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odd engine problem - misfire, lack of power - 1.6VX

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not on my car. a friends car...


its a 1999 1600 8v VX. weber 40 dcoe.

overnight it develops this fault. have tried EVERYTHING to remedy it, but just cant seem to get it right.


it starts and idles fine. it sits in the garage and revs fine. it sounds ok. the mixtures are ok. the jets are the right sizes, and the carbs have been stripped/checked/cleaned.


we have swapped out the coil, the distributor, the plugs, and the ECU from my car. no change.


the head has been off and been skimmed, and new gasket.

the valves have been checked.

the temps are fine, the coolant does not leak, and the oil pressure is sound.


and yet STILL, the ****3R wont run right. *mad* ☹️ 🙆🏻


you can warm it up, balance the carbs, tune them up them and it all sounds sweet in the garage - but take it out on the road, and it farts and stutters and misfires each time you ask it to accelerate, regardless of gear or revs.

it will build up speed if gentle throttle is applied and its not hurried. it seems happy to drive at 70 or so once it gets there -and all sounds well.

but try to "accelerate" up to that speed, or put the foot down in any gear at any revs and its misfiring, farting and spluttering....


WTF!! *mad* *confused*


the carb-to-inletmanifold O rings may not be perfect [this is known] but the WD40 trick wont reveal an obvious air leak, so I dont think its that serios. i just dont know where to look next ☹️


all 4 cylinders appear to be firing because as stated it runs fine when stationary. the plugs are all a good grey/black colour so its not terribly lean or anything. ALTHOUGH I have noticed on a couple of the plugs, the electrodes are black [suggesting rich] and yet the core electrode is WHITE [suggesting lean, and HOT] ?? thats weird, no?


please help *thumbup* *thumbup*

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i personnnaly have not changed the HT leads, despite suggesting it on a couple of occasions, Iam assured that they have been checked, and swapped out with a set of leads from another car [not a 7 vx] probably an old vw polo or something...

i WILL swap the HT leads next time I have the chance for my own peace of mind.


Crankshaft sensor. this was discussed some time ago. cant remember the conclusion.


can you direct me towards it? is it an electrical sensor that I can swap out or check the connection to? or is it buried in the block?

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my own current thought is that we need to swap out the fuel pump.


all the symptoms seem to suggest its fuel related - so thus far we have concentrated on the carbs and on the electrics connected to the spark.


but could the fuel pump be faulty 🤔 *idea*

it ticks away merrily and delivers enough fuel at idle and when revving stationary. but under load, could it be under-fueling and causing the misfire?


this might explain how the plugs could be black [strong idle mixtures] and yet the core electrode could be white [lean and hot under load]


any thoughts *idea*

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The crankshaft sensor as i remember it is a Black lead on the nearside of the engine.


The most common faults are poor connection between component. Are the connections all clean and well fitting? Have you checked the distributor cap and rotor arm?


Underfueling i thought would occur also if you are revving the engine. The pump jets squirt fuel in when revving so i should imagine if the fuel pump could not keep up it would show up there also.


Sounds to me maybe more of a ignition problem as it is happening under load. The system should be advancing or retarding the ignition depending on the load and revs and maybe this is not happening, The cranshaft sensor helps to control ignition point and you so this could be a good port of call.



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A few possibles:


Pump jet operation - blocked/pumps not working etc look down the carb and operate

throttle cable linkage forcing carbs off the o rings causing air leak

carbs hitting chassis tube and forcing carbs off o rings

thackeray washers/bobbins knackered/carbs not flexibly mounted

coil/ignition breakdown

fuel pressure too high/low


These are the best kind of problems - once you've fixed them *tongue*




Green and Silver Roadsport 😬


Edited by - Mr Locust on 22 May 2005 11:52:18

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ok, I'll locate and check the connection to the crank sensor.


surely the car uses more fuel revving under load, than it does revving stationary? so would a lack-of-fuel issue maybe not show when stationary, but raise its head under load?


distributor and coil both swapped out for my items, no change.


we tried to create an air leak by pressing and pulling on the carbs, during fast idle, while spraying wd40 around the carb/manifold connection. no raise in the revs to indicate drawing air.


what are the thackery washers/bobbins 🤔

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Webers/dellortos are not meant to be rigidly mounted to the manifold as it causes frothing of the fuel in the carbs. Behind the mounting stud nuts (which clamp the carbs to the o-rings) there will be rubber bobbins between cupped washers or thackeray washers which are like an extra long spring washers.


Personally I think the bobbins are just that but hey that's just my opinion. I also prefer the webcon supplied 'misab' plates instead of the plastic o ring carriers.


