2.4 w/Webers, wierd backfire

[Roger 911]

OK, I have had Webers on my 2.4 w/ E cams for years. Car ran well, idled pretty well too. I decided to rebuild the carbs over the winter. Why? Not sure. Restless? Anyway, I hooked everything back up, followed all the usual procedures. I have the mixture screws turned out two turns, air correction are out two as well. Balanced. But, I'm getting a backfire through the exhaust at idle and poor off idle response. I checked timing with and without the vac retard connected, 5 atc and 5 btd respectively. No significant difference. It will sit there at idle and just backfire out the exhaust.

My first reaction was too rich. But if I turn the screws in, it will stall. So I thought maybe the floats were too high, allowing fuel to spill in. Checked levels with car running. If anything, they are too low, with the meniscus falling just below the 2nd of the top two lines on the PMO vial gage. Next I thought, fouled plug, So I pulled the plugs. They are clean and gray, after sitting and idling, popping back though the exhaust. This seems weird to me. I really expected the plugs to be black, from an overly rich condition. The gray sure looks lean to me, not rich. However, I think conventional wisdom says lean conditions cause backfires up through the carb, not the exhaust.

So, where do I go from here? Should I just turn the screws out more (like three turns) and see if it gets better? Turn the air correction screws all the way in? What about the float levels? Would slightly low float levels cause this? Any suggestions are appreciated.

Thanks.
Roger

[1QuickS]

Try this (from my web site):

Dull popping from exhaust (at idle):
•One or more cylinders too lean (open mixture screws)

Weber Carbs Technical

I like to use a Colortune tool just for this reason, it allows a visual verification of idle mixture and enables the tuner to set all six exactly the same. Exhaust gas testers can do this as well but they average exhaust readings so determining what each cylinder is EXACTLY is a bit less direct. The "two turns out" method for mixture does not provide uniform mixture adjustment from cylinder to cylinder due to variances in clearances, wear, etc. Two turns out for the idle air screws is trying to assure the first hole of the progression circuit is not uncovered at idle. The two turns out for the air screw is just a starting point, a STE synchrometer or similar is required to get idle airs matched for flow which will then throw off the mixture settings (change the air flow and the mixture is affected.)

Hope this helps.

[Roger 911]

Thanks. I'll give this a try. However, I must say it is not "dull popping". It is really loud backfires that could singe the hair off your legs! I have M&K track mufflers so the backfire comes straight out the pipe.

[1QuickS]

I have open pipes on my engine as well and have experienced your backfiring issue but not to the degree you describe. I occasionally get this during initial run-in after remanufacturing a set of carbs when mixtures, individual air balances and side-to-side air balance are out of whack.

I really recommend a Colortune tool; tedious but reliable for diagnosing issues like yours. I've used mercury sticks for air balance (a great tool as all six air flows are viewed simultaneously and works with air cleaners installed) and a Unisyn but have given over to using a STE.

Perhaps another forum participant will have a better solution.

[Eagledriver]

Back firing usually is from a lean mixture. The mixture is so lean that the cylinder doesn't fire and the raw fuel goes into the muffler where it detonates. Clean your idle jets, often they get a spec of dirt in them, especially when you have taken the carbs apart and messed with the fuel lines.

-Andy

[Roger 911]

Paul,
Whether my float level is related to the backfire or not, do you think I should adjust it up a bit, or leave it? Again, it is just below the botom of the top two lines.

Andy,
Yes, it certainly could be clogged idle jets. However, all the plugs are exactly the same appearance. I think it would be unlikely that all the jets would get clogged at the same time.

Thanks for the help.

Roger

[1QuickS]

Hi Roger,

First, float levels need to be the same and adjusted on a running engine, this allows the vibration to release any "stiction" that will otherwise affect level readings. It also allows your fuel pump to operate at its running pressure; 3.5 psi at the carb inlet is the factory specification for fuel delivery pressure.

Fuel level affects the timing of the main circuit activation, lower fuel level delays the main circuit and higher levels accelerates its activation. Also, to a lesser degree it affects fuel delivery in the progression. Fuel delivery is dependent upon vacuum from the running engine and this vacuum must draw the fuel up from the fuel well and back down into either the progression circuit or the main circuit. So, for a lower fuel level more vacuum is required to get the fuel flowing.

Since the carbs were apart you may have upset the relationship between needle valve tip and the corresponding tab on the float. The needle valves will create a divot on the tab of the float and swapping floats between the wells may cause a new divot to be created. This old divot has the potential to create side load with the fuel valve so it is recommended to remove the old divot during a rebuild. This may be done by peening the bottom of the tab to level the top surface. When I rework floats I use a point file to flatten the tab and then adjust float geometry to factory specifications as they are never right, new or used.

Quick tip: the distance between the two lines on the fuel level vial is approximately equal to a shim thickness of 0.010".

So, to answer your question; I'd set float levels to be at least within the two rings and drive the car to be sure there are no transition issues.

[Roger 911]

Paul,

I set them on the bench, with a fuel line hooked up to the pump. Based on the info on your site, and other threads here, I thought that the level may actually rise once they were on a running car, due to increased voltage and/or vibration (guess not). While my lean idle is probably contributing to drivability issues, I had a feeling that the low fuel level may also be causing hesitation off idle in the transition region. I will raise them.

Thanks for your help.

Roger

[Roger 911]

Progress! I reinstalled the plugs, with a slightly smaller gap, .040 versus .050 (I have an MSD), and I twisted the screws out 3 turns. It idled much better. I balanced the carbs at idle, adjusted timing, and twisted the screws out a bit more as needed. Idles nicely now with no loud backfires.

Float levels are next, then final linkage adjustment and balance.

[turbo23dog]

I had a 73 2.4L with webers and I had to clean the carbs almost once a month due to constant backfiring, especially on throttle lift. It took over a year to figure out that root of the problem was rust in the gas tank. Seems that some rust particles were getting past the fuel filter and clogging the jets. Just a thought for something else to keep an eye on.

[1QuickS]

There are sources of rust other than the fuel tank. Most efforts to stop debris from entering the fuel bowls revolve around good fuel filters and fresh fuel lines with a tank cleaning. All these are good but there are other sources to check:

1. tank filter screen cleaned
2. vent pipes in top cover of Webers rust internally and drop flakes of rust directly into the fuel well; wire brush and apply "Extend" or equal; or remove, replate and reinstall
3. spark arrester in OEM air cleaner housing (25mm diameter, right angled tube) is a good source of rust
4. also the trough where the seals are installed will collect water, rust and pinhole after time which allows flakes to be drawn into the fuel well (I suffered this one for awhile before I found and fixed it)
5. poor sealing of air cleaner housings (OEM and after market also) with sealing gaskets allows other debris to enter your fuel well; apply a thin strip of oil resistant, closed cell, self-adhesive foam
6. spark arrester hose may harbor debris which may be drawn into the system

Assuming a well sealed and cleaned air cleaner and the other issues listed are corrected, there will still be crud entering your fuel bowl; the smallest orifice in the Weber carb is the idle jet. Anything smaller than that will either stay in the bottom of your bowl or be delivered to your engine and spit out the tail pipe. Also, the air cleaner operates with a low pressure so it is natural for atmospheric air to be sucked past poor seals and to bring crud with it.

[Roger 911]

More progress! Reset the float levels higher, much better in the transition off idle. Still bucks a little between 2-3K. Probably just need to fine tune the balance between the carbs. Unfortunately, November has returned to Chicago, so I won't be able to finish this up this weekend. Two weeks till Road America!