Backfire through intake EFI & Carburetor

Tuning concepts, methods, tips etc.

Moderators: jsmcortina, muythaibxr

svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Backfire through intake EFI & Carburetor

Post by svocapri »

I put a ms2 v3 on my 85 Mercury Capri back in 2009. It ran OK with a Holley 4bbl TBI and ford Duraspark distributor with timing locked and megasquirt firing the coil. Last year after sitting for three years, I dropped in a freshly rebuilt 302. From the beginning, it has had a problem with backfiring through the intake below 2500 rpm. It does it when I open the throttle as if it doesn't have enough acceleration enrichment.

Currently, it is running MS Extra 3.4.2. The new engine has a factory roller cam, mildly reworked Dart Windsor heads and a Weiand Stealth manifold. It has the same distributor from the previous engine, and the same Hollley 4bbl TBI.

This is what I have tried so far to fix it...

Added acceleration enrichment until the car would stumble from being rich on acceleration
Loosened the valves until they were clacking thinking I might have had a valve too tight
Adjusted the distributor phasing thinking the spark was jumping between cylinders

Finally I have reverted to an old carburetor that I had running on the car before the megasquirt and it is still backfiring. I have a datalog attached from today with the carb on it. I don't have a TPS on the carb but if you look at the points where the MAP goes over 100% those spikes are the backfires. I even removed the secondary linkage on the carb so I am only dealing with the primaries, and still no better. The megasquirt is still controlling timing with the carb on.

Can anyone offer me some troubleshooting advice? Is there something physically wrong with this engine?
slow_hemi6
Super MS/Extra'er
Posts: 4122
Joined: Fri May 07, 2004 3:33 am
Location: Australia

Re: Backfire through intake EFI & Carburetor

Post by slow_hemi6 »

It has to be firing at the wrong time/cylinder. Is the new engine HO firing order or the older firing order? how did you set/check the rotor phasing?
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

The new engine has a factory HO cam with the HO firing order.

For the rotor phasing I...
Spun the crank over until the timing pointer showed 25degres btdc
Adjusted the distributor so the rotor lined up with the terminal
Tightened the distributor hold down bolt
Rotated the engine back to 10 degrees btdc
Adjusted the VR sensor in the distributor so it lined up with the tooth on the reluctor.
jamies
Master MS/Extra'er
Posts: 523
Joined: Tue Apr 07, 2009 12:22 am

Re: Backfire through intake EFI & Carburetor

Post by jamies »

do a compression test incase the intake valves are being held open - ie incorrect pushrod length
whittlebeast
Super MS/Extra'er
Posts: 2221
Joined: Tue May 04, 2004 8:20 pm
Location: St Louis
Contact:

Re: Backfire through intake EFI & Carburetor

Post by whittlebeast »

Regarding timing, I would start by checking the timing with a timing light at the crank on a running motor.

The motor needs to be tuned.

Andy
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

I am catching a theme that I need to look at the ignition. I was reading this forum yesterday and ran across this post referencing a mustang with a similar VR distributor pickup that was getting interference from the tach-output wire and causing the car not to run.
I was about to bypass the MSD box and saw the tach-output wire (for the dash gauge cluster) running along the megasquirt trigger wire. Idk if it makes sense for that to be an issue, but I unplugged that wire and the car now idles and revs a little.
I have my ignition trigger( bip373) mounted in a box under the hood and grounded through the distributor. I think I need to move that ground over to the intake manifold. I noticed in my logs that the RPM's were jumping wildly whenever the AFR would go lean. Is it possible that the coil is requiring more power to spark across the lean mixture and introducing noise that's falsely triggering the VR circuit?
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

Well, that didn't quite work out. I re-mounted my coil and ran new wires to it so they are about 6" separated from the distributor pickup wires. I also grounded the ignition module to the engine block instead of the distributor. It seems a little less prone to backfiring but it still does it.
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

Can someone tell me if this is a normal amount of RPM noise? The log is attached if the extra data helps.
2017-02-12_20_rpm_noise.png
turbo conversion
Super MS/Extra'er
Posts: 1281
Joined: Thu Aug 19, 2010 6:22 pm
Location: White House, TN USA

Re: Backfire through intake EFI & Carburetor

Post by turbo conversion »

Two things I would try after looking at your data log.

