Low compression on commando 273 with 13000 miles??

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xLURKxDOGx

xLURKxDOGx

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I tore down and rebuilt a 66 273 with egge pistons, commando specs and a comp cams 20-306-4 cam about 1 year ago. Recently ive noticed a bit of blow by and now it seems be running a bit sluggish over the last month and a half or so. I just did a compression check across all cylinders and it seems low but not entirely sure what they should be with that cam. I set the ring gaps accordingly so a little stumped no how its possibly worn down under 13000 miles.

1-135 2-110
3-120 4-120
5-118 6-115
7-110 8-120

Ive been working on a 360 magnum and the fact the its low has me worried about what i could have done wrong and what i need to do differently so this isnt a problem for my new build. Any insight would be greatly appreciated.

Thanks,
Jake
 
sluggish and low- don't know your compression ratio but with that cam I think with the throttle open she should be around 150. was it bored and honed? could be leaking by the valves
 
x2 Marcohotrod I would retorque the head bolts ..If that doesn't work check the valve seals . Hope they are PC seals . If that doesn't work check crosshatch on cylinder walls . check for a blown intake gasket . Hope this helps .
 
Marcohotrod said:
sluggish and low- don't know your compression ratio but with that cam I think with the throttle open she should be around 150. was it bored and honed? could be leaking by the valves
Click to expand...
My thought was possible the heads. It was bored and honed. These are 920 closed chamber heads that were just shaved to make flat, they were listed as 10.5-1 but i know thats not right without shim gaskets which i didnt use. Thanks.

383Scampman said:
x2 Marcohotrod I would retorque the head bolts ..If that doesn't work check the valve seals . Hope they are PC seals . If that doesn't work check crosshatch on cylinder walls . check for a blown intake gasket . Hope this helps .
Click to expand...
I retorqued the heads a few months ago, i used felpro gaskets i think that were .40 torqued. Thanks for the response.
 
Any possibility the rings are upside down? Any oil use? I could see a Chinese box being mislabeled or instructions being wrong.
 
fratzog lover said:
Any possibility the rings are upside down? Any oil use? I could see a Chinese box being mislabeled or instructions being wrong.
Click to expand...
Anything is possible but i remember keeping them with the dot facing up. It is using oil, i forgot to mention that.

Jake
 
Yeah, it was around 146. Im hoping its the heads...........

Jake
 
Was the engine warm when you did the test? Also, did you hold the throttle wide open? Did you crank it enough to build pressure (at least three to four revs)?

I'm no expert, but I think your next step is to squirt some oil into the spark plug hole and see if the compression increases. If it goes up it indicates rings.

You could also perform a leak down test to see if the valves are leaking, but if it were valves (or head gasket) I would not expect the compression to be down evenly in all cylinders. Maybe do a leak down on 2 and 7? On mine I was able to hook my compression tester fitting to a compressed air source and then listen/feel for air escaping from the exhaust pipe (exhaust valve), carb (intake valve) or oil filler cap (rings). You should get some leakage past the rings, so you have to decide if it's excessive.

Do you have an oil pressure gauge, or just a light? It might be worth getting an oil pressure gauge hooked up to confirm you've got good oil pressure.
 
Leakdown test and go from there.... It is odd that all cylinders are down. Whatever the problem is will be abroad. If your leakdown seems OK could it be a valve timing issue?

JW
 
SLOPAR72 said:
Leakdown test and go from there.... It is odd that all cylinders are down. Whatever the problem is will be abroad. If your leakdown seems OK could it be a valve timing issue?

JW
Click to expand...
In other words your timing chain may have stretched. Turn the motor over until the time marks line up at 0 then pull your dist cap off, turn the crank in the opposite direction, until the dist rotor just starts to move. look at your harmonic balancer. How many degrees do you have. 16 degrees would be a badly stretched timing chain.

A leak down tester is best. but if you don't have one, a little oil will tell you if it's rings or valves that are leaking.

All three of these things can lower your compression.
 
PocketAces said:
Was the engine warm when you did the test? Also, did you hold the throttle wide open? Did you crank it enough to build pressure (at least three to four revs)?

I'm no expert, but I think your next step is to squirt some oil into the spark plug hole and see if the compression increases. If it goes up it indicates rings.

You could also perform a leak down test to see if the valves are leaking, but if it were valves (or head gasket) I would not expect the compression to be down evenly in all cylinders. Maybe do a leak down on 2 and 7? On mine I was able to hook my compression tester fitting to a compressed air source and then listen/feel for air escaping from the exhaust pipe (exhaust valve), carb (intake valve) or oil filler cap (rings). You should get some leakage past the rings, so you have to decide if it's excessive.

Do you have an oil pressure gauge, or just a light? It might be worth getting an oil pressure gauge hooked up to confirm you've got good oil pressure.
Click to expand...
Actually, it wasnt wide open, maybe that is the problem. Ill go back and test it again, i knew i was forgetting a part if it. I did crank over a bunch of times as well. It does have an oil gauge and reads 40 at idle when warm and 75 in the higheer rpm's. I think this was because my clearances were on the tight side.

SLOPAR72 said:
Leakdown test and go from there.... It is odd that all cylinders are down. Whatever the problem is will be abroad. If your leakdown seems OK could it be a valve timing issue? Its strange, i had my timing set at 34 total 18 initial and when i went back to check it 5 months later it was at 44 total and 28 initial, maybe it did skip. It does bounce around a lot with the light around 22-2400.

JW
Click to expand...

Cudafever said:
In other words your timing chain may have stretched. Turn the motor over until the time marks line up at 0 then pull your dist cap off, turn the crank in the opposite direction, until the dist rotor just starts to move. look at your harmonic balancer. How many degrees do you have. 16 degrees would be a badly stretched timing chain.

A leak down tester is best. but if you don't have one, a little oil will tell you if it's rings or valves that are leaking.

All three of these things can lower your compression.
Click to expand...
Thanks, ill look into that as well.

Jake
 
Ok, new erections. Ive got pretty much 150 across all cylinders, the lowest shows 143. Im starting to think this is timing, I took out the timing light again and seems that its where it should be roughly, 19 initial 35 total. The chain looked ok, a little bit of play but not too much play.

Jake
 
What rings did you use? Cast? Moly? What degree cross hatch did you hone to? 60*? 75*?


Without knowing this, we are at a disadvantage. If you have moly rings and honed to a standard 60* for example, there's your problem.
 
Cudafever said:
How did you get your compression to come up?
Click to expand...
By doing the compression check the right way, i failed to apply the open throttle the first time........

RustyRatRod said:
What rings did you use? Cast? Moly? What degree cross hatch did you hone to? 60*? 75*?


Without knowing this, we are at a disadvantage. If you have moly rings and honed to a standard 60* for example, there's your problem.
Click to expand...

I believe these were moly though i cant seem to find them on their site. Dont know what cross hatch either, i left it to them and maybe that was the problem.

Jake
 
Could be. Different material rings require different cylinder finishes in order to have correct break in.
 
I will also add that we sent block in to install new cam bearings.
Sometimes you roll the dice and ya get lucky in our case we did.
 
4spdragtop said:
I will also add that we sent block in to install new cam bearings.
Sometimes you roll the dice and ya get lucky in our case we did.
Click to expand...
I did the bearings and double checked that they were right, that doesnt guarantee they were though my pressure has been good.

Jake
 
You did measure the compression with all of the plugs out right ?
Also I would try the oil squirt test to make sure you havent burnt your valve seats.

Did you put hardened ones in ?
 
I did it both ways, same result. I dont believe they put hardened seats, so im hoping its in the heads.

Jake
 
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