I need some advice and oppinions here...i drive a 79 f-150 supercab 4x4 with a c-6 and a mostly stock 400. i know what youre thinking, thats not a hot-rot. i know but its all i have so i try my best to make it one the engine is mostly stock with a few performance add ons. it has an edelbrock performer and lately ive been experimenting with carbs. heres the question having to do with a quadrajet i was running awhile back. it was the first one i tried with my new intake and it seemed to run alright. it didnt get a whole lot RIGHT off the line but once it got moving then it would really kick in and start roaring (im thinking thats due to the vacuum secondaries and heavy weight). the only things i didnt like about it was one, i couldnt really "race" it because after it would hit around 3500, the engine would start starving. i dont know if there was something wrong with the carb, the fuel pump, or if i shouldnt have been running a pressure regulator...but it just wouldnt work. it was so bad sometimes that if i put it to the floor and held it it would never get to second gear because it would keep lurching around at the top of first. the other problem was poor tunablility and bad gas milage. i then dropped on a big holley and the starvation problem was totally gone. (it wasnt mine though so i used it for a day and then tried something else) i then tried out a carter 600 which is what im running right now. it has mechanical secondaries and idles even smoother than i could ever get the stock 2bbl carb to run. it gets almost decent gas milage (10.5) and is an all around smooth carb. it really moves right off the line (im thinking because of those mechanical secondaries) but once it gets going, it really doesnt seem to have the boost that the quadrajet had. it almost felt like turbo boost on the q-jet and now the carter just runs the same throughout the rpm range. so really the question is...was the q-jet i was running bad or what was the problem with the starvation. and also what kinda of performance differences should i be able to expect assuming that both carbs were in good running order? any enlightenment on the subject would be greatly appreciated.

 

Keep the Edlebrock/Carter! try opening up the secondary jets to something around .101-.104..
Open up the top of the carb (takes about 5 min.) see what is in there now.
Go to smaller meetering rods in the primary (2 screws to loosen there they are) a number is stamped on them. go SMALLER like .070x.037 (number on rod is like 7037.. Mine anyway)

__JOHN__Õ¿ö
Member-
"Crusin North" Car Club
Santa Rosa, Calif.
72 F-250
ICQ#6030753
http://clubs.excite.com/72FORD-F-250

 

I'd go with the Carter, because it's easier to get the parts needed to tune it. It's possible to tune the Q-jet to work perfect on your engine but the jets/needles are harder to come by. What is the number on the side of the Q-jet? There should be a number that starts with a 170 or 70 that is located on the drivers side of the carb on the main body. I just want to check to see what it came off of and how closely tuned to your motor it is.

 

hey ben, thanks for the reply. sorry it took so long to get back to ya on the #. it says on the side, 17056240 , then underneath that it says 2685 ADZ (i dont know if those last ones mean anything) ya i was just curious as to why the quadrajet was running the way it was...it was strange. there was the whole thing with starvation if you kept your foot in it and then there was another wierd problem that happened with it on occasions. a few times i was cruising along and then stomped it to the floor, and it made the "whine" sound of the secondaries kicking in but it had a sudden loss of power and wouldnt do anything. then i let back of the gas about half way and it lurched forward and started to accelerate again. as i said earlier, i like the carter for milage, smooth idling, easy tunability, and good power for the first second or two off the line, but in the future i may want to throw the q-jet back on if i wanna race any thoughts on why it could be acting that way? is there something wrong with the carb or is it just not suited well for the engine?

 

oh i forgot to give some better details so you know what im dealing with, the electric fuel pump is rated at up to 9psi, im still running the stock fuel pressure regulator, the engine is bored .040 over but has all stock internals, and the heads are from an early 70's 351 with no emissions junk and are lightly ported and polished. it has close to none of the emissions controls hooked up,an edelbrock performer, and some other small performance stuff like coil, wires, and air filter and exhaust.

 

Maby you have a float level problem in th Q-jet? Seems like you have the clasic fuel starvation problem. Once you back off the fuel catches up to the carb`s needs.
Edlebrock carbs like 7psi MAX....BUT.... there has to be enough fuel FLOW for anything to happen.
__JOHN__Õ¿ö
Member-
"Crusin North" Car Club
Santa Rosa, Calif.
72 F-250
ICQ#6030753
http://clubs.excite.com/72FORD-F-250

 

According to my book the carb is for a '76 Buick, can't tell what engine though. What size secondary rods and hanger do you have? I'd check the float level and the fuel filter in the inlet of the carb if it's still in there. Also the problem when the secondaries open up could be several things. The secondary air valves(flapper doors on top) could be opening too soon, not far enough, not opening smoothly, or there just isn't enough fuel in the float bowl to sufficiantly supply the secondary system. Check to see if the blades open smoothly and don't stick open. One other thing, on some Qjets there is a mechanical linkage that prevents the secondary air valves from opening when the choke is on. If yours has this check to see if this is working properly. I remove this linkage on all my Qjets by driving out the roll pin that holds it all together.

