Backfire through carb under load

jboulay

Recruit
Joined
Apr 19, 2005
Messages
5
Re: Backfire through carb under load

Not to sure if you found the problem yet but I have a 87 Formula w/ twin 350's and last year i had the same problem. For me it was a costly repair because i tried everything in the book also rebuilding the carb. It ended up being a stuck valve in the head. Backfiring throught the carb under load. I ended up blowing out the head and the rings on the other cyl. Hope it will help it is worth looking into before damage
 

Mahoney

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

Ordered another carb that is going to be here on Friday. I am pretty sure it is a lean backfire as I get the backfire at different RPM, sometimes not at all, depending on how I monkey with the carb settings.
 

trog100

Senior Chief Petty Officer
Joined
Dec 1, 2004
Messages
751
Re: Backfire through carb under load

now what did i say mahoney.. about fifty zillion posts ago... he he he <br /><br /><br />
cwm42.gif
<br /><br />trog100
 

Mahoney

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

yah yah, I can take the ribbing....if this fixes it:)
 

cny boater

Cadet
Joined
Feb 19, 2005
Messages
9
Re: Backfire through carb under load

As mentioned, both the secondary air valve wind-up (controlled by the small spring on the underside of the RH side of the air valve shaft) and the choke pull-off work in conjuntion with each other to regulate the secondary air valve opening rate.<br /><br />Did you check to see whether the choke pull-off diaphram is working properly? If it isn't, I doubt you'll ever be able to adjust the wind-up to be able to consistently give good performance.<br /><br />Adjusting an old Quadrajet is more of an art than a science, as you are finding out.<br /><br />BTW, as a former Rochester Products employee, I can tell you that the reason for the different secondary metering rod hangers is to compensate for production tolerances from the air horn to the secondary metering jets. As the carb went down the assembly line, a gauge was inserted into the secondary jets to measure the distance from them to the top of the air horn. This measurement dictated which hanger to use, as the secondary rods had to sit at a precise depth for proper mixture. <br /><br />If you do get a "new" carb, expect to have to make adjustments to it also. HTH's
 

Mahoney

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

I am still working on the carb side of things got a new carb, that was not rebuilt well. They are sending a new one. Upon placing this carb on the engine, I got the lean backfires at different stages of throttle that I didn't get previously. Decided to hook up the vacuum gauge again and it ready steady 15 lbs at idle. The only common denominator with these backfires and changing when they happen or don't happen has been influenced on Carburator adjustments.<br /><br />I believe CNY is right, tuning those puppies definitely is a science, and I am definitely working on it!
 

cny boater

Cadet
Joined
Feb 19, 2005
Messages
9
Re: Backfire through carb under load

Bigdaddy, ironic that 2 newbies are from Syracuse, but I'll have to disagree with ya about pulling the heads w/o first diagnosing the problem. There may very well be something wrong with the heads, but they can be acquitted or condemned without removal in this case.<br /><br />There was talk of camshaft issues and fuel pump volume issues earlier.<br /><br />Typically, if a SBC has a worn exhaust lobe, then the symptom will be a repeatable, rhythmic backfire under load. <br /><br />Fuel volume is not the issue here either. If there was a fuel volume problem, the resulting symptom is usually a smooth, gradual loss of power. A backfire caused by a Quadrajet's secondary air valve opening too soon is caused by a lag in fuel, not a lack of fuel. <br /><br />The float bowl has fuel in it, or the engine wouldn't be running. When you open the throttle all the way, the air enters the secondaries faster than the secondary fuel system can supply fuel. This lag causes a lean backfire. It doesn't take much fuel to prevent the lean backfire. A fuel pump volume problem will have different symptoms.<br /><br />By adding a secondary air valve that is dampened by a spring and a choke pull-off diaphram, the incoming air is slowed down enough to allow the secondary fuel system to provide fuel and prevent a lean backfire.<br /><br />There are also 2 discharge ports mounted high and in front off the secondary air valve. They act as a secondary accelerator pump, as a drop in pressure pulls a fuel shot (similar to a primary accelerator pump shot) to further aid in preventing a lean backfire.<br /><br />3 questions:<br /><br />1. Can you accelerate through the backfire? That is, after the backfire occurs, does the engine run fine at WOT?<br /><br />2. Did you check to see if the choke pull-off diaphram on the original carb holds vacuum? You can also look for the secondary "pump shot" if you have someone drive the boat for you. Be mindful about sticking your face too close to the carb...<br /><br />3. Maybe I missed it, but did you do a compression check?
 

Mahoney

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

Thanks cyn<br />1. Yes and no. To further troubleshoot the problem, I adjusted the secondary rod hight so they provided more fuel right off the bat, that seems to aleviate the lean backfire severity I was having before when I even touched the secondary throttle valves. But it seems there is a tricky medium in there. When adjusting enough to preven the backfire, it bogs horribly(rich) when secondarys are engaged. I have to bring the throttle back down to the intermediate circuit to get maximum RPM. I am going to work on the air valve tension.<br /><br />2.Choke diaphram holds vacuum and lets it go upon throttle up, seems to be fine.<br /><br />3.Did a compresion check and all cylinders have compression. ~135, one cylinder had almost 150 and one had a bit above 120. This was done before oil change, oil was filthy and before any real sustained running, lubing rings, etc.<br /><br />If I increase RPMS gradually with no load on the intermediate circuit, it runs great, no backfire up to 3000 rpm, this is as high as I like to have it turning for a few seconds without any load. NO sound from valves(ticking, knocks, etc.)
 

