About Us

WildBill79 · 10-14-2011, 06:35 PM

I performed a cold compression test today since that is what I had time for. I know it is not as good as a warm one, but I figured it would still show me if something is wrong. Here are the results
#1 112 wet = 118
#2 122
#3 115
#4 121
#5 115
#6 118
#7 114
#8 121
All numbers are +- 1-2 psi due to difficulty reading numbers on my gauge

On all cylinders the maximum numbers were achieved within 3 compression strokes.

The problem during the test I noticed was each time the engine turned over the numbers would rise, which is correct. However, once it got close to the maximum number the gauge would start bounce about 30-40psi. For example, each time the engine turned over the gauge would go 40-90-120 then bounce between 90 and 120 each additional time it turned over. It did this for all cylinders so I am not sure what to make of it.

PontGuy · 10-14-2011, 08:00 PM

I don't know what your CR is, and don't know what effect checking it cold would have.. but those numbers are a lot lower than any that I have ever gotten. Also I never saw the pressure bounce like you are describing. Are you sure that your gage is OK?

Interested in hearing what others think.

WildBill79 · 10-14-2011, 08:31 PM

Since I don't have another gauge, I'll check the gauge out on another engine tomorrow.

According to the service manual the compression ratio is 7.6 and should have between 120-160 when cranking, which I assume would be with a warm engine.

Stuckinda60s · 10-14-2011, 10:50 PM

The guage is supposed to trap the pressure so it builds to a peak and stays there. Those pressures are low, but the guage is probably at fault.

WildBill79 · 10-15-2011, 10:40 AM

Well the gauge is bad. When it jumps it doesn't stop at the highest reading, it goes down several psi. Since all the numbers are pretty close and the gauge responded the same in all cyclinder I don't really think compression is the source of my problem. I'll get a different gauge just to verify, but until then does any body have any other suggestions?

Greg Reid · 10-15-2011, 12:06 PM

You may have moved beyond this already...but just for the sake of being able to elliminate another suspect....Did you ever check the fuel pressure coming from the mechanical pump? It doesn't need much but it needs to be there and be steady.

WildBill79 · 10-15-2011, 05:22 PM

I just checked fuel pressure and it bounces between 8 and 9 psi at idle. I also tried spraying some carb cleaner around the intake and carb and that had no effect. I am still at my original problem. It will run rough, then die once it warms up. Also, the timing mark jumps around and vacuum is low and erratic. Any other ideas?

Greg Reid · 10-16-2011, 11:39 AM

I'm stumped...unless it's a vac leak you haven't found..like cracked intake, warped carb base or something.

torqjunki · 12:00 PM

Wonder if it has some valves sticking when the motor gets warm? Maybe a compression test quickly after it starts acting up will show something.

Greg Reid · 10-16-2011, 12:15 PM

One other thing to maybe narrow it down...maybe you can put your hand behind the exhaust to feel the pulses just to see if it's maybe only one side that's missing and running rough...?

WildBill79 · 10-16-2011, 06:25 PM

I just did a compression test with the engine warm and got pretty much the same numbers as I did with a cold engine. It looks like my gauge may be worthless.

As it was warming up and running rough I listened to the tail pipes and they both sound the same even with true dual exhaust, no h or x pipe. It really sounds like it is running on seven cylinders now so I'm not sure how the exhaust could sound the same on both sides if that was the case. Not sure if running on seven cylinders would cause it to die when warmed up either.

Also, when I pulled the plugs, seven of them looked good and one of them was really white. I think tomorrow I will warm it up again and try pulling plugs until I figure out which cylinder is the problem. Hopefully I am on the right track.

Stuckinda60s · 10-16-2011, 06:34 PM

The one which was really white is lean. That probably means you have a bad intake gasket at the port for that cylinder.

Do you know which one it was? If not, you should be able to see which one it is if you bring the pistons to TDC, one at a time, and look at the piston top. Use a flashlight, if necessary.

Greg Reid · 10-17-2011, 06:55 AM

Quote:

Originally Posted by Stuckinda60s

The one which was really white is lean. That probably means you have a bad intake gasket at the port for that cylinder.

Do you know which one it was? If not, you should be able to see which one it is if you bring the pistons to TDC, one at a time, and look at the piston top. Use a flashlight, if necessary.

If it's pulling air in, that's going to make it pretty much impossible to get the carb tuned, right?

And by the way, WildBill, just for future reference regarding pulling spark plug wires...it's a lot easier to use your timing light. Just hook it up normally and move the high tension clip lead to each plug wire in turn. Any plug that's not firing or firing erratically will be easy to spot.

ta man · 10-17-2011, 12:48 PM

Spray around the gasket to see if the idle smooths out.

Stuckinda60s · 10-17-2011, 12:52 PM

Quote:

Originally Posted by Greg Reid

If it's pulling air in, that's going to make it pretty much impossible to get the carb tuned, right?

And by the way, WildBill, just for future reference regarding pulling spark plug wires...it's a lot easier to use your timing light. Just hook it up normally and move the high tension clip lead to each plug wire in turn. Any plug that's not firing or firing erratically will be easy to spot.

Yeah, the mixture will change at different rpm levels due to the vacuum changes leaning the engine out.

WildBill79 · 10-17-2011, 07:44 PM

Tried a different compression gauge today and got pretty much all the same numbers.

I also used the timing light idea Greg mentioned and didn't really see a difference between cylinders.

Finally, I sprayed around the manifold and carb again and didn't notice any change.

I am not sure if there is anything else to do other than tear the motor down more.

Stuckinda60s · 10-17-2011, 07:59 PM

Check your coolant level! You may have a blown head gasket. When you posted about the white sparkplug, there was something nagging at me and I didn't realize what it was until just now.

Greg Reid · 10-18-2011, 05:25 AM

Do you have a leak-down tester? If not and you can borrow one might be worth the trouble. You could start by checking that one white plug's cylinder when it's hot.

WildBill79 · 10-18-2011, 08:14 PM

Made my own leakdown tester tonight. I'm not sure if it is working correctly since the regulator gauge and leakage gauge drop the same amount when I turn the air on. Either way it is letting me put air into the cylinder so I can check for leaks. Only problem I am having it how to figure out when I am on top dead center for each cylinder. Is there an easy way to tell, what order do they go in?

Greg Reid · 10-18-2011, 08:31 PM

Quote:

Originally Posted by WildBill79

Made my own leakdown tester tonight. I'm not sure if it is working correctly since the regulator gauge and leakage gauge drop the same amount when I turn the air on. Either way it is letting me put air into the cylinder so I can check for leaks. Only problem I am having it how to figure out when I am on top dead center for each cylinder. Is there an easy way to tell, what order do they go in?

Well, an easy way to do it is feel for compression at the same time the rotor is pointing to that cylinder's distributor tower. You just want to make sure it's actually at the top of the cylinder so the air doesn't slam it back down. Doesn't have to be exact like with a degree wheel but it needs to be at TDC.

Menu

About Us