Low compression in one cylinder

[bmw8890]

!963 Flavia convertible has uneven idle so I checked the compression on the cylinders. Compression check revealed uneven compression ranging from 180 to 90 psi. The car doesn't burn oil and the 90 psi cylinder compression went up to 150 psi after oil was added to the cylinder indicating that the compression rings are bad.

What is recommend for an engine rebuild for these cars? The engine has about 90K miles on it.

Thanks,

Brian

[grifo63]

Hi Brian, is your convertibile with 1500 or 1800cc engine? in the first case if you still have the original two carburator configuration in my experience the idle unbalancing may be more related to a bad alignement between them .

However you are right to say that 90psi is very low but in such condition I might expect some more problem trying to get the higher performances then just at idle.

Just for your reference on mine I've measured 154 (1), 162 (2), 125 (3), 133 (4) psi; even if it is not at the best with these parameters by a fine carb tuning my engine run reasonably smooth at all speed condition

what about the spark plugs? can you see someone considerably worse the the other?

[bmw8890]

It is a 1500 engine with dual 35 PII carbs. cylinders 1 and 2 are 180 and 3 (90) and 4 is 150. when I add oil to 1 it goes up to 215 and the 90 goes up to 150 plus. All four spark plugs are a nice light brown color so I know I got the carbs adjusted pretty close to the right mixture. I sent the distributor out to get rebuilt so that should further improve the idle. Car runs great at high speeds.

Just wondering if I could replace the rings and hone the sleeves to get the compression back to spec

Also dealing with a tranny issue. Got a bad clunk sound coming from the intermediate shaft so it needs to get pulled and checked out.

[grifo63]

90psi is definetevely very low but I still not understand why in your case only idle is affected; sorry if I insist but I had similar issues with this engine (815.100) and until I was able to trim very very carefully, measuring by an airflow meter (debimeter), each cilinder I had been never able to reach a good balancing at idle and low rpm.

Unfortunately also, Solex C35 PII doesn't allow an individual trimming for the two ways of the same carburator and sometime one trottle valve may be a little bit more weared out then the companion one on the same axe (incidentally this was my case); moreover the arms which couple the two side might be affected by some others tollerances and also this could affect badly the idle/low rpm performances.

I found also interesting to compare your figure with mine; in both our cases the best cilinder are 1 and 2, 4 is good or accettable and 3 is the worse.... perhaps is just a coincidence but anyway it sound curious.

Regarding to replace the rings and overhaul the sleeves (if I undestand correctly what do you mean with "hone") I'm afraid you have to replace the pistons too with a larger size; anyway you can refer to the attached to determine if it would be necessary or not.

[racing]

Maybe a problem with the valves. Check the clearence of them.
Regards Andreas

[Bert Ewalds]

Why don't you simply try to just replace the piston rings, no honing of the sleeves.
This can be done without removing the engine from the car
Do check whether your pistons are still Original size or an oversize,else you mignt fit the wrong rings

The link below downloads a PDF with all the Flavia Technical data in English

https://dl.dropboxusercontent.com/u/659 ... asheet.pdf

Bert E