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MG MGB Technical - Engine Sluggish When Hot

I did an engine rebuild on my 77 MGB this past year. When cold the car runs well with the following exceptions; there is a surge in power that occurs randomly, I can not get the car to go much beyond 55 mph even with my foot to the floorboard on a straight away. I am able to pick up speed when going up a hill with no visible lack of power. Once the engine warms up the following occurs: I have no power, even on a straight away I will end up in 1st or 2nd gear to keep the car from dying out due to a complete lack of power. Once I am going down hill I can begin to pick up power but as soon as there is any incline I begin losing power again. I have confirmed the following: timing, vacuum, and carburetor settings, choke setting and fuel flow into the carburetor. I have removed most of the smog control devices on the engine. Those that could not be removed have been plugged. Any ideas on what may be wrong will be appreciated as I am at a loss on what to check next. Thanks,
Jim

What type carburetor is on the car? What do the spark plugs look like after the engine warms up? Have you checked compression with a hot engine? Did you have the problem immediately after the rebuild? Answers to these questions may help us help you.

Clifton
Clifton Gordon

Choked exhaust?

What are the plugs like after running badly for a while? The colour/conditon should tell you if it's mixture or not.
Paul Hunt 2

Quick thought - could it be an air leak on the carbs?. I recently rebuilt my engine and found it hard to start and then when running would rev and die, rev and die.... could be the same?
Mike

OK first thanks for responding. The carburetor is a straight Zenith Stromberg nothing fancy purchased from MOSS. The carb worked without any problems prior to the rebuild. Pulled all four plugs they were a soft gray, number 1 was darker and had more build-up then the other 3 plugs.

Compression was within tolerance per the engine specification when last checked. I will try and check it again tonight or tomorrow just to be sure it is still in spec.

I do have an additional question. I have the breather hose hooked up, the one that runs from the oil separator to the carb. I think I had it installed previously but am not sure. Since I removed most of the emission controls should I plug them as well or should they remain installed?

As for the carb leaking or sucking air I am leaving that out as a possible problem as the carb runs well with no load on the engine, ie. it does not rev or die.

Again thanks for any suggestions on what to check.
Jim McDowell

I had a similar problem on my '78 - the catalytic converter had clogged. It idled well, worked great when first started, but after 20 miles on the freeway it could no longer maintain freeway speed even at full throttle. Pulled over, and it still ran *perfectly* except that it had no power. If that's what's going on, and you have a catalytic installed, chances are you've toasted it.

If you don't have a cat, there's one obvious culprit from where I'm sitting: is your advance system working? You've checked your timing at idle, ok - but make sure the timing advances properly as the engine is revved. A stuck advance in your distributor can leave the engine totally gutless at speed, yet idling just fine. Check out what your timing mark does when you rev the engine, and make sure it behaves as it should. Might want to at least check this before shelling out for a new cat.

You're right to rule out an air leak - they cause horrid idle but don't really reduce your full-throttle power. Doesn't match your symptoms at all.

The positive crankcase breathing system is a great thing: it removes blowby gases from inside the crankcase, prolonging the life of your motor oil (and thus, your engine). The stock breather works great, and should not be messed with. Since you don't have a vacuum problem, the breather cannot be a culprit in this.
Sam Good

Mixture looks OK, so possibly ignition or choked exhaust as first wondered.

What does the burning on the rotor look like? In theory it should cover most of it but leave a clean bit each end, which indicates the points are phasing with the rotor/cap contact position correctly. In practice mine have the burning going from the middle right to one end. If the phasing is off, under certain vacuum advance conditions, the rotor could have not reached or be moving away from the cap contact when the coil fires, which will cause HT problems. Mine must be just OK as I don't have a missfire.

There should be a hose from the carb PCV port to the tappet chest front cover. The port on the rear of the rocker cover should be fitted with a small filter, or sealed off altogether and a ventilated oil filler cap fitted. The EGR port on the carb must be sealed off if the EGR valve has been removed, or it can be used for carb vacuum in preference to manifold. The EGR, distributor (if it has been moved to the carb) and any other manifold ports left unused if emissions has been removed must also be sealed. The carb float chamber must be piped down past the exhaust and left open to atmosphere. The tank vent that went to the charcoal canister, if that has been removed, must also be left open to atmosphere. If you get *any* of these wrong it will cause running problems with incorrect mixture and/or timing.
Paul Hunt 2

i have a m reg peugoet 405 glx diesel every now and again i lose power when engine is hot changed fuel filter still same can anyone help thanks
fugazi

On US-spec models the float vent goes to the charcoal canister, not to the atmosphere. This recovers fuel that would otherwise evaporate away on a hot day. (I'm assuming that the charcoal canister is still there, as there's nothing to be gained from removing it, and without it the car would reek of fuel when parked on hot days.)

Agreed that leaving both the float bowl vent and the tank vent open to the atmosphere for a day's drive is a good troubleshooting step, as you'll find out if either of these is pressurising or pulling a vacuum. (neither should be.)
Sam Good

"On US-spec models the float vent goes to the charcoal canister, not to the atmosphere"

From the factory it goes to armosphere *via* the canister. The canister recovers *fumes* from expansion of the fuel in the tank and float chamber. If the float chamber floods it runs into the canister and from there onto the ground. People often remove the canister as part of 'desmogging' as it makes the engine bay considerably easier to work in, and in that case the float chamber vents *must* be piped down past the carbs and not left to leak onto the exhaust manifold.
Paul Hunt 2

This thread was discussed between 26/09/2007 and 01/10/2007

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