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Backfires thru carburetor when starting cold engine
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Author:  sam03v [ Wed Aug 05, 2009 6:15 am ]
Post subject:  Backfires thru carburetor when starting cold engine
I've got a Carter BBD on my 225 and when trying to start the engine cold it is backfiring thru the carb. It's not a loud bang,just sounds like starter fluid being ignited backwards thru the carb, then the starter spins. After 3 or 4 tries the engine will start. Once it is hot/warm it will start fine.

Is it just the carb bowl going dry? I only use the vehicle about once a week and the temp outside is in the low 90's this time of year.

thanks in advance,
Author:  Reed [ Wed Aug 05, 2009 8:16 am ]
Post subject: 
Sounds like a timing problem and/or a valve adjustment problem.
Author:  Aggressive Ted [ Wed Aug 05, 2009 10:28 am ]
Post subject: 
Advance your timing......sounds like your too close to zero degrees or TDC. Try setting it at 10 degrees.

When was the last time you lashed the valves? If one or more valves are hanging open too long and or not closing all the way it will also cause the backfire until it's warmed up.
Author:  sam03v [ Wed Aug 05, 2009 10:44 am ]
Post subject: 
Actually I think the timing probably is set at about TDC. I couldn't get the idle below 1100 rpm after putting a super six setup on it and finally discovered the timing mark was way off. So...I rotated the engine by hand until the distributor rotor was pointing at the #1 plug, marked the harmonic balancer and called that TDC. So its far from accurate. After that it would idle much better, but I got the cold back fire problem as a trade off. I'll try advancing the timing a bit and see what happens.

Thanks!

By the way this is 82 slant so it doesn't have adjustable valves...
Author:  Aggressive Ted [ Wed Aug 05, 2009 11:01 am ]
Post subject: 
Sounds like you need to confirm timing and get another damper.......to eliminate the backfire........

As for the idle not coming down after the Super Six install, you may have a vacuum leak. Either under the carb, like the base gasket, or the intake manifold gasket. Which intake gasket did you use? You might try a little spray around the gaskets.
Author:  Reed [ Wed Aug 05, 2009 11:18 am ]
Post subject: 
(1) make sure the throttle cable isn't holding the throttle open on the carb. This is a common oversight when doing a super six conversion.

(2) in the short run, get yourself a positive stop tool and use it to determine where true TDC is on your motor. Install the tool in the #1 spark plug hole. Rotate the motor by hand until the piston comes into contact with the tool and the motor won't turn over any more. Mark the dampener at the TDC mark. Rotate the motor the opposite direction until it stops turning and mark the dampener at TDC again. The midpoint between your two marks is true TDC on your motor. Mark true TDC and reset your timing (remember to remove the tool and reinstall the #1 plug). Most slants aren't designed to run at TDC. Double check what your factory timing is supposed to be.

(3) check your idle vacuum. Refusing to come down from 1100 RPM is likely a combination of misadjusted timing and the throttle problem described above. A correctly set up slant should be able to idle smoothly down to about 500 RPM (I have had motors idle even slower).

(4) report back your results.

You say the slant is an 82. Does your motor have any sort of computer controlled carb or ignition system?
Author:  slantfin [ Wed Aug 05, 2009 11:38 am ]
Post subject: 
Quote:
Rotate the motor by hand...
Disconnect battery ground first so she doesn't start up unexpectedly. Remote but unpleasant possibility.
Author:  sam03v [ Wed Aug 05, 2009 1:52 pm ]
Post subject: 
Thanks, I'll get a positive stop tool and readjust.

I don't think I have any leaks, i did initially after swapping manifolds but after many frustrating hours got them sealed. I cheated here though, I bought one of those Australian gaskets, and it leaked, so for grins I put a new Autozone gasket on top of the Aussie gasket and the leaks sealed. I sprayed copious amounts of oil around the manifold gasket(s) and did not here any increase in engine speed.

It's an 82 motor in a 80 pickup truck. The carb is definitely not computer controlled, but it does have electronic ignition.

I put a new throttle cable on it and it is good to go. I picked up the proper throttle cable bracket off ebay so the cable lines up to the carb nicely and it is not holding open the throttle.

