Cylinders 3 & 4 Not Adding Much

[Donie]

Hi Guys & Gals

It's been a few months since I got time to spend on the MGB 1978 which hadn't run in more than 17 years.

Following paying attention to the SU pump, and several areas of tired fuel line, I got it to run in the winter past, but the noise from the water pump was making it difficult to sweeten up the running.

A new water pump helped tremendously, and I now I find that, on tick-over, cylinders 3 and 4 don't bring very much to the party.

The surprising thing is, that with both these plug leads off, it will still run, but naturally it's rocking energetically.

I'm thinking that the rear SU carb is not playing it's part.

My previous B's, back in the day, had earlier SU's so I'm not very familiar with the HIF4 (I presume that's what's fitted to a 1978) can anyone offer some words of advice, before I start into it.

[Vic Butler]

First thing is to carry out a compression test. With the engine at working temperature, remove all spark plugs, disconnect the ht lead from coil to distributor, screw in the tester and spin the engine over with the accelerator pressed to the floor.
Readings should be around 150 and within 10% of each other.
HIF carburettors aren't the best. I ditched mine and replaced them with HS6's.
If the compression test is satisfactory then the next thing to do is to synchronize the carburettors. The workshop manual shows how to do this.

[Ian F]

With all the plug leads fitted I would run the engine until up to temperature - as little choke used as possible. I am guessing that you can't drive it anywhere so a stationary test will have to suffice.

Then check each plug in turn - lay them out on the bench. Are they all the same? Are they all firing or are some wet? They may look a little sooty/rich because you hadn't been out on the road, but it should highlight any differences between the cylinders.

Ian F

[Donie]

Hello Guys and Gals,

Thanks for your input Ian, and Vic.

I did a compression test, and only no 4 was a little below the other three.

Still suspecting the rear SU, I slackened the coupling nut on the accelerator linkages, in an a effort to try and balance them, although I don't have an ST balancer.

Having removed the air cleaners, and with the engine running I lifted the piston in the rear SU with the tip of my finger, but only by a hair, and hey presto it's stops it's rocking, and runs smoother.

So I'm guessing I have a needle problem of some sort in the rear SU. Maybe a worn needle and jet tube.

I didn't proceed any further 'till I run it by you gurus out there.

[Vic Butler]

HIF's have biased needles which rub against the jet and wear it. However, remove both dashpots and check the jets are level. If not, screw them both flush to the bridge and then back off a couple of turns and take it from there. I presume the throttle and choke is synchronized between the two. The needles should have the shoulder flush to the piston.

[Donie]

Hello Vic,

Thanks for that helpful information.

I better start studying the workshop manual, to get familiar with HIF''s, or start hunting for HS6's.

[Vic Butler]

HS6's are 4 stud fitting and need a different manifold. HS4's are a straight swap.
BFS in Salisbury are best for SU spares and are very knowledgeable.

[Bumpa]

You don't need HS6 carbs on a standard engine, in fact they can reduce the performance. A pair of HS4s will be ideal.

[Donie]

As expected the rear SU was obviously over lean, so now I have it running reasonably well.

There was noise from one tappet so I ran through all tappets (or at least I thought I had), but with the rocker cover back on, the loud tapping is still there.

Did I just miss one ( I don't think so) or is there something else that I'm missing?

[Vic Butler]

It could be a cam follower.