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MG MGB Technical - Timing

It's been a long, long time since I have looked at timing on any car. Nearly 20 years in fact.

I've decided that may be what's needed on my '70 BGT as it has some symptoms of the timing not being "right".
It could also be the valves need adjustment, but I'd rather check timing first.

This has a stock 1800 that was rebuilt, de-smogged, and seems pretty strong after a "wake up" from a 15 year nap in the PO's garage.

I'm pretty happy with the power I get out of the motor, but the valve train produces a bit of racket (not real bad, but audible) and the fuel economy is less than what I want after a full tune up.

When I look at the timing, where should it be? What's the 'sweet spot' generally on these motors?

Other thoughts about increasing economy while retaining power?
Rick Bastedo

I recently asked Peter Burgess for his recommendation on this. He suggested 28 degrees at 3000 rpm, without the vac advance attached.....this is what I'll be aiming for when I set my timing
S Longstaff

I was actually going to start a thread to update the board on how my ported advance to non-ported advance conversion turned out.
To compare my results I noted the rpm at idle using my dwell meter and it was right at 700 rpm. I didn't bother with a timing light because my setting was way past the factory indents on the balancer (advanced).

I then used the fitting that was originally for the PCV valve. I had to order a special plastic elbow that goes from 9/16" to 1/4". I know that the hose is 5/32" , but it fit with some effort. I might pick up another reducer to go from 1/4" to 5/32" in the future.

What I did then was loosen the distributor hold down, start the engine, and then adjust the idle back down to where it was before (700 rpm). I test drove it up a hill and there were no rattles from the engine at all. But I had to test what the new real limit was in order to see if how much advance was gained at idle. I bumped the static timing until the engine started to rattle again (on the hill) and then I backed off a tad.
Well, it turns out that I gained a little over 100 rpms. Its now idling between 800 to 900 (closer to 900) rpms.

So it worked! I might turn the idle screw on the carbs down in the future just a tad to take advantage of the additional vacuum advance, but I'm happy with the results. No rattles and I gained some advance where I need it.
BurgerCokeFries

BurgerCokeFries

What additional vacuum? It is my understanding from the recent threads on this BBS that the basic difference between the vacuum from the ported (carb) and the manifold is that, at idle, the manifold vacuum is higher, and at higher revs they are essentially the same. Also, the use of manifold vacuum was initiated to meet US smog requirements at idle.

Larry
72BGT
Larry Hallanger

Someone here has a web site (I don't remember who) who tested this with a vacuume gauge and what it showed was that the vacuume is higher at idle but its also a little higher through the midrange than the ported test. The top end is pretty close to be the same (non-existent).

I can now turn my carb idle screw down if I want to save gas, and the car runs just as good if not better as it did before - and keep in mind I had my static timing way up beyond the factory indicators before.
BurgerCokeFries

that midrange vacuume reading was just a tad higher than the ported reading in that test.
BurgerCokeFries

I'm going to head back to Rick's original question.

I'm not an expert, but I've found a few things that seem to work for me. First of all, compression is next to godliness on these engines. Please warm up the engine and do a compression test. The values should be within 10% of each other, and on most gauges, about 150 to 170 psi. Secondly, the valves could very well be out of whack. The B series engine loves to breath, and if the valve timing is off due to adjustment, setting other things won't help much. Once you have these two things nailed down, (and since you say you've done a 'tune-up', I won't advise you to change sparking plugs, wire, cap and rotor, and the condenser if you have one, a lot of these cars have an aftermarket electronic ignition) then you can check the timing. I completely concur with the Peter Burgess recommendation which S Longstaff offered. Have fun, your timing marks are probably on the bottom side of the engine, adjacent to the harmonic balancer. You will be laying on your back, looking upward. Get some help for this.

Then, and only then run your car for a tank and see what kind of fuel milage you are getting. Chances are your carbs are set correctly (they hardly ever need attention) but if they leak around the throttle shafts they could be set rich to compensate. The original aircleaner set with the radius inlets are ideal for our engines, get a set and put them on if they are missing.

good luck,
dave
Dave Braun

Yes, I've replaced all that normal tune up stuff (plugs etc) and my inlet setup is shown in the image.
(Shown with one filter off to illustrate)

Let me clarify: I don't have any knocking - just a bit of noise. Most likely just worrying too much. Mainly I'd like to see an improvement in the fuel economy, It's in the low 20's (mpg) now & it would be nice to get it to the mid to upper 20's.


