Backfire through carb under load

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Yup Don, checked that. That cam is only operated when the actual airchokes are open on top(and all seems fine), which I couldn't check for sure because I had the flame arrestor on. Didn't want to be to careless regardless of how frustrated I am getting:)<br /><br />Plugs are always black when I take them out, not wet, just black. This should indicate a rich condition, which got me thinking I might want to relook at my timing.<br /><br />I should probably start a new post for it, but here it is. I have always noticed when I am adjusting timing that if the engine is running at ~700 rpm and I set the timing by rotating the distributor to where it is supposed to be to make the timing mark match up to 8 deg BTD, I can turn the distributor and it makes more RPM and the engine runs smoother.<br /><br />I think I might do this next time and keep adjusting then backing off throttle till I am back at my idel rmp range and check timing again. Not sure if this would cause the backfire through the carb I am experiencing, but I am running out of options.
 

Reel Poor

Vice Admiral
Joined
Jan 29, 2005
Messages
5,522
Re: Backfire through carb under load

You can simply advance or retard your timing by 2 to 3 degrees and see what happens. If it gets better suspect balancer slip, wrong balancer, or wrong timing cover.
 

dolluper

Captain
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Jul 19, 2004
Messages
3,900
Re: Backfire through carb under load

Well Mahoney amazing I tracked you down here,s a site you should check for your acc pump and so on adjustment's click on in carb section<br />Rochester Quad Manual M4Mc +++++<br />get ready for your neck exercises Ben There<br /> http://www.oldcarmanual.com/
 

Mahoney

Chief Petty Officer
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Aug 2, 2004
Messages
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Re: Backfire through carb under load

Haha, thanks again dolluper(ben there)I replied and thanked you in your other post. As you can see I have been hitting everyone up for suggestions. I think with everyone pitching in like they have we will get this thing licked.
 

dolluper

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Messages
3,900
Re: Backfire through carb under load

Got a question for you when you got this boat did it have AC s/plugs in it if so what # were they if not post the # and kind ,if AC post that #
 

Mahoney

Chief Petty Officer
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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I can check on that, I know now I have ngk plugs from cross referencing the old ones at Schucks. I still have the old plugs so I will check them out when I get home.
 

dolluper

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Messages
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Re: Backfire through carb under load

Post AC #'s if you can I have an attack plan
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I also found some other stuff I think I overlooked. There was a wire going to the distributor when I had my original point system that I double checked with pertronix. I had it not attached to anything, and it needs to go on the + terminal on the coil. That should be fine now(could definitely contribute to advance timing problems I guess)<br /><br />Also reading through the manual you sent me on the carb, I am questioning weather I have the needle valve plunger retaining clip hooked up correctly. It says it need to be connected on the center of the float arm, but it also notes it SHOULD NOT be connected to the 2 holes on top of the float arm. Well with the new float I got in the rebuild kit, these are the only places to connect it, so I am going to get the old float and double check this.
 

dolluper

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Messages
3,900
Re: Backfire through carb under load

That was one of my taughts it is tricky the first time you do one but you will never forget it now the plugs should sort out any if any timming problems it will ID the motor better to get the spec's, since in a boat you will time it as if it was a standard [gearshift] car not autos more RPM and some motors less degree's, by the way if I were you I would stick to AC's
 

fendersfender

Petty Officer 1st Class
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Mar 23, 2005
Messages
317
Re: Backfire through carb under load

i would have to agree that the carb is not the problem. i have the same problem with my '79 searay with 305. i have discovered the hard way (bought a new carb) that it is a miss that shows up under load only. it runs fine in driveway except for barely noticeable miss, but under load it misses and backfires. i have narrowed problem down to cam, lifters or valve problem----not a fuel issue
 

Mahoney

Chief Petty Officer
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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I think my problem is a lean condition, I can choke the carb by placing my hand over the primary choke and it picks up revs quick. Compression test, vacuum test and timing all check out. I seem to have severly misadjusted the carb upon rebuild and am redoing it from scratch to see if that helps.
 

weatherz

Seaman
Joined
Oct 9, 2004
Messages
56
Re: Backfire through carb under load

Mahoney, I was just curious if you had double checked to see if two plug wires got crossed somewhere along the line like I had mentioned in an earlier post. I have seen this and it acts exactly as you have described. Good Luck
 

Mahoney

Chief Petty Officer
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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I am rebuilding the carb to specs again today. I think there were some things out of whack, but yes I am going to double check wires. I would be ticked if that was it. I learned that lesson the hard was long ago.<br /><br /><br />Thanks Guys.
 

rodbolt

Supreme Mariner
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Sep 1, 2003
Messages
20,066
Re: Backfire through carb under load

hello<br /> did this carb ever run correctly on this motor? sounds like either the secondary air valve is not adjusted correctly or the main well or mainwell feed holes may be stopped up. could be the secondary air bleeds or incorrect secondary metering rods. will it pop and slow down or pop and contiue on up in rpm or constantly pop when holding the throttles open. also not much can be done unless the engine is under a load as all the power circuits are vacum operated and with no load vacum signals will be incorect.
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Ok, redoing carb, but found out something with ignition as well. TDC is TDC, balacer is right on, when I took off the dist. cap I noticed a bunch of red dust from the cap. Apperantly the rotor that cam with the ignitor kit uses screw to hold the contact strip on and when the rotor spins it shaved a little groove in the molding, no damage, but I replaced with the stock rotor, and it fit like a charm.<br /><br />Another question. I have a ballast resistor. I called pertronix and they told me to just place the black wires from the stock wiring harness on the positive side of the coil and the grey(tach) to the negative. Then hook up red from ignitor to + and black to -. seems to work. I found the old instructions though and it says to splice the red from the ignitor to the ballast resistor.<br /><br />My existing ballast resitor shows a slightly different setup than in the ignitor instructions. I have 3 wires ging into the resitor mounted on top of the intake man. 2 on one side and one on the other. The ignitor diagram only shows one and one for stock. Do I just get a fitting and put the red wire into the terminal where the 2 wires are attached?
 

Mahoney

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Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Well it was a lean mixture. Even though the carb was running prior to storage I am told. I decided to give a whirl and prop up the secondary rods a smidge to open the jets ever so slightly even during the intermediat circuit, and low and behold--NO BACKFIRE when accelorating.<br /><br />Apperantly they were seated so snug in the jets that the transition never happend fir them, and subsequently never reduced vacuum enough to release the vaccuum secondary for the butterflys to open.<br /><br />I am sure I have some tuning to do, But alas!
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I wil prolly have to get a richer set of metering rods, but it is WAY better now. I need to try it out on the water and do little adjustments, those things are finicky for sure.
 

Olds Eddie

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Jan 30, 2005
Messages
194
Re: Backfire through carb under load

I used to work on Q-jets all the time when they were used on new cars. The hanger that holds the secondary metering rods come in different heights to address the problem you found. Sometimes when you put a carb together from the scrap pile you run into these things. I probably have several different hangers in my spares box. I own a 69 Hurst/Olds which uses a Q-jet so I never throw any of this stuff away. Let me know the measurement from where the hanger mounts on the cam follower to the holes for the rods and I'll see what I have. I also have plenty of metering rods. I belive the most common ones were marked AJ.
 
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