> For the past 4 months, I have had an occasional problem with my MGB.
Year? Type of carbs (HS, HIF, or Z-S)? Anything unusual about the
engine (cam, dual-point distributor, twin-cam alloy head?)
> When it started happening, I decided it was time to replace parts of
> the ignition. [...] All to no avail. Once
> it warmed up, it ran great, but occasionally in the mornings, boom, no
As a general (but not ironclad) rule, temperature-related problems
are more likely to be fuel-related than spark-related, particularly
on the later cars which have bimetallic strips in the floats. It's
never a *bad* idea to replace the ignition system, but it doesn't
sound as though that's your problem.
> Now, I disassembled those things 6000 miles ago, and refilled them.
> This problem only occured after that.
Bingo. Something that happened at that time is causing a temperature-
related sticking problem in your dashpot, which allows air and fuel to
enter the engine. When the car warms up, this problem normally goes
1. Did you get the dashpots backwards? (I assume that since you say
"them" that your car has dual SU carbs of some flavor.) The piston and
dashpot of every individual SU carb is a precision-made matched set.
You can't use the pistons from one carb in the dashpot of another.
2. Could a piece of dirt/rag/some other contaminant be stuck in one
or the other of the dashpot pistons? Those things have such tight
clearances that the smallest piece of obstruction can make them
stick closed (or less frequently open).
3. If you have the fixed-needle carbs, your needle might not be
centered in the jet, and could be causing the dashpot to hang up at
some point. The later carbs have spring-loaded needles that do
not require this. I've never centered needles, as I only just
began using replacement carbs that have fixed needles, but it's in
the usual SU source materials.
It could also be a vacuum-related problem, one in which an orifice
opens up when a piece of rubber shrinks from the cold, but which
closes when heat makes the rubber swell. (No, Mark, I'm NOT trying
to start the "do holes shrink when you heat them?" argument! For
the record, holes expand when you shrink them, and if you don't
believe me, DON'T mail the whole list, I've proved it with a hair
dryer and a set of pistons...)
Why did you have the carbs apart 4 months ago, and what did you
do to them at that time?
Your problem appears to be a heat-related carburetor issue. Try
removing the dashpots one at a time, wiping the piston and inside
of the dashpot down with a rag soaked in cleaner (or spray them
with brake cleaner and wipe them down), then reassemble them
You just might also have a piece of dirt in one of the float
bowls. That will cause the car to run on two cylinders if it
sticks the float valve closed.
And you shouldn't be going 6000 miles without looking at the oil
level in those carbs, especially if they're the later style with
holes in the damper plugs. Hard acceleration makes them squirt
oil out of those holes, so you need to replace it once a week or
> This afternoon I will check the
> oil levels in them. I tried to get the problem to occur this morning,
> but of course they never happen when you want them to. A few more
> days of commuting will tell me.
A few more minutes spent in the garage will tell you more, and more
quickly, and make those days commuting that much more fun!
You can't ignore an MG. Either you do the work to keep on top of
what they need to operate, or you do the work required to make them