Can compression be tested without power on the engine

Hi all,

I have a spare engine that I want to swap into my '85 Maxim X. However, I'd like to test the compression. Is there any way to test compression without power? Or, at least without a starter switch? I have a good battery. Starter is good. Just nothing connected on the front end.

Incidentally, I did try cranking it by hand. Could only read 30 psi... I'm really hoping that is because trying to turn the crank with a wrench won't build pressure. Not sure though.

TIA

 

You can Jason. Just hook a battery directly to the starter to turn the engine.

 

yep. as long as you got a starter and battery hookedup. negative to some ground on the engine. and all plugs pulled.

and if the carbs are mounted. you want wide open throttle.

 

Hmm... Ok. So, I looked at the starter, and it looks like there is only one wire connected to it. That wire runs to a solenoid, the other side of which goes to the + side of the battery. Do I just take + directly to the connector on the starter, and bypass the solenoid?

 

I'd put a switch of some kind in there some where, otherwise as soon as you hook it to the battery the motor will start spinning. A toggle switch at least, that way you can have everything hooked up, flip the toggle for several revolutions and then flip the toggle off, give you some control over the operation.

 

positive to the starter terminal. negative to the engine anywhere. and yes. have a second person. you don't want that engine flipping around on you.

 

Thanks! I'll give it a shot tomorrow.

 

Damn! So you have to have the throttle open, all the plugs pulled and then the starter turned over?

No wonder I wasn't getting readings worth a crap when I was doing it.

8O

 

with the plugs pulled, the engine cranks over easier. wide open throttle allows more air into the cylinders. you won't get full compression readings with closed throttles.

the correct procedure would be to have a warm engine. since heat expands metal. the pistons make a better seal when warm. but i don't know how much difference there would be between hot and cold. since you don't have the engine mounted.

keep us posted on the readings.

 

Don't know if it will make a difference on a stone cold motor, but after you get your first readings- make sure to write them down, don't try to remember them- put a teaspoon of oil in each plug hole you are testing and write down that number. The difference, if significant can tell you if you have ring problems or valve problems. How well this will work on a cold motor though, I'm not sure. Most compression tests are done on a warm motor like snow said.

 

The most important thing is the comparative readings. I always do a compression test on ANY bike/motor I buy, and cold is better than no test at all.

My "project" 650 Seca wouldn't run but blew 150+ across the board, cold, one of the reasons I bought it.

By all means do a compression test; and don't fret if they might be down by 10%-20% You can still do a "wet" test although it wouldn't be as conclusive.

 

it be wise to check out BEFORE mounting. you don't to waste your time mounting a weak engine.

 

you need a leak down test
http://www.dietersmotorsports.com/Compr ... wntest.htm