14 replies to this topic

[Ouster]

I have a fairly consistent misfire at around 1800 RPM and as I accelerate slowly up the rev range to about 3000 RPM, which is making the car undriveable around town. It was recently (September) set up on the rollers at Slark, nothing under the bonnet had been touched until the problem arose. I've tried a new coil and points since and still have the same problem; I have two questions:

1. Could carp in the carb cause the problem? I don't have a fuel filter installed yet.
2. Can carbs "unadjust" themselves over time?

I'm at a loss at what to do really!

[Dan]

I would say it's more likely to be ignition related. Did you change the condensor with the points? If not why not? The condensor is the component that is more likely to have trouble in a particular speed range than the points are.

[Ouster]

Sorry, forgot to mention I also changed the condenser at the same time as the points - made no difference at all.

Edit: And spark plugs!

Edited by Ouster, 30 December 2008 - 05:55 PM.

[blacktulip]

dodgy ht lead? what about the coil connections?

Edited by blacktulip, 30 December 2008 - 05:53 PM.

[Ouster]

Pretty sure the coil is connected properly - I did it the same as the old one, and a 1990 is non ballasted isn't it?
I'll try and find my old HT leads tonight and try them tomorrow morning - are HT leads known to break down after around 6 months of useage?

[Dan]

Well I'd still stick to looking at the ignition for a bit yet. Check over the dizzy, make sure the dizzy drive isn't worn. How do the lobes of the dizzy shaft cam look? If the dizzy is wearing then electronic ignition could cure the problem. Check the timing as accurately as you can (check the dwell angle too instead of checking the gap) and check that the bob weights in the dizzy are free to move and the springs are intact. Make sure the baseplate moves properly and it's all clean inside. Don't get me wrong, it could be down to fuelling I just think that if the fuelling was set properly as recently as september it won't have changed that much by now and it would have had to change a fair bit to cause a misfire. Does the car still misfire in this speed range when you are sitting in neutral or only when under load? Carbs don't really adjust themselves, whatever some people might tell you! If the carb is in good condition, is clean and the screws are all tight and properly set then it won't wander (I suppose it could be worn but I would expect a good RR would have told you about that). The thing is that can be the very problem, the engine's requirements change over time as parts of it wear and the carb needs to be adjusted to compensate. A few months running isn't going to cause that much of a difference in the engine's internals though! One thing you could look for is air leaks, although I would expect an air leak to be present at all speeds. There are some things that are known to leak only when the engine is loaded and straining against its mountings however. Things like the brake servo pipe can be pulled against as the engine twists and cause small leaks.

The other thing it could be is that if the fuelling has been very accurately set on the RR, or if they re-timed the cam (unlikely I'll admit) you might need to have the dizzy curve adjusted. Again I would have expected a good RR to point that out to you, or do it there and then.

Edited by Dan, 30 December 2008 - 06:36 PM.

[Dan]

Sorry, just noticed the replies that came in while I was typing.

No, a 1990 is very much ballasted. There should be two connections to the coil positive, one pink/white and one white/yellow. A non-ballasted coil in a ballasted car will give you a weak spark at all speeds but the effects would certainly be more noticeable as the speed increases. The fact that the mis-fire cures itself at higher speeds makes me doubt it's the coil causing this. Do fit the proper coil though and go for a test, you never know.

It could be a lead but that's another thing that would normally show up at all speeds and is independant of load.

Edited by Dan, 30 December 2008 - 06:11 PM.

[ANON]

1. Could carp in the carb cause the problem? I don't have a fuel filter installed yet.

depends on what type of carp it is, koi can cause rough running problems. ghost koi are quite hard to spot when you are looking into the carb so make sure you use a good light

[taffy1967]

No, a 1990 is very much ballasted

But the wires on my 90' Mainstream Cooper are all white or black white, i.e. there's no pink wire at all: -

[benb12]

Was this before or after the petrol cap was nicked?

[Ouster]

Thanks for the reply Dan, that gives me some stuff to be looking over tomorrow - been busy up until now!
My wires have been painted over so I can't tell what colour they are, and after Taffys's reply I don't know what to think!

This was before the petrol cap was nicked by the way. Watch this space!

[AzMaN]

1. Could carp in the carb cause the problem? I don't have a fuel filter installed yet.

depends on what type of carp it is, koi can cause rough running problems. ghost koi are quite hard to spot when you are looking into the carb so make sure you use a good light

aww i was gona make a similar joke

[Dan]

But the wires on my 90' Mainstream Cooper are all white or black white, i.e. there's no pink wire at all:

Aren't Mainstream Coopers electronic ignition? Was I talking about a Cooper? The car in question in this topic is a Flame.

As above it can be caused by junk in th carb I just think that a good RR would have ensured the carb was nice and clean and the fuelling really wasn't setup all that long ago. Very little junk gets drawn into the carb and while it's common in an old carb that's been sat there in an undermaintained condition for many years it's not likely if it's been cleaned recently. If the fuel lines are breaking down there could well be junk in the carb but again fuel blockages only get worse as the speed builds, they don't fix themselves and they tend to be independant of load.

[Ouster]

Update: turns out the base plate wasn't returning to it's position - I guess the bob weights/springs were at fault. Luckily had a spare distributor so just swapped it over. Problem solved! Thanks Dan.

Now I just have to worry about driving with all the salt and ice on the road.

[mini-obsessive]

i may be pointing out the obvious here but have u made sure that no moisture is getting into ur points etc... common problem in mini's and especially mine! Infact mine broke down this morning for this reason... RAC was called out and by the time they arrived points had dried out and it started... Typical!!!!