The first thing I would do is turn off EAE until you get your AFR and VE maps dialed in.

EAE will only make things worse trying to tune AFR and VE maps.

The other thing is your TPS is stuck at -14.8 even when the throttle is being moved.

I find it odd that even when TP AE is triggered TPS stays at -14.8 at 369.896s, not sure why.

David

EDIT: word
1976 Datsun 280Z L28ET Garrett GT35R T3-T04E stage3 50 trim 63 A/R housing custom grind cam 2000-6000 rpm 440cc injectors intercooled 18 lbs. boost
3" exhaust turbo back LC-1 o2 sensor Hallman manual boost controller EDIS 6 ignition batch fire 60mm throttle body 5 spd T5 borg warner 3.54 lsd
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

David,
Thanks for the input. The reason for the TPS at -14.8 is because it is currently running a carb with no TPS and therefore no EAE in play either. I never could get the fuel dialed in without it backfiring through the intake so I resorted to a carb that was pulled from a similarly equiped 302 mustang which didn't exhibit any significant stumbling or backfiring problems. The datalog you are looking at is with that carb, and it is backfiring through the intake on this engine.

I am thinking that the ignition, which is still being controlled by the Megasquirt, is too noisy and when the RPM's are going haywire that is when it backfires through the intake.

Can anyone shed some light on weather my RPM graphs are noisy or am I barking up the wrong tree?

Thanks,
Julian
jsmcortina
Site Admin
Posts: 39587
Joined: Mon May 03, 2004 1:34 am
Location: Birmingham, UK
Contact:

Re: Backfire through intake EFI & Carburetor

Post by jsmcortina »

It sure sounds like false triggers on the tach input are the source of your problems.

James
I can repair or upgrade Megasquirts in UK. http://www.jamesmurrayengineering.co.uk

My Success story: http://www.msextra.com/forums/viewtopic ... 04&t=34277
MSEXTRA documentation at: http://www.msextra.com/doc/index.html
New users, please read the "Forum Help Page".
turbo conversion
Super MS/Extra'er
Posts: 1281
Joined: Thu Aug 19, 2010 6:22 pm
Location: White House, TN USA

Re: Backfire through intake EFI & Carburetor

Post by turbo conversion »

svocapri wrote:David,
Thanks for the input. The reason for the TPS at -14.8 is because it is currently running a carb with no TPS and therefore no EAE in play either. I never could get the fuel dialed in without it backfiring through the intake so I resorted to a carb that was pulled from a similarly equiped 302 mustang which didn't exhibit any significant stumbling or backfiring problems. The datalog you are looking at is with that carb, and it is backfiring through the intake on this engine.

I am thinking that the ignition, which is still being controlled by the Megasquirt, is too noisy and when the RPM's are going haywire that is when it backfires through the intake.

Can anyone shed some light on weather my RPM graphs are noisy or am I barking up the wrong tree?

Thanks,
Julian
I remember reading that in the first post days ago but forgot it when I posted a response. :oops:

David
1976 Datsun 280Z L28ET Garrett GT35R T3-T04E stage3 50 trim 63 A/R housing custom grind cam 2000-6000 rpm 440cc injectors intercooled 18 lbs. boost
3" exhaust turbo back LC-1 o2 sensor Hallman manual boost controller EDIS 6 ignition batch fire 60mm throttle body 5 spd T5 borg warner 3.54 lsd
5.0Thunder
Experienced MS/Extra'er
Posts: 256
Joined: Sat Mar 07, 2015 4:46 pm

Re: Backfire through intake EFI & Carburetor

Post by 5.0Thunder »

svocapri wrote:I am catching a theme that I need to look at the ignition. I was reading this forum yesterday and ran across this post referencing a mustang with a similar VR distributor pickup that was getting interference from the tach-output wire and causing the car not to run.
I was about to bypass the MSD box and saw the tach-output wire (for the dash gauge cluster) running along the megasquirt trigger wire. Idk if it makes sense for that to be an issue, but I unplugged that wire and the car now idles and revs a little.
I have my ignition trigger( bip373) mounted in a box under the hood and grounded through the distributor. I think I need to move that ground over to the intake manifold. I noticed in my logs that the RPM's were jumping wildly whenever the AFR would go lean. Is it possible that the coil is requiring more power to spark across the lean mixture and introducing noise that's falsely triggering the VR circuit?

