carb backfire

[str8up720]

I'm having some backfire troubles on my 318. Engine fire up well but won't steadily idle for long, and when it dies, the carb spits fuel back out. Is my distributor of 180*? Would it even start up if if it was? Check all the wires and are in the1-8-4-3-6-5-7-2 order. Any ideas?

 

[Roughbird72]

could be.
could be valvetrain problems.
i've had engines with bad camshafts do this ... lobe/lobes gone

 

[bm02tj]

is this a new motor or an old motor
is the rotor pointing at the contact or between two of them in the cap
how is the dist bushing

 

[str8up720]

The motor isn't new, but I just had it tore down not to long ago for inspection, and mainly to replace main bearings and rear main seal. All the internals checked out fine, including the camshaft. I just finished re installing the distributor making sure motor was at tDC compression stroke, and that the rotor was pointing at #1 tower. Battery is charging, so will have to come back and fire it up again later....

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is this a new motor or an old motor
is the rotor pointing at the contact or between two of them in the cap
how is the dist bushing

Which is the bushing you are reffering to?

 

[coloradodave]

If the motor started and ran, the distributor is NOT 180* out. Put a timing light on it and set initial timing at approx. 12-16* BTDC with vacuum can disconnected and hose plugged.

 

[bm02tj]

Bushing in the distributor that will let the dwell wander and cause all types of erratic things
also did you replace timing set or check for slack

 

[The Horvaths]

Backfire out carb can certainly occur with way too much advance, as in a cylinder too soon.
Of course there may be another reason but this is a good start.

 

[charger318]

u could be running real rich on carb, I was messing with my 318 in charger and when I would try to rev it up quickly in park it would stall and something back fire through carb because my carb is holley vac sec.( so sec butterflys wont open with no load like being in park as I was) but the accelerator pump would get press so the extra fuel was to much and cause a stall and BACKFIRE through carb. so make sure u aren't running real rich good to start bad for running?

 

[str8up720]

So here's where I am at...I feel like I made progress. I played around with the distributor a bit more and checked the bushing, which was covered in rust! I cleaned it up a bit and installed everything back on the way it was. Car fired right up and stayed running at pretty high Rpms. No more backfire! Now the issue seems to be the high idle. I set the timing light and while idling at 3500, while backing out the idle and high idle screws barely enough to keep it running, timing was at about 12*. This seemded strange to me but nonetheless I continued advancing timing, and Rpms continued rising. I also started noticing the headers getting hotter noticeably as I advanced the timing. Any thoughts or ideas? I have a feeling my timing light may be bad, so I may need to get another to verify the timing, in the mean time I'm open to suggestions...

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Bushing in the distributor that will let the dwell wander and cause all types of erratic things
also did you replace timing set or check for slack

How can I check for timing chain slack? I think I have read that you can check with the distributor somehow..?

 

[coloradodave]

A couple of problems: 1) 3,500 rpm is not idling. 2) Normally, when headers get 'hotter' (are they glowing?) the motor is too far retarded, that is, the exhaust valve is partially open when the cylinder fires and the air/fuel is ignited in the header.
In order to check the initial timing, you have to get the motor to idle down to 700-900 rpm, depending on the motor, and have a timing light that works (or see below). Check the obvious reasons that might be making the motor rev to 3,500 rpm. Vacuum hose connections, missing carburetor base gasket, leaking intake gaskets, choke fast idle cam stuck or misadjusted, choke blade incorrectly adjusted, etc.
You can get the initial timing close by pulling #1 plug and turning the motor over by hand with your finger over the plug hole. When the compression tries to push your finger off the hole, stop turning and look at the balancer. The timing mark should be approaching Top Dead Center (TDC). Continue turning the motor until the mark on the balancer is aligned with the 10* BTDC mark on the timing tab. Then look at the #1 tower on the distributor cap. Take a sharpie pen and make a mark on the side of the distributor case straight down from the #1 tower. Remove the distributor cap and turn the distributor until the rotor is perfectly aligned with the mark on the distributor case. Tighten down the distributor hold down bolt. Put the spark plug back in and fire it up. The timing will be very close. Good luck and keep us informed...

 

[HT413]

Same thing happened to me. My timing was like 10 degrees retarded, I dunno i guess I put the dizzy back in a little FUBAR'd after I had the cam out and top end apart. Carb backfiring like a son of a b. Straightened that out and it still ran like hell. turned out the main vac port at the back of my holley 750 was wide open. I think one of the carb backfires blew the plug off the vac port. Now I have very little carb tuning experience and that double whammy took me quite a while to work through lol.

 

[bm02tj]

At 3500 you should be at full advance about 34 to 36deg

 

[miller]

Full advance is usually in around 2000 RPM.

