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MG Midget and Sprite Technical - I WILL cure this backfire! (1500)

Sigh - My midget 1500 starts, runs, and drives very well indeed. Fires up on the button, settles into a nice steady idle just under 1Krpm, behaves itself wonderfully, but frequently backfires on the overrun.

It's really getting frustrating, and I drive it pretty gingerly around town just to avoid the Big Bang. Everything's adjusted and tuned to a tee. Timing and mixture should be fine, etc. Setup is near factory-spec, with the single ZS carb, stock manifolds, air pump, EGR valve, and catalyst, although the latter is an empty canister. Ignition is via a Mallory distributor with a pertronix and an MSD coil.

So what in the world could be causing this? For what it's worth, a few years back I had the intake and exhaust manifolds planed together so the mounting faces are dead flat, and reinstalled it all with a new manifold gasket. So there shouldn't be any leaks, and I haven't heard any hissing or blowing from that area.

I would love to get it corrected once and for all. It's great fun to give the Midge a bit of welly every now and then, but not if it's going to sound like a drive-by shooting.

Thanks in advance for any suggestions!

Gryf Ketcherside

ahhhhhhhhh, I just love a good bang or raffle from the back side going downhill. (in holland....?)
to bad it stopped doing that with another exhaust.
Alex G Matla

>>> I just love a good bang or raffle from the back side going downhill. <<<

Yeah, but a deafening BANG as you're driving past City Hall? Not so fun.

-:G:-
Gryf Ketcherside

Could a faulty egr valve cause this?
Trevor Jessie

>>> Could a faulty egr valve cause this? <<<

Good question, Trevor, and I certainly wouldn't rule that out. It's the original EGR, and I assume it works, but can't be sure. I can easily remove it, plug the hole, and see what happens, I guess.

Thanks,

-:G:-
Gryf Ketcherside

Gryf. The EGR (exhaust gas recycle) valve has relatively little effect on the problem you describe. What does have a direct influence is the gulp valve, which needs to be examined.

The most common cause, almost the exclusive cause, of the problem you describe is fuel droplets which have fallen out of suspension and cling to the sides and bottom of the intake manifold. Under normal conditions, standard intake manifold vacuum, the droplets just sit there and the mixture remains constant. Under other than normal conditions--the brief, much higher vacuum created by the sudden closing of the throttle, these droplets are sucked off the bottom and sides of the intake manifold passages and carried into the cylinders, resulting an a quite excessively rich mixture. Thus, the gulp valve. Normally in the closed position, when the vacuum reaches a certain level inside the intake manifold, the gulp valve opens, allowing the air pump to force extra air into the intake manifold resulting in a significantly weakened mixture. This, combined with the fuel droplets being sucked into the cylinder head from the intake manifold results in a relatively standard mixture being induced into the combustion chambers--one which will not be leaving a lot of unburned hydrocarbons which will then collect in the exhaust system causing a back fire.

You will find that the problem is encountered more often when the intake manifold is not fully warmed up. When the intake manifold is cold/cool the potential for the droplets falling out of suspension is greater. When warm/hot the potential is decreased. If this is what you are experiencing, check out the function of the gulp valve system and the air pump.

Les
Les Bengtson

Thanks for the info, Les! Unfortunately, the '78 Federalized Midget wasn't fitted with the gulp valve. At least they weren't in the fall of '77, when my car was built. I think it was a California thing.

There is one thing I can try here, I guess. The Decel Bypass Valve on the front of the Zenith can be adjusted to regulate the amount of fuel mixture allowed to bypass the throttle plate during deceleration. It opens when there's a high delta between the vacuum downstream and upstream of the throttle plate, and prevents a sudden chop of intake air/fuel, and the resultant spike in manifold vacuum. I may have mine adjusted too tightly, or maybe it's stuck shut. BTW, you can tell if the Decel Valve isn't tight enough - regular intake vacuum can pull it open and raise the idle to 2500 or so.

All of that to say that loosening the Decel Valve a bit will lessen the vacuum spike on throttle closure, a side effect being that revs might drop more gradually when throttling off.

