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Discussion Starter · #1 ·
Hi,
I am getting a popping in the exhaust when decelerating (seems like fuel ignition in the exhaust pipe) in the range of 3000 rpm and lower. At higher rpm this doesn't happen (decel from 5000 rpm for example, the popping starts only around 3000rpm and lower)

I have a wideband O2 sensor, and in the popping condition it is lean (like 15 or 16 to 1 AFR).

My engine is a 2000 Nord, with DCOE32 carbs, JF4 dizzy set to 4° BTDC. The idle jets are 50F8. The cams are C&B 12mm 292° cams (quite lumpy - I suspect this is contributing to the problem). At idle the AFR is about 13 to 1.

I am assuming that there is too much fuel going through the engine at higher rpm with the throttle closed (decel condition); should I try leaner idle jets? Is my assumption even correct? Does the ignition timing have anything to do with this? Anybody have a good solution?

Thanks in advance
 

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IMHO, the cam isn't contributing to the problem; it is the problem. In my experience, this is simply a typical characteristic of aggressive cams.
 

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Have you ever set your timing at the M mark at 5000 RPM? Then let it come back to idle and use that as your initial timing?
Retarded timing can cause backfires thru the exhaust on decel. Many times backfires are blamed on a rich mixture when in fact they are caused by a lean mixture, suach as your 15 or 16 to 1. How many turns out are your idle mixture screws? If more than 1 turn, you might increase the size of the idle jets.
 

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Discussion Starter · #4 ·
I readjusted the idle mixture, this time arriving at just under 1 turn from closed (15/16 to be exact), which is a tiny bit richer than where they were before. This has significantly reduce the amount of popping on decel. I guess I had in my head that it was too much fuel causing the popping, so I was always trying to lean out the idle. Also since the AFR at idle was 13 to 1, it didn't seem logical to want make it richer...
Regarding the ignition timing, I havn't measured at 5000 rpm, I just advanced as much as I could without knocking at high rpm/full load, and left it there. I haven't gone back to see what the static advance is (I'll do that for info).
Despite the lumpy cams, the engine is reasonably polite now below 3000 rpm. I recently switched the dizzy to a Bosch JF4. With the old Marelli, I had no luck getting the engine to behave at lower rpm (I think the advance springs were a bit weak).
I'm still curious why a lean mixture would cause ignition in the tailpipe?
Thanks
 

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Same reason why an air leak in the intake manifold will burn a valve or piston: lean is HOT (think acetalene torch and how it acts when oxygen is added)

It seems counterintuitive, but lean in the pipes will more easily mix with whatever air may be present, and as lean mixtures are often easier to ignite than rich, the net result is backplop as the stuff ignites and burns off.

A bit richer content in the exhaust stream is just enough to prevent ignition and is a bit cooler in general which also helps alleviate the problem as it cuts down on what could be potential hot spot ignition points.

'Course a good inspection of the exhaust system as a whole wouldn't hurt either, as any pinhole leak at a joint or whatever introduces air into the system, which in turn leans out whatever air and unburned fuel mix that may be present, and makes for backplop yet again, even if the mixture would normally be fat enough to prevent it if there were no leaks.
 

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Try turning the mixture screws out 1/4 turn more and see what happens with the popping and also idle quality. It really is best to set the timing for total advance at 5000 RPM. Retarded timing can cause some backfiring out the tailpipe also.
 

