Carburetter Tuning

[patrickswan]

I have just rebuilt my carburettor (new seals, needle,float valve etc) to hopefully cure very bad running/stalling when hot. I now need to get it set up and the car running. Any tips on how to get the basic settings right before I try and fire her up? Thanks.

[eisdielenbiker]

As a P5B-owner I don't know your carburetter exactly. But this should apply too:
www.mg-leipzig.de/downloads/SU_Tuning_1.pdf

Good luck
Mark

[Phil Nottingham]

The 3 Litre uses the HD unllike the H & HS type which is dead easy to tune and set up - Make sure jet is Centred by checking it dros easily after lifting the pin. Refer back if it does not as procedure is different to H/HS types (or look in WSM!)

Turn fully closed but NOT TIGHT the big slowrunning valve on top and then open 3.5 turns.

Undo fast idle screw so that it clears throttle bar with choke fully in

Raise jet lifting screw fully so it jsut clears jet lever and then screw down 2.5 turns.

These are inital settings to start and fully warm up engine ready to tune mixture in usual SU method by lifting piston and observing engine speed.

Adjust idle speed second and recheck mixture

Set fast idle only when 1 & 2 complete - make sure when pulling choke out it firsts opens throttle without moving jet down and the Yellow Coldstart light is OFF. Light should come on immediately jet commences lowering so adjust cable to suit.

Beware using Colortune with type of carb as idle mixture is set by the big screw and doing things in wrong order will result in incorrect mixture at running speeds

Bad hot starting can be ignition caused (coil or timing) and also blocked economiser valve on carb or carb inlet

[patrickswan]

Thanks for this very thorough reply, some info of which I have also found in a P4 workshop manual. ( incidently it recomends using a cigarette paper to measure the gap of the mixture screw, presumably an essential part of every mechanic's tool kit in the 1950's) Could you clarify your advice on the 'economiser' ie what am I looking for and what do I need to do with it? Many thanks.

[Phil Nottingham]

The Economiser is the rubber tube from the T piece on top of float chamber to air filter elbow. The elbow has a small hole which can block, clear carefully without enlarging it as size is crucial)

The narrow metal tube on the other end of the T can also block/kink etc as can teh hole in the carb flange.

The rubber tube perishes/leaks/collapses - use standard rubber/nitrile petrol pipe NOT vac hose. There should be a special fibre washer under the float chamber T piece as well - this is vital a plain one will not work.

The reason why soft petrol pipe is used is to test the Economsier.

Do this when the engine is fully warmed up, mixture and idle set to not more than 600 rpm (preferaby 520-550 rpm if it will run) - squeezing the pipe closed should immediately stall it. If it does not then its not working.

Too high idle speed or rich idle mixture will also cause hot starting difficulties as will coked up head/valves and too tight tappets.

A well sorted 3 litre engine should idle silently and smoothly at revs down to 400-450 rpm and it should not stall egaging drive if auto - V8's cannot even approach this low speed.

I do not recommend such a low idle speed though as sump oil splash is insufficient.

Later P4's have an identical system except some had the smaller HD6 carb

[patrickswan]

Philnottingham, what would we all do without you! I can remember mine idling almost totally silent 500rpm as you say. I will look at the things you suggest.
Many thanks.

[Phil Nottingham]

As I said P5B owners do not know the meaning of silent/smooth as regards engine performance when compared to a 3 litre. Only just a bit slower anyway on top speed and accleration is quite adequate for modern roads even though the V8 will outperform it

[Deleted]

If you read James Taylor, page 189, you will find that the top speed of P5b is 115 mph but the top speed of a Mk II coupé (overdrive) is 115,3 mph. It also tracks better on the motorway.

Regards

Lars

[patrickswan]

I've had my 3 Litre at a ton on the motorway no problem, and the old girl realy shifts on overtaking (enough torque to pull a house down in top gear). It's just hopeless at a traffic light racing start, but I'll leave that to teenagers in Citroen Saxos.

[dorsetflyer]

At least you are safe in the knowledge that the P5 will outlast any Citroen Saxo.

[stantondavies]

Dec 6, 2008 10:14:51 GMT patrickswan said:

hopeless at a traffic light racing start.

The car needs tweaking Patrick. My 3 litre frequently leaves other cars as a small dot in the mirror when starting from traffic lights - no effort, just a dab on the throttle. If I lock down the BW35 the car moves so quickly I can struggle to control it. And exiting a junction, I have to be careful not to spin the wheels on occasions.

See how you get along after tuning the carburettor. Has the distributor been rebuilt? That can make a big difference.

By the way and for the record, the carburettor is an HD8 - SDs are at an angle, not vertical. Otherwise Phil's post is superb, as always.

I'll look out for you when I'm over Northwich way, though in the last ten years I've seen only four P5/P5Bs on the road.

[Phil Nottingham]

Quite correct not sure where the SD came from?

Perhaps I better retire!

[patrickswan]

I've got the carb tuned reasonably well, enough to not cause serious problems anyway, and also checked and tested the economiser and all was ok there, but I still have the same old problem. The car will just about get to warmed up running temp then get lumpy, then conk out alltogether, particularly if I blip the throttle. It then totally refuses to start untill fully cooled down. It's had all new coil and plug leads as well as me having rebuilt the carb. The only thing I can think of is philnottinghams suggestion of the valve clearences. I haven't checked these since I had the head off about six years ago, and to be honest it's been a bit iffy when warm since then. I do perhaps have a tendancy to set them tight to quieten them down. Now the plate on the rocker cover says the adjustments can be done hot or cold, the workshop manual says do them at running temp, which I suppose in this instance would be the most benificial. Failing this being the problem I haven't got a clue

[Phil Nottingham]

Check your valve clearances - inlet must be hot bit its safer to do the exhaust cold which is likely to be the problem anyway cold - err on the salck side (do you use additive?)

Also check the coil connections "+" should go to the dizzy and "-" to ignition (white) assuming you have not converted polarty and have a new coil (old was CB/SW respectively)

[stantondavies]

Yes, DO check the coil connections. I had precisely this problem - car would start cold but when I stopped for petrol it just would not restart untill cooled down a bit. The coil connections were reversed.

[Phil Nottingham]

NOTE VERY WELL that running temperature for tappets does mean hot and not just warm - this is crucial for the inlets and tightening down to eliminate noise will cause burning in the end.

Noise is either caused by a worn camshaft/worn roller tappets (they get a flat on them) or hopefully worn rocker pad(s).

A special tappet adjuster (Gunsons now do the SPQR one of old) will eliminate rocker wear noise.

If an engine will start easily from stone cold in this weather which it should then not much serious will be wrong (bearings apart)

Have you set dwell and timing by hand - Dwell meter and strobe light is the best way even on these engines?