MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Carb cutting out during race

While racing last weekend at the Pittsburgh Vintage Grand Prix, I had a strange problem with my front carburetor and hope someone might have some ideas. I run a tuned XPAG with a shaved and ported iron head, Derrington header, stock but ported intake, 1 1/2" Mark II carburetors with Grosse jets, and Facet fuel pump with a new fuel filter. The front and rear header tubes have exhaust gas temperature probes in them to monitor mixture and balance.

Among the 20 or so turns at PVGP, the course features several sharp right turns. If I dropped to second gear, as I powered out of the tighter right corners, the car would completely loose power and the EGT for the front carb would drop to the bottom of the scale. It really seemed as though there was no gas in the carb but I suppose it is possible it was completely flooded. After a short coast, things would start to come back, a few sputters and then full power. If I stayed in third and slogged along, there was never a problem.

In between sessions, I took the float lever and jet apart and blew everything out, lowered the float level a bit because it seemed high, made sure the angle between the carb body and float body where 90 degrees, checked the fuel output at the carb. Everything looks fine. I'll take out the Grosse jets and switch to standard float needles but really don't know where else to look. Any and all suggestions would be appreciated.

It's going to be difficult to test until my next track event. Terrorizing my neighborhood with an open exhaust car is frowned upon.
Kurt Byrnes

I don't have a clue but I'ld love to take a lap with ya! Anywhere near Pottstown? Might be heading there next fall.

Can you hear me drooling yet? Sorry - couldn't resist.

Ed
'50 TD
efh Haskell

I just had a similar problem with my TD. Found that the fuel filters were full of junk. hitting the throttle and it would cut out and then come back when I backed off a little. Filter would suck the stuff into the thing and then drop it when pressure dropped off.
Tom Maine (TD8105)

Ed - The closest venue I can think of would be New Jersey Motorsports Park. VSCCA, VRG, and probably SVRA have events there so... it could happen. Attached is a shot from last year's PVGP, bottom of the serpentine.

Tom - I thought about that too but I have a brand new filter in-line after the pump and confirmed at the track that I took out the thimble screens from the carb inlets.

Kurt Byrnes

The Fawcett pump probably puts out about 6.5 psi and can overpower Grosse Jets, and standard needles for that matter. Put in a pressure regulator and regulate down to 4 psi.

You could also, as Tom states, have debris in your fuel lines. Disconnect the fuel line at the carbs and run the pump to clear the debris. Then flush the lines between the carbs. Clear the filters in the float bowl lids.

In an ideal situation the level of the fuel would be level with the bridge in the carb, but cornering and accelerating would be impossible as fuel would spill into the bridge and hence the airstream. So, once you have your fuel pump and lines straightened out, set your fuel level so that it is at .120 to .200 inch below the bridge of the carb (Go for .160, if you can, lower if you have problems with acceleration and turning due to fuel slop while racing). You can fiddle with the jet on choke to find the fuel level, and then measure the depth of the jet below the bridge with a dial caliper to find the level of the fuel. Then, set your jets to .070 inch down and adjust your mixture from there, adjusting each nut the same number of flats for max RPM. Then enrichen one to two flats. Depending on the needles you are running you will end up between .063 to .080 down from the bridge.

warmly,
dave
Dave Braun

Kurt, I've read about fuel starvation in hard cornering with later SU's with the hinged floats. The fuel pools up to the outside of the bowl due to the G forces and shuts the flow off. In your case you might be seeing something of the same issue even though the bowl is not hinged. I think changing the grose jet out would be a good start. I believe you can replace the jets with a larger diameter version which should also provide some relief. You could also try the mga twin cam float bowl to lid spacers avaiable from Moss although it's a mixed bag of opinions on value.
LaVerne Downey

Dave, I'm actually running a Facet Posi-Flo low pressure pump rated at 1.5-4 psi but I did not check the pressure at the carbs. Good idea and I will do so. I'll need get some BSP adapters to hook up a fuel pressure gauge. The pump is rated for 23 GPH, do you think that's enough flow?
For carb setup, I follow a procedure from Craig Chima which sounds very similar to what you're describing but Craig says to set the jets 0.060-0.065 down and never fiddle with the nut. If the mixture isn't right, change needles or move them up or down in the piston. I usually try to get an EGT reading around 1200-1250 so I don't detonate. I did try the MGA rubber float bowl mounts with bolts but struggled with the float levels and gave up.

