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Re: Miss Under Load

To: <>, "MG List" <>
Subject: Re: Miss Under Load
From: "Paul Hunt" <>
Date: Tue, 21 Nov 2006 16:45:48 -0000
How did you validate the fuel flow?  It should deliver *at least* 1 Imperial 
pint per minute into a container from a pipe disconnected from a carb, with 
negligible bubbles.

Someone else mentioned the vacuum advance.  If the vacuum pipe is connected 
to the inlet manifold and not a carb when you disconnect it the idle revs 
should drop, possibly even stalling, even when the port on the manifold is 
blocked off with a finger as soon as the pipe is removed.  With the pipe 
disconnected and that port blocked off, by sucking on the pipe to the 
distributor (needs a pretty good suck) you should be able to speed up the 
revs.  With the distributor cap removed you should see the 'points' plate 
twisting clockwise an anti-clockwise as you suck and release respectively. 
You should *not* be able to draw a continuous flow of air from the pipe, and 
if you suck and block the end of the pipe with your tongue it should hold 
the vacuum (for a few seconds at least) until you move your tongue.

With the vacuum still disconnected and (ideally) an adjustable timing light 
plot the centrifugal advance against revs.  For a US (I'm assuming) 73 you 
should have an 18V 672 or 673 engine with a 25D4 41491 distributor.  This 
originally had a static timing of 6 degrees which results in a strobe 
setting of 11 degrees at 1500 rpm.  Additional (to the static figure) 
centrifugal advance was 16 degrees at 1500, 32 at 3825, and 39 at 4800.  The 
vacuum capsule should be stamped '10-15-5' which means vacuum advance starts 
at 10 in. Hg., is at a maximum at 15 in. Hg., and is a maximum of 5 
distributor degrees i.e. 10 crankshaft degrees.

Also check the timing for any timing light erratic flashing as the revs go 
up or it comes under load (i.e. someone dragging the clutch with the brakes 
firmly on while you watch the timing light, head *not* under the bonnet!

If the problem tends to happen with revs rather than throttle i.e. its OK up 
a hill in 4th but starts missing in 3rd then I'd say it was ignition.  If it 
does it when you open the throttle above cruising level, regardless of gear 
and revs, then I'd say it was fuel.

But first of all you should do a full setup of valve clearances, plug gaps, 
timing including checking vacuum and centrifugal advances, and check for any 
blue flashes round the HT leads in the dark.  When you have eliminated any 
possibility of the problem being in the ignition, then do a full carb setup 
from scratch.  If you have to move one or both carbs very far from the 
mixture starting point (2 full turns of the jet nut down from flush with the 
bridge for HSs, two turns of the screw in for HIFs) or one ends up with 
significantly different setting from the other for correct mixture and 
balance, then problems with one or other carb or the intake manifold are 


----- Original Message ----- 
>I am new to this list and have recently purchased a 1973 MGBGT.  Hopefully
> someone can help me sort out an engine miss I am experiencing at highway
> speeds when under load.... 

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