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MG Midget and Sprite Technical - decel backfire causes/solutions

I've been enjoying the midget but would like to tame the deceleration backfiring a bit. I understand air leaks in the exhaust can cause this. I regasketed the manifolds a while back. Is there a trick to doing this? Double up on them or use something along with the gasket? Could also be in the pipe joints but I've redone the checked them a few times now.
1098 with header straight to Monza exhaust. If I decelerate quickly it pops something fierce, if I back off very gradually it won't. I can also hear what I believe to be a leak from the cockpit. Plugs look good, don't think it's too rich.
J Van Dyke

Check your timing. It may be advanced too much.
Martin

What carbs are you using?
cheers
Brad
B Richards

I did play with timing a tad, didn't seem to make a difference but I can tweak it again. I tried to get it at about 8 BTDC but the silly upside down pointers are a bit imprecise, no?
Weber 32/36. Not sure how it's jetted. Could be a bit rich but plugs seem to look good. I should probably go through the whole carb adjustment routine. I have not touched the Weber (other than remove and reinstall, turned down the idle a tad and redid the throttle cable linkage as it was binding a bit).
PO did give me the original SUs too.
J Van Dyke

J. The primary mechanism causing backfire on deceleration is condensed droplets of fuel, inside the intake manifold, being sucked into the combustion chambers by the increased vacuum when the throttles are closed. These droplets, then, create a brief period when there is an excessively rich mixture--more fuel than can properly burn during the standard firing cycle. This leads to an increased amount of "unburned hydrocarbons", the HC reading on an exhaust gas analyzer, and backfiring on deceleration.

The two methods which MG used to control this problem were the HIF carbs, having the poppet valves in the throttle plates, and the "gulp" valve. Both were methods of adding air to the incoming mixture, when the throttles plates were in the closed/near closed positions, resulting in a leaner mixture and less chance of backfiring on deceleration.

At one time, back in those murky days of pre-history, i.e. back when some of us were young and before all of the emissions requirements, backfiring on deceleration was simply considered a part of owning a car with a "hopped up engine". They ran a somewhat rich mixture, providing better power, rather than a somewhat lean mixture which provided better fuel economy. With today's computer controlled "engine management" systems, things which we considered common and acceptable--back firing on deceleration, rough running when the engine is cold, problems when moving to different operating altitudes, the engine management system corrects all of these "problem" for more efficient running. No so back in the old days when we used to swap needles in the SU carbs, and jets in other brands of carbs, when moving from sea level to higher elevations.

I cannot help you with your problem because I do not know how your engine is set up, nor do I know how severe your problem is. But, at one time, a sports car, decelerating to take a sharp curve, would sound like a machine gun going off as it backfired through the exhaust system.

Les
Les Bengtson

I see you have been posting while I did. The information of the Weber is of particular significance as they tend, due to the different manifold design, to be the worst offender for backfiring on deceleration. My cars having the Weber will backfire on deceleration, but only to a very minor extent.

Look at the intake manifold, which is U shaped and examine the portion directly under the carb. Does it have a water heater attachment? Riverside used to make an attachment that fit the Pierce intake manifolds and helped to warm the carb end of the intake to cut down on the droplets of fuel being condensed out of the mixture onto the cold legs of the manifold. For the MGB, the TWM intake manifolds had the water heater as part of their design and they tended to both begin to run smoothly earlier than the Pierce manifolds and to have less backfire on deceleration. If you do not have a water heater, adding one would make things somewhat better.

