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MG MG Y Type - Carburetter problem

After engine rebuild, a strange phenomenon: Cold, the engine starts willingly, with little choke needed, knob can be pushed back in after less than half a mile or so. Then for something like a quarter of an hour the engine just runs perfect. However, when hot, low rev torque starts to disappear. Sometimes I have to de-clutch, rev the engine up, to be able to accelerate at all. And when I let it idle, it just goes out, but re-starts immediately on full throttle. It has become worse over this summer. Basic carb settings look OK, the "lift-the-piston" test is correct. Now, is this a case of vapour lock, or do I need a different carb needle, to suit it to my ported TF head (bigger valves), and 8.5 compression ? Before rebuilding my engine, I never had this problem.
Remo Peter

Does the car run rich when having problems (black smoke from exhaust)? If so I'd look carefully at the float level (and/or petrol level in the jet).

On the other hand it could also be running lean, caused by ill fitting gaskets or loose manifold bolts?

Is the petrol pump delivering enough fuel?

You might need another needle but it won't cause these problems as all suitable needles have the same stationary setting.

To prevent vapour lock (which I don't think it is?) a heat shield and rerouted fuel line away from the manifold are helpfull.
Willem van der Veer

sounds like an ignition problem to me. Change the coil and condensor.The coil could be breaking down under a heating condition and the condensor is also doing the same.
Sandy Sanders
Sandy Sanders

I tuned S.U.'s, 'by the book', for years, but after adjusting with a 'Colour-Tune' kit I found my settings were too rich the old way.
Good luck!
Scott Barrow

Thanks a lot for your help.

Smoke: no
Loose connections: nil, everything checked
Float level OK
Jet petrol level: I thought when float level was right, then the level in the jet is automatically correct, too. Am I wrong here ?
Petrol pump: I noticed rapid, irregular pumping noise when very hot (when the engine misbehaves). Cold starting. a few ticks, then quiet. So I think it's not the valves in there. Rather pumping something not liquid (air sucked in, but where? vapour?)
Coil, points, condenser: all renewed upon engine overhaul ("sports" coil fitted). Perhaps a case of "if it ain't broke, don't fix it", i.e. new parts worse than what they replaced? Have to check them all.
But it's not misfire: I set the idle a bit higher, then the engine didn't go out when I stopped the car, and idled regularly. But stepping on the accelerator (no gear engaged) simply did not make an rpm increase for 3 seconds or more, the engine just kept idling with the throttle open, before slowly picking up. Above something like 2000 rpm throttle reaction was normal again.
Plugs have greysh deposit, like they always had, I think it's from the lead substitute.
Remo Peter

Because of the irregular pumping I'd focus on the fuel delivery by the pump for now AND the condition/free movement of the float-needles.

<<Jet petrol level: I thought when float level was right, then the level in the jet is automatically correct, too. Am I wrong here ?>>

No, you're not! By looking at the fuellevel is the jet you can double check the correctness of the float level.
Willem van der Veer

Willem is right. If you switch on the fuel pump and then remove the dashpot and piston, you can look down the jet and see the fuel level. It should be about 1mm below the top of the Jet/bridge. This is the ideal fuel level regardless of where you think it has been set in the float bowl.

Since the original settings for float levels were written in 194?, the fuels have changed quite a bit.

Does anyone know the specific gravity of pump fuel in 1946 compared to 2006 ?.
Tony Slattery

Now that's interesting. That's the first time I hear fuel might have changed so much to be unsuitable for standard carb settings. I also noticed the stuff smells different since a while. Would this mean that conventional carbs, like Webers, would now need different jets, too ?
My float level has been set using the famous rod under the fork (I even have a special carb tool set from the 1960s that includes such a rod, which doubles as lifting tool for the piston to check idle mixture: Just remove the damper and stick the hollow rod into the piston: you have a handle and don't have to remove the duct from the air cleaner: very clever!)
I will check my fuel level and report back.
Still I'm puzzled why the engine should behave properly when half-warm, so badly when hot.
Remo Peter

