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More Holley Tunning. Carb Guru's a moment of your time please.
Hey guys, I know there are 999999 topics on holley tunning, and yes ive read them all. But I would like some suggestion's on my setup.
Motor is a 355ci SBC with Flat Tops, 9:3-1 compression, AFR 195cc heads, Comp XS282 Solid (244/252@0.050 .520/.540lift 110LSA) RPM intake, 750DP (73/82 jets and 3.5 PV).
Probillit, 6al, Blaster coil, 8.5's 20inital 38 total all in by 3000rpm.
Car is a 3600lb IROC-Z with Th-350 4000stall and 3:08's (4:10's soon)
My problem is, im getting a HUGE lean stumble and a off idle and part thorttle stumbles. Every time I read the plugs they have perfect tan color, no soot, white marks, just your text book prefect plug color. Im thinking my next move is a higher number powervalve. Idle Vaccum is 7-8 inches.
Im also having to crack the throttle plates (front and back) open a fair bit to keep a decent idle, Im thinking of drilling the butterfly's, if so how big of holes should I use?.
Also Alt here is anywhere from 4000-5000ft.
Any suggestions and comments welcome.
Thanks in Advance.
__________________ 1986 IROC-Z - Its Getting There.
How much clearance do you have on your accelerator pump arm and the linkage? Sounds like it may be a bit tight, try about .020. Also how far out do you have your idle mixture screws?
Just reread first post again, are you blocking off the rear PV? Reason I ask is rear jets are QUITE a bit bigger than the front and that would also be a reason since it's getting alot of fuel dumped as soon as you leave the gate. Try getting it squared up. Personally I would go back to about 76 or 77 squared, make a few passes and see where your at and go from there.
The 3.5 PV should be about right, but trying a 4.5 is worth a shot.
That 7-8" of vaccum was with the car in gear right?
Your 3.08 gear is another factor that is leading to off the line bog, your set-up is a upper rom package and the 3.08 is not letting it spin up fast enough. I'd be willing to bet getting the 4.10/4.11 installed will help a bunch too, then it's back to retuning
When your talking about your altitude, is that actual or "theroritcal"sp off a weather station? If that's the case maybe jetting a little lower like 74 squared may be in order. What part of the plug are you reading, dont go by the porcilen to read your plugs!!! All the porcilen is usually used for is showing if your having detonation problems.
Originally posted by IHI Just reread first post again, are you blocking off the rear PV? Reason I ask is rear jets are QUITE a bit bigger than the front and that would also be a reason since it's getting alot of fuel dumped as soon as you leave the gate. Try getting it squared up. Personally I would go back to about 76 or 77 squared, make a few passes and see where your at and go from there.
The 3.5 PV should be about right, but trying a 4.5 is worth a shot.
That 7-8" of vaccum was with the car in gear right?
Your 3.08 gear is another factor that is leading to off the line bog, your set-up is a upper rom package and the 3.08 is not letting it spin up fast enough. I'd be willing to bet getting the 4.10/4.11 installed will help a bunch too, then it's back to retuning
When your talking about your altitude, is that actual or "theroritcal"sp off a weather station? If that's the case maybe jetting a little lower like 74 squared may be in order. What part of the plug are you reading, dont go by the porcilen to read your plugs!!! All the porcilen is usually used for is showing if your having detonation problems.
Paste that link check it out, ever since I started tuning this way as well as my close buddies, cars have become faster AND more consistent.
Wow thanks for the info.
I set the Accel pumps front and back to .020 and made all the timing come in by 2400rpm, this helped alot.
The Stumble is more of a "BrapBrapBrapBrap" sound and the motor will not rev, it just sits there going "Brap Brap Brap" and shaking. This is only if you mash it hard from a dead stop after some cruising. Im starting to think the car is having fuel system problems cause it gets worse with heat, if the coolant temp is 180 or less, I can usally get away with stalling it to 2500+ and launching it, although it leaves lazy and has some stumble (brapbrapbrap and I have to back out of it to get it going). Anything 190-210 coolant temp and its horrible
yes the rear PV has a plug in it. It came like this outa the box (DP 4779) Should I Square Jet it to 74 all around and run 2 3.5pv's?.
