1974tii Backfire Deceleration

[BCbryan]

So finally getting my car back on the road after a bunch of winter projects.  I recently put an AFR in and went through all the throttle linkage.  Car is running well, but I have a slight pop/backfire when I decelerate or coast in gear with foot off gas peddle.  I’ve read just about ever post on here with similar problems.

 

Valves have been adjusted to .007

Timing is bang on

New plugs, wires, cap/rotor.  Running a 123

AFR readings of high 12’s a idle, low 13’s at cruise and low 14’s at WOT under load.

 

I probably have a bit more fine tuning of AFR.  I have been doing this primarily with the intermediate shaft.  Factory settings were pretty lean, so I have now partially covered the hole.  Better readings at cruise, but did not seem to effect WOT much.

 

I feel like the backfire should be exhaust leak related or because it’s running lean.  I don’t think it’s a lean issue.  In fact it seems worse now that I richened up the AFT a bit.  I have inspected exhaust extensively and can’t seem to find anything obvious.

 

Any advice?

[jimk]

It IS running lean on deceleration.  If really lean it will not ignite fuel (flamout) in the cylinders and fuel builds up in the exhaust, when the throttle is tipped in, the fuel in the exhaust ignites and hence the exhaust pop.

[BCbryan]

Well this makes sense and what I suspect.  The AFR goes high when you lift off gas...16ish and then settles back down at idle.

 

So what is the fix for it running lean on decel?  If I richen it up more at the intermediate shaft then I’m rich across the map.  Richer idle?  
 

I’m going to play around with the tuna can mixture screw.  Richen idle more and see if I can make it go away.
 

 

[BCbryan]

Also wondering if it could be a vacuum leak.  I just replaced all the runner gaskets, hose from WUR, ect...  

[ray_]

What exhaust do you have?

 

I have read somewhere that tii's cut fuel on decel.

 

You won't hurt the motor with this specific issue; my S14 map cuts fuel on decel and the AFR gauge pegs lean.

 

Cheers,

[irdave]

My '72 tii would pop on decel.  Changed the exhaust manifold gasket and problem stopped...  Anecdotal, but worked.

[BCbryan]

I just replaced the header gasket with an E21 version with the shield.  Had the header out to put in AFR.  Put all new studs in and new copper bolts.  I guess it’s certainly possible that something is not perfectly sealed there.  Certainly not obvious.  The old gaskets has black RTV on the header (Metal) Side of the gaskets, but consensus from everyone I talked to was no RTV on re-install?

[Mike G]

Lean on lift is normal but you want WOT around 12.5-13. You’ve got an AFR so the verboten screw is fair game. Get WOT where you want it then tune in the idle/cruise with the tuna can. 

[jimk]

EFIs shut the fuel off completely, so no raw fuel passing thru.

If the fuel does not shut off completely the flame could go out as per my previous post.  Driver doesn't notice the non firing.

[jimk]

6 minutes ago, jimk said:

EFIs shut the fuel off completely, so no raw fuel passing thru.

If the fuel does not shut off completely the flame could go out (above 17:1) as per my previous post.  Driver doesn't notice the non firing.

 

[flagoworld]

I leaned up my 20kpa row on my VE table to make this happen on purpose. It’s a glorious sound ?

[irdave]

10 hours ago, flagoworld said:

I leaned up my 20kpa row on my VE table to make this happen on purpose. It’s a glorious sound ?

 

We need an emoji that's an old guy shaking his head at the kids.  Kids these days- Gimme the pops!  :)  Whole reason dude cracked the e46 325 ecu and open sourced it (well, he wanted to turbo as well), but for the pops.

[g_force]

"Pops" are a baked in feature for the sport mode on a lot of new cars. It is cheezy in my opinion. . . unless it's accompanied by flames shooting from the exhaust tips.  Then, it's Adam West Batman cool.

 

As an aside, I think AW's flames were supposed to suggest the Batmobile was turbine powered, but I still stand by my point. Exhaust flames are Adam West Batman cool.

Edited June 8, 2020 by g_force

[ray_]

3 hours ago, g_force said:

"Pops" are a baked in feature for the sport mode on a lot of new cars. It is cheezy in my opinion. . . unless it's accompanied by flames shooting from the exhaust tips.  Then, it's Adam West Batman cool.

 

As an aside, I think AW's flames were supposed to suggest the Batmobile was turbine powered, but I still stand by my point. Exhaust flames are Adam West Batman cool.

 

 

Yes.

 

That!

 

Cheers,

[AustrianVespaGuy]

22 hours ago, BCbryan said:

low 14’s at WOT under load

This ain't great; go richer and see if both improve!

FWIW, I 'experimented' a little with lean overrun in my Megasquirt, but found it REALLY tricky to get throttle tip-in just right so that it didn't STAY too lean when rolling back on the gas.  My conclusion was though kinda fun, wasn't really worth it ?