Compression Test

[msampsel]

Updated the tech wiki to have both throttle plates and pistons propped open.

S

I know this is an older thread but....What is a good, safe way to "prop the piston of the carbs wide open"?
We are talking about the Dashpot pistons in the SU carbs.
I don't want to do it with something that will block the flow or with something that would get sucked down into the cylinder.
I would say use my fingers to hold them up but I think they will be turning the ignition key.

Would a shortened clean painters stick for stirring paint work, propped up vertically and with the long length horizontally coming out of carb area?

What else would work better / safer ?

No mention of doing this with a warm engine either.
The engine should be warm so the rings expand as the would normally during most running states.
I just propped the throttle wide open and did nothing with the dash pots.

But seems the safest and easiest is to remove the pots (4 screws) and then pull the piston out entirely.
And making sure you know which is front etc.

For me it is a heck of lot easier then taking on and off the fricking air cleaner.

[redroadster]

You need a completely COLD engine room temp anyway for an accurate compression test
You do not even want to drive it in the shop the heat created builds air pressure to give a better reading than what it really has ,ive seen a 35lb difference

[bobd]

Updated the tech wiki to have both throttle plates and pistons propped open.

S

I know this is an older thread but....What is a good, safe way to "prop the piston of the carbs wide open"?
We are talking about the Dashpot pistons in the SU carbs.
I don't want to do it with something that will block the flow or with something that would get sucked down into the cylinder.
I would say use my fingers to hold them up but I think they will be turning the ignition key.

Would a shortened clean painters stick for stirring paint work, propped up vertically and with the long length horizontally coming out of carb area?

What else would work better / safer ?

I my mind, I'm picturing credit cards trimmed to fit with a notch to prevent ingestion.

A compression testis for me, a relatively quick way to see if an engine will run - usually anything over 100 psi, but a leak down test will give much more information.

[msampsel]

Cold vs hot, seems most on Internet agree with warm engine. Pistons rings etc are in the as used state. Hot is where the sparks fly and the fuel air mixture gets compressed.

Cold only occurs briefly for a few seconds.
But hey if it does not start than cold would be the only way.

[Gregs672000]

You don't need to open the carb pistons (slides? Whatever) only the throttle plate. The engine will draw what it needs, and a slow spinning engine will not need that much air. The throttle plates are your real obstruction, not the carb pistons.

[svwilbur]

I have always read to do it with a warmed up engine (and the prior directions mention that too) and hold open throttle.

Sounds like maybe dashpot pistons may not matter. Maybe I will test that.
Credit card mutilation might be a good way (cutting to fit the hole and still be big enough on one end to keep it from being swallowed).

I know that at idle the dashpot pistons come up a little off their down state. But if you are just cranking I doubt they come up very much.
I wonder if they do? So I would think that it would make some difference.

Taking the pistons out might be a good way as msampsel said. That is easier than the air cleaner and all the hoses.
But if you are going to do a carb balance afterwards you might as well take the air cleaner off.

Also they usually say do the test warm followed by doing it all over again with a squirt of oil in each cylinder to see if it comes up a lot. The prior directions mention that too. If it comes up a lot then you may have ring damage. It normally is suppose to come up a little with oil but not dramatic.



I may try a leak down test after the compression if one is way out of range. I suspect #4 from a prior reading with Previous Owner.
Maybe one of the auto stores has a loaner. Or I may purchase one. But it is a tool I would hardly ever use.

If I recall if the percentage leak is 20% or less you are kind of ok. More than that is an issue.

From the web on Leak Down Testing with directions and results:
https://mobiloil.com/en/article/car-mai ... kdown-test

Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.
Piston rings : Whistling or hissing out of the PCV valve, oil filler cap hole or dipstick tube means the air is pushing past the rings.
Suspect ring or cylinder wall wear.
Head gasket : Air bubbles in engine coolant seen at the radiator filler cap could mean air escaping into the coolant past the head gasket.
Cracked cylinder head : Bubbles in coolant or coolant being pushed up out of the radiator neck can also indicate cracks in the cylinder head
or cylinder walls.

[redroadster]

how do you get a warm engine ,per a compression or leakdown test ?
as the exhaust valve head & seat area goes to 600-800 degrees in 75 ' of driving and stays hot for hours
about 300-350 for the piston top and cly head all this in a closed 25 CC area ,heat expands air molecules ,thus you have a higher reading than just the compresion will give .

