"Back Fire" out the exhaust

[Nunyas]

Well, this is sorta new to me. I picked up a smog pump from Kragen and I was surprised it actually looks like the stock unit. The non-functioning one that was on my car turned out to be a Nissan part. It still worked for the core charge though. /ubbthreads/images/graemlins/smirk.gif

Anyways, since I've installed the new pump I've picked up a back fire out the exhaust. It happens when I shift under moderate acceloration, and sometimes when I go from steady state driving to full throttle lift. I've noticed the loudness of the back fire is in direct relation to how far the throttle lifts. For example, half throttle acceloration gear shifts result in 2 to 3 clearly audible and distinct pops out the exhaust. 1/4 acceloration will give me a some what subdued but still noticeable pops. Full throttle... haven't tried that one yet, I'm afraid it might sound like a gun shot. It does NOT happen while I'm engine braking on long down hill roads though.

I assume this is because of the momentary richness associated with sudden throttle lift, in conjunction with a faulty Gulp Valve. Could it also be because the carb is set too rich?

I still have more emissions parts on the way. Even though I find this back firing out the exhaust some what entertaining right now, it's gonna get old quickly. I also doubt that the "Smog Technician" will let the car pass with it sending reports out the exhaust. /ubbthreads/images/graemlins/nonod.gif

 

[Stewart]

When was the last time you checked the oil in the carb?

 

[Bret]

Hey Rob,

This might be a silly question but when you set the air fuel mixture of the ZS are you bypassing the air pump as described in the manual? I only ask because if you don’t disconnect & plug off the air pump you could be making it too rich.

In fact if you use a CO gas analyzer to set the mixture for the proper CO% (5.0% +/- 0.5 I think?) reading. Then recheck after you’ve reconnect the pump the reading should be almost zero CO%.

I’ve known a few guys that made these adjustments with the smog pump still connected and it induces false readings on a CO analyzer and in fact be way too rich.

Disclaimer: this is assuming that you’ve checked and adjusted everything else correctly. Timing, valves etc.

My $0.02

 

[tony barnhill]

Good points so I'll only add this: Nissan Emissions parts off the old Z cars interchange with MG parts quite readily (& might be less expensive).

 

[Nunyas]

well, inexpensiveness might have explained the Nissan smog pump being on the car, but the cost of the factory looking part was only $60. So, I don't see how much less expensive the Nissan part could have been. Either way the new one is working beyond any shadow of a doubt.

I haven't messed with the mixture yet, because when I installed the new pump I didn't think that anything else would have to be changed. I guess I should have known better, ay? I'll have to do that tomorrow.

I haven't checked the oil in the carb in a couple of weeks. I'll have to do that in a bit before I take off to take care of my appointments today.

I'm gonna have to pick up more hoses too. The pump to air rail hose is long enough to conntect the two devices but it's about 1/2 an inch too short to fully seat on both ends.

 

[Nunyas]

Well, I re-topped the dash pot this morning before leaving, and ... uhm ... that seemed to make the throttle lift back-firing louder. However, I did find that if I slowly let up the accelerator before shifting I could eliminate the back-firing all together. I think the new Gulp Valve (anti-back fire valve in some parts books) that I have on order might actually fix this... or atleast I'm hoping it will.

 

[Nunyas]

well, small update on the backfiring issue. I grabbed my spare Gulp Valve and fit it to the car, and it eliminated the back firing on throttle lift. So, I guess that was the problem, but this valve perpetually leaks and lets the front two cylinders burn leaner than the rear two.

Hopefully, it'll burn more evenly between all 4 cylinders when the new valve arrives.

 

[tony barnhill]

You can get them new through Moss - or go down to the Nissan dealer

 

[Nunyas]

yeah... I'm waiting for the one I ordered through you to arrive. /ubbthreads/images/graemlins/smile.gif

 

[tony barnhill]

Oh yeah, forgot about that - it should be here in a day or so & I'll get right out to you

I can also drop ship directly from warehouse if people need parts quickly

 

[Tom_Fisher]

Rob:

I've read that the MG gulp valves are of a notoriously bad design - no wonder they fail so often. Don't know about the emission requirements in your state but I removed the air pump, injectors, and gulp valve from my 69 MGB about a year ago. It now runs A WHOLE BUNCH BETTER and never backfires, either.

 

[tony barnhill]

[ QUOTE ]
no wonder they fail so often

[/ QUOTE ]

Interesting comment, Tom....my '79 MGB w/100000+ one owner/driver (me) miles has never had it replaced! Everything is 100% original.

Granted, however, they do run better without that stuff!

 

[Nunyas]

unfortunately I have to have that stuff on the kar ... thanks to Kalifornia... /ubbthreads/images/graemlins/mad.gif

I think the only problem I'll have with the new bit is where to find that little restricter piece that goes in the hose between nthe pump and valve.

 

[tony barnhill]

Tell me the Moss number - I might have a good used one.

 

[wkilleffer]

I'm having this problem as well, without any smog gear. But I'm also having some ignition issues, which should be cured early next week. Curing that should help with getting the carbs properly in tune.

I bought a gulp valve and air rail, but haven't bought the air pump and may not ever.

 

[Nunyas]

Tony- according to the Moss Online catalog it's part # 367-025. They seem to have two different part numbers, one for pre-75 and one for post-75. The part number I just listed should be for my car... I hope. /ubbthreads/images/graemlins/smirk.gif

William- sounds like yours might be iggy related. My backfiring out the exhaust didn't begin until I started pumping fresh air into the exhaust manifold. I suspect it was reacting with the hot unburned fuel entering the exhaust during throttle lift and then boom in the manifold with the report going out the back... heh

 

[tony barnhill]

Lemme look tomorrow

 

[kennypinkerton]

The only car I ever owned with just a cat and a air-pump similar to MGB was a 1981 Pontiac Sunbird 2.5L 4 cylinder... Had a very similar air injection unit. I lost the cat one night due to a rotting rubber exhaust mount. Gone, as in it disappeared into a ditch one night, and I had to tie the short downpipe to the body so I wasn't dragging the pipe on the ground. After calling many shops the next morning, and finding out nobody would replace a cat with either a straightpipe, or on a car in which the cat was missing, I finally made my own pipe, and had some muffler shop weld it up for me.

Main point - while the pipe was loose - about 3 weeks - If I let off the gas pedal at any point, I'd get seriously loud backfire (from the exhaust)... If I was under hard acceleration and let off - It would sound like a shotgun went off inside the car - then a long series of less loud exhaust pops until I started back into the throttle.

 

[sparkydave]

/ubbthreads/images/graemlins/iagree.gif Mine was making the occasional bang when letting off the gas, but after I ditched the cat it was ten times worse. Took the air pump belt off - problem fixed. So far it's still exempt from emissions testing (knock on wood).

 

[Nunyas]

I envy you exempt guys.... /ubbthreads/images/graemlins/smirk.gif