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Discussion Starter · #7 ·
Hi Guys,
Thanks for the good tips. I reset the timing, the previous values were off because the F and M marks were for a spica injected engine, so I redid everything relative to TDC (the P mark). Now the timing is set as follows:
1000rpm->10° BTDC
1400rpm->12° BTDC
2500rpm->27° BTDC
3300rpm->31° BTDC
5000rpm->41° BTDC
(of course everything is +/- a couple of degrees)
Now that the ignition timing is set (by the way new condensor, points, rotor, cap and plugs (Golden Lodge HL),and points gap set to 0,015"), I went back to the idle mixture.
At idle, with previous setting of 15/16 of a turn from closed, the lambda was 0,9 (13,2), and it was idling at 1200 rpm (the idle needs to be a bit high wit these lumpy cams I find). I turned all the screws out by a quarter turn, this dropped the lambda down to 0,78 (11,5), and the idle dropped to about 900rpm. I used the idle adjust screw to bring it back up to about 1000rpm, and went for a test run. The popping is much much less, just the occasoinal light pop, so on this front it seems to be much better. Seems like the method of "fastest lean idle" is not necessarily the way to go.
By the way the exhaust system is pretty tight and tidy, i.e. no leaks.
I am still plagued by a sputtering/hesitation (sometimes), just as I come off idle. Seems to be more frequent when the car and the outside temps is hot (like today around 30°C). Before trying to compensate with the mixture settings (which seem pretty good now), I am just going to go check the float levels, a friend suggested that the float levels might be misadjusted since the car really misbehaves when it is not flat (like trying to climb a steep slope out of a parking lot - it sputters and coughs like crazy) I've never been able to get an AFR reading during this, but based on the smell of raw fuel... I am sure the gauge would read "RCH" (i.e less than 10). I'll keep you posted.
thanks - andrew
 

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Discussion Starter · #8 ·
Float Level Measurement

It seems that the fuel level in the floats are a bit high, at least according to the advice from centerline. I measured the fuel level in the chamber (after removing fuel and letting it refill), and I get 1,032" on the back carb, and 1,057" on the front carb (fuel level to the top machined surface where the jets screw in - see attached photo). The measurement is supposed to be 1,14". Is this the accepted way to measure the float level? Could this be the cause of sputtering on slopes (and in hard corners)?
thanks
andrew
 

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On the same centerline site under webers where you found the measure the fuel in the bowl info, on the previous page is the float setup. I would perform that method prior to measuring the liquid. The 8.5 mm setting they use is correct for your 40DCOE32s

I'm interested in what you find because my 1750 with DCOE32s also pops when decelerating--same symptoms as you. I have the centerline distributor set at M (36 deg) which means its set at 10 deg advance at static (george Willet--confirmed by my timing light). I richened my idle jet which fixed the flat transition point but didn't affect the popping. I can set my static timing to 2 deg ATDC as called for a 1750 with webers and the euro cams (48s) and it still pops but doesn't run as well. Mine will backfire at times like a rifle shot when backing off in 2nd, 3rd or 4th rolling through the gears to stop. Sounds like it's coming out of the end of the tailpipe.

Mine could be an exhaust leak--if so it's not readily apparent from inside or outside the car. My plan is to put it on stands this weekend and look for small leaks.

If you cure the problem let me know and I'll do the same.
 

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Andrew, the float adjustment is critical. You may be pleasantly surprised with your performance once they are set properly. I'm sorry I didn't suggest that first. Pete
 

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Where in the exhaust is your EGO sensor placed? At the tail pipe, the exhaust gas can easily get diluted with atmosheric air when there is low exhaust gas flow (idle).
From memory, my Alfetta manual said 4%CO at idle. That is about 12.5:1.

Also, some of the ignition advance figure you have stated seam very low at idle. Are you using the standard distributor advance curve?
Alot of what you're discribing sounds to me like ignition that is taking place too close to the opening of the exhaust valve.
Not really helpful to you, but most NA engines with computer controlled ignition timing will idle anywhere between 15 and 30* BTDC with similarly advanced timing for low loads and mid/high engine speed.
 

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My buddy asked me to help him tune the Spica on a customer's car and he had a portable exhaust analyzer he wanted to use. The mixture ended up way off and we finally set it the old fashioned way and it ran great. Duk's on the right track as far as I see things. Sometimes technology ain't all it's cracked up to be. Your popping is most likely related to the pilot circuit of the webers, as turning out the mixture screws has indicated. Float level affects mixture also. After setting the float levels properly, you should check that the carbs are synchronized as having one carb with throttle plates slightly open on decel can cause popping also.
 

