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MG MGB Technical - Power Loss at WOT

Am driving a '79 that hasn't been DETOXed, 100+ K miles, completely standard daily driver. Tooling along one day on the freeway, started up a grade, incresed throttle and began to experience power
problems, seemed to cure itself and went on my merry way. Within the next 100 miles the problem
became persisent at any throttle setting above about 2/3rds in any gear or RPM. As long as I am not getting on it (ie) driving around town, the problem is completely invisible. The TCSA valve is failed open - but I don't believe that to be the problem since this is its normal state in 4th gear and the problem happens in fourth as well as the other gears. I also plugged the vacuum advance line and the problem continues in the lower gears. While researching the archives the explanation for this seems to fall in the mixture or timing tuning issues but I am at a loss to figure out what could have failed or fell out of adjustment so quickly. Thanks in advance for all your expert advice.

Dan

Dan deCamp

This isn't expert advice, it's just a shot in the dark really, but it happened to me...

Has one of your float chambers worked loose? Actually I guess you'd have a ZS carb so there's only one, but is it well attached? One of my SU float chambers became wobbly once and at high revs it would vibrate and the vibration was enough to stop fuel flow to the jets. If I backed off enough to lower the revs, the problem went away.

If it's not that, my other suggestions are:

1) Suddenly lots of carbon has gathered in the combustion chamber (get a bad tank of gas?) and you are experiencing pre-ignition... easy way to tell is, does it do it with the engine cold? If so you can rule out this one...

2) Your fuel pump's failed and you're getting fuel starvation. The pump is good enough to run the engine at lower power though.

3) Coil/centre HT lead/disi cap has gone kaput so that you're getting a weedy spark that won't light the fuel/air under high pressure conditions. Check everything/dry it/swap on known good stuff until it goes away.

That's all I can think of for now :o)
--
Olly S
Oliver Stephenson

Thanks for your quick reply Olly. Cold or hot it loses it at high throttle settings. I hesitate to dump this
on fuel starvation due to pump or other restrictions simply because the power failure is instantaneous with
throttle movement. My Sprite had pump that was prone to failure when it got warm. As the fuel level dropped in the bowls, the mixture would lean and the engine would run rough as the power output dropped off. The B just feels like it just quits, almost like you've turned off the ignition but it is still producing power just not much. Will continue to trouble shoot ignition components - currently the coil has 1.5 ohms resitance across the primaries and about 6.5K ohms across the HT post to ground this is a little off from what I've heard but the late model Bs have a ballasted coil so this might be correct. Swapped HT leads - no joy. Will try to find my old dizzy cap and rotor and give that a try tomorrow.
The float bowl set up on the ZS is intergral to the body unlike the HSs and seems secure. The only thing loose on the Carb is the heat mass for the auto-choke. It rotates in its mount, I generally understand the HSs but the ZS is serious black magic to me as of yet. Can that lead to some terminally rich mixture setting at high throttle settings ? Anybody.

Thanks Again
Dan
Dan deCamp

I had a 75 (new then) which did this exact thing twice. All the complicated solutions were wrong. Trouble was a fuel filter. With an electric pump, this is exactly the way a partially plugged filter acted. Hit the gas on a grade, it just seemed like someone took the power away instantly. For a quick fix, I reversed the filter under the hood and left the engine side disconnected. (Don't know if this will work on a 79). I bumped the key and the filter shot a load of crap into a can I held under the filter. Reversed to normal and everything worked fine. This held me until I got to a parts store for a new pump.
Arthur Shaffer

Arthur, Olly thanks alot for your input. Simple was right, I did swap out fuel filters, it was due, but it didn't solve the problem but when I swapped out the the old rotor and dizzy cap that did the trick. By the way that was the last MX performed-new cap, rotor, wires, and plugs. I should have known. Now to attack that pesky clunking from the back end of the car - thrust washers or suspension bushes?
Dan deCamp

My money's on the thrust washers.
Paul K

You can probably tell from the tone of the clunk but I guess it would be quite hard to describe :o)

The axle that I've just removed had a great clonk - partly due to the thrust washers in the diff, but mostly due to my having worn a fair amount of metal off the crownwheel and pinion. About 10° of backlash on the pinion between those two gears. Ouch.

If your clunk is fairly dull inside the car but louder if you take the batter cover off, then it's more likely to be your axle. If the clunk is quite sharp anyway, it's something touching the bodyshell.

Like Paul says, it's far more likely that your thrust washers are worn than a bush has failed enough to allow metal-to-metal contact. The kit to change them only costs a tenner or so and doesn't take too long to install.

ttfn,
--
Olly S
Oliver Stephenson

Your ignition problem could also have been caused by the capacitor or condensor. I had similar problems with a car after I had replaced the points and condensor. When I put th eold one back the car ran just fine.
AndyT

I have found that when you lose at WOT you are running too lean. This can be from float adjustment, weak fuel pump or partialy plugged fuel filter. It seems like you just run out of power and if you back off just a bit on the throttle it gets better.This is because you are cutting back the amount of air flow slightly so it richens the mixture up just a bit. When there were electrical problems like a weak coil, bad plug or weak condenser it always showed up as a miss rather than a flat spot.Just my expierence, for what it's worth
gerry masterman

Gerry,
I have been having just such a problem lately(flat spot at WOT)As well as an Idle that gets so slow the car wants to die.

I notice that after running it hard. I stop and look at the fuel filter. It is always pretty empty with just a trickle going thru. It seems to me that it should be at least half full if normal.

I checked my fuel delivery for both volume and pressure. Volume seems great 1 pint in less then 20 seconds. The pressure was 1.5 lbs. Does this seem to be enough pressure?

John Fraioli '79 B
John F

This thread was discussed between 02/07/2002 and 03/07/2002

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