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MG MGB Technical - Timing question

Got my 67 B GT out and on the road. Had a valve job done on the head this winter. Because of the burnt valve, I found that the front carb was now way off when I put the unisinc on it. Readjusted carbs and reset timing to factory 13 degrees BTDC.

Car started and ran good, but was somewhat flat on acceleration in all gears.

I used the timing adjuster on the distributor to change the timing(20 clicks towards the firewall) and power is much better. Have not put a timing light on it yet.

I know some say adjust the timing until the engine pings and then back it off a little, but is there a way of determining what is the best timing other than just trial and error?

I do not know if cam is original. Head had been converted to unleaded in the past and cam could have been changed. I noted pistons are stock, so don't think it has been rebuilt.

I did clean and lube the distributor (original for 67). All parts were in good condition and tight.

Bruce-C

Bruce - Here is a method that works well. Cheers - Dave

IGNITION TIMING
With Tachometer and Vacuum Gauge

Connect an electronic tachometer and a vacuum gauge to the engine (See illustration). Set the engine speed to 1000 r.p.m. with the idle speed adjusting screw. Loosen the distributor mounting screws and slowly rotate the distributor back and forth until the highest vacuum reading is obtained on the gauge. If the engine speed changes, readjust to 1000 r.p.m. After the highest vacuum reading has been obtained, rotate the distributor in a retard direction until the vacuum gauge has dropped exactly one-half inch and then tighten the distributor locking screws.

If engine speed drops more than 20 r.p.m. after retarding the timing, one or more of the following conditions exists: Loose distributor shaft, improper cam angle, faulty advance mechanism, poor point condition.
If this ignition timing procedure is followed, the possibility of detonation will be eliminated and power settings of the timing curve will be up to standard.
David DuBois

Super, thanks Dave. I will set it tonight.
Bruce-C

A vacuum gauge can be very useful as it takes account of your engine and your fuel, which can vary from engine to engine even of the same spec, whereas the manufacturers figures assume the worst scenario i.e combination of tolerances most liable to pink plus give a safety factor. The vacuum gauge shows the early onset of pinking by it's little occasional flicks, and the instructions on mine say to set timing so there are just no flicks, then retard a further 3/4 in. Hg. On a 73 A-series engine it gave me noticeably more advance and hence better performance and economy. But on my 73 MGB (which are notoriously prone to pinking especially on unleaded) it gives the same results or slightly worse than spec, so I have to use the 'most advance just short of pinking at any combination of throttle, revs and load' method. The vacuum gauge also only takes account of conditions at idle, if you have the wrong curve from either an incorrect distributor, worn springs or indeed changed characteristics due to modern fuels, then it may still pink at part throttle, which is what mine does. Ironically opening the throttle wider stops it. Presumably the bigger charge cools the cylinders better, and that pointed fin between the valves in the head which is what causes the problem. It's not just the loss of vacuum advance at larger throttle openings, as I have a system whereby I can turn that off anyway.
Paul Hunt

I've tried many different approaches to this problem over the years. But, in the end, I wound up using the "road timing" system. My dial back timing light is very handy, but does not take all the revelant data into account. Running a supercharger, the timing becomes an extremely important thing. RAY
RAY

Ok, I hate to say it but in my garage full of tools, I do not have a diagnostic tach. I went to two different automotive parts stores and even Wallmart and it appears that diagnostic tach/dwell meters are a thing of the past like our cars. Ebay to the rescue.
Bruce-C

You surprise me. Usually we can't get the specialist tools here in the UK that Americans often mention, but there are several automotive multimeters including tach and dwell functions available here.
Paul Hunt

To Dave DuBois

Dave, I couldn't find your illustration. Please tell me:
1. Where to connect the vacuum gauge. For example, to the vacuum port on the rear carb where the vacuum advance for the distributor is connected or to the intake manifold directly?

2. Should I disconnect and plug the vacuum advance hose to the distributor?

PS - my engine is a early 3-main fitted with a PCV valve, if this makes a difference.

Thank you for your regular participation in the BBS your advise is always professional and helpful.
Frank Grimaldi

This thread was discussed between 13/05/2009 and 14/05/2009

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