In 87 I a tech at a Nissan dealer here , the DSM and an instructor from Chicago had me to help give instruction on compression and leakdown tests , I was the wrencher part as the other techs from area dealers looked on . they had sent a 2 page instruction booklet that I was to review , I had looked it over . when they came to my bays and wanted to do it I when out to get the car to be used and drove it in , getting out they both came up and asked if I read the procedeur ? then pointed out the 1st step the engine must not have ran for 4 hours .

I have done a very warm comp test when I know a cly is low ,to see if it's valve or cly. too but to do a very accurate test it needs to be "cold or unrun "
At Mitsu in 03 , a customer lady brings her 98 montero in saying it chugs intermitantly . 98 Monteros had a head defect in some cars caused by a machining bit with no way to tell which ser # were affected , I would find out later .
I always got it around 3:30 pm , I drove it and it ran very good to me ,had good power , but she protested it would do it just drive it more , she got a loaner and the serivce Manager took it home & gave it back the next day . days later she is back I drive it and at a light here is the chug seems like a dead cly . I let it cool down with a fan on top of the engine and did a compression check on the cly it is 7psi off on that cly but that should not cause the condition , next day swapped out the lifter, swap a injector & of course plug and wire every time going out to test drive & see if it set the #4 miss fire code .
No change , I got on the horn with techline You never seem to get the same instructor when you call . I let them take control as to what course to procede , one wants to do elect cks on FI wiring all over the car / truck still no change
they want it hooked to OBD3 so they can see data the thing is in for a week or more , with much of the dash down to get to the FI connections kind of torn apart finally one instructor wants to do the compression test again says to park it inside do not drive it and see what you got in the morning . I did it had about 15 psi lower compression ,he looking at the OB3 data says wait ,why is it been driven . I say it wasn't as far as I know , he says yep at 7:18 for 80 sec looking at data .
A porter moved it to clean ... later doing it dead cold ,the 4 cly was 27 psi lower than others because of the head defect a slightly warped lifter hole

[svwilbur]

redroadster - so maybe testing both ways (Hot and Cold) may make sense based on your experience.

This afternoon I tested mine after running the engine for about 15 - 20 minutes. I needed to turn it around to have it heading out of the garage for some future work and I did a compression check after I was done warming it up and listening to things. This was after a valve adjustment I had just done. I was checking for noises and smoke and stuff. She still smokes some at idle. May be the carbs too rich at idle on #3 and #4. Those plugs are still burning dark. I have a new set of carbs to install soon. I'm done messing with these ones.

And there is a ticking sound that I have always had I guess which I think seems to come from the front of the valve cover. maaybe it is the mechanical fuel pump?

I tried to listen with one of those stethoscope things with a rod on it but can not tell much other then it is a little louder in front I think.

I took some video to listen to but it is 1GB in size. The ticking sound seems much much louder on the video than in person. The smoking looks bad in the closeup with the lighting just right. Not so bad zoomed back. I don't know if that is a lifter or valve or what that makes that noise. To me it sounds like just one valve or something. Maybe I need to recheck my valve adjustments. or mabe it is the fuel pump.

Below are some compression numbers that I got for readings on my 1970 2000 with 108,077 miles on it. A rebuild was done at 88,375 in 1981 so 35 years ago. but "only" has 20,000 miles on the rebuild. I am using a cheap auto store tester made by Actron CP7826 about $28.00. seemed to work fine.

TEST #1 distributor wire and plug wires pulled, spark plugs all pulled, WOT wide open throttle by my foot. Cranked until no more increase in pressure, about 7-9 chugs from starter.

Cylinder
#1 - - #2 - - -#3 - - - #4
190 - 188 - 182.5 - 180 first check
190 - 188 - - 180 - - 180 second check
-------- ------- --- 180----- ---- Third check with NO throttle(NO difference)



TEST #2 distributor wire and plug wires pulled, spark plugs all pulled, WOT wide open throttle by my foot. Cranked until no more increase, about 7-9 chugs from starter. (PLUS ON THIS TEST I REMOVED DASHPOTS TO KEEP THEM FROM BLOCKING AIR FLOW TO CYLINDERS)

#1 - - #2 - - #3 - - #4
211 - 205 - 200 - 198 first check, about a 7 count on chugs from starter, cylinder #4 about 3.4% lower than the average of the others. 6% lower than the highest



