Bogging and misfiring

[Crockett]

Well, after having some success helping to change the engine mountings on my son's 2000 and then spending the morning helping to re-align the geashift on his car, I decided it was time to spend a little time on my own. All I really wanted to do was to drive it around a little, having been laid up for a few months over the winter, and to charge the battery etc.

It started quite well (considering it hadn't been run for a couple of months) and drove smoothly and normally. Everything seemed to be going fine. I left the engine running just above idling (about 1500-2000 RPM) for about halk an hour. Everything was fine until I came to take it back to the garage.

It was then that the "stoneleigh problem" (as it has become known since it once stopped me from getting there) came back to haunt me. I thought I'd gotten rid of this.

A description of this would be a particularly bad misfire and lack of power which gets worse the harder you depress the accellerator, Easing off the accellerator seems to improve it slightly although by then it's hard to make much progress using only 1/3 throttle. Any attempt to use more will cause coughing and spluttering and loss of any engine power.

I originally thought this might be due to a weak spark so I have tried replaced the plugs, the coil, the distributor cap, the leads, every spade connector in the ignition circuit under the bonnet (some were quite corroded), the car also now has an Aldon ignitor (which it didn't used to have) and I cleaneed out the filter in the glass-domed fuel pump but still this problem comes back whatever (and disappears just as quickly), again and again - without warning.

could it be something to do with grounding of the distributor baseplate (which might explain why it's worse under heavy throttle, as that's when it'll moves?).

I wondered if anyone has any ideas about this one? I'm running short just now and it's very frustrating.

Either this is a much more complex problem or I am missing the point entirely.

Regards
Daryl.

[Alan Chatterton]

Sounds like fuel to me.

Have you checked.......

Fuel filter?
Fuel filters in pump?
Jets?
Float chambers?

[Crockett]

I should have mentioned I have also (to try to localise this problem);

o Cleaned out the fuel filter in the pump.
o Fitted an in-line filter (for good measure).
o Replaced the needle valves (which were worn and causing the car to leak fuel when parked on a extreme hill).

You mention float chambers. What else might I be looking for with the float chambers? Since the engine tuned as expected (after the needle valve replacement) I'd assumed the float chanber levels toi be correct.

I also found myself reasoning this way:

Since the misfire is worse the larger the throttle opening, it would appear to be that the increase in fuel / air mixture cannot be properly ignited for some reason.
As the ignition system appears to (now) be in order, this could only be because the mixture is either becoming over-rich or over-lean at full throttle opening.

The problem also doesn't change when the choke is pulled out (it makes no difference at all) which seems odd as it should either improve things (if over-lean) or make things worse (over rich) unless there is simply no more fuel available for delivery. That seems to me like it might be a fuel starvation issue. That in turn could only be either the needle valves (replaced), a blockage (which intermittently clears then reoccurs?) or the fuel pump not being able to deliver enough, possibly when hot or when the fuel level is lower.

One other thing, it also seems worse when going up a hill, although this may be more noticeable because that's when you need to use more throttle.

I could try a fuel pump replacement as I do have a spare available (attached to my son's recently recovered [from his write-off accident 2007] engine, which is only about three months old and is an easy replacement. The problem is that because as this problem last occurred in February 2008 (as I log all these issues), I may have to wait another year before I can be sure it fixed the problem!

Regards
Daryl.

[Alec]

Hello Daryl,

it does sound very much like a fuel problem. Could you have some debris in your fuel tank that is blocking the outlet?
I don't know what model you have but, S.U.'s are quite easy to do a quick check of fuel level and see how much is in the float chamber when the problem occurs. The other option is to remove the air filters, whether Zeniths or S.U. and look down the choke tube as you rev the engine. There should be a visible amount of fuel coming from the jet. If not then it is fuel and it is a matter of determining the cause.

Alec

[wild bill]

You don't have something in the tank-cloth etc being drawn onto the outlet pipe by suction?Quick check is to hang a petrol can above the carbs,connect a pipe to the carbs& syphon/gravity feed the carbs.Always worth a try-and it don't cost money! Good luck.Bill.

[Crockett]

It is fitted with SU HS4s. Checking the fuel levels might be quite tricky though since as soon as I pull over due to this problem, the engine starts running as normal again, so I wonder if the fuel level may appear normal at that point.

Revving the engine also does not seem to cause the problem, it's only when under load and acceleration that this starts to happen. It'd been idling away today quite hapilly for nearly an hour, until I tried to move off.

It seemed to "recover" when it had been idling for a while, then started to cough & splutter again further down the road. That does sound like a fuel deprivation problem now that I think about it. The question though is: Why?

Cheers
Daryl.

[Alan Chatterton]

Try the same (ticking over, revving then driving off) with the fuel cap removed......

I'm wondering if its fuel starvation due to a blocked breather?

[David Withers]

As already suggested by Alec and Bill, I would first ensure fuel is getting from the tank to the lift pump.

I once came across almost exactly the same symptoms with a Hillman Avenger. The cause turned out to be paint coming off the inside of the tank and being drawn against the outlet when the engine called for more than a dribble of fuel.

[CAR]

I had this sort of thing once when I had replaced the tank after cleaning and painting it. I tried all sorts to fix it (not the ones from Bassetts!), but eventually I realised that the little triangle of unused space, just between the spare wheel and the inner arch/boot floor hump for the axle, was not as ideal as I first thought for storing a can of puncture repair.
It turned out that this was squashing the fuel line, and although it wasn't enough to stop the car starting, it was enough to stop the car accelerating!

Colin.

[Crockett]

Certainly the consensus appears to be some problem with fuel delivery. That makes sense to me as I'd have expected pulling out the choke to have a bigger positive or negative effect, at the time. I believe backfiring / spitting back is also a sign of way too lean a mixture?

What bugs me most is the intermittent nature of the problem, which makes it very hard to find and "catch in the act", so to speak. Could fuel pump valves occasionally sticking (gummed) open possibly account for something like this? The fuel filter could be in a better place too IMO. I'm going to move it to the input of the pump rather than the input of the carbs, where it is now. It gets a lot of air / vapour in there (you can see it as it's glass and is never full) especially when it's hot. It may not be connected but it'll at least eliminate something if I move it.

I'm going to go over the fuel pump (and possibly replace as I have a nearly new one available), breather hose and fuel filter, probably at the weekend and see what I find. Something else which may not be connected but could be, is that the car (even when running well) often has a tendency to be hesitant at full throttle, dropping it back a little regains some power and that's actually quite similar to the "Stoneleigh issue", only nowhere near as bad. I guess that too could also point to a fuel delivery problem in general.

Cheers
Daryl.