Rick Bastedo

I found the web site and here are some quotations...

"...a steady 20mph on the flat in 3rd gear. Even with the very small throttle opening carb vacuum is already at 10 in. Hg. with manifold at 19. Incidentally this manifold reading is higher than at idle as the engine is operating more efficiently"

"The next pair are at about 25mph on the flat in 3rd gear now the throttle has opened a bit more carb vacuum has risen to about 13 but manifold has fallen to 17"

"The final pair are at about 30mph and carb and manifold vacuum are virtually identical at about 14 in. Hg"

http://www.mgb-stuff.org.uk/curvesframe.htm


No doubt the results would have been different if the readings were taken in a different gear (speed to rpm relationship), but as you can see 9 to 4 inches of vacuum is nothing to sneeze at. It makes perfect sense that the increased non-ported vacuum will just taper off with a stronger signal than the ported.

BurgerCokeFries

Rick, on the 70 MGB I care for, we routinely expect 25 mpg plus on high speed (70 mph) trips. When we started our process, we were exactly where you are. i.e. we were in the high teens to very low 20s mpg, and while the car was fun to drive, it lacked oomph. We didn't have 'knocking' we had valve train noise, which on these cars is very difficult to eliminate entirely. Our aircleaners were Wieglands without the radiused inlets.

We basically did the steps I outlined, the increase in power once we got the valves correct was amazing. You could tell the car was breathing better. We still had leaky throttle shafts, and after setting the timing (at that point we used 14 BTDC at idle with the vacuum line plugged) we didn't notice a lot of improvement. We rebuilt the carbs, adjusted the mixtures by driving and making minor adjustments and retesting over a wide throttle range; and added the original aircleaners. Since then, we've driven about 20,000 miles. We are running with the air pump removed and the rail openings in the head plugged.

I can't tell from your picture if your carb inlets are radiused or not.

dave
Dave Braun

Valve clearances must be checked before adjusting timing. Likewise the condition of the plugs, points, HT leads, cap and rotor must be good, and the points dwell/gap correct. The centrifugal and vacuum advance mechanisms must also be working and in good condition. *Then* you can set the timing!

Basically with today's fuels the highest advance you can run without pinking at any combination of revs, throttle opening and load is best. However on low compression engines you may not get audible pinking, and so the original figure is as good as any. On American spec engines from 1968 the prime requirement changed more and more from performance to low emissions, and performance and economy suffered as a result. The easiest way to get maximum performance and economy is to fit a UK engine and distributor, from any year! Otherwise you are down to modding your engine to get closer to that, or fitting modified heads, carbs, exhaust, supercharger in various combinations.

My conclusions on the difference between carb vacuum and manifold vacuum were that apart from at idle tiny throttle openings and very low speeds, the two were the same, i.e. all normal driving for all intents and purposes.
Paul Hunt 2

I have noticed that without the manifold vacuum advance tube connected, my car requires the idle screws turned in quite a bit further, and the idle is nowhere near as smooth, as compared to when the tube is connected. From what I understand, ported vacuum is virtually non-existent at idle. This leads me to believe that manifold vacuum advance improves efficiency (lower idle setting) and overall makes the car run smoother.

If the car makes a light tapping noise at all RPMs, and at light, medium and wide-open throttle openings, then I doubt it is a timing issue, but more likely valves that are either too loose, or there is wear in the engine. I concur with the others, that accurate valve adjustment can make a large difference in performance in a B engine - my seat-of-the-pants dyno proves it.
Remember that B engines always make some valve noise (at least if in relatively stock form) - if they don't, the valves will be likely to burn and lead to loss of compression.
Erick Vesterback

Yes Dave those are the original radius type inlets that came with the car. I counter sunk the bolt holes and fitted with hex head fasteners to get the bolts out of the air path. Then the cone K&N's fit to the outside of the inlets with clamps. I feel I'm getting the max amount of air possible.

I did adjust the carbs a bit, turning them up about 6 flats (one turn) - finding out that was too lean then turning them back down about 3 flats each.

When pushing up the pin they speed up slightly then drop back down but don't stumble.

