Backfire through carb under load

rodbolt

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Re: Backfire through carb under load

hello<br /> whoever did the carb last knew some about the q-jet. q-jets are incredibly simple carbs that have been about since 1965 or so. the epoxy on the passage plugs is something I always doo. make sure its still there and in good shape when the dunking is done.<br /> the red seal is not a seal.<br /> its a retainer. the seal is between the bore and the jet holder body. engine vacum holds the spring loaded jet holder down. when the throttles are opened and engine vacum falls the spring pushes the holder up and the tapered rods retract from the priamry jets and more fuel flows through the primary circuits. like I say its a very simple carb but requires some knowledge on the circuits and how each works.
 

Mahoney

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Re: Backfire through carb under load

Rod bolt, this is great information. There was a seal missing from this "brass shaft"( bore and the jet holder body)I am guessing this is what you mean.<br /><br />Basically you are saying there should be a seal under where that red retainer is, on the rod holder?. There is not one there.<br /><br />I think I have the seal for that area.
 

Mahoney

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Re: Backfire through carb under load

I posted a new pic, entitled Carb passage left. This little hole, left in the pic, seems to be the only one I can't blow out on both sides of the primary jets. What is this for, and should it be able to blow out clean? It could be that the passage is so small that I can't tell through the over spray.<br /><br /> http://community.webshots.com/photo/311524204/311877248SZZIFB <br /><br />Thanks!
 

trog100

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Re: Backfire through carb under load

yes my slamming the throttle open and hand choking isnt a real test.. but it is the kinda thing i would do after rebuilding the carb and sticking it back on before going to the trouble of carting the boat off to the lake for the real test.. it might just save half a days messing about and dosnt take long..<br /><br />trog100
 

Mahoney

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Re: Backfire through carb under load

Almost ready to put her back on, I answered my last question, those holes do go down tto the primary metering jets, all blown out. I am going to retorque the intake manifold while I got the carb off, to eliminate vaccuum leak potential there as well.
 

Mahoney

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Re: Backfire through carb under load

back on, still trying to find fitting to do an adaquate fuel pressure test. Good hint on the opening up throttle thing, even after totally cleaning carb, I am getting the blowback. I did a vacuum test, and all seems good, the only troubleshooting thing it hit on was indicating defective spark or small spark plug gap. I pulled the plugs, they are dry, but extremely black from carbon.
 

trog100

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Re: Backfire through carb under load

weak mixture due to air leak or restricted fuel supply mahoney or retarded ignition which isnt as likely.. will it rev/run okay if u nurse it past the backfire spot.. say at 4000 rpm.. ??<br /><br />restricted fuel supply due to weak pump or blocked filters should only come in when the engine is drawing gas and under load.. under no load your float chamber should be full enough to provide enough gas for the throttle opening test.. i think its still a carb problem.. <br /><br />trog100
 

Mahoney

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Re: Backfire through carb under load

yes, if I go easy on it, or even rev it up pretty quick, just not snapping it to 3000 like I was to get the pop, it seems to run very smooth, I can hold it at 3000-3500 rpm and no pop.<br /><br />I am going to let it set a bit, and clean and regap the plugs, then go to the hardware store for some fittings so I can pressure test fuel system.<br /><br />Ill keep updating my progress. thanks for the hint on snapping the carb open, that saves a lot of time trying to go back and forth to the water!
 

trog100

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Re: Backfire through carb under load

when your boat is running under load that fuel pump and filters have to shift up to ten gallons of gas an hour.. doing what u are doing all u are running on is whats already in the float chamber the fuel pump or filters aint really having to do a great deal at all..<br /><br />i still recon its the carb.. check it all thow and keep us informed.. he he<br /><br />trog100
 

Mahoney

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Re: Backfire through carb under load

did a pressure test on the pump and it only read 4 lbs at 1000RPM, it is supposed to read 7-9. Took it apart and checked it out, looked brand new inside. Its a mechanical pump so not much to it besides levers and diaphram, all looked fine.<br /><br />I am going to check it again now that I got it back on and see if anything has changed. I also checked the fuel feed hose and that was not plugged either.
 

achris

More fish than mountain goat
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Re: Backfire through carb under load

Have you checked the volume output of the pump?
 

Mahoney

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Re: Backfire through carb under load

Just checked the volume and it seemed pretty good. It flowed enoug to fill up about 16 oz(pounder) in about 15 seconds, maybe a bit less.<br /><br />I was also wondering, with the fuel pressure reading only 4, can that be due to a carb problem as well?
 

