How to check compression?

[Jim Bronson]

What is the quickest/easiest/cheapest way to check compression on a TC?

[hogdoctor]

remove spark plugs and install compression tester in one cylinder, open throttle fully (and lift slide if it's a CV) and crank it ten  revolutions (or until the tester isn't picking up pressure any more).  Note pressure and  repeat for rear cylinder.

[Tsani]

Don't forget to remove the air filter too.

[Jim Bronson]

Thanks. About what should the readings be for a 95" with SE heads, 030 Cometic head gaskets? How much difference is normal from one cylinder to the other?

[Ohio HD]

About + / - 10% is what's considered acceptable as far as variance.

To figure your approximate cranking pressure, go here http://www.bigboyzcycles.com/TwinCamComp.htm and build your motor with all of your specs. Should be close to what this tells you.

[Jim Bronson]

Wow, that's a lot of info I don't have, so I just left the default values in there except for the 203 cam and 95" displacement and cometic .030 gasket. It also has SE Performance heads, if that makes a difference. It works out to 9.35:1 for the combo and 178.5 for the comp. Does this sound about right? Should I check it both cold and hot?

Thanks!

[Tsani]

Check cold and hot. That will tell you how it is sealing up. If there is a big difference in the reading between cylinders, first check your valve adjustments then try again. Then move on to putting a wee bit of oil in the cylinder and check again. I have found the online calculator to be closebut remember it is only a ball park figure. Your location also plays into it. I am running 185 measured in both cylinders at 68' asl with a build that close to yours. The calculator had me at 183.

Oh, and make sure you have a fully charged battery. Makes a difference too.

[sfmichael]

remove spark plugs and install compression tester in one cylinder, open throttle fully (and lift slide if it's a CV) and crank it ten  revolutions (or until the tester isn't picking up pressure any more).  Note pressure and  repeat for rear cylinder.

Ten seems a bit much, I usually go 4-5, but I'm usually checking more cylinders

[Jim Bronson]

Will any auto tester work?

When I entered my altitude (429'), I get 8.75/176.4.

Thanks.

[Ohio HD]

I assume you mean a compression tester for a car? Not something automatic or?

Yes, a compression tester will work for any engine, as long as it has the range on the gauge that you need.

[Tsani]

Yeah, a car one will work fine. Last one I bought I got at Pep Boys and It it has indicated very well and repeatable when I cheaked it against a racing buddy's gage. Cost was like 39.00. Just be sure it has the adapter for your spark plug size. Not all do. 12mm X 1.25mm.

[Jim Bronson]

OK great. I'm off to Autozone. Thanks all!

[Ohio HD]

Yeah, a car one will work fine. Last one I bought I got at Pep Boys and It it has indicated very well and repeatable when I cheaked it against a racing buddy's gage. Cost was like 39.00. Just be sure it has the adapter for your spark plug size. Not all do. 12mm X 1.25mm.

I put a quick disconnect shop air line coupler on my testers hose, and then I weld an air fitting in the appropriate spark plug body that I gutted of the porcelain. Then use an o-ring on the spark plug threads. Works great, and you can get to tight areas easier to thread the fitting in.

[sfmichael]

I put a quick disconnect shop air line coupler on my testers hose, and then I weld an air fitting in the appropriate spark plug body that I gutted of the porcelain. Then use an o-ring on the spark plug threads. Works great, and you can get to tight areas easier to thread the fitting in.

Good idea.

I recently bought a pretty nice OTC m/cycle one of eBay for less than $50 and it had all the weird (to me:) smaller sizes and it still works on cars as well.

[Jim Bronson]

Yeah, a car one will work fine. Last one I bought I got at Pep Boys and It it has indicated very well and repeatable when I cheaked it against a racing buddy's gage. Cost was like 39.00. Just be sure it has the adapter for your spark plug size. Not all do. 12mm X 1.25mm.

I put a quick disconnect shop air line coupler on my testers hose, and then I weld an air fitting in the appropriate spark plug body that I gutted of the porcelain. Then use an o-ring on the spark plug threads. Works great, and you can get to tight areas easier to thread the fitting in.

