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MG MGB Technical - adjusting ignition timing 'by ear'

I took my '68 roadster in for MOT last week (it passed). The guy there said that the engine seemed out of adjustment, he could smell it was running rich and a bit lumpy, so he offered to adjust it. He has a history of hotrod racing and has certainly worked on A series engines in his youth! He tweaked the mixture using his gas analyser and then proceeded to adjust the ignition until tyhe engine ran smoother. Afterward the car seemed to run better with no pinking and there was no run on when I turned it off (which it used to do.
I checked the timing tonight and it was around 30 degrees at idle! This seemed a little (!) high so i turned it back to 14 degrees. It maxed out at 28 degrees. So my question is can you adjust the ignition at idle by turning the distributor for max revs then back it off a bit (which is what this guy did)?
Steve Church

I am not sure about the MGB but when I had a BSA Bantam many years ago, a mechanic ( who was a professional racer too) set mine up by starting the engine and turning the bit of engine casing holding the points until the engine ran at its smoothest. There was no advance mechanism to worry about but he said he rarely set the timing statically unless it was to start the engine!
AdrianS

Steve,

Are you absolutely sure that the timing mark on the pulley is indicating TDC?

In our Dutch MG magazine was an article about an MGA pulley of which the rivets had loosened. The DPO had repaired this by welding the pulley. Alas he did not check the orientation of the timing mark....

Willem vd Veer

Yes I did check the timing mark with TDC. I will check again though.
Steve Church

Steve, are you still running that "unknown" dizzy?
To answer your specific question "can you turn the distributor etc"; Yes. But it is a imprecise setting and usually comes with a different set of issues like difficulty starting from cold (ask me how I know).
I hope you don't take this next bit the wrong way but I think you need to get your car to someone you trust to set it up properly.
I've followed your posts for a while now and pitched in when I have felt able to do so. I'm now thinking you haven't been listening or understanding what we have been advising. Your engine with a suspect dizzy, none standard cam, over bore and performance head is VERY UNLIKELY (scratch that...) WILL NEVER give it's best with standard settings. A quick check of the resources available and you will see the max advance for your engine in standard form, on leaded petrol is 30deg. Which means (if you are getting 14degs on the dizzy) you should set at around 16deg at idle (600 rpm). Now if your engine idles smoothly at 600, I'd start to question just how "hot" the cam is and is it running actually at 600rpm? If its slightly faster a setting of 14deg will be a bit too retarded.
Considering what you've spent on the engine so far, get the 123tune fitted and get it to Peter or someone local with a RR and some B experience.
THAT WILL BE THE ONLY WAY YOU WILL GET THE BEST OUT OF IT BEFORE YOU KNACKER ALL THE GOOD WORK YOU HAVE DONE

Alternatively you can continue tinkering and running it before doing all the work again in 20k miles.. at least than you'll know exactly what to do....

sorry for the rant

MGmike
PS. I hope he rechecked the mixture AFTER setting the timing?
M McAndrew

Hi Mike,
Thanks for the comments. I take all comments on here as constructive criticism so no probs with your advice. I am planning a trip to see Peter next spring. Our availability dates didn't coincide this autumn so I've had to put it back until next year. It will be an excuse for a long weekend away with the wife. I shall probably get a 123tune from Peter and ask for some advice on an initial curve before I see him next year. I don't expect to be doing too many miles over the winter. It will give me something to play with! I'll put it on my Christmas list.
Steve Church

I assume you disconnect and plug the vacuum pipe when adjusting the dissy?
Mike Howlett

You cannot adjust multi carb setups with an exhaust gas analyser.
c cummins

I have a Moss supercharger on my '67B and the only way that I could get it run up to expectations was to "road time" it. This consists of taking the car out for a drive, with the distributor just loose enough so that you can turn it by hand. Run the car through the gears and observe its performance. Pull over to the side of the road and advance the timing a small amount. Drive the car again and repeat until the engine starts to ping under load. At this point, back off the timing just enough to eliminate the pinging. Tighten down the distributor clamp. You should now have an engine that will give you increased performance and mileage. When I worked on cars in the '70s, it was common for Ford, among other manufacturers, to recommend this procedure due to each engine being a little different from the others. I also used this system on boats that used inboard engines. I'd take the boat up the river, at full speed, and advance the timing until I detected pinging. Backing off the the timing and tightening down the distributor produced the most power available from these craft. A lot of contented customers. You may also be getting a false reading, from your harmonic balancer, as these tend to delaminate over time, and deliver inaccurate timing light readings. RAY
rjm RAY

FYI re Ray- That is the reason I went to the D-25, recurved by Jeff Schlemmer, which has the advance/retard nut. Approx 12 clicks = 1 degree. I also fine tune the timing per Ray, on the road, then on the side of the road: Only effective way really. Re ccUmmings: if you match the piston fall on 2 Su, per John Twist U-Tube video, then set both jets about 64 thou below the deck to start, an Innovate AFR with analog gauge will in deed dial in the mix over range. Just be sure to richen or lean both carbs equal flats. Again, the only effective way to get the right power mix ( 11-13 or so:1) over 1500-4500 RPM I think. Cheers, Vic road.
vem myers

Hi everyone, Steve an interesting thread...may I ask what engine pinging/pinking is. I've seem the term many times before but do not understand what it actually is / what the symptoms are. Can anyone explain?

/Moss
Moss

In my simple laymans understanding: as the engine spins faster the precise time the spark is generated needs to change (advance)in relation to where the piston is in that cylinder. If you advance it too far it sparks before this optimum point (piston further down the cylinder and rising. Rising piston is then confused by the combustion charge which tries to push it down. Up meets down = bad news.

