K&N air filters flow more air than the stock ones. It sounds as though you
have upset your air/fuel mixture at the upper end, when your engine needs
more fuel to maintain the car's speed.
Try the stock filters just to see if they resolve the problem. If they do,
and you really want to use the K&Ns, a needle that is richer at it's bottom
(the skinny) end should work.
You may also have a point that your fuel pump is not keeping up with demand.
Make sure all the filters are clean and that the pump is making 2-1/2 to
3lbs pressure at the carburetors, and that the gas cap vent is clear.
By the way, I hope you have overdrive in your car. Driving a fresh (less
than 1000 mile) engine in a non/o/d at 80 mph is certainly one way to make
sure you'll have to rebuild again in a few years! Even with overdrive, 80
m.p.h. is 4,000 rpm and that's a bit quick for top gear on a new engine.
Lawrie
British Sportscar Center
-----Original Message-----
From: Dirk de Boer <deboer@duke.usask.ca>
To: mgs@autox.team.net <mgs@autox.team.net>
Date: Monday, November 16, 1998 1:44 PM
Subject: MGB misses at 70 to 80 mph. Possible causes?
>Here's a question for all of you:
>
>My 1966 MGB has a fresh engine. So far, I have put about 500 miles on
>it. When I rebuilt it I replaced the entire ignition system (plugs,
>wires, points, cap, rotor, condensor) and I put on a Lucas Sports coil
>and K&N air filters. When I take it up to 70 to 80 mph, I get a miss
>once in a while. It sounds sort of like a muffled sneeze, and I can also
>feel the car hold back when it happens. In every other gear at the same
>rpm's and in my garage at idle everything works fine of course. Timing,
>point gap and dwell are all on spec, and I get advance when I rev the
>engine. Points are and look new. So in the end, the engine works great.
>It's wonderful to drive away without leaving a blue cloud, and without
>the clutch slipping. Except for the miss in fourth at 70 to 80 mph,
>everything is perfect. Does anybody have any ideas about what could be
>causing this? Could it be the fuel pump cannot keep up with demand?
>
>Dirk de Boer
>1966 MGB snowbound till next Spring
>
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