Intake backfire during cold starts

[X-forces]

My first post on the forum. I am new to the Megasquirt world, and I was hoping to get a bit of insight.

I am currently working on the first phase of my MS project. Currently I am running a MS2 3.0 v2.920, the subject vehicle is a 1984 Buick Regal with a 3.8l v6. My set up is ignition only at the moment, as I am finishing up the last portions of the FI conversion.

The MS is configured for VR input, and the high current ignition driver. The VR signal is coming off of a stock GM HEI dizzy that was originally installed in the car. The dizzy is a 7 pin module type, but I have removed the module and I am running directly off the reluctor pickup. The MS is triggering the coil through a MSD StreetFire CDI.

My problem is that when I start the car after an overnight cold, it will backfire slightly through the intake during cranking. Once the engine starts, it runs like a champ. After the engine has run for about 3-4 minutes, I can start it all day long without any backfiring. I have narrowed down the issue to something the MS is doing to the ignition properties.

I removed the VR input from the relay board and wired it back directly to the MSD, the engine fires up without a hitch, with no backfiring during the cranking. Even with the dizzy locked at 8deg, which is where the offset is set, the engine started and ran. Once I hooked the VR back to the MS, and the MSD trigger to my S5, the engine would backfire again upon starting.

I am not certain what property of the ignition is being changed by the MS, but I have changed just about every setting in the Ignition Options that I can find, and none seem to resolve the issue.

I first though that I might be getting some signal noise from the starter, causing the VR to trigger the ignition erratically, however, when I look at the traces in Megalog there are no spikes in RPM. Megalog also shows that the timing never goes over advanced during cranking, and I have attempted to set the starting advance at 8deg by manipulating the spark table.

The one "abnormality" I have noticed is that while cranking my battery voltage drops to around 9.0-9.5, and the dwell goes up to 16.8ms. I am not sure if the increased dwell could be causing the problem or not, and if so how to address it.

I only have the battery voltage compensation set to increase the dwell by .9ms for -4.0v, so I am not certain why the dwell would shot up so high. I have the max dwell set to 7.5ms, however I had edited the .ini to allow for the 25.5 selection, and I have never edited it back.

The other ignitions settings -
Falling Edge
Trigger Rise
Standard Coil Charge
Going High(Inverted)

Has anyone encountered a situation like this? Better yet, anyone have any ideas?
Thanks in advance

[Bernard Fife]

X-force,

Back-firing into the intake usually means too lean a mixture. You might want to richen up your cranking pule width at low temps.

Lance.

[X-forces]

X-force,

Back-firing into the intake usually means too lean a mixture. You might want to richen up your cranking pule width at low temps.

Lance.

Thanks Lance,

However, this is an ignition only issue. My set up is only ignition at the moment, and when the ignition is not controlled through the MS, the problem goes away.

[X-forces]

Attached the wrong file in my first post.

[Bernard Fife]

My set up is only ignition at the moment

X-forces,

Whoops, sorry, I missed that in your original post...

Lance.

[X-forces]

My set up is only ignition at the moment

X-forces,

Whoops, sorry, I missed that in your original post...

Lance.

No problem buddy, I am sure that an ignition only set-up is probably a pretty rare bird round here. I just wanted to give the ignition side a test run before I dropped in the injection.

I managed to get it all sorted out (Still can't believe it), so life is good. I am looking forward to the next step in the next week or so. Thanks for trying to help.

[Alain V.]

Can you post what the solution to your problem was. This would be helpful for others who are searching for answers to similar issues.