Either way you should be able to waggle the air filter end of the carbs up and down by c. 3/8" when they are installed i.e. they don't bolt up tight. If you can't move them you are likely to get spurious crap running type faults but ony which correspond to vertain vibration modes/frequencies of engine operation.




Green and Silver Roadsport 😬

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I would have thought the crank sensor is a waste of time if it is running i.e. it does or doesn't work. The only possible issue may be interference if the ht leads run anywhere close to the sensor wires.




Green and Silver Roadsport 😬

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thanks locust,


the crank sensor lead is routed along a chassis tube, well away from the HT leads, for most of its journey toward the bulkhead. it does pass close to the distributor and coil though as it rounds the end of the head....

i'll check and re-route it to be sure. thanks.


i agree that a sensor normally sesnes, or it doesnt. unless the connection is bad. but having just inspected mine for reference I can see that it is a sealed unit, ie sensor and cable are moulded.

I'll check that my friends car is the same. *thumbup*



thanks for the description. i now understand what the thackery bobbins are.

I was aware that these are to allow a certain amount of movement. these have been adjusted to give 1.5mm between each of the 2 copper cups that hold the rubber grommit.

I will recheck that these have not been tightened.


our carbs are mounted with an ally spacer plate between the carb and the inlet manifold. there is an O ring on each side of this plate, and for each intake.

we do know that one or more of these O rings is suspect, and will be changing them all as a matter of course.



anything else worth looking at, or swapping over that could cause a lack of power and lots of popping/farting but little forward motion.

it sounds for all the world like a misfire. but I know that all 4 cylinders ARE firing. and on time.


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I went through a very similar frustrating exercise on the Locust which used 1500 Triumph engine and twin DHLA Dellortos. The rubber bobbins seemed to loose their resiliance and even with movement as described the problem persisted - so I wouldn't just trust to adjusting them. In a relatively short space of time, I found that even new ones compressed to a fraction of their pre-fitting length and stayed that way so they weren't springs any more.


What I did see was the carbs hit a noticeable vibration resonance when the problems happened i.e. they went all blurry! Simply resting a finger on the carbs was enough to change the frequency/damping of the system and make the problem cease.


Only on replacement of o rings and carriers with misab plates and bobbins with thackeray washers did the problems on my particular installation cease. I too had substituted distributors, coils, fuel pumps etc and become thoroughly hacked off with the whole piece. I was using the car for work at the time and annoyances such as these don't sit well with 300miles/wk.


I also had problems with brittle thackeray washers due to embrittlement from crap plating. See if you can get black/natural finish if you go down that route. You will be surprised how much more spring there is in the assembly with this setup.


Having used brand new carbs on my car which were rather expensive even 10yrs ago, it was really frustrating to find that 15p components are the key to the whole thing working wonderfully or being a pain in the 🙆🏻 whenever you try and drive the car. Once I sorted it I didn't touch the carbs for another 10k miles or so when the car got written off by a kind sort in a Vectra anyway. Balancing the carbs wouldn't have sorted that out.


Keep us posted.




Green and Silver Roadsport 😬

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I had similar issues a while back with a 1600 VX I used to own. The car started and ticked over fine, it would accelerate gently fine, and would cruise on a light throttle OK too. Once the throttle was pushed down with any vigour the car would misfire. Just occasionally everything would clear for a few seconds. Does this sound similar?


In my case one of the chokes within the carb had come loose. The choke insert was twisting in the barrel of the carb, and cutting off the main jet. When the car was on the idle circuit it ran fine, as soon as you needed the main jet to accelerate it missed.


I was surprised it could be driven at all in this state (with main jet circuit out of action). I drove 120 miles home with it like that, cruising at 60. A blatchat 'help' thread elicited a response from Weber God Roger King. With the aircleaners removed I could move one of the chokes within the carb - the other three were tight and would not move.


The one that moved was realigned, and a small screw under the carb (with a locknut IIRC) tightened up. Problem gone.


I suspect a simple blocked main jet could cause similar symptoms.



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jonathanG, thanks for the reply. the symptoms you describe sound almost exactly the same.


i think we are getting somewhere now. through discussions last night with lawrence and my father, and the suggestions on here and scottish sevens, we're pretty sure its a fuel shortage issue.


perhaps not being caused by the fuel pump not delivering enough, as I was beginning to expect - but by something causing an insufficient amount of fuel to be "injected" when required. ie under load.


we are now proceeding to check:


pump jet and main jet blockages.

progression drillings blockage.

acellerator pump diaphragms.

correct operation of accelerator pump mechanism.

choke alignment and secure location.


as well as changing the O rings between carb and inlet to ensure no air leaks.


im pretty sure that one or more of the above factors is going to cure our problem. *thumbup*

will keep you posted.

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