Proper way to do compression test on 1976 1150 thunderbolt

[oldguy60]

Hi all.
I'm new here and new to boat repair (although I have done my share of repairs on our vehicles over the years).

My question is: How do I "safely" do a compression test on a newly acquired 1976 Mercury 1150 Thunderbolt?

Having read through all the posts on compression tests I come away with some questions and some things that need clarification.

For example - I have a remote starter switch and one poster said that if you connect each alligator clip of the starter switch to each side of the stater solenoid (the large wires not the small ones) then you don't need to disable the ignition module for fear of damaging it - can anyone confirm this on this motor and ignition set up?

Secondly - if I do have to disable the ignition module what does it look like and how do I do that?

Thirdly - do I have to ground the wires and/or plugs to the motor frame or just leaving them in the end of the spark plug wires will suffice (after they have been removed from the cylinder head of course)?

Won't the sparking plugs ignite the fuel mixture coming out of the open spark plugs holes when cranking the engine if they are just dangling there sparking?

Finally I read both methods of testing for compression (cold and hot method) are fine - what is the most accurate method? I have the plugs out already and the engine was run a few days ago with the ears - will that be fine to just go ahead and do the test or do I have to put the plugs back in and warm it up first (besides it was a bit of a chore to get the compression gauge threaded and seated properly in the bottom #6 cylinder because of the cowling) and I have not cranked it over yet until I get some questions confirmed - I can' afford to do damage to this engine.

I started quickly and ran fine with the ears on it and idled fine but I have read that these in line 6 cyl. engine can seam to run fine when only running on 5 cyl.
Any help would be much appreciated! If the compression checks out fine then I will be moving on to the impeller and water pump service/change.

 

[emckelvy]

Re: Proper way to do compression test on 1976 1150 thunderbolt

You'll probably get accurate enough readings although IMHO more accurate when done warm. But I've done plenty cold readings and if you find a compression issue cold it likely ain't gonna get better warm!

One thing, don't know what kind of remote starting switch you have but it'll have to have Big Honkin' Wires if you are able to connect it across the main battery connections on the starter solenoid! Hundreds of amps going thru there and if you have the wrong switch all it's gonna do is burn up!

I suspect your remote switch is probably the type that's meant to supply control voltage to the solenoid and hence wouldn't need to carry many amps. If it has regular, small-sized alligator clips/wiring this is what you've got.

To use this type of switch, connect one lead to the "hot" (Battery) side of the starter solenoid (or connect directly to the Pos terminal of the battery.

Find the terminal on the solenoid with yellow wire connected. This is where you'll hook up the 2nd wire from your remote starter switch.

Note on the ign: you need 12V to both red and white wires on the Stbd side of the CD ign box to get spark (I'm assuming you have a distributor model). If the key is off, you ain't gonna get spark, you won't have to worry about damaging the ign, and you can do anything you like with the spark plugs/wires.

When accomplishing the compression check, be sure the throttle is advanced to fully open the carb butterflies. You need to pump a lot of air thru the motor in order to get valid compression readings. This won't happen with the carbs shut.

Crank the engine over until the compression gage needle stops moving. That's your highest reading.

Repeat until all are done. If compression readings are within 5 psi of each other, you probably have a very good motor. More than 10 psi I'd start worrying!

Or do a decarbonizing treatment followed by another compression check. Read all about decarbonizing with Seafoam in the maintenance FAQ section of the motor repair forums.

HTH & let us know how it works out.........ed

 

[JB]

Re: Proper way to do compression test on 1976 1150 thunderbolt

Check out the compression test sticky at the top of the JohnnyRude Forum.

 

[oldguy60]

Re: Proper way to do compression test on 1976 1150 thunderbolt

Yes I have the type of starter switch you describe and I do have a distributor.
So when you say to connect one clip to the red wire on the solenoid you mean the large wire (as that is the only red wire on the solenoid) and the other end to the yellow wire (of which I have 2 yellow wires) One of which is the big honkin wire so the other end must go to the "small" yellow wire??
Is the CD igition box the one that is mounted on the front of the engine across from the carbs? Mine has four wire on one and three on the other. The side with four has a red wire and a beige wire that have a red and white paint swatch on the connector block it self so I assume that is the red and white wires you are referring to?
So what you are saying is as long as the key is in the off position there wont be any power to the ignition and no concern of damaging anything ignition wise??
By the way what does the CD in the ignition box stand for?

 

[oldguy60]

Re: Proper way to do compression test on 1976 1150 thunderbolt

Also - I was wondering if there was a big difference between the Seafoam product and say the Evinrude version of decarbonizer which I have used when I owned a 1976 90HP rude?

 

[emckelvy]

Re: Proper way to do compression test on 1976 1150 thunderbolt

CD="Capacitance Discharge". The spark energy is stored in a capacitor and the sudden release of this stored energy causes a very hi-output spark to the plugs.

