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Hi guys,

Hoping maybe for some comments on the latest test run above with a picture of the "dash" while running and backfiring still. I ran it up to 4,000 rpm and backfiring still existed.

Suspicious it might be a mechanical issue? Anything that comes to mind or that I should test for?

Thanks
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Rutager West

1977 911S Targa Chocolate Brown
Old 11-05-2017, 03:00 PM
  Pelican Parts Technical Article Directory    Reply With Quote #221 (permalink)
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AFR 13 + exhaust leak = backfire.
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Old 11-05-2017, 06:09 PM
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Quote:
Originally Posted by jpnovak View Post
AFR 13 + exhaust leak = backfire.
Thanks Jamie,

I'll pressurize the exhaust and get out the soapy water and see. A little suspicious of the clamp on O2 sensor, but if you remember, I had the backfiring before I removed the CIS and it came on suddenly at that time, so I'll check at gaskets etc.

Almost scared to ask, but any chance of a broken head stud or valve that would let air in?

Rutager
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Old 11-06-2017, 01:30 AM
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Okay guys,

I put 8 lbs up the tailpipe using a plumbing cap and the hose from an eBay homemade smoke tester (smoker part doesn't work). I heard air coming out the nozzle on the air cleaner cover. Turning the motor over with a wrench got it to a point where I heard no air and it took over a minute for the air to bleed down when I removed the compressor hose and put my finger over the fitting.

I believe because of the test, that my exhaust is sealed, correct?

So, can the exhaust connect to intake directly? If not, how can the air get there, broken ring, open valve?

Suggestions for tracking down an issue, or maybe there isn't one?

Thanks,
Rutager
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Old 11-07-2017, 03:55 PM
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If you are in overlap you will hear air in the intake thru the exhaust as both intake and exhaust valves are open simultaneously.
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Old 11-07-2017, 04:14 PM
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Are you sure ignition side of things is OK?
Old 11-08-2017, 02:27 AM
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Quote:
Originally Posted by rwest View Post
Quick note about the O2 bung; car suddenly went lean on me after last drive and backfires out the tailpipe, so I won't drive it somewhere, I don't know how to weld and don't have any friends that do either.

So, two options: one take off exchanger and bring it somewhere or hire a mobile welder to stop over. I'm sure bringing the exchanger to a muffler shop for welding would be super cheap- breaking an exhaust stud would get expensive and add extra complexity to the project, so I think paying a company to do it in place is the smart option- probably be a couple hundred bucks, but in my case the better way to go.

Any MN Pelicans that can weld and want to make some money, I'm in Saint Paul near Maplewood!
Bit late in the day but that's Rich misfire not lean. Popping out of the exhaust is sign of a rich mixture, spitting out of the intake a sign of lean. When you get a misfire due to overly rich mixture the wideband will see unburnt oxygen and show a lean condition (falsely )
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Old 11-08-2017, 02:42 AM
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Have you set EGO step and EGO limit to 0 yet? You NEED to do this, or the computer will be stepping all over anything you try to do. Setting them to 0 means that it won't try to tweak anything while you're tuning.

I've left them at 0 permanently, because I don't care about emissions, you really only need them if you have a modern engine that is designed to sit at 14.7 through 90% of it's use.
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Old 11-08-2017, 07:58 AM
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You say the backfiring was there prior to the bitz, correct? Have you adjusted the valves previously? Sounds like you may have a valve or two too tight.
Old 11-08-2017, 09:18 AM
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Originally Posted by Pazuzu View Post
Have you set EGO step and EGO limit to 0 yet? You NEED to do this, or the computer will be stepping all over anything you try to do. Setting them to 0 means that it won't try to tweak anything while you're tuning.

I've left them at 0 permanently, because I don't care about emissions, you really only need them if you have a modern engine that is designed to sit at 14.7 through 90% of it's use.
Hi Mike,

No, I haven't done that yet, but will next time I get the chance.

Thanks for reminding me.
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Old 11-08-2017, 02:39 PM
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Quote:
Originally Posted by eastbay View Post
You say the backfiring was there prior to the bitz, correct? Have you adjusted the valves previously? Sounds like you may have a valve or two too tight.
I did adjust this summer, so have been driving a bit without issue, but after I do a couple simpler things, I'll pull valve covers and investigate.

Thanks
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Old 11-08-2017, 02:41 PM
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Quote:
Originally Posted by Sub8 View Post
Are you sure ignition side of things is OK?
Well, I do want to check each wire to make sure I'm firing and will replace plugs just in case they have fouled.

I completely swapped out the MSD, cap, rotor and coil and had the same results. Wires are Clewett and have only a couple thousand miles and a year on them, distributor was rebuilt just 6 miles before the backfiring started, so I was suspicious of it, but the timing is right on.

