In my sig is the set up I am running right now. When I get on the throttle it stumblesover itself for a second and then straightens out. What causes this??? Carb adjustment? Or timing?? please help! thanks

 

Your carb may be out of adjustment OR

your fuel pump may not be delivering enough fuel Try running a stronger fuel pump for that bored over motor

 

fuel pump isn't the problem. make sure your throttle blades are adjusted right, as well as the idle mixture scews.

the problem is probably the accelerater pump cam, or the squirter. try a shorter and more aggressive pump cam profile, or step up to a bigger squirter on the primary side.

 

IS there a kit I can buy that has an assortment of accel pump cams or squirters?? and where can I buy them

 

I'd strongly suggest getting the throttles closed all the way, before messing around with the accel pump.

Take the carb off and turn it upside down. You will see a couple of hloes and slots on the front side of the throttle bore, that the blades cover and uncover as they transition from idle to open. You will notice that there is a long vertical slot. That slot is called the "off-idle transition slot". (Sound sort of like what you're having trouble with?) The front edge of the primary blade should be below as much of that slot as possible. You accomplish this by adjusting the secondary idle stop screw, which is only accessible from underneath, in most Holley carbs. Open the secondaries with that screw until they are nearly as far open as the primaries now are; then back the primary idle screw out until the throttle blades are at the very bottom of that slot.

Put the carb back on, see where the idle speed is, tweak it as required, and re-adjust the idle mixture as necessary; drive the car, and see if the off-idle transition is any better than it was. If the idle is too high even with the screw backed all the way out, pop it back off, and back of the secoondary one a little; keep it up until you get it right.

Then see this thread https://www.thirdgen.org/techbb2/sho...hreadid=244930 for instructions on how to get it to run right and get good fuel mileage.

 

alright I did all of this and set it on the engine and came out with about the same idle as I had when I took it off. Anyways it cured part of the problem but the stumble is still there.

 

Update! I still have the stumble along with lots of black smoke coming from the exhuast. It never did this until I adjusted the secondaries. Even with the idle mixture screws it still doesnt cut back on the smoke. What do I do now???

 

Replace the primary power valve. Make sure you use the right PV gasket.

 

The power valve is already gone??? Damn it I havent even run this carb for a week yet!! What a PITA, thanks for the help.

 

One spit-back through the carb can pop it.

 

How do i know which one to get? Is there anyway to prevent them from blowing so often?

 

"Which one to get" is a tuning thing, determined by your carb's jetting and idle vacuum. I'm guessing, just looking at your motor, that by the time you get it all dialed in right, you'll have about 10-11" of vacuum idling in gear; so I'd try a 8.5 PV first and see how that goes. Check your vacuum to make sure, and get the one that's about 2 numbers lower than your idle vacuum. One that's too low of a number will cause a big flat spot off idle, if the carb is jetted for modern street driving.

There's a "PV blowout preventer kit" you can get from Holley. It's a kind of check valve thing that goes in the vacuum passage that leads to the PV. Some carbs come with it already installed, but most don't. Especially older ones.

 

RB83L69,

Thanks for all of your help I am glad we have a wizard on this board like you.

 

Thanks! But, get it running good before you promote me from sub-moron all the way to wizard in one step!!!!

 

Alright man will do...But atleast you have me going in the right direction.

 

Well to my amazement my carb already has the check ball thing installed. I find this out after I recieve my checkball parts

 

I guess you're lucky.

Go ahead and change out the PV and go from there. You might want to have an assortment of them to try out.

 

Hey RB,
I need some more help I am gonna pull the carb off and give it one more try. what I need you to help me with is setting my jetting or atleast getting me started in the right direction.

Here are the specs for my carb,

700cfm,
Primary jet-122-69
Sec jet-122-78
primary power valve-125-65 but I changed it to a 8.5 PV after you suggested it
Primary discharge nozzle-.028


Do you think my jetting is alright or should I start with something new? I am willing to try something out cause this stumble is driving me nuts and the car is using ALOT more gas than it should be using.

 

....

 

ok another stupid question. Are the primary and secondary blades supposed to be closed after I set them or are the secondaries supposed to be partly open. I have them set partly open and my car wont run for crap it will just barely idle and while its idleing black smoke is just rolling out the exhaust. It works good as a mesquito screen though I think I am seriously screwed up in the head. I am missing something bad in this post and its prolly obvious as can be.

 

well I set it with the secondaries completely closed. Car runs better with more crisp throttle response. It still has a stumble but not near as bad. I mean before it could kill the motor it was so bad but now it seems quite abit better and it seems that I can prolly work it out with the right PV and jetting. Plus I am gonna order a edelbrock air/fuel monitor so I can set it to the best of my ability and to keep up with my air/fuel ratio when i install my NX hitman system next week.