This was in my post and that issue/solution has me and a few other experts confused, but it fixed it so I'm just going about my business. Lol I have an MSD Dizzy with VR sensor and the VR wires are running separate from any other wires, shielded as well. the Megasquirt trigger wire (from BIP373) was running alongside the Tachometer wire from the MSD 6AL and that is where the issue was. I, and a few other people, didn't think noise was a problem for the Megasquirt trigger wire but as soon as I removed the Tachometer wire from the MSD 6AL, all noise issues went away and the car ran.
1990 Mustang: MS3Pro Gen 1, 306ci, 72mm turbo, 80lb inj, 36-1 + Cam Sync + D585 coils, World heads, TFS1 cam, GT40 intake, auto, Speed Density
1987 Mustang: MS2v3.57, direct coil control, MSD VR Dizzy, SBE, Weiand 174,190cc heads, F303 cam, TBI w/ (4) 160lb inj, GT500 MAF
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

Well, I finally have made some progress on this. I re-wired the distributor pickup to the Megasquirt with a higher grade shielded wire, and I re-wired the ignition module to get the high current wires away from the distributor pickup. I was thinking that maybe there was some electrical noise causing my problem. I also swapped my coil to a MSD blaster. None of these fixed the problem.

I had a friend come over and we put the Holley throttle body back on. When I went to check for fuel leaks we found a leak between the injector pod and the throttle body. Four new o-rings fixed that and then things started to improve.

We drove for about an hour and let TAL tune the fuel map. It added fuel between 1400 and 2000 RPM above 95%VE, and that really helped a lot. The way we got to those cells was by running in fourth gear about 1200RPM and rolling into the throttle so that it would hit full throttle with no more than a 200 TPSdot. The first few times it really wanted to break up and sputter, but as it bumped the VE over 100% it got smoother and smoother.

I am confused by the very high VE between 1400 and 2000. It seems unreasonable to me, but the engine will backfire through the intake with anything less that 100%VE in that area.

Can anyone shed some light on this?

Julian
Screen Shot 2017-04-28 at 11.00.45 AM.png
slow_hemi6
Super MS/Extra'er
Posts: 4122
Joined: Fri May 07, 2004 3:33 am
Location: Australia

Re: Backfire through intake EFI & Carburetor

Post by slow_hemi6 »

It is not just adding fuel there it's adding fuel in the 3200/3500rpm 40 to 65 kpa range as well.
My guess is you have over estimated your injector size (which varies with supply pressure) this is producing a smaller required fuel calculation which means the VE numbers are ending up bigger than you might expect.
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

slow_hemi6 wrote:It is not just adding fuel there it's adding fuel in the 3200/3500rpm 40 to 65 kpa range as well.
My guess is you have over estimated your injector size (which varies with supply pressure) this is producing a smaller required fuel calculation which means the VE numbers are ending up bigger than you might expect.
I have suspected that was going on so I re-scaled my VE table and required fuel to bring the largest cells in my VE table closer to 100%. I guess I should be asking if it is normal for the peak to be over 25% greater than RPM's on either side of it? Also is it normal for there to be an area where the VE is greater at part throttle than at full throttle like the 3200/3500rpm 40 to 65 kpa range? I have run TAL on full throttle 97kpa area from 1500 to 5500 well over 30 times and I am reasonably confident that those cells are close to where they should be. Likewise, I reduced the 3200/3500rpm 40 to 65 kpa range below 100% VE and sure enough then next time I auto tuned that area it came back up.

If all of this is just some sort of resonance making certain areas of the map extra efficient, I am fine with that, but I am just concerned that I have something boogered up.

Thanks for you help.

Julian
billr
Super MS/Extra'er
Posts: 6828
Joined: Sun May 15, 2011 11:41 am
Location: Walnut Creek, Calif. USA

Re: Backfire through intake EFI & Carburetor

Post by billr »

If you have "rescaled" the req_fuel value from the correctly calculated one, then yes, you have something "boogered up". Req_fuel is a value defined by your engine displacement, injector flow, and fuel type. It must be that fixed and calculated value, or else many other settings will not follow normal logic, and you/we will be in a long process of trial-and-error guessing.