Thanks again, and more opinions are certainly welcome,

-:G:-
Gryf Ketcherside

Have you tried the WD40 trick, warm up the engine and spray around the intake and carbs looking of a speed up of the engine, just to make sure there is no small airgaps getting in.

when was the last time you changed spark plugs


Prop
Prop

Another thing you might look at, I dont know if its effecting you guys but our fuel here in jeff city all changed in the last 2-3 months.

regular unleaded is now at around 81- 83 octane, and super unleaded is at 89-91, you might have a look around and see what your guys octan rating is at the pumps there in st. louis.

if thats the case you may have to re-adjust the timing a bit

Prop....Its getting bad out there...Prop
Prop

Gryf:
Faulty air pump? Might explain a lean condition (hence backfire) upon overrun, though I'm not sure how one might check the air pump for failure. Maybe sticky check valve on the air pump manifold? Might produce the same symptoms.
R
Richard 1979 1500

Okay, I've been thinking about this again.

Just a bit of theory - the Zenith choke is different from the SU in that the choke is a separate unit that screws on to the carb; it incorporates a needle valve to regulate fuel flow, and this needle valve includes a little O-ring to shut fuel off completely when the choke is closed. This needle valve can be actuated either automatically by coolant heat, or (in my case) manually by a cable.

The needle valve regulates fuel flow through a passage connecting the float chamber and a jet just upstream of the throttle plate, and when the choke is open, flow is initiated by the difference in pressure between the upstream and downstream sides of the throttle. Thus, the choke really only works at idle, or when the throttle plate is closed, creating the pressure difference.

So I'm starting to think that my choke isn't sealing well, possibly because the little O-ring has failed (again). This would send a shot of raw fuel into the airstream when the throttle closes, maybe causing backfire on the overrun. I want to pull the choke unit off for a look, and I'll report back. We Zenith guys need to stick together...

Cheers,

-:G:-
Gryf Ketcherside

In the interest of not leaving a thread hanging, I'll mention that I removed and disassembled my choke a few days ago, and found it to be in perfect shape. The little O-ring that shuts off fuel flow through the unit looked like new, and all parts moved very freely. So I buttoned everything back up, made sure the choke unit was tightened firmly against the carb body, and am still getting backfires.

Stay tuned...

-:G:-
Gryf Ketcherside

Well thats bad to be you, I was just thinking about you yesterday, I figured you where out speed demoning around town.

Did you mention this to john, Id be curious as to what he says.

I really got to start reading up on the 1500s and there intricasies....any chance its getting around a butterfly shaft, maybe a little wear in the shaft and bushings....btw, did you look to see what your octane rating is there, If it changed here im certian it changed there also...I know my truck sure loves octane booster


Is there any chance you getting mositure into the igniton system somewhere....when my truck was having over heating issues sometime back and I would pull the cap and a gysor of molten coolent shoot in the air all over everything it would make the truck back fire untill it dried off the coil and dissy parts


Hang in there,

prop
Prop

Gryf,

We had a similar problem a few years back with our '74 B/GT, and the fix was a new gulp valve. I think you are correct concerning the gulp valves being used on the 1275 only. The 1500's had what they called a diverter valve connected the intake manifold to the air pump. With new diverter valves difficult to source, and wonder if the earlier gulp valve could be used here?

A final thought concerning the air pump itself would be the check valve between the air pump, and the air injection pipe/manifold. We've replaced many of these over the years on our B/GT.

Regards,

Larry C.
Larry C '69 Midget

Make sure you don't have an exhaust leak. Exhaust manifold to head probably. Sucks oxygen in on decel and all the unburned fuel in the exhaust ignites in the pipe.
J Van Dyke

I think backfire is exacerbated by lean condition, not enriched?
Richard 1979 1500

Detonation in the exhust normally caused by un-burnt fuel being pulled into hot manifold (as pointed out by Les). It's exacerbated by the high vacuum in the inlet drawing excess fuel past the jet without any additional air to burn it.

Some SUs are fitted with pop-valves on the butterflies to provide a short blast of air to burn this fuel.

I'd suspect the Zenith is letting in fuel without air under high vac - but as I don't know the principle of this carb so can't comment further.

A

Anthony Cutler

Thanks for the additional comments. The '78 - at least the Federal version - was fitted with neither a gulp valve nor a diverter valve. The California folks got all the extra bits, but in my case I can rule those out.

Anthony, I'm familiar with the poppet valves in the SU butterflies, but the Zenith doesn't have those. What it DOES have is a Deceleration Bypass Valve that allows some intake air flow around - not through - the butterfly via a small port and a spring loaded diaphragm that in effect does the same thing as the SU arrangement, but is adjustable via a small screw on the outside of the valve unit. I may have mine set too tight, although that would have the same effect as soldering the poppets on an SU, and that seems to be recommended, with no mention of backfiring as a result.

