High cylinder compression in herald 1200

[Tipidave]

I am somewhat befuddled having just tested the compression in my 1966 1200 herald engine.

First a little history with the car. First owned from 1988 to 2014.

during this time the head had a light skim 5 thou if I remember correctly and the head was never modified to unleaded valve seats. The engine was getting a little tired and from memory had around 110-120 psi cylinder pressure.

car then sold due to divorce and rediscovered and repurchased in 2018.

recently had cause to check compression (had a cylinder not firing). Using the same compression tester ( albeit many years later) and found compression to be 150-160 psi.  book value suggests max 133psi.
 

so my question are...

could this just be a faulty compression tester?

if during the years away from my ownership the head was removed and reskimmed could this have such a large effect upon compression?

if so, then would this require a different set up in terms of ignition timing etc?

is there any way to know for certain how much has been skimmed?

and if (it hasn’t been converted to unleaded seats) and I wanted to do this, would it be able to have another slight skim if required.
 

As always, I await the wisdom of the “collective” with eager anticipation.👍👍

 

[Pete Lewis]

agree thats on the high side for a std 1200 

if you get a aldi lidl endoscope you might get a view of the valve seat via a plug hole  inserts are only  normally fitted to exhaust seats 

with a valve open you may get sight of the insert ring    never tried that 

with a high CR timing would need a little trial and error to stop pinking and really must use a 97/99ron fuel  

i dont have any head depth data ,   the main result is are they all within a small % of each other the gauge could well be out of calibration 

there is much lead memory in the casting so dont worry about having inserts fitted untill you really need the head off it will do many 100s miles quite happily 

Pete

 

[Tipidave]

Thanks for the reply. Interestingly I have found the engine to be happiest on shell v max 99RON.  It is running v nicely with 20 degrees of advance at fast idle (without vacuum). Nothing seems to be ‘book values’ any more but I guess that a combination of wear, modern fuel, and now  the higher compression would account for that. 
 

I like the idea of an endoscope and will look out for the next time they are on sale 

 

[Pete Lewis]

20 at a fast idle  is excessive .....whats a fast idle  the dizzy has no advance with just below normal idle like 600rpm 

Pete

[Tipidave]

850 rpm

[Colin Lindsay]

Sounds like the leads are too far round on the cap; try swapping them all round by one terminal and see what the timing reads.

[Pete Lewis]

you can add 1 to 5 deg  @ crank  at 900rpm from the dizzy test data  so with a 1200 having a  static of 15 deg  your 20deg is sounding well  ...correct  

so sleep easy on that one  

Pete

 

[DanMi]

Compression testers may well not be accurate, they are not calibrated what you are really looking for when doing a compression test is variance ie if 3 cylinders report 160 and 1 reports 90 there is a problem with that cylinder. or if compression is 110 dry but when oil added to each cylinder it jumps to 150 (say) this would point to worn rings

[JohnD]

Good point from DanMi!   It is misguided to say that any compression guage readings are 'too high' or 'too low' if they are consistent across the block.  Or that someone else's are too high/low.  The gauges are not precison instruments and as Dan says, not calibrated.    If all your cylinders are the same, TipiDave, +/- 5-10%, then put the gauge away and pursue some some real problem!

If you really want to compare, then get two cars together and use the same gauge on the same day.     A session at your next Twiddle Day, Pete?

John

[Pete Lewis]

1 hour ago, JohnD said:

A session at your next Twiddle Day, Pete?

will there ever be another twiddle day   we await ................freedom 

as Im getting older  can we do tappets with zimmer input   Ha !   

its in mind and needs good weather , John  youve got a lot further to drive than myself to get to HQ  and Berns burgers 

if summer comes  lets see 

Pete

[Tipidave]

Thanks for the replies. I wasn’t going looking for problems but was surprised when using it for diagnosing some thing else. My reason for puzzling was that if it means that the head has been off and the valves replaced/refaced etc. Then it is one job. That i can take of my list of planned work.

 

[johny]

I think with those readings (ok for peace of mind you could always check them with another tester) you can take removing the head off the list of jobs. If you eventually get poor running, and it turns out to be from low compression, then it will be worth removing the head to see what the situation is👍