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Loose Power, Dizzy Vac Unit?

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#1 walter103

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Posted 01 March 2013 - 06:26 PM

1992 998 Stage1

When I put my foot down (especially in 1st) it drastically looses power and the revs drop for a couple of seconds, builds up again, repeats in each gear change.

I was told that this was due to a broken distributor vacuum unit, which I have replaced. Problem persists.

I can overcome the problem (sort of) by applying the choke.

Any input would be greatly appreciated

Cheers

Edited by walter103, 01 March 2013 - 06:30 PM.


#2 Ivor Badger

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Posted 01 March 2013 - 06:49 PM

Since the vacuum unit is a fuel economy device to lower fuel consumption on a light throttle, how would a failed one give these symtoms?

Edited by Ivor Badger, 01 March 2013 - 06:49 PM.


#3 walter103

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Posted 01 March 2013 - 07:10 PM

no idea; I shall slap the mechanic. Any suggestions?

#4 lrostoke

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Posted 01 March 2013 - 07:11 PM

How longs the stage one kit been fitted ???

What needle do you have...when I first fitted my stage 1 the AAU needle that came with the kit gave same results...I switched to AAA problem solved.

#5 KernowCooper

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Posted 01 March 2013 - 07:47 PM

The vacuum advance unit is redundent when you apply power and as you say you've replaced it and made no difference answers that one, as your able to improve the problem by richening the mixture with the choke it points towards a weak mixture, whip the needle out and have a look at whats in there now and post the info.

In the mean time list the full spec of the engine i.e carb/air filter/exhaust and any other mods

#6 crock

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Posted 01 March 2013 - 08:49 PM

You almost certainly have a mixture which is too lean. It is being caused by a vacuum leak, the wrong jet needle, or the wrong dashpot oil/spring combo. Mr Kernow Cooper above has the right idea. Post your specs and somebody can solve the problem for you.

#7 walter103

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Posted 13 March 2013 - 05:46 PM

Ok. I did have a ABU needle, which i have just replaced with an AAU. the problem has worsened. The car now only ticks over and runs with quite a lot of choke on. I have fiddled with the most forward adjusting screw on the carb but this has made little difference. As to listing details of the rest of the mods im unsure. I only know of the RC40 exhaust, K&N element (holes in box).

The exhaust was on the car when brought, I added the K&N. The problem has occurred since I had the timing adjusted as is was running too advanced. This is when I was told the vaccum unit needed replacing

Any more advice???

Thanks very much for the help so far!!!

#8 KernowCooper

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Posted 13 March 2013 - 06:23 PM

Its no good just swapping the needle without setting the base mixture at idle, did you do that after the needle swap? set the new needle to 2.5-3.5CO at idle

Just looked at the needle charts and the AAU needle is richer just off idle and very much so from position 7 on the needle chart, are you able to check the timing yourself? if not then I'd stay away from the previous mechanic and find one who understands basics about distributor ignitions and setting up SU carbs.

You need to make sure the engine is setup correctly before trying to do any carb mixture setting and thats points/plugs/timing/tappets, but with out equipment like a rolling road its all down to experience and thats the hard bit

Quote from Mini Spares on the AAU

The favourite and best needle for Minispares stage one kit. Tested on a rolling road by Peter Baldwin and Keith Calver this baby gave the best performance when using our RC40 system, Minispares inlet and exhaust manifold plus a K&N filter.

If using other manufacturers components this needle never seemed to work as well from results obtained or given to us over the last 5-6 years, but it is a good starting point.

Edited by KernowCooper, 13 March 2013 - 06:26 PM.


#9 walter103

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Posted 14 March 2013 - 08:08 PM

Thanks very much for all the input, but its way over my head. So, I took it to dc minis. Whats the worse case scenario?


#10 KernowCooper

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Posted 14 March 2013 - 09:12 PM

Check engine tune and components = plugs point condensor, unless electronic cap/rotor then timing and then Find out why it appears weak? needle or air leak I suspect is the path they will take





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