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MG MGB Technical - Hard starting mgb

My Mgb runs fine , but if it sits for one day, it is very difficult to start, when it finally kicks over it runs [or barely runs] awfully bad. However when it warms up in about 5 minutes it runs great.
Could this be the choke not setting correctly? If so how do I diagnose and fix this problem? thanks
Kevin Donahoe

You will need to tell us about your car - age, type of carb(s), how much anti-polution equipment has it got, etc, etc.
Mike Howlett

My car was had always been a bit of a pig to start ever since I bought it. It would only start if 1/4 choke and slight pedal pressure......then I read good old haynes, balanced my carb and set up the choke which hadn't been set right so mixture wasn't richened etc. now its set is on the button and runs great.
S Longstaff

SU, Weber or ZS carbutretor?

Clifton
Clifton Gordon

I have the same problem with my 74 with twin SU's. I usually have to pull the choke about half way, pump the gas peddle a few times until I can remove my foot without it stalling out. I leave it to run for a minute or so and then push the choke all the way back in. This has only started the past month or so when I tweeked the idle to get it down to just under 1000 rpm's

As soon as it starts getting colder I may take it over to the shop and let an "expert" deal with it! There is a garage about 5 miles away that deals primarily in LBC's
GG Ginty

Sorry I should have said it is a 1971 mgb with twin su carbs, and it runs great once warm , just extremely hard to start cold. thanks
Kevin Donahoe

Yes the choke adjustment could be the cause of hard starting, but you could have some other problem. You said it runs well after warming up. A quick check of the choke is to pull the choke out to it's limit and look at both jets at the bottom of the carburetors. Both jets should be pulled down equally around 3/8" to 1/2". If not a carburetor adjustment/repair is needed. This article should help.
http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/basic4.html
This John Twist You Tube may also help, it covers HIF carburetors but the general procedure is similar.
http://www.youtube.com/watch?v=ASeMfXfjNpw

Clifton
Clifton Gordon

Ok, got to ask. American carburettors have a pump that activates when you hit the gas pedal. I am not aware that SU's have anything similar.

What affect would pumping the accelerator have on a B?
BEC Cunha

BEC,
When you say pump, i assume you mean the high speed choke that engages in cold weather? you hit the gas to the floor once, start the car, let it idle a couple minutes, then hit the gas again to kick it down into low idle speed.
this isnt a pump. its part of the automatic choke setup. the auto choke works by flat steel wound up and connected to the butterfly of the choke. when cold, the choke is closed. when hot, the steel expands and opens the choke. there's a spring that engages on the cold setting that increases the idle speed.

not sure how clear that is the way i explained it. B's dont have this. the manual choke takes care of it, similar in principle as the manual choke of american cars before auto chokes came around. of course, the carbs are completely different.

Don
don g

Don,

(reaching back in my memory) a lot of carbs from the 60s and 70s (what Bruce and I were driving in high school) had actual plungers which would bring fuel out of the float bowl to enrich the starting mixture, much as a pump diaphram does on a hand pump at the 'farm'. Replacing this plunger (pump) was part of the rebuild procedure. Kicking the accelerator pedal would both reset the fast idle AND pump in fuel. This pump feature was sometimes called the acclerator pump because it would work when the pedal was suddenly floored as well.

warmly,
dave
Dave Braun

Pumping the throttle pedal with SUs or Zenith does nothing useful at all. However it does cause premature wear to the throttle spindles, bushes and cable.
Paul Hunt 2

You will have to keep the throttle open (ie pump the pedal) with twin SUs if you only pull the choke half way out - because doing so would not be enough to lower the jets and hence richen the mixture. You are lucky it starts at all!

If you have additional throttle return springs (required for motorsport) then pressing and holding the pedal down before pulling the choke knob can help ensure that you get maximum movement of the jets - the pull of the throttle return springs can be greater than you can apply with the choke knob, particularly with HS6s as the choke linkage is not so good...

Neil

Neil22

This thread was discussed between 07/11/2007 and 09/11/2007

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