Yes the 4:10's will be in there soon.
The Alt is Actual Weather Station Measurement.
That is a new way to read plugs, I will forsure use it from now on.
Im not really sure what way to go with the tune. If I had to "reset" everything so to speak. What would you suggest.
One fact i can attest to is the same stumbling after awhile of cruising, even though my motor only gets to abouot 160, the head thrown off our motors and collected under the hood is enough to get the fuel hot and not atomize correctly. Add to that the fact you have removed the rear PV and the problem gets worse yet since you are dumping in more fuel than needed while cruising so you basically have 2 problems.
1. the heating of the fuel with all the under hood heat.
2. A really rich condition with rear PV removed that are basically trying like helll to foul out your plugs.
While I personally have'nt tried the rear PV block off yet and have heard a more consistent time is to be had, the negatives effects from doing this in a street/strip car are just not worth the headaches when you decide to cruise it.
What type of spacer are you using, aluminum or plastic? I've heard from guys going round robin that the aluminum spacers seem to get hot and the added heat changes the signal being received from the carb going to the intake and this has produced some "stumbling" also. The first thing I did was remove my 3 day old 2" aluminum spacer in favor of the phletonic?sp 2". When I went back I still had the same stumble and finally installed afuel pressure gague under my cowl and found out fuel delivery was the main culprit for me. My A2000 pump fell victim of poor tank cleaning from the shop that sumped it and a peice of welding wire found it's way in and bound up the impeller so I had a Holley Red in the car until my new A2000 came back. Once I had the fp gauge I seen that when I mashed it the fp dropped right down to 3psi. Installed a Holley Black spring into this Holley red pump and the stumbling problem went away since it then carried 7psi all the way down the track.
If it were me, the cheapest easiest change to make would be remove the PV block, get the 3.5 or 4.5 PV back in then install like 76 squared to start and jet from there. I'm thinking this may cure your stumbling blues somewhat. I still run both PV's, but I'm jetted for WOT down the track so when I do street drive it, the carb is very rich for the stoplight to stoplight thing. And even with my set-up I still get the stumble. I think it's mainly a rich/fouling plug condition since I've stopped by buddies houses before and the car has completely cooled, then once the engine has reached op. temps and I stand on it the stumble still exists.
Im gonna try running 2 PV's (3.5's) with 72-74's all around, 76's would be to rich up here, then maybe hit the dyno.
The Carb Actually came from holley with the rear PV pluged off. Stock Jetting was 71/80 with a 6.5PV.
Im also thinkin about running locked out timing (like 34-36 degrees) since the car has 4:10's with a 26 inch tire and a 4000 9 inch converter, it doesn't stay below 3000 for long and I think that kinda timing would clean up the idle alot.
Yes the underhood heat in these cars is brutal, Ived tried all the tricks and such, they have helped but its still a problem. Im going to try and insulate all the fuel lines with Steam Pipe insulation from the local oilfeild supply shop.
Also the Fuel System im my car is a Carter 120GPH 6 PSI mech using stock LG4 Tank Pickup and line's. (Maybe a Holley blue hooked up to use the Return Line would help?).
Also I am not running Any spacer. I have a stock hood and very little clearance.
I used to have a problem with the underhood heat effecting things. I'm running a 1/2 inch phenolic spacer with the nitrous plate on top of that then the carb. My fuel line is wraped up in a heat reflective fibreglass cloth that I ordered up from Summit. Both things really helped and problems have all but gone except for when it's really hot and the car is parks for a little while. Even then it's only on startup that it acts up.
Im gonna try running 2 PV's (3.5's) with 72-74's all around, 76's would be to rich up here, then maybe hit the dyno.
The Carb Actually came from holley with the rear PV pluged off. Stock Jetting was 71/80 with a 6.5PV.
Im also thinkin about running locked out timing (like 34-36 degrees) since the car has 4:10's with a 26 inch tire and a 4000 9 inch converter, it doesn't stay below 3000 for long and I think that kinda timing would clean up the idle alot.
Yes the underhood heat in these cars is brutal, Ived tried all the tricks and such, they have helped but its still a problem. Im going to try and insulate all the fuel lines with Steam Pipe insulation from the local oilfeild supply shop.
