Cutting out after hard acceleration?

BX Tech talk
adamskibx
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Post by adamskibx » Mon Apr 10, 2006 11:58 pm

Thanks guys for helping me out; even looking up diagrams in the Haynes manual! Very much appreciated.

Firstly, in my last post I got completely the wrong idea about what a ballast resistor is. I thought it was those brass colourd things on the carb with wires coming off (the idle cut off thing is likely to be one of these). As you say Richard, I found exaclty the same thing; if I disconect the "one" wire I have that goes to the brass thing on the carb, then the engine will not idle, but will start and run so long as the accelerator is pressed a bit (bypassing the idle cut off valve). However, I have two of these brass things on my carb. From what your saying Richard, one is likely to be the idle cut off valve/solenoid. And Jeremy, as you point out, one is likely to be an electricaly operated ventilation valve; however, the odd thing is that the original GT carb only has one of these brass colours solenoids attatched to the carb, whereby the newer engine's carb has two. I only have one wire on the GT loom, but the TRS the engine came from had 2. It seems odd that I can connect this single wire to either of the two connections, and the engine will idle fine each time. This leads me to think that they are both idle cut off valves, but one is definately positioned in a way which makes me think its doing something regarding ventilation as there is the vent hose to the front of the car, and one to the air filter positioned right below the unit. My thoughts about changing to the GT's carb was that the single "brass solenoid" as opposed to two, would be more in keeping with the original GT wiring, and that the fact that two exist, must be for a good reason, which cannot be forfilled with the GT's wiring loom.

Ellivie: Thanks for the congrads on the MOT; i was rather nervous at the station having put all that work in lol! I havent yet got the timing dead on, so will get a strobe at some point soon; im inclined to agree with you on the posible incorect ignition timing not causing the problem, as I have taken it to both extremes with the same result occuring.

And Geoffrey, thanks for that explaination on ballast resistors; it explains a lot; I remember what one is now as the old GS had one, but on the BX, like you predict, there is no sign of one anywhere near the coil etc, so it probably hasnt got one.

This is my plan of action:

I have ruled out the air filter as this is new (mind you its the circular GT one, not the flat later type one as fitted to the car that the engine came out of)

I have ruled out fueling as the symptoms arent quite right and theres always fuel at the end of the line when it cuts out.

Firstly, clean all conections up, and clean all ignition parts.

If no joy, then ill wait for the problem to happen again and see if the coil or module are excessivly hot; I suppose a hot coil could be caused by a tired module though :?

Failing all that, ill try swapping the carb over for the original GT one, and certainly before the purchase of an expensive ignition module (if they even exist still).

Thanks for all the help guys; its given me some useful pointers and understanding so hopefully ill have it sorted reasonably soon.

Adam

adamskibx
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Post by adamskibx » Tue Apr 11, 2006 12:08 am

Just a thought: As the idle cut off solenoid just requires power which is suplied with the ignition turned on, chances are that the other solenoid jsut requires power too- could it be posible perhaps that if I split the single wire into two and connected it to both of these solenoids, that things would sort themselves out?

Cheers, Adam

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Post by jeremy » Tue Apr 11, 2006 12:22 am

Have a look at this thread - does this look anything like your carb?

http://bxclub.co.uk/forum/viewtopic.php?t=2179

jeremy

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Post by ellevie » Tue Apr 11, 2006 2:02 am

When I said that I would be strongly inclined to eliminate the timing as a possible cause, I meant that you should set the timing correctly now to check if it is actually the cause of the problem. In other words, I really meant that timing could definitely be the cause of the problem. Sorry for not making this clearer.

You could have the timing set at a garage for about £25 but it would obviously be much cheaper if you can do it yourself. Timing lights are available on eBay at the moment for about £8 inc postage. Just think of all the fun you'll have trying to spot the little white line jumping about on the flywheel. As an aside, a timing light also doubles up as a handy tool for checking for the absence or presence of an ignition spark because it flashes brightly when the spark is present.
David

BX19TRS 118K E Reg 1992-2008
BX19TRS auto abs 96k F Reg
BX19TXD 150k K Reg

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Post by Geoffrey Gould » Tue Apr 11, 2006 10:41 am

Hi I hope all is going well, just a couple or three small things, could it be that the extra valve is a safety valve in the event of an accident it stops the fuel from emptying from the carb over a hot engine?
If so then putting a loop of wire linking the two valves together may be a good idea. Taking this a step farther if it is a ventilation valve, and it does look like it is, if it blocks off a vent as the fuel is used it would tend to cause a vacuum in the float chamber and that would upset things a treat. Also it may be an emission thing (spit).
I apologise but I may be throwing a herring (red) in the works but where is the ignition amp. if it's mounted on a plate like a 16V then it could be that its getting over heated by the heat sink compound between it and the plate "drying out" over time and not doing its job. The plate can get horribly corroded as well. Remove amp. clean off plate ontill it's smooth and bright, carefully clean off face of amp, go to somewhere like Maplins and get a tube of Heatsink compound and spread a thin layer over the face of the amp. and bolt back together. This is well worth doing anyway and at the same time the plug/socket can be cleaned as well.
I was going to dive off into the distributor end of things but This is probably enough at the mo.
Good luck.
Geoff.

