Compression test results

[AUSMHLY]

I just got my carbs back from having new throttle bushing installed.
Ready to do a tune up.

I ran the following questions by a very competent Healey guy yesterday and thought I'd run this by the forum, just in case someone else had/has this same situation.

I removed 6 spark plugs yesterday to find they are all black, which I understand means it's running rich.
(Not sure if that's related to a possible vacuum leak or if I tuned them too rich. I can cross off throttle shaft leak from here on out.)

I found oil on plugs 4,3,2... thread only, not the tips.
I did a compression test. (6)145 (5)140 (4)140 (3)145 (2)130 (1)148
Should I be concerned about 130 pounds of compression on #2? Why is that?
Why do I have oil on the threads of plugs 4,3,2.

I've never had any luck getting as an even a pressure flow out of the left exhaust pipe as I do with the right (facing the rear of the car).
The left exhaust pipe connects to the front carburetor.

Which cylinders are for the front carburetor?

I understand when you adjust one carb it affect the other. I'm assuming because I can't get a smooth pressure out of the left pipe it's related to the front carb?

 

[red57]

How are you measuring the 'pressure flow' from the exhaust pipes?

Plug colors don't look too bad to me, just slightly rich. To know more do a rolling plug cut - cruising at freeway speeds, kill engine, coast to stop, check plugs for color. Right now you may be seeing more 'idle/slow speed time' rather than normal running time.

Dave

 

[John Turney]

How are you measuring the 'pressure flow' from the exhaust pipes?

Plug colors don't look too bad to me, just slightly rich. To know more do a rolling plug cut - cruising at freeway speeds, kill engine, coast to stop, check plugs for color. Right now you may be seeing more 'idle/slow speed time' rather than normal running time.

Dave

In our area, freeway speeds are slow speed.

 

[AUSMHLY]

Pressure flow, not sure what's it called. Would someone please inform me of the correct terminology.
Maybe this photo will help.


According to the photos, all plugs are black, C.

I place my fingers over the each tail pipe to feel how the exhaust is coming out, like the photos.
Right tailpipe exhaust, feels like B, smooth and consistent.
Left tailpipe exhaust feels like A, erratic.

No matter what I do, I just can't seem to get the left tailpipe feel to like C, smooth and consistent.
Is it related to cylinder #2?

 

[Keoke]

Well TH:

To get balanced flow out of the tail pipes usually requires adding a cross over pipe to the system.

 

[Deleted member 19315]

IMHO 130 psi on cylinder 2 is too low. 10 % variance highest to lowest is an accepted norm. Make sure you are testing compression with the throttle wide open , same number of revolutions each time. If numbers are adrift do a leak-down test; listen at the exhaust the inlet and the rocker box for the leaking air. A squirt of oil into the cylinder will isolate ring leaks or valve/ seal leaks.
Do these checks first as a recognised means to establish cylinder seal and any leak paths.
Exhaust pulsing can be misinterpreted easily but is usually poor carb' set up or an ignition fault IF the cylinders are properly sealed.
The secret to fault finding is one step at a time and making sure its repeatable.

 

[RAC68]

Hi All,

Roger, although my compression readings are much higher, due to a shaved head back in 1970 or so, my low reading among cylinders is also Cylinder #2. However, the low reading is quite a bit less then 10% different between the High reading and Low.

It seems odd to me that a number of Healey friends in the UK and Europe seem to have easy access to rolling roads for diagnostics and dynamic tuning. Many have this task scheduled once a year with the rest addressing the expenditure only when crossing the frustration line.

Back when this was one of those things on my "to-do" list, I looked for a rolling road facility reasonably close by (in the state) with a non-astronomic service charge. On both levels, my search did not pan out. Why is this type of service facility more available, not to mention reasonably priced, outside the US?

Just my thoughts after trying to secure a stead, under 1K idle, unsuccessfully when I have good free operating performance,
Ray(64BJ8P1)

 

[steveg]

....Just my thoughts after trying to secure a stead, under 1K idle, unsuccessfully when I have good free operating performance,
Ray(64BJ8P1)

I rebuilt my HD8s with new teflon bushings and still didn't achieve that. When I had my carbs delrin-bushed by Tom Bryant, the problem was resolved. Now I can set the idle at a steady 500 or 700 or whatever I want with the idle mixture screws alone.

