Backfire through intake/ stall on acceleration

Knebel

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Whats the general rule when you rev the engine and it pops out the carb and/or stalls?

I recently installed an open spacer and now when i rev the engine a little while its its under 100°, i get this backfire cough through the throttlebody. I could actually make it stall once or twice.

I have a fuel injection that also controlls timing. Don't let that be an issue because it works like the same principle with accelerator shots and so on just on a wider range of adjustments, just works a little different.

Did my carb spacer make the whole thing leaner in the accelerating department? It is a 1/2" open spacer on a dual plane intake. The open spacer is supposed to be better for the EFI to distribute fuel.

I run 18° at idle.
34° wot 3000rpm and 6000rpm (High load high rpm)
1100rpm wot is also 18° (high load low rpm)
39° are cruise values at 3000rpm and 6000rpm (low load high rpm)
22° at 1100 cruise (low load low rpm)

this stuff is based on vacuum/map values so even in free rev, the map value reaches the so called "high load" pretty quickly even though there is no real load.

Anyways, I have the possibility to adjust accelerator pump shots at various temps (20, 65 and 170°).

Is this backfire through the carb rather fuel related or should i look at taking timing out anywhere? Not sure how to approach this since it backfires only under 90-100° through the carb!?
 
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What brand of EFI did you add? I presume the situation you describe started after the EFI was installed or just when the additional spacer was installed?

On the 3000rpm light load timing amount . . . Most engines in the later '60s and such, in the 2500 rpm cruise mode (of 2500rpm "free load" mode) would usually have a total advance figure in that mode of about 50-54 degrees BTDC. WOT (high rpm, high load) distributor+base timing would be where the desired 38 degrees BTDC timing would be optimum, with appropriate fuels.

With these EFI systems, is the type of intake manifold (dual or single plane) important for correct results from the EFI OR is it the depth of the plenum instead. Just curious.

Just some thoughts,
CBODY67
 
Assuming that your engine did not backfire thru the throttle body before you installed the spacer, I think it is likely that:
a.) There is a vacuum leak at the plate due to a bad gasket or warped plate.
b.) The addition of the adapter may have messed up the fuel ratios by moving the throttle body up from the intake port. Could be a turbulence issue. If the car is firing back thru the throttle body, that suggests that the system is running too lean, you can try fattening up the mixture settings to see if that solves the problem. Be sure there are no vacuum leaks at the throttle body and adapter plate first! Try a squeeze bottle with a small amount of fuel around the edges of the plate and throttle body, if the engine speeds up, there is likely a vacuum leak. Try these and report back.

Dave
 
Vacuum leaks can be important, but I've observed that small vacuum leaks can be compensated for by the computer reading the A/F ratio, enriching the injector amounts automatically until the desired A/F is maintained.

Just a thought,
CBODY67
 
Thank you for the suggestions. I will try the spraybottle test probably tomorrow. However, CBODY67 you are correct, the computer adjusts til its got the AFR correct.

Anyother way I would have seen a vacuum leak would be the Idle Air Control valve. It would basically close more when some air comes in elswhere. Thats not the case and a quick test i did was plug the hole for it with my thumb and the engine almost choked, so I would say 95% not a vacuum leak. But I'll check anyways!
 
What brand of EFI did you add? I presume the situation you describe started after the EFI was installed or just when the additional spacer was installed?

On the 3000rpm light load timing amount . . . Most engines in the later '60s and such, in the 2500 rpm cruise mode (of 2500rpm "free load" mode) would usually have a total advance figure in that mode of about 50-54 degrees BTDC. WOT (high rpm, high load) distributor+base timing would be where the desired 38 degrees BTDC timing would be optimum, with appropriate fuels.

With these EFI systems, is the type of intake manifold (dual or single plane) important for correct results from the EFI OR is it the depth of the plenum instead. Just curious.

