Backfire through carb under load

Mahoney

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Re: Backfire through carb under load

Yes, Bondo I feel a bit stupid for the last couple posts, this is the original engine, just when I went searching trying to decode that block it came up with chevy truck.<br /><br />I am checking iboats for the applicable fuel pump for this engine, and came up with 18-7268. The flange number on the pump that is on there is 407778.<br /><br />Would you mind double checking this for me? I am getting some conflicting results when searching around for the part. Also if you had a carb model number for that it would be fantastic! I learned a lot about my engine in the last coupe days, and don't want to go ordering the wrong parts.<br /><br />Thanks again bondo!
 

Bondo

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Re: Backfire through carb under load

Well,....<br />In My NAPA book,..with it's Serria Part #s,.. Which I believe iboats uses......<br />Your fuel pump with #407778 flange ID#,.. Should be replaced with #18-7263.....<br />#18-7268 is Also for the 307cid, but Flange ID# 41415....<br />My Guess is they're the Same, Except the ports have been moved to line up with the fuel lines....<br /><br />Also listed in the Same Book,......<br />The Carb. Should be,...OMC # 382821..... MFG. # 7028280...... Code,+ Model , R4 4MV..... Part#18-7605.....<br />Carb. Kit # 18-7072.... Float part # 18-7201......<br /><br />Good Luck...... ;)
 

Mahoney

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Aug 2, 2004
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Re: Backfire through carb under load

Thanks Bondo, thats a life saver, it looks like the carb was replaced with a similar but not correct carb at some point as well.(7059285)<br /><br />Off to order!,
 

Reel Poor

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Re: Backfire through carb under load

A worn out cam lobe will cause this type problem also. Might pull a valve cover and at least look. It could be that an exhaust valve is not opening but just a little.
 

Mahoney

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Re: Backfire through carb under load

Still stumped. New fuel pump got here and I am still only getting 4lbs pressure. I bypassed my fuel water filter and ran feed directly from tank to pump and still no dice, also loosened gas cap as well. Manual is reading this should develope 7 lbs pressure, and I have been getting 4 with both old pump and new pump.
 

trog100

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Dec 1, 2004
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Re: Backfire through carb under load

a thing creates pressure by applying a certain force against a given resistance.. the force would be the pump itself the resistance would be the little valve that is sometimes closed sometimes open that lets the gas thru into the carb float chamber.. unless there is some kind of pressure relief valve built into a mechanical fuel pump i cant see how the pressure would remain constant anyway..<br /><br />anyways mahoney.. if a new one performs exactly the same as the old u only have two possibilites.. nothing wrong with the old one.. or the new one is faulty in exactly the same way..<br /><br />i go for option number one here.. he he <br /><br />could the manual be wrong..????<br /><br />trog100
 

Mahoney

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Re: Backfire through carb under load

Little more information. I have found that the backfire occurs(and only occurs) when the secondary butterflys start to open, I can snap rev wherever I want all day as long as the linkage doesn't hit the secondary butterflys.<br /><br />This would further indicate (as talked about in this post) a lean condition as when those 2 big butterflys open its popping back through the carb.<br /><br />Any suggestions with this most recent info?
 

Dunaruna

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Re: Backfire through carb under load

Just a thought - test your pressure gauge.
 

KaGee

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Re: Backfire through carb under load

Interesting read to say the least.
 

trog100

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Re: Backfire through carb under load

yes the jets that feed the secondary are not supplying enough gas.. it isnt the fuel pump its the carb as i have kept saying..<br /><br />if the backfire is the instant the secondary is opened its directly at the carb jets.. u are still running off the float chamber gas.. for it to be the pump it would need to be held open longer and be under load and drawing real amounts of gas.. the float chamber would need to empty and not be supplied with gas quick enough from the pump.. lack of fuel due to pump or filters would take longer to come in..<br /><br />whats happening is simple.. as u open the secondary choke air rushes in without enough gas from the secondary to go with it.. result instant weak mixture and backfire.. not being overly familiar with the carb u have i cant say exactly why.. but its not fuel pump or filters look at how the secondary is fed with gas.. thats your problem..<br /><br />i would try and grab another carb if it dosnt cost too much.. i recon the old one has been "looked" at enough.. <br /><br /><br />trog100
 

Mahoney

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Re: Backfire through carb under load

