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Backfiring and loss of power at hgher revs

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Beat Weibel
Posts: 134
Joined: Sat Feb 27, 2016 4:13 pm
Location: Munich

Backfiring and loss of power at hgher revs

Post by Beat Weibel » Sat Aug 17, 2019 8:43 am

Hi experts

As mentioned in my earlier post, I made long drive with newly acquired chassis No. 100164.
Already on the autobahn I felt that it lacks power at higher revs. Looking at it closer at home, I figured out that it backfires at revs above 3500 / 4000 rpm.
Up to 2500 / 300 rpm it is running very smoothly.

My suspect is ignition timing. Maybe the automatic advance control doesn't work properly or I just have to advance ignition a bit with help of the micro-screw at the side of the disitributor - or should I retard it?

Any suggestion is very welcome
Best regards
Beat

Ian Slade
Posts: 456
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: Backfiring and loss of power at hgher revs

Post by Ian Slade » Sat Aug 17, 2019 9:32 am

Probably the usual the advance drive seized to the main shaft, quite common.
Owner since the 70's, Genghis is slightly to my left.

silverdart
Posts: 160
Joined: Sat Feb 13, 2016 10:49 am
Location: West Midlands

Re: Backfiring and loss of power at hgher revs

Post by silverdart » Sat Aug 17, 2019 10:23 am

If you have a conventional distributor, I would first replace the condenser. I had similar symptoms to yours and changing this, cured the problem.

Dave.

Tony250w
Posts: 19
Joined: Sun Feb 14, 2016 11:54 am
Location: Leicestershire

Re: Backfiring and loss of power at hgher revs

Post by Tony250w » Sun Aug 18, 2019 6:07 pm

Additional to the above the vacuum advance may well be inoperative

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heh101353
Posts: 32
Joined: Sat Sep 30, 2017 11:34 am
Location: Basel Switzerland

Re: Backfiring and loss of power at hgher revs

Post by heh101353 » Wed Aug 21, 2019 11:04 am

Same problem here. Last week mine started "machine gun fire" at 3000 revs and above (under load). Yesterday hardly could start the engine and running was very rough at low RPMs.
Culprits condenser and points. Points showed quite some pitting and the condenser was "gone".
I'd suggest, if you change the condenser change the points as well. If the condenser gave in the points are most likely gone too because the condenser could not stop the arcing anymore.
Hans

John Hitchins
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Posts: 211
Joined: Mon Feb 08, 2016 10:52 am
Location: Wellington Somerset

Re: Backfiring and loss of power at hgher revs

Post by John Hitchins » Wed Aug 21, 2019 9:08 pm

Is there an easy quick way to check the condenser, I have wondered for years on how to check the odd ones in my old box of bits.

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Vortex O'Plinth
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Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Backfiring and loss of power at hgher revs

Post by Vortex O'Plinth » Wed Aug 21, 2019 10:49 pm

John Hitchins wrote:
Wed Aug 21, 2019 9:08 pm
Is there an easy quick way to check the condenser, I have wondered for years on how to check the odd ones in my old box of bits.
MGAguru.com is, as the name suggests, a site dedicated to the MGA. It does however contain a wealth of technical information relevant to not just the MGA but to many cars of the same period.

This page explains how to test a condenser with an analog ohm meter. Many meters are digital these days, but you can still buy a simple analogue instrument for around a tenner.
Nick

"Open the pod bay door Hal".

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Kbeal
Posts: 75
Joined: Mon Feb 15, 2016 9:01 pm

Re: Backfiring and loss of power at hgher revs

Post by Kbeal » Thu Aug 22, 2019 9:04 am

If none of the above is the cause, then it may be warped heads. I had this years ago, everything was fine up to 3000 rpm when the engine just ran out of breath, misfired etc. The way to check is get someone to rev the engine hard whilst you spray WD40 or similar along the head gasket line. If it bubbles you have a problem. Hope it's not this as the heads have to come off for a re skim.

Kevin

Beat Weibel
Posts: 134
Joined: Sat Feb 27, 2016 4:13 pm
Location: Munich

Re: Backfiring and loss of power at hgher revs

Post by Beat Weibel » Thu Aug 22, 2019 10:32 am

Thanks to all for the great advice. I checked the condenser and the HV advance mechanism. All ok. Increased the advance setting at the distributor slightly. Now it runs much better - no back firing at higher revs.
BR
Beat

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Vortex O'Plinth
Posts: 826
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Backfiring and loss of power at hgher revs

Post by Vortex O'Plinth » Thu Aug 22, 2019 12:33 pm

Beat Weibel wrote:
Thu Aug 22, 2019 10:32 am
Thanks to all for the great advice. I checked the condenser and the HV advance mechanism. All ok. Increased the advance setting at the distributor slightly. Now it runs much better - no back firing at higher revs.
BR
Beat
Glad your problem seems to be sorted.

When adjusting ignition timing at the distributor it's worth remembering to verify that the points dwell is correct, since this can affect timing. This is particularly important on the Daimler V8 where two sets of points control the dwell, since although the total dwell may be correct, if it is not shared equally between the two sets, misfiring can be a result. The article below illustrates the problem...
V8%20Dwell_zpsebfxwpty.PNG
To obtain the correct dwell precisely a dwell meter will provide a more accurate answer than relying on setting the points gap with a feeler gauge. Unfortunately setting the correct dwell on the V8 Lucas distributor can be laborious. On distributors with externally adjustable points, dwell setting with the engine running is a doddle, but the Lucas has to have the points manually adjusted with the engine static before refitting the cap and restarting the engine and checking the dwell. With the total dwell correct and the individual points dwell identical the timing can then be adjusted. A timing lamp will allow a precise analysis of the elements contributing to total ignition timing - static timing, centrifugal advance and vacuum advance.
Nick

"Open the pod bay door Hal".

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