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MG MGB Technical - Timing Options

Yes, I'm back, still trying to better understand and solve the ole' timing & gas mileage question.

In 2009 and before I averaged 15-19 mpg (72 MGB) and it smelled rich. In 2010, I realized my vacuum advance wasn't working. As a compromise/temporary fix I simply set my timing from 10 BTDC to 20 BTDC at 800 rpms. My mileage immediately went to 22-24 mpg, and rich smell disappeared. Car ran fine.

During winter of 2010-2011 I delivered my distributor to Jeff @ AdvanceDistriburos. I received back a distributor that was so polished it looked like a work of art.

I installed distributor and set timing at 8-10 BTDC without vacuum connected at 800 rpm. I am getting vacuum from a port on rear carb, not intake manifold. I have SU HF4 Carbs AUD 265F.

The car has never run so smooth, and it starts instantly, only requiring the choke for a few moments. No rich smell from exhaust, and exhaust sound & air flow is much smoother too.

However, my gas mileage went back to 16-18mpg. I'm running full synthetic oil now, and replaced air filters.

Last night I changed timing to 20 BTDC at 800 rpms. Time will tell if this helps.

I also checked advance last night. With vacuum attached it appears to peak around 35 BTDC at 4000 rpms.

I have no vacuum at 800 rpm, thus no advance at idle. I have 5" at 1800 rpm, 12" at 3000 rpm, and 16" at 3600 rpm. These readings are from vacuum port on rear carb, not intake manifold, where vacuum would be a steady 16+.

I am again baffled that it runs so smooth, no rich smell, starts and idles much better, and drives fine; but mileage has dropped back to teens.

Your thoughts. And thanks for your patience on this topic.

R.W Anderson

RW,

Have you verified that the marks on your crank pulley / timing cover are correct? It could be that you either have some mismatched parts or that your harmonic balancer has slipped.

I think you have a typo in your carb identification. Do you have HS4 or HIF4? The HS has the separate float bowl bolted on and the HIF has an integral float bowl.

Not that it helps me figure out anything, but I'm not convinced that your intake manifold vacuum would be constant.

Charley
C R Huff

TDC on timing mark appears to coincide with TDC on corresponding pistons, rotor, etc., whenever need has been to align things.

Typo on the Carbs, SU HS4 - external floats.

Clarification on manifold vacuum, I should have wrote steady 16" at idle.
R.W Anderson

Well, I don't know what was done to the distributor, but your timing figures are all wrong.

All figures with vacuum disconnected.
OE dist 41370 (1972) is:
Static 10BTDC
16BTDC @ 1500

Dist mech advance (crank degrees) is:
20 @ 1700-2000
35 @ 3300-3900
41 +/-2 @4800

Vac advance is:
start 7" hg
End 13"hg
Total crank deg 6 +/- 2

Since your engine has been mucked with, it is hard to say what it needs. The engine basically wants the above probably. Since it has the incorrect carbs, things may be somewhat different. But the OE dist calls for manifold vac, and carb vac is very different.

Might want to call Jeff to find out what he says, or to clear up some misunderstanding of the instructions.

FRM
FR Millmore

We have been over carb v manifold vacumn several times. Once you are away from idle there is hardly any difference. The change was made for emissions reasons. With US gallons an MGB should easily achieve 25MPG. You seem to have done most sensible things. A rolling road session may sort things out.
Stan Best

I'm running the 25 model distributor.

Not quite sure what is meant by "my timing figures are all wrong".

I had it originally set to 10 BTDC, which matches your list above. And it appears I'm around 35 BTDC at 4000 rpm. But it is hard to estimate the timing once it is past the timing mark gauge.

I could go back in and see what the timing is at the various rpms you list to see if they are within range you list.

Interesting that you suggest I should be getting vacuum from manifold not carb. I've been told specifically to not take it from the manifold. Is that the case even if as you suggest, I have the wrong carbs too.

But I've been wrong before, especially when trying to remember what it is I thought I may have heard someone say.

I'm also checking with Jeff.

Thanks for input. I'll use it as a guide tonight.

As to what was done to distributor, it was a total rebuild.
R.W Anderson

P.S.

Most of the emission control stuff for US was removed well before I acquired car. All that is left is the carbon canister plumbed to the gas tank, valve cover and intake manifold, as originally laid out.
R.W Anderson

You can try doing what I did to many cars in the '70s. Take it out for a drive and keep advancing the timing until you start getting pinging. Back off the timing a bit and lock down the distributor. This is what I had to do with my supercharged engine to get just the right amount of advance while returning good fuel mileage as well as performance. With the supercharger, I average around 28 mpg. RAY
rjm RAY

I tried a variation of that last evening.