I agree, I double and tripple checked avery fuel delivery section in my boat and ruled all of them out to the carb now.<br /><br />Its a rochester quadrejet that I had to take apart, clean, blow out(i can be sure all passages are clean) and reassembled with new float and accelorator pump plunger. It is possible I miscalibrated something.<br /><br />Does anyone have any ideas on what I might look for before I break down and buy a new carb, which after my buying a new fuel pump for no reason, isnt sitting so well:)<br /><br />Thanks
 

trog100

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Re: Backfire through carb under load

"buying a new fuel pump for no reason, isnt sitting so well"..<br /><br />dont worry about that one too much its par for the course.. something we have all done.. he he <br /><br />trog100<br /><br />ps.. what a less than honest but not that uncommon professional would do in similar "go for the wrong part" circumstances would be leave the unneeded pump on.. fit the needed carb and charge the customer for both.. he he he
 

Reel Poor

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Re: Backfire through carb under load

I think you have a mechanical valve problem. Do a compression test. If it is to specs, [a worn exhaust cam lobe will not show as a problem during compression test or cylinder leak down test] then pull the valve cover and watch rocker travel.<br />You mentiomed in an earlier post that you were going to "RETORQUE" the intake manifold. Has it been off????? If it has check for proper intake bolt length and placement. Two of the SBC intake manifold bolt holes line up with two pushrods,[one on each side, but the same position on each head] if the bolts have been replaced they should be 1 1/4 inches long. Most bolts are sold in 1/2 inch increments and wont work for intake bolts 1 inch is to short and 1 1/2 inch is long enough to go through intake and head, and drag on the pushrod. :eek: :eek:
 

Mahoney

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Re: Backfire through carb under load

Valve bolts were replaced where needed with same size and lenght bolts and torqued to spec. I think if it was a valve issue I would see this behavior prior to the secondary butterflys opening on the carb. Wouldn't I?<br /><br />I think it is too much of a coincidence that as soon as the secondarys are opened I get this behavior, and never before, which is why I could idle in the drive at such high RMP's then have it pop under load when I had to give it more gas, opening the secondaries. <br /><br />I am doing my best to research more about carbs, but will probably end up taking it somewhere to get professionaly rebuilt.
 

KaGee

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Re: Backfire through carb under load

Most shops around here won't do the carb unless they do the complete job.<br /><br />*If you are sure the ignition and timing is set right. <br />*And you are getting proper spark advance.<br />*And the vacuum readings are good.<br />*We know that the fuel pump is good.<br />*And Compression on all cylinders is good.<br /><br />Then that still leaves lean fuel as the probable issue left to deal with. Which leaves the carburator as the suspect. Confirmation of that to me is the problem arises in sequence when the second butterfly opens. <br /><br />I wouldn't mess with it anymore.<br /><br />Ebay has a ton of 4 barrells right now. Some re-manned, some used, some new. With a little homework i would think you could pick one up there that would work and not hurt the wallet much more.
 

Mahoney

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Re: Backfire through carb under load

This weekend I am going to go through the checklist again. I have petronix ignitor ignition, no problem shifting. I am not sure if there is a shift assist module on that year, but I don't think so, when I shift there is no engine hesitation at idle.<br /><br />I will also do a compression test, but everything still seems rather suspect to me that the backfire only occurs as the secondarys open. I have never got it to backfire reving it with only the primaries opperating.<br /><br />Timing was good, advance was also working<br />pump is new<br />lines are all clear<br />vaccuum test was good<br /><br />thanks all, hopefully when I get this running again I can proudly post the solution, this one has everyone I have talked to stumpped.
 

trog100

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751
Re: Backfire through carb under load

the solution will be replacement carb mahoney... he he he<br /><br />
cwm42.gif
<br /><br />trog100
 

Dunaruna

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Re: Backfire through carb under load

Damn trog, you're one ugly dude :D <br /><br />Mahoney, before you go and start spending more money check your spark. Should have a nice blue crisp color and a "snap"ing noise. <br /><br />Aldo
 

Mahoney

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Re: Backfire through carb under load

thanks Dunaruna, Ill add that to the list of double checks as well.
 

Don S

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Re: Backfire through carb under load

Well, Im not going to read all the replies to find out what you've done, but did you check the small cam that lifts the secondary metering rods up when the secondaries are open? They can break and then the metering rods won't lift up when the secondaries open.
 
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