The Stovebolt Site Forums The Shop Area The HiPo Shop Need help with duel carb tuning

Just got my 56 235 rebuilt and in the 46 1/2t. Still working out the bugs. I bought the duel fenton manifolds intake and exhaust,2"cherry bombs duel exhaust,HEI dist, and (2) 32dft weber carbs from Langdons.The truck runs and drives ok but if I use engine braking, like decelerating down a hill, the engine backfires and then wants to die when I come to a stop. Feathering the gas as I brake keeps it lit.
I bought a carb syncronizer from edlebrock and need advise on tuning the carbs.

After engine warm do I back out the idle screws and set the idle mixture to each carb and then set the idle with the syncronizer or visa versa? What's the best way to set the idle mixture on the webers? Is there something else that I should look for?

Thanks,
Rick.

I would start by checking the timing.

Which synchronizer are you using? UniSyn? They are not too helpful on Webers based on their design. Go to a cycle shop and check out a Carbtune model. They are more expensive,$90, but maybe you can rent one or even have the shop tune the carbs for you. Both are British made by the way.

I have tried several carb synchronizers over the years (did a lot of foreign multi carb cars) and never found one that worked as well as a piece of hose and your good ear.

Set the idle air screws the same (1-1/2 turns or whatever is correct for the type)

When the engine is warm, loosen one end of the linkage so the carbs are independent.

Balance the air flow in the carbs with the butterfly screws. When the rpm is correct and the carbs are syncronized (balanced), tighten up the linkage connection. You are done with the syncronizer.

Now adjust the idle air and main jet (if adjustable) one carb at a time.

X 2 on that. You need to advance it a bit.

I set mine with a vacuum gauge.

Set timing to maximum vacuum.

Disconnect linkage between carbs

Adjust mixture on each carb to maximum vacuum.

Adjust the idle speed on each carb, a little on each, going back-and-forth between carbs, until you get the idle speed (RPM that you want).

Reset the mixtures.

Reconnect linkage between carbs.

Reset/check the timing.

If it backfires through the carbs you're running too lean. Get the timing dialed then mess with the carbs. I set mine with vacuum gauge as well. Fuel pressure might need checked also.

Do you or Tim know why running lean causes backfire through the carbs?

It's detonating fuel too early. Is your cam in decent shape? I've seen lossy exhaust lobes cause this issue. The combustion is exiting the intake runner. Maybe the exhaust lobe is worn on the cam.

An intake leak can contribute to the problem.

Dennis

Mechanically I can understand the worn out cam explanation for backfiring thru carb, since valve is late/not fully closed (+ any timing issue)..but still struggling w/why running lean would cause it. Detonation is via plug, not compression, what am I missing? Maybe Professor Jerry HL would like to chime in?

Detonation by it's very nature is NOT TIMED. A hot spot like a bit of carbon can ignite the incoming charge while the intake valve is open. That will back out the intake real fast. Lean condition will make the hot spot hotter and that will make the detonation issue worse. This all gets worse with higher compression, think about the carbon build up for that. And last but not least, if the mixture is lean the plug will get hotter and cause the incoming charge to ignite while the valve is open. So detonation can have more than one cause. Most of the time it is related to the timing, but it can be just a hot spark tip, rough valve edge, or hot bit of carbon. And lean will make it worse.

Rick (elmer46),

Is it backfiring through the carburetors or in the exhaust system?

Pull the plugs. Could be one cylinder. You want a nice mocha color on the plugs. Also check to see the heat index of your plugs themselves. I just went through this issue on my carbs. Takes 10 minutes to re-gap some plugs. Also a little spray bottle with water, set it to mist and spray around the carb and intake to check for a leak.

All this is assuming your timing is correct.

I am not familiar with the weber, but I'd bet your initial idle is high - 750rpmish - so that when decel with engine brake the throttleblades are closed and the vacuum high. The transfer slots will be exposed and the highvacuum will call for fuel, in brief fuel is raining into the engine via both idle and transfer slots and this raw fuel is causing the backfireing and general poor running.

What happened to Elmer? Want to make sure you get fixed up, but while we're waiting on your answers to several post above, I'm still struggling with a little bit of carbon (and thus decrease in volume which increases pressure) increasing cylinder pressure enough to cause premature ignition and backfiring while intake valves are open (ie through carb).

You guys are the experts, but it seems to me that if that could happen, why wouldn't the engineers just increase compression ratio on these gasoline engines and get rid of plugs, and ignition system? How do we know that actually happens?

That's a Diesel engine. What fun would these trucks be without the little quirks?

It looks like Elmer is referring to the Holley-Weber Ford 740 Weber DFT32 sold by Langdons (new carbs - progressive 2-barrel; originally used as the single carb on 1600cc engines). Mine would easily and smoothly idle at 400rpm.

Wow 400rpm. How much vac at idle?

It had to be between 19-21 (I've never had less, and never had more in my various 235/261 engines).

I'm at 18-21 cruising, also running a huffer... Truck gets crazy mileage if you're cruising. If I hit any boost then it's gallons per mile.

