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CF 250 2.3 petrol slant compression

Post by webby » Tue Sep 11, 2018 10:40 am
Joined: Wed Jun 06, 2018 2:44 pm
Posts: 12
Hi Everyone
Just had a compression test and here are the results:

DRY
Cyl 1-130
Cyl 2-130
Cyl 3-105
Cyl 4-125

WET
Cyl 1-145
Cyl 2-140
Cyl 3-135
Cyl 4-140

The garage said cylinders lower than expected especially Cyl 3 needs engine rebuild.
What are your thoughts I would spend out on a rebuild as it does spit oil out of the tail pipe and backfires in 4th under load, I just wanted an experienced 2nd opinion and any advice.
Many thanks
Webby

Re: CF 250 2.3 petrol slant compression

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Post by Phil Bradshaw » Club admin » Tue Sep 11, 2018 12:08 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3669
130psi give or take 10psi is good for the 23L (low compression) Vauxhall OHC engine.

No. 3 cylinder compression lower than the rest is a reason for further investigation but not necessarily an engine rebuild; pistons and bearings can be hard to find these days...

Compression figures indicate No. 3 valves not seating properly, most probably the exhaust valve.

This isn't necessarily a reason to remove the cylinder head just yet: the valve seats are soft enough for the valves to hammer them back into shape if given a chance to.

Adjust valve clearances leaving No. 3 exhaust valve slack by 0.003 inch (one full turn of tappet adjuster screw).

After 100 mile and 500 mile (roughly) re-check cylinder compression.

If No. 3 starts to show some improvement then adjust valve clearances again at 500 mile (No. 3 wide still - let the tappet chatter until compression doesn't improve any more) and schedule for adjustment check every 3000 mile (assumes lead replacement additive being used).

Once no more adjustment is left under-size tappet adjuster screws will be needed, or a thicker camshaft to cylinder head gasket can be fitted, but only until valve stem protrusion (H) above the head reaches maximum of 1.13 inch (28.7mm).

Image

If clearances cannot be adjusted without fitting a thicker camshaft to cylinder head gasket then it may be more economic in the long term to remove the cylinder head to have hardened exhaust valve seats fitted.

Seats should be finished to restore valve stem protrusion above the head to 1.06-1.07 inch according to condition of original tappet adjuster screws - replace any under-size screws with standard size for maximum service life.

Spitting oil out of the tailpipe without accompanying clouds of smoke sounds more like sooty condensate (water) to be expected until the exhaust is thoroughly warmed up, more so with later exhaust with silencer box at the rear (Facelift models).

Backfiring under load is more likely to be an ignition system fault, or something rolling around the carburettor float chamber (debris or water droplets), or carburettor icing, or a combination.
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  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: CF 250 2.3 petrol slant compression

Post by webby » Tue Sep 11, 2018 5:33 pm
Joined: Wed Jun 06, 2018 2:44 pm
Posts: 12
Thanks so much Phil, I don't know what I'd do without you and this forum probably get ripped off at a garage I imagine.

You have wrote a very informative post I will follow the advice and hopefully save the cost of an engine rebuilt it certainly puts my mind at rest, even if I just need the head rebuilt in the future that will be a lot cheaper than the whole engine.

Thanks again

Webby (Allan)

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