Pump Shot Help
Pump Shot Help Engine is an LS3, milled stock heads, 230/242 cam headers, GMPP dual plane intake and a holley 770 Street Avenger. Truck is a 69 C10 short bed, T56 trans, 3.73 rear and 28" tire.
I have been chasing a lean condition at tip in forever. Worse at take off, but occurs at cruise as well. Carb has a 50cc nozzle, 50cc pump kit, and yellow cam installed in no 1 position. With this setup, the condition is at its "best" but its still lean. Confirmed with wideband and physically feeling the stumble. With that said, I think the size of the shot is right since the engine recovers quickly, but I need it to hit sooner.
Any suggestions besides changing carbs would be greatly appreciated.
I have been chasing a lean condition at tip in forever. Worse at take off, but occurs at cruise as well. Carb has a 50cc nozzle, 50cc pump kit, and yellow cam installed in no 1 position. With this setup, the condition is at its "best" but its still lean. Confirmed with wideband and physically feeling the stumble. With that said, I think the size of the shot is right since the engine recovers quickly, but I need it to hit sooner.
Any suggestions besides changing carbs would be greatly appreciated.
What shooter is in it now? What power valve? How much vacuum do you have at idle? How far out are your idle mixture screws? What's the LSA on the camshaft? On a L92 single plain 35+ front shooter is the norm IMO.
Remember, this carb is a single pumper. 50cc shooter
Stock 65 power valve
Will have to confirm idle vac. It was around 15" last time I looked
Screw are 1.125 turns out
LSA is 113
Stock 65 power valve
Will have to confirm idle vac. It was around 15" last time I looked
Screw are 1.125 turns out
LSA is 113
I can tell you what worked for me. Adjust the idle feed restrictor and idle air bleeds to keep the AFR right on the money with very slow throttle movements. You shouldn't need any pump shot under these throttle conditions.
Second make sure the butterflies are set so that 20 thousands of the transfer slot is visible on both the primary and secondary. If you need to tweak the idle speed make sure you adjust the primary and secondaries equally. If this causes less than 20 thou of the transfer slot to be exposed, then back off on the idle timing by lowering the 500 RPM setting. Only tweaking the 500 RPM setting lets you come right back into more timing when you get off idle.
If you do those two things you will most likely be able to switch back to the 30 cc pump. I had a 50 cc pump for a good long time cause that is what everyone on here said needed to be done. All the larger pump did was mask the mistuned transition circuits.
One last thing, make certain the accel pump arms are adjusted so even a slight throttle movement causes a good strong squirt down the throat. There should be no slack in the linkage so that you are quick on the trigger.
By the way, if the IFR and air bleeds aren't adjustable on your carb, then drill them out and replace them with set screws you drill yourself. I had to do that on the double pumper. Scary to think about but really quite easy to do.
Second make sure the butterflies are set so that 20 thousands of the transfer slot is visible on both the primary and secondary. If you need to tweak the idle speed make sure you adjust the primary and secondaries equally. If this causes less than 20 thou of the transfer slot to be exposed, then back off on the idle timing by lowering the 500 RPM setting. Only tweaking the 500 RPM setting lets you come right back into more timing when you get off idle.
If you do those two things you will most likely be able to switch back to the 30 cc pump. I had a 50 cc pump for a good long time cause that is what everyone on here said needed to be done. All the larger pump did was mask the mistuned transition circuits.
One last thing, make certain the accel pump arms are adjusted so even a slight throttle movement causes a good strong squirt down the throat. There should be no slack in the linkage so that you are quick on the trigger.
By the way, if the IFR and air bleeds aren't adjustable on your carb, then drill them out and replace them with set screws you drill yourself. I had to do that on the double pumper. Scary to think about but really quite easy to do.
Air bleeds on this carb are not adjustable. I'll look into the transfer slot. I set it at .020 on the primary when the carb went on last time, but I don't recall messing with the secondary setting.
