Backfire through carb under load

[Mahoney]

Re: Backfire through carb under load

Thanks Oldseddie. I was wondering about buying a couple richer meters and some higher hangers. They are sure finicky to bend slightly. Ill have to check the stamps. I believe the rods are stamped CG if I remember.

 

[rodbolt]

Re: Backfire through carb under load

cool on the olds<br /> I have a 70 w-30 myself . no q-jet anymore though. the airvalve on the secondary which contains the cam to operate he rod hanger is adjustable. if you will note the seconday throttle valve is mechanically linked to the primary. sometimes the uninformed like to bend or remove the link. you can adjust the air valve tension for earlier or later valve opening. if you note the offset on the airvalve shaft to valve placement it becomes clear..<br /> the q-jet was the most widely used 4bbl in auto history and probably the least understood by the people that opened them up. you have to understand the relationship between throttle valve opening and engine vacum to properly set one up or rest it after its been monkyed with.

 

[Mahoney]

Re: Backfire through carb under load

yah, problem was the secondary throttle plates were opening, but there was absolutley no fuel in there to start the siphon from the secondary passages, so the secondary air valve never got a chance to open, being held by the secondary vaccuum choke.<br /><br />Letting a little gas past the rods in the secondary seems to have solved that. Ill play with the airvalve spring tention as soon as I get the hanger hight correct.

 

[dolluper]

Re: Backfire through carb under load

Did you have to tilt your head alot to get the spec's I gave up at 85 my neck couldn't take it I have up to 77 but missing 78-79 in my carb cataloge, was the float sitting on the holes? Sounds like your almost a happy camper Be rocking forsure soon Happy for you bud [ben there]

 

[Reel Poor]

Re: Backfire through carb under load

Mahoney<br />Im glad I was on the wrong track and it was just one of those "SIMPLE" problems.<br /> Happy Boatin

 

[rodbolt]

Re: Backfire through carb under load

mahoney<br /> ya got the operational theory all wrong. the vacuim pull off serves a dual function. it lightly opens the choke at engine run and assists the airvalve in its operation at rapid throttle openings. as engine vacum drops on throttle opening it takes sometime for the pull off diaphram to extend. thereby allowing engine vacum to build slightly again before the air valve opens. I still think your problem is either in the secondary air bleeds or the accelerator pump adjustment or passages. if the rods worked before they will work now. by holding them open your allowing the secondary fuel passages to fill and stay full. if the bleeds are plugged the engine vacum will empty the passages and cause a lean pop when first opened. after that it will pick up and roll. there are some nice books out on the q-jet and how and why they work and work so well.

 

[Olds Eddie]

Re: Backfire through carb under load

Mahoney,<br />If you need a hanger and some richer rods I will be glad to send them to you. All I ask is for you to refund the shipping cost. I have intirely too many spare parts and like to see them put to good use. People give me old Q-jet parts all the time. I'm not sure how to communicate with you directly and I don't recommend posting e-mail addresses on a forum for spam prevention reasons.

 

[olbuddyjack]

Re: Backfire through carb under load

Make up a coupla hotmail accounts just for this. You can either communicate there or exchange other e-mail addys in private.

 

[Mahoney]

Re: Backfire through carb under load

Oldseddie, thats really generous of you. You can email me at shawn.mahoney@gmail.com and we can work something out for shipping, thats more than generous on your part.<br /><br />I am going to try and see how much it is to get a tuning at a carb shop, but having extra kits I can use to do a bit of tuning in the field would be wonderful regardless of if I get a new carb or not. <br /><br />I was actually starting to add up the cost of 4-5 sets of rods, hangers, primatry, jets etc, and eventually it would add up to the cost of a newer carb, which I would no doubt want to tweak anyhow!<br /><br />Thanks again.<br /><br />Shawn

 

[Silvertip]

Re: Backfire through carb under load

Just a long-shot. Quadrajets had "air valve secondaries". When the primaries are opened to a certain point the air valves are unlocked and free to operate on a demand basis. Vacuum beneath them caused them to open in proportion to demand. There is a small spring on that assembly that can unhook or be incorrectly adjusted causing the air valves to open too quickly or under little load. As the air valves open, a cam lifts the secondary metering rods. If the air valves are not adjusted properly the engine will pop back. With the throttle open about half way, gently push on the air valves. If they operate really easy that spring is not tensioned properly. The air valves should exhibit a little resistance.