While holding the rpms at 4000 I slowly turned the distributor clockwise and counter clockwise to see where I got the most rpms. With vacuum attached.

I then checked the timing and I was right back to the 35+ BTDC, that I had when simply setting the timing at idle at 10 BTDC.

I'm presently awaiting word from AdvancedDistributors to see how much I'm mucking up his good work.

I could be like others, and simply give up on getting good gas mileage. But mpg in the teens suck, and don't make sense with the running so well.
R.W Anderson

Bob-
US cars 72-on have low compression in addition to all the visible crap, as well as leaner idle/low speed mixtures. This generally requires more advance, especially at light throttle conditions - idle and cruise, just what affects fuel mileage most (other than a heavy foot). Bitsa engines may be all over the place, and our pals on the other side have not been afflicted by this compression issue

"I had it originally set to 10 BTDC, which matches your list above."
"set timing at 8-10 BTDC without vacuum connected at 800 rpm" Your statements.
"Static 10BTDC; 16BTDC @ 1500" My quote from the WSM
"Static" means engine stopped, not running. Once it is running, the mech advance is coming in at some rate, and there are no published figures for what it is at 800 for this distributor. Typically it is around 3 but possibly as much as 10deg between 600-1000rpm plus the static setting. So, setting at "8-10 @ 800" is not the same as 10deg static.
The 16@1500 figure lets you check this, BUT I've found that this range of speed is where the mech advance is changing rapidly, so if you are not reading rpm with an accurate tach, you may be off significantly. The in car tach is not a precision device!

"around 35 BTDC at 4000 rpm"
My quoted figures are for distributor advance only. You need to add the static to this to get the actual timing number. So "35 @ 3300-3900" + 10 means you would be around 45-46 @ 4000, without vac. You can measure 10 deg off the pointers and make marks on the damper to extend your scale.

The manifold/carb vac issue depends primarily on compression, and dist advance curves and vac capsule. Cam timing can be an issue, but all cams until cat equipped cars were the same. Cam wear and timing chain wear may be an issue, and both would likely require more advance at low throttle conditions, favouring the manifold vac system. I have worked on all possible combinations of dist/carbs/heads, and find that some respond best to one vac source, but the next car might be reversed. I rarely know what is inside the engine, and sometimes it is bizarre. I do find that the vac capsule/source can seriously change throttle response just off idle, had one car that would stall as you opened the throttle with the vac unit it had; it was much worse with carb vac, better with manifold, but ran like a scared bunny when I tried an Austin America vac unit connected to manifold.

I think 16" vac @ 800 is low. This can be retarded spark or retarded valve timing/worn cam.

All of this depends on what the "total rebuild" of the distributor included. I know Jeff adjusts things for customer engines; if you gave him the right & complete info, then he may have modified curves and vac capsule to work with what you have, if you follow HIS installation instructions. I repeat: "HIS instructions" - any body else knows less than you do. I can only give you published figures and general principles.

Best, and keep us informed.

FRM


FR Millmore

It's easy to get in a one horse race with the dizzy and timing when there is a whole host of things which can affect fuel consumption. 16 to 18 us gals is pretty poor, I very seldom see less than 30 imp gals per mile. Your dissy mechanical advance should give 10deg max, (check the figure stamped on the advance mechanism, under the top plate), that's 20deg at crank. Add that to the static/idle setting to give max advance.
AUD 265 were fitted to '68 cars with FX needles.
The carbs are the most likely cause,wrong/worn needles, worn jets,incorrect mixture, weak and unbalanced springs, inefectual damping, blocked filters, inefective heat shield, to name just some things to look at.
Also check; tyre pressures, tyres rubbing body work, tracking and all wheel alignment, binding brakes, leaking tank/pipes/unions.
Is this a high compression engine? Low compression commercial lumps have often been transplanted when the original one has expired.
Oh and don't use synthetic oil. These modern "super" oils don't do these old engines any favours at all. Stick to good old 20w50,specifically marketed for classic cars.
Allan Reeling

Andy, to read the timing past the meager locations offered by the timing marks, simply add another timing mark PAST the current mark on the crank pully, and read it when you are past the existing timing marks, adding the new value to the first. For example, if you can read 20 degrees of advance on your marks, add one more mark on the pulley 12 degrees past the factory mark. When that mark is lined up with 20 degrees, you are actually seeing 32 degrees. Note that your original pulley mark will be past the timing marks.

warmly,
dave
Dave Braun

Apart from at idle, and just coming off it, manifold and carb vacuum sources give identical vacuum and hence advance for all usable throttle openings when under way. Hence you would definitely *not* get a constant 16 in Hg from manifold vacuum - there would be absolutely no point in having manifold vacuum if that were the case, apart from the physical impossibility. Have you checked the advance at those vacuum readings?