Just some thoughts,
CBODY67
It is a Fitech system. The timing is newly added and i did have some issues with noise interference that i had to sort out. I actually have not driven the car in 2 monthS because of this (dont wanna stall in traffic...and thats EVERYWHERE). however, i do not get this interference anymore just runnig it in the garage or going up and down the road here.
To the timing, in theory i could put 50° in the computer..yes. However the computer has a range of about 30° so i would need to stab and phase the distributor at 20°. that could be difficult and creat crossfire withing the terminals in the cap! and the issue is that idle timing needs to be above the initial timing the distributor is stabbed in. So there are benefits but also limitations to it.

The intakes do matter, yes. The Fitech runs fine on dual plane intakes, however a single plane is "supposed to" be better for overall fuel distribution since all 4 barrels open at the same time. I had a 4-hole spacer first which was said to be the worst that can be done with the EFI so i added an open on and that is supposed to even out any fuel distribution issues on the dual plane. Thats all I know from my research. Oh and the darn thing is HIGHLT sensitive to EMI/RFI.. that definately caused me a lot of grief and i have lost trust in the reliability a little.
 
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Where is the RFI and such coming from? Internal vehicle or external to the vehicle? Thick aluminum foil can be an effective interference shield, sometimes.

How tall is the plenum divider in the dual-plane intake? Single-planes should be better, in relation to fuel distribution, as all cylinders feed off of a common plenum. Except for #5 and #7, with #7 firing right after #5, and pull from the same corner of the plenum.

Trying to consider the dynamics of this situation.

CBODY67
 
Where the rfi is coming from i dont know but the wire from the distributor to the ecu picks it up. I imagine from the ignition. I used copper wire shielding and gounded it. Also used a ferrite core around all high current wires and also on said pickup wire.

Now to the backfire: I did remove the spacer and it got better. I did adjust my accelertor pump shots back down where they were. Now i got something different happening, if i stab the throttle and let go of it, my AFRs go crazy rich to the point the car wants to die. If you know any EFi, do you know whats responsible for that? there is a decel fuel cut bit bit only works at cruise rpms. I also have "tipout" setting for various temps but i know too little about what they do!?
 
One reason for the "fly by wire" EFI on OEMs is that the rate of throttle opening can be modulated by the computer. Rather than with a manual throttle cable, when the throttle is suddenly WOT, enrichment CAN be too much at that point in time, which hurts emissions and fuel economy.

Remember that with EFI, the microsecond the throttle linkage moves, AFR is affected and altered by the computer's TPS signal. No matter of rpm. On a carb, when the throttle is slammed open, nothing happens (outside of the accel pump shot) until air velocity through the carb's throttle bores increases. This is why a steady throttle foot is necessary for best EFI fuel economy.

Also, with all FOUR throttle blades opening at the same time, that's probably toooo much potential air flow way too quick for the engine to use. Much less need. Effectively "smothering" it with air and fuel mixture (as if it really needs it) at too low of an rpm to use very much of it at all. Think of the secondaries on a 4bbl opening at 1500rpm without an air valve mechanism to modulate the air flow levels.

When the earlier TBI units were coming out, many were "4bbl" units with advertised flow capacities of 1000cfm, supporting over 500 horsepower with the supplied injectors. I'm thinking that's neat, but it's the same flow capacity as a Holley Dominator carb, a "race only" item. Too much air flow for most engines to handle, much less need, much less the reasonably stock engines many could be installed on. As a result, other than the elevated price back then, I passed for a Holley 2bbl TBI that I bought later on, before the feedback loop computers were in vogue. Be that as it may.

Keep us posted on your progress.

CBODY67
 
There are definitely drawbacks on the "all 4 barrels opening" especially on a smaller displacement engine. It doesn't give a good MAP resolution because it jumps to 95kpa relatively quick. However that just requires some adjustment on the breakerpoints. I don't see the air/fuel as much as an issue because you set what kind of AFR you want and thats exactly what it injects...

