What is the proper way to do a compression test on my flathead.
Never done one before and I want to do right the first time.

The instructions on the sheet said with all the plugs out. I never read that
before so now i'm confused.

Thanks.

 

Joe -

I'm not sure what the "proper" way to do anything is...but, I can tell you how I do a compression test:

I use a cardboard box bottom and cross-cut 8 holes in it and label the holes #1 thru #8. I pull all the plugs out, taking care to put them in the box hole for the cylinder they came out of.

I then screw my compression tester into #1 cylinder and reset the tester to zero. I crank the engine over (at least 4-6 times). Check the tester dial and write down that number next to the plug for that cylinder. Go to the next cylinder and repeat. (If all your readings are identical, you didn't reset the compression tester to zero...don't ask how I know that).

Once I have all the readings, I take the box bottom with plugs and compression notes over to a good light and magnifyer I have to inspect the plugs to tell me how the cylinders are firing: black = too rich; white = too lean; tan = good; greasy black (wet) = oil leaking into the chamber (valve stem seals or rings); no color usually means that I have a head gasket leak and the plug is getting steam cleaned.

I look for the compression readings to be within 10% of each other. If I have one or more cylinders suspiciously low, I squirt a slug of oil into the spark plug hole and compression test it again. If the reading is higher, the rings are getting tired. If the reading basically doen't change, a valve is leaking (usually the exhaust).

I then throw a new set of properly gapped plugs back in and have a frosty one.

 

Do I have do anything with the plug cable connects to the center plug on the distributor that comes from the coil???

 

Joe -

Plug (and coil) wires are a completely different issue. There's no connection to compression testing.

Are you also trying to do a tune up? Or are you hijacking your own thread?

I don't mess with wires until and unless the engine starts missing. Then I check the wires out as a part of the diagnostics. Generally speaking, the wires last a long time. If yours are original wires, they have definately made their "long time" and it would be prudent to change all the plug wires. Whenever I change plug wires, I also change the coil-to-dizzy wire. They come in nice colors now. I like colors...

 

hold the throttle plates in the carb wide open also helps so theres no air restriction and the cylinders are filling completely, if possibale do the test at operating temp

 

hold the throttle plates in the carb wide open also helps so theres no air restriction and the cylinders are filling completely, if possibale do the test at operating temp, removing all the plugs just alows the starter to spin the engine easeir

 

JOe,
RJ has given you sound advice. The only thing I would add is I always put a brick on the accelerator to hold the carb in the WOT position. It's supposed to let the engine "breathe" so it can pull in as much air as possible for an accurate reading.

Good Luck!

*note to self* learn to type faster!

 

No randy. The compression tester device I have said in the instructions to also take the plug that goes from the coil and connects to the center distributor plugs, pull it and ground it. After all it feeds the electrical current to the plugs and since you said you do pull all the plugs why send the electrical current from the coil to the distributer.

I'm just wondering since you didn't mention it for the flathead, I was wondering if this is something you do for the more modern engines. I wasn't changing subject or hijacking the thread. Just wasn't sure thats all.

 

Joe - OK, now I understand the coil wire thing. Sure. Pull it out.

I don't bother doing that...and nothing has ever blown up.

A compression test is a pretty generic thing. I haven't done one on a flathead, but the procedure should be the same.

 

It doesn't hurt to pull the coil wire so you know your plug wires cannot arc on anything that they might hit.

 

grounding the coil wire is done for later model electronic ignitions, should need to be done on a flathead or point type systems

 

Lets see here, 8 plugs = 8 frosty beverages + a couple of extra frosties for the planning stages, that brings us up to 10, add a couple of more for post compression test bragging and obligatory patting yourself on the back for finishing a project brings the total up to an even 12. Now if you have friends helping you might wan to bump that up exponentially, keeping in mind that frosty beverages are genearlly sold in mulitples of 6.........

..........I do like this modern math

Bobby

 

yep I know this is an old thread but, Really good info Randy! I'm printing this out.
but Also wondering if you or someone knows what the compression should be for a flathead 8? (120 psi ???)

I'm hopefully going to look at a 53 flathead running in a f-600 on Friday. would that be the 8ba/eab (same thing?) I also assume it would have the rear sump oil pan? anyone know for sure? I would like to drop this in my 51 f1 if it checks out. Also someone at MSN flatheads mentioned checking the vacuum? and buildup of oil etc on draft tube, can anyone comment on that?
Oh, Bobby. I'll try to remember the frosties I almost missed that step!

Thanks so much
Josh

 

At sea level, 120 is about right for a stock flathead. As far as I know all flatheads have rear sump oil pans, are you asking if it has the truck-style removable circular plate?

Checking the vacuum? If it runs, I guess you could. It would show a burned or sticking intake valve.

 

Thanks Ross, Yes I was wondering if they would have the "removable circular plate" on the oil pan.
Is there a formula to determine the changes in pressure/compression per elevation change? Truck would be between 4000 and 5000 ft above sea level.

This engine is running so I would like to check everything I possibly can before paying for it.
I found my compression tester, now I need to go find a place to buy a vacuum tester, any particulars I should be aware of when buying one? saw a few online for $20-30 for fuel pressure testing and vacuum testing.

Thanks again
Josh