hi i have been fiddling with this engine for while now, finally got it to where i can drive it and get the performance tuning and stuff going. so at lower rpms in pretty much every gear it will be sluggish and when you put the pedal down at low rpms it'll stutter or bog. also will backfire every once and a while when you put in the clutch and let the rpms drop. is it a jetting problem? factory jetting on my holley 670 cfm is 68 primary and i believe 70 secondary. is it the accelerator pump not allowing the fuel it needs? do i need a steeper pump cam? i do have a .520 lift on the cam i am running for the record. any help would be great thank you!
does it back fire out the exhaust or the intake? a rich idle mixture screw can cause low rpm stumbling.
it is an exhaust backfire. and okay that is something i will look for today. ill back it out to 1.5 turns and see what it does
Do yourself a favor and hook up a vacuum gauge to start tuning based on finding the highest engine vacuum reading. My quick guess would be a a lean stumble caused by insufficient fueling or even misadjusted pump cam arm. With a cam like that.. inadequate base timing is probably adding to the issue as well. On Holley's without a secondary pump shot or power valve.. jet spread should be at least 6 numbers bigger on the rear side. Although you didn;t give us much info to go off of here.. here's some ballpark spec's to start with. 14-16 initial advance with quicker centrifugal sweep that "all in" and limited to around 38 degrees by about 2,800 rpm. Vacuum advance pot hooked up to FULL manifold vacuum source and adjusted to tighten up the pots diaphragm which limit its contribution to around 8-10 degrees. The allen wrench size should be around 5/32 and fits into the pots nipple once the hose is removed. Best to test the diaphragm for leaks first too. Jetting should be in the neighborhood of 68 front/ 74 rear. Pink pump cam with at least 34 shooter size.. but could be much bigger depending on gear/trans/induction system combo. PS. A lot of the guys I've helped around here lately have also had misadjusted throttle plate openings(front AND rear) which prevents insufficient transition slot exposure. The front side is what matters the most when bigger cams are used and sometimes means closing up the rear a bit tighter to get it opened on the front side(more slot exposure) without having the idle rpm go sky high. That issue will also cause stumbling as well.
okay so everything you have said makes perfect sense looking at all the little problems I've already had. i just got a vacuum gauge so ill use that with my timing light to set my timing, idle mixture, etc. and as far as my vac. advance i was never really certain that i had tuned that right so i will take a look at that as well. i have a t5 trans N/A planning on spraying it. and the throttle blades i think you have something there as well, the secondary blade may very well be too far open (i thought it was). what kind of increments should i close the secondary blades in?
light should just barely be visible when looking past the blade to bore separation. Maybe around.010-.015 or so and you don't wnat the blades touching the bores or they'll stick and wear into it. Carb has to be off to adjust the small set screw on the baseplate.
Also when warmed up the car wont start unless you put the pedal to the floor while cranking, no matter how much i pump it prior. Is that normal? This is my first build so i am not sure what is normal and what isnt
The last sounds like float height. Pull the carb and make the blade adjustments as groberts indicated. Primary should have .020 use a feeler gauge. Secondary like groberts said with visible light around the blades but not touching. Then put it back on.....start it and set your timing as groberts said. Once timing is set use the vac gauge and no back and forth on your idle mix screws using 1/8 equal turns side to side until best vac is reached. Then drive it.....stumble or cut out coming off idle would be to small a pump cam or squirter size. Have to play with those to get the best performance. Then start on your vac secondary.....get a spring kit and go lighter until you feel a hesitation ( u feel the secondaries coming in ) then go back to the last lightest spring u didn't feel the secondaries. Then most importantly have your wife film your first burnout and post it here
And Wtf.....why does volley make the secondary set screw impossible to get to with the carb on......... Thanks holley
Wow thanks for that mustangnut! That should help a lot! And i know why under the carb for the secondary screw! I'll be sure to post the first by out vid on here! Lol
Mine is actually accessible with a small screw driver with my 2" spacer and intake..... Just noticed that..... Dangit lol
Yeah groberts is probably the biggest help I've had on this site! Seriously he's so helpful here. Well thank you both for all the great info I'm gonna try all this today. Already adjusted my dist to 18 to give me 38 degrees total advance and i put my idle mixtures down to 1.5 turns out and it has helped immensely! No stutter, rarely ever backfires now. Now I'll do what you told me and I'm stoked to see how she runs. I'll post on this thread to let you know how it goes and if i had any more questions!
