View Full Version : compression results

02-15-2005, 04:30 PM
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 :)

02-15-2005, 04:43 PM
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...

02-15-2005, 04:46 PM
148psi 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.

02-15-2005, 07:23 PM
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?

02-16-2005, 09:00 AM

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?

02-16-2005, 11:03 AM
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.

02-17-2005, 03:04 PM
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.


02-17-2005, 03:23 PM
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.

02-17-2005, 04:23 PM
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.


02-17-2005, 08:47 PM
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.

02-18-2005, 12:47 PM
I must have something happening with the carb because I can screw the mixture screws all the way in and there is little, if any, effect on the idle. I'll recheck the float level but they look right on to me. If the mixture screws are all the way in and the throttle plate is covering the transfer slots where would the fuel be coming from for the motor to idle? Something must be haywire in the carb I'm just not sure what.


I'll do the compression test this weekend I hope and then try to figure the carb out.


02-20-2005, 01:34 PM
O.K. I have completed the compression test. Here are the results.

Cylinder #3 and #7 135 lbs
Cylinder #4 140 lbs
All other cylinders are between 145 and 150

That seems like it should be O.K. to me but will wait to hear from the people who know a lot more about motors than I do.

The test was done with the motor stone cold as it has not run for about a week.

I did notice that the exhaust ports were all very sooty, except for #7, which was soaking wet, as if that cylinder was not firing at all. I will put in new plugs and check the wires before running it again.

I removed the exhaust manifolds to do the test. There is no gasket between the manifold and the block. What is recommended as a sealant when I reassemble?

Just for the record, as this is a long thread now, I'm trying to diagnose a low vacuum situation and an overly rich mixture. Currently the mixture screws on the carb have no effect on idle. The carb has just had a renew kit installed and the float levels are correct (I think).

Well that's it for now. If anyone has any suggestions I will be glad to hear them.



02-20-2005, 02:56 PM
#7 concerns me more than anything. The 135# was only that high because of all of the gas in it. The wetness was gas, right? How much wetness came out while you cranked the motor? #7 may be dry enough to fire, now so I would dry the plug, run it and retest the compression on #3 and #7. If #7 still comes up wet, you may have a bad head gasket. This isn't the hardest thing to replace and it can definitely be done without removing the motor, it just needs to be done right. Place something on the carpet so you don't get it full of oil, grease or other dirt.

If the head(s) come off, this is a great time to freshen them up. Find a decent machine shop and for under $100, they should be able to get you some added zip. Have them magnafluxed first.

02-20-2005, 07:17 PM
Hi Jim,

The wetness does appear to be fuel and not oil or water. The plug was soaked and smelled like gas and the port was mucky. I still have to check the wire and I think I will put new plugs in as well since it's so much easier while the manifolds are off.

So you're recommending running it again and then retesting the compression. Is there anything else that can be done to determine if the head gasket is goofy?

Luckily at this point the boat has no carpet so I don't have to worry too much about keeping it clean.


02-20-2005, 08:10 PM
Is the compression of the adjacent cylinder lower than most? This could indicate that the gasket material between the two is weakening and starting to leak. Also, check the oil for water. If it's cloudy, water is getting in. If it smells like gas, you may have bad rings, but it won't be easy to tell definitively unless the head comes off. Then you'll be able to see the cylinder walls, too.

02-20-2005, 09:10 PM
Is there anything else that can be done to determine if the head gasket is goofy?
One option would be a cylinder leakdown test. You do need equipment for this and might have to take to someone with the device to do the test. In the test, compressed air is pumped into a cylinder, usually at TDC, through a fitting that screws into the spark plug hole. The amount of air leakage is measured. If leakage is excessive, you can hear the air leak through the exhaust if it is a burned exhaust valve or through the valve cover via the crankcase if the rings are leaking. You would hear the air coming into an adjacent cylinder if the head gasket had a leak or you might also see bubbles in cooling system....that could also indicate a cracked head or block :(

02-22-2005, 01:41 PM
O.K. I reassembled the exhaust (wow those things are heavy) cleaned the plugs, adjusted the carb and fired it back up. It ran much better. I let it warm right up at a fast idle then brought it right back and attempted to adjust it with the mixture screws. No luck. I'm sure now that a large part of the carb problem is that I have the secondaries open a bit at idle. I think they are open too much so I'm going to close them down completely. I have a Holley tech book that recommends having them open at idle a bit as it should increase air flow, but not add much fuel. I think it's dumping in too much fuel however as I can't get the idle under 1000 even with the the idle speed screw backed all the way out. After deciding that I still need to fix the carb, I shut the motor down and got out the compression guage again. I was really only worried about #7 as it was the 'wet' one last time. This time the plug was sooty but not wet but all the plugs are sooty as it's still running rich. The compression this time was 142, a little higher than last time, but the engine was still warm. I also pulled the plug wire off #7 when the engine was running and it caused an immediate miss so that cylinder is firing O.K. now.

