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MG MGB Technical - Mysterious Revving

Hi guys,

I have a problem. I have a 2099cc 5 bearing breathing on twin HS6 carbs. Needles, etc. all fine and it has been tuned by a proper tuner. It fires up on the button and ticks over at about 800rpm. Oil pressure sits around 70psi pretty quickly.

However, the problem: if you touch the throttle, it goes happily to 3000+rpm and stays there, regardless of throttle position, occasionally continuing to rev higher and into the red line with no foot on the throttle.

The only way that I have found to stop the mysterious revving is to give the throttle a blip. Whilst this sounds nice on the drive, it is a tad worrying when driving.

I have checked the carbs, and on release of the throttle pedal, they return to their idle position instantly, yet the engine keeps on revving until I blip the throttle.

Today was the first day that the car has been out of the garage since its rebuild and I was hoping to take it to the Bath Motor Pageant tomorrow, but I'm not sure if I want to chance it.

Any help or recommendations of a good mechanic in Bristol (UK) would be most appreciated,

thanks in advance,

Grant :-)

PS I have also cross-posted this to the MGA side of things as it is an MGA, but has modified 5 bearing engine
G Hudson

just to encourage more knowledgeable replies I'll post

quick thoughts -
. any electronics fitted (rev limiter?)
. did you check carbs with air filters off
. checked position of and for catching carb linkage
. some sort of vacuum or leak issue

sorry this is too late for the show but you should be giving your car plenty of shakedown runs if it's just been rebuilt extending the mileage of each journey

obviously this needs sorting but you should take advantage of the weather this week and drive your car as much as you can
Nigel Atkins

Other possiblities:

Overrun valve (if fitted) in throttle disc not seating properly.

Throttle disc loose in spindle.

Double check the individual throttles are fully closing.

Paul Walbran

Whilst a vacuum leak will cause an engine to rev higher for a given throttle opening, it will only be slightly higher. To rev up to the red line with the throttle closed lots of mixture has to be getting past the butterflies somehow. HS6s do seem to have poppet valves, so that must be the first suspect. These would be clearly visible with the air cleaners off, pistons lifted, and throttle opened wide. Secondly loose butterflies (both as Paul Walbran has mentioned), but I would have though the symptoms of that would be more erratic. The other thing is, does the engine rev (throttle released) normally, i.e. firing on all four cylinders, or is it obviously misfiring? If it is running normally then that implies *both* carbs are equally affected, which seems unlikely for poppet and butterfly problems, which points back to the linkage not closing the butterflies when the throttle is released.
Paul Hunt

Sounds like the butterflies are opening on the throttle but not being returned. I'm assuming HS6's use the same linkage, if so there is enough spring action to close a fairly stubborn butterfly. Have you got 3 return springs? Is the spindle pin in the fork of the shaft? I picture would help a lot.
Allan Reeling

Hi Guys,

Nigel - No, Yes, Yes, possibly but can't tell.
Paul W - Not fitted, no, checked and they are
Paul H - possibly, but cant tell
Allan - lots of spring action only 2 return springs (one on either side, but I doubled that to 2 per side giving a total of 4 and that made no difference)

I've chatted to a few MGB bods at the show today and a couple recommended the same garage, so I'll give them a go.

Many thanks guys,

Grant :-)
G Hudson

only other thought that enters my empty mind (apart from ale) is dashpots matched and dry on piston edges/dashpot oil in dashpots (I'm a convert to "90 weight gear oil" as per John Twist) - but you'd have seen unevenness if they remained high - so really I'm ouuut (never was in really just trying to get you replies)
Nigel Atkins

Give us a picture of the set up.
Allan Reeling

This thread was discussed between 05/07/2013 and 06/07/2013

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