Even 700 RPM is kinda high for a stock motor. Initial timing has to be checked at IDLE speed. Much over and the advance starts kicking in, and throwing your timing read off.

Like the guys say, do a hand check on setting your timing at 10 BTDC. For the idle, back the idle speed screw out, until it is off the cam arm...then, turn it back until it just makes contact, and 3/4s of a turn more. That will simply 'crack' open the throttle plate, and the thing should idle, if the timing is close.

If, and when it does start, check the idle timing and idle RPMs, adjust as needed to get it basicly set. You've also fooled with the mixture. Is it a four-barrel? If so, you've got 2 mixture screws. Once your timing and idle speed are set, if your mixture is off, one mixture screw at a time, slowly turn one in until the RPM starts to drop, and back it out to the highest RPM. Shut the motor off, gently turn that same screw in until it touches the seat, counting the number of turns it takes.
That number of turns is your basic mixture 'set'. Back that screw back out that number of turns. Then do the same thing with the other mixture screw (if you have two), gently seating it, and back it out the same number of turns as the other screw.

Start the motor, and check the idle RPM. If it's off, reset it to the right RPM. Mixture can affect the idle speed. Idle speed affects timing. They all go together, and needs to be right, to set each one.

 

[str8up720]

Thanks for the help everyone. As bad as I want to get my Mopar on the road asap, I won't be able to get to it for a couple of days. I will keep your advice in mind and keep you all posted!

 

[str8up720]

So here is where I stand. I just wanted to verify with all you my steps before I startit up again.


This is where my timing is set (14* btdc compression stroke)



Rotor pointing at number one wire




Order of my wires

I have my idle mixture screws backed out 1 1/2 turns, idle and fast idle backed out all the way. Will adjust idle screws as I see it. anything else i should adjust while im at it?

 

[str8up720]

So I got my hands on another timing light and confirmed mine was damaged. I ended up setting timing to 35* advanced at 2500 rpm. I am almost at where I want to be to be able to drive this thing. Only issue I am having now involves the choke. I noticed when I let it open, idle increases. When I hold it closed with a small amount of opening, it idles just where I want it to at around 850 Rpms. I feel like my choke is misadjusted, although I did set it via edelbrock specs. ( by the way the carb is an edelbrock 4 barrel #1406 with an electric choke). My question is how does the choke operate? Does it have to open after warming up? Can I just tighten the choke valve to where it keeps the choke plate closed?

 

[miller]

Suppose there's no way to see the timing at idle, until the carb is right.

Is that a rusted part in the distributor?

All the choke does, is exactly as it's named. It 'chokes' the air supply to the carb, forcing an overly rich mixture. You do not want that rich mixture all the time. It would effectively 'wash' oil off your cylinder walls, and piston rings, and flat wear them out.

It's been way to long since I've messed with an electric choke. Can't recall exactly how they work. Somebody else jump in here.
Off the top of my head, the choke adjuster should be backed off, until the choke plate is fully OPEN. Then, you turn it back in, until you have a measured gap at the top edge of the choke plate. That gap you'll need to look up.

Last...you do not want to adjust any timing, or carb settings, until the engine is at least warmed up...choke fully open!

 

[coloradodave]

The electric choke operates with a bi-metallic coil, that is, a spring under the cap that is wound of two dissimilar metals. It uses 12 volts to heat the spring and 'unwind' the choke blade at a given rate. There are 2 contacts on the plastic choke cap. One contact gets grounded (one of the cap hold-down screws will work) and the other gets an ignition operated 12 volts (can take off the ignition side of the ballast resistor). When the motor is stone cold you adjust the blade by loosening the 3 plastic cap hold-down screws, opening the throttle a bit (to release the fast idle cam), and turning the cap CW while watching the choke blade close. When the blade is just closed, tighten down the 3 screws.
This will get the adjustment close. The fine tuning of the choke including the setting for the blade opening and the fast idle will are below, which is from the Edelbrock Owner's Manual.
Everything else in your timing and plug wire pictures looks perfect.
Good luck...

 

[The Horvaths]

A key point here is that the engine should be tuned with the engine fully warmed up and choke wide open. Then you can worry about the high idle choke setting.

 

[coloradodave]

A key point here is that the engine should be tuned with the engine fully warmed up and choke wide open. Then you can worry about the high idle choke setting.

Absolutely. So, to set the choke blade wide open, disconnect the hot lead, use tape to insulate it from ground and loosen the 3 cap screws. Then slightly open the throttle and turn the cap CCW until the choke blade is open with a little spring pressure to keep it there. Then tighten the 3 screws.
This assumes that your car will start without a choke....if not, see my original post.