The vacuum reading also improved to about 16 at idle, which is O.K. and moves to a steady 18-19 above idle so I think everything is good there. I suspect I will see higher idle readings when I close the secondaries as well.

So luckily I don't think I will have to explore any deeper into the motor at this point. I still need to get a handle on the carburetor adjustments. I did take it off last night and closed the secondaries right down, then reassembled it, but it was too late to run it.

I really wish the 4160 had externally adjustable floats.

Thanks for the input from all those who replied to this lengthy thread. I am out of town for the rest of the week so I won't be able to fiddle with the carb until next week. If anyone has any carb tuning tricks for the Holley 4160 I'd love to hear them.

Bruce :o

02-22-2005, 06:50 PM
I would be looking for vacuum leaks, at this point. Under the carb, along the intake manifold, the PCV hose and valve or any other places where air could get in.

02-23-2005, 04:20 PM
I remember hearing that a good way to locate the source of a vacuum leak is to use a propane canister, and by placing the nozzle around the engine at various possible leak sites, the propane will get sucked into the engine and cause an increase in the rpm. I've never tried it. Good luck. Maybe this can help. Bill Z

02-23-2005, 06:52 PM
Similar to the propane, you can use a little starter fluid / ether. Spray it on the various leak sites and see if the engine revs.

If you do use this method, I do suggest keeping your fire extinguisher handy.


02-23-2005, 08:10 PM
Now that's two real mechanics tricks! Scares me just reading about this!

02-23-2005, 08:11 PM
Checking for vacuum leaks is a good thing to do, but I think your low vacuum is a result of poor tuning. Even though you have seen a good improvement from basically a plug change and a (very)little carb tinkering, I think you still have a long way to go -with regard to tuning.

You need to get those secondaries closed! They should not be "interfering" w/your idle, and by being open, that's exactly what they 're doing....interfering with the performacne of the primary side and the idle circuit. Once you get them closed, it will make your primaries and the idle screws in particular, MUCH more responsive to adjustments, and you WILL see changes in engine running w/adjustments to your idle mix screws. Also, you won't have that "hanging" 1000 RPM idle. Right now, I believe that you do have a vacuum leak: it's through your secondary throttle plates.

After you get the carb sorted out, you need to get after the ignition timing. Just like adjusting the carb, it's trial and error, and you need to "feed the motor what it wants". I suspect several degrees of advance will do your engine well.

I'm very confident that after you get the carb and ignition properly TUNED, you will see at least 18" of vacuum when warm. At least 18". Your compression readings are w/in 10% so cylinder seal-wise, you're fine. The actual numbers aren't as important as the consistency is. Also I (respectfully) disagree w/Jimn about the #7 cylinder being wet, and raising your compression. Conversely, I've seen that the gasoline washes oil off the cylinder, piston, and rings, causing a LOWER compression reading...IMO.

I agree the leak down test would have told you a lot, but you are past that now, and have gotten the info you need I believe. I think from what you have posted that you motor is "sound" and you just need to get the tunning correct.

1. Close secondaries
2. Adjust primary idle mixture screws
3. then set the idle speed
4. optimize ignition timing
5. Reset idle speed
6. Smile about 18+" of vacuum, a super smooth idle, and great throttle response.

02-23-2005, 08:56 PM
One thing that hasn't been mentioned yet is - remove the throttle cable before you start it next time. If the cable is hanging up in one direction or the other, it will affect the idle quality because the throttle will not be where it's supposed to. For example, if the cable hangs up going from high RPM to low and the idle speed is set at this time, the next time it goes back to it's normal position, it won't maintain idle. If it hangs up going from low RPM to high, the next time it sticks, it'll be too high. It sounds like the secondaries need to close, though. You may have some worn linkage.

Get it to idle without the cable attached. While it's running (or before, it really doesn't matter), you can check the throttle for added friction or slop.

02-28-2005, 03:16 PM
Thanks again for the input. I closed the secondaries down completely, set the mixture screws open 1 1/2 turns and the idle speed screw in 1 1/2 turns from the stop. Then fired it up. What a difference. It instantly ran much better, even at cold idle. It wasn't lumpy like before, the exhaust was much cleaner and it required no feathering of the throttle to keep it running. I let it warm up and then adjusted the idle to about 800. Now if I turn the mixture screws in it stalls the motor, like it should. It seems to be fine at about 1 1/2 turns out so I think I will leave them there. The vacuum is much better at idle, but still slightly below 18. If I pull the revs up to 1100 or so then I get a steady reading of 18.

