87 johnson 150 compression low

[rickdog67emtp]

another question about this problematic O/B,. Hot, this motor has 75 psi of compression, cold is 85 or 90. Is this ample?

 

[moderator1]

Re: 87 johnson 150 compression low

Are they all prtty close or do they vary alot from cyl. to cyl..

 

[Walker]

Re: 87 johnson 150 compression low

Try another test with a different guage. 75 is pretty low. If the motor hard too start?

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

the cylinders are all within ten easy to start. Gauge has been checked against another

 

[Walker]

Re: 87 johnson 150 compression low

If it starts easy you have enought compression.

 

[Dhadley]

Re: 87 johnson 150 compression low

The actual number has a lot to do with which heads are on it and which head gaskets. Thats why OMC never gave an actual number and just said to look for no more than 15 lbs difference. <br /><br />It also has a lot to do with how coked up the rings are. If the set up allows a top rpm in the low 5000 range, theres probably some stuck rings. Especially if you havent decarbed in a while. When did you decarb last?

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

I just bought it havent done a decarb/ I put new impeller assembly, head gasket, thermostats, and changed the l/u oil. Its weak on the top end after heating up, but I hav e 2 bad coils. One has a broke tower, the other is arcing thru the tower to a ground (just found this out). What do you use for decarb?

 

[Dhadley]

Re: 87 johnson 150 compression low

I use Engine Tuner. You can get it at any Evinrude / Johnson dealer. Follow the instructions on the can. Do not let it sit in the motor longer than they say.

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

i only got 3800 rpm btw

 

[moderator1]

Re: 87 johnson 150 compression low

OUCH!!!!!!<br />Maybe with 2 coils down, it would be that low...<br />When you get all taken care of, I think we need to be in the 5500-5700RPM range atleast....maybe a bit higher..

 

[dajohnson53]

Re: 87 johnson 150 compression low

FWIW, my early 80s Johnson V6 has compression in the low-mid 90s (even, +/- 5 or so). It really runs great, easy to start (once I learned - and adhered to EVERY time - the exact method of steps and incantations), and runs strong at factory recommended max RPMs. My mechanic who is pretty good told me this is close to normal, new compression for this vintage/type of engine. He is quite familiar with the engine having taken care of it since it's birth as a re-man in the late 90s, to present.<br /><br />I'd be interested to hear what your specs and performance are after the decarb and replacement of coils. BTW: there's a decarb FAQ on this site which gives you a couple options for the procedure. Good luck -please post back.

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

thanks for the reply. I plan on picking up some decarb tomorrow and trying it. My two coils should be here friday. I know I have one cylinder that is a little weaker (between 65 and 70) instead of 75 b/c of a slight score in the wall from a blown headgasket. But, I figured its still within limits. When I first got the boat, it would really fly, this sudden drop in RPM is pretty recent. Hope the coils are to blame.Ill let y'all know what happens after the decarb.

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

Update on motor . I havent decarbed yet, but have get the spray from the dealer. I ordered the coils for the motor and low and behold, theyre not the right one! 18-5179 is the part # but that has two snap on towers. The ones I have one snap on tower for the spark plug wire, the pos and neg come out of the back. So I researched again, come to find out the welch plug on the port side of the top of the motor has J185TXCRS which translates to a 1984 185hp. Buth that should still take the two tower coil instead of the single tower with the wires coming out of the back. The number on the transom bracket is J150TXCOS (85 150hp). I m assuming the two tower coils need to be sent back and the other ones purchased instead. Any thoughts?

 

[rickdog67emtp]

Re: 87 johnson 150 compression low

BTW those part # are sierra

 

[dajohnson53]

Re: 87 johnson 150 compression low

Rick - I'm guessing this is a rebuild or remanufactured engine? I have a reman. engine (has a large sticker from a remanufacturing company.) The cowl says "175 VRO, it has no transom bracket, and the engine model number translates into an 82 235 HP. My understanding is that in '82, VRO was not even offered?? It does have a VRO fuel pump, but I pre-mix. <br /><br />The same mechanic who I use also did the work on this engine for previous owner and he said that until he figured this out, it was a bear finding parts. Now it's no big deal. My understanding is that the mechanical parts and carb correspond to the '82 235, but the ignition parts, and the VRO fuel pump, are different.<br /><br />Based on my/our experience, you might need to order parts based on what's on the engine rather than what either the transom bracket s/n or engine s/n call for.<br /><br />Good news is that inspite of all of this, the engine runs perfectly nowadays.