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.
Cylinders 3 & 4 Not Adding Much
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- Forename: Vic
- Surname: Butler
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- Location: North West Hampshire
Re: Cylinders 3 & 4 Not Adding Much
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.
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.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Re: Cylinders 3 & 4 Not Adding Much
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
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
MGCC member
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
Re: Cylinders 3 & 4 Not Adding Much
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.
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.
-
- Posts: 787
- Joined: Wed Dec 21, 2016 6:07 pm
- Forename: Vic
- Surname: Butler
- MGCC Member: Yes
- MGOC Member: Yes
- Location: North West Hampshire
Re: Cylinders 3 & 4 Not Adding Much
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.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Re: Cylinders 3 & 4 Not Adding Much
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.
Thanks for that helpful information.
I better start studying the workshop manual, to get familiar with HIF''s, or start hunting for HS6's.
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- Posts: 787
- Joined: Wed Dec 21, 2016 6:07 pm
- Forename: Vic
- Surname: Butler
- MGCC Member: Yes
- MGOC Member: Yes
- Location: North West Hampshire
Re: Cylinders 3 & 4 Not Adding Much
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.
BFS in Salisbury are best for SU spares and are very knowledgeable.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Re: Cylinders 3 & 4 Not Adding Much
You don't need HS6 carbs on a standard engine, in fact they can reduce the performance. A pair of HS4s will be ideal.
Mike
1969 MGB GTV8 3.9 Conversion. Many classics have passed through my hands but the faithful BGT V8 is the last man standing.
1969 MGB GTV8 3.9 Conversion. Many classics have passed through my hands but the faithful BGT V8 is the last man standing.
Re: Cylinders 3 & 4 Not Adding Much
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?
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?
Last edited by Donie on Thu Jun 30, 2022 9:40 pm, edited 1 time in total.
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- Posts: 787
- Joined: Wed Dec 21, 2016 6:07 pm
- Forename: Vic
- Surname: Butler
- MGCC Member: Yes
- MGOC Member: Yes
- Location: North West Hampshire
Re: Cylinders 3 & 4 Not Adding Much
It could be a cam follower.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine