Backfire “trama” post rebuild

[mike sampsel]

Well I’ve got 330 miles now on my rebuilt engine (was a good challenge).
I’ve got CIS, fab speed cat bypass, and 20/21 cams, went from 3.0 to 3.2 with 98 mm 9.5 CR pistons. Oh and added a Carrera sway bar and Carrera muffler too.

Car is a lot loader than before, it’s loud like a turbo 3.0 with sport exhaust (maybe a bit louder).

The car runs hungry, wants to go. Acceleration is fine, on deceleration I get some put put sounds. Seems a put put backfire on deceleration with a fab speed cat bypass is fairly common, and not a concern?

On occasion when I run up to 5000 and shift from second to third I get a load exhaust backfire. Car pulls strong and has no issues just a bit embarrassing roaring by and then bang! I leaned my mixture a wee bit (1/8 turn CCW) and its still present. After a 50 mile run I tried three times to make a boom by going to 5000 rpm in first and then shifting to second (no boom).

Sounds like there is not a extreme concern for this backfire itself. But it would be desireable to remedy the issue.

Oh yes and my fuel is probably pretty clean.

And so it begins.

[Paulporsche]

You may have taken care of things by leaning the mix.

You should still have your mixture verified, though. 1/8 turn is actually quite a bit, maybe something like 1 1/2 or 2%.

You should also check to be sure your fuel and control pressures are in spec. If your WUR hasn't been reset since 1978, it's unlikely it's correct.

[mike sampsel]

Originally Posted by Paulporsche

You may have taken care of things by leaning the mix.

You should still have your mixture verified, though. 1/8 turn is actually quite a bit, maybe something like 1 1/2 or 2%.

You should also check to be sure your fuel and control pressures are in spec. If your WUR hasn't been reset since 1978, it's unlikely it's correct.

Yes, I‘lll likely do this although all seemed fine in April prior to start of rebuild. And it tested out fine in 2016 or so and 5 k miles ago.

[Paulporsche]

OK then. Your WUR is probably good, although you may want to verify your CO% just to be sure.

Also, do you know the field test for checking mixture?

If you raise the sensor while running and the revs rise, you can go richer. If you raise the plate and the revs fall, you can go leaner. No change indicates a good mix.

[Rawknees'Turbo]

When making engine changes like you did, it would be a good idea to check AFR readings at idle, cruise and full throttle, and then adjust your control pressures accordingly (assuming you have an adjustable WUR).

[mike sampsel]

Originally Posted by Paulporsche

OK then. Your WUR is probably good, although you may want to verify your CO% just to be sure.

Also, do you know the field test for checking mixture?

If you raise the sensor while running and the revs rise, you can go richer. If you raise the plate and the revs fall, you can go leaner. No change indicates a good mix.

Thanks for the reminder, I was aware of the test were if you just touch the sensor plate down a tad and the revs rise, too rich and if you raise it a tad and the revs rise too lean.

I'll give this and your method a shot.

[mike sampsel]

Originally Posted by Rawknees'Turbo

When making engine changes like you did, it would be a good idea to check AFR readings at idle, cruise and full throttle, and then adjust your control pressures accordingly (assuming you have an adjustable WUR).

AFR is ?Fuel Regulator?

Alas I do not have an adjustable WUR. Can they be set for various rpm?

[High Life]

You probably already did this - but I would verify your timing. Mine was off a bit and I put it into spec.

AFR is Air Fuel Ratio

When I had backfire issues - I "cured" it by resetting the AFR mixture also (using both the idler bypass screw and the 3mm allen screw on the bypass plate) as others suggested.

I installed an AFR gauge in order to get this dialed in.

Roughly - my idle (AFR) is in the 13 range (depending on air temp )..

[mike sampsel]

Originally Posted by High Life

You probably already did this - but I would verify your timing. Mine was off a bit and I put it into spec.

AFR is Air Fuel Ratio

When I had backfire issues - I "cured" it by resetting the AFR mixture also (using both the idler bypass screw and the 3mm allen screw on the bypass plate) as others suggested.

I installed an AFR gauge in order to get this dialed in.

Roughly - my idle (AFR) is in the 13 range (depending on air temp )..

Thanks,

Okay AFR is lambda. Timing was in spec, set at 7 deg Before TDC, maybe I should try 3 deg BTDC. 5 +- 2 is the spec at 950 rpm.

[crashmy911]

I would think going to better cams and increasing bore you would be lean. How did you adjust cis mixture from start?

[crashmy911]

I would buy a AFR gauge and install for peace of mind. You've spent lots of money and time with your build. Don't take any chances

[Rawknees'Turbo]

Originally Posted by mike sampsel

AFR is ?Fuel Regulator?

Alas I do not have an adjustable WUR. Can they be set for various rpm?

AFR stands for air-to-fuel ratio.

With an adjustable WUR you can tune the control pressures in order to get AFR numbers you are looking for (cold pressure, warm pressure - which includes cruising RPM, and full throttle enrichment - not sure if all WURs have a full throttle enrichment function, but someone such a boy911t/Tony or RarlyL8 would know this).

Your current WUR can be made adjustable - RarlyL8/Brian is an option for that.

Tuning via the WUR requires that you have an onboard AFR meter installed - shows you the air/fuel ratio of your car's exhaust gasses and then you can make control pressure adjustments accordingly (raise pressure to get a leaner mixture and reduce pressure to richen).

[mike sampsel]

Originally Posted by crashmy911

I would buy a AFR gauge and install for peace of mind. You've spent lots of money and time with your build. Don't take any chances

Well this is a complicated question (refering to your how did you question). I had a slight air leak at first so as I enriched the mixture to get to the stumble point the car died. After fixing the leak, from there is was turning the wrench until stumble then just back off. As with anything, I think one needs to acquire a feel to get this right.

I might search for a meter, just to have another toy.

[mike sampsel]

Well, I have a wideband sensor from innovate sitting in my garage!

I assume I can screw it into the bung on the cat bypass?

Then check the AFR at 950 rpm and others too.
At idle what should I try for a reading 13 - 14 reading?

I'll check my bentley for an SC I guess, some said it says "13.2 to 13.9". Sound reasonible?

[High Life]

This thread has valuable info on AFR installation and readings. Im assuming you have a Lambda sensor and a relay located under your passenger seat? What year is your SC?

AEM Wideband O2 gauges

[mike sampsel]

Originally Posted by High Life

This thread has valuable info on AFR installation and readings. Im assuming you have a Lambda sensor and a relay located under your passenger seat? What year is your SC?

AEM Wideband O2 gauges

No O2 sensor it's a '78

[Rawknees'Turbo]

Originally Posted by mike sampsel

Well, I have a wideband sensor from innovate sitting in my garage!

I assume I can screw it into the bung on the cat bypass?

Then check the AFR at 950 rpm and others too.
At idle what should I try for a reading 13 - 14 reading?

I'll check my bentley for an SC I guess, some said it says "13.2 to 13.9". Sound reasonible?

Yes to the cat bypass bung.

With no Lambda control on your system attempting to force an idle AFR of 14.6 (basically too lean for e10 gasoline - right on the edge of lean misfire/rough running), I would attempt to get about 13.4-13.6 at idle (very smooth running then). Of course, you can experiment by going a little richer or leaner and see what your engine's sweet spot is.

And yep, drive it and see what your cruise, and full throttle AFRs are. Around 14.5 - 14.8 is a pretty nice cruise AFR (smooth running without wasting a bunch of fuel). I am not familiar enough with non-turbo full throttle enrichment to know what is considered good. With the turbo system, anything leaner than 12 during full throttle/max boost is playing with fire, but that might be too rich for a naturally aspirated engine with CIS. Hopefully someone else here with chime in about that.

[mike sampsel]

Originally Posted by Rawknees'Turbo

Yes to the cat bypass bung.

With no Lambda control on your system attempting to force an idle AFR of 14.6 (basically too lean for e10 gasoline - right on the edge of lean misfire/rough running), I would attempt to get about 13.4-13.6 at idle (very smooth running then). Of course, you can experiment by going a little richer or leaner and see what your engine's sweet spot is.

And yep, drive it and see what your cruise, and full throttle AFRs are. Around 14.5 - 14.8 is a pretty nice cruise AFR (smooth running without wasting a bunch of fuel). I am not familiar enough with non-turbo full throttle enrichment to know what is considered good. With the turbo system, anything leaner than 12 during full throttle/max boost is playing with fire, but that might be too rich for a naturally aspirated engine with CIS. Hopefully someone else here with chime in about that.

Rawknees'Turbo,

Good stuff, thanks. I'll start with some various rpm readings without moving/load.
I'd rather know the readings with load, but routing of the cables is not going to happen during this good driving weather. .
Too bad my sender and gauge don't have their own WiFi!

PS: My Knees are raw too from being on them while working on cars

[mike sampsel]

Well it was a good exercise! I discovered part of my loudness was the missing bung plug in the Cat bypass .... doh! Anyway, changed the reading on my guage from 13.8 to 13.1-13.3 toggling at warm idle and I'm excited to give it a go!

One lesson learned:

louder backfires occur when the bung plug is missing!

Update:
and a drive with the wideband sensor in my bypass hole made for a backfire free drive today! Sounded much better too.
I new it was loud, I should have suspected the plug came off when I was looking for exhaust leaks!

[3rd_gear_Ted]

Just a suggestion and something top end rebuilds should include IMO
Spend $250.00 to rebuild all your fuel injectors