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Lumpy SC at low revs

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racingred
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racingred PCGB Member
2021/07/24 13:59:15 (permalink)

Lumpy SC at low revs

Hi. My friends SC is very 'lumpy' at low revs but appears to clear at around 2500rpm...I understand it did have new plugs and dizzy cap but this has made no difference. He was going to get a new set of leads next but I was wondering if it is the fuelling side...any thoughts/advice welcome....

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chas3.2
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chas3.2 PCGB Member
Re: Lumpy SC at low revs 2021/07/24 14:04:03 (permalink)
Hi 
Check inlet manifold gaskets all vacuum pipes.
could be sucking in too much air.

Current 911 997 2S 2008.
1987 924 S.
3.2 Tech Advisor.
Previous over 40 years Cayman 2.7
993 targa
2. 3.2 Car...> 2.944 2.5 lux
3.944 Turob's 250bhp
1.3.0 911 SC.
1.New 2.5 Boxster
1.964 1992
356 B super 90 1962
(262 > max. 255)
Old and Slow
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Re: Lumpy SC at low revs 2021/07/24 17:44:13 (permalink)
I had to change all the plug leads on mine - they are screened and they break down inside; whether this causes lumpy running, I'm not sure.  I also replaced the auxiliary air valve - apparently (according to a Service Note I have) the early one can open uncontrollably at low partial throttle opening and cause the engine to "buck".
Also, it could be a fuelling problem - ethanol absorbs moisture which can condense in the tank, so you might get a fuel pick-up problem.  I use a fuel additive to compensate for the low octane.
Hope this helps.

Old and slow
richard.stephens-eosemi.com
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Re: Lumpy SC at low revs 2021/07/25 19:09:52 (permalink)
Ignition and vacuum leaks are the common cause. Make sure none of the pipes have fallen off or cracked. If its backfiring it could be the warm up regulator.

Rich
911 SC Technical Advisor
racingred
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racingred PCGB Member
Re: Lumpy SC at low revs 2021/07/25 20:15:39 (permalink)
chas3.2
Hi 
Check inlet manifold gaskets all vacuum pipes.
could be sucking in too much air.




Many thanks - project for next weekend.
racingred
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Re: Lumpy SC at low revs 2021/07/25 20:24:40 (permalink)
Many thanks for the suggestions - will get to grips with it shortly. All the best!
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