MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - cold-no start

hi again
79 mgb zs carb
it's getting cold and she dowsn't like to start. i believe the mixture, timing, plugs, compression, whatever, are all o.k.
this has been going on for several winters.
in really cold nights i have a 60watt bulb on under the carb all night to warm things up for the morning.
i must have overlooked something really important, so , any suggestion will be more than welcome.
the induction heater never worked and was removed.
thanks.
john
john sutter

My B (1972 with twin su's) was always a pig to start from cold until I checked the choke and discovered it was only giving fast idle and not moving the jets to richening the mixture.

Once adjusted correctly it starts on the key. Might be worth a check.
S Longstaff


p.s. d.c isn't that cold.
thanks to s longstaff, i'll check that out.
john
john sutter

I second the comment of S Longstaff. I left the south for a time in 1967 to attend school in a northern state. My '66 wouldn't start (with key) at any temp below 40 degrees F. Long story short - my choke linkage wasn't adjust properly and I was only getting 1/4 choke.

I have no experience with adjusting/checking choke function on a ZS carb, but that is where I'd look first.
Steve Buchina

There's no choke linkage or cable on a Z-S carb to adjust. Here's a link to Rick Jaskowiak's treatise on the Z-S choke. I haven't found anything better on the subject.

http://www.sterlingbritishmotoringsociety.org/files/choke.pdf

Paul K

*Originally* the Zenith carb didn't have a manual choke, but I understand quite a few have been converted to manual choke because of difficulties with the auto choke. Many years ago I had a Scimitar with an auto choke for which again a manual conversion was popular. However I persevered with it, there were several different adjustments for temperature, enrichment and fast-idle, they all need to be right to work properly, and bit by bit I did get it right and it was fine thereafter.

That link looks excellent, and right at the end Rick describes having to tweak the heat mass one side or the other to get the correct enrichment for the temperature, and tweaking the fast idle and slow idle screws.
Paul Hunt 2

hello again.
sorry!!
i forgot to mention that i removed the automatic choke long ago, now have a manual one.
also, air pump, gulp valve, air injection manifold, etc all removed.

so, the question is what to do if have manual choke.
any additional information really appreciated.

right now it's cold and rainy out so i can't do anything until it clears.
sorry, again, for not remembering to include all the
facts in the first post !!
john
john sutter

sorry, part 2

while i'm at it do the settings of the coarse and fine adjustments for the idle mixture have a critical adjustment for a manual choke?
by the haynes manual, the coarse setting seems to be 2 turns up and the fine 2.5 turns up from clockwise limit. this centeres everything, whatever that means.
does this apply in my case or for automatic choke?
john
john sutter

Slightly off topic: throw a can of starter fluid or carb cleaner in your trunk.

















Dan Robinson

What weight oil do you use? Here in NY english cars with 20w50 do not crank so well in the cold weather.
ASH Andrew

I've been using ATF for decades. Sometimes go to a heavier oil when a piston damper seems a bit worn.
Steve Buchina

andrew. i use castrol 20/50 year round. i would guess this might cause a problem with cranking in cold weather.
i usually go to my chem lab and get a bottle of ethyl ether, i few c.c's of this usually does the trick. be careful to store it safetly outside.
john
john sutter

Yes, coarse and fine idle adjustment idle mixture has to be correct for manual choke as for automatic, although if incorrect this is more likely to affect normal running than cold starting with a choke. The settings will be the same irrespective of the type of choke.

You would need to check your manual choke and make sure it interfaces correctly using the images in the above link. Presumably the manual choke cable engages with the lever shown in picture 21, and moves it one way or the other to both enrich and give fast idle.

However slow cranking can be a contributory cause through reduced voltage and hence reduced HT spark, and as well as cold weather and thick oil (20W/50 should be good down to 15F, for temps down to -5F you should probably be using 10W/40) bad connections between battery and starter are a common cause. If you connect a volmeter (analogue is best) between the 12v battery post (not clamp) and the solenoid stud, disconnect the coil, and crank the engine, you will see a voltage displayed that is lost voltage. Then do the same between the battery ground post and the starter body. It is possible to get these down to about 0.25v per 'leg' with all good connections. Anything approaching 1v is worth looking at, and I have seen as much as 3v lost. As well as battery connectors and solenoid conenction there is the battery ground strap to body, interlinking cable for twin 6v batteries, and the engine/gearbox ground strap that can all soak up some of the battery volts. You should be getting about 10v still on the brown at the fusebox during cranking, and of course a 79 should have the coil boost circuit which increase coil voltage from half battery voltage (about 7v when running) to full battery voltage when cranking (i.e. the aforementioned 10v) which actually gives a *stronger* spark during cranking than running. Spark plug condition and gap, and timing as well as carburation will all have an effect on starting performance, as will dampness in the atmosphere leading to beads of moisture all over the engine. In this latter case I have found pulling the HT lead out of the coil a little bit while someone else turns the key increases the gap which causes the HT voltage to rise, which can also allow an otherwise non-starter to fire.
Paul Hunt 2

This thread was discussed between 06/12/2007 and 10/12/2007

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now