Thanks
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Old 11-08-2017, 02:45 PM
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So, Have you tried to lean out the mixture and see if the backfiring stops? Have you tried any tuning?
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Old 11-08-2017, 04:19 PM
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Quote:
Originally Posted by jpnovak View Post
So, Have you tried to lean out the mixture and see if the backfiring stops? Have you tried any tuning?
Hi Jamie,

The only tuning I have done was to get everything adjusted for the stock tune for my motor and the LC-2.

I can't quite get my head around how I lean out the mixture with the tuning. I know you said that once I get in there, it becomes somewhat obvious. So can I change the fuel type number, or should I figure out the VE table?

I was focusing more on mechanical issues since the backfire was present on CIS and it seemed like the stock tune was working for everyone else.

It will be a few days until I can start the car again as I bought a shed kit and a massive box is blocking the car in!

Appreciate all of your help and suggestions
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Old 11-08-2017, 04:56 PM
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Hi Rutager,

next time you start it, try to run it untill it is fully warmed up, so that the WUE is not affecting anything anymore,
make sure you set EGO corrections to 0 so you can get correct feedback.
(i'm driving with EGO set to 0 all the time, because the car feels a bit better that way, no tiny corrections, so no tiny hunting engine movements while cruising)

The stock map will run the engine Ok, but not perfectly, and anyway, every engine is different so you will need to tune a bit.

First thing you should do when the engine is warm is trying to get it to idle at 950 rpm (you can use the hand screw near the throttle body to do that) and tune the VE table so that you get around 14 AFR, maybe even a little bit richer. The aircooled engine doesn't like to idle around 14.7 at all. You will hear when the engine is happiest.

The way i tuned it is put the fuel number to the 3.0L specs (in my case), and then never touch that anymore and tune via the VE table.

Once you have a smooth idle, try to go for a drive and try to follow the AFR gauge while you're driving. You should be at around 12.5 at full throttle, less rich at less agressive throttle input. When you're not under load, being rich or lean is not dangerous, so you can leave that alone for now.

I went to a dyno after a couple of weeks, just to get the WOT perfect (gained 12hp in comparison to my base settings) and a couple of cruise settings, this costed very little, very much worth it. All the rest of the finetuning i do myself. It's becoming wintertime and i'm experimenting with WUE and cold accel to get cold running perfect.

good luck,
Jan
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Old 11-10-2017, 10:06 AM
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in your specific case: try to get it warm so that WUE is finished. This will take at least 10 minutes. Probably more.
if you still need to lean it out at that time, hook up the laptop, open the VE table, and look where the engine is at that moment (you'll see something hovering over the VE table or the cells light up, can't remember). Adjust these specific cells down (click on the cell, and press the down arrow) to lean out, adjust them up to make richer.
Note that the computer extrapolates between the cells. e.g. There's a cell for 900 RPM and one for 1500 RPM, so if you are running 1200 RPM, then you have to adjust both adjacent cells at the same rate to make the adjustments.
Just try it, and change the cells 1 number at a time, you'll hear the engine react immediately and you'll see the change on your wideband gauge. Small changes make an impact quite quickly. (so don't go from 50 to 40 in one go for example....go from 50 to 49 or 48 or something)
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Old 11-10-2017, 10:13 AM
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things i noticed at idle: if you go too rich, the engine starts stumbling, going up and down (and you'll smell it!) ...if you go too lean, the engine will start to misfire. you need to aim somewhere in the middle of these 2 conditions.
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Old 11-10-2017, 10:16 AM
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Thanks Wrecka,

Good info; fortunately they are forecasting some forty and fifty degree F weather next week- was 11F this morning!

Should be able to move/assemble the shed and get some tuning and troubleshooting done then.

Rutager
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Old 11-10-2017, 02:09 PM
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Guys,

Got around 40F this afternoon, so after work I spent some time on it:

I set the EGOs to zero and burned to ECU; started car- still backfiring and slightly rich, so I started to adjust the VE settings and got it to lean out, then it started getting too lean, so I went back- whole time getting backfires: 13-15AFRs, didn't matter.

Got car up to 120 coolant temp and it kept getting leaner, so I put all the VE values back and it didn't really move down much.

Other "interesting" thing is my timing backup about another 5degrees! I still suspect that thing. I did source another distributor from a Pelican for a great price because it is a bit rough- no side play, so I may try installing and see what happens.

Here's some screen photos- backfiring intermittently all the time, sometimes going over a minute between them.

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Old 11-14-2017, 04:10 PM
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rutager,
There is a start up/cold start sequence that adds fuel (over and above the VE table) for cold starting/running and then reduces the added fuel as the engine warms up. So, if you are changing VE settings, while the system is running thru the WUE (Warm Up Enrichment) cycle...you will be chasing your tail a bit. Wait until the engine is warmed up past the WUE temp to achieve the proper AFRs in the VE table and then use the starting sequence to provide a richer fuel mix, based on engine temps. Heres a link to a decent explanation of the starting sequence.

https://www.diyautotune.com/support/tech/other/megasquirt-startup-tuning/

regards,
al

PS: If you already know all this.....never mind.
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Old 11-14-2017, 04:22 PM
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