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MG MGB Technical - 1971 MGB - Loss of Power

Does anyone have any ideas as to why my new little MGB loses power at about 3000 rpm? It starts and idles beautifully, but as soon as you press on the gas, kaputt put put. You let the gas off, and it resumes idle.
We have replaced the fuel pump and it pumps well into the carbs. We have checked the distributor and it seems to be okay. We have checked the vacuum pressure off the manifold and it is an adequate 20.
Valve timing may be the ticket, but I would expect a bad idle if that was the case.
Any ideas very much appreciated as it is now in the garage, and ready to be ripped apart ;)
Thanks
Kerry
Kerry

Any oil in the dashpots??
Chris at Octarine Services

Hi Chris,
I haven't checked those yet. Can you elaborate a bit?
Thanks
Kerry
Kerry

Kerry,
Check your timing. If it is off the idle seems fine but the engine will not have any power to handle the torque.

Ron
R Garcia

Kerry,

Unless there is sufficient oil in the dashpots ( so that you can feel the resistance when pushing the damper down to screw it up) then the pistons will rise too quickly and give a lean mixture and no power.
Chris at Octarine Services

If you're stepping on the gas while stationary, it's likely timing or oil in the carbs.
If you mean when you're trying to roll it out of the garage under it's own power, check that the plug wires are back in order - 1-3-4-2 counterclockwise. It's amazingly simple to reverse 2 and 3, and end up with 1 and 4 carrying the whole load.

Steve
Steve Aichele

Kerry,

Also check the plug wires for cracks. I had the same problem with a MGC. I lived in Miami and drove to Utah and spent a few months there working. On the way back, the car ran fine until I needed higher RPMs. I stopped at a MG dealer I knew in North FL (1974), and the service department tested the car and found nothing wrong. I left the dealer, went two blocks ddown the street to a NAPA store, and bought new wires. Problem solved. My guess is that the wires dried out in the dry Utah climate and then cracked and started leaking spark. It's worth a look.

Chuck
Chuck Hassler

Check that the fuel filter is not plugged.

Jim
74B Roadster
James Conner

Thanks for the tips. We started going down the road of valve timing, but due to the 'normal' idle, we backed off. Actually, we got hung up on removing the crankshaft case (with the 1 5/16 " ratchet). We couldn't get enough torque on the ratchet without turning the pulley. We had crowbars wedged in every angle and still couldn't get the bolt off.
Any ideas are very much appreciated!

Thanks
Kerry
Kerry

Thanks for the tips. We started going down the road of valve timing, but due to the 'normal' idle, we backed off. Actually, we got hung up on removing the crankshaft case (with the 1 5/16 " ratchet). We couldn't get enough torque on the ratchet without turning the pulley. We had crowbars wedged in every angle and still couldn't get the bolt off.
Any ideas are very much appreciated!

Thanks
Kerry

PS - the loss of power occurs both when in motion and in neutral when you accelerate
Kerry

Not to rain on your parade, but IMHO you're barking up the wrong tree. removing the timing cover and getting anything done in there is a messy/cumbersome undertaking, and fairly unlikely to result in much unless the cam was wrong at the rebuild.

Check the basics - set valve clearances, get your timing set up, make sure your points are in good shape, make sure your carbs are getting gas and have oil in the dashpots. A clogged fuel filter/line or a weak fuel pump will sometimes behave this way - enough gas to idle, but not enough for high RPM. Then step through the carb tuning as described inthe Bentley manual.

If you get to pulling the timing cover (as a last resort...) an impact wrench works great if you can get it, or chock the wheels, put it in a high gear, and get the biggest bar you can on the bolt. As a later resort, rotate the engine to just after TDC on #1, pull the plug out of #1 and feed in a bunch of soft rope. This will keep the engine from turning while you wail on the bolt.

Steve
Steve Aichele

Just to remind, there is only supposed to be a very small amount of oil in the dashpots. Also, just a suggestion, use Dexron 3 ATF in the dashpots versus motor oil. Itll stay the same no matter what the temperature
Jarrod Hills

Jarrod,

No - the piston tubes should be full of oil - any excess will work its way down the outside of the tube.

The SU manual actually shows that oil should be added till it just overflows the piston tube.
Chris at Octarine Services

Don't remove the timing cover to check the valve timing just put No1 on TDC at which point the two valves on cyl 4 should be "rocking" i.e one just opening, the other just closing. If you are hell bent on removing the cover put a socket on the pulley nut and the bar on top of the LH chassis leg. Remove the LT wires from the coil and hit the starter.
Iain MacKintosh

What Chris says is right in that some diagrams in the manuals show the oil level above the top of the cylinder, but if you try and maintain it at this level you will be topping-up after every trip. If you unscrew the damper cap and lift it up, and press it back down and can feel the resistance of the damper before the threads touch, you have enough. How much before indicates how much of a 'reserve' you have. Interestingly this is the method given in the books for checking the level on the Zenith but not the SUs, but it still applies. You shouldn't have to top up in several thousand miles, but whereas HS cylinders I blind bored, I believe, and so cannot leak HIFs are through bored and then plugged. I have known this plug to come adrift meaning the oil leaks out in short order.
Paul Hunt

Yes, that is what I meant. I was saying not to fill it up all the way because I have seen a pair of carbs with the ENTIRE dashpot filled with motor oil...guy didnt know why it was running so rich. Anyway, that is what i meant
Jarrod

I had a siimilar problem with my 1500 MGA. It would idle and run great up to 3500rpm then start to sputter. It turned out to be a broken piece of the float bowl grommet that blocked the fuel flow in one carb.
Kris Sorensen

This thread was discussed between 20/09/2004 and 24/09/2004

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