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Discussion Starter · #13 ·
Setting Float Levels

Hi All,
I took the covers off to measure the distance from the float to the cover surface and got the following:
Back Carb:
0,345" high position
0,548" low position
Front Carb
0,335" high position
0,616" low position

the specs from the centerline bulletin are 0,33" high position and 0,59" low position.

Given that my fuel levels were measured to be a bit on the high side, shouldn't I expect these numbers (particularily the high position as it is this tab that operates the needle valve) to be smaller than spec, allowing more fuel in the float bowl?

Attached is a photo of how I measured, it seems a bit arbitrary, since the surface if the float is somewhat irregular. Also do the floats look normal (tinning etc)? I don't have a scale to measure 26grams to confirm the condition of the floats.

Thanks!
andrew
 

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Those settings are close enough (did you include the gasket in the measurement?) Are you sure the needle valve stops the flow when the ball is depressed?
 

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Good point flivesay. Try holding the carb top in front of you while looking into a mirror. Blow throught the fuel inlet and lift the float. When it is parallel to the carb body, you shouldn't be able to blow through the needle and seat anymore if they are seating properly and in good condition.
 

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Discussion Starter · #16 ·
Hi, sorry for the time out. I did not include the gasket in the measurement, as it conviently drops down out of the way. I inspected the needle valves and they seem to be in good/new condition, I replaced them when I serviced the carbs about 10000km ago (haven't had a chance to do the check as per pete's reccomendation).
For the issue of the transition coming off idle, I've always figured that the cams could really affect this, because when using a sync meter (air flow) at idle, you can really see that the flow through the head is poor as the needle is jumping back and forth (presumably because of the 292° overlap). Also as the engine sputters a bit then "jumps to life" as the rpm increases, this leads me to beleive that it is related to the cams and the flow through the head. Perhaps trying to lean out the transition stage would be a good idea? Does this make sense or am I on the wrong path?

thanks again for the great help, I very rarely get a "pop" out of the engine now! what a pleasure :)

andrew
 

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Glad to hear of your progress.Richening the idle screws decreased the pop, right? How did the richening affect the "transition", or did it? At this point, how many turns are your idle mixture screws turned out?
 

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Discussion Starter · #18 ·
thanks pete, richening the idle indeed reduced/eliminated the popping, it is not at all an issue anymore (many thanks for that, not that I was embarassed to drive an Alfa, but.....). The screws are now just under 1,25 turns. The transition issue I don't think got better or worse, it seems to be about the same. It does seem much worse when the ambient temperature is hot. Cool weather seems to be OK (like 15°C - 20°C outside is OK, 30°C and the sputtering on transition becomes annoying).
thanks
andrew
 

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Discussion Starter · #19 ·
Oups, just checked the notes I took and the idle mixture screws are in fact at 1 and 1/16th of a turn open. The idle speed is 1200 rpm (thanks to the idle adjust screw), and the O2 sensor is fairly steady at 0,84 to 0,86 ( 12,3 to 12,6 : 1). It is important (for the benifit of others) to note that this is not the fastest lean idle; tuning by that method gives the "pops" on decel.
Some other measurements that I took, just for fun:
@ full load/full throttle
4000rpm -> lambda = 0,73
5000rpm -> lambda = 0,84
6000rpm -> lambda = 0,90
(reasonably steady state figures)

and then decelerating (closed throttle)
3000rpm -> lambda=1,16

and light load (4th gear throttle barely open, constant speed)
4000rpm -> lambda=0,9

I would say that the behaviour of the engine above 3000 rpm is fantastic, seems like it is perfectly in tune. (135 main jets by the way). Squeezing the throttle at 4000 to overtake is very exciting!!

If I turn out the idle adjust screw so that the idle is 800 or 900rpm, the idle gets a bit too rough, more like a stock engine at 600rpm. I figure this is because of the cams; I guess this is where the whole transition circuit starts to get a bit messy....

cheers
andrew
 
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