From 1971 American engines were low compression anyway, not that that would explain why the mpg changed so significantly with a distributor change. If you set the static/dynamic timing to the same figure for both distributors, and didn't change anything else, then it can only be timing differences through the rev and throttle range. Lack of vacuum advance will cause poor cruising economy.
PaulH Solihull

Okay to recap all this...

2009 - 15-18 mpg w/old distributor @ 10 BTDC dynamic at idle.

2010 - 22-25 mpg w/old distributor @ 20 BTDC dynamic at idle.

2011 - 15-18 mpg w/rebuilt distributor at 10 BTDC dynamic at idle.

2011 - ???? mpg w/rebuilt distributor at 20 BTDC dynamic at idle. I need to run a few tanks through to check this.

All timing set with vacuum hose off dizzy and plugged. Although this didn't matter pre-2011, as the vacuum advance did not respond to vacuum, now with new modual it does.

Car runs amazingly smoother with rebuilt distributor.

Vacuum for dizzy is coming from rear carb port, 0 vacuum at idle. Vacuum reading from carb port increases with rpm, while parked - no load, to the 16" the manifold port has at idle. I have not checked what advance is for each variation in rpm.

If I change vacuum source to manifold, there will be apx 16" of vacuum advancing the timing, I have not documented how much advance.

I'm not entertaining changing out the carbs to something that was original for 72, as I got better mileage with the old distributor set to 20 BTDC dynamic. I do acknowledge my carbs could use a rebuild, but still I got better mileage last year with old dizzy at 20 BTDC. And the car runs so smoothly with rebuilt dizzy.

I may be missing something, but I do not read from above that I should change vaccum source to manifold instead of rear carb?

I checked the timing once on my 2000 Honda, just to make sure it was right, 140k later it is still dead on.

This sure is a lot of fussing for a car I only get to use for 2000 miles per year. Maybe I'm not destined to be a real "enthusiast", I expect things to sort out, and stay sorted out, not keep getting more complicated.

But maybe I'm just getting too frustrated over the mileage issue, and should just drive.



R.W Anderson

Having just read the posting under "carbs" I see that my 25D distributor typically has its vacuum coming off the rear carb. This posting also suggests that the HS4 were matched up the 25D distributor. Not sure if my HS4/AUD 265 was the HS4 in mind, as one posting above suggests the AUD 265 was original on the 68 model, not the 72.

So I'll leave vacuum from port on rear carb.

I guess I've opened up a new can of worms in figuring out what the difference is between the various HS4 carbs, assuming my AUD 265 even has the proper parts in it.

I'm still leaning towards, simply driving and not looking at gas mileage.
R.W Anderson

More advance will give better performance and economy, as long as it is short of pinking or stalling the starter. Take any engine and rotate the distributor clockwise t idle, and the idle speed will go up - same fuel going in per stroke, bigger push on the pistons from the burn. It's because more of the burn occurs coming off the top of the stroke and less is wasted as heat further down. However you can go too far of course - into the pinking/stalling zone.
PaulH Solihull

I set mine the way Ray mentioned. Kept advancing and going for test drives until it pinked then dialled it back a little. The car goes well but I am not sure of my mileage but to be honest I don't drive it in what could be called and economic manner! If you are driving it more aggressively now than 2 years ago it might make a difference?

Paul, what do you mean by stalling the starter? Mine sometimes seems a little hard to turn over. It's as if the starter loses power. Doesn't do it all the time and usually the next try it fires straight up. Never have worked out what causes that. I suspected my battery voltage was dropping on cranking but that's not the case.

Simon
Simon Jansen

I just got my 25D distributor back from Jeff at Advanced Distributors and can speak first hand to the performance difference.

On Jeff's recommendation I set the timing high (he recommends 12 to 14 degrees BTC at idle and vac line detached but I set it to 15). My mileage on the first tank after the change over went from 20 to 25.