[/quote]It looks like Elmer is referring to the Holley-Weber Ford 740 Weber DFT32 sold by Langdons (new carbs - progressive 2-barrel; originally used as the single carb on 1600cc engines). Mine would easily and smoothly idle at 400rpm.
[/quote]

Thanks, I took a look at it. I didn't see throttle bore/blade size but they look too large to me.
The 'progressive' part is that linkage rod underneath and the blades open independantly at different rates? Is that right?
Seems to me a pair of them would be much for a mild 6cyl.

I meant Carter-Weber Ford 740 DFT32 - sorry.

I have found that the pair of them is about right for 235ci and too little for a 261ci.

That carb was originally used on a 1600 cc engines (about 100ci).

Two of the Holley-Webers that Langdon sells would be probably too much for a 235.

Here I am. Sorry been out of pocket. With regards to the backfire it is through the exhaust. I put on a fuel pressure gauge and it runs about 5lbs. The sychro I used is from edlebrock. The timing is on the "ball".

I still have the same prob. The truck will run fine for a while but then it will start burbling and backfire thru the exhaust and then run a low rpm at idle about 300 to 400 and just run rough at Idle. I've tried to duplicate the symptom and I think it is after a high vacuume event like down shifting. You can blip the throttle and the motor will spool up fine but then drop back down to low rough idle. I have even seen it run low and rough and then slowly creep back up to a normal idle about 600 to 800 and smooth out.

I've hooked up a vaccume gauge but I must have it in the wrong place. The port above the throttle plate where I have the vac advance reads 0 at idle but reacts with a flip of the throttle. The ports below the throttle plate react the same, 0 vac at idle but reacts with a flip of the throttle. It's the same on both carbs.

I am totaly baffled.

Rick.....

Lets break it down to basics.

Backfire - out of the exhaust - Bang, Bang or Pop, Pop, Pop.

For this to happen, unburned fuel has to get through the engine into the exhaust system in sufficient quantity to be able to ignite. It also has to have enough oxygen for combustion. There has to be a an ignition source.

When the engine is fully warm, you can produce a backfire by turning off the ignition for a couple of seconds (at speed, down hill works best) and switching it back on. Should give a good bang. Run it off too long and you could blow the muffler off.

How to have it do it on it's own:

Loose primary wire that cuts the ignition on and off.
Sticky set of points that fire randomly.
Sticky exhaust valve that hangs open.
A rich mixture and an air leak in the exhaust ahead of or in the muffler.

That is a start.

Do you have a timing light with an advance function? I went through this very issue with mine. Did some of the same things you have trying to chase down a "fuel problem", until I happened to have a chance to talk to my uncle, who had a virtually identical setup in a '49 Chevy Business Sedan back in a mid-'50s. I told him what it was doing, he told me to advance the timing about 5 to 10 degrees. Sure enough, that fixed the problem. Haven't had an issue with it since.

What timing do these motors like? Never worked on an i6. Only efi and carburated v8's. I know the v8's like about 14 degree advance at idle and 32 all in. Mostly high hp also.

The i6 are awesome. Just trying to learn more about them.

Generally speaking, with stock specs, they are usually set to 0 degrees base timing when set on the timing ball embedded in the flywheel, though that can vary a bit depending on what it was in originally. Without knowing what ignition system elmer46 is using, I can really only give a ballpark figure of 5-10 degrees advance for the setup he has. In mine, which has a very similar setup, seems to run best with 15 degrees advance at idle. I will mention, mine does not have a vacuum advance on it.
I'll agree with you on the old I6, much better engine that most folks give them credit for. I used to get people ask me all the time when I was going to "put a V8 in it", but once they hear it run, they tend to change their minds. My Uncle Max, who I mentioned in my other post, used to race with the one he had. He really liked beating up on the 392 Hemi cars. Most of them couldn't believe they got beat by an old Chevy 6.

At 15 degrees in do you need to run premium?

Timing is critical on them, on a similar setup I never did figure out exactly what the timing was but kept jacking it around until it was just right. You have got to let the engine tell you what it wants, do not get a specific number stamped into your head that will lead you down a false path. Remember you have changed how the engine runs and you must let it tell you what it wants.
before you do it you have to have all else perfect, new plugs, wires, coil, carbs etc - it has to be perfect or you will use the timing change to cover another problem (I do this professionally and see this often) and things spiral downhill from there.
I don't know where you are but weather will affect things as well, I do the supertune in warm weather. If I tune an engine in Feburary then it'll run good in Feb and will need to be redone in August.

Truer words on timing were never posted, ojh

The original timing marks only get you close (you hope). Too many things have changed since that setting was marked (if you even have the original flywheel).

1. set timing statically to the timing mark
2. run and warm-up the engine and the set timing to maximum vacuum
3. do a "run and strain" and reset-timing by the old-time ping test/technique

No, I run 87 octane E10. The compression on it is fairly low, 7.:1 stock, I have about 8.5:1 with the overbore and having the head lightly milled to true it up.