Only one problem with that. I have no welder and no skills 

[14Frisco]

Sears sells a kit as well: http://www.sears.com/craftsman-compression-test-kit/p-00947089000P

[skunk]

14Frisco
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Posts: 706
 

Re: How to check compression?
« Reply #15 on: Today at 09:15:47 AM »
Quote
Sears sells a kit as well: http://www.sears.com/craftsman-compression-test-kit/p-00947089000P
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I bought the same one at Sears a couple of years ago and it works very well. I took it to work this year and had the tech's in the instrument calibration lab check the calibration on it ( cost= 1 breakfast buritto ) found it to be within 1psi at midscale and within 2 at low and high ends of the scale. I would say it's worth the money.

[Jim Bronson]

OP Update:

I picked one up from O'Reilley this morning (I like Sears but they're quite far from here).

Readings are: Front: 183, Rear 190.

Can I just run it in the garage for a few minutes for the hot check or does it need to be up to full operating temperature?
Also, when I removed the air filter, a couple of drops of oil dripped down on the cam cover. It was between the rubber filter seal and the filter mounting flange. I know the breather deposits a little oil into the intake, but should there be this much? I'm always careful not to overfill the crankcase when I change, so that's not the problem.

If you haven't guessed, I'm troubleshooting an oil consumption problem. It's been an educational experience so far. Never did this before.

[Ohio HD]

I would ride it until its at operating temp.

It's pretty normal to see some oil in there, some bikes get more than others. If you run one pretty hard, and the oil is just at the full line, it tends to suck some into the air breather. I vented my motor to the underside of the bike, like a lot of guys do, and no more oil in the air breather, and my bike mysteriously stopped using any oil too!   

[Jim Bronson]

I would ride it until its at operating temp.

It's pretty normal to see some oil in there, some bikes get more than others. If you run one pretty hard, and the oil is just at the full line, it tends to suck some into the air breather. I vented my motor to the underside of the bike, like a lot of guys do, and no more oil in the air breather, and my bike mysteriously stopped using any oil too!   

Is there some kind of kit for that? Wouldn't the oil consumption still be the same, except it is venting to the ground instead of through the intake? Or is there enough vacuum in the intake throat to pull some of it out of the heads?

[Ohio HD]

I would ride it until its at operating temp.

It's pretty normal to see some oil in there, some bikes get more than others. If you run one pretty hard, and the oil is just at the full line, it tends to suck some into the air breather. I vented my motor to the underside of the bike, like a lot of guys do, and no more oil in the air breather, and my bike mysteriously stopped using any oil too!   

Is there some kind of kit for that? Wouldn't the oil consumption still be the same, except it is venting to the ground instead of through the intake? Or is there enough vacuum in the intake throat to pull some of it out of the heads?

I believe that to be the case, the suction of the intake while running pulls the oil out at times. Now if you were over full on your oil, it might still come out the hose under the bike, but if the oil level is correct, and the motor is in good mechanical shape, you shouldn't see any oil, or just a drop or two. Mine quit loosing any oil once I vented it that way.

There may be a kit that does that, I didn't bother looking, I just did it myself. Here's a couple of photos.

Also here's a write up from Wurk_Truk on how he did it. Mine is pretty much the same.
http://harleytechtalk.com/htt/index.php/topic,50630.msg531934.html#msg531934


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[Jim Bronson]

Thanks. Looks like a nice rainy day project (if we ever have one).

Here's what mine looks like. It has the rubber thingys that press onto a fitting and route into the throat. I think I can just fill the half moon channel with RTV and find some hose that will press onto the fitting and route under the bike. What am I missing?



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[Tsani]

Your within 5% difference between front to rear. Heat er up and check it and see what ya got. How did your plugs look? What year and model bike?

[Jim Bronson]

Hot compression is: 200 front, 205 rear. The only thing left to do is check again when cold while watching the gauge to see how long it takes to pump up.
Bike is an 05 Dyna with 95" SE kit, SE Performance heads, and 203 cam.

The plugs look pretty good. There's some black residue on the insulator of the rear plug that's not on the front. Looks more like carbon than oil to me.



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[Ohio HD]

Thanks. Looks like a nice rainy day project (if we ever have one).

Here's what mine looks like. It has the rubber thingys that press onto a fitting and route into the throat. I think I can just fill the half moon channel with RTV and find some hose that will press onto the fitting and route under the bike. What am I missing?

That sounds like it would work fine. What I might do instead of the RTV, is see if you can find a piece of rubber or something that you can RTV in place where your hose goes now. Then the air filter will help hold and seal it in place.

As long as you can get a tight fit with the hose on those fittings, and maybe lightly clamp them, you should be good.