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Old 02-15-2005, 04:30 PM
Hunterb Hunterb is offline
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compression results

One for the engine guru's,

I am in the process of rebuilding a 1990 PS 190 with the SO Ford 351. I have been working through a few problems with the motor and am gradually sorting things out. I have been wanting to do a compression test as part of the diagnosis of the motor. It is producing lower than ideal vacuum (about 12 when hot and I would like to see 18) and seems to struggle at idle. I have only been able to connect my compression guage to two cylinders so far (#1 and #8) because the exhaust manifold is in the way on the other cylinders. Those two cylinders show 148 lbs, which I think is pretty good. I have a friend with a different guage that I think will fit under the manifold coming over tonight. If all cylinders show in the same range as the two tested already my questions are:

1) Is that considered poor, fair, good or excellent compression ?

2) If that is decent compression does anyone have any suggestion on what to look at to diagnose the low vacuum reading?

Thanks for any info you may have.

Bruce
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Old 02-15-2005, 04:43 PM
ktn_cmu ktn_cmu is offline
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Is this engine stock? A big cam can help to produce lower vacuum readings. A carb set up to be lean can also help with lower vacuum readings as the throttle plates will have to be open more to get the same rpm thus reducing negative pressure. A plain old vacuum leak, check the base of the carb or the intake maifold gaskets...loose spark plugs...
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Old 02-15-2005, 04:46 PM
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Thrall Thrall is offline
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14si sounds very good. Tom or the other Ford guys will chime in what it should be.
Low vacuum is probably caused by a vacuum leak in one of several areas. Intake manifold gasket, carb base gasket, any vacuum lines from carb or intake manifold. Probably looking for a cracked line that is leaking a little, but not enough to keep the engine from running.
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Old 02-15-2005, 07:23 PM
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JimN JimN is offline
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If you aren't running the motor at this point, you may want to remove the exhaust manifolds. It will make a lot of things easier. Or, go to an auto parts store and see if they have any kind of adaptor. I wouldn't recommend using the compression tester that has the rubber nose on it, just the kind that screws in. Are you going to tear the motor down completely? If so, have it magnafluxed.

Re: the low vacuum- since you haven't checked the compression in all cylinders at this point, it may be a funky valve seat or stem. How lumpy is it at idle?

Do your compression test dry. Pull all of the plugs before you start this so no gas is sucked into the cylinders. Do one test cold and one hot. If you get ~148 across the board, that's not bad at all. What's the history of this motor? Do you see any gasket sealant where the head gaskets are?
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Old 02-16-2005, 09:00 AM
ktn_cmu ktn_cmu is offline
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Jim,

If he has good compression in two of the cylinders or more, but not good in other cylinders...wouldn't that make for an erratic vacuum reading?
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Old 02-16-2005, 11:03 AM
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JimN JimN is offline
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That's what I was referring to in my second paragraph. Could be a gasket issue, too. Too early to tell since not all cyclinders have been tested.
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Old 02-17-2005, 03:04 PM
Hunterb Hunterb is offline
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Thanks for the replies,

I came down with the plague the other day so have not done anything on the motor since. I will get to it this weekend. The history of this motor may be significant as it sat for at least a year before I bought it. The hour meter says 630 and it does work. The motor runs O.K. in the driveway, but struggles to idle when cold and is running rich for sure. I put a rebuild kit in the carb (holley 4160) but it still seems to run very rich. The water coming out the exhaust has unburned fuel in it, the exhaust smells rich and the plugs are very sooty. I have re-checked the float levels and they look good to me so I'm not sure why it would be running rich. Once I determined that I had low vacuum I replaced the gasket on the base of the carb and between the spacer and the manifold.

I decided that before I get too carried away I need to determine the internal condition of the engine, hence the compression test. I may take Jim's suggestion and pull the exhaust manifolds off to do that. I suspect that many of the gaskets are suffering so it wouldn't hurt to put new intake and exhaust gaskets on as it's quite easy to do, and may solve my low vacuum problem.

I'll try to do the compression test this weekend as that will probably determine what I need to do next.

Thanks again for the input. I'll post the results as soon as I have them.

Bruce
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Old 02-17-2005, 03:23 PM
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JimN JimN is offline
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If you do remove the manifolds to do the compression test, lay an old blanket or tarp over the carpet and gunwales so you don't mess them up and do this test with no spark plugs in it. If it's low on some cylinders, turn it over by hand and listen for hissing so you can identify the source. You will also be able to look at the exhaust valves and hear any leakage from those. If you can, do a cylinder leakdown test. It's more definitive than just a compression test.

Remember to drain the water out of the block if you decide to tear the top end off. Look for replaced freeze plugs, too. The motor may have been overheated or frozen, so there are quite a few sources for this kind of problem. Not the end of the world it it had either situation, though.

Try adjusting the fuel mixture screws, too. This could be part of the reason it's running rich.
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Old 02-17-2005, 04:23 PM
Hunterb Hunterb is offline
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Thanks Jim,

It's great to have someone with your knowledge to bounce ideas off. I do have a compresser so I may be able to do a leak down test but I haven't done one before. I assume I would need an adapter to go in the sparkplug hole, turn the motor so the valves are closed, then pump up some pressure in the cylinder. Do I then listen for the leakdown or watch the pressure drop or will if be really obvious?

I think I will take the manifolds off as I have found other gaskets on the motor in poor shape so I'm sure they could use renewing. I can't seem to make any difference with the mixture screws on the carb. I tried opening up the secondary throttle plate a bit to allow more air in, but it doesn't seem to make any difference. The primary throttle plate is covering the transfer slots.

I may have a bad cylinder, which would explain the fuel in the exhaust, but not the overall richness.

Thanks again. I will let you know what I find with the compression guage.

Bruce
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Old 02-17-2005, 08:47 PM
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JimN JimN is offline
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The gauge takes care of the test. You may be able to rent/borrow one. Some car parts stores loan them. It'll come with instructions, too.

The richness can be from low compression. Some unburned gas may be scavenged from the bad cylinder to a good one during intake/exhaust valve overlap. There isn't a lot, but it's there. Also, hot exhaust gases can burn off unburned gas and since it's not under compression, the combustion isn't complete. If you turn the screws in too far, you should still be able to kill the motor.
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