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MG MGB Technical - WEBER SAVVY

My 79 mgb roadster is now running quite poorly.

Adjusted the timing on the Lumitronix distributor from a retarded position to an advanced position to overcome acceleration lag. The car pinged, exhaust backfired and bucked. So I backed the timing off about 3-4 degrees. This helped and the lag is gone. The bucking and backfired is relatively minimal.

However, I changed the plugs and they were black as well as the tail pipe.

The car does not idle well and the exhaust smells "rich"

I am now suspecting the carburetor.

The car has a Weber downdraft 2-bbl carb with electric choke. The choke is open when warm and the electric choke warmer is working.

I see no mixture adjustments for this carb. I admit I am not educated as to the characteristics of these Weber carburetors.. So if someone out there can lend your knowledge, I would be greatful as what direction to go.

Thanks in Advance

cheers

Gary :>{D

79 MGB
gnhansen

I'm not familiar with that particular Weber, but is the air filter clean?
Dave O'Neill 2

I have the weber on a 72 and 79, I have had the same kind of problems before, but not with the weber.

1. I had a bad vacuumn leak, hose on carb and vacuum advance, one leaked from broken hose, other clamp was loose.

2. Timing, I had to advance until no pinking, it worked at 20 degrees BTDC.


Two adjustments, idle screw behind throttle linkage, and mixture adjustment inside of that toward manifold.

Mine are working great.
James Huggins

Gary, try accessing Bob Ford's BRIT-TEK website at www.gerardsgarage.com/GarageContent/Tech/Weber or you can call him at 1-800-255-5883 or 1-603-437-1584.

He was a lot of help when I did my Weber conversion.

Cheers, Davy
Davy Crocker


Ignore the stumble for now. Something else is up.

Weber DGV carbs do not idle poorly. They have their faults, but a rougher-than-stock idle isn't one of them. So, if a Weber DGV can't idle right, you know that something very basic and very important is not right.

So. To get this sorted:
- Start with your timing at its stock setting.
- Check that your ignition advance/retard is working properly: a sticking centrifugal advance can cause big mixture issues. Connect a timing light, rev the engine, make sure the timing mark moves as described in the manual for your distributor. In brief, it should advance when revved up to about 2500-3000, then go back to resting position as your revs drop. If the mark sticks badly, fix your distributor promptly. If it moves nicely, all is well.
- Verify that your valves are adjusted within reason and your engine compression is basically okay
- Make sure you haven't got a vacuum leak (intake manifold leak sounds highly likely with these symptoms, but check everywhere - I would be surprised if nothing is leaking, given the symptoms you describe)
- Now. The Idle mixture screw is a flathead screw with a spring on it, that screws into the body of the carb. On mine it's on the lower corner of the carb beneath the throttle cable stuff. If you have no idea where to begin, screw it all the way in and back it out 2.5 turns, then adjust from there. Engine warm and idling, turn it in till it leans and stumbles, then back it out enough to smooth the idle again.
- if it still won't idle well, just take the carb apart and put it back together, cleaning and adjusting as you go. Amazing the amount of good that can do. You'll find and fix clogged jets, or floats that don't, or whatever else might be wrong in there. The rebuild kit includes enough instructions for a reasonably mechanically inclined person to do it in an afternoon, if you take your time. You'll need a clean workbench and some patience. You'll be rewarded with a freshly rebuilt carb and a full understanding of what's in there.
- Further mixture adjustment is done by replacing jets. Don't do this yet.

You should now have a smooth idle and stock timing. That's a good place to start.

And your car will still stumble when taking off from a standstill.
That's okay.

The off-idle stumble is actually a very common tuning issue with the Weber. Get everything back to stock and regular adjustment, idling properly so you've got a good baseline to work with, and then look up the weber stumble issue in the archives on this BBS. Plenty of people have tackled that one before you, so you won't need to reinvent the wheel on it.

Best of luck to you!
Sam Good

Sam, Davy, et al: Thanx for the info.

Davy: contacted Bob Ford @ Brit-Tek and got good info

Sam: Your advice is right in line with Bob Ford's. I will get into the car at a later time. Again great info.

My thoughts are along the lines of a vacuum leak as well as carb adjustment.

Yes the plugs are new and the air filter has been cleaned and dried. Able to se light through it after cleaning (K&N)

Will share the outcome as soon as I return to the car.

thanx to all

cheers

Gary:>{D
79 MGB
gnhansen

OK, reset timing to 17 BTDC -BIG improvement.

Next, found the mixture adj screw (that took a bit)It was WAY out. As per Bob Ford, turned the screw full in then backed out 1-1/2 turns, started the car and worked from there. HUGE improvement there also .

Tested the advance on the dist. with the timing light and it functioning fine.

Still is not a smooth idle, but much improved.

Took the car for a good 20 mile run on the Northway at 70mph arond 3800 RPM. The lag from start through the gears is gone and more power now.

As I ran out of daylight, I called it quits, I'll pursue the vacuum leak angle another day and will update the outcome of that adventure.

Thanks to all for the guidance - good info.

A timing question. This engine was rebuilt, bored .030 over, mild race cam and Lumitronix ignition kit installed. How does one set the timing after those mods? Is the 20 degree BTDC still apply or...?

cheers

Gary :>{D
79 MGB
gnhansen

Ah yes a nice Saturday and what could be better than to work on your MGB.

Went over the timing again. 2 sources of info

The timing marks are 5 degree increments beginning at the "top" being 0 degrees progressing to 20 degrees.

So the Haynes MGB repair manual states for the low compression engine (which applies in my 79) the timing is 13 BTDC @ 600 RPM.

The Owners Handbook reads out the setting at 10 BTDC @ 1500 RPM.

I went with the Owner's Handbook, and set the timing as stated.

Reset the idle back to 1000 RPM

Went back to the carburetor and worked the mixture set screw until the engine smoothed out at the max RPM level at the current idle setting.

Reset the idle back 1000 RPM.

Reset mixture screw leaner until the RPM began to slow.

Went for ride and the original problem of "lag" through the gears is gone, good power, good idle, and pre-ignition dieseling upon shut off is gone.

The only thing now is the engine seems to struggle slightly under load. Perhaps the plugs are a bit fouled from all the maladjustments.. A few trips to work and back hopfully will clear out much of the carbon build up done by the over-rich carb.

That will be another thread at another time.

Again, thanx to all who commented.

cheers

Gary :{D

79 MGB
gnhansen

This thread was discussed between 24/09/2007 and 29/09/2007

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