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Holding onto RPM far too long

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Holding onto RPM far too long
#1
deodra Avatar
deodra Jason Buis
Ferdinand, IN, USA   USA
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1977 MG MGB "Midge"
For some reason, after you rev up, RPM's remain high and very slowly start winding back down. Throttle cable is not blocked or slow to slide, I know there are some air leaks, but they were minimal, and the butterflies seem good and are pretty responsive. I'm wondering if it could be the carb adjustments. Would running rich from the mixture screw cause this?


1977 B Project - sold

1977 B needing a little TLC

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Re: Holding onto RPM far too long
#2
Rod H. Avatar
Rod H.
Amity, OR, USA   USA
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1964 MG MGB
1968 MG MGB GT
Weak springs on the poppett valves in the carb butterflies can cause this. What carb(s) do you have?


Friends talking around a fire is the history of mankind.

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Re: Holding onto RPM far too long
#3
deodra Avatar
deodra Jason Buis
Ferdinand, IN, USA   USA
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1977 MG MGB "Midge"
In reply to # 3475691 by Rod H. Weak springs on the poppett valves in the carb butterflies can cause this. What carb(s) do you have?


It's weird. It just started and I'm pretty sure it's my stupid mistake somewhere. I just installed a new distributor and have been resurrecting the car. After the install, this started happening. I'm thinking the carb setup was completely wonky because the PO was trying to mask some other issues I've found along the way. Now that the car is actually running and several things have been fixed properly, the air/fuel/idle is all out of whack.

Weber 32/36 electric choke with Cannon


1977 B Project - sold

1977 B needing a little TLC

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Re: Holding onto RPM far too long
#4
RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
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1967 MG MGB "My Girl"
Does the new distributer have vacuum advance? If so, try disconnecting the vacuum line and observing the engine speed. RAY

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Re: Holding onto RPM far too long
#5
Rod H. Avatar
Rod H.
Amity, OR, USA   USA
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1964 MG MGB
1968 MG MGB GT
Given it's a Weber, Ray is onto something. Manifold vs ported vacuum issue perhaps.


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Re: Holding onto RPM far too long
#6
barry s Avatar
barry s Barry Stoll
Alexandria, VA, USA   USA
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1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
There are no such things as "minimal" air leaks, in part because one cannot quantify with any confidence.

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Re: Holding onto RPM far too long
#7
mac townsend
Fairfield, CA, USA   USA
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1973 MG MGB MkIII
the DGV has what is called a ported vacuum for distributor vacuum advance. It must be matched to a distributor designed for ported vacuum, not manifold vacuum which is what a 77 would have. The result of using a wrong matchup would be such like you are seeing.

So I think that if your replacement was bought "for a 1977 MGB" you ought to replace it with one "for a 69 mgb" or install a vacuum port in the DGV manifold and use it then block the ported vacuum take off on the carb.

Or, temporarily, disconnect the vac advance (plug the carb port or stick a golf tee in the vacuum hose at the distributor end...after disconnecting it) and see how that works.


1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

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Re: Holding onto RPM far too long
#8
spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Canton, IL, USA   USA
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1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1976 MG MGB "Cecil"    & more
engines don't rev without fuel.

A vacuum leak isn't the problem.

the car will wind down with or without the ported advance, this is not the issue.


Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.


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