What fuel pressure are you running? My book says these carbs like no more than 6psi and no less than 4psi. If you are serious about running a Qjet on your truck I think you should use the Qjet you have now for parts and picking up a non-emissions one. Smogger Qjets are too complicated and hard to tune. I think your Qjet is not giving your engine the gas it needs under full throttle. It's fine for sunday driving but when you put it to the floor it's choking your engine. I recommend going to your local salvage yard and picking up a 1974 or earlier model off a big block Oldsmobile. That should be very close to what you need, it has the straight out fuel inlet and should be already tuned pretty close for your engine. Just slap on a rebuild kit, you might have to rejust the idle and idle mixture though. Also make sure it's super clean before putting it back together. I also suggest converting to manual choke, I don't trust the electric chokes on these carbs and they are a bitch to get to work properly. One more thing, I suggest you pick up a copy of "Rochester Carburetors" by Doug Roe. It has everything you need to know on rebuilding, repairing, and performance tuning Qjets. I have this book and I highly recommend it.

 

I just thought of something. There is a vacume diaphram on the pass. side of the Qjet. Make sure the diaphram is working properly. If the diaphram on the carb doesn't have an air bleed hole you can test it by unhooking the vacume line and push the plunger in until it seats then put your finger over the hole where the vacume line was connected. Then let go of the plunger. It should not move until you remove your finger. It might move a little but as long as it stops and then moves all the way out after you remove your finger. If it moves all they way out like the seal is shot when you have it plugged then the diaphram is junk.

If your diaphram does have the air bleed hole, then cover it with a peice of tape or whatever to cover the hole. Then just do the same test as explained above. I think the air bleed holes are in the tube where the vacume line hook onto. If you have two diaphrams on this side of the carb, then just buy the book cause it's alot of typing to explain that one. Good luck!!

 

hey ben, thanks a lot for all the good info. where to start...the rods and hangers? i have no clue, i havnt really torn apart the carb yet, i was just trying to get an idea of what needs to be done to it before i get too far. ya the uppper flapper is opperating perfectly, also i did hook up a manual choke back when i was using it and made sure the secondary hold back thing (whatever you want to call it) was staying out of the way. i think youre right about there being no fuel in the bowl... i would guess thats maybe a float problem. it seems to me like the carb dumps plenty of fuel but once it uses up all that was in it...it takes awhile to get it back. now im not sure whether thats an actual problem withing the carb, or a problem with the fueling system before the carb, or just that the carb is working fine and wasnt intended to suck so much fuel out so fast. my pump is rated between 5-9 psi (give or take a little i think), but i am running a stock regulator that was for the 2 barrel so maybe it needs to be removed or tuned? that was interesting that you said it takes about the same pressure as an edelbrock though, because the edelbrock that im running right now has no starvation problems. hmm, about that diaphram thing? what exactly is it's purpose? I recall playing with it when i first put the carb on...(not knowing at all how it worked) and if i remeber right, i just pulled and plugged everything. i did that because i noticed that when it idled, the diaphram was out (or in, i dont remeber) which moved a linkage back to the secondary flapper to keep it closed. a mechanic friend said that under acceleration, the vacuum opens (or closes) the diaphram allowing the secondaries to open...i jsut figured id make sure they woudl be opening so i disconnected it. when i fist put it in, the secondaries never opened, but after doing that and the manual choke to keep that little stopper out of the way, it ran hard (other than the starvation thing). anyway, thanks a lot for all the info guys.

 

I think I have figured out the reason why it would hesitate when the secondarys would open. The vacume diaphram that you unhooked controls how fast and how far the flapper doors open. At first you would think that having the doors open as fast and as far as possible would be a good thing, but it's not. When you would open up the secondarys by flooring the pedal, the flappers doors almost instantly flew open. This would lean out the fuel mixture causing a lean "bog". In some cases the doors can actually open and close very fast, causing a very noticable jerking motion in the car. Also, the doors actually create a low pressure area inside the carb which sucks out the fuel from the float bowl. The low pressure areas are at their strongest when the doors just start to open til somewhere between 1/4 and 1/2 open. This is because it needs to suck the fuel all the way up the little passages inside the body of the carb. Once the fuel reaches the discharge ports and starts pouring down into the intake it doesn't take as much to keep it going and by then the doors would be alomost all the way open. A good example would be siphoning gas from a car(without a pump). The hardest part is getting the gas up the hose but once it's going it don't take much. But what your carb did is this. When you floored the pedal the flapper doors opened 100% as soon as the throttle plates opened, this leaned out the fuel mixture your engine "bogged" down and continued to do so until the fuel reached the discharge ports or until you got sick of the engine spitting and sputtering and you let off the pedal. The best thing to do is reconnect the diaphram, if it works properly this should fix the stumble when you stomp the pedal.