Also the Fuel System im my car is a Carter 120GPH 6 PSI mech using stock LG4 Tank Pickup and line's. (Maybe a Holley blue hooked up to use the Return Line would help?).
Also I am not running Any spacer. I have a stock hood and very little clearance.
Thanks again for your help.
Couple of things. I take it this is not a street car and that street performance is not an issue? If it is then that camshaft is simply too large. But for the strip it should be OK.... once you change to the 4.10's. With the 3.08's you simply will not be able to get it to run correctly. Too much of a component mismatch as you well know.
One critical problem that I see. 9.3 to 1 is not enough CR with that cam. You would need a minimum of 10.5 to 1 ( or even higher ) to get enough dynamic CR. 10.5 to 1 can be run on the street with pump gas with aluminium heads. With your cam specs you could probably even run 11 to 1 on the street. That is a big cam for a 355.
I don't agree with installing a rear PV. Holley designed that carb to be used without a PV. Emulsion circuit and air bleeds are all sized to be used without a PV. Adding a PV could result in an excessively rich fuel curve on the Secondary side at high RPM's. The provision is there ( mainly for Road Racers who use Dual PV's ) to add the raer PV, but it is for the benefit of Professional tuners who know how to resize the high speed air bleeds and make the necessary modifications to the emulsion circuit
Another problem with a rear PV is it's height in the float bowl. In a Drag car, the rear PV can be uncovered by fuel slosh at the launch. Slicks and a good chassis make the problem worse. Lots of Drag people add jet extensions to prevent the jets from being uncovered....but they forget about the rear PV. The front PV is not a problem, the fuel " climbs " the metering block wall, so it remains covered.....but a rear PV has the fuel dropping away from it. IMHO do not run a rear PV. Not needed, not the cause nor the cure for your problems.
If this is a strip only car, lock the mechanical advance at 38 deg total and be done with it. If a dual purpose car then it gets trickier.
One thing you haven't told us is what color pump cams and what size shooters you are using. Could you provide this info please? Most important.
The 3.08's and the low CR are killing you. Gears the easy fix. CR could get expensive. Might be a cheap way out though. Felpro has just released and " Extra Thin " head gasket. .015" compressed tickness compared with the regular .039". Part # 1094. Check to see if it is compatible with your Aluminium heads though ( Aluminium heads require special, Head gaskets with a softer " Fire Ring " than Iron heads. )This should bump CR a bit. I'll see if I can do a calculation later on to find by how much. What head gaskets ( Part# please ) do you have in there now?
Edit: You've somehow got to get a phenolic heat insulator under the carb. Even one a 1\4" thick will help. There is a better way though. Take the manifold to a machine shop and get them to mill some material off the top. 1\4" should enable you to run a 1\2" spacer no problem. If you can take off 1\2" safely then even better. Run a four hole spacer , not an open spacer, with the RPM manifold.
Other option is to run a hood scoop.
Insulate all of the fuel lines with heat insulation sleeving. DEI and ThermoTec make lots of products for this.
Last edited by Chickenman35; 08-12-2004 at 01:33 PM.
Originally posted by EvilCartman I used to have a problem with the underhood heat effecting things. I'm running a 1/2 inch phenolic spacer with the nitrous plate on top of that then the carb. My fuel line is wraped up in a heat reflective fibreglass cloth that I ordered up from Summit. Both things really helped and problems have all but gone except for when it's really hot and the car is parks for a little while. Even then it's only on startup that it acts up.
That's basically what I have doen. I also run a Cowl induction hood and an homebuilt aluminium airbox that seals to the bottom of the hood. Really made a difference. Last night I just covered the whole box in self adhesive heat reflective sheets. Readily available from places that deal with Circle Track and Road racing.
I also have some Custom hood louvers that I installed in the hood. Out of a mid-90's Pontiac Grand Prix. Look nice and really let a lot of hot air escape.
Last edited by Chickenman35; 08-12-2004 at 01:45 PM.
Here ya go. It's not the prettiest...but it works very well. Since the picture, all of the sides exposed to engine heat have been shielded with heat reflective insulation.
