Thread: Need some help tuning

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Ok,.. Here's my setup;

350 SBC with Vortec heads (NAL 19210007) camshaft is hydraulic flat tappet with .435 in. intake lift and .460 in exhaust lift with 212 degree open duration intake and 222 degree open duration exhaust lobes at .050 in. ground with a 112.5 lobe centerline. Holley 83670 670cfm Vacuum secondary carb with the stock 65/68 jets and 6.5hg power valve. Edelbrock Performer RPM intake manifold. MSD 8360 Distributor with 12^ initial timing + 21^ in at 3400 rpm. .44 Plug gap. 6-7 PSI fuel pressure and 12-13 in Vac at idle warmed up. TH400 with 2800 rpm stall and 3.80s out back. Vehicle weighs 2800lbs with full tank and me. I live in S. California about 1000' above sea level.


Ok,.. here's the issue;

Big bog on taking off. In drive or with no load the engine stumbles when the loud pedal is depressed. Past the bog the car runs fine and idles great. This is a brand new carb and I have tuned the four corner mixture screws for highest vacuum. Accelerator pump IS working at the crack of the throttle.


Ok,.. My thoughts;

Not getting the correct pump shot. I am posting pics of the pump discharge nozzle as I cant figure out the size. There are just two "lines" that look like machine marks on it and no other stamping. The pump cam is still is what came on the carb and in the factory position. I thought was odd for it to be in the #2 slot? I believe (both my wife and I cant really tell as it looks kinda brown) the cam is the orange one. I was thinking of increasing the size of the discharge nozzle a couple of sizes but cant tell what size I have now.

Ok,... What do you think / suggest? Thanks for your time.



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This is from nastyZ28 forum......

There are 3 problems on the Street Avengers that cause the stumbles and hesitations:
1. Lean Jetting. The S/A carbs are jetted very lean on the primary side, and extremely lean on the secondary side. Bump primary jetting up about 3 jet sizes and then make the secondary jets 8 sizes larger than the resulting primaries. To save a few bucks on jets, take the jets out of the secondary side and install them in the primary, and then just buy a pair of jets for the secondary side. This gets the carb jetted about right and will do wonders for throttle response and power.
2. Accel pump lever not touching pump cam. I see this constantly on the S/A carbs, and it's really lousy assembly work at the factory: The accel pump lever that connects between the pump cam and the accel pump arm does not ride on the pump cam at idle. The lever is riding on top of the throttle shaft spring and is not touching the pump cam. Only after you rotate the throttle a bit (with no accel pump action) will the pump cam touch the lever and produce a pump shot. To fix this, remove the lever and bend the lever a little more right where you see the "kick-up" bend in the lever where it's supposed to ride on the cam. By giving the lever more of a bend, the lever will not hit the shaft spring and will ride on the pump cam right where it should. This will produce an instant pump shot off idle, and will solve most of the hesitation/stumble issues associated with these carbs. Keep in mind that you will need to re-adjust the accel pump lash adjustment after you bend the arm.
3. Inadequate secondary idle speed setting. On most S/A carbs, the secondary throttle (idle speed) is fully closed. This causes the primary idle speed setting to get cranked up too far in order to achieve a reasonable idle speed, thus exposing too much of the primary transition slot. This, in turn, will cause an off-idle stumble due to lack of transition fuel enrichment as the throttle is opened. The primary and secondary throttles should be open the same amounts at idle - they should be balanced and see equal airflow. See my Holley Setup Paper on how to do this.

Forget about swapping pump cams and accel pump nozzles - that's not the issue, and it won't solve your problems. Fix the pump lever geometry instead...

Now, as far as fuel pressure, no modern 4-bbl needs or wants more than 5 psi at the bowl and most of them would operate better with 4 3/4 psi. More pressure than that can overpower the needle and seat in the bowl and allow the fuel pump to blow raw fuel into the intake manifold, creating a tuning nightmare for you.

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Ran out and checked, the pump moves with every movement of the throttle. It couldn't be any more "right now". I have glass fuel bowl sights and the fuel level stays put just under 1/2 way in the window but I will back the pressure down to 5-6psi. I am almost exactly one turn in on the idle speed screw from completely closed plates and she idles well at about 7-800 rpm. The jets will have to wait for another day. I had this same issue (stumble) with a 1406 Edelbrock and that's why the Holley is new. lol

Anyone have ideas on what the discharge nozzle size is?

Mine was doing the same thing, read the article above and checked the plugs,they showed lean. Re-jetted the carb to 68 primary, 76 secondary and instantly no bog!

I have my primary jets currently at 70 primary, with the secondary still at 76, my cam is a little larger than yours so I had to drop the power valve to 2.5 (usually you want your power valve at half your idle vac) you may want to drop yours to 5.5, if it needs it. You want the power valve to open at WOT, not before or you'll get a stumble from it before the secondaries open. When the power valve is correct you 'll feel the secondaries open without any stumble and you'll know when they open. Mine runs real good now and plugs read correct color. You better be holding on now when I accelerate. LOL

Also make sure you have the accelerator pump adjusted correctly. Mine was way off, it had a big gap between the plunger arm and the actuator at idle. Set it so actuator (bolt with spring) contacts the plunger arm firmly at closed throttle (idle), then hold the throttle at wide open and make sure there is at least .020 between the actuator and the plunger arm, should be able to slide the feeler gauge in with only slight resistance (this must be done at WOT).

