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MGB MGB 76 lack of power/fuel consumption

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don62

don62

Senior Member
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Hi,

I drove for the first time last weekend my MGB.
I was making not more than 18 miles/gallon.
It was starting well when cold but when hot I had to keep my foot on the gaz (black smoke)otherwise it wouldn't start.
On deceleration it was dying. I had a hard time to go over 60miles/hour, it was falling on his knees, like having too much fuel. My manual choke doesn't lock anymore, do I have to replace the cable? Could this be a fuel/air mixture problem? Choke or carbs adj?

Thanks

Don
 
Some things we need to know to help us get a feel for the car:

It should have one carb, a Zenith Stromberg. The fuel pump should be an SU electric unit, mounted half-in, half-out in the front of the trunk floor on the right.

Some "usual" causes of what you describe:

If the pump is an aftermarket one, it may be supplying more pressure than the needle/seat in the carb can regulate. No more than 5 pounds per square inch, at a maximum... Float damaged or float needle not seating can also cause it.
 
Is the cat still installed & when the car acts like that is the cat glowing red?
 
Hi

The car has a dual carbs,HU Type HS4.
The fuel punp is original.
The cat was removed.

Thanks
 
Step one then would be to check floats and needles/seats.
 
Step 2 if 1 doesn't solve it is to look at the timing.

Now, a question: do you still have the brake booster? If so, somehow put a piece of double sided tape or something on the rear carb fuel bowl, something that'll transfer to another piece of metal if the 2 pieces come in contact - powder, whatever.....get her out on the road & torque the engine (I mean load her up in the power range)...when you get home, look to see if the tape or powder, or whatever has transferred from the fuel bowl to the brake booster....a problem with HS4's a boosted brakes is, when the engine lays over under torque, the fuel bowl knocks on the booster - over time it can crack the bowl....one of the reasons I use HIF-4's for that modification.
 
Interesting tip.
 
Ahhhh, Another tip to put in the memory banks. Who would ever think?
confused0031.gif
 
Yes, I do have a booster.
The coolant temp guage is always in the cold.
Should it be in the center when hot?
Is there a temp sensor? Could it be the reason why the mixture is too rich? The spark plugs are dry and black of carbon.

Thanks
 
Temp sensor is in the head just under the thermostat cover. Gauge can be "field tested" by grounding the wire and turning the ignition to the first "on" position. Needle should go "full scale". If it does, the sender is faulty. If it doesn't, the problem is in the gauge or wiring. It wouldn't have any effect on the way the engine runs, though.

Sounds like a rich running mixture so far. Could be the choke levers are sticking open slightly.

Best first step is to get a Bentley manual for the car.Then you can identify the parts and have a good proceedures book to study.
 
Thank you all for your help.

I still have the same problem but I realised few problems.
Even if the choke is off, the jet head on the first carb is slightly on. The feed tube was cracked(loosing some fuel.
There is some fuel appearing on the shaft of both carbs between the jet locking nut and the adjusting nut. Is this normal?

Thanks

Don
 
No, it should be dry. Most likely the seals have aged and the carbs need rebuilding. The most reliable supplier for such kits is Joe Curto in NY. I don't know about Canadian suppliers, but Joe is the one I'd suggest for about anyone.

As for the jets, they should normally be recessed somewhat in the bores as you view them from the intake. To get a close mixture the adjusting nuts should be used to bring the top of the jet body level with the "stage" in the intake then lowered twelve "flats" on the adjusting nut. This will get a good approximate setting to work from if ~all the other~ parts of the carbs are functioning as designed. If you have evidence of fuel externally something else is happening and needs attention.
 
the jet head on the first carb is slightly on[/qute]

That's an easy adjustment you'll want to make. However, Doc's correct - carbs probably need a good going through.

Oh, when you get around to setting your carbs, set the one to which fuel comes first; then set the 'secondary' carb
Click to expand...
 
Thanks again you made my day!

Afetr talking to the last owmner, he told me that the carbs were rebuilt last fall. So, I just had to replace the broken feeding tube to the carb. Re-install properly the jet(one of them was stiff, thus keeping the choke on a bit. Finally retuning the jets and trottle. The car runs great and starts A1 and no more black smoke. I looked a bit weird driving the top down at 25 degree but I had a great time,lol. I brought her up to 85miles/hour, what a feeeling.

Don
 
I fine tuned the carbs and as described above, they were rebuilt last fall. I still have some fuel leaks from the jet heads eventhgough I recentered them as described in the procedures. The jet heads moves freely and the piston as well. Help, I'm out of clues....

Don
 
Check the float valve heights, should be about 1/8th I think if less you will drain fuel to the carb. The fuel level in the float chambers must be below the choke/seat for the needle.
 
I Checked the float valve heights, it was more than 1/8th. It's still loosing fuel from the jet heads only when the car is parked and not running for an hour or more.

Don
 
It is happening all the time if at all. You just don't notice when the engine is running as it is burning the fuel. Susprised that you have not noticed that it will not idle down properly.

The only thing, and I repeat, the only thing that can cause fuel to come through the needle seats is gravity when the engine is off. Check the float levels again please. And double check the manual to insure that you are checking them correctly.

I had this same problem on my SU H1s and must have checked half a dozen times till I caught my problem. This is a real critical adjustment.
 
I will recheck the float levels again. If it caused by the gravity, does it mean that the fuel levels in the chambers should be lower than the top of the jet heads?
Should I adjust my choke cable 1/16th free as soon as the spring tension gets on it or if it just as soon as the jet heads start opening?

Thanks

Don
 
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