The reason for looking at the fuel level at the bridge is that there have been several thickness of washers used between the float body and the body of the carb. Even though you have set the float level correctly with the bar it can be off. The thickness of the mounting washers determines the actual level then. I remove the piston and needle and look down the jet to see the fuel level. This is with the key on. I have used 1/8" below the bridge as a setting and have had good sucess with it for the past 25 years.
Sandy Sanders
Sandy Sanders

This is new info for me, thanks. Neither the period manuals nor the one issued by Burlen Fuel Systems mention the possibility of different thickness washers. I wonder how many SU's apparently set up correctly have their level wrong.
Remo Peter

The density of water is 1 kg./cu. M, or 1000 gm/ cu. M
The density of gasoline is 737.22 gm / cu. M
The density of Ethyl Alchohol is 789.2 gm / cu. M
There is approximately 15% alchol by volume in U.S. gasoline in my neighborhood, so the density of the mixture would be around 745 gm / cu. M.
Scott Barrow

Now if we can find out what the S/G was for pump fuel in 1948, then we might have some idea about variation in float levels from design to today.

Those Y-Typers with twin carbs should check their float levels by the bridge level method, as it is critical for twin setups to be balanced accurately.

Back in the mid-80's, I had a Midget 1275cc that had an annoying hesitation/hunting while at cruising speed. After tuning the carbs dozens of times "by the book", I tried the float level setup by the bridge method and WOW, what a difference & so simple when you think about it.

The further down the jet the fuel level is set, the greater the energy (in the form of low air pressure) that is required to draw the fuel into the airstream. It needs to be below the bridge/top of jet to avoid flooding, but not so far down that it is not easily drawn into the airstream.

Try setting your float levels by the bridge method, you will notice the difference !.
Tony Slattery

If you do remove the dashpots though, be sure to mark them (I use snowpake/white out) as to which is left and which is right. Also be sure to mark them so that you know which side is engine side and which is not.

Paul
Paul Barrow

I also made a little plywood stand for the dashpots, marked front & rear, to hold the dashpots upright to save the oil and to prevent damage to the needles.

I'll send Paul a photo for the website - it not a pretty thing, but it is effective.
Tony Slattery

I'm not yet among those with a twin-carburetted Y (anyone out there with a spare in/ex manifold ??). But at least I removed all steps between the different parts of the induction. My dashpot has punchmarks to show its correct way round. Must have been made by a PO, SU apparently never cared to mark the dashpot/body position or to make installation foolproof by some notch/tab arrangement. To protect my needle, I have stuck an old winebottle cork over it.
Remo Peter

From reading old articles about, 'Pool Petrol' which was the common lot in in Britain in the late '40s, it seems 'Pool' was a mixture of whichever grades of gasoline were available, and quite different from Regular Grade fuel here in the U.S. at the time. TCs have a micrometer adjustment for the distributor timing which was to be set by the operator in order to compensate for the weekly fuel rations' particular octane rating.
Even the lowest grade of unleaded fuel now available in the U.S. is far superior to the 'Pool' of the 1940s. The shop manuals for our cars stress the need for frequent, 'de-coking' of the cylinder head and valves, the result of very incomplete combustion. I think 'Pool' was probably 'natural' gasoline with little or no lead additve and being a admixture from various refineries.
Scott Barrow

Remo

I recently got an email from Shadetree Motors in the US telling me they had "used Y manifolds and carburetors" available - price for the manifold and carbys was $US200.

Their email address is kelsey@value.net if you want to follow up.

They also had a "used Y grille" (where would you find a new one?) for a measly $US1000 :-0
David Robley

OK, I'm at my limit here so I'm depending on you folk to check my work.

We are trying to establish the variation in float height from 1945 to 2006, due to variation in fuel quality.