I am just starting to get fed up with it, didn't want to introduce too many electronics and expected this to be bolt on and go. I am getting close to eventually go back to carb... just really frustrated with this efi.
 
There are some YouTube vids on the FITech system's quirks and such. MANY of them, from what I've seen. Seems there's an "advanced" screen you need to get into?

Will the car start/run/idle reliably as long as you don't get too wild with the throttle movements?

CBODY67
 
oh yeah I am more in the "advanced advance mode now" I am waaay beyond just enabling the pro tuning. I had to manually midify the VE table with the pro tuning computer program. mind you I had this system for 3 years now and had some tipin lean conditions, horrible gasmileage and such..starting issues.the thing "never ran 100% right". So i decided, lets add the timing control so I can be 100% i have the right ignition advances and dont need to mess with springs and such. I added this and then my IAC motor coincidentally died giving me "random stall"... it idles and suddenly its like you turned off the key. Fixed that, car ran fine, i take it out a week later take off from a light and the car instanly stalled. Lucky I got it home but gave me crazy rpm readings because of noise interferance. Added all the shields and ferrites but haven't really taken it out because I do not trust it anymore to be reliable and i can't just have it randomly stall whith all the texting idiots driving around me here at any time of the day/night.
 
Hate to hear of all of the accumulated frustrations over such an extended time! How did you finally get around the fuel economy issue? Just curious.

CBODY67
 
Hate to hear of all of the accumulated frustrations over such an extended time! How did you finally get around the fuel economy issue? Just curious.

CBODY67

I didn't. Last time I got 6mpg. The well tuned holley i had before had no issues getting 11mpg around town. But again, the carb only lets so much fule through as the jets allow. the efi squirts in whatever condition to reach the target afr afr or what "it thinks it needs to squirt". I bought this efi to get better economy because of fuel shutoff when closing the throttle. that wont help me if that only enables at 2500rpm!? I put around town mostly at or below 1800rpm. At 2500 i drive 80mph... the care sees this speed close to never.

Anyways, I was hoping to get some better economy from having the timing control. Can't say anything to that yet since it has not been running reliable enough to go for a real drive.
 
I thought these FiTech (and others) were self-tuning and almost plug-and-play? I didn't believe that hype 10 years ago, but I thought they would've gotten there by now.

There are 2 books out there on EFI tuning by Greg Banish, get them if you can. Lots of good insight in them, he is a former (or current?) GM EFI calibrator. 1 thing he said is that fueling and timing will never be right/will cause confusion if the airflow modeling isn't right.

Also - do you have your 12v power to the EFI directly at the battery? I've read in numerous instruction manuals that connecting them anywhere else can cause strange anomalies due to voltage fluctuations (which the battery buffers out).


Now to the backfire: I did remove the spacer and it got better. I did adjust my accelertor pump shots back down where they were. Now i got something different happening, if i stab the throttle and let go of it, my AFRs go crazy rich to the point the car wants to die. If you know any EFi, do you know whats responsible for that? there is a decel fuel cut bit bit only works at cruise rpms. I also have "tipout" setting for various temps but i know too little about what they do!?
I wonder if you are getting too much fuel film on the intake walls, from having MAP go low with high throttle opening, yet not enough airflow to carry it? (kinda what Cbody67 said). I also

I didn't. Last time I got 6mpg. The well tuned holley i had before had no issues getting 11mpg around town. But again, the carb only lets so much fule through as the jets allow. the efi squirts in whatever condition to reach the target afr afr or what "it thinks it needs to squirt". I bought this efi to get better economy because of fuel shutoff when closing the throttle. that wont help me if that only enables at 2500rpm!? I put around town mostly at or below 1800rpm. At 2500 i drive 80mph... the care sees this speed close to never.

Anyways, I was hoping to get some better economy from having the timing control. Can't say anything to that yet since it has not been running reliable enough to go for a real drive.
 
