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MG MGB Technical - MGB carb and timing settings

Here's one for you more experienced enthusiasts out there:

I have a 1976 MGB that has the following upgrades: Weber 32/36 DGV carburetor, modified cam and a Mallory dual point distributor. When I did a timing check, I noticed the timing is off the scale (advanced) about 20 degress - total of about 35+ degress from TDC. The car runs / accelerates / idles fine but I have to pull the choke out about half way (even when warmed up) or else it bogs when taking off from a dead stop. The carburetor is leaned to about one full turn out. The car also runs cool (it does have an oil cooler) - I've never seen it get above 1/3 way on the gauge. What I'm getting at is are my settings okay or what should I adjust? I've played with the mixtures (richer and leaner) but that doesn't seem to make it run any better. If I set the timing closer to TDC (I believe 15 degress is the factory setting) the car runs like a three-legged dog. Any help would be appreciated. Except for the Weber, the car came with the upgraded cam and distributor so I don't know what else was done or the specs on the cam.
Joseph Maggiore

My guess? You hit on it when you said it never gets over 1/3 warmed up.

A cold motor needs richer mixture and runs better with more advance. This relates to your bogging until you pull the choke out and running so well so much advance.

I'm going to guess that your thermostat is either broken or missing.
Matt Kulka

The thermostat is new. I replaced it when I replaced the water pump and hoses. Any other suggestions would be appreciated.
Joseph Maggiore

Joe,

it may be the timing chain tensioner is broken and has to be replaced.

rn
RN Lipow

Joseph, my theory is fuel starvation: needs more advanced timing to get a decent burn, if you pull the choke out it is better, kangaroo behaviour on take-off, probably OK at idle and steady high revs?

I don't know anything about Webers but in addition to checking that for air leaks you need to check the vacuum advance and any other inlet manifold connection for air leaks. Your timing should be at factory spec unless you have converted to unleaded which should need an extra 3 degrees advance. Richard
Richard Evans

The Weber, if I recollect correctly, also has a separate idle jet circuit and an accelerator pump. The idle jet deleivers fuel at near closed throttle openings and overlaps the main jet circuit. If the idle jet is not the correct size there would be a momentary hestitation until enough air is moving throught the venturi to fully use the main jet circuit.
A weak or leaking accelerator pump would be another cause. Its purpose is to inject more fuel into the venturi at rapid throttle opening.
Andrew Blackley

Joseph, I agree with Richard, my car had the same advanced timing as yours and I found the plug in the manifold was loose. When it was tightened the timing went to the correct position. Redline Weber has all the info for setting-up the carburetor on their website: www.redlineweber.com.

Doug
D. Cook

Ron,
What are the usual symptoms of a broken timing chain tensioner?
Frank

Joe- New thermostat? What temp, 165? V
vem myers

Thermostat is 180 and new. Weber is brand new and The plug on the manifold is nice and tight. Accelerator pump is working fine. I'm leaning towards the theory that my car is running too cold but that still doesn't explain the timing being so off the marks. Another theory presented to me was that the harmonic balancer could have seperated and thus the mark on the pulley does not reflect true TDC - I will check this soon. Also, could it just be that because of the performance cam, weber carb and Mallory dual point distributor (with mechanical advance) affects the timing scale?
Joe

Hi,

Check that the vacuum advance is working. Grab the vacuum hose end that is attached to the carb or manifold and try to suck on it if there is no resistance there is probably a hole in the vacuum diaphram and you are not getting any off the line advance. Replace the vacuum canister and you will have a smooth transition from idle to power.
Alan

Joseph: Have you checked to see that when the piston in number 1 cylinder is at TDC the pulley timing marke is in alignment with the TDC mark on the timing chain cover? If not you need to verify where true TDC is located and go from there. The parts you have added may need some fine tuning but they will not require 30 or 40 degrees more timing than stock settings. It sounds like you may have a crank pulley for an early engine with tinming marks at the bottom. You cannot get your timing right until you know where your timing marks are located in relation to TDC. I also agree your engine shouldn't run as cool as you say, you can check that with a meat/kitchen thermometer. Your gauge may be defective. The crank pulley probably is not delaminated if your timing readings are consistant. FWIW, Clifton
Clifton Gordon

Try tuning with a vacuum gauge. Check out this site: http://www.centuryperformance.com/vacuum.asp
Also, where are you conecting your vacuum advance line to, on the manifold?
Andrew Blackley

I think his Mallory is a mechanical advance distributor, no vacuum advance. Clifton
Clifton Gordon

As stated before, the distributor is a mechanical advance, so vacuum is not an issue. I plan on checking for TDC this weekend. The pulley theory could be correct as I noticed that the car had the older style coolant hose setup when I got it. Who knows what else is not correct for the year? As stated before, the car runs GREAT except I have to have the choke on or else the power dips then takes off when I step on the gas from a dead stop. I have plenty of uphill power too. I did notice some burbling though the exhaust pipe (not quite a backfire) when letting off the gas under load but I thought this might be due to either the performance cam and/or or flow through exhaust (it has a header with one muffler instead of the stock two (and no catalytic converter).
Joe

I'm almost in the same situation Joe. Tried to time to fact specs but after Webber, header, and brit spec distributer with pertronix, had to advance almost off the scale. No new cam (yet) and do have vacuum advance.
Interesting that it appears the harmonic balancer was also replaced on my car by P.O.
I'm interested to know how you make out.
Dan H.
Dan Hanson

Joseph,
Don't discount the possibility that your new thermostat is defective..... Just a thought.
Rob Edwards

This thread was discussed between 30/09/2003 and 07/10/2003

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