 

[Mahoney]

Re: Backfire through carb under load

Yes, that is one of the eadjustments I was tweaking with, unfortunately there are a miriad of others that go along with it. Just when I think I got it down, something else shows to be aout of whack. I had it not backfiring under load, but instead it was flooding out and bogging down when the secondary valves opened. I could literally see gas shloshing around those barrels when the air valves opened.<br /><br />When that book gets here and I get some parts(thanks to Oldsie) I can do away with my mini vice grip hanger bending solution to the issue.

 

[Mahoney]

Re: Backfire through carb under load

I called a local carb shop on getting the carb back to its factory specified tuning. They want $85/hr, which I think is the going rate. They usually can get it done in an hour to hour and a half.<br /><br />Would you recomend this? I know I have the book coming and more knowledge on the way, but I think that having it professionally looked at might be a better starting point than what I have right now.<br /><br />What do you guys think?

 

[dolluper]

Re: Backfire through carb under load

Wait for the book you get the specs it's not that hard , it's very important to get the choke set right or lots of secondary problems, and secondary lockout adjustment , take special note of choke and secondary linkage as I have had carbs brought to me with the choke housing and linkage on wrong very simple to mess that up

 

[Silvertip]

Re: Backfire through carb under load

Here is a long shot. Quadrajet carbs used "air valve" secondaries. The air valves at the top of the carb were not linked mechanically to the throttle linkage but relied on a vacuum below them and atmospheric pressure above them to open them, depending on demand. As the air valve opened, a cam raised the secondary metering rods to add fuel. There is a little spring (adjustable) that often slipped its anchor, or was in this case, was misadjusted during a rebuild. If the air valves open to quickly, the mixture goes lean and hence the pop back. With the throttle about half open, gently push on the back half of the air valve. If they move really easy, that spring is not adjusted properly or is disconnected. You should feel some resistance on the air valves. It's been a long time since I worked on one but I do have service literature for specific settings if you feel this is the problem.

 

[Mahoney]

Re: Backfire through carb under load

I had that calibrated correctly per rebuild specs. I think is it to tighten the screw then go another 1/4 turn and sinch it back down, However the metering rod hanger hight and taper also have so much to do with this. If I made them open later, it tended to bog down, sooner and it would pop, or they would open, turn the metering rod cam and dump too much fuel in the engine.

 

[dolluper]

Re: Backfire through carb under load

But hooked to the choke by the secondary lockout that is why you have to adjust it, so in fact connected to choke linkage maybe Upinsmoke can look up your cast # and give you the total carb adjustment spec's

 

[Silvertip]

Re: Backfire through carb under load

Sounds like you've played with the air valve adjustment. My info (GM) would not likely be any different than the spec sheet in the kit. I'll dig out my old books and see what it says. As old as the carb is, the spring may have lost some of its original tension so further tweaking may be in order. This is a fairly fine adjustment. Q-jets were also known for worn shaft bores in the base which may contribute to a lean condition. These bases can be re-bushed to eliminate the slop. As for the four PSI pump pressure, it don't think that's that problem either. You might check with some car friends, perhaps a local car club or even one of the marine shops to see if they have a Q-jet you could borrow for a bit just to see if it is a carb problem.

 

[Silvertip]

Re: Backfire through carb under load

Sounds like you've played with the air valve adjustment. My info (GM) would not likely be any different than the spec sheet in the kit. I'll dig out my old books and see what it says. As old as the carb is, the spring may have lost some of its original tension so further tweaking may be in order. This is a fairly fine adjustment. Q-jets were also known for worn shaft bores in the base which may contribute to a lean condition. These bases can be re-bushed to eliminate the slop. As for the four PSI pump pressure, it don't think that's that problem either. You might check with some car friends, perhaps a local car club or even one of the marine shops to see if they have a Q-jet you could borrow for a bit just to see if it is a carb problem.

 

[Silvertip]

Re: Backfire through carb under load

Sorry for the double posts gents. Guess I missed the posts on the spring adjustment as well.

 

[Buttanic]

Re: Backfire through carb under load

Quadrajets are just too complicated and tempermental. Chunk it and get a Holley spread bore replacement. Plenty of parts available to tune to your hearts content. 99.9 percent of racers and hotrodders can,t be wrong.