The Engine has a rough idle and backfire trouh the carb if accelerate.


I Have change :

Carb and new gasket
Complete disributor with wires /12 @ IDLE
spark plugs
Fuel pump with lines
Vacuum gauge shows 13" mild comp camp (like NEW)


i have NO idea what can be the reason......maybe a burnt valve ....valve spring broke ....but thats a rare option.
## Spark plugs look always black so it must be any leak ......let me know what you think !!

 

Perhaps just a bit more information would assist others in helping you to diagnose.
(1) Manufacturer & size of engine.
(2) Manufacturer of vehicle and model of vehicle.
(3) Type of ignition system...HEI, contact points, etc.
(4) Tuneup:
(a) Dwell Angle
(b) Timing Degrees

 

Did you disconnect the vacuum advance when you set your base timing?

 

455
HEI
12 initial

 

Jeep

 

Before the items in post #1 were changed, was there any other changes to your engine? Is it an intermittent back fire through the carb, or a rapid fire type as you rev it? Was the HEI recurved to use the 12* initial?

 

The complete story:
Engine idle great and run strong with a EDE 600

Than I change to quick fuel 750, install a fuel pressure regulator with new braidet 6AN fuel lines and correted fuel PRESSURE AND float level.

Result :
# backfire and rough idle which go a way with some adjustment on the accelerator adj. Screw.

A few drives later same scenario.
​​​I have try to adjust the accelerator pump like first time but without results.

I change back to EDE... Idle a little smother but backfire to the carb

Sparks always look really black

Changed everything...

The two distributors run really great... I don't think timing is out.

 

Why did you install a fuel pressure regulator?

 

I have a Regulator laying around and read holley aka quick fuel carb are sensitive to wrong fuel pressure....

 

A few weeks ago my engine suddenly ran a bit rough and would backfire through the carb with light throttle application. I figured it must be ignition related and remembered I had used the distributor kill switch recently and thought there may be a poor connection that resulted in lower voltage to the HEI distributor. So I turned the switch off/on/off/on multiple times and the backfiring and rough running went away.

So maybe check the voltage at the distributor + terminal to ensure there is full voltage before pursuing more complicated possibilities.

 

​​​​​****.... You bring me to another symptom which came at the same time as the backfire begin.
I notice that red light from the charging system begin to flash... I want to fix this problem after I fix the engine problem... Maybe only the connection at the alternator.

But can this really be the problem????

 

Spark plugs should not be black as described. There does exist a delta between when an engine runs lean vs. when an engine runs rich. Normally, IIRC, a cold engine (with an operating choke) would be fed more fuel during a start-up (cold operation) and as the engine warms up less fuel is to be supplied resulting in a more lean condition. If the spark plugs continuously demonstrate a black appearance, I would suspect a rich running condition. A rich running condition might suggest fuel which has not completely burned (ignited) and which is igniting either in cross contamination between cylinders, or igniting outside of the cylinder itself.

Does the backfire occur via the exhaust or via the intake manifold (e.g. carburetor orifice)?

 

Carb....

 

IMO, that's a tell-tale sign of a rich fuel mixture.

 

Does it spit and burp through the carb when it's cold or warm; or, the same amount of dragon breath in both cold and warm conditions?

 

I think one thing we could agree upon is the fact that the intake valve is open when the backfire occurs since it's burping and spitting a backfire through intake manifold (carb) since if it were an exhaust backfire it would be belching out the exhaust and indicate the exhaust valve was open during the backfire. Hope it isn't a worn cam. Generally though, I always thought a backfire from the carb (intake manifold) was from a lean condition and not a rich condition. Albeit, my questions regarding when the backfire occurs.

 

Fix alternator issue, then return to carb/ignition.

 

x2 on the alternator issue. Your timing @12* is not advanced enough, bump it up to 16/18 and disconnect the vac advance temporarily.

 

Can you hear any mis at the exhaust? Is the mis at the same cylinder or random cylinders ?....Tedd

 

thats the PLAN ...yes

 

My Base setting is 16 yes....i have try with Timing @ 12/16 and with without better results ..can´t remember if i rev and test with hose disc. .....i try it today

 

What are the specs on the motor? It sounds like a timing issue but could be a vacuum leak, especially doing it with both carbs. Of course you may even need more advance if it is a low compression Olds V8.

 

Low comp. BB
Lift: .456''/.456"
Duration: 268°/268°

I also think about vacuum but the gauge say 13"....like before the engine run god

 

 

Approx 8:1
Yes adj. Valve train

 

Low compression explains the vacuum somewhat. Did you adjust the valves since installing the carb? Too tight of adjustment on the intake valves will cause rough running and backfiring.

 

Chrysler are very acceptable to engine problems that are caused by voltage fluctuations. Lower voltages less then 11 volts start causing minor problems with the ecu sending or receiving bad data. Less the 10 volts the problems really start to get bad. I seen many running problems from alternators not putting out the right amount of voltage or the alternator putting out AC voltage which will cause some really weird stuff to happen.
​

 

I disconnect the ECU on my 65 Olds ....

 

A few things. 1 is it. A lean pop ? You may need to adjust the accelerator pump / pump by adding cams that allow more fuel off the line. Last summer I did a bunch of carb tuning and after fixing an internal fuel leak on the carb it was so lean that it would cough pop and spit fireballs out the carb while it was running.

​​​​Check your timing. My sbo with the same cam liked 12 degrees initial.


You mention adjustable valve train. How much did you preload the lifters . Half turn ? Less preload is good. Just FWIW. Tight lifter preload will give a rough idle quality.

A flat cam will give a back fire out the carb when the exhaust lobe is going.