The internet and Scott have provided the following.
The density of water is around 1.000g/cm3
The density of current fuel is around 0.745g/cm3
The density of natural gasoline (1945 pool fuel) is around 0.711g/cm3
The weight of a carb float is around 22.75g (I weighed 8 floats at 182 grams total)
The diameter of a carb float is around 47.4mm

By my calculations, 0.566mm depth of float displaces 1cm3 of volume.

So in water, 22.75 x 0.566 x 1 = 12.876mm depth of displacement.

In 1945 fuel, 22.75 x 0.566 x .711 = 9.155mm depth of displacement.

In 2006 fuel, 22.75 x 0.566 x 0.745 = 9.592mm depth of displacement.

So the fuel level will be 9.952-9.155 = 0.437mm lower than it should be.

I think I have now de-bunked my theory that changes in fuel density since 1945 have had a marked effect on designed float levels.

Or have I missed something ?.
Tony Slattery

I just checked the density on a drum of 102 Octane Racing Fuel - 0.778.

Racing Fuel 102, 22.75 x 0.566 x 0.778 = 10.018mm

10.018 - 9.155 = 0.862mm

If I can move the decimal point, I'd have the answer I was looking for !!!!!!!.
Tony Slattery

Keep them coming, I really like these theorized calculations! (and I thought my tire-ratio calculations were far-fetched....)
Willem van der Veer

Dear David Robley
Re. the used manifolds: I already have a spare original one (and a rad grille on my partner's living room wall, bought for something like $220). To fit dual carbs, I'd need the bits for a TD/TF. But I am reluctant: that big oil-bath filter on top gives the engine that "vintage" look...
Remo
Remo Peter

Now the weather is cold, no more problems with the engine's behaviour. 95% proof that the problem was related to high underbonnet temperature. So, I'll order that instruction on how to make a heat shield, put that on, add an insulating induction flange, and wait for days getting hot again.
Remo Peter

Peter

We are looking at adding a section to this for adding a heat shield to Twin carburettor engines too. Currently the data sheet only addresses the single carburettor.

Paul
Paul Barrow

I dismantled a Y-Saloon some time ago (YA2413) which came from a warm part of Australia (Darling Downs - I know there are warmer places !).

Interestingly, the fuel supply line for this car went around the rear of the battery box to the fuel pump and not between the engine and battery box. The pipe was well made and would have passed as an original if one had not noted that it's route was unusual. It is considerably longer than the original and is one piece all the way from the fuel tank.

Perhaps another attempt at reducing under bonnet temperature in the fuel supply. I do know that SU pumps are not good at sucking vapour !.
Tony Slattery

An interesting observation Tony, and certainly this may have helped as a fair amount of heat would be avoided. They would still have encountered the main problem though of heat from the exhaust manifold though. Interesting though!

Paul
Paul Barrow

I improved mere a few in SU carburetor of 1-1/4(Type H2 Spec.456). If original SU pulls a choke lever, a throttle and a Jet will move simultaneously. In this case, a plug may become poor. I attached the cam to reference for SU carburetor of other types. Only a throttle moves by the 1st operation and a throttle and a Jet move by operation which is the 2nd time. This improvement is very good when the winter engine has got cold completely.

http://roppatu.hp.infoseek.co.jp/cgi-bin/img-box/img20061213205029.jpg
XPAG Masaaki Sakaguchi

I have had far too much experience with SUs (Cooper S, Jags,Aston DB5 etc). Setting the needle closing forks is at best a rough guide. Other factors are the fuel pressure as higher pressure will require the float to come higher in the bowl to overcome the force down on the needle. I have encountered the washer thickness problem on a Mk 2. Also, when cars are 50+ yrs old who knows where the bits have come from? My Aston has identical floats and bowls but when set with the bar the front is overflowing the bridge and the back one is way down the hole. (Middle carb has different fork setting (5/16") anyway so the gifted amateur without the manual will be completely bamboozled.)
Question: When people say 1/8" or 1mm below bridge/jet top, which do they mean? Jet top can easily be more than 1/8" below the bridge, in which case setting 1/8" below the bridge will have the top of the jet submerged.
Doug Young

