Consistent MFI Backfire, Ideas???

[50/50]

I have a consistent backfire at 2800-3000 . Just a slight popping. mostly off throttle. On throttle it rips up to redline.
Its a 2.2S with MFI , no cold start, headers, and an after market sport exhaust (maybe Monty).

To date the fuel system has had:
> The tank checked and it was clean
> Fuel screen at tank cleaned and it's an 8mm discharge
> Had the flexible line to the pump replaced
> Fuel pump checked
> Flexible lines in engine bay changed
> Fuel filter replaced
> Fuel solenoid relay replaced
> MFI pump refurbished and calibrated
> Throttle bodies rebuilt and calibrated
> MFI timing belt replaced and checked
> injectors checked and cleaned
> Hard fuel lines cleaned and replated
> throttle linkages rebuilt and checked for lengths
> Added ahelper spring to assist throttle shut down off throttle

And on the Ignition:
> New Partsklasick CDI box
> New Partsklassick coil
> New ignition wires
> New points
> New RPM Transducer
> Timing checked and rechecked
> Various plugs tried

What's left? Exhaust leaks? Multiple techs have said no.
Bum Tach? Although I may have had this rebuilt, need to go to the record book
Over the years, fuel additives, 110 octane fuel and other potions

What say the jury?

[gled49]

My MFI cars have always had a “burble” at steady throttle, not sure there is anything wrong with yours.

[Jeff Higgins]

As gled49 indicates, this really isn't a "problem" with MFI. Most of us see it as a "feature".

I see you mention a new RPM transducer, so I assume you are still running the shutoff solenoid. If it is working properly, and you are off the gas at 2,800-3,000 RPM, there should be very little to no fuel delivery, so nothing to make it pop and backfire. Maybe take a look at that shutoff solenoid, the switch that trips it (near the #1 cylinder intake stack), and the attendant wiring.

Many of us remove this solenoid, the trip switch, and the wiring. Another assumption on my part - if you have already removed the cold start solenoid, are running headers and a sport muffler, "originality" may not be all that important to you, so maybe removing this stuff won't be a big deal. It's just one more thing to complicate matters and possibly fail. These cars run great without it. You begin to appreciate the popping and burbling on decell...

[66S]

Id disable the fuel cutoff and use a stock muffler. If your tach is acting erratically it could be caused by the alternator.

[HughH]

at around 2800 rpm there is the switch over in the MFI from the first rack to the second one - notoriously hard to get that switch over exactly correct even if both the low revs rack and the high revs rack appear to be calibrated correctly in my experience - hence often a little spot there that appears a bit lean or rich despite the rest of the range being great. As Gorden says a part of the character of the cars so long as it is not too intrusive - but if you get really worried about it you could attach an air fuel meter to it while driving and see what the reading is around that range - however IF it is slightly out be aware that once you start fiddling around with changing the rack (need the side of the MFI pump off each time to make the adjustment - it is not done by clicking the adjustment screws) it is like 3 dimensional chess and it can get a lot worse very quickly even IF a MFI specialist is working on it. It took me 6 months of small adjustments with driving between each one with the A/F meter and computer attached to get mine "almost right" and even then it varies slightly depending on the type and brand of fuel I am using as well as where I am driving (country v city) but admittedly mine is hard to tune exactly as it is a 2.7RS engine in all respects (including the space cam in the MFI pump) except for the E cams

[50/50]

Thanks all. The odd part is that it developed some what quickly. Also I have a second 2.2S that is smooth ,up and down, through out the rpm range. Maybe they are human like?
Hugh sounds like you truly sweated out the details with less than perfect results.
I hope that your country gets some soaking storms.

[Jeff Higgins]

I must say that I'm a little bit confused, Hugh. What is this second rack to which you refer? Maybe it's a matter of terminology, but there is only one "control rack" that runs between the plunger units. And which side cover do you remove, and what sort of an adjustment can be made through that side cover? As far as I know, the only adjustments that are typically made on these pumps are via the main control rack adjustment screw on the control rack head and the idle adjusting screw. If one wants to dig just a bit deeper, one can remove the cover plate that holds the idle adjusting screw to expose the low and high speed screws in the capsule on the end of the centrifugal governing unit, but this requires draining the oil from the pump beforehand.