Apologies for being a broken record on this, but it really does cure the idle problems.

AUSM - Your plugs look like mine did before my rebuild a couple of years ago. I had at least a couple of oiled spark plugs at the time. With a tired engine, you may have it tuned as well as can be done under the circumstances. Make sure your needles are centered and unworn. If not centered replace jets and needles. Suggest evening out the main jet drops with a dial caliper and making sure the float levels are the same.

Cyls 1 - 3 run off the front carb. FYI - there's a divider in the intake manifold between 3 & 4 with a balance hole in it.

Good source for SU tuning process, including making your own wire pointer synchronizers:
https://thosbryant.wordpress.com/2015/10/04/su-carburetor-tuning/

 

[mgtf328]

Are you sure your carbs are balanced? Perhaps this is the cause of the pressure difference. Is there any oil leaking down from the rocker cover into the plug recesses causing the oil on the threads?
My plugs "soot" up like yours. I found that the mixture was too rich. I backed it off until it stopped "sooting" up but it didn't run as well so I put it back. Bill Rawles told me that BJ8's like to run rich when I bought my car. Being a clever guy and an engineer to boot I didn't believe him! I had to find out the hard way!
AJ

 

[steveg]

....Is there any oil leaking down from the rocker cover into the plug recesses causing the oil on the threads?....
AJ

Above each plug is a threaded hole from one of the rocker pedestal studs. Mine all leaked onto the plugs until I sealed the studs with thread sealant. You have to remove each plug to see the hole.

 

[BOBBYR]

Hi Guys , I was just reading these post and I am assuming that this compression check was a dry test for compression . Did you do a wet test on this motor ?? I would do a wet test under the same conditions and that might give you a brighter picture of your engines condition . Hope you are all doing well .
BobbyR

 

[Deleted member 19315]

Yes good point dry is the first test then wet can eliminate ring seal as an issue leaving the valves seal as the prime suspect...

 

[twas_brillig]

Haasad recomended ensuring the throttle was wide open when doing a compression test. With the SUs, would it also be appropriate to lift the pistons? Thanks, Doug

 

[Bob_Spidell]

Wouldn't hurt.

 

[Tom Bryant]

Wouldn't hurt.

Wouldn't help, either. In fact, I've done numerous compression tests, on many different types of engines, and consistently found that, as long as your throttle plates aren't completely closed; i.e., as long as your idle screws are holding the throttle plates open a "normal" amount, you will get the same compression reading as you would get with the throttle plates held wide open. If closed throttle plates don't make any difference, then holding up your pistons (which are already held up a smidgen by design) certainly won't.

The reason that it makes no difference is that, at starter cranking speed, air can get past the slightly open throttle plates fast enough to result in nearly atmospheric pressure inside the cylinders at the bottom of the intake stroke. Typically the intake valves do not close until the piston has moved about 30% of the way back up on the compression stroke, and air is pushed back out of the cylinder during the upstroke.

That means that if the cylinders are anywhere near atmospheric pressure at the bottom of the intake stroke, they will certainly be at, or even slightly above, atmospheric pressure by the time the intake valve closes. So, no matter how wide open your throttle plates are, you're going to start compressing your air/fuel mixture at roughly 30% of the way up the cylinder, and with the charge at virtually the same (near atmospheric) pressure.

Try it, and you will see.

Tom Bryant

 

[Bob_Spidell]

OK. Always kinda wondered (when I've forgotten to make sure the throttles were open). Note on HD carbs if you 'RTFM'--in some books, anyway--the throttles are supposed to be fully closed at idle and the engine running on the slow run valves only (I open mine a bit). Any idea if the slow run valves let in enough air?

 

[Tom Bryant]

Any idea if the slow run valves let in enough air?

"Slow run valves" are the functional equivalent of opening the throttle plates a smidgen at idle. Their main advantage is that they permit more accurate, and more consistent, adjustment of idle speed. The end result is that there is "exactly" as much flow area for air to get into the engine as there would be if the throttle plates were used to trim the idle speed. Thus, the effect on compression readings would be the same; there is no need to open the throttle plates when checking compression.

Once again, you can easily prove this to yourself simply by trying it.

Tom

 

[Bob_Spidell]

... Once again, you can easily prove this to yourself simply by trying it.

Tom

Why bother. You've convinced me.