Question
My baby and my jealous
Hello, My 1979 MGB w/dual SU HS4's dies at high speeds...but let me back up and tell a little about what my mechanic has found and fixed and what I have found and fixed:
First, it seems that back in ‘89 a previous owner had a mechanic overhaul the engine and even had the block “sent out to have cleaned and {unreadable}…”; “Camshafts, valves, lifters, oil pump and head replaced”, etc. Now it seems that the shop replaced the stock head with a “used” cylinder head from a different year MGB. It has a slightly offset groove/notch where the block and head (or head and valve cover?) meet. This notch is designed to allow the oil to move around the engine/valve cover properly (?) {I’m going on what my mech said}. He also said he was amazed that the car ran for so many years with this “odd head”. I’m thinking that the car didn’t record very many miles with speeds above 55, as it only had 22xxx miles on it at the time of the overhaul and now has 36xxx on the odometer. I'm certain that the car sat for many of it's years, as I am the third owner. My mech said that he fixed the problem by replacing the rear rocker pedestal…did something with the oil pump/pressure sending unit….something…so that the oil pressure was good again. He also:
• Replaced the timing chain and reset the cam timing, advanced the cam timing (by hand) because the original had been replaced with a high performance cam.
• Replaced the ignition with high performance coil and wires.
• Set all valves to .13”
• Adjusted the points
• Reset both carbs: Replaced the floats, needle and seat (upgraded, actually, to the Grose Jet w/a ball instead of standard all brass needle and seat).
I left his garage and drove her for about 1/2 hour at 60-80 mph, stopped for dinner and when I got her back on the highway, she died (after 5 mins) like fuel was not getting to the carbs. I waited 10 mins with the hood opened and she got me home. The mech. made a house call and he set the timing and tuned the carbs (!?) to a nice low idle (just under 1K). I thought it sounded rough but she got up and got zippy after she warmed up...died again after getting really warm (without the mech around, of course). I found my vapor canister in the boot had lost the top screw and fell 90 degrees, so I propped it back upright and changed the fuel pump for good measure. Still had problems staying running after high speeds and/or warming up.
I researched "vapor lock" problems and everyone says "doubtful with SU Carbs” but I'm leaning toward the heat shield failure causing the fuel to boil, either in the fuel line between the carbs or in the float chamber… or there is a vacuum problem somewhere. Here is the tricky part(s):
• Most of the emissions equipment has been removed
o No air pump, no air manifold components
o No EGR Valve, Carbon Canisters (and no second fuel filter that I can find)
o There is, however, an oil separator hose to a “Y” and one hose to the front carb and one to the rear carb. Do these even need to be attached or just plugged at the source…or should they be attached to something else (float chamber overflow tube instead of carb body overflow)? It seems strange that this is still attached if all other emission equip is off. Also, the last owner replaced these hoses with “pretty” blue tubes that seem to be “pretty flimsy”. About a year ago, my mech had the fuel tank cleaned and added a new bladder (lots of rust in there). He states that he “flushed out the fuel system”. Is this enough or should I consider replacing all hoses from the fuel filter forward and add steel mesh over the lines, as many bloggers say?
Also, I watch the oil pressure fluctuate A LOT! I’ve never been an observer of oil pressure and its affects (partly because this is my first MG and I was MUSTANG guy in my past life…never saw this many problems before!)
Can you help? ….I swear, if it is just a matter of replacing the air filter, or using a lighter oil in the dash pot, I will probably pass out - Ha, ha!
Best,
Kristopher
AnswerWow, you have had a lot of trouble.
For your information, there were two slightly different designs of cylinder head, one with a direct feed to the rockers and the other with an offset feed. The only difference is they need a different rear support bracket for the rocker shaft. If you had the wrong one, no oil would be delivered to the rockers, the rocker shaft and the rocker bearings would wear out pretty quickly, and the tops of the valves could be damaged. This is not major, and easy to fix.
UK cars have a mechanical oil pressure gauge, not electrical. My gauge is calibrated up to 100 psi, but the normal pressure is 40 psi. It never varies while I am driving. With a hot engine it can drop to 20 psi on tickover. Anything less than that needs investigating. I never use modern thin oil. I only use 20W/50.
The pipe from the front tappet chest must be routed via a plastic Y-piece to the twin SU’s. If you don’t connect them, too much air will be sucked into the carbs, giving you a weak mixture.
I am aware that modern fuel is causing lots of trouble with older cars. Every day I get complaints about fuel vapourisation, so it is possible that you have that problem. I assume that your twin SU system has a big heat shield?
I still think you have a fuel delivery problem:
Pull the fuel delivery pipe off the carburetter and put it in a jam jar. Switch the ignition on for 30 seconds and switch off again. How much fuel is in the jar? An MGB at 75mph does about 20 mpg. That equates to 3.75 gallons per hour, or 30 pints per hour. So it needs half a pint per minute. Is there at least a quarter of a pint in the jam jar? If not, check the following:
1. Take the fuel filler cap off and look at the back. Does it have the word VENTED stamped into it? I think that emissions cars had non-vented caps. Your car had the emissions equipment removed, so maybe you have the wrong cap.
2. Inside the tank, attached to the suction pipe, there is a plastic filter. It could be clogged. With the tank less than ¼ full, remove the fuel gauge sender from the side of the tank. The filter will come out on the end of the sender.
3. Inside the base of the fuel pump there are two valves. One of them will have a metal gauze filter over it. This must be clear.
4. There should be an inline fuel filter in the flexible pipe going to the carburetters. Is this clean? If not, replace it.
I hope this helps.
