tbi backfiring

[chevyman_400]

I am about to pull my hair out..... I have an tbi set up I have installed on an 1972 amc 304. I orderd the kit from afi in hopes that it would be plug and play as I am going this for a customer and wanted it to be fast and esy. I have now decided that I should have built my own like i did on my onwn cars. that said I am tied of dealing with afi and not getting any where. I have tuner pro and an apu1 from moats..... so on to the truck. it is a stock amc 304 in a stock jeepster. it starts and idels relitvely smooth but on heavy tip in it backfires throught the tbi. the current tune I have on it is a stock 305 tune with the egr and vss flags shut off. I got rid of the afi chip due to it running the timming up to 41degs at idel. I thought that was my problem. how ever it was not. if I increase fuel press (from 11 to 25 ) the severity gos down but it is still there. sorry for the rambleing and spelling. If there is any other info I missed let me know Thanks for the help Joe

[RobertISaar]

if more pressure helps, then i'd say you're going very lean on tip-in, since all other things being equal, adding more pressure causes more fuel to flow out of the injectors.

[PJG1173]

I am about to pull my hair out..... I have an tbi set up I have installed on an 1972 amc 304. I orderd the kit from afi in hopes that it would be plug and play as I am going this for a customer and wanted it to be fast and esy. I have now decided that I should have built my own like i did on my onwn cars. that said I am tied of dealing with afi and not getting any where. I have tuner pro and an apu1 from moats..... so on to the truck. it is a stock amc 304 in a stock jeepster. it starts and idels relitvely smooth but on heavy tip in it backfires throught the tbi. the current tune I have on it is a stock 305 tune with the egr and vss flags shut off. I got rid of the afi chip due to it running the timming up to 41degs at idel. I thought that was my problem. how ever it was not. if I increase fuel press (from 11 to 25 ) the severity gos down but it is still there. sorry for the rambleing and spelling. If there is any other info I missed let me know Thanks for the help Joe

IDK increasing the fuel pressure from 11-25 is a huge jump. I would think that is masking a bigger problem.

[EagleMark]

Timing at 41 degrees at idle? Did you set timing to 0 with wire disconected? What ignition/distributor is it running?

Could be so many things... first was the engine sound before you started? Compression check? How is vacuum now. what reading at idle and is it steady? Fuel pressure should be close to 13 PSI and a stock engine really needs no more. Was the TBI unit rebuilt? Does it have new and properly working regulater? Were injectors serviced? Is the MAP sensor hooked to port on back of TBI between fuel lines? Is hose to MAP a big thick solid enough hose that it does not collapse?

What your experiancing can also be a vacuum leak... so, so common on conversions. When I worked at AFI the kits used to come with adapter plate and TBI bolts but bolts were to long and went through adapter plate and hit intake causing a vacuum leak. Then we added a washer with bolt and lock washer to prevent the problem. Last year I worked on one and sure enough the washers were gone and had a vacuum leak caused by bolt coming through adpter hitting intake.

[chevyman_400]

Timing at 41 degrees at idle? Did you set timing to 0 with wire disconected? What ignition/distributor is it running? Could be so many things... first was the engine sound before you started? Compression check? How is vacuum now. what reading at idle and is it steady? Fuel pressure should be close to 13 PSI and a stock engine really needs no more. Was the TBI unit rebuilt? Does it have new and properly working regulater? Were injectors serviced? Is the MAP sensor hooked to port on back of TBI between fuel lines? Is hose to MAP a big thick solid enough hose that it does not collapse? What your experiancing can also be a vacuum leak... so, so common on conversions. When I worked at AFI the kits used to come with adapter plate and TBI bolts but bolts were to long and went through adapter plate and hit intake causing a vacuum leak. Then we added a washer with bolt and lock washer to prevent the problem. Last year I worked on one and sure enough the washers were gone and had a vacuum leak caused by bolt coming through adpter hitting intake.
I tried to answer in order
that was set to 0 degees with the wire disonected( afi said just leave the wire connected and turn it down to baout 25. That is why I have started over witha stock tune which now reads 25 at idle) the distributor is the one afi sent with the kit it apperers to be a holly diss with a magnetic pickup. that is hooked in to the gm modual

The engine sounds good and even cranking. compresstion is in the 135-145 range. Vacuum is 17 at idle and very steady. fuel press is at 11 right now. the tbi looks to have
been rebuilt as it came from afi the regulator seems to be working and holding steady. they sent it with 4.3 liter injectors i have sence put new 305 injectors in it. the map is hooked to the factory location in the center back of the tbi. I will check the hose it is a new hose. but that does not mean much

I will check again for a vaccum leak around the tbi.