As to the backfiring into the intake, I see no way for that to happen unless an intake valve is leaking or the spark timing is bad. Mixture quality shouldn't matter much, if at all. Yes, there is a bit of valve overlap when both the exhaust and intake valve are open, but I feel it is very rare that an NA engine would still have enough active combustion at that point to ignite the intake mixture unless spark timing was horribly retarded.

Check you compression, maybe even leak-down on all cylinders, but valves probably aren't the problem. Something probably isn't quite right with that basic-trigger dizzy system, and the most practical way to "fix" it is to convert to a missing-tooth crank wheel with wasted spark. If you want to continue with the dizzy, though, then spend a lot of time with a (simple, not dial-back) timing light; using it on all cylinders.
kjones6039
Super MS/Extra'er
Posts: 1986
Joined: Sat Jul 22, 2006 8:02 pm
Location: Eureka, NV USA
Contact:

Re: Backfire through intake EFI & Carburetor

Post by kjones6039 »

I hate to ask the question but.............. You sure you don't have a couple plug wires crossed?

Ken
1979 Corvette - 383 CID SBC w/ Holley Pro-Jection 900 CFM TBI, 4-85 lb lo-z injectors & Walbro 255 pump
MS2 v3 w/extra 3.4.2 Release
36-1, Delphi LS2/7 coils in wasted spark, driven by v2.0 logic board from JBPerformance
Spartan Lambda Sensor from 14point7
TinyIOX from JBPerformance
slow_hemi6
Super MS/Extra'er
Posts: 4122
Joined: Fri May 07, 2004 3:33 am
Location: Australia

Re: Backfire through intake EFI & Carburetor

Post by slow_hemi6 »

VEAL will also add fuel to compensate for an ineffective Acceleration enrichment. This obviously makes some unusual bumps in the VE table.
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
svocapri
Helpful MS/Extra'er
Posts: 51
Joined: Sat Nov 29, 2008 7:43 pm
Location: Rome, Georgia

Re: Backfire through intake EFI & Carburetor

Post by svocapri »

Just for clarification, the backfiring seems to be gone now that I have a richer mixture between 1400 & 2000, and I have increased the accel time from 0.2 seconds to 0.4 seconds and increased the taper time from 0.0 seconds to 0.4 seconds. If i slam the throttle to the floor I can make it stumble, but i no longer see the MAP exceeding 100KPA during the throttle transition which I read as a backfire.
kjones6039 wrote:I hate to ask the question but.............. You sure you don't have a couple plug wires crossed?
Yes, i am sure the plug wires are not crossed and that the firing order is correct. I can even run overdrive at 25mph (900rpm) give it moderate throttle and have it smoothly accelerate.
billr wrote:If you have "rescaled" the req_fuel value from the correctly calculated one, then yes, you have something "boogered up". Req_fuel is a value defined by your engine displacement, injector flow, and fuel type. It must be that fixed and calculated value, or else many other settings will not follow normal logic, and you/we will be in a long process of trial-and-error guessing.
Bill, maybe you can help me with the required fuel conundrum. I emailed Holley the part number of my injectors and they replied that my injectors are the 65pph at 21psi. I have my fuel pressure set to 15psi according to the fuel pressure gage plumed into the throttle body which should be 55pph flow. When I calculated required fuel based on four 55pph injectors and then ran autotune, a very large portion of my VE table came out over 100%. Because of this, I multiplied the entire VE table by something like 0.8 to bring the highest value down to 100% and then I multiplied the required fuel by 1.2 to compensate.

Is it possible I have my peak and hold settings wrong for these injectors? Could that cause this discrepancy?

I have also tried to get the dead time set properly for these injectors. Based on the specs from Holley, I generated this spreadsheet to calculate the dead time and voltage correction which is what I am currently running in TunerStudio. When swapping from eight squirts alternating to four squirts alternating, the VE table doesn't seem very far off using these values.
InjectorDeadTime.png
Post Reply