Testing, testing.......

[Andy K]

Hi all.

 

I'm hunting down a misfire and power loss. Been through everything three times. My own gauge tester had given roughly 180 PSI across all the cylinders after testing the engine by turning it over. A pal lent me a compression tester with which you apply compressed air to each cylinder at TDC and measure.

 

I've come up with 75-80%, 85%, 85%, 85%, 85%, 85% across 1 to 6 front to back

 

No 1 is clearly off, but is 85% "efficiency" as cited on the device OK?

 

There is no bubbling at the rad and no transmitted sound down the exhaust system or at the carbs.

 

So is the air escaping through the rings or a failed gasket? Could it be a failed valve seat?

 

Am I taking the head off to find out?

 

Your thoughts as always, gentlemen, would be appreciated!

[RogerH]

Hi Andy,

not sure about your figures but to establish whether its rings or valves squirt oil into the #1 pot and re-do the compression test

(throttle full open, spin 5 times).

If the compression comes up it could be the rings.

If no difference it's probably the valve seat.

 

Roger

[PILKIE]

Hi Andy.

 

is it a pi car?

test injectors for 1 sticking shut or not pressurising and dribbling?

std ign?

check plugs points cond rotor cap coil etc

Check all spade elec contacts throughout the ign circuit

does you rev counter reading drop just as it misses

[Andy K]

Good evening gents, back with some more numbers from another compression test.....

 

Numbers are rear to front, dry/wet:

 

175/175 175/180 175/180 180/180 175/180 165/175

 

There is something going on here at number 1 cylinder as previously.

 

The car is twin SU, all ignition components replaced/swapped for known good items.

 

My feeling is a sticking valve or damaged valve seat. The head is an unleaded one with exhaust inserts, supplied with the car as a rebuilt item for fitment.

 

Any comments?

[demon-jules]

Hi - get a vacuum guage on the inlet manifold - it is a wonderful tool for diagnosis!!

 

Regards

 

Julian

[stevefmyers]

Hi Andy

I don't know if it helps but I had exactly the same problem last year. Part of the solution was the reversion to standard ignition parts (it seems the electronic ignition wasn't working as it should have done). Part two of the solution was to replace the Strombergs with a pair of SUs. You already have SUs so replacing them won't help (although it could be they need servicing?) but perhaps the fault lies in the ignition?

Steve

[john.r.davies]

Good evening gents, back with some more numbers from another compression test.....

 

Numbers are rear to front, dry/wet:

 

175/175 175/180 175/180 180/180 175/180 165/175

 

There is something going on here at number 1 cylinder as previously.

 

 

No, there isn't, IMHO!

That's well within normal variation.

Misfires are rarely due to leaking rings or valves, and you don't have any, anyway.

Look elsewhere - injectors (see above), ignition, mixture.

Do you have electronic ignition? Not ECU, just something a bit more modern than a spark gap. Optical, magnetic, Hall effect, there are many different systems that offer far more reliability than the original contact breaker.

And how are your dizzie cover and rotor arm? Signs of tracking? Loose contact arm (use a Red one!)

Good luck!

John

[Nick Jones]

I'm with John on this. Those compression figures look pretty good to me - maybe slightly leakier valves / rings on no.1 but nothing that's going to cause noticeable power loss or rough running.

 

It's hiding somewhere in the fuel or ignition IMO. Can you be a bit more descriptive about the symptoms? Is the misfire random or regular - worse under load or at part throttle?

 

Nick

[Andy K]

The car is standard for a late car apart from:

 

UK early cam (150 not 120) We were on the way to either PI or 3 X SUs or three X ssh-you-know-wots or even EFI!

 

unleaded head (hence the concern about the valve/valve seat)

 

SUs - all overhauled some time ago and looking good

 

Standard ignition - distributor dismantled and OK

 

New coil, points, condensor, rotor arm, cap, leads, plugs

 

All gaps as should be

 

timing fine

 

done the vacuum test - OK

 

As I say, just going in circles with this. There must be something I am missing and I can't spot it.

 

The car was going really well and then started running on 5, worse under load.

 

I'm off to give it a good talking to and a hard stare now. Perhaps show it a picture of a 302 crate engine.

[RogerH]

There has been lots of postings on dodgy rotor arms and capcitors(condensors) lately - what is the pedigree of yours.

 

Roger