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Post by ernst stavro blofeld » Tue Apr 11, 2006 2:26 pm

As the man says, make that a thin layer of heatsink compound. The thinner, the better.

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Post by jeremy » Tue Apr 11, 2006 8:45 pm

Just for a moment to return to the original problem of the car dying after hard acceleration - this may be a classic case of fuel starvation. car accelerates - and burns loads of fuel - which initially is fed from the carburrettor supply in the float chamber - which eventually gets exhausted and - the engine dies.

This can and often is simply a fuel supply problem which could be a filter problem or a fuel pump problem. On your car you have a low pressure pump mounted on the engine - and it could be the source of your problems.

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Post by adamskibx » Tue Apr 11, 2006 9:23 pm

Right I see. Thanks for clearing that up ellivie; I think ill get the timing sorted first then, then change the fuel filter, and then apply heat sink compound to the ignition module. Firstly ill see what happens when I attatch that wire to the other solenoid aswell, and then service everyhing else as a precaution.

The carberettor in the pictures from the other post is very similar to mine but not quite the same; the accelerator pump is positioned differently etc. The intake mouth is exactly the same shape ect though.

It looks like im going to be spending another few days under the BX pretty soon! Ive got this to sort out, an imitation oil seel that doesnt seal (i suspect) to replace again, as well as fixing the heater water leak (ive bridged the matrix for now)

Cheers for all the help guys; it'll be interesting to see what the problem is when I finally get to the bottom of it.

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Post by mat_fenwick » Thu Apr 13, 2006 11:22 pm

I've had a similar problem before, which turned out to be a defective coil. My speculation is that under load, there is greather combustion pressures, hence the spark will find it more difficult to jump the gap at the spark plug.
It will then 'seek out' any alternative path, such as short circuiting the insulation in the coil (some I believe are oil filled). It tended to be more frequent with the engine warm and in warm weather.
If you have a rev counter, check to see if that falls when the engine splutters, if not you can rule out a low tension ignition problem.

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Post by adamskibx » Fri Apr 14, 2006 1:04 am

Yes thats a good idea about looing at the rev counter Mat, Ill give that a try tomorrow. I am however, at this stage thinking it could well be the igntion timing causing problems. I tried disconecting the vacume advance/retard hose and plugging it today, and briefly took it out and the problem had almost disapeared. I will check out that low tension circuit check by watching the rev counter next time it happens. Id imagine that, if the rev counter doesnt drop off relative to actual engine speed, then I can outrule a fault in the low tension systems, but I suppose timing errors or faults with the vacume advance would escape this test, as even if the igniton was totally wrong, it would all be happening at a speed directly proportionate to the engine speed. Also, with my car, the problem happens after the engine has been worked hard, not necessarily during. For example, today, I drove the car about 30 miles down a motorway; Having worked the engine hard driving up the sliproad, the engine developed this problem; I had to turn the ignition off, let the engine stop, then turn the igniton on again, then lift up the clutch to start it, all at about 70MPH; oddly, this time the engine ran on badly even after the restart, and the only way to not let the symptoms appear was to break the motorway speed limit! The engine was only happy while working hard, and I have a feeling the second choke opening up in the carb was the point at which everything seems OK again; the point at which the accelerator pedal feels like its suddenly working two springs rather than one, and the exhaust note deepens. Im bidding on a timing strobe on ebay at the moment. When I get it ill do the timing. I know the standard advance at idle is 10 deg BTDC, but I think it has to be 7 for unleaded fuel. I only know this because there was a note in the hand written service history that came with the donor car, saying that "ignition timing set to 7 deg BTDC for unleaded petrol", in 1992. If this cures the problem, great, if not then at leas thats another thing eliminated. One thing I still dont understand about these early electronic ignition systems is the job of the TDC sensor; surely any data that can be collected off the flywheel on an old engine like this, can be collected from the distributor? If it does have a useful job, then the fact that mine is a bit oily due to the leak might not help, and I did guess the air gap when I put the TDC centre back in after having to swap it over or something. All I can tell from the wiring diagram in the Haynes, is that it definately connects to the diagnostic socket (cam timing check?), and posibly, has something to do with the rev counter (im useless at reading wiring diagrams btw).

