Holley 870 cfm,vacumn advance, timing initial 18 total 38. when accelerating from a stop i get a hesitation and also after each shift. plug color looks perfect so it doesn't appear to be running rich. i tryed adjust the idle mixture screws but the hesitation is still there, i tried adjusting the timing advanced then retarded again no improvement. If i really push it in second it appears to cough out the carb for a split second. I also forgot to mention i adjusted the accelerator pump also. Any ideas, Thanks Alan
hesitation starting to drive me crazy
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AGREED , buy a set of discharge nozzles and go larger until it stops ,or start drilling yours out (small steps) small bits , until it goes away . You have the common holley lean bog ! If this doesnt cure your problem ,get back to us.
what size engine is this 870 on? auto or stick? rear gear? cam and manifold?
unless that thing has a 4.11 that carb is too big. causing the secondarys to open faster will probably make it worse, not better. try putting a stiffer spring, not a softer one in the sec vacuum pod.
65lkey, I've had lots of vacuum secondary carbs that didn't have any hesitation at all, no matter what I was doing with the throttle pedal. Where did you come up with that bit about the vac secs causing a hesitation? That sort of fails the sniff test.
65lkey, I've had lots of vacuum secondary carbs that didn't have any hesitation at all, no matter what I was doing with the throttle pedal. Where did you come up with that bit about the vac secs causing a hesitation? That sort of fails the sniff test.
18/38 timing if fine,leave it there.
Teher Are many issues that could cause it caugh out the carb like maybe partially foule plug/s or other ign issues but i have to assume they are ok becasue you stated that.
so with that said it's likely issues with too small of a accell pump shot along with accell pump ajd even though you did states you checked it,some times excessively lean primary fuel calibration can also add to that problem when just out of/above the idle circut whicl still transitionaing to a higher throttle position too.
So what mains are you running,what p/valve/and whats the idle vacuum in gear for auto trans?
If you wondering why i ask about idle vac and p/valve it's becasue you also state when you push it meaning to me going for more throttle it caugh's. That shows excessive lean when the throttle is opening more and intake vacuum is dropping when the p/valve should start to crack open for more fuel when motor is under considerably more load where it needs more fuel it and isnt getting it the motor with caugh and incorrect p/valve & mains can also collective add to the problem too when it caughs mid stream in mid throttle when out of the idle circut. The larger squirters will only get rid of an off idle stumble but a much with a caugh when out of the idle circut and mid throttle,thats more so issues mains and p/valve setup too lean for application. And in some extreem cases the idle bleeds need to be changed too but most times the larger squirters with fix off idle stumbles and larger main jets and correct p/vlave will correct a mid stream/part throttle (out of idle circut) lean fuel calibration caugh like your experiencing but the larger squirters will help a little there too,it just depends on lean the prim fuel calibration really is .The larger squirters may fix it or you may have to also fatten up the mains and change the p/valve,thats what dialing in your motor is allabout but its good that you got your timing correct 1st ,its perfect at 18 base and 38 total and i assume total is all in by 2800-3k rpm when you stated the timing your running.
Scott
Teher Are many issues that could cause it caugh out the carb like maybe partially foule plug/s or other ign issues but i have to assume they are ok becasue you stated that.
so with that said it's likely issues with too small of a accell pump shot along with accell pump ajd even though you did states you checked it,some times excessively lean primary fuel calibration can also add to that problem when just out of/above the idle circut whicl still transitionaing to a higher throttle position too.
So what mains are you running,what p/valve/and whats the idle vacuum in gear for auto trans?
If you wondering why i ask about idle vac and p/valve it's becasue you also state when you push it meaning to me going for more throttle it caugh's. That shows excessive lean when the throttle is opening more and intake vacuum is dropping when the p/valve should start to crack open for more fuel when motor is under considerably more load where it needs more fuel it and isnt getting it the motor with caugh and incorrect p/valve & mains can also collective add to the problem too when it caughs mid stream in mid throttle when out of the idle circut. The larger squirters will only get rid of an off idle stumble but a much with a caugh when out of the idle circut and mid throttle,thats more so issues mains and p/valve setup too lean for application. And in some extreem cases the idle bleeds need to be changed too but most times the larger squirters with fix off idle stumbles and larger main jets and correct p/vlave will correct a mid stream/part throttle (out of idle circut) lean fuel calibration caugh like your experiencing but the larger squirters will help a little there too,it just depends on lean the prim fuel calibration really is .The larger squirters may fix it or you may have to also fatten up the mains and change the p/valve,thats what dialing in your motor is allabout but its good that you got your timing correct 1st ,its perfect at 18 base and 38 total and i assume total is all in by 2800-3k rpm when you stated the timing your running.
Scott
( Stock for the avenger P valve .40 mains 76 primary and i think 82 0r 84 secondary,I have to check the vacunm, i am starting to wonder about that.) Thanks i am going to check it out this weekend and i will let you guys know what i find out. I also called holley because i wasn't sure which discharge nozzle was stock (.40) and even they suggested going up two sizes as you guys suggested. Thanks, Alan
With approx 226 deg [email protected] in A 396 THE 4.0 P/Valve can setup a too lean of a siutation for shure but the 76 prim is plenty,may actually be too much when you get the correct p/valve installed.
I would expect that cam to have approx 12"-13" + intake vac at idle in neutral and lower in gear so with that said you should use a 6.0-6.5 p/valve.
