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Re: [Mgs] too rich or too lean?

To: MG List <>
Subject: Re: [Mgs] too rich or too lean?
From: Max Heim <>
Date: Mon, 26 Oct 2009 09:52:57 -0700
I'm going with the fuel pump crapping out intermittently. When my SU pump
died, it was a long drawn-out process with very similar symptoms. The fact
that it happened on a 3-day rally only added to the aggravation.


Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires

on 10/26/09 8:02 AM, Aaron Whiteman at wrote:

> I'll try to incorporate other suggestions in to this reply too.  I
> found myself to be shockingly tired all day yesterday, so I didn't get
> out to do a whole lot of troubleshooting.
> On Oct 26, 2009, at 1:51 AM, Paul Hunt wrote:
>> Could be almost anything, especially since you have recently worked
>> on ignition and carbs.
> I reset the timing to 15 degrees BTDT (from 19), but I ended up not
> changing the carbs.  They seemed to be behaving correctly (small rise
> then settle when barely lifting the piston) without adjusting.
>> What does the tach do for a start?  If that is flicking all over the
>> place then it is an ignition LT problem.  If it is relatively steady
>> then HT or fuel.  As it happens when cruising you may be able to
>> reproduce it with your head under the bonnet, if so clip a timing
>> light onto the coil and plug leads and watch the flashes.
> The tach is stable.  I did drive to get groceries yesterday, so I was
> able to drive for about a mile at 35 (in third).  It only seems to be
> happening when cruising at speeds greater than 45mph.
>> Erratic flashing on the coil lead indicates coil or possibly
>> condenser problems.  Steady on the coil lead but irregular on the
>> plug leads (remember the coil lead flashes four times faster than
>> any one plug lead) indicates that the rotor or cap are breaking
>> down, although that would be more likely under acceleration than
>> cruising.
>> While on No.1 lead point it at the timing marks to check that it is
>> about right, and not jumping around.
> Hmm.  I didn't check yesterday, but the timing mark has always bounced
> around.  Even after replacing the timing chain, it bounces more than I
> think it should.  It's been "an issue" for years.
>> If all that is OK then it looks like carbs. Unlikely to be fuel
>> starvation as that would be worse under acceleration. Could be the
>> aforementioned vacuum leak, test with propane or carb cleaner, or
>> possibly float valves, a slight leak causing mixture problems under
>> light fuel demand but not under heavier demand.  If you have an SU
>> pump turn on the ignition but don't start the engine, and listen to
>> the clicks.  Once the float chambers have been filled the pump
>> shouldn't click more than once every 30 secs.  More than this
>> indicates a leaking float valve or possibly a leaking one-way valve
>> in the pump inlet, although this latter is more likely to cause
>> problems under high fuel demand than low.  Remove the float chamber
>> vent pipes to see if one or other carbs does eventually start to
>> overflow, this can take several minutes, so disconnect the coil to
>> prevent it overheating.
> I'll hopefully be able to check for leaks tonight.  I don't have the
> SU pump, so I'll just have to wait for it.  I did check the plugs
> yesterday.  The #1 plug was a bit white, but otherwise all plugs were
> a nice light brown ashy color (I'll pull that #1 again and take a
> closer look at it)


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