Hey folks,

So I finished re-assembly of my first ever QJet rebuild, and with from the fine folks on this thread, I managed to rebuild and rejet the Ebrock 1901. Per Lar's recommendation the pri jet/rod is 44/73. So first the good news: After a few cranks to prime the bowl, she started right up. Choke worked perfectly, no leaks.

Out test driving with the primaries, the car runs great, with the exception of an off idle stumble. Other than that it screams. But it does have the stumble which is concerning.

With the secondaries set free, it falls down pretty quick with them open, which starts with a bog, and ends with a back fire. This is worse when I floor it at low rpm. If I gradually work my way up to WOT, or open it up at 70mph takes off. So I'm thinking I got too much secondarie action. I also have the points @28* and may open them up a little more.

Any idea's on what to try would be appreciated. This whole project started with a WOT breakdown issue so at this point I havn't made much progress on the core problem

Thanks again for all the help.

 

Sounds like it may be the secondary air valve adjustment. On the passenger side of the butterfly on the top of the secondaries is an allen screw that faces down toward the manifold. Loosen it while holding the little screw that adjusts the spring that holds the butterfly shut with a screwdriver. Tighten the adjusting screw 1/8 turn, retighten the allen screw and test drive. If it is better but still there, tighten another 1/8 turn until the bog goes away. If that does not fix it, it may be an accellerator pump issue. God bless, Sensei

 

Sounds like an accelerator pump or timing issue perhaps related to vacuum line. With engine off open up the choke in the primaries and look into the carb then open the throttle plates less than 1/4 of the way...you should see fuel squirt into the venturies when you just crack the venturies like that. If you don't see a squirt then, but it does squirt out when you open the throttle more, then it's the accelerator pump adjustment...if the pushrod is on the outer hole then move it to the inner...if it's already on the inner then you will need to carefully bend the rod straighter. If you don't see any squirt out at all then something went wrong with the rebuild.

If the accelerator pump is fine, but there's still hessitation then check your timing.

 

Ok, I have come to the conclusion that my 74/43 combo on the primaries is too rich. (I think). I adjusted the accel pump, and it is in spec and squirting just fine. (I broke the pump rod in the process, and had to make a temp from coat hanger wire).

From going through Lars doc, I must be rich, with sec's locked I still breakup at WOT, and stumble a little just off idle, but cruise is good and has good response. I verified all the timing, and I think I have too much vac advance when connected to the manifold port so I need to play with that as well. Mech advance is good 34*@2800, jumps to 48 with VA on manifold.

It runs well enough to go to the cruise in tomorrow, and next week we try again.

 

Agree with the above,..your Air Valves are opening too quickly causing the bog. You'll need to tighten the adjustment which will delay the opening of the AV's a bit.

I think your jetting is ok,..I run 71-72 jets with 41 or 42B rods and my engines and spark plugs like this combo.

If your accel pump is ok, the hesitation and backfiring could be a vacuum leak,..might listen for one or close off vacuum ports one at a time until you ID the prob.

Good luck!

 

I seriously doubt the 74/43 combo is causing a rich condition..perhaps quite the opposite as you are claming 250 HP
Thats almost the same setup Im running on a stock 190 HP L48....... 73/43 with DR secondary rods which runs perfect all around

 

From everything I have read and tested it seems to be a to tad rich. No Vac leaks, Sec Rods right now are the AY which is on the leaner side. Anybody know the dimension of a DR rod? The tuning paper lists the DR but no dimension. I'll keep playing with it.

The 250hp is embellished a little. NOM. From researching the engine casting number, it is a 1992 GM Crate 350, 4 bolt main, "hencho e mexico". Rated at 233hp. I gave myself 17hp for the intake and exhaust. I truly don't know exactly what it puts out...not enough ! I can tell you that. (is it ever enough)

thanks again for all the help.

 

Dr = 0.0570

Code P/N Dia of Tip Tip Length (Short, Medium, Long)
AX 7033549 0.0400 S
BG 7034822 0.0400 M
AH 7033812 0.0530 M
AU 7033655 0.0530 L
CV 7045984 0.0530 L
CH 7045779 0.0570 S
AN 7034320 0.0700 S
DP 17053531 0.0690 S
DR 17053659 0.0570 S

 

Too rich how? Is that too rich in cruise or too rich at WOT. You have to remember that even though the primary jets and rods work together they are two separate systems. It is very easy to have WOT lean and cruise pig rich. At WOT with a lean mixture the engine will begin to break up and misfire. On the primary side the ONLY thing that controls this mixture is the jet size.