Here's another idea - what part (if any) would the EGR valve play in all this? I know what it does; allows a brief shot of exhaust gases into the inlet manifold under certain conditions - but I don't really know WHY it does that, and what the effect is meant to be. I don't think mine is functioning anyway. I was just out in the garage a little while ago tinkering with the Midge, and noticed that when I throttle the engine by hand, I can see that the EGR pin isn't moving. I know it used to, but it's not now. Any ideas about this, relative to the backfire?

-:G:-
Gryf Ketcherside

Ha! Closure.

I just spent some more quality time with the Midget. With the car idling in the driveway, I loosened the Decel Bypass Valve to the point where if I blipped the throttle at the carb, the revs wouldn't fall all the way back to idle, but would hold around 2000 rpm... meaning that the valve was loose enough that idle airflow was causing the valve to float and allow bypass mixture.

I then tightened it back in steps, giving the throttle a blip each time, until the revs fell back to idle - a bit more slowly than before, but they still got back to 900-1000 in a reasonable time.

Then I went for a spirited drive. By the time the engine had reached full operating temperature, the Decel valve was floating again at around 2000 rpm... engine braking was nearly nil, but I had a nice cruise control at slow speeds.

So I pulled off into a convenient parking lot and did the tighten-and-rev routine again until the revs would fall back to idle normally. And in spite of some spirited acceleration on the way home, I didn't get a single backfire, nor even a hint of one.

So I'm considering this one solved.

Cheers,

-:G:-
Gryf Ketcherside

I hope so gryf, that sounds complex, will it revert back when the engine is cold...Hope not.

how wet did you get?

Its been raining here almost non-stop scence 11am...I got things to do.


prop
Prop

>>> how wet did you get? <<<

Not at all actually, but it had rained pretty heavily earlier in the day. When I left on my test drive, it was lovely out, but things were overcast again by the time I got back.

-:G:-
Gryf Ketcherside

Hi Gryf

Soldering the poppet-valve on the SU carb may not induce back-fire - and indeed the better carbs don't have them. But the SU carb maybe has an advantage in over-run compared with fixed jets: that is when the throttle is shut, the piston/needle returns to its lowest position and almost shuts off the jet completely so little fuel emerges.

Fixed jets can't do this.

HTH

A
Anthony Cutler

Interesting point Anthony, but I'm not sure what you mean. The piston/needle in the ZS also falls all the way back down to the bridge on throttle closure, and it seems this would have the same effect. Don't the SU jets move only when pulled down by the choke cable? Or do they move in regular operation? I didn't think they did.

Cheers,

-:G:-
Gryf Ketcherside

Zenith = ZS = Zenith Stromberg = crappy version of SU with the diaphragm instead of the piston.

All becoming clear(er).

Was thinking fixed jet until I realised!

SU operates as you say.

Zenith must some how have been pulling fuel through without air.

But glad to hear it's sorted.

IIRC the strombugger wasn't used generally by Austin/Morris/MG; it was Triumph (and Jag on the V12?) that seemed to like them. I recall friends cars with split diaphragms etc.

IIRC 2 the UK 1500 Midgets had SUs as well; so I assume USA ones had/have Strombuggers - well you lot deserve(d) better is all I can say!

A
Anthony Cutler

>>> ...with the diaphragm instead of the piston. <<<

Actually, the piston in the ZS is very similar to the SU one, the main difference being that instead of relying on a precision slip fit in the vacuum chamber, it uses the neoprene diaphragm to ensure a positive air seal. Personally, I've never split a diaphragm. (I've had three hernias, but that's different...) Now, it's possible to damage the diaphragm due to improper maintenance, such as attempting to adjust the needle without the special tool that prevents that piston from rotating with the adjuster screw... also, use of starting fluid will damage the rubber. But with proper care, they seem to last a long time. At least mine has. And even though the ZS carb draws a lot of criticism, there are some features that really aren't that bad, such as adjustable bypass (mentioned above), a neoprene air seal around the throttle shaft, positive vacuum seal via the diaphragm, and so forth. Mixture setting is achieved by raising and lowering the needle, not the jet.

Whatever... I've had pretty good success with mine, having spent a few years digging into the theory, rebuilding it once, and tuning it. It performs fine, given it's only a single carb, and I'll admit that I'd rather have a less restrictive intake. But otherwise, the only serious issue I had with it was due to a mod I'd made that may not have been advisable; fitting a Grose Jet instead of the standard float valve. Mistake, but again, this wasn't due to the inherent design of the ZS, and once I returned to the older-spec float valve, I've had no further issues with flooding.

-:G:-
Gryf Ketcherside

This thread was discussed between 05/05/2009 and 26/05/2009

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