948 - poor performance

[Cam]

Hi all,

I wonder if you knowledgeable folk could help as I'm tearing my hair out.

A branch member came to me with his car (Series II with 1098 engine & box) as it was burning a lot of oil and was in need of a rebore/rings/pistons, so to cut a long story short he decided he wanted me to fit a supposed known good 948cc engine (eBay special).

Anyway, I fitted the engine as well as an alternator conversion and electronic ignition. The car now suffers with a lack of power.
I converted back to the points and it's running ok at tickover but I feel it's not pulling as it should.

I have fitted the dizzy and carb, manifold exhaust, etc from the 1098 engine.

Just to fault find, I swapped the carb over from my everyday 1098 car and no difference to either car (mine still runs fine and the 'project' still lacks power). I statically set the timing (and checked again) to 5 degrees BTDC as mine is set to. After a run, the plug electrodes are the right colour (grey/brown tips). The compression is about 140psi on all 4 cylinders.

I have set the rocker clearances as per the book as usual.

I have also replaced the rotor arm, dizzy cap, points and condenser.

Any ideas? This one has me scratching my head. I have previously fully rebuilt 1098s no problem, but not had much experience with 948s

[David W.]

Is the vacuum advance on the dizzy working properly?
That cured my similar problem.

[kennatt]

May be you are comparing one engine with the other,the 948 was a good enough engine in its day,but the 1098 was obviously a lot better performance wize.the 948 can be made to perform but at a cost and with a lot of work.

[ampwhu]

have you checked the exhaust manifold/pipes for an kind of blockage?

[ampwhu]

May be you are comparing one engine with the other,the 948 was a good enough engine in its day,but the 1098 was obviously a lot better performance wize.the 948 can be made to perform but at a cost and with a lot of work.

I would disagree. I would suggest the 948 is a stronger better revving engine that is more workable with upgrades.

[liammonty]

Agree with Ampwhu on this one - a good 948 won't be much slower than a 1098.

Cam - I'd play around with the timing a little bit more. There's always the chance that the timing marks are wrong (I've heard of that happening on other BMC cars before), so why not have a go at advancing it and seeing if it has any impact on performance. The other thing I would check is that the dizzy is advancing correctly mechanically (as well as the vacuum advance already mentioned) - get a strobe on it and you should be seeing something in the order of 30 degrees max advance at the crank, by around 3000 rpm. Good luck!

[SteveClem]

I enjoyed my 948 but since it was swapped with a 1098 I must say the car is much more able to cope with modern traffic.
Maybe it's not so much about the engine size but more about its state of health?

[Cam]

Well, I checked the timing advance with a tachostrobe and guess what? No advance at all when going up to 3000 rpm.

Turns out the bob weights were seized and also the vacuum advance unit was faulty.

So, I rebuilt the 948 distributor with working bob weights and working vacuum unit, set it statically to 5 degrees and test drove it.

Pulls like a train now. On my 'test hill', going up previously had to drop down to 2nd on the steep bit with only me in the car, now she sails up in 3rd with 2 adults in the car! Brilliant.

So, thanks liammonty for the heads up on the advance!

[liammonty]

Fantastic news Cam!! Really glad it's going OK now - the 948 is lovely when it's going as it should - not as powerful, but so much more eager than a 1098. Well done saving the old dizzy! Hope the owner enjoys it.