Am I OK - compression reading

[bassa9]

I just bought a 1991 70hp johnson that seems like a good deal, but after looking at the readings I am not sure if I have a problem. The readings are 100, 99 & 101 and my concern is that they are too low. Is this range OK?
Thanks in advance for all replies.

 

[kenmyfam]

Re: Am I OK - compression reading

100 ish and even within 10% of each other should be O.K. Compression testers vary so a different one may well read in the 115psi range.
If you get the motor give it a decarb and check again for improvement.

 

[kenmyfam]

Re: Am I OK - compression reading

By the way, welcome to iboats. Great place to be for all things boating and much more.

 

[stylesabu]

Re: Am I OK - compression reading

won't decarb lower compression? unless rings are sticking.but readings are close enough together,engine is wearing evenly. how does it run?

 

[1946Zephyr]

Re: Am I OK - compression reading

No, decarb will clean things out and get things working better. Your compression readings are pretty good and close to each other. That is the main thing. I think you'll be okay with this motor.

 

[kenmyfam]

Re: Am I OK - compression reading

won't decarb lower compression? unless rings are sticking.but readings are close enough together,engine is wearing evenly. how does it run?

Decarb is routine maintenance. Compression may improve after one is done. It was merely a suggestion to the OP

 

[paultjohnson]

Re: Am I OK - compression reading

Was engine warm, cold ? Did you try it cold then warm to see what the diff was ? Did you hold the throttle wide open, This can increase your readings.. Did you have all the plugs out or just the cyl you were working on ? Did you crank the engine over till the gauge stopped moving. Sometimes 6 or 7 revs at least. And as others have stated, its good that there is uniformity among readings. and that the gauge might be innacurate. Also, you could put a teaspoon or two of oil in the spark plug hole, turn the engine over a few times to coat the cylinder walls and retest, If readings go WAY up it can indicate cyl/ring wear as the oil temporarily creates a better seal.

 

[eavega]

Re: Am I OK - compression reading

You should be good to go given the variations introduced by the gauge itself, your technique, etc. I have a '76 70 HP, and the compression readings were 105-110 on all three cylinders. After a decarb it really didn't go up that much, maybe more evenly 110 across all three cylinders. Decarb definitely blew out a lot of black sludge which I assume was carbon buildup on the cylinders/pistons/etc. The real test is to put it on the water, properly set up for your boat, and see how it performs. If you can get the motor turning at the right RPMs and it puts out adequate power, then you are good.

Rgds

 

[bassa9]

Re: Am I OK - compression reading

I feel better already, thanks for all the help. I bought the motor last week and plan to install at the end of the month. I am having fiberglass work performed on my 1965 glasspar and she will look real good with the more powerful motor. I'll post a photo after all is complete with an update on the Johnson.

Best to all!

 

[boobie]

Re: Am I OK - compression reading

The big thing about a decarb is it will free up your bottom rings on the pistons which will give you a better idle.

 

[bassa9]

Re: Am I OK - compression reading

Update & still need a little help!

Well I know more than I did about motors a few months past, but am still in the learning stage.

Changes:
1. Installed tach on boat to measure outboard performance
2. Changes plugs to champions and gapped for high altitude - 35
3. Changes gear case oil
4. Sea foamed carbs
5. Performed new compression check, 121, 120, 120 I think the numbers I got from the dealer were a guess
6. Water tested multiple times
7. Changes prop to 14 x 13
8. Now run only 91 octane gas
9. Checked timing and it is at factory specs

Remaining issues:
1. Max wot rpm's 5,050 ish
2. Dies when I troll
3. Max speed is 24/25 mph checed by gps

Any suggestions for next steps?

Thanks for all the past recommendation

 

[levi_tsk]

Re: Am I OK - compression reading

time to rebuild your carbs ... get a can of dip and three carb kits your WOT rpm is a little on the low side but that could just be it not running right the fact that it dies at trolling speed tells me your low speed and midrage circuits in your carbs are probably gummed up while your at it and as a preventative maintence you might wanna rebuild your fuel pump too kits are only 15bux from BRP on you carb kits DONT SCRIMP go ahead and get the BRP ones also

 

[bassa9]

Re: Am I OK - compression reading

Thanks Levi, This will be my 1st rebuild effort and I am looking forward to the task, I hope thats not folly on my part. I can't find the BRP kits for 15.00, but did see them for 32.00. I will also rebuild the fuel pump, but only after I ake sure the card rebuild is OK.

I'll post update after the rebuild. Thanks for the help!

 

[Rick.]

Re: Am I OK - compression reading

Perhaps the seafoam ungummed your rings and that is why the compression numbers are up. I suggest you take a few pictures of your carb. linkages etc. before you pull them. It can be most helpful if your memory is short like mine. Best of luck. Rick.

 

[bang4dabuck]

Don't fight it. Buy the ORIFICE TOOL.

Don't fight it. Buy the ORIFICE TOOL.

It is a rip off but it ain't worth fighting ... take it from me.

 

[levi_tsk]

Re: Am I OK - compression reading

Thanks Levi, This will be my 1st rebuild effort and I am looking forward to the task, I hope thats not folly on my part. I can't find the BRP kits for 15.00, but did see them for 32.00. I will also rebuild the fuel pump, but only after I ake sure the card rebuild is OK.

I'll post update after the rebuild. Thanks for the help!

thats too high check ebay

 

[Monte1961]

Re: Am I OK - compression reading

Parts is Parts! The question is, How fast do you want them?

 

[bassa9]

Re: Am I OK - compression reading

Well, the challenge continues. I supplied the rebuild kits and had an outoard mechanic rebuild the carbs, but the outcome was not what I hoped for. I picked up one mph and about 100 rp's.
I now get 5,200 rpm's wot. The motor will still not troll and will die if put in neutral after running.

Any ideas on what I should try next?

 

[wilde1j]

Re: Am I OK - compression reading

You mentioned earlier "high altitude" ... how high? Did motor come from sea level or close to your operating altitude? If motor came from sea level and you're at 2500'MSL or more, the carbs need to be rejetted. See OEM shop manual for jet details.

BTW, you mentioned gapping spark plugs for altitude ... I've never heard of this one before. I spent lots of time in Santa Fe at ~ 7500 ' MSL.

 

[bassa9]

Re: Am I OK - compression reading

Jim, I bought the motor from a dealer that took it in on trade from a Colorado boat. He purchased
the motor new. I boat at 6,000' altitude average and it was recommended to gap the plugs at 35.
I will look into rejetting the carbs and see if I can get any history of changes/adjustments from the past owner.

Thanks for the help

Morry