Originally Posted by Duece coupe

The jets will have to wait for another day.

READ YOUR PLUGS! Too lean will cause serious motor damage, jets are a lot cheaper.

Just looked and I have a set of 73s laying around. I'll take the 68s from the secondaries and flip them. Now,... if I just had bowl gaskets....

Haven't had it out much to get a good read on the plugs yet. but I will!

Changing jets doesn't fix a tip in issue. You need bigger squirter nozzle and possibly a squirter cam change. Making the carb rich will band aid the problem but foul plugs everywhere else.


Put a 35 nozzle in it and a blue cam. It will fix the issue without a band aid.

Just an FYI. This is the Holley response to the same questions I posted;

"Yes the engine combo with the Vortec heads will need more fuel I would try 68/72 jets and see how IT reacts and if it still needs more pump shot try a .035 should be a .031 stock. Read the plugs and tune as needed based on plug color and engine reaction."

I'm expecting my order today which includes jets from 70-78, 5.5 power valve, gaskets and 33 & 35 discharge nozzles. I already have the pump cams and secondary springs. This will give me the parts and a starting place to tune it up. I plan to lower the fuel pressure and try the 68/72 combo first then read plugs.

Thanks to all for sharing your experience. I will post with results as soon as I have time to tinker with it.

Got a chance to play today (for a while at least) and changed out to the 68/72 jets. No other carb changes. 10000000000^10 X improvement. Gained at least 1in. Vacuum (about 14in) too. Idles at about 6-700 in gear smooooooooooooooth. Still a hint of a bog about .5 seconds into WOT but it recovers. I get a little ping after about 1-2 seconds into WOT. I may need to jet up a little more on the secondary but I think I'll take just a hint of timing out first.

I took her around town a little and onto the freeway for a couple miles and I'm letting her cool off so I can take a look at the plugs. Not sure I ran it enough (about 15-20mins) to change colors. I have one of those "Summit by Holley" fuel pump / regulator combos and I'm chasing the fuel pressure all over the place. Set it at 5psi get out and the gauge shows 3psi. Set it back up to 5 and its 6-7psi. Thing is all over the place. I forgot how freaking loud that pump is too.

Always interested in other's ideas. I will post with further results as they come. Hopefully it may help someone else.

Thanks to techinspector1, 36 sedan and camaro_fever68 for your help.

"I get a little ping after about 1-2 seconds into WOT."
Ping is DETONATION.

Fatten up the carb until it make tears in your eyes standing behind the car, then start leaning it down from there. You can't hurt the motor by being too fat, except for fouling spark plugs, but you can sure hurt it by being too lean. Alter the timing curve to bring in all the advance (34 degrees initial and centrifugal) by 2800. To read the plugs, you need to make a full-power blast, go to neutral and chop the ignition, coasting to a stop and pull the plugs right there. Any further driving after the full blast run will re-color the plugs to reflect the driving you did to get home, not the full-blast run. Tee off at the fuel inlet at the carb bowl and run a temporary 1/8" line back to the firewall and up through the lip of the hood to the cowl. Mount a mechanical, liquid-filled 0-15 psi gauge on the cowl temporarily with duct tape, cardboard and tie wraps so you can read it through the windshield as you drive. The carburetor needs only 4-5 psi, just enough pressure to keep the bowls full and as we have discussed before, more pressure can overpower the needle and seat and allow the fuel pump to blow raw fuel into the intake manifold, creating a tuning nightmare for you. Volume is more important than pressure.

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Ya, I think your right. After looking at the plugs I think I could fatten it up a bit more. This was after a couple of blasts but then drove normal city streets for a couple miles.
There were NOT new plugs.

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By changing to a lighter spring in the secondary vacuum you should be able to eliminate the the bog at WOT.

Originally Posted by NTFDAY

By changing to a lighter spring in the secondary vacuum you should be able to eliminate the the bog at WOT.

I maybe wrong, but wouldn't a lighter spring open the secondaries sooner, which could cause more of a bog. I believe the power valve moderates the fuel for the instant WOT, then the secondary jets come into play. Again I could be wrong, but too small of jets at the secondary will cause a bog at WOT also.

My reading of the plugs (and I maybe wrong), looks like the idle is lean and the timing a little advanced. As tech said adjust your timing and fatten the jets.

The color change on the ground strap looks good, but you might want to try one step colder plugs if it continues to color at the same place once you get the whole rigamarole dialed in. No big deal, just experiment with it.

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Originally Posted by 36 sedan

I maybe wrong, but wouldn't a lighter spring open the secondaries sooner, which could cause more of a bog. I believe the power valve moderates the fuel for the instant WOT, then the secondary jets come into play. Again I could be wrong, but too small of jets at the secondary will cause a bog at WOT also.

My reading of the plugs (and I maybe wrong), looks like the idle is lean and the timing a little advanced. As tech said adjust your timing and fatten the jets.

Sorry, you're right, must have been a senior moment. Stouter spring since secondaries are tipping in too soon.