I have one question that may be important to truly solving the problem: If you open the throttle very slightly and hold it there, does the lean condition linger? Or, is it just a brief condition?
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Im very simple minded, so don't hold this comment against me. Those heads love fuel and air, so why the single pump avenger ? I think your asking a mild street carb to effectively feed a monster. Even with double pumpers, lean tip in is an issue with LS3 heads. You have to run what you run, so good luck. These guys that are helping you know their stuff and if anyone can help get it to work, they can.
Because I didn't know any better. I called Holley tech support and this is what I ended up with. I've runs carbs in the past, but not on anything I would call a performance motor.
In their eyes, my truck is heavy and needs Vac secondaries.
I'm hoping to find someone local that has a 650 or 750 DP that I can throw on and test drive to see if its worth buying another carb.
In their eyes, my truck is heavy and needs Vac secondaries.
I'm hoping to find someone local that has a 650 or 750 DP that I can throw on and test drive to see if its worth buying another carb.
You might check to see if the pump cam and actuator spring are adjusted properly. With the air cleaner off, try to ever so slightly open the throttle and look to see if the squirter nozzles shoot any fuel. I have seen people have the accelerator pump linkage too loose causing a delay in pump actuation and subsequently a stumble.
Because I didn't know any better. I called Holley tech support and this is what I ended up with. I've runs carbs in the past, but not on anything I would call a performance motor.
In their eyes, my truck is heavy and needs Vac secondaries.
I'm hoping to find someone local that has a 650 or 750 DP that I can throw on and test drive to see if its worth buying another carb.
In their eyes, my truck is heavy and needs Vac secondaries.
I'm hoping to find someone local that has a 650 or 750 DP that I can throw on and test drive to see if its worth buying another carb.
Mine took 45 squirters, and may still want more. My 60ft went from 1.41 to 1.35 now, stumble was awful. Look at the size of that intake, the length of that runner......lots of air in there. Gotta throw some fuel in there with it.
An out of the box 750 DP will do the same thing until you tune it properly. You have the dual plane which is less susceptible to this sort of thing than the single plane. Read through the 100's of posts on this issue.
If you want to buy a bolt and go carb, Holley Ultras are reported to work better on LS motors out of the box than the old school carbs.
But any carb you buy may need to be tuned.
If you want to buy a bolt and go carb, Holley Ultras are reported to work better on LS motors out of the box than the old school carbs.
But any carb you buy may need to be tuned.
You might check to see if the pump cam and actuator spring are adjusted properly. With the air cleaner off, try to ever so slightly open the throttle and look to see if the squirter nozzles shoot any fuel. I have seen people have the accelerator pump linkage too loose causing a delay in pump actuation and subsequently a stumble.
An out of the box 750 DP will do the same thing until you tune it properly. You have the dual plane which is less susceptible to this sort of thing than the single plane. Read through the 100's of posts on this issue.
If you want to buy a bolt and go carb, Holley Ultras are reported to work better on LS motors out of the box than the old school carbs.
But any carb you buy may need to be tuned.
If you want to buy a bolt and go carb, Holley Ultras are reported to work better on LS motors out of the box than the old school carbs.
But any carb you buy may need to be tuned.
What you have is the slow boat to china carb. Chuck it. 750/800/850 cfm (stock fi cfm?) Double pumper (with extra cc pumps). If you want to play games for ever use what you have or for more fun a Rochester or trips lol......Been there. I always liked adding the secondary adjustable plate for a 4 corner tune also. Good luck.
Right. This is also a street only setup. I rarely go to the track and when I do, its for grins, not competition. The 770 in its current state works fine for me outside of this tip in issue.
ok as you said that you have your shot of gas just right and it stumbles? That says double the sqirters. If your worried about fuel go and jet the two squirters down til u get your bog again. With that huge cam and huge head this is what you have to do. That carb was made for something with 180 sbc runners or peanut head 396 truck . Both with mild cams.