It is the tinkly sound you can hear if you accelerate vigorously (and /or uphill) in too higher gear. Why does it need to advance? It's a bit like panning with a camera when photoing cars passing, on on my new fangled digital thing pressing the shutter before the race car actually get to the corner! The relationship between the moving piston and the time the dizzy generates the spark. There is a tiny but finite time between the spark being generated and the combustion charge going bang!

Others may explain it better M
Michael Beswick

"You cannot adjust multi carb setups with an exhaust gas analyser."

You can *if* both carbs are already fully balanced for mixture and air flow and all you want to do is get the emissions below the limit, by adjusting both carbs in the same direction by the same amount. But you certainly can't get correct adjustment of the carbs otherwise.

The spark should occur as the piston is still compressing the mixture, which starts the burn, which compresses the mixture even more. If this starts too soon then the mixture reaches the point of self-ignition through the heat created by compression, and so the remainder of the mixture explodes instead of burns, before the piston reaches TDC. The optimum is to raise the temperature through compression, both from the piston rising and the burn, just short of self ignition. That way more of the burn occurs just as the piston goes past TDC and is higher in the cylinder. If ignition occurs too late then more of the burn occurs when the piston is further down the cylinder, so pushes on the piston less, and the energy is converted to heat instead of forward motion.

There is a way of tuning by 'ear' (actually by eye) by twisting the distributor and that is with a vacuum gauge. Advance it until you get small flicks of the needle, which you might just be able to hear, then back it off by 3/4 in. Hg. That takes account of both your grade of fuel and many other aspects of your engine and distributor. All engines are slightly different when new (and a lot more different 30 years later) because of component tolerances, and when the manufacturer quotes a timing figure it takes a worst case into account and adds a safety factor. So many engines are able to run safely with more advance, and that is what the vacuum gauge allows you to do, giving you better performance and economy. I tuned my BL engine that way in the 70s and really noticed the difference. When under warranty it came back from dealer servicing running like a dog by comparison, until I reset the timing with the gauge. However my MGB came out at just about the book figure, so I didn't bother with the vacuum gauge anymore. That was on leaded, it has to be retarded slightly on unleaded, even with 97, 98 and 99 octane.
PaulH Solihull

Something cant be right here, 30 static degrees would guarantee pinking on unleaded. Have you checked with dowel in the spark plug hole the piston is at TDC when the timing mark says it is?
Is the vacumn advance working?
Stan Best

Stan,
It came out as 30 degrees dynamic (using my timing light with advance dial). Vacuum disconnected. And yes I thought it would pink at that setting. I even suspect my timing light!. I've checked the TDC with a dowel and that's correct (to within a degree or two). The vacuum advance works as I checked that by sucking on the end of the pipe and watching the timing mark under the strobe.

I've set it back to 14 degrees and may try advancing it in steps until it pinks. I do have a vacuum guage attached but that is a blur at idle speeds. I posted on here a while ago about that. I will try putting a small air resevoir in the line to damp out some of this. However I fear it will also damp out Paul's 'little flicks'.
Steve Church

If you've got a dial-back light, set it for about 32-34 all in (i.e. >3500 rpm, vac disconnected) and see where you fall at idle. That'll be your best setting assuming the distributor is in good nick.
Rob Edwards

You need to restrict the pipe till the vac gauge is fairly steady otherwise the flickering will be too much to give any meaningful info.

What timing light are you using to tell you 14 degrees or 30?

The most important thing is dynamic timing, what does the advance go to at say 2000, 3000 and 4000 rpm? If the advance curve is malfunctioning maybe you have 14 everywhere whereas 30 everywhere may have been better. Check with no vac.

You have to state accurate measurements to get help online as all is guesswork unless we are playing on a level playing field. Even then it is impossible to tell you what curve you need. Timing variation of a couple of degrees can make or break the performance of an engine. Your max advance requirement will fall, roughly, into a band of 25-40 degrees depending on your specific engine needs. Anyone that tells you exactly what it should be is talking rubbish.

Peter
Peter Burgess Tuning

Thanks for your input Peter. I'm using a gunson unit with advance feature..one of these: (http://www.gunson.co.uk/item.aspx?cat=662&item=3442)

I'm not sure how you check if the timing light advance function is working correctly (it is plainly changing but whether it agrees with what the dial says is another matter). I did have it all apart on my workbench connected up to a signal generator and 'scope but that was inconclusive without a circuit diagram.

The advance on the dizzy works as I stated earlier that it maxed out at 28 degrees when I set it back to 14 degrees at idle. I daren't do the same at the mechanic set 30 degrees. Figures I did earlier this year are:
800 rpm 14 deg
2000 rpm 20 deg
3000 rpm 26 deg
4000 rpm 28 deg

On an earlier thread it was decided I had the wrong dizzy for the car (41155) so I will be getting a new dizzy shortly (hopefully a 123tune when I can find the pennies!). Then up to Derbyshire next spring to get it all sorted on Peter's RR.
Steve Church

Steve, no rant today :0)
As mentioned previously, I would still run a little more advance at all speeds with that dizzy. Move the 800rpm out to 18 deg which should give you 32 deg at 4000rpm and see how it runs. You might even get away with 34deg at 4000 (or 20 at 800) any more than this and starting from cold might become an issue.
Alternatively, don't run it at all until you save the pennies ;0)
MGmike
M McAndrew

This thread was discussed between 01/11/2012 and 08/11/2012

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