Reel Poor

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

Your compression test alone suggest exploritory surgery may be necessary. <br /><br />With all the carb work that has been performed and replacement with rebuilt unit, odds are not in your favor that this is a carburator issue. Although Q-jets are finiky, your engine on muffs is not. Your engine, when not under a heavy load does not require such delicate fuel control. Your problem has been to consistant to keep working at this. It time for deeper diagnostics.
 

Mahoney

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

Well I put the old carb back on after making some tweaks and a little more cleaning, and I can not get it to backfire anymore while in the drive. This hasn't ever happened before, and I am anxious to try it out this weekend.
 

trog100

Senior Chief Petty Officer
Joined
Dec 1, 2004
Messages
751
Re: Backfire through carb under load

your compression testing results are a little too variable between cylinders to guarantee that all is well cam/valve wise mahoney.. things get more complicated..<br /><br />i still think u have a carb problem.. but it could be exaggerated by other things not being quite as they should be.. plus the over-complicated type of carb u have.. <br /><br />cry boating seems to to know his stuff about them.. his post goes a long way to explaining the variouse bits and what they do..<br /><br />if it dont cost too much and involve too many other changes.. swopping it out for a more simple one (carb) might make sense.. i dont know enough about what would be involved to say any more than that..<br /><br />trog100
 

Mahoney

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

I plan on pulling the heads if the next outing goes south. I decreased the advance timing to about 8degrees for my engine instead of 10. Upon checking the manual and the timing with unleaded gas, this is the recomendation. I am guessing at higher RPM, one of my problems could have been spark was igniting to advanced and not firing mixture, leaving it to blow back out on the next stroke.
 

olbuddyjack

Petty Officer 1st Class
Joined
Oct 23, 2003
Messages
318
Re: Backfire through carb under load

I kinda doubt the timing will matter much. I've run up over 10 and only get a spark knock.<br />If you decide to pull the heads check for a busted valve spring when you get the rocker covers off. You can replace valve springs with the heads in place.
 

trog100

Senior Chief Petty Officer
Joined
Dec 1, 2004
Messages
751
Re: Backfire through carb under load

it wont be your timing mahoney.. and if anything firing off the mixture later would make the condition worse.. your thinking is back to front on that one..<br /><br />retarding the ignition by two degrees is an anti-knock thing.. it wont affect much else..<br /><br />a backfire thru the carb.. (assuming valve and ignition timings are vaguely right) is caused by the mixture firing poorly and still going off on the next induction stroke.. its a weak mixture thing.. the mixture fires but not properly..<br /><br />the momentary total misfire (flat spot) is also a weak mixture thing as the carb lets more air in but not enough gas during acceleration.. in this case the mixture dosnt fire at all..<br /><br />the mixture should be about 12 to 1 for ideal firing conditions.. carbs can have problems maintaining this thru the entire operating range..<br /><br />as u move away from this ideal range in a weaker direction.. its becoems very critical.. the mixture reaches a point where it suddently dosnt fire at all.. basically on the weak side of ideal there isnt much leeway at all..<br /><br />going the other way.. too rich.. there is a lot of leeway and no sudden cut off point.. i think u can richen the mixture by about 20% and get a 5% gain in power.. if out and out power was your sole desire this would be the way to go.. richen it any more and the power starts to drop off slowy.. it dont cut off all at once like the too weak condition does.. it just slowly and smokily runs slower till it eventually spluters to a stop..<br /><br />apart from the carb not doing its job properly and a possible weak spark.. i cant see any other major players in your current problems..<br /><br />u have your new ignition.. that should take care of the spark problems.. your new fuel pump.. that takes care of the supply problems.. your new carb.. errr.. that one hasnt seemed to work.. to be honest i thinks its the type of carb now causing your problem.. its a hard one to get right..<br /><br />trog100
 

Billnicole

Petty Officer 2nd Class
Joined
Jul 26, 2003
Messages
134
Re: Backfire through carb under load

Check your gas it may be bad I have had that happen too me and it drove me nuts :mad:
 

Adam13

Cadet
Joined
Aug 20, 2005
Messages
16
Re: Backfire through carb under load

I have a 1993 four winns with a 5.0L HO and it backfires in the carb and wont run full throttle without it bogging. I agusted the lean and rich screws on the carb and never backfired again but still wouldnt run full throttle. took it in and they found out that there is a filter in the gas tank that was pluged so it wasent getting all of its fuel. Went and put it in the water and it started up good and went to give it full throttle and did the same thing so i put it in the harbor and was going to take it out and take it back and it wouldnt start it wouldnt even turn over so i checked the oil and it was 1 inch over the safe line. Found out the the exaust flappers were bad and got water in the block so the pistons couldnt turn over so it was stuck.
 
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