With the 4.0 p/valve you have to basically be allmost WOT at over 3/4 throttle or @ WOT to get it to open with the cam you have so it will be running lean when getting into the throttle hard. Thats because the 4.0 p/valve wont start to open like it should at that time in a moderate throttle position that's less then 3/4 throttle to richen things up even though the motor is under a fairly big load at like close to or at 3/4 throttle neediing more fuel then the main/primary jets alone can provide so try a 6.0 or 6.5 p/valve.
BTW,a 6.5 p/valve is stock for most holley perf carbs not the 4.0 you mentioned ,that's for a motor with a lrg cam that idles with approx 6"-8" of vacuum.
Keep the timing at 18 base and your 38 total with the vac adv hooked up to full intake vacuum all the time.
I bet that will fix the problem or at least get rid of most of the caugh/stumble along with a smidge larger prim accel pump squirter but replace the p/valve first because i know the 4.0 is no correct for your cam & motor.
Remeber you may have to back off the prim jets from the 76 it currently at to maybe 72-73 after getting proper 6.5 p/valve installed or it may be fat/rich. That's because the carb will now be giving the proper enrichment the motor needs with the 6.5 p/valve at mid throttle whereas prior to installing the 6.5 p/valve someone tried to cover up the stumble/ caughing carb extreem lean problem at mid range to WOT with more prim jet then whats needed when running the proper p/valve that enriches the primary fuel calibration when required so keep that in mind.
Scott
I would expect that cam to have approx 12"-13" + intake vac at idle in neutral and lower in gear so with that said you should use a 6.0-6.5 p/valve.
With the 4.0 p/valve you have to basically be allmost WOT at over 3/4 throttle or @ WOT to get it to open with the cam you have so it will be running lean when getting into the throttle hard. Thats because the 4.0 p/valve wont start to open like it should at that time in a moderate throttle position that's less then 3/4 throttle to richen things up even though the motor is under a fairly big load at like close to or at 3/4 throttle neediing more fuel then the main/primary jets alone can provide so try a 6.0 or 6.5 p/valve.
BTW,a 6.5 p/valve is stock for most holley perf carbs not the 4.0 you mentioned ,that's for a motor with a lrg cam that idles with approx 6"-8" of vacuum.
Keep the timing at 18 base and your 38 total with the vac adv hooked up to full intake vacuum all the time.
I bet that will fix the problem or at least get rid of most of the caugh/stumble along with a smidge larger prim accel pump squirter but replace the p/valve first because i know the 4.0 is no correct for your cam & motor.
Remeber you may have to back off the prim jets from the 76 it currently at to maybe 72-73 after getting proper 6.5 p/valve installed or it may be fat/rich. That's because the carb will now be giving the proper enrichment the motor needs with the 6.5 p/valve at mid throttle whereas prior to installing the 6.5 p/valve someone tried to cover up the stumble/ caughing carb extreem lean problem at mid range to WOT with more prim jet then whats needed when running the proper p/valve that enriches the primary fuel calibration when required so keep that in mind.
Scott
BTW,a 6.5 p/valve is stock for most holley perf carbs not the 4.0 you mentioned ,that's for a motor with a lrg cam that idles with approx 6"-8" of vacuum.
(Sorry for the confussion i was refering to the squirter size of 40 not the p valve.)
Wow,17" at idle with that cam,at what idle rpm and is it a hyd or solid roller bothof which could have better idle/low engine speed vacuum due to different lobe design then a std flat tappet cam would in most cases of like/= cam profiles?
The 228 deg @.05 cam on a 112 lsa i had in my 396 pulled approx 12" at idle and the cam i have in my 396 now (222 deg [email protected] on a 112 lsa) pulls approx 14-14.5" .
Both were hyd flat tappet cams and the motor was idling at approx 900 rpm idle + - 50 rpm.
Anyway to ans your question yes the higher idle intake vacuum does have a bearing on your p/vlave selection.
If i remember correctly your currently running a 6.5 p/vlave in prim side,if thats correct your good to go . So then the stumble /heasitation could to be casued by an out of adj accell pump,need larger squiter ,accell pump possibly bad,and possible lean jetting on the prim side too which i bet sure your already aware of.
You checked for vacuum leaks that could possibly be leaing out the fuel calibration,correct?
Scott
The 228 deg @.05 cam on a 112 lsa i had in my 396 pulled approx 12" at idle and the cam i have in my 396 now (222 deg [email protected] on a 112 lsa) pulls approx 14-14.5" .
Both were hyd flat tappet cams and the motor was idling at approx 900 rpm idle + - 50 rpm.
Anyway to ans your question yes the higher idle intake vacuum does have a bearing on your p/vlave selection.
If i remember correctly your currently running a 6.5 p/vlave in prim side,if thats correct your good to go . So then the stumble /heasitation could to be casued by an out of adj accell pump,need larger squiter ,accell pump possibly bad,and possible lean jetting on the prim side too which i bet sure your already aware of.
You checked for vacuum leaks that could possibly be leaing out the fuel calibration,correct?
Scott
Scott,
780 idle rpm,Hydralic Roller (Crane) I am not sure which p/valve is in the 870 cfm avenger. I did check for vacuum leaks, couldn't find any. Alan I checked with holley and that carb has a 4.0 p/valve so i guess i will change it out to the 6.5 as recomended.
Thanks Scott,and all who contributed.Added the #45 squirter and that did the trick, no hesitation even between shifts. I pulled the plugs after driving around and they where perfect so i am not running rich. Thanks again. Alan
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