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MG MGB Technical - Surging on overun

My 1979 roadster has started surging on deceleration when hot. Engine braking pulses as the car slows and the revs rise and fall on the tacho. The car acelerates well and the plugs are a nice grey / brown colour. I've checked for vacuum leaks and the vacuum advance on the distributor is also ok. My HIF4 carbs have the original poppet throttle discs and the springs may well be weakened through age. I've read this can cause a fast idle but not heard of anyone experiencing surging. (The idle is Ok). Rather than soldering the poppets I have ordered replacement earlier HS4 solid discs. I am very curious if anyone else has heard of this problem and would welcome any suggestions for a cure.
K P Hamer

It is an unusual problem. The surge means the engine is getting more air and fuel than your right foot demands. Either the nedles are being pulled by vacuum to let more fuel through the jet or you might have a leak through the choke mechanism.
The colour of the spark plugs suggest the fuel mixture and timing are correct at least while steady running so I would focus initially on the dampers. If the oil level is OK the dampers need to be very worn not to work. The vacuum chambers and pistons are easily removed so the springs in the vacuum chamber can be tested upside down. With the chambers off the base, you might check the pistons are a reasonably tight fit.
The chances of the choke mechanism leaking fuel when decelerating can't be ignored but then you might expect to produce black smoke. If you do remove the carbs from the manifold you might be well advised to replace the O-ring designed to stop any leaking {unless it has perished or water in the fuel has corroded the sides of the chamber}.
I have heard of a very slack timing chain having quite dramatic effect on the running of older engines. I dont think it likely with the layout under the timing cover of the 1800 but the distributor advance mechanism still can't be ignored. I would still look at the dizzi after the carbs
Roger Walker

Pulsing on the overrun can be caused by the OD when disengaged, the solenoid valve not opening far enough to fully release pressure, the residual pressure causing the OD to repeatedly partially engage (so you are in effect free-wheeling) and disengage gently, and is a known problem albeit not widely. My V8 started doing it occasionally after 20 years and 100k, then after the engine and gearbox came out and went back, no work on the gearbox, it hasn't done it again. Both Stephen Strange and Paul Walbran have commented on this in the past, the latter used to have a web page on it but the link is not working now. Adding a second gasket to the solenoid cover plate may stop it, but instead of having the large hole of the standard gasket as shown it should only have a small hole large enough for the solenoid plunger to pass through, and that should allow the valve to open further.

With the butterflies closed it seems unlikely that the pistons are pulling the needles out of the jet, which would make for a rich mixture and more likely cause stalling, as well as cause much more obvious problems when running. But anything's possible.



paulh4

Thanks for that Guy's. The frustrating thing is it only occurs after around 15 to 20 miles from cold. It's was also more pronounced during the hot summer. The car has done 61000 miles and has an accuspark electronic ignition. With the introduction of e5 then e10 I installed SU overhaul kits in both carbs and replaced all rubber fuel lines 5 years ago. I'm hoping the plain HS4 throttle discs will eliminate the problem although I'll probably wait until spring to remove the carbs. If I've understood the HIF4's correctly the only way the needles could be lifted is by leakage around or through the closed discs. If poppet springs loose their temper due to heat, might that not account for the problem?
I've thought about the overdrive; this sounds plausable but would it account for the tacho rising and falling?
I'll keep you posted.
K P Hamer

Yes, the tacho does rise and fall if it is the overdive, and is the best clue that this is probably the first place to look. Check by seeing if it happens without the overdrive engaged and not when engaged. If so, then it is definitely overdrive related, as a tune/fuelling issue would be there regardless of gearing.

Paul, thanks for pointing out the broken link, a few pages went missing when my website was redesigned and I am still trying to track which/where they are.

If it looks like a tune problem, I had one car here, similar year, which solved it after much investigation by going a tad leaner and opening the plug gaps to the specified 35 thou. Both changes were done separately from each other and each made a noticeable reduction on its own. In combination, back to normal.

It could well be that minor leakage past the o-ring was contributing, though in the several years since then there has been no indication of it deteriorating further as would be expected.
Paul Walbran

It'd all depend on how far this tacho reading is fluctuating I think
I had an e type jag that did a weird missfire surge as the throttle was eased to off at cruise ,drove perfectly otherwise and that ended up being carburettor air bleed adjustments slightly out---but no tacho issue
If you're getting little minor tacho flicks it 'might' be ignition related but if the tacho is actually moving a few hundred revs when this surge is happening it's sounding more like the overdrive issue
willy
William Revit

Good point about the ignition flucker will, I stand guilty of being myopic!
As you say, ign or od depends on what subtle flavour the tacho fluctuation is.
Paul Walbran

I've found the overdrive article:
https://www.mgclub.org.nz/site/information/technical/overdrives.aspx
The more I think about it the more I'm convinced Paul is right. The surges were about 2 seconds apart with the revs rising and falling approx.100 to 200.
It could well be free wheeling on and off. The 15 miles from cold would account for the time taken for the gearbox oil to get warm and seep past the O ring.
I will also make sure to not disengage on overun from now on.
I will take it for a drive to see if it happens with overdrive in & out.
My freindly classics garage will then get the job along with gearbox oil change as I've not got access to a hoist.
K P Hamer

Thanks for that link, I've corrected my web page.

As part of investigating the pumping or surging on my V8 engagement became intermittent and the cause was the spring contact 'wings' at the valve end of the coil having become flattened so not making a good electrical connection to the inside of the solenoid body, they should be sticking up as in the left-hand part of the attached.


paulh4

This thread was discussed between 21/09/2022 and 23/09/2022

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