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MG MGB Technical - Double Ignition Timing With A Weber DVG?

Iím trying to run the bugs out of my 77 B Roadster with hopes of using it for a run from Kentucky to Utah this coming weekend. Iím trying to get the tuning correct both for the pleasure and to optimize the fuel economy. It has been sitting since the summer of 2010, and I just bolted the accessories back on it a few days ago, and fired it up.

To the best of my knowledge, the engine is stock except for the 32/36 Weber DVG and an Allison electronic package for ignition. Cranking compression is 142 to 148 last I checked.

Iíve been looking through the archives, and there seems to be a lot of people indicating that the timing should be set to about 19 BTDC. The factory spec for mine is 10 BTDC at 1500 RPM. At the moment I have it set at 15 BTDC, and there is no spark knock (pinking) even on the gas that is more than 2 years old. I believe that for both performance and economy, the greater advance, up to spark knock, the better. But, my cautious side says that doubling the timing advance seems a bit extreme. So, I would like some fresh feedback on the subject. One thing I would like to understand is why does the carburetor change make so much difference to the desired timing.

I have now burnt most of the old gas out of it, and am heading out to the gas station to fill it with high test. Then Iíll adjust the valves, check to see if the TDC mark is true, and consider further timing advance depending upon the feedback I get.

Thanks,
Charley
C R Huff

Charley. The carb has no effect on the ignition timing that I am aware of. I have run the Weber DGV on three cars and have never had to change the ignition timing because of the carb change.

Your Allison Optic-Electric system is a replacement for the internals of the original distributor which should have been the unreliable Lucas 45DE4. Installing the points replacement system would have no effect on the operation of the basic distributor's mechanical and vacuum advance systems. Do you know if both of these systems are working properly?

Recently, in a response to someone's e-mail, John Twist recommended disconnecting the vacuum advance and setting the overall timing to 32 deg BTDC at 4,000 rpm with an engine similar to yours. If you do that, check the timing at 1,500 rpm and see what it is. That would be your new base setting for timing.

Les
Les Bengtson

Thanks Les,

Yes, both the mechanical and vacuum advance work. That sounds like the right distributor number. I think that was the one with the OPUS system, which had a bad reputation? I'm not much of a believer in eliminating the vacuum advance, especially before a long road trip.

Here is a sample of some of what I found in the archive regarding timing and the DVG:

Charley

gnhansen, westport, ny, rosegary@optonline.net Posted 20 November 2008 at 19:51:17 UK time

So, the timing set to 15 degrees BTDC @ 1500 RPM was a great help. I believe the ignition is about as good as it gets.

Andrew in Austin, TX
Bob,

On my '79 roadster I used the Lucas 45D Eurospec from Brit-Tek with a pertronix electronic module installed for about 3 years in conjunction with a Weber DGV. It worked well. Other than the Weber's flat spot at below 2,000 rpm, I had no complaints with the set-up.

I did have to set the timing to 19 degrees BTDC at 1,000. At the time, I was and I'm still using a Piper HR270 camshaft along with the pre-74 stock cast iron exhaust manifold connected to a Falcon free flow stainless steel exhaust system. In addition to the previous mods, I'm also using the earlier pistons that raised compression. I've since switched over to HS-4 SU's, which eliminated the flat spot.
C R Huff

Charlie-
You are comparing oranges to horse apples and buckeyes here.
Archives of unknown people relating experiences on unknown cars is not a good path.
The carb should have nothing to do with timing, except that very lean mixtures will require more advance. Thing is, it was already quite lean with the ZS, but if you have a flat spot it might be even leaner now - or it needs more advance.
The second example you give is a completely different engine than a stock late car. Distributor, Cam and cam timing, compression, manifolding, exhaust all change things. It has about as much to do with yours as a 1935 White Mustang or a 54 Buick V8.
The John Twist recommendation is safe but by no means optimal, especially for this 75-on engine. I have found that they are so insensitive that it is nearly impossible to ever get it to ping, even when it won't start from too much advance.
If you have all stock, plus maybe some cam retard from chain wear, and maybe some lost valve lift from wear - all common - then it will take even more advance, but it won't matter a whole lot.
The spec to set it at 1500 is brain dead, as it is a bad place on the distributor advance curve to set it. The curve is steep here and a few rpm makes a bad reading. Even slight temperature or mixture changes change the engine speed and timing enough to throw it all off. It all had to do with emissions testing.