Cheers, Adam

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Post by Way2go » Fri Apr 14, 2006 2:32 am

Reading your problems it seems more likely that it is fuel starvation. I think you may be fooling yourself by turning the ignition on and off because if your pump operates like mine then when you turn the engine over on the starter the pump is continually pumping petrol which probably replenishes your float chamber quickly for your restart. With the ignition on but not starter and engine stopped no fuel is pumped.
Mine is injection so pump may operate differently but thought this may give you food for thought.
1991 BX19GTi Auto

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Post by Geoffrey Gould » Fri Apr 14, 2006 11:30 am

Hello The plugging of the vacuum tube started the alarm bells ringing loud. Have had problems in the past on BMC and Vauxhall motors with a wire from the body of the distributor to the "points" fracturing inside the insulation, when the base plate moved as the vacuum altered the wire would go open circuit then make again when the engine had stopped so that it would start again as normal. Used to happen with a Viva, usually going up hill. The base plate earth leads on Lucas/BMC used to do the same and cause missing and bad running.
Just a thought. I assume that yours does not have any points as you mentioned a amp. but as you have a vacuum capsule then something has to move.
Cheers.
Geoff.

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Post by ellevie » Fri Apr 14, 2006 5:25 pm

Adam, I'm glad to hear that you are getting a timing light. You will have to set the timing sooner or later, so you might as well do it now and set your mind at rest. I can see from your post that you are still a little concerned about the exact operation of the timing adjustment mechanism so here is my fairly simplistic take on it to hopefully help when you come to do it.

There is a magnetic sensor on the outer casing of the distributor which is in very close proximity to a star shaped a four pronged rotor mounted on the distributor shaft. Each time one of these prongs passes the sensor, a tiny signal is sent to the amplifier (ignition module) where it is amplified and conditioned and then used to switch the ignition coil to make a spark.

Now if we cause the flywheel to rock forwards and backwards by nudging the bumper with the car in gear, the distributor shaft will rock to and fro in sympathy because it is mechanically locked to the motion of the flywheel via the crankshaft, cambelt, camshaft, etc. There is a mark on the flywheel called TDC which when aligned with a 0 degree mark on a small triangular timing plate above the flywheel, indicates when the cylinder is at the top of its stroke.

Now by slightly loosening the two bolts that fix the distributor to the engine we can rotate the outer casing and hence the magnetic sensor independently of the distributor shaft which contains the magnetic rotor. Thus by rotating the outer casing we can move the timing of the spark backwards and forwards relative to TDC. The spark is used to trigger the flash gun so that the TDC mark appears to be stationary thus allowing us to adjust its position to coincide with the 10 degree mark on the timing plate, by rotating the distributor housing.

For simplicity's sake I have not mentioned the centrifugal governor or the vacuum advance which are effectively in series with the distributor shaft and modulate the timing in accordance with engine conditions. I had a look at the timing on my 19TRS a few weeks ago when we were posting about it and at first I had some difficulty spotting the little white line on the flywheel. It turned out that it was directly under a cable which crossed over the timing recess; you should look out for this and move it aside if needs be.
David

BX19TRS 118K E Reg 1992-2008
BX19TRS auto abs 96k F Reg
BX19TXD 150k K Reg

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Post by Geoffrey Gould » Fri Apr 14, 2006 7:54 pm

Hi looking at the reply above by ellevie then it looks as if anything connected with a wire is stationary ie. bolted to the distributor body, sorry if I may have missled you, never seen one having a diesel myself.
Cheers.
Geoff.

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Post by adamskibx » Sat Apr 15, 2006 12:58 am

Thanks guys. Geofrey, your idea about a wire not making complete connection at certain vacume advance is plausable I reckon. There is a fair bit of circuitry and a few wires in there; I took it apart the other day to see what happens etc- tried sucking the vacume hose to see if it all moved-it did. Today I had a quick look over it all and changed the fuel filter; ill do the pump next as a might as well put the 40k pump on instead of the current high milage one. BTW I tried looking at the rev counter when it happened today- it definately reads correctly even when most power is lost from engine, only fluctuating fractionly as the engine gets bursts of power.

Ellivie; thanks for that explaination of the timing system. The only thing im left to understand is the job of the TDC magnetic pickup sensor that picks up a dip in the flwheel which is independent to the smaller timing mark used to time up against that litle dial that lets you time the engine. Its a litle black thing with a wire coming off it which leads into the loom, and it definately travels to the diagnostic socket.

Thanks again, Adam