Trying to understand a general carb problem

[Ian_Fearn]

We've got 2 carb'd cars. The 1.6 auto BX and the 3.5 V8 Range Rover.

The BX has an autochoke and the RR a manual choke.

Both cars suffer exactly the same problem.

Both start perfectly from hot or cold BUT if you start the engine from cold, both on choke, run them for a few minutes, perhaps a few hundred yards and then switch off neither will restart without loads of cranking.

This is a problem for me when leaving work, driving off site I often get searched where the official rule is you must switch off the engine.

I'm assuming the engine has too much fuel in it and is essentially flooded. Am I correct and can it be prevented??

[chriscarterstevens]

i seem to recall the following from my old 16 trs (manual), which was on a citroen sticker on the windscreen:

starting from hot, you must depress the accelerator - not sure why?

i did have the same trouble with mine. but was led to believe it was 'normal'

[DavidRutherford]

I'm assuming the engine has too much fuel in it and is essentially flooded. Am I correct and can it be prevented??

It is indeed flooded to one extent or another. Essentially the mix is too rich to support combustion, and so you need to crank it loads to get some air in.

Not a lot you can do with the autochoked car, but on the manual choke, a good idea if you know you're going to have to turn the engine off is to press the choke all the way back in and allow it to stall. This means you won't have petrol everywhere when you go to restart.

[paul001]

Could be an issue with the bi-metallic choke or the fuel solenoid shorting out.

As for the Rangie is it the twin strombergs ? [Pot style carbs] they suffered badly with membrane failure inside the carbs or the needles stick on those if they arent correctly oiled. I had a 3.5 that suffered with the problems ripped out the carbs and replaced with a webber conversion no problems again till the rear cross member rotted out along with the sills and floor pans

[citronut]

the BX carb dose suffer with drops of water getting into the emulsion tubes/jets, this usualy happens if the cranck case breather is blocked as it causes mouistur to build up on the under side of the rubber air intake hood over the top of the carb, then you get droplets going down into the emulsion tubes/jets,

the other common fault is the floats get badly adjusted, or bent on re/fitting the carb top,

another prob they suffer with is the auto chokenot openning properly sometimes becaues the coolant level is low, so the thermo capsule dose not heat up to open the choke flap

regards malcolm

[mnde]

We've got 2 carb'd cars. The 1.6 auto BX and the 3.5 V8 Range Rover.

The BX has an autochoke and the RR a manual choke.

Both cars suffer exactly the same problem.

Both start perfectly from hot or cold BUT if you start the engine from cold, both on choke, run them for a few minutes, perhaps a few hundred yards and then switch off neither will restart without loads of cranking.

This is a problem for me when leaving work, driving off site I often get searched where the official rule is you must switch off the engine.

I'm assuming the engine has too much fuel in it and is essentially flooded. Am I correct and can it be prevented??

My 16 (and the previous one) used to be like this and it would drive me mad.

I just need to ask one thing about the RR - when you come to restart it after 100 yards/a few minutes is the choke still fully out? If so, therein lies your problem. Your air/fuel mixture will be far too rich with the choke flap "strangling" the air supply.

Same with the autochoked BX. I used to find short manoeuvers/journeys a pain because the car would invariably refuse to restart. When I investigated, I found that the choke flap was always in the "full on" position and, at first, the only way to get the car restarted was to hold it open artificially with a screwdriver to admit more air. Then I found a similar (and less faffy) method was to floor the accelerator and keep it there while cranking. This activates the deflooding mechanism (i.e. cracks open the choke flap by about 8 mm).

If the coolant is not yet warm enough to activate the choke waxstat to progressively open the choke flap, as I understand it, you are reliant upon a number of things:

1) sufficient vacuum to operate the pulldown mechanism. This is reliant on the diaphragms in the choke deflooding capsule being in good order. On mine with a Solex carb with a black vacuum chamber on the side (two stage pulldown) there is also a short hose that sucks the air from the chamber (which must not leak). Basics of operation: as soon as engine has fired, the choke flap must open a smidgen otherwise the engine will instantly die (flooded). Vacuum pulls on the diaphragms an unspecified amount (stage one), which opens a passageway to allow air to be sucked from the black chamber; the additional tug moves and holds the diapragms up against an adjustable stop. Under correct conditions (Solex) this should mean that the choke flap is now opened by 6 mm. If the flap doesn't open this amount, the engine starts to run rich (throbbing), the idle speed drops and it can cut out.

2) Correct setting of waxstat mechanism so that (like the 6 mm measurement above) it's at a certain position at an ambient temperature of 20 degrees C

3) Free operation of the choke pulldown rod emerging from the deflooding capsule (i.e. that the vacuum action pulls upon). I found dirt/lack of lubrication gums this up which interferes with the action of opening the choke flap.

4) Sufficient fast idle speed. Set with a tiny stop screw which pushes on a lever attached to the throttle mechanism. The action of the waxstat capsule opening the choke flap also progressively returns the idle to the normal speed. I found, especially in winter, the fast idle speed from cold was insufficient. This meant, along with running slightly too rich, the engine would slowly falter and die while I was still scraping the ice off the windows . More revs = more air = more vacuum = faster warm-up.

5) Correct air/fuel mixture and ignition timing. A couple of years ago, I found that slight tweaks to the timing had noticeable effect on cold start performance.

Since my car had a tuneup at a garage recently, and since I have adjusted the various carb mechanisms above, I've had no trouble restarting a few minutes after an initial cold start, or after short journeys. Since I don't yet have access to an "ambient temperature of 20 degrees C" all my adjustments are only approximate - which means my fast idle is probably too fast (c 1700 rpm - but that seems to work well!) and the pulldown probably opens a tad more than 6 mm, so it does stall after the first start of the day unless I pump the throttle as soon as the engine's fired.... EDIT: but not if it's a hot day.

Sorry this is long, but I hope it's helpful

Someone might be able to repost up the link to the scanned in pages from the Solex/Weber Haynes manual which gives all the above adjustments in more detail than the Haynes BX manual

Mark.

[Tim Leech]

i seem to recall the following from my old 16 trs (manual), which was on a citroen sticker on the windscreen:

starting from hot, you must depress the accelerator - not sure why?

i did have the same trouble with mine. but was led to believe it was 'normal'

As you say the factory sticker states that if the engine has already been started "do not hesitate to depress the throttle fully" and to "engage starter for up to 10 seconds" when starting from warm.

All the 16 petrols and 2 out of the 3 19s have behaved the same way and I guessed this is "just the way they are". Regarding the R/R Ian the Sd1 is a similar setup and performs fine and only needs full choke to start from cold. Once the first "cold start" is out the way it starts first time every time even if relativly cold. Maybe changing from strombergs to su's (same as the Sd1) on the R/R may help?