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MG MGB GT V8 Factory Originals Technical - V8 tuning question ...

After many hours of finagling, I've got my 4.2 Rover topped with a Carter 400 running just beautifully, but for one thing: when I decelerate, after a few seconds, as the revs come down, the motor sort of "hiccups" for a second, the car shudders, then keeps running fine although the tone's subtly different.

What the heck could this be? We're talking about very high vacuum situations, so the metering rods and springs shouldn't be relevant (the springs I finally selected pop up at 5 in/hg, which isn't much). It's a very consistent, repeated behavior which if nothing else is just getting on my nerves!

(One thing which I've heard can bring this on is a ram air/cold air box setup, because it keeps trying to shove air into the carb. But I certainly don't have one! I just have your basic old low-rise air cleaner on top of the car.) Any ideas?
Bill Withum

It certainly is a puzzle and to be honest I have no idea what the cause may be.

Perhaps a rolling road dynomometer may be useful in determining the nature of the problem?
Cheers, Pete
Peter Thomas

Bill - like you I suspect the ignition. The engine is very forgiving to incorrect timing - I would start again with the basic timing - run with and without vacuum advance before going onto leads, the coil and the amplifier. Amplifier problems come in different forms.

But I am not familiar with the carter or what it does in the over-run situation

I always though hiccupping, poping and crackling on decel was to timing (too much advance?) and or to rich on fuel? I would say hit timing 1st, if the car runs good accrost board otherwise I would not want to mess with the fuel settings unlesss you have too..
Larry Embrey

Roger, you know, that makes good sense to me. Timing is spot on but I bet something's up with the vac advance. I have the diss limited to 36 degrees total advance, but what I may be doing wrong is that I have it hooked up to manifold rather than ported vacuum. Since the throttle discs close on decel., manifold vacuum is very high but ported is low.

I can't wait to check it out. (Anyone remember where the ported vac takeoff is on a Carter/Weber/Edelbrock carb?)
Bill Withum

Could be a vacumn leak. Check the headers are tight, and also check that the plugs are fully seated. Some spark plug spanners won't sufficiently tighten the spark plugs as the head of the unit can't go fully into the block. This same problem had me for ages! (duh) When I found it, I had to throw away the plug spanner and grind down a socket instead.

Used to show up especially when backing off into roundabouts from around 50mph, then getting back on the throttle - hiccup - rarrp rarrpp - few seconds later all is normal.. Just an idea. :)

74 V8
Neil Cotty

> Used to show up especially when backing off into
> roundabouts from around 50mph, then getting back on
> the throttle - hiccup - rarrp rarrpp - few seconds
> later all is normal.. Just an idea. :)

That's it **exactly**! However, plugs are tight (13 ft/lbs; brand new; gooped w/anti-seize too) and as they say here in NASCAR country "I'll be dipped in sh*t" if I can find a vacuum leak. Headers on there perfectly. The only leak I've ever heard under there is a very slight hiss from the dipstick tube -- it appears to seal just a little less than optimally.

(To Roger's earlier comment, the Carter doesn't have a ported vacuum takeoff so I can't try that. Sure made sense to me though.)
Bill Withum

This thread was discussed between 30/09/2002 and 10/10/2002

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