Les
Les Bengtson

Good stuff. It seems to run well and if that's the way it's "supposed to be" then I don't mind a bit. I'll look into the timing again and look at the intake for that connection. Thanks.
J Van Dyke

I rotated the dist. anti-clockwise just a tad (that's retarding it no?) No effect other than it was less peppy I did not break out the timing light and all that, just a quick road test, pulled over on the way to work and reset it (advanced it a bit, rotated slightly clockwise back to where it was). Checked manifold, it is a Pierce but didn't see any fittings anywhere for a water heater. I'll tear the carb top off and pull some jets to get sizes, should know anyway, judging by how it runs and idles and how the plugs look though, I'd guess it's close.
I'd say it's pretty severe as any sudden decel and it pops alot, sometimes very loudly. If I ease off the throttle very gently I can mask it easy enough but only if the guy behind me doesn't mind a very slow decel, and all of them mind! Shifting between 2-3, 3-4 usually results in a pop or two as well. Worse when all warmed up, as I'd expect.
J Van Dyke

One other item might be worth checking (if the other guys haven't got the correct diagnosis already) - I am not sure if you have a vacuum advance on your distributor. If its an old part its probably worth checking the advance mechanism is working correctly - pull the cap off and watch the points plate move when you suck on the pipe (if you don't mind the taste)

I know when they get old they stick and become erratic - could lead to issues with a late burn and the back firing - I assume we mean back firing - late burn - leading to some combustion late when the exhaust valve opens?


One clue if its rich on overrun - would be to get some one to follow you and watch for black smoke - or if you go on a chassis rolls - this would be even easier to spot.

Does it get better when the engine gets fully hot? I woul dave thought the manifold wouldn't support too much condensed fuel when its fully hot?

Either way - good luck..!
John

John. The SU intake manifold warms up quickly and the short runners doe not have much space to allow fuel droplets to collect. The Weber DGV, on the other hand, has U shaped manifold with significantly longer (as well as curved) runners. With the SU carbs, it is a straight shot through the venturi and into the cylinder head. With the Weber, the sits on top of the intake manifold with a plenum beneath, the runners going from the plenum to the cylinder head. Thus, the mixture has to make a 90 degree turn between as it comes from the venturis, into the plenum, then around the curve of the runners and into the cylinder head ports.

The Weber intake manifold sticks further out than the SU intake manifold, sufficiently so that it does not need to have a heat shield installed. The portion where the carb sits is significantly cooler than the portion where the SU carbs sit on the factory manifold.

That is the reason that the better designed manifolds for the Weber incorporate a water heater under the plenum--to warm it up quickly and keep it sufficiently warm that the fuel mixture does not cool as it is passing through the intake system, causing condensation of fuel droplets.

The Weber DGV is not a bad carb when replacing the Z-S carb. But, the twin SU carbs are better in almost every way.

Les
Les Bengtson

Interesting. Maybe I'll go through the SUs and put 'em back on. They're in the box of parts, not sure how much is missing....
J Van Dyke

J. I am somewhat familiar with the Weber DGV because, here in our area of Arizona, emissions testing is required on an annual basis. The original Z-S carb, intended to be rebuilt and replaced on a regular basis, even when new, can seldom be tuned to meet the emissions test requirements. But, the State of Arizona does not really care what carb is on the car--all their books show is that the MGB and Midget's of that era had "a single carb". The Weber DGV is, with the manifold, header, carb, and linkage, less expensive than the SU replacement for the Z-S carb. (At least for the MGB, which I am most familiar with.)

Thus, many of us run the Weber on our rubber bumper cars and it is not a bad set up. But, it will be an exceptionally cold day in Arizona when I replace the twin SU carbs on my 68 with a Weber.

The Weber relates to the SU as "Mad Dog 20/20" relates to a good vintage first growth Bordeaux. Adequate for the intended purpose, but not something anyone would wish to discuss such usage of with friends.

