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Discussion Starter #1 (Edited)
Ok, I have been through everything that I can think of and can't figure this out. I have developed a back fire/sputter through the carb. I am pretty sure it is something simple and nothing serious like a flat cam or a valve or spring issue but I could be wrong. It only does it intermittently. Very rarely does it happen below 4k rpm or so and once I really get on it then it happens every so often, sometimes more sometimes less. The noise in the carb is not a POP like a backfire but sounds more like air is being blown back out the carb with a momentary loss of power. I am thinking it is my coil or 6al box but not sure. Any help or suggestions would be great. Thanks.
 

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have you checked timing,ignition.

i would check my timing to eliminate that possibility.even if you haven't moved it.good luck these kinds of things are a real pain in the ass:|errtravis
 

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What the Hell is That?
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I'll be watching this one. I have the same problem.
 

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Had the same problem a year ago. I ended up pulling the bowls off, and squirting carb cleaner ( using the tiny straw) through all of the passages. Sure enough, one air passage was plugged, and after really holding the straw firmly in the hole, it popped the crud loose. As soon as it came loose, I knew it was it. I put the carb together after cleaning, and she has run perfect ever since.




Darrell.
 

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When was the last time you looked inside you dizzy cap?
x2, or there's a plug goin to shit. i'd also wanna know the condition of the valve springs, are they weak?
 

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x2, or there's a plug goin to shit. i'd also wanna know the condition of the valve springs, are they weak?
X3. Replace your dist. cap. Then, if that doesn't work, buy or rent a valve spring pressure tool. You can test springs right on the motor. Good luck.
 

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Check the Dizzy cap for cracks too. If the center deal in the cap is wore off it can cause problems.
I would also check the pick up in the Dist. its 20 bux for a new one.

Just before the 6 AL took a dump in the Cheena I had a pop at around 3 to 4 K till the box died. Sent it MSD and they fixed it for a fee. Less than 50 bux with shipping.

I got a spare Box in the boat when I'm on the lake.
Got to have a perfect ground with the 6 al box. That and getting the box wet will kill em.

Hope its something simple.
 

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It looks good to me. What should I be looking for if it were bad?
other than cracks and carbon tracks, the center post inside the distributor is carbon, and will wear down over not too long a time. i'd also put a timing light on it, check timing up to 5k or so, just to make sure it doesn't start jumping around at upper rpm...
 

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Discussion Starter #14
Thanks guys. All good ideas. I am truly stumped. I have gone through all the internal possibilities and fuel possibilities. Now I am thinking it is electrical and the dizzy is my next step. Thanks again. Anymore ideas would be appreciated. I am going to replace the cap and rotor tomorrow. I have had a new cap, rotor and wires for two months but have not had time to make up the wires but it's time. I am also putting new plugs in. Hopefully this will take care of it.
 

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Thanks guys. All good ideas. I am truly stumped. I have gone through all the internal possibilities and fuel possibilities. Now I am thinking it is electrical and the dizzy is my next step. Thanks again. Anymore ideas would be appreciated. I am going to replace the cap and rotor tomorrow. I have had a new cap, rotor and wires for two months but have not had time to make up the wires but it's time. I am also putting new plugs in. Hopefully this will take care of it.

I've noticed lately the MSD rotors, do not have enough spring pressure on the cap, this results in the carbon button burning away rather quickly.
 

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Thanks guys. All good ideas. I am truly stumped. I have gone through all the internal possibilities and fuel possibilities. Now I am thinking it is electrical and the dizzy is my next step. Thanks again. Anymore ideas would be appreciated. I am going to replace the cap and rotor tomorrow. I have had a new cap, rotor and wires for two months but have not had time to make up the wires but it's time. I am also putting new plugs in. Hopefully this will take care of it.

What carb are you running? Had a Holley DP acting same way, found the squirter adjustment way to loose. Motor was starving for fuel on initial hit until the squirter cam caught up. Check arm adjustment, squirter pumps and for blocked passages.
 

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My 18' jet boat did the exact same thing! Sonetimes when I got on it hard over 4000 rpm a slight pop through the carb. Most fuel pressure guages are on the carb lines, but at WOT is where my problem was. I do agree with others cleaning the carb but it sounds like you have. I changed my mech. fuel pump to a Clay Smith and never have had a pop again, my basicly stock 454 turns around 4500 rpm up and down Parker all day POP free! Good luck, hope this helps.:D
 

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Discussion Starter #18
Well

After racking my brain for days and not getting any results from anything external I finally bit the bullet and decided to pull the heads. Looks like my problem was a blown head gasket. I did a compression test and all cylinders were within acceptable range but I left the other plugs in so I decided to take all the plugs out and try again and #3 and 5 were 90 and 60 compared to 180 on the rest. I figured I had valves not seating properly but the gasket between 3 and 5 was shot. Looks like the plug in number 3 held the pressure for 5 and vice versa. So off to lopers to get some head gaskets. Hopefully this is the problem and I didn't just catch this before it became a problem. I'll lose my hair if it still does it after the new gaskets are on.
 

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Discussion Starter #19
OK, so I am losing my hair as I type. The problem ended up being burned exhaust valves on 3 and 5 and the little bits that broke off tore up the pistons. So looks like I am getting all new valves and springs and I am going to try to file and sand down the pistons so they will at least make it until the new motor is done and I will rebuild this one to a 496. This really blows because this is the second motor already this season that has had to be torn down. Well teaches me not to flat foot it if I don't want to spend money. I'll put up some pics tomorrow.
 

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What the Hell is That?
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OK, so I am losing my hair as I type. The problem ended up being burned exhaust valves on 3 and 5 and the little bits that broke off tore up the pistons. So looks like I am getting all new valves and springs and I am going to try to file and sand down the pistons so they will at least make it until the new motor is done and I will rebuild this one to a 496. This really blows because this is the second motor already this season that has had to be torn down. Well teaches me not to flat foot it if I don't want to spend money. I'll put up some pics tomorrow.

Not really the answer i wanted to hear
 
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