The engine had been professionally rebuilt some time before I bought it, for some reason the PO had kept it garaged for about 15 years without driving it much.
It ran but obviously needed a tune up and fluid change out. I did that, it runs pretty good now. Idles right about 950 consistently after a few minutes of warm up.

One thing it does that bothers me and might help in diagnosis:

When I coast downhill the rpm will slowly drop down until (if I let it) the engine will die. It idles fine for about 15 - 20 seconds, then starts dropping and within about 30 - 45 seconds it dies. If I coast to a flat area before it dies it will idle again, so this has something to do with the car pointing downhill. I've tried it with the tank full and the tank half full and nearly empty with the same results.
Rick Bastedo

It sounds to me that your float levels are too high or that they are hanging up due to fuel contamination. Ray
RAY

Manifold vacuum give near maximum advance at idle, whereas carb vacuum gives none. The greater vacuum and hence advance at idle *does* result in a higher idle speed for a given throttle opening, which is why the idle screws can be reduced with manifold vacuum conencted and have to be increased when manifold vacuum is removed i.e. for adjusting the timing.

The 'efficiency' gains only occur at idle, and are emissions related, as a car with manifold vacuum will idle with a smaller throttle opening than one with carb vacuum, and hence emit less emissions. Off-idle they are exactly the same. You may wonder why vacuum wasn't always connected this way, and the reason is that high idle vacuum and advance means higher cranking vacuum and advance, which makes for more difficult starting.

The irony is that with all the emissions rubbish that American-spec cars got lumbered with, and the running problems that some of them caused, they ended up with vacuum advance in 4th gear only (Transmission Controlled Spark Advance or TCSA), and so the idle emissions came back to exactly the same as carb vacuum cars!
Paul Hunt 2

"It sounds to me that your float levels are too high or that they are hanging up due to fuel contamination. Ray"

Thanks Ray, I wouldn't think I have fuel contamination after running through a dozen or so tanks of gas.
I can take the tops off and see if theres anything bad looking in the bowls.

How would I check for & fix "float levels are too high"?
Rick Bastedo

I dont believe that. If their were no midrange vacuume (tapering off) I would have noticed the difference there. Thats why I mentioned the speed to rpm relationship, it really comes down to load.
BurgerCokeFries

Well I know my vacuum advance works now.
I removed the hose from just behind the rear carb and stuck it on the manifold. RPM sped up significantly.
Should it be connected close to the carb, or to the manifold? I assume right behind the carb is correct.

I pulled the tops and the rear bowl had significant amounts of very fine rust at the bottom. I cleaned it all out and reassembled, but with no change to how it ran.

Ideas?
Rick Bastedo

I chose to use the existing pcv valve fitting and I like that its dead center of the manifold, because the vacuumed might be a more steady signal. I dont know for sure if it makes a discernable difference, but I wouldn't doubt it.

I laid it all out and the facts don't lie. The physics of it is pretty plain to me.
BurgerCokeFries

Rick - if your rear carb has a port then I'd use it. If not I'd use a manifold port. It is desirable to use either as it improves cruising economy and mid-range acceleration. Of the two carb vacuum is preferable.

With the throttle butterfly open just a few degrees the carb port is only an inch or so away from the manifold port, which is why the two give the same signal from that point (of throttle opening) on.
Paul Hunt 2

Thanks for putting the cookies on the bottom shelf for me Paul. That is an explanation I understood!
Rick Bastedo

Rick, With the float chamber lids removed invert the lids and check how much clearance you have between the lid and the top of the float. It should be 1/8 inch. You can easily check this by inserting a 1/8 inch drill bit between the two. I would remove the float pivot pins and check the floats for leaks and the needle valves for any wear which would prevent them from seating fully. This would also be a good time to replace the in line fuel filter since those rust particles came through the old filter. Ray
RAY

Rick, the test for leaky throttle shafts is to spray some carb cleaner in the vacinity of the shafts and see if the additional combustables in the carb cleaner temporarily enrichens the engine. When your engine was rebuilt 15 years ago, chances are they rebuilt the carburetors, and with little use, the shafts are fine right now. But you want to find out.

Another thing that comes to mind is that your engine may not be bedded in as well as it could. Good hard driving will seat your rings and improve everything.

