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MG MGB Technical - (hopefully) straight forward timing question...

Hopefully this is straight forward for the collective wisdom...

Background:
1971 BGT 18V series. pertronix ignitor I and pertronix coil installed with freshly rebuilt Jeff Schlemmer 25D4 dizzy. Running 98 (RON) gas and using air filters and HS4's with (AFAIK) standard needles, and otherwise stock setup.

Problem:
When the timing is set at a stock 12-13 degrees at 1500rpm, the motor's got good low-end and high end go, but pinks under heavy acceleration in 4th from 30-50mph - you know - the 'classic test'. Also pinks a bit in 2nd and 3rd under heavy acceleration.

The pinking won't (just) go away unless I've retarded the ignition so much she's got no grunt at low revs. I think the fuel mixture's pretty close, although the front carb might want a bit of a look at I think there's a very subtle vac leak there somewhere.

Question:
What's a bloke to do?
Curtis Walker

Curtis: You didn't mention if you have done any work on the engine beyond the ignition system. If your BGT has any miles on it, you may need to pull the head for the classic job of 'de-coking' the head. These B-series engines do build up carbon in the combustion chamber and over time it will both raise the compression ratio and provide lots of places for pre-ignition both of which will cause the 'pinking' you are hearing (in the US we say 'pinging' instead of 'pinking'). While the head is off, performing a light polish of the combustion chambers to remove the sand casting roughness will go a long way to preventing carbon buldup in the future.
John Perkins

If it was me, I'd simply loosen up the distributor lock nut and retard the ignition slightly - just enough until the pinkng stops. Turning the distributor counter-clockwise will accomplish this.
Tom Fisher

Curtis:

After re-reading your original post, I see that you already have done what I suggested. Actually, IMHO, a teeny tiny bit of pinking under load is okay.
Tom Fisher

A simple remedy for de-coking that I have used for many moons (50) is to remove the air filters.Then with the engine running slowly pour water into each carb. you will have to hand open the throttles during this. The water will turn to steam and clean the piston tops. following that I pour ATF fluid into each intake with the engine running. Pour it until the engine stops. Let it rest overnight. When you start it be certain to cover the exhaust with a heavy duty cloth so as to protect your driveway. It will make a long black dirty streak if you dont. This has softened all the carbon and blows it out the exhaust.
I use at least a quart of water and a pint of automatic trans fluid . The cheapest works well.
You will be surprised as to how well it works. I use this system to preclean and engine prior to doing a valve job also.What ever is remaining of the carbon is a lot softer and scrapes off easily.
A visual indicator of how well it works is to look at the piston tops when you remove the head and if you see a clean streak across a piston top it indicates that you have a beginning leaky head gasket.
Sandy
Sandy Sanders

Curtis,
Sounds as though you have too much advance at higher revs, if this is the case the mechanical advance in the dissy needs looking at, firstly to check its max adv (there is a stop on the mech adv plate which allows a certain amount of total adv) and secondly to check the adv springs, they could be weak or incorrect allowing too much adv.
Suggest you buy or borrow an advance type timing light, then with the help of an assistant to operate the throttle you can plot an advance curve and see whats going on. Max adv on a std B engine (carotid shaped chambers) should be 34 degs (according to PB's book).
Assuming you are starting with 10 deg static I guess you should be looking for something like 20 at 1000, 24 at 1500, 28 at 2000 and 30 at 2500, in each case this is made up of 10 static plus 10,14,18 and 20 mechanical adv.
good luck
Graham

Graham Cherry

Thanks for the advice, guys!

In retrospect, and driving around a bit more today, she really is running nicely, and I'm convincing myself that the pinking really is only a small amount, and only under inappropriately low revs!

I'm pretty sure the dizzy is up to spec, as it's a fresh 25D4 by Jeff Schlemmer, all recurved & respringed etc.

She's certainly at a nice 13-14 degrees at 1500 (mechanical advance only). I'll get the front carb attended to and report back!

John: I certainly haven't done any work beyond the ignition system, but as noted below, I'm not 100% sure about the motor's history.

Regarding the coking issue, I think the engine is fairly fresh after a down-to-metal rebuild before I got the car, but to be honest, I'm not sure. I'm a bit leery about pouring water into the cylinders, I have to admit! Next time I pull the head I'll look at getting this done, along with a few other bits & pieces (polishing, hardened valves etc).


Curtis Walker

I'll agree with Sandy, but I'd use "Seafoam"...the best de-cocking product on the market
R Dougherty

Curtis. What is the measured mechanical advance curve on the distributor? What is the overall advance at 3,000 rpm? Should not be more than about 32 degrees mechanical.

If you have a "dial back" timing light, it is very easy to plot the mechanical advance starting at 1,000 rpm and going up to 3,500 rpm at increments of 500 rpm. This will tell you if your curve approximates the factory curve or not.

As for a "little pinging", even that is too much. Damages the pistons and may damage the rod bearings depending on when it is happening. It would be better if you did a little more investigation before deciding to drive the car any extensive amount. Could save you an engine rebuild.

Les
Les Bengtson

Les, the ignition curve I put in my performance rebuilds is matched to the engine of the car. Normally I reduce the amount of advance, sometimes drastically, never the other way around in an MGB. The curve is similar to the early Bs, with a couple of twists to help reduce the chance of pinging.

Curtis, if its on the fringe of pinging, retard the timing 2 degrees. Try 10-11 BTDC. Because of how your ignition curve is set up, you can now check the timing at idle. The marks should be rock solid. Check your advance at idle before making any changes and use that as a reference.

Pinging can also be caused by an excessively rich mixture or a lean mixture. If there's a shop you can use to check your air/fuel ratio, that's the fastest and most effective way to get the car running very well! An overly lean mixture that's difficult to adjust will signify a vacuum leak. Any leaks need to be addressed before you can continue to try to tune the engine! A lean condition WILL do damage in short order. You want 14.5-15:1 at idle, 12.5-13:1 under a heavy load.
Jeff Schlemmer

Curtis said: "Regarding the coking issue, I think the engine is fairly fresh after a down-to-metal rebuild before I got the car, but to be honest, I'm not sure. I'm a bit leery about pouring water into the cylinders, I have to admit! Next time I pull the head I'll look at getting this done, along with a few other bits & pieces (polishing, hardened valves etc)."

Hi, Curtis:

I can appreciate your concern about this as reported by Sandy above. Sandy is correct, however, in that this does work for removing carbon buildup. Just be aware that this is an old practice that is sometimes used by "fly by night" tune up shops. They will do this and then hand you a bill claiming they replaced spark plugs, rebuilt carbs, etc.

It really does pay to know who you are dealing with.
Tom Fisher

This thread was discussed between 05/03/2008 and 06/03/2008

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