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Vac advance, how much is too much? Is that why it's backfiring out the carbs?

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Vac advance, how much is too much? Is that why it's backfiring out the carbs? #1
AmishIndy Seth Jones Glendale Heights, IL, USA Sign in to contact 1971 MG Midget MkIII "Guenevire" 2007 Mazda 3 "Porco Rosso"	Topic Creator (OP) Aug 31, 2014 10:47 PM Joined 13 years ago 5,849 Posts

Just tested Eric's car the other day with a timing light. At idle, we set the dynamic timing to 10º BTDC with the vac advance disconnected. When I reconnected the vac advance, timing went to between 25º and 30º BTDC at 1000 rpm. Every sop often it spits unburnt fuel out the forward carb. We've checked the points, adjusted the valves, installed a new fuel pump, changed the throttle shafts and discs, and also shimmed up the float valves so that they both have a 1/8" clearence as per factoroy specs. The rear carb is missing the idle adjustment screw (there is one on the way). At idle it runs on only the front carb. Is that timing or mixture related? For the moment both jets are turned 0.070" down from the bridge (standard starting point). We havent tuned them yet because of the missing screw.

Seth Jones

1971 MG Midget

www.SpridgetGuru.com

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Re: Vac advance, how much is too much? Is that why it's backfiring out the carbs? #2
S1 Elan Kurt. Appley Akron, Ia., USA Sign in to contact 1964 Lotus Elan 1965 MG Midget MkII	Sep 1, 2014 08:32 AM Top Contributor Joined 11 years ago 8,014 Posts

Sound's like a lot of advance at 1000rpm. Distributor could use work???

Kurt.

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Re: Vac advance, how much is too much? Is that why it's backfiring out the carbs? #3
littlecars David Bassett Nashville, TN, USA Sign in to contact 1963 Chevrolet Corvair "Cheeto!" 1965 Chevrolet Corvair "Ski Team Transport SOLD!" 1965 MG Midget MkII "Buffoon --- SOLD!!!" 1966 MG Midget MkII "Swiss Cheese...SCRAPPED" & more	Sep 1, 2014 08:37 AM Top Contributor Joined 12 years ago 8,389 Posts

Awaiting more responses to your query Seth. I'm having the same problem on my, technically, rebullt SUs from another car. Did all the same things you did.

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Re: Vac advance, how much is too much? Is that why it's backfiring out the carbs? #4
refisk Rick Fisk Frankenmuth, MI, USA Sign in to contact	Sep 1, 2014 09:51 AM Top Contributor Joined 12 years ago 14,662 Posts

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Vac advance, how much is too much? Is that why it's backfiring out the carbs?

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