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RarlyL8 RarlyL8 is online now
Crotchety Old Bastard
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Join Date: Jan 2000
Location: Huntsville, AL
Posts: 13,563
New (?) Running Issue

Two steps forward and one step backwards, that is the credo for restorations isn't it.
A new (?) issue surfaced on Friday. My son has been wanting me to take him to school (he is a sophmore in HS) in the slant so his buddies can see the car. I've been putting it off with all the fuel system issues leaving me less than confident driving the 35 mile round trip in rush hour traffic. Well I've adjusted the WUR to compensate for the weak pumps and the car has been running pretty good so I thought we'd go for it. Fired 'er up on the chilly morning and off we went. Car ran flawless all the way to the school and about 1/2 way home. On the final stretch I fealt a slight hesitation that progressively worsened. At steady cruise it would periodically buck like you were switching the engine off and quickly back on. When I rolled up to stoplights it would idle perfectly. Pull away and the hesitations start again. I coasted down a mountain with the tranny out of gear and the idle was rock steady. Put it back in gear and the bucking returned. When I got near home I decided to boost it and see what would happen. Power was strong as usual but the bucking became expectidly violent when it occured. When I let off after boosting through 2nd gear the bucking was full blown bad and often. Nursed it home and put 'er away. This morning I put an LM1 on it and fired up. Started perfect and instanly, idled perfect. OK lets go for a ride. Ran perfectly for a couple minutes then very subtly the hesitations returned. Once fully warm the bucking began. Boosted it and the bucking became horrible.
So this is a repeatable failure of some sort. Here's what we have observed:
> No bucking at idle cold or warm.
> Bucking begins as engine warms and progressively gets worse.
> AFRs are good. Does not run lean at any point in the range.
> Power is good.
> Bucking is like you are cutting off the key quickly. Frequency is every 10 or so seconds during onset and every second to several times a second after boosting.

Have any of you seen these type of symptoms? Would be great to have my suspicions validated or disproven. This is a tech forum so no better place than to solve it here and post results.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 10-27-2012, 09:06 PM
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