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MG MGB Technical - Acceleration Hestitation

I have rebuilt the Weber Carbs, Gapped plugs at 28, timmed at 20 degrees Idle at 800 rpms, have a petronics coil and electronic ignition and all is fine except upon accereration there is a noticible hesitation in all gears....any ideas? I have a '74.5 MGBGT. Thanks in advance!

JW Colson


Is 20 degrees at 800 rpm correct for the engine?

I don't know which engine you have, but I think the bone stock book settings, depending upon which engine, run from 10 to 15 degrees set at between 600 and 1500 RPM. I don't mean just anything in that range. I mean that different engines had a different advance set at a different RPM. For example, 12 at 600, 11 at 1000, 13 at 1500, etc.

May not be your problem, but might be worth checking into. From what I have heard, I think the advance should max out at around 32 degrees BTDC.

C R Huff

When I had this with a Solex carb it turned out to be the emulsion tube blocked. This tube is full of fuel with air bubbles in it and is very easily vapourised it is pulled into the choke when manifold pressure drops to give you the rich mixture you need when accelerating. worth checking with a gunsons "see thru" spark plug that the flame goes yellow as you blip the throttle.
Stan Best

Do these Weber carbs have an accelerator pump? Is it working? Is it adjustable? Have you tried a richer setting? This was the case in a Scimitar GTE.
Paul Hunt

You say weber Carbs, plural. Are you running twin DCOE's? I am not familiar with them but the DGV does have an accelerator pump that Paul mentioned.

Go through ALL the ignition settings first, 20 deg BTDC at idle is far too much advance. The most, in the manual, is 15deg for 18V engines, but remember this is at idle WITH vacuum pipe disconnected and "plugged". After that start suspecting the carb. You don't say the type of Weber. But most do rely, to an extent, on the correct idle mixture to make even transitions from idle to primaries, as well as the accelerator pump, which on down-draughts, has 3 adjustments. Also downdraughts have a "step-up" needle and jet. The springs are easily changed to change the staging of this needle. Since all this is dependent on vacuum, check for air leaks on the carb gaskets.

I am with Allan, It's too much for an idle advance setup.
Allan, yous say "vacuum pipe disconnected and "plugged". " Do you mean plugged on the carb side?

It's not the pipe that is plugged on the carb side, but the carb (or manifold) port. However if you removed the pipe from the vacuum capsule you would plug the pipe. The purpose is to avoid a vacuum leak while adjusting the timing.
Paul Hunt

Thanks, guys, this has been helpful. How in the heck did I get the 20 degree timing when researching and on this post 15 and 11 degrees are the ticket?! Yet when it was set on 20 degrees (sitting)the engine ran faster and smoother???? Thanks again!

JW Colson

When ticking over, on a very little fuel, the mixture needs lots of advance to get decent combustion. When you set the timing you are adjusting it an arbitary figure to get the best combustion, without pinking, under all circumstances. WHen at idle the vacuun unit gives the extra advance necessary, when the engine is revving the "bob" weights also advance the ignition. If you set too much advance at idle you will get far too much advance when these systems are operating, not good for the health of your unit, lots of pre-detonation!!!!!!

Many thanks Paul, as usual really accurate infos. I did not express myself right: when i told plugged onto carb side, I meant plugged the carb vacuum intake.
best regards.

"When ticking over, on a very little fuel, the mixture needs lots of advance to get decent combustion"

Not strictly correct. MGBs (and many other cars) originally had *no* vacuum advance at idle (carb vacuum source), full vacuum advance was applied rapidly as the throttle starts to open, then gradually less as it is opened further.

Then someone realised that if vacuum advance were applied at idle (manifold vacuum source) it *does* makes for better combustion, so you get a higher idle for the same throttle opening, so you turn down the idle to compensate, which uses less fuel, i.e. creates less pollution. So yes it makes for better combustion, but it is purely an emissions measure and not a case of the engine *needs* it.

When setting the timing dynamically on manifold vacuum cars you *must* disconnect the vacuum pipe and plug the manifold pipe or you will get grossly *retarded* timing.
Paul Hunt

This thread was discussed between 24/07/2008 and 08/08/2008

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