Spitting through HS4 carb

[Steven collins]

Would a sticky float cause spitting through a carb When increasing the revs Or will it just be a case of the carbs not set up right ie too rich/lean?
The car prior to engine rebuild ran well (apart from the main & big end noise!!) since rebuilding it starts ok (123 electronic ignition) bit erratic on tic over, then when increasing the revs spits from one of the carbs (rear one)
Many thanks.
S

[Vic Butler]

I presume the plugs on cylinders 3 & 4 are firing OK?. Spitting back through the carb is symptomatic of several things including a sticking inlet valve. I'd rule out retarded ignition because it's only the rear carburettor.
It could be a weak mixture too, or a sticking carburettor piston. Are the carburettors more or less balanced? Does the linkage open both throttles at the same time? Does the piston move freely and is there oil in the dashpot? I use ATF in the dashpots on my MGB GT as its viscosity doesn't change with heat.

[Steven collins]

Lots of things to look at, the timings fine that I know I adjusted the airflow so that is ok, linkages are opening together so ok as well, dashpot oil refreshed, will recheck piston is not binding I have already cleaned them up very careful not to touch damage the needles. I do not have a colourtune to check mixture so may just adjust it slightly and see if that makes a difference.
If it is a sticky valve how would I check - apart from removing the head?

[Vic Butler]

To get a starting point the jets on both carbs should be screwed up so they are both level with the bridge. Each jet is then lowered by 2 1/2 turns on the adjuster. Any further adjustments can be done when the engine is warmed up by adjusting each carburettor by the same number of flats on the nut, up or down as required.

[Steven collins]

Ahh the proverbial 12 flats (2 complete turns) will start there and see how it goes...
Right, I have measured the jets using a vernier calliper (probably slightly over the top but hey!!), there was a difference of 15 thou between them so have adjusted them so they are the same, I will start the car up in the morning, once the car is warm I will balance the airflow and see if there is any improvement both in running and tick over.
Watch this space....

[Ian F]

Steven, using a digital Vernier is what I always do when setting the SU jets.

Firstly I find that both jets do not sit equally on the bridge when fully screwed up, so I check then.

Secondly after doing the 12 flats initial set up and then adjusting until I am happy with the running, I check again and make sure they are both set to the same distance down. Invariably I find that they are not - despite taking care to adjust each jet the same number of flats.

I now set them a set distance down, measured in thou of an inch.
They are the original HS4s by the way, but have been rebuilt over the 42 years of my ownership of course!

Ian F

[Steven collins]

Thanks Ian,
Do you remember what measurement you ended up with?
When I measured mine they were 085 thou and the other 100 thou, balanced them at 085 thou for now. I know not everyone’s set up will be the same (too many variables within our engines/set up) Just interested as to what yours are currently running at.
Incidentally, I checked and can confirm that the butterflies are set correctly and working in tandem as is the choke mechanism.

[Ian F]

I will check the measurement this morning and report back

Ian F

[Ian F]

Steven, here is my current set up:

Jets down about 66 thou each

BUT needles are AAA. This is a slightly richer needle than standard

I have tried several needle profiles and these seem to be best for my engine, although I have not had it set up on a rolling road

The engine has an overbore to +20 thou, a Piper 270 cam, K&N filters, and a head reworked to the spec in Peter Burgess's book on the MGB. It is a 1972 car so already had the larger valves which are still fitted, with an unleaded seat conversion.

This all works well for me!

I used to run a Triumph TR2 in hill climbs and sprints, this had no choke fitted and until fully warmed up would spit back through the carbs ferociously! (ie when the mixture was too weak for the engine). I have always associated spitting back with a weak mixture, assuming everything else was more or less correct of course

Ian F

[Paul Hollingworth]

I believe the default setting for the position of the jet is 0.060" or 1.6 mm down from the bridge. I find that although this might not be the optimum setting for every engine they normally run OK at this. Getting the mixture right on both carbs is the trickiest adjustment, each carb should be similar in jet position but not necessarily the same. We have the lifting pin method which is imprecise. I follow john Twists advice and lift the piston 1/32" (guess) with a screwdriver down the throat. If the engine speeds up its too rich and if it slows down it too weak. Its said that it should hiccup slightly then settle down to the same speed. This is all setting the mixture at idle (ish) and at no load. Ultimately plug condition is the best guide and that is not as easy now with unleaded.