Could you maybe point out what you are referring to as this second rack? Thanks.

[HughH]

Jeff

Maybe we are talking about the same thing with your description of "expose the low and high speed screws in the capsule on the end of the centrifugal governing unit, but this requires draining the oil from the pump beforehand." I was just using the terminology that my MFI guy uses. Mine is a 73 pump if it makes any difference

Both "racks" are accessed from the cover plate on the RHS looking at it from the rear when installed in the engine. (when you open it up you do have to drain some oil first to access). I will have to see if I can find a diagram or ask my MFI guy for an explanation - I am certainly not the expert on this I just watched my MFI expert do it many times (unfortunately) as we tried to fine tune my car.



In the words of my MFI guy the "first rack" covers the fuel delivery over the rev range from idle up to about 2800 rpm and then the second one from about 2800 RPM up to the maximum. Both are adjustable independently and both then get impacted proportionately by the control rack head screw in the front of the pump (ie the normal one that is used to adjust the rack)

Hi Jeff - additional info

I just talked to the person who does the work for me It is the govenor that gets adjusted. There are a number of screws on it - a center one that adjusts the idle (which was not touched) and then two either side: one that adjusts the fuel delivery between 1000rpm and 1700RPM and the other between 1700RPM and 2800RPM before the main rack takes over by itself at 2800RPM. He says it is like some of the fine adjustment screws on a carburetor.

[Jeff Higgins]

Thanks, Hugh, it was just as I suspected, a matter of terminology. The four screws underneath the cover to which your MFI guy are referring are, in fact, the low and high speed adjustment screws in the governor mechanism. Here are a couple of photos showing the cover over them that holds the spring loaded idle adjusting screw, and the other showing the cover removed. With the cover removed, you can see the governor assembly capsule that holds the two pairs of adjustment screws, one pair black (which are the low speed, but almost too dark to see) and the other pair white (high speed). The large center screw is the idle adjustment.

I've found these screws to make very little difference in fuel delivery in their assigned RPM ranges. Adjusting them changes preload on three concentric coil springs of increasing tension. These three springs arrest the movement of the flyweights as they open with increasing rpm, with the lighter of the three controlling flyweight deflection at idle, next heavier at low speed, and the heaviest at high speed. I've included a photo of the capsule and the three screws behind it.

This whole thing is, unfortunately, rather imprecise regarding any adjustment's effect on mixture at its assigned RPM range. Ultimately, these adjustments are controlling how far the space cam moves fore and aft with varying RPM and, therefore, where the stylus that controls the position of the control rack lies on that space cam. Lots of overlap between adjustments, and lots of variables between those adjustments and their actual affect on where that stylus is reading. Kind of seat of the pants, voodoo black art in getting it "right", really. That's why these things may display some "personality" at times, at various RPM ranges. It's what keeps it fun...

Oh, and sorry that the photos aren't exactly in order. I may have MFI somewhat figured out, but this darn photo uploading continues to confound me...

[HughH]

Hi Jeff
I was my language that was not precise enough - not being an expert on this and having the procedure occur a number of years ago (in fact I think in 2013) When I talked to him today and he mentioned governor adjustments I instantly remembered

It was a long slow process to get it done - (oil out, adjust, put back together - drive in the country at night with the A/F meter sometimes attached but mostly with me doing the same predetermined circuit with different conditions of throttle use and "feeling" the difference and then repeat - a couple of times a nights and then over multiple days).

There was definitely a big difference over time with these adjustments but it was a slow process and sometimes the change made it worse. towards the end we did tests in the shop with the A/F meter but found that we needed to do them in the car when driving to get it completely right. However I would argue that my "feel" adjustments were better than what we achieved with the A/F meter.
the upside though was that we have not had to touch the MFI at all since then and it runs really well and even gets great fuel economy on the highway (but not in the city or if driving hard )