Yeah, I forgot the wire going to the starter is yellow, too (just a bit bigger than the other one!). You want to supply +12V to energize the solenoid, so connect your starter switch to the small terminal with yellow wire attached.

Here's a link to wiring diag for your motor, this makes it all very clear how the stuff works:

http://www.maxrules.com/oldmercs/Wiring/1966ona/50.jpg

The red & white connections which are powered up for sparking are on the Stbd side of the CDI box. There is always battery power on the red terminal of the box. When you turn the ign key to "ON", the white terminal is powered-up as well.

The 3 connections on the other side of the box are for the distributor and should never have power applied to them.

If the key is off there's no way to get spark. As long as your motor stops when you turn the key off, you're OK. It doesn't hurt to stick the spark plugs back in the wires, and lay against a metal part of the engine. So, if you feel like doing that, go for it. I just leave the key in "OFF" and have never had issues with that.

Don't know if there's a Big diff with OMC Engine Tuner and Seafoam, I've used both and they do a good job. I think the OMC stuff is a bit more of an "extreme" cleaner, the Seafoam a bit gentler but still very effective. Haven't priced a can of Engine Tuner lately but I expect it's not gettin' any cheaper. But then again, Seafoam used to be $4.99 or less on sale and last time I checked Wal-Mart it was more than $7 a can! Oh, the Humanities!!!!

At any rate, if I had my Druthers I'd use Seafoam, but if I ran out and had a can of Engine Tuner handy, I wouldn't make a special trip for more Seafoam.

HTH........ed

 

[oldguy60]

Re: Proper way to do compression test on 1976 1150 thunderbolt

Well what I ended up doing is removing the red wire and the beige wire from CD ign. box it self at the connection block (the ones that have the connection block itself colored red and white). I didn't get any spark crackling from the leads so I guess all is well. I opened the throttle all the way and here are the readings done cold:
#1 - 140 psi
#2 - 155 "
#3 - 155 "
#4 - 155 "
#5 - 155 "
#6 - 150 psi
I though maybe a decarbonize job might bring #1 cylinder up a bit??
- a local mechanic said I could also try the stuff you spray into the carb and/or the spark plug holes and you let it sit for a while and it melts all the carbon then fire it up and it blows it out?
- either way it's a good sign from these compression readings that this motor might have some life in it yet and now I thought I would move on to changing the impeller, a kit in the water pump, a kit in the fuel pump, and change the timing belt. The rotor and cap look fine, and the plugs had a bit of carbon on three of them but no metal shavings or such (although I will also go with new plugs).
Any other areas I should be concerned with?
thanks for all your help!

 

[Chris1956]

Re: Proper way to do compression test on 1976 1150 thunderbolt

A couple of notes. The rotor is non-removeable. It is replaced as a unit with the dissy shaft for $200+. The cap is $100+. The dissy trigger is $200+, so make sure you have good spark, before spending $ on other stuff.

Make sure the carbs are clean, and the ignition timing is spot on, with a max spark advance of 21* BTDC. Use a 50::1 gas-oil mix. Adjust your idle mixture screws in the water, with fresh fuel.

 

[oldguy60]

Re: Proper way to do compression test on 1976 1150 thunderbolt

Well I don't have a timing light or spark tester so I talked to the local yamaha dealer and he checked my spark and timing and only charged me 1/2 hr. labor. He said the spark is really strong from one end to the other and the timing is spot on. The carbs look very clean and it idles fine on the ears. The previous owner replaced the rotor at some time recently and I cleaned up the rotor and contacts in the cap. (good thing as It appears the rotor is no longer available from Mercury and it was very expensive). I changed the impeller and shaft seals and the old impeller was in great shape for being in service since 1976. I was thinking of just buying a fuel pump kit and having it on board so I could change it if something failed as the engine seems to run fine now (but I haven't tried it in the water yet)?
I have a few more questions please:
1) - I got some decarbonizer carb cleaner from the local Merc dealer and they say it's what they use to clean out the carbon by spraying in the carbs about 2/3 of the can while idling at 1200rpm and then increase the idle to 1600-2000rpm and stall engine with remaining 1/3 of can and then let sit for 15 min. or longer and then run engine at full throttle for 5 min. This would have to be done on the water or in a tank of course but if on the water it would be kind of difficult to change/clean spark plugs if they become fouled by this. My question is: wouldn't it be harmul to the engine (because wouldn't this cleaner clean the protective oil from the fuel mixture from the cylinders as well as the carbon)? The name of the product is: Mercury power tune engine cleaner.
2)- can anyone tell me the rpm operating range of this 1976 1150 engine as I have two extra props for it that came with the boat and I want to try them out but I don't know the correct rpm operating range.

Sorry for being long winded!