Trouble with Carbs

[914obsession]

I have a set of Webers that I have rebuilt, and then re-checked on a newly built 3.0 that I am working on. It's running CIS cams and pistons. The I've blown air through all the passages, everything looks really good. The carb bodies were ultrasonically cleaned before the rebuild. I am running 160 mains, 175 air correction and 65 idle jets.

I'm running an MSD 6AL ignition system, and the carbs are getting 3.5-4.0 PSI of fuel. I cannot get the engine to idle without turning most of the air compensating adjustment screws out more than 1 full turn. The idle idle adjustment screws are each 1/2 turn out. With this setting the mixture screws have no effect, I can screw all 6 of them all the way in and the engine runs the same. The spark plugs are all VERY black, the engine is running very rich. One cylinder (#3) does not seem to be firing at all. The plug is getting a spark, but once removed, the plug looks completely clean and is wet with fuel.

My thought is that the engine is running on 5 cylinders, therefore causing me to let more air in via the air compensating screws which then messes up the idle circuit. Once again, all passages were checked and completely clean. The #3 cylinder is getting compression.

Any thoughts? I've pulled the carbs off twice now to check all the passages and haven't come up with anything. The engine sounds healthy but obviously has some issue. Any thoughts would be helpful, this is starting to drive me crazy.

Thanks

[Steve@Rennsport]

What size main venturies do you have?

65 idles are FAR FAR too rich for your engine & configuration. Start with 55 idles before using 60's.

[914obsession]

I'm running 34mm venturis. I have tried a set of 60 idle jets with the same results. I'll scrounge up a set of 55s and try those. Can someone answer these 2 questions:

1) wouldn't turning in the mixture screws all the way basically cut off the fuel flow entirely to the carbs?

2) how far out can you turn the air compensating adjustment screws on the carbs before you've gone [I]too[I] far? By opening these up more than a half turn or so, are you taking carbs out of the idle circuit?

Thanks,

Dan

[Steve@Rennsport]

Originally Posted by 914obsession

I'm running 34mm venturis. I have tried a set of 60 idle jets with the same results. I'll scrounge up a set of 55s and try those. Can someone answer these 2 questions:

1) wouldn't turning in the mixture screws all the way basically cut off the fuel flow entirely to the carbs?

2) how far out can you turn the air compensating adjustment screws on the carbs before you've gone [I]too[I] far? By opening these up more than a half turn or so, are you taking carbs out of the idle circuit?

Thanks,

Dan

Dan,

With 34mm venturies, 55's are the right ones.

1) Not completely,..A very small amount is drawn by the main circuit; the bulk of the idle fuel comes through the idle jets.

2) Nossir,..those are for balancing airflow when the throttle plates are at their rest position. The carbs run on mainly idle jets from idle to 3K, at small throttle openings. For example, cruising down the highway at 65 MPH, steady throttle, the carbs deliver most of the fuel through the idle jets with the rest coming from the main circuit. Opening the throttle beyond 10%, starts transferring fuel from idle to main jets.

[914obsession]

Steve, thanks for all the advice! So I have figured out why I can't get the car to idle without either opening up the air compensating screws too much or turning the idle set screws in much more than 1 turn. I think I'm running on 5 cylinders...

1) the number 3 cylinder is not responsive to any changes with the air compensating screws,

2) while the engine is running, 2-3k, placing my hand over the air horn on that cylinder doesn't do much, the other cylinders suck my hand down pretty good,

3) the spark plug looks totally clean, no black or white soot, and it looks wet with fuel,

4) the header coming out of that cylinder warms up a bit but not as quickly or as hot as the others.

I've checked the valve timing on both the intake and exhaust for cylinder 3, everything was spot on. There is a spark coming out of that spark plug. While the engine is running and the #3 spark plug was out (while I was checking for spark) the engine definitely made sounds consistent with healthy compression. The timing is set to 35 degrees. And, as stated above, I've checked the passages in this carb 3 times, everything checks out.

Any thoughts?

Thanks,

Dan

[flat6pilot]

Could it be so rich that it can't fire it? (#3 cylinder?) Plug gap ok?

[Steve@Rennsport]

Dan,

Confirm compression on that #3 using a compression tester or better yet, leakdown tester.

Make sure all cylinders have a nice fat blue spark while cranking. This confirms the wires and plug connectors (troublesome, if you have OEM ones).