Jeff explained that the 73 cars were specified at 11 degrees before TDC at 2500 rpm in order to meet US emissions requirements. When he rebuilds a distributor he uses springs that alter the advance curve for the car to more performance related standards. With the emissions components removed from my car that makes it more in line with pre-emissions vehicles. This would be why your timing is showing differently from factory specs.

I have HIF4 carbs and my vac line is off the manifold.

Correct me if I'm wrong but doesn't the 25D distributor have both mechanical and vacuum advance? The mechanical kicks in at low rpm and the vac takes over when the rpm's get higher. That is why I'm setting the timing at 800 to 1000 rpm with the vac removed.....I assume with a some mechanical advance taking place.

BH Davis

BH Davis

The mechanical advance works throughout the rev range. The 2 sets of springs and weights give a "break point" in the advance re RPM as a rough fit to the ideal curve. The vacumn advances and retards according to throttle input. I always set my car to 10 deg BTDC static using the set up on the crank (18GB engine) then as above advance it as far as possible from there. The fuel has less time to burn as RPM go up, hence the advance.
Stan Best

Simon - it's something I've heard of with low compression engines. You can keep advancing the timing but it never pinks because of the LC, but if the spark occurs too far before TDC the burn will try and push the piston back down the compression stroke against the force of the starter.

25D and 45D distributors come in both vacuum and non-vacuum versions. The non-vacuum are for competition use where cruising economy is irrelevant, and it makes it much easier to determine and achieve the ideal curve for a particular engine on a rolling road. Sadly far too many people are sucked in by the competition or 'performance' label and think putting one on their bog-standard MGB is going to transform it. It doesn't (unless the old distributor is knackered and/or the wrong one of course).

Centrifugal advance progressively increases the advance (between lower and upper limits) through the rev range. In combustion the mixture doesn't explode but burns at a fixed rate, a flame front moving across the combustion chamber. As the mixture burns the combustion chamber pressure increases, which is what pushes the piston down on the expansion stroke. However if too much pressure develops too soon i.e. if the piston is still too high, you get pinking or worse detonation. This is when the pressure gets so high that a spontaneous explosion of the rest of the mixture occurs, like in a Diesel engine, you get extremely high combustion chamber pressures, which can destroy pistons and bottom ends. Because the mixture burns at a constant rate, as the revs go up the piston passes through TDC sooner, so ignition can occur sooner without causing pinking or detonation. If it didn't advance, more and more of the burn would occur too far down the expansion stroke to be of any use, generating excess heat rather than forward motion.

Vacuum advance performs a similar function but based on throttle opening. The bigger the throttle opening the more mixture is pulled into the combustion chamber so the greater the pressure rise when the burn takes place so again the greater the chance of pinking. But on part throttle the pressures are lower, so less chance of pinking, so the timing can be advanced more than at full throttle. Again this allows more of the burn to occur with the piston nearer the top of the cylinder, so converting more energy into forward motion and less lost as heat. Effectively you can cruise at a given speed with a smaller throttle opening, so saving fuel i.e. greater economy. Vacuum advance is applied through the throttle range, again between upper and lower limits, and benefits part-throttle acceleration as well as steady state cruising.

The two advance mechanisms are continually varying independently as revs and throttle position change, both adding to the amount of static advance. MGB distributors can give anything from 15 to 41 degrees centrifugal advance, and 6 to 24 degrees of vacuum advance. It is possible to get anything from 31 to 50 degrees total advance depending on the distributor under high revs and a light or trailing (depending on whether the vacuum source is carb or manifold) throttle.
PaulH Solihull

Just as a side question, and with high regards and respect...

With all of this knowledge and understanding of MGs and car engineering in general seeming to flow forth so smoothly and clearly, I wonder if those of you that appear as wizards to all these issues, are still able to retain enough common knowledge and understanding to enable tying of one's shoes or have some had to resort to loafers!

Apparently I'm taking up too much brain space with environmental and contract law issues to squeeze some of this MG wisdom in! Maybe when I retire, I can squeeze out the budgets, contracts, environmental knowledge and make way for MG knowledge.

Thanks again for all the insight.
R.W Anderson

The low comoression cannot achieve the same economy as the HC one due to thermodynamics. The model of the ideal heat engine doesnt read directly across to an ic engine but does give some guidance.
http://en.wikipedia.org/wiki/Heat_engine
The key point is that efficincy is a function of the difference in temperature between the heat source and sink. Temperature is controlled by compression ratio at the high end. So maybe if a free breathing gas flowed HC MGB can achieve 40 mp imperial g maybe an LC one will only ever return 20 per US g?
This is why variable dispacement works (eg the 300C Chrysler) which runs as a 4 cylinder at low power demands.
Stan Best

Excellent description from Paul - I suggest everyone copy and keep in a safe place!
A niggle - the non vac advance versions of 25D & 45D respectively are 23D & 43D.