The OE distributor mech advance spec is:
13-17 @ 2000
28-32 @ 3500
33-37 @ 4500
Set at 10 @ 1500
The initial setting is subject to all sorts of error, but you can see that if you set it at 5 static (doesn't work!) you will be at up to 37 @ 3500, or 42 @ 4500, far past Twist's number. It will absolutely be a pig if you set it as Twist says - if he said that in reference to an engine like this.
I have also found that maybe 15-20 @1500 works as well as can be had.

All this with vacuum disconnected, per usual. And Twist was not suggesting that you leave the vac off, only that you set it that way.
The vac spec (Federal cars) is start @ 3"Hg, end @ 11"hg, total 24 degrees. The 24 degrees should be distributor degrees, as that is the convention; so it would be 48 degrees crank, a hell of a lot. The WSM is murky here - check the vac capsule, which should be marked as 3-11-12 or 24. The third number is vac advance in distributor degrees, crank degrees is double.

Anyway, you can see that especially under part throttle, the thing is running crazy advance, and it NEEDS it!

I see no need to use High Test in these things - they work fine on panther piss if stock.

And call me if you want (email for number - I am up very late indeed), but have a fine trip, a favorite of mine. (go up the Missouri River valley from St Louis US40, MO94, US50 at this time of year, and US 50 across Colorado/Utah.)

FRM
FR Millmore

Hello Fletcher,

I was hoping you would jump in on this. Somehow I missed that Twist did not mean to leave the vac adv off while driving. The whole apple to oranges thing you brought up is why I asked the question in the first place. I know it was old info, but the examples I gave were only a couple out of about a dozen I saw.

You mentioned that 15 to 20 adv was good in your experience, and I am now sitting on the low end of that scale. And, the car is running much better.

I'm glad to see that you recommend US 50 across Colorado. That was the route I was planning for this trip. Typically, I run the Interstate to about Topeka, KS, and then jump off on the 2-lane to the west coast. But, this time I'm not going to the coast, and I haven't taken 50 through CO for many years. Usually I take 36, 34 & 40, and then 50 through Utah and Nevada. I'm going to Zion this time, so I will be going a little farther south. I'll probably take 50 till it runs back into 70 at Grand Junction, and then take 70 to Green River, UT, and the take 12 through Capitol Reef and on down through Bryce to Zion.

I think you and I both have the love of the 2-lane. I have read you speaking of it before. We probably have that love for the same reason. You and I both used to be owner/operators, and we burned ourselves out on the Interstate.

I'll shoot you an e-mail with my phone numbers, and I would very much like to have your number. If I get in a jam on this trip, I can't think of anyone better than you to talk me through it.

Thanks,
Charley
C R Huff

I guess with this talk about a trip to Utah, I should throw in a photo of the car. So, here it is.

Charley
C R Huff

I guess with this talk about a trip to Utah, I should throw in a photo of the car. So, here it is. Well, somehow it seems I mangaged to screw that up. I'll sign off and try again.

Charley
C R Huff

Okay, One more time. Here is the pic.

Charley

C R Huff

Ahh, I see that you have the life boots, whoops I meant boats, on the bonnet, in case she stops.

Herb
Herb Adler

Herb,

You are more right than you know. Back in the 60s and 70s I used to drive $25 cars cross country while wearing $100 boots. But, in this case, I took the pic to send to my sister who is also going to Utah. I just wanted to let her know that I got the hiking boots she recommended, and that I got the car together.

Charley
C R Huff

Charlie: My 79 MGB has a Weber and a Lumitronix ignition system. After experimenting, I got a good combo with 17 degrees BTDC @ 1500 RPM. Runs on Regular 87 octane and 26 mpg - no pinging. Pinging beyond 17 degrees BTDC. (That also has a Crane 270 cam).

Try it out. I found it's the best of both world for me.

Hey, Did you ever get that Aeronca airborne??

Cheers

Gary

79 MGB

PS Your B looks just like mine except I have Rostyle wheels.
gary hansen

This thread was discussed between 16/10/2012 and 25/10/2012

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