In your first post you say that the primary jet/rod combo is 73/44 yet in this post it is 74/43. We need some clarification here. It is very possible that 74/43 is too rich at cruise.

There are two different kinds of off idle stumble or sag problems and they each have a different solution. The first is pulling away from a 4 way stop with a police car sitting at one of the other stop signs. In other words, very light on the throttle. You will be moving the throttle very little so the accelerator pump won't be doing very much to cover up the hole in the mixture as the car begins to move. The primaries don't come on line until you get to about 25 or 30 MPH so this is all idle circuit and transition slot territory here. Now, about half way through the intersection you can feel the engine lay down just a little. This is because the idle circuit and transition slot take there fuel through the "idle channel restriction". This restriction is large enough to feed the idle system but it can not do that and the transition slot. The Edelbrock 'Jets are lean everywhere. Idle, off idle, cruise, WOT all lean. Even the choke was set lean. I would think that the idle restriction could stand to be opened up a tad. This is not easy however, and is beyond the novice level of tuning ability. It is not easy to reach this restriction and it is very easy to screw it up. You may be able to back the idle mixture screws out a bit to cover this up.

Then there is idle to WOT just as fast as you can get your foot to the floor. Hopefully, the police are no where in sight. THIS IS THE MOST DIFFICULT MANEUVER FOR THE CARBURETOR TO PREFORM. The accelerator pump MUST do its job otherwise there will be massive backfiring through the carburetor. If there is any weakness when doing this maneuver it is most likely a problem with the accelerator pump.

I don't recommend trying to tune the secondaries until you have the primaries done, however, you could try a richer rod or hanger here. A good idea here would be for you to list all of the tuning parts you have so that we know what you have to work with. Main jet numbers, main rod numbers, power piston spring numbers, secondary rod letters and secondary rod hanger letters.

BigBlockk

Later.....

 

Hey...thanks for the reply..

ok..first off, the current combo is 74/43. The first post I swapped the 4 and 7 inadvertently. Sorry for the confusion.

So, here are the settings, or as close as I can get them:

Pri 74/43
Sec AY, hanger set @ 41/64 as stated in Lar's doc..
Power piston spring is the orange 5in-hg
I used the high capacity needle and seat that came with the kit. Float = 3/8.

Current troubles. short off idle stumble. I worked this out a little by increasing the accle pump rod. The pump rod now measures 1/3" and it got better, but did not eliminate it. I'm going to try another 1/16 and see if I can tweak the rest out.

Mixture screws are 1/2 out from max vacuum.
With pump disconnected, my lag seems to go away, but it's really hard to tell without driving it. If I cover the choke horn with my hand the engine stalls. This is what led me to believe this was a rich condition. I drove it out today, and all in all it ran pretty good. I still have that little stumble, (getting from idle - to ~1300) and breakdown at WOT when I mash it like you said Big Blockk. If I work up to WOT in more of a linear fashion, (gradual pedal increase), it's pretty smooth and has decent power, but sill start to break up a little around 5000.

My cruise is beautiful and responsive. So from reading your post, I'm thinking that another tweak on the pump and my stumble will be all gone. If your theory of still being lean at WOT is on, then I should try a 75 jet with a rod combo that gives me the same cruise metering area as the 74/43 combo, so try 75/45?

One thing I have not done is change the plugs. I also have a pertronix to swap out the points.

thanks again for all your help Big Blockk. This project would be a mess without your guidance....and the knowledge I'm gaining is invaluable.

 

A 75/45 will be just a touch leaner in cruise than what you have now. A 75/44 will be just a little richer in cruise than what you have now. Your call.

When you try your WOT runs are you locking out the secondaries? If not, you really should. Tuning these things is tricky enough without adding complication to it.

It sounds like you're going in the right direction. Go slow, keep records and don't get discouraged. It's going to take a while. Once you get the primaries to where they don't break up at WOT you can start tuning them with timed runs.

It's a slow process. Keep at it. The next one you do will go quicker.

BigBlockk

Later.....

 

Good job on making do with the materials at hand. I've done the same thing myself. But, it's not a permanent fix...clothes hanger wire is too mild a material...it will weaken greatly with heat and use and you will soon be back with a stumble. Replace it with the correct rod as soon as you can. If you want to make another one then some materials which are handy can be found in hobby stores which carry some use nice rod stock of the necessary diameter. Harware stores might have some rod stock that size as well. Some people have had good success with welding rods too, but they are more expensive than plain steel...they get used simply because they have a welder and it was handy at the time.