BurgerCokeFries, I had a hard time following your comments, much less Ricks, because of your thread jacking. In the future, starting your own thread would be helpful to us older guys with limited ability to multitask. Thanks in advance!

warm regards,
dave
Dave Braun

Thanks Dave,

"Good hard driving will seat your rings and improve everything"

Yes, that was my feeling as well so I have been doing that. I've seen a lot of improvement since I first got the car, but I feel there's still a little room for more.
When it warms up a bit here I will adjust the valves.
We will see then just what else it needs.

I did spray with carb cleaner and didn't find any problems. The carbs seem to be running just fine. I will put an inline fuel filter in there, it obviously needs that.
Rick Bastedo

Here is a twist, I am sure the answer to this question is pretty simple:

I have forgotten just what exactly the little adjuster on the distributor below the vac advance unit does. Or rather - I believe it advances the timing a bit, or retards it a bit. How does it work, what is the desired function and how or why would you turn it?

I turned it and one way the idle speeds up and the other way it slows down. I adjusted so it sped up from an idle of 900 to about 1200 RPM and the annoying problem of the car wanting to die while coasting downhill completely went away. What does this mean?
Rick Bastedo

The factory designed the adjuster on the distributor to allow the owner to make adjustments to the timing due to variations in fuel octane levels. By adjusting the knob so that the idle has increased by 300 R.P.M you have advanced the timing. This is accomplished by the point plate moving in relationship to the amount the knob has been rotated. Ray
RAY

Thanks Ray, I thought it was something like that.

When I was young, and had no kids, no wife, had no "this old house" kind of home to maintain... I really knew all about these things.

Now I've found I've forgotten so much that I'm having to re-learn that it just isn't funny.
Someone reminded me of things I did back in the early 80's like stripping paint off of wood without any protection - those chemicals were pretty bad stuff, I don't think they are even legal any more.

Oh well, I guess that just means I get to learn a lot of "new things" again.
Rick Bastedo

Whilst advancing the timing may well have stopped a tendency to stall by increasing the idle speed, this really should have been done with the idle screws on the carb as advancing the timing may have undesirable side-effects like causing pinking/pinging under load. That assumes the timing was correct and not retarded to start with, and is why all aspects of the ignition including timing must be correct before adjusting the carbs.

But it really shouldn't be stalling at 900 rpm anyway, and neither should you be coasting down hill! :o)
Paul Hunt 2

Thanks Paul, I agree with you and value your advice (when I understand it) 8-)

I believe my timing was retarded a bit, I get no pinking or pinging at all with it "advanced".

I dug out my old timing light, but it was non operational for unknown reason. The last time it worked was over 20 years ago, so I assume something in there was broke. Maybe it was just it's time to go.

I know I shouldn't be coasting downhill, but it was weird that it would act that way so I was merely trying to figure out why. After cleaning out the bowls and following all the recommendations here it still did it.
It would stay at about 900 for 10 seconds then slowly decrease until it stalled. Advancing the timing a "little" caused that problem to cease. I want to fine tune this, and only advance it just enough that it runs good and doesn't pink or ping and continues to idle weather pointing uphill, downhill or sitting flat.

I should adjust the valves, though there's nothing that tells me they are very far out. Just to be sure I should check them anyway. But it's too darn cold out right now!
Rick Bastedo

Hi Rick - does the tendency to stall only happen when coasting downhill? I.e. does it also happen when *stationary* but pointing down the same hill? How about on the flat? Or pointing uphill?

They *should* all be the same, of course, but it is possible that there is some debris in one of the jet pipes that is tending to block the pipe with the car pointing downhill. If it only happens coasting downhill, and not stopped when pointing downhill, that would be something else! Sometime ago there was a long thread from someone in Oz whose car always spluttered to a halt on Sydney Harbour Bridge, I think, don't know whether that was ever resolved. And also the famous one of the car that wouldn't restart after the driver had bought chocolate-chip ice-cream from the local shop, but did if he bought vanilla!
Paul Hunt 2

Come to think of it yes, while sitting at a stop sign when pointed downhill it was difficult to keep it running. I think you are onto something there.