[RichHawk]

If that cylinder does not suck just as hard as the others, then you've got something other than carburetor problems (But I bet you know this already).

Lack of intake suction means one of two things:
1) the piston has a hole in it, or compression rings are broken, or detonation has cratered the cylinder
2) the piston is drawing in the air, but not all through the intake port: Bent exhaust valve. leaking intake boot.

Any which way, you need a leakdown test. all the compression test is going to tell you is what you already know. something is wrong with #3.

Richard

[Eagledriver]

I believe you are also running way to rich on the main circuit. I have 150 mains and 200 aircorrection jets with 38mm venturies. Buy a cheap wideband AFR guage. I bought one and it was the best investment I've made for tuning the carbs. They are around 200 dollars for a basic one.

-Andy

[2.70Racer]

914obsession,
Think about your statement for a minute.
"With this setting the mixture screws have no effect, I can screw all 6 of them all the way in and the engine runs the same."

If the engine runs the same, where does the fuel come from?

The idle fuel mixture screws closed completely, blocks fuel to the lower idle fuel port.

So fuel is entering the engine from another source, not the idle fuel port.

Simple answer, the butterfly's are not completely closed. They are in fact probably held open by the idle screw setting enough that fuel is entering the engine from the second fuel port located just above the closed butterfly.

Remove the carbs from the engine. Inspect the butterfly's from the bottom of the carbs.
With the idle speed screw backed off completely, all three butterfly's should be closed.
And closed all the same. Not 2 open and 1 closed.
If you can see the fuel port above the idle fuel port, that is the the lowest port of three ports under the brass screw, this is where the uncontrolled fuel is coming into the engine.

Readjust the butterfly shafts so all 3 B-fly's are closed.
There will still be a small opening, the B-fly doesn't completely seal inside the venturi.
Look at the carb base, you'll see a square notch. This is the opening for the idle air into the engine with the butterfly's completely closed.
This air passage bypasses the butterfly's.
The idle air through this square port is controlled by the idle air correction jets.
Once the B-fly's are fully closed, the idle fuel port below the B-fly is the only source of idle fuel.
Then the idle air is controlled by the idle air bypass screws.
Open the idle fuel screw 1-1/2 turns. Open the idle air correction screws 1 turn.
You will now have control over the idle fuel and idle air.
Back off the idle speed screw so it is not holding the butterfly's open at all.
Make certain the idle jet holders are not leaking air, as well as the large brass screws covering the 3 ports above the butterfly.
If they are leaking air due to old hard, worn out O-rings replace the O-rings.
The engine is trying to suck fuel through the idle jets and air leaks will prevent idle fuel from flowing. It will act like blocked idle jets.
Worn B-fly shafts will also allow uncontrolled air into the carbs, making an idle setting difficult and erratic. Fix the loose shafts if you expect a decent idle.
Make certain the carb gaskets are in good shape and not leaking air.
Once you have the idle circuit working you can then begin to properly balance the carbs.
There is much more, but this will get you started.

[Number1_dog]

I am just completing a top end re-build on a 79 SC short stroke with SSIs. This year I will be running with the CIS, but have started collecting stuff to convert to Webers next winter.

My question is is it worth while to install O2 sensors in my exhaust, I'd do one in each side, to aid in monitoring the carbs operation?

Thanks
dog

[E Sully]

Have you checked to be sure your float levels are correct?

[Eagledriver]

Dog,

I recommend installing ports for an O2 sensor in your exhaust. You can plug them with a bolt until you are ready to install the sensor. I just put one on the passenger side and assume the other side is the same. I do plan to put one on the drivers side in the future to check it.

-Andy

[barryr]

I ran with Webers for about 15 or so years before I changed to electronic fuel injection. HUGE difference in real world reliability.

Anyway. If the carbs are still on the car, get a mirror and a flash light and while you are looking down the throat of number 3 have someone turn the ignition on but DO NOT start the car. If the throat starts to get wet or you actually see fuel starting to drip the problem is the float is either stuck down, the needle valve is in upside down, missing, stuck or damaged. If it really pours then you may have tried to reuse the old gasket because you misplaced one.
All of the above learned the "hard" way
If the carb is off, take the top off and check the above.
Once it is running, absolutely go with the O2 sensor and meter but get 2 sensors so you can switch between left and right. This really helps in checking the balance while under load.
Barry