 

What's your vacuum idling in gear? You probably want a PV about 2" below that. The 8.5 PV should be good for idling in gear vacuum of about 11". If the vac is higher than 11", raise the PV; if it's lower, lower it, because it's probably opening at idle, and that will definitely make it too rich.

Tune your primary jets for good mixture cruising (probably close to 14.7:1, but who knows); and tune the sec jets for best ET.

 

how do you tune the secs for best et??

And I will have a Vacuum gauge friday so I can check it.

 

have you tried adjusting your accl pump .... a stumble can be caused from to rich or to lean conditions try going 2 turns either way try see if it helps .... most stumble problems i have seen can be helped by tuning the pump

 

Wow, 78 secondary jets are huuuuuge. This likely accounts for your WOT issues.

Think about going down to 72 secondary jets for that engine and setup.

The ideal way to do jetting is on a dyno with wideband O2 sensor. The less-than-ideal way is reading plugs.

 

Thats why I am gonna get an air/fuel monitor so I can see where I need adjustments and to see whether to go richer or leaner.

 

Good idea, and start gathering up jets. Like I said, 78 is a gigantic secondary jet for a bored-out 350. Biggest I've seen on a 350 is 74, and that was with 70 primary jets...on a 650cfm Holley double-pumper.

 

i think I might jet it down to 74 jets cause I will be adding nitrous and dont wanna take to many chances.

 

Do you know what your timing is set at? You can set the initial a little higher to help burn more of the fuel at idle and it will also make the engine a little more responsive. Also, what is your fuel pressure and float level? These things can cause the engine to run too rich.

I just remembered that I found a website that helped me a while back. Here it is.

http://www.bob2000.com/carb.htm

 

my timing is so screwed up its not even funny. I had a cam, heads, ect ect put on the car and now I cant line up any of the timing marks and if I do the car wont start or will back fire. I didnt change the timing cover or tab or anything just what I stated above. I cant time it. Fuel pressure I dont know yet cause I am installing a autometer fuel pressure guage with an isolator so I can install it in the cab of the car. float level checked fine.

 

Are you running vacuum advance? Have you played with the mechanical advance yet? What ignition are you running?

 

Yep I am running vacuum advance, I have played with the mech advance which helped some. I am running a MSD 6 ignition with MSD 8.5mm wires, NGK plugs, Accel distributor, Super coil.

 

Have you pulled out the distributor and lined up the mark on the balancer to the 0 mark on the tab? It kind of sounds like the thing is a tooth or two off. Line up the marks like I said before and then put the distributor back in. See if this helps the problem.

 

We did that and it wouldnt fire and if it did it would shoot out flames out the carb

 

Could be too much vacuum advance on the stock can. they pull loads too much for performance applications and if you try and recurve it will "chug" everytime you step on the gas when cruising. Crane adjustable vacuum advance will fix that or at least eliminate the VA from the equation. Only 25 bucks and it will allow you to dial your curve right in so its a win win. Kind of hard to nail down a carb problem if your timing is whacky. Rmember to adjust one thing at a time only and you will get it on elimination process. Good luck

 

my ACCEL performance distributor already has an adjustable vac advance.

 

 

whats that for?

 

Thast for me feeling foolish on advising you about something you already have.

 

its alright man you didnt know and I dont think I said anything. But thanks for the help.

 

O.K. It sounds like 180 degrees off. Will it run at all or can you not get it started? If it's 180 off just do like I said before and line up the marks. Then turn the distributor rotor 180 degrees and stab it back in. Also, how did you adjust the rockers? Are they too tight? This can also shoot flames out the carb. Oh, when you put the distributor back in make sure that it fully seats onto the intake. I'm sure that you know this already but I'm just trying to help.

 

Thanks for the help so far. My distributor is set right where its supposed to be but I just cant line up the marks. My car runs fine just cruising around but when you press the gas in rapidly it will stumble over itself for about a sec then takes off. I got a jet kit and it now has 65 primary jets from 69 and now 74 secondary jets instead of 78. The stumble is still there but its not as bad as it was.

 

Thats your vacuum advance man. The can is set for way to much. Adjust it down to about 9 degrees total and it should work fine. If you can't read it with a timing gun just adjust it down to the minimum and start adding one turn at a time and try until it "chugs" and then back it off a turn. That should fix it up

 

I had this problem with my Edelbrock carb. I just found a buddy with a similar aftermarket carb and threw it on and there was no stumble. That way I new it was the carb. So I rebuilt the carb and no more stumbling.

 

11secformula,

I adjusted the accel pumps down 2 turns each. The low end seemed alot more crisp and seemed to be more powerful but if you got to cruising rpm's it surges and stuff. I am getting ready to try it the opposite way out.