It is possible to tune these diaphrams so they open faster but still slow enough not to open too fast. There is a restriction inside of the hose nipple. On some models it can be reached but on others it can be on the wrong side of the bend. If you can get to it you can drill out the oriface in .002" increments but don't go too big(about.025") or the flappers will open too fast. It takes some tiny drill bits to do this, and you have to use a pin vise to hold them because they are about the size of a pin.

I still think the float needs to be re-adjusted and the flapper door problem more than likely contributed to the problem. There might also be a restriction in the fuel inlet or somewhere in the carb not allowing enough flow to keep the bowl full. Also can you describe this regulator your using? I have not seen a "stock" fuel pressure regulator for a 2bbl carb before. Where did you get it? Or was it on the truck when you bought it? I am not awhare of Ford using pressure regulators on carbureted applications.

 

i totally agree with you that the secondary lag due to opening to fast was going on in my truck. i just dont think thats what was the cause of of the top end starvation is. one thing i didnt mention was that if i stomped it off the line the rpms's would actually drop for a split second and it would almost die...then the fuel would catch up as you just described and it will really start screaming. i think if or when i throw the carb back on, hooking that back up will be the first thing i do to it. i had wondered what that could be and that sounds like its definitely it.

well as for the regulator, ya i used the wrong word when i said "stock". i had forgotten that it came with a mechanical pump and whoever installed the electric one mustve put it in...i have no idea what its tuned like but i guess itd be worth a try. the old electric pump, that was on when i got it, broke down awhile ago and when i put the q-jet on, i put a new one on. i started out with one that was rated at 4 psi or so thinking it wouldnt matter. i used it for awhile but the top end starvation problem was very extreme. i figured it was the pump and got a higher pressure one...around 5-9 i think, which did help the problem slightly but didnt cure it. i was thinking about maybe pulling that regulator off but when i put on the carter and it ran perfectly...i figured it just mustve been the carb. quick question, is it possible that the q-jet uses a lot more gas than the carter under acceleration? also, what is the deal with pressure ratings...as in, will it hurt anything to be running 9psi to a carb thats not supposed to be mroe than 7? well thats about it. thanks again for all the great info.

 

If you run more psi than what the carb is supossed to have then you risk over powering the float. This means the pressure of the fuel coming in to carb is more than what it takes to shut the needle and seat, so the fuel would keep filling the bowl no matter what. Eventually gas would start coming out of the float bowl vents and all over the top of the engine, not a good idea with all those hot parts under the hood. And if it were to catch fire, the electric pump would keep feeding the fire and your truck would be toast if you didn't shut the pump off.

The stumble off the line you describe sounds like another bog. But I can't tell if it's getting too much gas or to little. When it would do this, once it started to go would there be a big puff of black smoke?

It is possible that the Qjet uses more gas than the Carter. I don't know for sure, but the Qjets are not known for their excellent mileage. I have a Qjet on one of my trucks and I get around 14mpg with a 3spd manual.

From reading your posts it sounds like you like the Qjet more than the Carter except for the problems you were having with it. The Qjet you have is for an emissions controlled car. The 1975 and later model Qjets(except for truck applications) are alot more complicated and it takes alot more work to get the to run right. I suggest picking up a pre-'75 carb from a junk yard(about $30 around here) and take it to a parts store and pick up a rebuild kit and a new float and on the way home stop at a book store and buy a copy of "Rochester Carburetors" by Doug Roe and have at it. An early seventies Oldsmobile carb will have the fuel inlet in the correct position for you application, you can also take off the long part that holds the filter and use a banjo fitting to clear the distributer and make it easier to hook up the fuel line. I think with this carb, and the knowledge you would get from the book you would be very happy with the performance of these Qjets.

 

Thanks again for the help ben. the carter is working really well for now and ill probably leave it till summer when im bored and wanna play with stuff again

 

You may not be aware of it, but the Edelbrock Performer runs out of snuff around 2600 rpm, its no wonder you are having problems by 3500! While it does sound like you have some carb issues, if you run in the 3x range you have got to swap out the performer for an rpm...its the difference between night and day. This is not just my opinion, do some searching and you will find it well substantiated. Just wanted to offer some advice as the intake dictates quite a bit...