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MG MGA - Lots of Pumping

After the car sits for more than a day I find I have to pump the gas, somtimes more than 10 times, to get the car to start, even with the choke out. When the car has run during the day and has cold down its starts fine. I have checked the choke cable and its set find (full choke when pulled out). The fuel pumps up quickly to a stop when I turn the ignition key on.
What are the possible causes?

Ken
L Caya

Ken, SU carbs do not have a fuel pump. All that happens when you "pump the gas" is open the throttle plate. So I don't understand why it should help! Unless you mean pumping the gas while the car is firing up.
Art Pearse

Ken, SU carbs do not have a fuel pump. All that happens when you "pump the gas" is open the throttle plate. So I don't understand why it should help! Unless you mean pumping the gas while the car is firing up.
Art Pearse

Ken. Actually, what Art means is that the SU carbs do not have an accelerator pump which pumps fuel into the throat of a car, such as the Weber DGV series, when the throttle is opened. The accelerator pump provides an extra rich mixture when accelerating. The SU carb provides this feature through the oil dampening of the piston, causing it not to rise as fast is it could and causing the mixture to enriched due to the increased vacuum under those conditions.

All "pumping the throttle" does with an SU carb is to open the throttle plates several times with no effect on the mixture. The choke is the primary method of enriching the mixture when the engine is cold. I would want to pull the damper covers off the carbs, then pull the pistons out (being careful not to damage the needles) and inspect the depth of the jets with the choke knob pulled out. Paul Hunt has some good information on the HS series carbs, which are very similar to the H series carb, on his website, "The Pages of Bee and Vee". You may find this of use. David DuBois has some technical articles on the fuel system and fuel pump on the MG section of my website, www.custompistols.com/ and they may be of use to you.

There are also some articles on the ignition system which may be of use. When the engine is cold, the ignition system has to work more effectively than when the engine is warm. Thus, a minor ignition system problem will normally show itself first when trying to start the engine in cold conditions.

Les
Les Bengtson

Les, as you described the oil dampening pistons function of enriching the fuel mixture, I realized I have not check the oil level in a while, could low oil level add to the problem?

Ken
L Caya

Ken. Only a very little, if at all. The main function of the oil dampening is to keep the pistons from rising rapidly as the throttle is opened more widely. This happens mostly when the engine is running, not when it is starting. The choke is the main mechanism for enriching the mixture when starting from cold.

Les
Les Bengtson

I am going to be controversial and say it's a battery past its best. I would suggest that when the car has sat for a day or 2, the battery has leaked away some of its power. What is left is used for the starter motor, leaving not a lot to create a spark. The engine is also high resistance with cold oil etc. Once the car has warmed and the battery recharged, all is ok for the rest of the day.

I have 2 x 12 volt batteries wired in parallel, either of which I normally leave isolated. When, in winter, I sometimes have this type of sympton I switch on the second battery to double the power (not volts) and the car starts immediately.

Steve
Steve Gyles

Steve,
Have you EVER had a fuel problem?
Ray.
Ray Allden

Ray

I had some problems getting the carbs initially set up correctly; I once had a sunken float; and the throttle shaft between the 2 carbs came disconnected earlier this year. Otherwise, they are so simple there is so little that can instantly go dramatically wrong. Over the years I have read articles, watched threads develop and have come to the conclusion that electrics are nearly always the culprit.

My 10 penneyworth.

Steve
Steve Gyles

Ken -

You said you get full choke when you pull out the choke cable. Have you checked to see that you are also getting movement of the throttle plates via the idle adjustment screw that is moved by the choke cam on the side of the front carb?
Steven B

Try pulling the choke out even further. When I still had SUs it used to take choke to the braided cable when really cold
dominic clancy

Choke adjustment and fast idle:
http://mgaguru.com/mgtech/carbs/cb102.htm

Cold weather starting (or starting from cold):
http://mgaguru.com/mgtech/carbs/cb103.htm
Barney Gaylord

Everyone is bringing up good points giving me plenty to investigate and tinker with this weekend. I think I'll read through Barney's web site and let you know what I find. I plan to drive 250 miles to Stowe for British Invasion on 9/14 so I need to do a complete tune also.

Thanks for your help,

Ken
L Caya

Will I spend 2 hours tinkering adjusting the rocker arms, setting points, plug gaps, syncing the carbs and adjusting the choke. The reason I had such a hard time starting after sitting for a while was the choke cable came loose and the choke barely opened.

Barney your site and assistance as always are great, as is all the help for this site. Tahanks again to all.

Hope to see you at Stowe

Best Regards,
Ken Cay


L Caya

This thread was discussed between 05/09/2007 and 12/09/2007

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