LaVerne, do you have any more information on larger diameter jets? I'm hoping I don't have to resort to the spacers, the front runners I know racing T's are not using them.
Kurt Byrnes

Kurt

When the factory was prepping these cars one of the modification was to add a spacer in-between the float bowl and the top or cover. This seemed to stop the fuel starvation under hard cornering.

Tom
T. L. Manion Thomas

Kurt - 1.5 - 4 psi should not cause a problem with the carbs, particularly if the Facet is mounted at the back of the car. Even if it is mounted in the engine compartment it still shouldn't cause a problem. If it does, there are some Facet pumps that have a maximum output pressure of 3 psi, which definitely won't cause a problem, or you could do something really unorthodox and put a SU pump in the car. From all the reports I have had over the years, I wouldn't mess around with a pressure regulator, they seem to cause more problems than they cure. For a listing of BSP filltings, see the chart, Fuel Line Fittings in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

It won't cost anything but a few minutes, so you might swap floats and see if problem slips to the rear cylinders, but I doubt it.

I do not like the pump location up high in front, where it has to suck fuel from the tank under a vacuum (actually, atmospheric pressure is pushes it) and gas can vaporize from factors such as heat, low fuel level and bad tank venting. I screwed a Facet right onto the elbow under the tank. Also stuck an SU under there for for primary pump since it is quieter, and leaves the Facet for spare pump and quick switch to reserve. They both run through a pressure regulator that dials pressure down to about 2 psi and serves as another set of check valves.

Thoroughly check out gas cap for venting. Checking gas flow can be misleading because the tank will be at atmospheric pressure at first and then can drop into partial vacuum later on.
Jim Northrup

I suspect he's running a fuel cell Jim.

Kurt I remember seeing a reference to using a larger float jet from Moss when they were developing the supercharger for the B. Seems the smaller stock jet wouldn't flow enough under full throttle applications to keep up with the engine. I believe I'd get ahold of Joe Curto and explain your problem. I bet he has an answer. I suspect whats happening is your presently getting a fuel shut off from the floats due to the G forces on the hard cornering/braking and a combination of insuficent recharge when getting back on the throttle. I'd have a good look at the bowls and make sure they don't have any scoring that might grab hold if pushed to the edges of the bowl. You could try and reorient the position of the cap to see if the movement of the float arm/needle provides any change. You might also consider splitting the lines before the carbs instead of running to the rear and then the front on the standard setup. It would then feed the two carbs equally instead of hitting the rear carb first. Just some ideas.
LaVerne Downey

Wow guys, thanks for all the suggestions. I'm not running a fuel cell yet but need to install one soon to comply with some of the race groups. I like the idea of splitting the fuel supply before the carbs. I'll run everything by Joe Curto and see if he has ideas and any better flowing jets.
I think LaVerne's suspicions are right but I did just remembered the front float developed a pin hole and sunk last fall and I replaced it with a spare from a set of 1 1/4" carbs. I thought I checked to confirm no difference in part numbers in the Moss catalog but could I have put in a wrong float? I'd heard new replacements were suspect and thought an original would be better.
Kurt Byrnes

Kurt, here is a link to the SU site with different float jet sizes. Looks like they show a .096 for competition TD's but I see a listing for .125 for super charged engines. Might see If Joe Curto has a couple.
http://www.sucarb.co.uk/ProductCategory.aspx?ParentId=257
LaVerne Downey

Hello Kurt
Sorry I'm a little late to this thread but have you given any thought to joining the MG Vintage Racers (http://www.mgvr.org ). There are several 'T'series racers in the group who I feel sure will be glad to help. I just wish I was actually racing my TD as opposed to still being in the build-up stage. But hay; Creg Seabrook called and I should have the rest of my wood order in about two weeks and my sprite seats are fitted and the fuel cell is on order, so's all right with the world. Hopefully I can join you at Pittsburg in a year or so, heard it's a great place to race.
Regards
Hiram
Hiram Kelley

Kurt, as Tom Manion suggested above look at Moss part # 370-505 fuel bowl spacers - they are designed to help eliminate the problem you describe.
BRIAN WARMUTH

Kurt-

Your questions might be better directed to the MG Vintage Racers group, who would have been well represented at PVGP.

Paul
VSCCA #52 MGTDMKII
Paul Gaynor

This thread was discussed between 26/07/2010 and 30/07/2010

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.