I'll report back after I get the tool...
thanks.
Author:  sam03v [ Thu Aug 06, 2009 12:12 pm ]
Post subject: 
Quote:
in the short run, get yourself a positive stop tool and use it to determine where true TDC is on your motor. Install the tool in the #1 spark plug hole. Rotate the motor by hand until the piston comes into contact with the tool and the motor won't turn over any more. Mark the dampener at the TDC mark. Rotate the motor the opposite direction until it stops turning and mark the dampener at TDC again. The midpoint between your two marks is true TDC on your motor. "
I've done a little reading about this method and have two questions. Will this work without a "degree wheel"? Do i need to worry about the valves hitting the stop?
Author:  daveca5150 [ Fri Aug 07, 2009 10:14 am ]
Post subject: 
You don't need a degree wheel and no you don't need to worry about the valves hitting the piston stop. Unless you already have a degree wheel, they are a bit expensive and not needed for this simple operation. The piston stop is essentially taking the same position of your spark plug, just a little farther toward the piston, which will not interfere with valves. Just use a white grease pencil, white out, thin tape or whatever else makes a good clear mark. If your two marks are really far apart and you're not sure where the center is, you can always back the piston stop out a bit and do the process again. Once your two marks are about 1" or so apart, you can get really accurate about where their center is. By the way, taking all of the spark plugs out will of course make it easier to turn. You still have the timing tab on your timing cover right? You want to make sure you mark accurately right at the '0' on the timing tab. I've seen these missing before so I just thought I would ask.

Good luck.
Author:  sam03v [ Fri Aug 07, 2009 12:04 pm ]
Post subject: 
Yeah the timing tab is still there. I should be getting the stop in the mail today, so I'm hoping to play with it this weekend.

thanks again.
Author:  1967 Dart [ Tue Aug 11, 2009 5:35 am ]
Post subject: 
I have almost smilar problems with 1976 225 with Holley 1945. Mine starts and idles perfectly when cold but hesitates and backfires when applying throttle. Accelerating is difficult, but idle at gear and steady speed driving works correctly. If you manage to get your speed over 35 mph, there are no driveability problems. When engine is warmed up, there's almost no hesitation at all, just feels little powerless (yes, I know, It's an emission-era engine).

The problem started when I rebuilt my carb. The previous owner had disabled the economizer by putting sealer into the vacuum passage - I cleaned it. One bolt out of three was missing from the bottom section of the carb. The top part of the carb was slightly warped - Filed a little and checked with the straight edge, now it's almost correct. Sealing the acceleration pump channel is still a bit of a problem, but with thick gasket, it's ok.

The acceleration pump squirts gas, but the squirting begins not until the lever has moved about 1/2". The acceleration pump piston rod have been modified somehow (the old rubber is completely different from the original) and the new pump rubber was moving up and down quite much when installed. The rubber is floating type but I think it shouldn't move that much. This could be one problem.

The nitrophyl float is bad. It's cracked and some pieces have separated. Ordered the new float.

Timing is set to factory specs, 2 BTDC with vacuum advance unplugged. Vac advance uses man.vac and timing is over 15 BTDC with vac advance present (vac advance pod reads 8.5x = 17 degrees advance?). Mechanical advance works. I'll try some more advance next, if nothing else helps.

Valve lash is ok.

If you have any ideas what to check next, I'll appriciate that.
Next thing I'd check again is the acceleration pump.
Author:  1967 Dart [ Tue Aug 11, 2009 8:22 am ]
Post subject: 
As I presumed, the acceleration pump was defective. I fixed it now there's less hesitation. There's still a dead spot in certain rpm, but only when engine is not totally warmed. When warm, it's operating normally.

There was a vacuum leak in EGR gasket, but fixing that didn't help.

I continue by adjusting mixture and timing again and checking vacuum reading.
Author:  sam03v [ Tue Aug 11, 2009 8:35 am ]
Post subject: 
I checked TDC with the stop and the original mark on the damper is okay. I don't know what i was looking at before. There are two grooves on the damper 180 degrees apart, I may have been looking at the wrong one to begin with. My battery died so i couldn't reset the timing last night, but will try again later today.
Author:  sam03v [ Wed Aug 12, 2009 5:40 pm ]
Post subject: 
i tried to get it running today and it wouldn't run worth a #@!$. When I finally got it to idle I set the timing to 10 BTDC, but the truck didn't have enought power to get out of the driveway, and it spit and backfired thru the carb like crazy. THe gas pedal would actually drop during some of the backfires!

I"m still blurry on how that TDC stop tool tells you TDC. How do you know if it is showing TDC on the compression stroke or the exhaust stroke?

I checked the compression in all cylinders and it was 135 for each. At some point during the testing I noticed the positive wire on the ignition coil had broke off so I put a new termnial on it and reattached. I thought that maybe it had been an intermittent contact causing the problems all along, but after repairing it the engine still ran terrible and didnt' have enough power to get out of its own way. I"m not sure what's going on here because the last time I had the truck running it would run fine once I got it started.

Any suggestions? I"m at a loss here...
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