I am also looking into putting a pertronix Ignitor & Flamethrower in this car. Any thoughts about that?
Rick Bastedo

"Pertronix Ignitor & Flamethrower"

Personally I wouldn't, with good original components you will see little if any benefit. The problem is that many people replace knackered points etc. with an electronic trigger, then say how big an improvement there was! I've never had any trouble with points in 40 years. Since checking dwell instead of gap at routine service intervals I haven't even had to adjust them between replacement times, and the current sets on both roadster and V8 are now well past replacement time, I'm seeing just long they *will* go. I did try electronic ignition in the 70s but saw no benefits, and although I still have it I haven't bothered to fit it to anything else. Points usually deteriorate over time and don't usually fail suddenly. And even if they do they can usually be tweaked back to get you home, and if not it is cheap and easy to carry spares and change them at the roadside. Points are very easy to diagnose with nothing more than a test-lamp or meter, electronic ignition can usually only be diagnosed by substitution. The *only* advantage is that in theory they should never need adjustment or replacement. But that is only an excuse for not getting the cap off and giving it due maintenance, which has its own rewards. America got electronic ignition because the requirements of the time demanded that the engine run in spec for at least 50k miles, something points would never do. Uk cars never got electronic ignition.
Paul Hunt 2

Thanks Paul, I guess I really should get out a feeler gauge and check the gap on those points!

I'm considering having the distributor rebuilt by Jeff at Advanced Distributors. I have no reason to think mine is worn out, but how long should a distributor go between rebuilds?
There's no record in the papers the PO gave me showing any service done to it. This doesn't mean there wasn't, just that there's no record I can find.
The engine was professionally rebuilt, a thorough job too. But they may have just stuck the old distributor back in if it was serviceable.
Rick Bastedo

Update:

I decided to go ahead with Jeff at Advanced Distributors, he is building a 25D for me today. He guarantees it will be "right for my engine". Everything people have told me about his service is good, and I found him very easy to deal with. He definitely works with you to come up with just what you need.

In my case I can't take this car off the road so sending mine in to have it rebuilt wasn't an option. Wish I had decided to do this before my "winter vacation" when I had 2 weeks off!

So he is building one for me, will send it to me so I can replace mine then I'll send the old one back to him.

It's been a long time since I have owned an MGB - this is the first one I've had in 20 years. As such, I have to ask what my reasonable expectations should be.

Currently the fuel economy is in the lower 20's (mpg).
The power is pretty good.

Example: when I enter a freeway onramp, generally in second gear and then in third gear it winds up to 5K rpm pretty quickly. Everything very smooth. It does not miss or sputter or anything like that. Idle seems a little rough, not terrible just a little rough.

Should I be hoping for more?
Do I have unreasonable expectations?

Rick Bastedo

Rick - sounds like you are pretty happy with it, which is the main criteria. A B is never going to perform as well as a modern car, the Top Gear program in the UK had their tame racing driver produce a faster time out of a modern Astra Diesel than the other presenters could get out of 80s supercars. having said that, drive with a bit of verve and you can keep up wth most modern drivers!

Mileage is more a factor of the driver and how they drive it than anything else. 20 doesn't sound too bad for American gallons. Original driving tests came up with high 20s for UK gallons, but I get that from my V8, and mid to high 30s from the roadster, and I don't hang about in either.
Paul Hunt 2

Well, just going by reason alone it's unreasonable to expect a 38 year old distributor to act like a new one.

Jeff built & posted my new distributor yesterday, I should have it by early next week. Hopefully I can lay my hands on a timing light by then too - as mine doesn't flash any more.

Thanks Paul, your thoughts are very helpful.

I am pretty happy with the driving of this car, but I know it could be better. I know I can't get that same feeling as I get in my midget (in terms of smiles per mile) or I used to get in my MGC but I know it could be better than it is. I've heard such good things about Jeff's rebuilt distributors that I'm really looking forward to getting one in this car.
Rick Bastedo

Rick, when it arrives, try 14 BTDC for your timing setting and you may need to readjust your idle a bit at that point. See how it responds and listen closely for any pinging. You should be pretty safe at that setting and get the power you're looking for!
Jeff Schlemmer

Thanks Jeff, I will be picking up a working timing light on Wednesday. This should work out very well.
I know I'll have to retune the carbs a bit after I get the new distributor put in, but thanks for the reminder!
Wish I had asked for your timing tape & pointer - it would be easier to do this from a standing position!
Rick Bastedo

Wow - the new distributor from Advanced Distributors just arrived! That was really quick!
Here I am without a working timing light.
Maybe I'll try to eye-ball it.
Hmmm...
Rick Bastedo

This thread was discussed between 19/12/2007 and 07/01/2008

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