Engine problems around 4000 RPM

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Daniel Worthington - 26 Aug 2004 18:34 GMT

Hi

I have a 1991 Mini Cooper S which has a 1275cc engine with twin 1.25" SU
carbs. It has the standard cooper airbox.

When driving, usually in 3rd or 4th gear, at around 4000 RPM the engine
starts to splutter and the rev counter needle behaves erratically bouncing
between 3000 and 6000 RPM. It feels as if it could be miss firing or running
to rich. In 4th gear with my foot hard down the car cant seem to get passed
70mph.

I thought it could because it wasn't getting enough air due to a blocked
filter, so I removed the filter and drove the car, this had no improvement.
I have also checked to see if the choke is closed properly, which it is. I
have also checked to see if the throttle cable is tensioned properly, which
it is.

I was wondering if anyone had any experience of this or any suggestions to
sort it out.

Thanks in advance

Dan

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Tim - 26 Aug 2004 22:06 GMT

Check your dizzy cap/leads and points. It sounds like a breakdown in the
ignition circuit. If the car has the original cap/leads etc then they are
probably long overdue for replacements.
A good and simple check is to when its dark, start the car with the bonnet
open and observe the dizzy and leads to see if there is any arcing (IE
shorts ).

> Hi
>
[quoted text clipped - 19 lines]
>
> Dan

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(-AD-) - 27 Aug 2004 14:28 GMT

And Elvis was sitting next to "Tim" <tim_lis(NOSPAM)@clear.net.nz> in the
spaceship, which I thought was kinda weird, but then they turned to me
and said:

> Check your dizzy cap/leads and points. It sounds like a breakdown in the
> ignition circuit. If the car has the original cap/leads etc then they are
> probably long overdue for replacements.
> A good and simple check is to when its dark, start the car with the bonnet
> open and observe the dizzy and leads to see if there is any arcing (IE
> shorts ).

It's likely a fault on the LT side of the ignition circuit, as it's
making the rev counter misbehave (rev counter takes its signal from the
ignition LT).

Possibly something that vibrates at that particular frequency at 4000 RPM
- sometimes you'll find a length of wire that vibrates madly at a certain
resonant frequency, and the copper inside breaks up near the
connectors due to fatigue.

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Graham W - 27 Aug 2004 14:58 GMT

> When driving, usually in 3rd or 4th gear, at around 4000 RPM the engine
> starts to splutter and the rev counter needle behaves erratically bouncing
> between 3000 and 6000 RPM.

Tachometer detects engine speed from the ignition signal. Your problem
is electrical.

> I was wondering if anyone had any experience of this or any suggestions to
> sort it out.

Stop interfering with the carbies and making matters worse! 8-)

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