I am a little further in investigating. It actually fouls the plugs. Also I know that it goes way rich whrn i hot the throttle, I just dont know why it backfires through the throttlebody. I am rechecking all the timing tomorrow and see if anything is off there. So far there has been nothing easy or self learning about this. I dont get why they cant make a software wherr you put your "accel afr" in and it adds/substracts fuel til it reaches that. I mean... it can do it steady state driving so why not!?
 
Does that thing ever seem like it's in "feedback loop" mode? Like OEM EFI does after the coolant temp or heated oxy sensor heats up?

Seems like the software should be better than it is! Apparently. If "stoich" is 14.7 AFR at cruise, then I'd think that should be a stated parameter. Then put in the CID and cam specs. I would think that with those "targets", the software should "learn" and customize the fuel map from there. Then let the timing map be adjusted to mimic an OEM curve, or perhaps a little quicker. Or "quick" until the detonation limiter might kick in.

I haven't read a FITech installation/tuning manual, but there are terms they are using that don't make any sense to be. Some of those values could be set to mimic general OEM situations rather than being customizable, I would think, which should simplify things greatly. Plus enhancing the stated "self-learn" operation.

With you and others mentioning the fuel economy issue, something's not right in their whole system, especially a self-learning system. Or perhaps the injectors are so over-capacity that they can't be throttled-down enough for a generally stock motor? Gotta be something, somewhere!

Those early Holley EFI systems had a computer that you adjusted manually for best results. The later ones could have an oxy sensor for feedback loop WOT control. Where you had to weld-in a bung for the oxy sensor, where many current units have a more clamp-in bung arrangement. I believe there's a particular angle the oxy sensor had to be at.

Just some thoughts,
CBODY67
 
well,a tuning manual doesn't really exist for the fitech. They have a manual explaining what certain parameters do but thats very not in debt. It does self learn and has the feedback loop of course. However it only learns steady state cruise AFR based on map, tps and rpm. You have to manually adjust your warmup, crank, accel fuel...all that. Their software is definitely not as sophisticated as holleys.

What ive done now is, reset the whole thing, im im gonna let it relearn and see what that does. If that wont change things, next summer my car will run with a carb again. Im fed up with it, might not even be the system but my inability to tune an EFI correctly. I tuned my carb with a wideband o2 no problem but this EFi is a challange...
 
Can you post your engine specs - CID, heads, cam 'class' - is this a stocker or modified?
Do you feel like your 11mpg with the Holley was reasonable for your car?

For the EFI -
I am wondering if the single-plane, open plenum is part of your problem?

For a carb, the dual-plane makes 1 cylinder breathe thru only 1 venturi of the carb to help with velocity and vacuum, then if secondaries open those kick into the same passages.

But with Fi-Tech and the others that have 4 injectors/4 throttle bores, all working simultaneously, you lose that velocity. The fuel added is the same (theoretically) because the AF and the MAP are handshaking (if in feedback mode), but MAP must surely be reduced because a cylinder is now breathing thru 4 bores. And an injector that can support 100hp may not be suitable for low-speed, low rpm work, as Cbody67 mentioned earlier. And a TB that can flow 100hp of air/bore could also have some tip-in issues.

In addition, the FiTech and others are dumping the fuel thru metered holes in the casting (thru some patented design?) while the older GM and Holley TBs spray the fuel onto the throttle blades, which helps whisk that fuel off. Does FT dump above or below the blades?

FWIW -
I am of the belief that a street car with an OEM-class of intake, whether port or TB EFI, does not have a huge benefit to a single-plane intake or open plenums. With a TB and wet intake the fuel distribution needs are still there, and with port EFI you still want the air to be balanced for each cylinder and have good velocity. Now with a highly-modified engine, with huge-port intakes, and heads, you probably want it uncorked instead.
 
I've been on a classic fast MPI since 2009. IMO you need to phase the dizzy and set it up to allow you to run up to 60* adv. I set mine in 60 btdc and checked the rotor phasing. Without this mpg is going to suffer.
 
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