loosing power/backfire through carb

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dart65

dart65

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I had been experincing a loss in power to the real wheels failry consistently for a few days, as well as a consitent popping back fire through my carb while letting off the throttle. I tried to figure what was wrong and was told from my mechanic friend my distributor was not putting out proper voltage (or something like that). I replaced it and the coil and a lot more power came back. However, I still get an almost inperceptible loss of power when cruising - power comes then goes pretty sporadically and the drop in power is much less than before, but still is there. Also the popping through the carb is still there. I restarded the timing a bit and the back fire dminished but still occurs less frequently.Oh backfire only occurs when in gear moving, not in neutral idling Any ideas??
Oh almost forgot
65 dart gt
273-v8 4barrel holley old style street dominator intake mild comp (20-220-3 )cam, eddy 500cfm carb, electronic ignition/mopar, doug headers, rebuilt with about 40,000 /904trans rebuilt with about 20,000
Any suggestions for finding out what is the prob? I have never had any engine trouble before
 
If you unplug the vacuum advance hose does it help more? If it does then maybe your initial timing to advance.
 
It could be a bunch of things including a vacuum leak, bad cam, burnt valve seats. Like asked we need more info to try and figure it out.
 
OK I did a couple of diagnostic tests and here are the results; compression is on average about 110 in each cylinder with less than a 10 psi difference between the highest and the lowest readings. Vacuum test on carb showed slight fluctuation at idle no more than 1 inch. At 2000rpm vaccuum was constant. On releasing the the throttle (letting of the gas) the vaccum maxed the gauge and then returned to orignal idle reading. I get hesitation at start and around 35-40 mph and occasionally a backfire through the carb when lettign of the gas. Only when letting of the gas quickly do I get a bakfire through the carb. However it is inonsitent sometimes it happens somtimes not, but when it does its only when realseing the throttle quickly. The hesitation is kind of a ru-ruum thing that is much more consistant than the backfire. Does this help? How about you guys tell me what test to do and I can post the results.

again the engine is a 273 was a 2 barrel stock but now,doug headers,
old holley street dominator manifold, eddy 500cfm carb, mopar electronic ingnition, msd coil, doug headers, xtreme energy comp cam 250 (800-4800rpm), bored over .020 whole thing rebuilt with about 40 or 50 thousand miles on it.
 
Check to make sure your accellerater pump is properly adjusted so it squits fuel as soon as the linkage starts to move.

Backfire thru carb when throttle is pushed means the engine is going lean until the carb can suck enuff fuel to ensure the right mixture.
 
yellowdartdave said:
Check to make sure your accellerater pump is properly adjusted so it squits fuel as soon as the linkage starts to move.

Backfire thru carb when throttle is pushed means the engine is going lean until the carb can suck enuff fuel to ensure the right mixture.
Click to expand...

It actaully backfires when the throttle is released quickly not when it is pushed. I'll check the acellerator pump thanks
 
Your compression isn't great but should be adequate. Try putting a timing light on each plug wire in turn and see if there is any crossfiring or odd spark sequences. All plugs should have a regular, even spark pattern. If that check out, I'd be tempted to swap out the carb with a known good one just to see if that makes a difference. Backfiring through the carb on decelleration is pretty rare.
 
cudamark said:
Your compression isn't great but should be adequate. Try putting a timing light on each plug wire in turn and see if there is any crossfiring or odd spark sequences. All plugs should have a regular, even spark pattern. If that check out, I'd be tempted to swap out the carb with a known good one just to see if that makes a difference. Backfiring through the carb on decelleration is pretty rare.
Click to expand...

thanks I will try that tomorrow.
 
Sounds like the timing is off wither its the distributor or cam. Back firing through carb means a valve is open at the wrong time as well as the spark plug firing..... at least I would think it would be.
 
Check the little screens inside the carb where the fuel flows into the bowls. Even though we use and change fuel filters, crap still gets by.
 
NSF said:
Check 5-7 wires to be reversed on dist.
Click to expand...
Welcome to FABO and good call on the wires! I guess we all presumed that he checked the firing order but that may be a bad assumption.
 
The only thing I can think of is the timing chain and gears. To check just pull off the distributor cap and rotate engine with a breaker bar or ratchet and see if rotor moves as you turn over the engine. You should turn it to the left with the ratchet or breaker bar.
 
How long has the carb been on the engine? Was it running fine for a long period of time, and this only started recently? Has anything been changed that may coincide with the drive-ability problems? Was the carb put on last winter, and drive-ability issues started since the weather has been warming up?

Check the simple stuff first.
Carb:
It could be going suddenly lean, which will cause spitting back through the carb.
1. How old is the carb?
2. How old is the fuel filter?
3. Any recent adjustments like jets, springs or metering rods?

Wires:
1. Look at the engine in a very dark place while it is running. If the wires glow, or you can see sparks anywhere, the wires are likely shot
2. Do you have a good ground from the battery to the engine and the engine to the body, not necessarily in that order?

Timing
1. Timing at idle? (with & without vacuum)
2. timing at 1500 rpm? (with & without vacuum)
3. Timing at 2500 rpm? (with & without vacuum)



Let us know the answer to these questions. It may be something bad like a cracked block or a wiped lobe, but it is more likely something outside of the engine, e.g. fuel and spark.
 
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