I have not adjusted the timing yet. It is currently set at 10 degrees advanced. It does advance properly when the motor is revved up. I will fiddle with the timing but I suspect I will not have to move it much from where it is.

I may still have to adjust the float levels in the carb, but I won't really be able to tell that until I can put the boat in the water and put it through some driving tests.

I hope everything is O.K. with the motor now. It seems pretty solid judging by the compression and vacuum tests. Now I just need to do a little painting on the motor, build the interior, have the upholstery done, install carpets, refinish the exterior, refinish the swim grid, bolt the tower back on and take it to the lake.

The trailer is another story but, like I told my wife, I don't ski behind the trailer.

Thanks for all the input. I'm sure I will have more questions.

Bruce :)

02-28-2005, 04:43 PM
Glad to hear that stuff worked. I still believe that you can get a little more out of it w/the timing. Again, though it's trial and error.

The float level should be adjusted to a spec. You shouldn't need to run it on the water to set that.

I'm glad to hear that it's running better for you.

02-28-2005, 07:57 PM

When you say 'get a little more out of it' do you mean vacuum, or just running a little better overall. It has an electronic distributer and is currently at 10 degrees BTC. Is it common to be more or less advanced than that?

The float level is supposed to be adjusted so the float is parallel to the top of the bowl (when inverted). It's not too precise as far as I can tell. I wish it had externally adjustable floats.


02-28-2005, 09:27 PM
I mean both. If you get the timing optimized for your particular engine, you will most likely see an increase in idle vacuum and power/torque from idle to WOT. The OEM purposefully sets the timing on the conservative(retarded) side to avoid detonation (pinging). Thing is, you're probably never going to run into detonation in a boat engine as they run much cooler than a car, and the compression is pretty low. Not YOUR compression, the compression on the 240 hp 351 engine.

What I did on my boat was I loosened the distributor bolt enough that I could rotated the dist. by hand, but not so loose that it would move by itself. Then I had a friend drive the boat on smooth water to WOT, and hold it there until the RPM's peaked out. Then, watching the tach and/or listening carefully to the tone of the engine, I rotated the dist. to achieve that highest possible RPM. You'll want to take some time doing this and experimenting so you know when you've found the "perfect" spot. When you've achieved the highest RPM you can get, your engine is naturally making the most hp it can. There will be a bit of a "plateau" where rotating the dist does little to nothing to the WOT RPM's. Set your final setting toward the retarded end of that "plateau" just to be safe. That is where you want the timing set. As far as 10* BTC and what that means, it doesn't mean much. Every motor and operating condition is different, so feed your motor what it wants. What ever degree number that ends up coming out to is what works best for your motor. I don't even know what degree mine is set at and I don't care. I know it's set at where it make the most power it can. I probably advanced mine about 4*-6* ahead of stock, but I never put a light on it to find out, because it really doesn't matter.

One addition to what I said about timing. If you really want to dial in your timing, you can adjust it as I described above, then put a light on it and see where your at at FULL ADANCE (now the number will matter). Write that down. Then at idle, rotate the dist to try to achive a higher idle. If rotating the dist. brings you a higher idle then shoot it again and write down the degrees at idle. Now what you need to do is set the dist. to the idle spec that you have writen down, then buy a re-curve kit (new weights and springs) for the centrifugal advance that will bring you to the total advance that you wrote down for the WOT run. You can subtract the initial timing number taht you wrote down from the total timing number that you have written down, to determing the total mechanical advance. This will be necessary when shoping for the re-curve kit. Follow this? This is really getting picky, but if you want it PERFECT, that is how you would do it. The same applies BTW, with points type distributor.

In the end though, I almost guarantee you that your engine will like some more advance both up top and at idle, so doing the WOT setting and giving it some more advance will most likely benefit the idle too and your engine will run better all around. So like I said at the begining, "I mean both". :)

You're right that the float level adjustment is pretty "cheesey". Just set it by eye to get it as level as you can and you're fine. The float level basically affects ALL the circuits. SO if you're too high, everything will be rich (idle, midrange, WOT, etc.) If it's to low, everything will be too lean. But it takes a LOT of change on the float level to make a little difference to the circuits, so if you are "close", the you're close enough. :)

Hope this is all understandable, adn helpful. If confusing, ask more. :)