Stan - LC engines do suffer a drop in both power and economy, but it is not huge, maybe 10% at most in the range we are dealing with here, 8.0-9.0 CR.
These losses are really a matter of effective CR, which is greatly reduced by bad breathing, increasingly at high rpm. Both lean mixture and low effective CR require more advance. US cars have very lean mix at idle, which is why they need and tolerate so much advance at low speed/light throttle, hence the manifold vac advance take off. The single ZS cars also have horrid breathing, so they need a lot of advance at high speed too, but they still run out of breath. It is a characteristic of IC engines that at open throttle loaded conditions, there will be an optimum advance at the point of maximum torque, which is at the most efficient volumetric efficiency. For a given engine, regardless of tuning variables, this point is at about the same speed, so this advance is primarily rpm dependent. But both the low speed/light throttle requirements and the high speed requirements vary a lot, hence all the advance curve fussiness. You can change the amount of mechanical advance versus engine speed, but once it is added you can't make it go away without reducing speed; and, you can use vacuum as a control, but vac diminishes rapidly with throttle opening. This turns out to have been a sufficient control until emissions became an issue, but then it got complicated. Some cars had both vacuum advance and retard, switched under various conditions. Possibly the greatest point of modern electronics is that you can have an advance table with various dips and bumps to accommodate all this.

While Bob's HS4/FX needles were the first emissions ones, they are not nearly as lean as the HIF ones, so his distributor needs to be setup more like the earlier cars.

Bob - Haven't worn lace-up shoes in the last 50 years!

FRM
FR Millmore

Paul,

Thank you for the very thorough explanation. It is right on topic with a concern I've been having.

Since setting my timing as described above I've been wondering if I'm putting undue stress on my over 35 year old never rebuilt 18v engine.

Even though my compression is in the 120 to 130 range the engine has plenty of power for the way I drive.

Do you think having advanced the timing to 15 degrees BTC at 800 to 1000 rpm will harm my engine?

Thanks,
BH Davis
BH Davis

Yes of course, 23D and 43D.

Oh boy, I hope my wife never sees the comment about no longer being able to tie one's shoe-laces, when mine come undone my granddaughter has said more than once that I should be using double-knots. Not quite there, but fast approaching. I don't mind that, nor leaving my zip undone, but I am worried about beginning to forget MG stuff ...

Whether 15 at 800-1000 will harm an LC or not is tricky. It's easy with an HC - advance it just short of pinking at any combination of throttle opening, engine revs and load. It's 3 or 4 degrees more than originally, but most engines should be able to cope with that. If you don't get pinking or a tendency to stall the starter when cranking, then it *should* be OK. It probably depends more on whether you use the accelerator pedal as an on/off switch or not i.e. full throttle or none, or drive it with a bit more sensitivity.
PaulH Solihull

I vote for trial-and-error ignition timing. The facts and figures should only be a starting point. The recommended settings may require adjustment due to the amount of carbon buildup on the pistons, the state of fuel delivery and type of fuel, whether the engine has been bored out or the head reworked, the type of exhaust system, state of valve adjustment, even driving habits.

Gas mileage is affected by all of these factors, too, and it varies from one car to another. I wouldn't get hung up on the timing as a solution to poor gas mileage.

One thing often overlooked when sorting mileage problems is the state of the spark plug wires and connections. I got an increase of 4-5 miles per gallon simply by replacing an old side-wire distributor cap and leads with a top-wire cap and wires with molded-on connectors. I'll grant that the original cap had a badly-worn center brush. But I was also tired of the spark plug wires getting pulled out of the screw-on sockets whenever I was working on that side of the engine.

As for knowledge filling the brain, I say keep forcing it in until you get a headache. I suppose that's the trial-and-error thing again. -G.
Glenn G

"I wouldn't get hung up on the timing as a solution to poor gas mileage." I would - potentially. Retarded timing will give rise to significantly poorer economy as well as poorer performance and higher running temps. This can be from non-functioning centrifugal or vacuum advance mechanisms as well as simply not enough advance for the engine and fuel. Of course there can also be many mechanical and carb issues causing the same thing.
PaulH Solihull

This thread was discussed between 09/06/2011 and 19/06/2011

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