Another expensive trap is to assume new carbs from Burlen et al will be built to original SU tolerances. I bought some new replica H8s - sand cast dashpots etc - for a C type engined XK140. Never ran well and misbehaved terribly on hot starts. Turned out there were 3 problems. 1. Dashpots not a snug fit on body (1/64" play in all directions) 2. Central guide in dashpot for piston not concentric with the inside of the dashpot, and 3 Needle boss not concentric with the rest of the piston. Centre the jet and piston fouls body of carb (but only when hot.
Resulted in piston sticking at different times and places on the bore and between the base of the piston and the body of the carb. Took years to fathom, Ended up removing lots of meat from the part of the piston which enters the carb body. Now doesnt stick anymore.
Doug Young


Dear all

Has anyone of you carburator professionals got a cut-away drawing of a SU carb, so one can see what you are referrring to, when talking about "bridge/tops" and the various levels.
As an alternative: are there any such drawings in either the Y or the TD workshop manual (TD for the twin carbs, as I restore twin YT-carbs).
Anton Piller

How long did the car sit during the rebuild? You mentioned the gas smelled funny. If it set very long, I would try fresh gas. Also, I put a new Sports Coil on my car. Failed almost immediately. Put the old coil back on, problem solved.
Larry Brown

Anton

I have exploded parts diagrams of the SU carbs as fitted to the Saloon and to the Tourer. I will dig them up and think about posting them in the Technical Data section of the Technical Centre as PDFs.

If you need one sooner - let me know

Paul
Paul Barrow

To try and explain without a picture (perhaps I should draw one and send to Paul - note to self !), the SU carb has a circular throat. Directly below the piston (and therefore at the needle and jet), there is a raised flat "bridge" accross the bottom of the circular throat.

The top of the jet when first installed should be at the same level or height as the surface of the bridge. Adjusting the idle mixture may lower the jet a little, but not too much (less than 1mm - if more than this you have an air leak around the spindles or manifold).

The aim is to have the fuel level 3-7mm below the bridge, so the least amount of energy is requiured to draw the fuel into the airstream through the carb. If too low, you will have a flat spot on opening the throttle to accelerate, and a fluttering of engine RPM at cruising speed (part open throttle).

Remember that the fuel supply is a simple syphon from the float bowl to the jet. The height in the bowl (controlled by the float height) is related directly to the height in the jet - 1mm higher in the bowl will give 1mm higher in the jet.

cheers

Tony
Tony Slattery


Dear Paul
Yes please, let me have the exploded view asap. I think I understand now, what "bridge" is referred to - Tony Slattery's verbal explenation helped. To veryfy this, I try and include a photo of a carb-housing in my return mail to Tony.

Please note!!! I am in the posession of pictures and ownwes details of two hithero unknown Y/Ts! I will folwaed very soon

Anton :o)
Anton Piller


Dear Tony

Thank you for your fool-proof explenation. I understand now. It is the machined flat at the bottom of the neck, whith the Jet hole in the centre.

Thank you
Anton :o)
Anton

Exploded parts diagrams now available on the site under <TECHNICAL CENTRE> and <Technical Data>!

Paul
Paul Barrow

Remo - For a Fuel Delivery Guide that may help you determine if you have a problem in that regard, see my article at: http://www.custompistols.com/cars/dave/ddFuelDeliveryTroubleshooting.htm
I would not worry about the specific gravity of today's fuel, if that was the problem, a lot of people would be experiencing the same symptoms. I would take a close look at the ignition side of the equation. Someone wiser than I once said, "if you are having problems with the SU carburetors, first check the Lucas ignition. Cheers - Dave
David DuBois


Thank you, Paul, for the exploded view of the carbs.

Best regards
Anton
Anton Piller

This thread was discussed between 25/09/2006 and 08/01/2007

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