Compression check method

[shadowrider]

I aquired a Suzuki 1987' DT85, and want to do a compression check. Do I need to ground coils or anything before doing comp. check ?

 

[rodbolt]

Re: Compression check method

yes,either ground them,pull the stop lanyard or make a spark tester so you can test spark while testing compression.
your motor isnt as dangerous as later models but its imperative to learn safty.

 

[JB]

Re: Compression check method

Howdy, Shadowrider.

Welcome to iboats.

*First, if possible, warm the engine up.
*Then remove the spark plugs.
*Make a small jumper to activate the starter solenoid without turning on the ignition.
* Ensure that the battery is fully charged.

1. Screw the compression guage into the first cylinder.
2. Jump the starter solenoid and turn the engine over about 5 times.
3. Record the reading.

Repeat 1-3 for each cylinder.

All cylinder measurements should fall within a 10% span.

Don't get paranoid about psi. If the cylinders all fit into the 10% you have a healthy engine.

There are about a bazillion variations on this procedure. They all work. There is no benefit to arguing over minor details.

 

[rodbolt]

Re: Compression check method

most old two strokes ,like the one in question DO NOT use any type of automatic fuel enrichment at cold start.
however anytime the flywheel is spun without disabling the ign OR controling where the spark occurs can lead to a stochiometric air fuel ratio being expelled from the spark plug holes.
on EFI and other carbed engines that use an auto enrichment,this can lead to FIRE.
not good.
best to get in the habit of controlling spark while testing compression.
that and it gives an indication of how the ign system is working.
most modern ADI or CDI systems require a 1 half to 7 sixteents gap to insure spark under compression.
like JB says, the methods vary however I trust nothing anymore and when fire may be involved even less.
google, boat fire colington harbor.
happened about 300 yds down the canal from my dock.
not only do I know that boats owner I knew the previous 4 or so owners and would not work on it .

 

[joey maneri]

Re: Compression check method

Im not sure about outboards but some car manufactures say to open the thottle to ensure a correct compresion reading. I would try it both ways just to see if theres a differance

 

[shadowrider]

Re: Compression check method

Thanks for all the replies, I dont think it is necessary to open up the throttle on 2 strokes, only 4 strokers. May get to it today if I can pull myself away from the fireplace !

 

[shadowrider]

Re: Compression check method

One more thing, this motor has old gas in the carbs, ( I can smell it), Should I pull the carbs first, or can I drain them without removing. I dont have a manual yet, as I have had the motor 1 week. This forum is so cool !

 

[steelhead2000]

Re: Compression check method

Hi Ancient Mariner, you seem to be very knowledgeable. I also have an older suzuki DT140 from the same era. I've been wanting to test compression as well on it and I notice you mentioned warming up the engine and then removing the spark plugs. I also read somewhere that people advised against removing the spark plugs after the engine is hot because it could strip the threads? Could you elaborate on whether this is a valid concern? Thanks

Howdy, Shadowrider.

Welcome to iboats.

*First, if possible, warm the engine up.
*Then remove the spark plugs.
*Make a small jumper to activate the starter solenoid without turning on the ignition.
* Ensure that the battery is fully charged.

1. Screw the compression guage into the first cylinder.
2. Jump the starter solenoid and turn the engine over about 5 times.
3. Record the reading.

Repeat 1-3 for each cylinder.

All cylinder measurements should fall within a 10% span.

Don't get paranoid about psi. If the cylinders all fit into the 10% you have a healthy engine.

There are about a bazillion variations on this procedure. They all work. There is no benefit to arguing over minor details.

 

[99yam40]

Re: Compression check method

In my opinion there is a difference in warmed up and a hot motor, so maybe that is what they were talking about. A warmed up motor just gets the cylinders,pistons and rings all up to a normal temp so that the rings will seal better and everything is lubed properly while checking the compression.
Opening the butterflies will allow the motor to suck air in to compress and will allow for a higher and better reading quicker, but you need to do it the same on all cylinders

 

[JB]

Re: Compression check method

I have found no difference. . .none. . .when opening the throttle or leaving it closed on 2 stroke engines.

Not only does it require at least three hands, it increases the amount of fuel that gets into the cylinders and gets blown out.

Leave the throttle alone.

Ooops. My bad. I said it wasn't worth arguing over details.

 

[JustJason]

Re: Compression check method

I agree with JB on the throttle position, it doesn't make a difference. I also say that temperature makes no difference either. If you have great compression on 3 cylinders and the 4th is dead because of rings/headgasket etc, temp doesn't make a difference. Also if your compression is low but even across the board (say 70psi). Warming it up isn't going to raise your compression any more than 10psi.

I also fully agree with rodbolt for his reasons and another 1 that i'll add. Compression testing is part of diagnostics. Checking for proper spark is also part of diagnostics. You can kill 2 birds with 1 stone if you use spark boards when doing a compression test.

I have a couple sets of the Stevens Instruments 4 wire boards that I use.