understand this must be a process done under power in a body of water. Currently the old boat is not in a "launch ready" status. Anyway, I guess you can't decarb in a shop environment in a tank?

Thanks to all who have provided great feedback on a well covered topic. It seems like there's quite a bit of subjectivity on compression matters. Different heads, designs, years, etc....I guess the older motors would run adequately on substantially less compression (i.e.< 100psi).

 

[jonesg]

Re: 200HP 1980 Johnson Compression?

We did mine on the hose.
I watched my mechanic spray seafoam directly on the pistons, let it soak for 20 minutes and fired it up, sprayed some into the carbs and kept it spluttering by blipping the throttle. Lots of carbon came out the prop exhaust and we picked up 10 psi after 2 runs doing that.
Actually he was using an aerosol decarb, but said its similar to seafoam.
Now I have 85 psi all even.

 

[Randy W.]

Re: 200HP 1980 Johnson Compression?

We did mine on the hose.
I watched my mechanic spray seafoam directly on the pistons, let it soak for 20 minutes and fired it up, sprayed some into the carbs and kept it spluttering by blipping the throttle. Lots of carbon came out the prop exhaust and we picked up 10 psi after 2 runs doing that.
Actually he was using an aerosol decarb, but said its similar to seafoam.
Now I have 85 psi all even.

Thanks! I'm going to look around for the aerosol stuff also.

 

[dieselcat]

Re: 200HP 1980 Johnson Compression?

just incase u havnt found the stuff ur looking for its called,DEEPCREEP it is seafoam in an aresol can.

 

[Randy W.]

Re: 200HP 1980 Johnson Compression?

Finally got back to this project! Seafoamed and now all cylinders are at 90psi, with one at 95! Much better than before. Still having a rough idle, but working through that...link and sync next. Thanks for the advice on seafoam from all of you. It's so easy to panic over compression. Hopefully it's high enough to get a smooth idle after other servicing?