Les
Les Bengtson

Well, I'm freshly repatriated MG owner (had a 1500 Midget back in college for a while Z-S single on it I guess). Current Midget is a '63 which originally would have had dual SUs I believe, why their books would say a single I don't know.
In any event, I've taken it that the Weber is easier to set up and "maintain", simpler in design I guess and may give a performance boost on the top end with a hit on the low end vs. the twin SUs. I've only had this car a couple months so I'm very ignorant yet. I've got enough "purist" in me that I'd prefer the SUs if they performed decently. I wonder why the PO put the Weber on? Rebuilding and setting up the SUs too daunting for him maybe? (me too quite likely).
I'll investigate the vacuum advance next, I did rework the line earlier this year. And that knob on the dizzy that the book says you can turn to adjust timing I don't quite get yet, I could not turn mine easily, may be all the way "in". It does make sense that if under high vacuum: deceleration, the vac advance should kick in and change timing, and mine isn't, then that could be a factor.
The popping is quite annoying and a bit embarrasing so I'm going to try and do something to reduce it.
My original idea of a leak in the exhaust doesn't seem to be a popular factor so maybe that's the wrong tree to bark at?
J Van Dyke

I pulled the carb apart to check jets.
Didn't pull the idle jet yet though.
Primary Main 115, Air 200
Secondary Main 105, Air 200

Retightened manifold bolts and added a shim under a washer on one. Seems a bit of carbon around that one.
No difference. Sure sounds like there's an exhaust leak there though.
J Van Dyke

It'll be the leaky exhaust.If it sounds like it's puffing away and you can see carbon,use a length of hose to find it. You should really never need to double up on manifold gaskets or use sealant or shim the washers as this means either the inlet and exhaust flanges are not in line or are of different thicknesses. Unfortunately both situations are quite normal with aftermarket suppliers. I'd suggest you pull the header off and file the flanges absolutely true. Anything out of line here and the manifold will leak and often fracture at the junction of the welds. Check the inlet flanges with a straight edge too, usually they are OK but just too thick, so mike up the thinnest flange and reduce the other to suit. I know some people file up individual stepped washers or mess about with shims but I wouldn't bother. The mating surface at the manifold is pretty marginal so the flanges need to be spot on. Consider also the way the exhaust system is supported. If it's held too rigidly or too loose the joint will fail prematurely.
f pollock

I've had backfiring problems in the past with my 1500, and it's recently started doing it again. I don't think it's timing, as I dialed it in pretty accurately with my timing light - right on the money. And it can't be the vacuum advance, since I'm using a Mallory distributor that's all-mechanical. I'm running the original ZS carb, but carefully rebuilt, with an appreciation for how it needs to be set up; and performance and pickup are as good as they're going to get with the single carb. A few years back I had the manifold mounting faces skimmed, so I know they're true and the gasket's good. And I don't suspect condensed fuel in the inlet manifold, since it still does it after the engine is up to temp, and I'm using the original water-heated manifold anyway.

So I guess it could be caused by an exhaust leak somewhere along the pipe. It's annoying occasionally, although I have to draw a strong manifold vacuum to make it happen; that is, accelerate to a pretty good RPM in 1st or 2nd and let off to shift.

Interesting discussion!

-:G:-
Gryf Ketcherside

I had a Yamaha Virago 1100 motorcycle that had decel backfiring, very common when guys put on pipes. Factory had an air injection system to add oxygen to the exhaust to reduce HCs. Fix was to simply plug the factory inlet, with a thimble!. Instant fix.
So, yes, there's an exhaust leak in my system I'm sure and I'm 95% it's the source of the popping. So I need to address the manifold fit I guess.
BTW my popping occurs immediately on a cold engine and could care less how much it heats up. Header (Pacesetter I think, was pretty chrome once) and Pierce intake look like they fit decently but apparently not. I tried to shim one yesterday with no effect. I think they need to come off for close inspection.
J Van Dyke

Gryf,
Sounds like you could have a burnt exhaust valve. Sometimes it will show up as a shadow on one side of the plug,but try a wet compression test. That would confirm it straight away.
f pollock

>>> Sounds like you could have a burnt exhaust valve. <<<

Eek! I hope not. Ah well, I suppose I could do a compression test anyway just to satisfy my curiosity. It's been a while.

For what it's worth, I'm running an empty cat converter (float valve failure years ago killed it, and I poked out what was left), but I still have the air pump connected. Maybe if I removed the air pump belt that would make a difference.

-:G:-
Gryf Ketcherside

This thread was discussed between 01/08/2008 and 11/08/2008

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