[chevyman_400]

the map hose is not colapsing and there is no leaks around the tbi

[POZE]

You didn't say if the engine is rebuilt but I have seen a worn exhaust cam lobe cause this. They will idle good and show good cranking compression but under load pop in the intake. On the exhaust stroke the cylinder builds pressure and releases this pressure when the intake valve opens. Just a thought.

[EagleMark]

That should show up on vacuum gauge as a shake/jump.

Sounds like they are using the Duraspark ignition module connected to an external EST module. Been through about a dozen bad brand new Duraspark modules this year so here's how to test them. They run even when bad but if any of these tests fail the pickup is no good. Same symptoms you are having with poping. Timing light seems steady at idle and goes nuts when you rev.
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?255-Dura-Spark-Stator-Testing-Diagnoses-TBI-Conversion

That's just BS to set timing with wire connected!!! Did you happen to check timing with engine running and see if it was advancing or retarding. Should be set correctly with wire disconnected to 0. Clear codes and reconnect and check timing, should be 15 to 24ish at idle and drop when accelerated and then advance after that. If it is backwards swap the purple and orange (Usual colors). If when checking timing with light and timing is not dropping a little at first acceleration then advancing smoothly. Like going nuts all over back to above DuraSpark module.

EST grounded and mounted on a heat sink?

[JeepsAndGuns]

When I first installed mine, it ran like POOP on all the stock chevy timing tables. It would backfire, spit, cough, just wouldnt go if I tried to hit the fuel even lightly. Has to accerate very slow. This was in my uber noob days and I had not learned just how much timing effects a engine. I finally found a 4.3 bin that had a timing table that was way diffrent (had more advance) than the V8 ones I was using. Diffrence was night and day, it would actually pull itself and not caugh, spit, and backfire. I then continued to smooth and build my timing tabe from that. One thing I found out, was my AMC engine likes a lot more timing than these chevy engines.

What ecm are you running? I have a bin for a 7747 I made you might try. Its got one of my early timing tables based off that V6 tabe. And it was before I started adding more timing to take advantage of higher octane fuel.

[chevyman_400]

here is a crappy cell phone pic of the diss. If this truly the druaspark I will run those tests in the am/.
http://i44.photobucket.com/albums/f8/chevyman_400/IMAG0253.jpg

I am running an 7747 ecm
I would love to look at a good running bin as I am very green in the tuning side of this and my boss is starting to get fustrated with me :)

[EagleMark]

That's the Duraspark! I'd check the gap too... But why does it have a vacuum advance on it? Does the rotor move like it has mechanical advance too? Shouldn't have either for spark control. Wonder if that open loop there is supposed to be grounded with a screw? Just looked at one I have here and no loop like that near rubber grommet for mount or ground. Is that what AFI sent you?

Once you get it running try to get the big cap adapter from Ford on there. It spreads out terminals further to prevent misfire.

You check that out and look at bin from Jeepsandguns and I'll throw a spark table in a 7747 bin for you too. Do you have knock sensor?

Here's the right way the kits used to come with...
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?21-Building-a-GM-EFI-Small-Cap-Distributor-for-TBI-Conversions

[chevyman_400]

That's the Duraspark! I'd check the gap too... But why does it have a vacuum advance on it? Does the rotor move like it has mechanical advance too? Shouldn't have either for spark control. Wonder if that open loop there is supposed to be grounded with a screw? Just looked at one I have here and no loop like that near rubber grommet for mount or ground. Is that what AFI sent you?


Once you get it running try to get the big cap adapter from Ford on there. It spreads out terminals further to prevent misfire.

You check that out and look at bin from Jeepsandguns and I'll throw a spark table in a 7747 bin for you too. Do you have knock sensor?

Here's the right way the kits used to come with...
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?21-Building-a-GM-EFI-Small-Cap-Distributor-for-TBI-Conversions

No knock sensor egr vss

this is exactly how it came from afi..... the vacuum advance is not hooked up. I just put a vacuum cap on it to try and stop some one else from trying. there is no mechanical advance on it. the loop is just a mount the ground wire is hooked up to the loop with the screw in ti on the other side.. what kind of air gap should I shoot for on the pickup as it moves fairly easy.

And thanks alot for every ones help.. it is nice to geet on a forum and get straight answers and honest help not just a run around

[droptopstng]

chevyman , Im not sure what ford speced the duraspark for as far as air gap in their service manuals but I have been setting them to 15 to 20 thou for ten years and havent had a problem but thank goodness these things are almost exstinct....... excuse the spelling please