Don S

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Re: Backfire through carb under load

It could be a fuel pickup problem. You can't make fuel pressure if you can't get fuel out of the tank. Could be because the antisiphon valve is plugged or maybe the pickup tube in the tank has a minor leak and is sucking air. Put a clear fuel line between the filter and the pump and see if you are getting a lot of air bubbles to the fuel pump.
 

Mahoney

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Re: Backfire through carb under load

Thanks Don, Ill double check that in the morning. I did take the feed tube out and made sure it was clear, as well as made sure the antisiphon valve wasn't stuck and everything looked good.<br /><br />I completely dissasembld the fule pump and re-attached with no change, still only 4 lbs. The small diaphrams in there looked brand new and seemed fine, the big bellows type diaphrams were intact but a little cruddy.<br /><br />Are there any tell tale signs that a fuel pump is bad by looking at the individual parts, or can those bellow diaphrams just loose there elasticity and be done? I didnt see a way to really replace them.<br /><br />I am pretty sure it will come down to a pump or carb, and would like to know which to purchase first(if lines are not sucking air) <br /><br />Troubleshooting is fun, but frustrating. Good thing its back to grey and rainy her in the northwest, otherwise this would really be driving me nuts!<br /><br />thanks
 

Don S

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Re: Backfire through carb under load

If you have low fuel pump pressure and everything looks good on the pump, and there are no leaks or plugs from the tank to the pump, replace the pump.<br />Didn't read all the replies, but did you check that the cam lobe for the pump is pushing the push rod enough?
 

Mahoney

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Re: Backfire through carb under load

I did check the push rod from the cam and it is operating properly and is not bent or tore up, same with the mechanical arm on the pump. This boat was sitting for almost 5 years when it was given to me, so I am guessing, even though everything looks good in the pump, the rubber diaphrams may have just degraded enough they dont work like the used to, I am looking for a nother fuel pump tomorrow.<br /><br />Ill keep you updated
 

achris

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Re: Backfire through carb under load

Hey Mahoney,<br /><br />I could get to like you. You answer questions asked and you post all the relevant info. Wish more people were like you.....<br /><br />The spec I have for a pump on that engine is 5 1/4 to 6 1/2 psi, and remain constant for speeds between idle and 1000rpm. Volume : At idle, the pump should deliver 1 pint in 30-45 seconds.<br /><br />Yes, after a 5 years sit, the diaphrams could well have lost some movement. Good call.<br /><br />If the carb was causing the low pressure, then you would have a rich condition to be troubleshooting, not a lean condition as you have.<br /><br />Good luck with the new pump.<br /><br />Chris...............
 

Mahoney

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Re: Backfire through carb under load

Thanks achris. I will probably end up replacing both Carb and Pump, Pump is cheaper so I will try and find that tomorrow. <br /><br />The info I have in my clymers manual shows 7-9 lbs for all V8's prior to 1980. Regardless, 4lbs or just under is too low. I am going to try Don's suggestion, weather permitting, then track down that pump.<br /><br />I am also a little conserned that doing research for my engine, the carb number is Rochester qaud 17059285, which I saw show up on some part cross reference charts as for a 454, possibly the original carb was replaced with an upsized carb?<br /><br />Valve covers on my engine say 225, and from all I could research they only made that in 1978 I believe. <br /><br />I mean to get the existing serial number on the engine as well as the fuel pump. I thought I could re-read the number on the pump after I put it on(forgot to grab pencil) but low and behold, it is in the one space I cant contort to find it!
 

rodbolt

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Re: Backfire through carb under load

hello<br /> reclarify the backfire occurence,, does it just occur on rapid throttle movement or at a steady speed and load? there is no "seal" on the primary jet assy. just a close fit between the barrel and the carb.I would have to go look but I belive the hole on the left that you say was plugged is the accelerator pump passage. may be for the cluster vent. watch the accelertor pump shot with the engine OFF the fuel bowl full and a rapid throttle openeing. you should see two large jets of fuel just beside the primary cluster.
 

Mahoney

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Re: Backfire through carb under load

All passages are good now in the carb, although the metering rods and jets are suspect. Engine runs great from about 550-2500+ as long as I dont snap the throttle open. It does idle a bit rough. All research is pointing to fuel pump at this point with a possible carb problem as well.<br /><br />Can't wait to nip this one!
 
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