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MGB Running out of gas symptoms

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Rut

Rut

Obi Wan
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Good morning all. I have a somewhat minor problem that I’ve been unable to solve on the MGB. When running along at 65+, in 4th or 4th OD for 10 minutes or so, the engine sounds like it’s being starved for fuel. It’s a performance build using a later block after I blew up my original 1970 engine...9.5:1, VP12 cam, Abarth exhaust system, big valves, etc. I’ve removed the vented gas cap, checked the fuel filters and verified that the fuel pump is working well. I have a new gas tank and rubber lines, but the original hard fuel lines. The carbs have been rebuilt and adjusted and it runs just a touch rich, but not bad and drivability is excellent. If I let off the gas for about 5-10 seconds it cures itself until I run another 10 minutes at speed. This has been going on for a couple of years, but hasn't bothered me enough to seek help...I thought I could figure it out. Now I need your expert trouble shooting to guide me!
Thanks, Rut
 
Just a thought, but you say the fuel pump is working well. Is it the right fuel pump (providing sufficient pressure) for the engine/carb setup you have?
Tom M.
 
Had this symptom with a car years ago....turned out to be a small rub on the hard line. It wouldn’t leak petrol when stopped, so was a joy to find.
 
I’ll check both the hard line and fuel pump for leaks, pressure and flow. Iirc, the fuel pump is a generic 2-4# variety that replaced the pump that was on the car when I bought it. Thanks for the thoughts and it could be either one of those issues, but never any smell of gas.
Rut
 
If you're still using the metal braided lines to the fuel pump inspect them for cracks and if they are hard. I kept loosing power randomly as they had cracked and would randomly suck air starving the engine of fuel. I cut the banjo fittings off and replaced the hoses with the gates barricade hose. Also it could be electrical. I had something similar in my gt and in the end it was a bit of corrosion at the big wire junction on the right side of the engine bay. Look for the white wires and give them a good cleaning.
 
Insufficient volume from the pump? Pump gets too warm after running at higher speeds and starts to fail intermittently? Full tank vs. less than half full?

My WAG would be to replace the pump with a Jag/Rolls "double-side" S.U. Not cheap but correct pressure and good volume through the whole range. That pump supplies V-12 Jag (four Zed-S carbs) and V-8 Rolls (two HUGE SU's) engines. It would keep up with what you describe as the engine build.
 
Thanks again and once I get my back up and running I’ll check all this out. I’ve got a few other things I need to check out that I’ve neglected over the last year, so I’ll add these to the growing list.
Thanks, Rut
 
Update...new fuel pump, filter and she runs well, but symptoms persist if I run at a steady high speed. I can let off the gas and it will catch up and run fine. The engine runs just a touch cooler than normal on the gauge and nothing to indicate anything is amiss, but I now have a little run on when I turn it off...same gas as always, timing unchanged, plugs look good, etc. I’ve got a friends MGA taking up the MGBs space in the shop and I’ll dig into it some more once the A is gone. Just for grins I’ll post some spark plug pictures.
Rut
 

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Off-the-wall idea here: Gas cap? Try running without one and see what happens.
 
Assuming good methodology (And I know ya Rut, so confident the plugs are in order!), odd the two middle ones appear to be running richer than the outer ones, and considering the firing order, could you be seeing a distributor/cap/rotor issue as a combination with coil/wiring contacts?

"If you've got carburettor problems, it's probably electrical!"

Just thinkin' out loud.
 
Dr,
They are in order and I thought the same thing. I had the distributor rebuilt by Advance and he set it up to match the engine mods and I’m using points. I’ve also removed the vented cap just in case, but the symptoms still exist...it’s a minor problem, but one that puzzles me. I’m going to install a dual AFR meter so I can get an idea of what’s happening and different loads and RPM...I’ve got one on the bugeyes 1275 and it really helps.
Thanks, Rut
 
Have you checked the hose from the tank to the pump for cracks or vacuum leaks? Had this same thing happen on our 72 and the flexible line from the tank to the pump had a crack in it and it was sucking a very small amount of air in. It ran fine until more demand was put on the pump and the carbs would slowly drop in fuel level, letting off the throttle the carbs would fill up with enough fuel to resume running. Just a suggestion. PJ
 
DrEntropy said:
Assuming good methodology (And I know ya Rut, so confident the plugs are in order!), odd the two middle ones appear to be running richer than the outer ones, and considering the firing order, could you be seeing a distributor/cap/rotor issue as a combination with coil/wiring contacts?

"If you've got carburettor problems, it's probably electrical!"

Just thinkin' out loud.
Click to expand...

I had a similar issue with Ms Triss - turned out to be the coil. I replaced it with a proper Bosch coil and the difference is like night and day. There is much interweb conversation about what makes a proper Bosch coil

this is the one you want https://www.busdepot.com/catalogsearch/result/?q=blue+coil&dir=desc&order=default_sort_order

here is the article

https://www.ratwell.com/technical/BlueCoil.html

Jeff used to sell them but it seemed like he couldn't get stock
 
PAUL161 said:
Have you checked the hose from the tank to the pump for cracks or vacuum leaks? Had this same thing happen on our 72 and the flexible line from the tank to the pump had a crack in it and it was sucking a very small amount of air in. It ran fine until more demand was put on the pump and the carbs would slowly drop in fuel level, letting off the throttle the carbs would fill up with enough fuel to resume running. Just a suggestion. PJ
Click to expand...

Paul,
Original steel fuel lines, but all new rubber hoses, pump, gas tank, etc. I’ll go thru to entire fuel system to see if I missed anything and the clear fuel filter that’s never full always freaks me out...I may change to a different style.
Thanks, Rut
 
JPSmit said:
I had a similar issue with Ms Triss - turned out to be the coil. I replaced it with a proper Bosch coil and the difference is like night and day. There is much interweb conversation about what makes a proper Bosch coil

this is the one you want https://www.busdepot.com/catalogsearch/result/?q=blue+coil&dir=desc&order=default_sort_order

here is the article

https://www.ratwell.com/technical/BlueCoil.html

Jeff used to sell them but it seemed like he couldn't get stock
Click to expand...

JP,
It won’t hurt to check since I have a couple in the shop and it’s easy to swap.
Thanks, Rut
 
Rut said:
Paul,
Original steel fuel lines, but all new rubber hoses, pump, gas tank, etc. I’ll go thru to entire fuel system to see if I missed anything and the clear fuel filter that’s never full always freaks me out...I may change to a different style.
Thanks, Rut
Click to expand...

Fuel filter won't be full - at 2-4 pounds pressure it doesn't.
 
Like JP I had an XJ6 that would cut out while driving to work. Pull over start and go to work. Same thing on the way home. Started watching exhaust and right before cut out, black smoke (rich?). Opened bonnet and coil extremely hot. Tightened electrical connections cured. Just a could be.
 
I had the same problem on my E-Type at higher rpms. It turned out the SU carbs were leaning out at higher rpms resulting in fuel starvation. The solution was to replace the carb fuel metering needles with a set that was richer at higher rpms. I went from the standard UM needles to UO needles which resulted in a 23% increase of richness at full rpm. Some use UB 11%, UI 16%, UE 23% (what I used) or UO 37%. All these needles have the same fuel mixture ratio at idle. I was told that today's gasoline is not as "powerful" and thus the carbs require more fuel at higher rpms. I am not sure which needles are standard in your carbs but a change to a more rich needle may solve your problem. It certainly solved my problem. Good luck, James.
 
Any progress on this, Rut?
 
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