Additional testing on cylinder #4
194 NO THROTTLE, BUT DASHPOTS ARE REMOVED STILL and a 7 count on chugs from starter
201 WOT and BUT DASHPOTS ARE REMOVED STILL with an 7-8 count of chugs from starter
200 WOT and BUT DASHPOTS ARE REMOVED STILL with an 8 count of chugs from starter, re-run



HISTORICAL COMPRESSION TEST RESULTS FROM THE YEAR 1982, 7 months after the rebuild at mileage ~ 91,000 so maybe 2000 miles on it.
I am not sure how they did the tests. It was just listed on a work sheet I got with the roadster.

#1 - -#2 - - #3 - - #4
180 - 185 - 185 - 170 Cylinder 4 seems to be low. about ~ 7.1% lower than average of the others. Those seem a bit off from my tests but it still shows #4 lower than the rest.


CONCLUSIONS:

Holding the throttle wide open has little effect on increasing compression and airflow if you leave the dashpots closed or have them still installed.
You need to remove the dashpots or maybe prop them wide open to have the throttle make an effect.
For highest compression numbers remove the dashpots and hold wide open throttle.

With the Dashpots removed and throttle open the compression numbers increase by about 20 psi per cylinder across all cylinders.
That is about 11% increase than with the dashpots installed.
Not having the throttle open but still having the dashpots removed decreased the psi by 6 or 7 psi or about 3.5%


But either way I consistently have slightly lower compression in cylinders #3 and #4.
but "only" about 1.4 to 5% difference depending on which readings you compare.

So I guess those are in the ball park?
OK?
NO worries?


I will try to do some DEAD COLD tests in the morning and see how they differ.
I also did not do any tests with squirting oil in the cylinders. I felt with them being somewhat close to each other and somewhat in the expected range that maybe it would be a waste of time?


Would you expect to see smoke from the tailpipe with any of these compression numbers?
I have always had an issue at startup and at idle. Like it floods up or something or maybe oil getting into some cylinders?

Should I maybe do a leak down test to see what it tells me??
Or are the compression numbers ok and a leak down test would be a waste of time??

Thoughts anyone????

[redroadster]

The first crank should be 70 percent of total compression
A battery charger is a good idea too , you likely lost quite a bit of voltage from the start to #4
In the morning start with 4 and work down
But then 4 is the cly that gets the hottest still not any thing. To worry about

[svwilbur]

The first crank should be 70 percent of total compression
A battery charger is a good idea too , you likely lost quite a bit of voltage from the start to #4
In the morning start with 4 and work down
But then 4 is the cly that gets the hottest still not any thing. To worry about

I actually did start with #4. because it was the one I suspected the most.
I tested each one several times as you see from the log above, yesterday.

[Gregs672000]

Good objective measures and interesting results. All good numbers that I would not worry about. As noted, a leak down will give more definitive measures.

[svwilbur]

just to finish up the compression testing.

TEST #3. Same as prior test, no plugs, WOT, dashpots removed, BUT COLD ENGINE.
#1 - - #2 - - #3 - - #4
205 - 205 - 190 - 190 first pass
205 - 203 - 192 - 192 second pass

Prior results with HOT ENGINE
#1 - - #2 - - #3 - - #4
211 - 205 - 200 - 198

So the results with the COLD engine have a bigger difference between the high cylinders #1 and #2 and lower cylinders #3 and #4.

I am going to run a leak down test in a couple days to see if it shows a leaking head gasket between #3 and #4, or some other issue.

if I can't determine any leaks then I guess I will forget about it until it gets worse.

[AidanDawn2000]

Great testing, analysis and conclusion Stacey. I did mine stone cold with no throttle or dash pot manipulation and got 165 acres the board. Wonder if I can break into the 190s...

[msampsel]

Stacy, amazing detail and notes. Guess I should expect this from a computer science dude!

I read somewhere a squirt of oil in the cylinder can show it might be rings if compression goes up significantly. And ring seal can and will be different hot than cold.

Just an option to the leak down test.
I wonder if rebuilder strive for better numbers and keep tweaking until they get them? ... Nah

[Gregs672000]

A compression test is good, a leak down is always better. Also, depending on the cam profile, compression numbers can be lower with a more high lift/duration cam, as they are designed to work at higher RPM and tend to allow more pressure loss at low rpms.