Originally posted by Chickenman35 Couple of things. I take it this is not a street car and that street performance is not an issue? If it is then that camshaft is simply too large. But for the strip it should be OK.... once you change to the 4.10's. With the 3.08's you simply will not be able to get it to run correctly. Too much of a component mismatch as you well know.
One critical problem that I see. 9.3 to 1 is not enough CR with that cam. You would need a minimum of 10.5 to 1 ( or even higher ) to get enough dynamic CR. 10.5 to 1 can be run on the street with pump gas with aluminium heads. With your cam specs you could probably even run 11 to 1 on the street. That is a big cam for a 355.
I don't agree with installing a rear PV. Holley designed that carb to be used without a PV. Emulsion circuit and air bleeds are all sized to be used without a PV. Adding a PV could result in an excessively rich fuel curve on the Secondary side at high RPM's. The provision is there ( mainly for Road Racers who use Dual PV's ) to add the raer PV, but it is for the benefit of Professional tuners who know how to resize the high speed air bleeds and make the necessary modifications to the emulsion circuit
Another problem with a rear PV is it's height in the float bowl. In a Drag car, the rear PV can be uncovered by fuel slosh at the launch. Slicks and a good chassis make the problem worse. Lots of Drag people add jet extensions to prevent the jets from being uncovered....but they forget about the rear PV. The front PV is not a problem, the fuel " climbs " the metering block wall, so it remains covered.....but a rear PV has the fuel dropping away from it. IMHO do not run a rear PV. Not needed, not the cause nor the cure for your problems.
If this is a strip only car, lock the mechanical advance at 38 deg total and be done with it. If a dual purpose car then it gets trickier.
One thing you haven't told us is what color pump cams and what size shooters you are using. Could you provide this info please? Most important.
The 3.08's and the low CR are killing you. Gears the easy fix. CR could get expensive. Might be a cheap way out though. Felpro has just released and " Extra Thin " head gasket. .015" compressed tickness compared with the regular .039". Part # 1094. Check to see if it is compatible with your Aluminium heads though ( Aluminium heads require special, Head gaskets with a softer " Fire Ring " than Iron heads. )This should bump CR a bit. I'll see if I can do a calculation later on to find by how much. What head gaskets ( Part# please ) do you have in there now?
Edit: You've somehow got to get a phenolic heat insulator under the carb. Even one a 1\4" thick will help. There is a better way though. Take the manifold to a machine shop and get them to mill some material off the top. 1\4" should enable you to run a 1\2" spacer no problem. If you can take off 1\2" safely then even better. Run a four hole spacer , not an open spacer, with the RPM manifold.
Other option is to run a hood scoop.
Insulate all of the fuel lines with heat insulation sleeving. DEI and ThermoTec make lots of products for this.
Thanks for the reply.
First off, yes its a huge cam, yes my compression sucks, there is reason behind this, I was in the process of building a 383, it didn't happen so I took all the good parts I had and build a 355 (had a good shortblock kicking around and couldn't stand the though of not ripping it up this summer). this is the result. I have looked into the .015 head gaskets, but i was told with the AFR-195's (68cc) they would not seal right.
Head Gaskets are Fel Pro #1003.
Yes this is a street car also. Although more of a toy and weekend/lets go for a hop kinda car. not a daily.
The holley is a box stock 4779DP with Pink Cams front and back on the first hole, I don't know the shooter size.
3:08's are on the way out ASAP, when I get some time off work to swap diff's.
yes im going to insulate the entire fuel system from heat. I also have some heat vents and such to get the air moving.
Yes I need a spacer in there, im gonna measure some stuff and see how much it needs.
Im thinkin if I leave the timming, swap the 4:10's, and then hit the dyno to figure the jetting, (after insulating the fuel lines and spacer) that would be a good way to go about doing things.
Please Keep the comments and suggestions comming, might just get this thing working awesome. well atleast till I can get the 383 plan back enroute.
Originally posted by IROC4LIFE Thanks for the reply.
First off, yes its a huge cam, yes my compression sucks, there is reason behind this, I was in the process of building a 383, it didn't happen so I took all the good parts I had and build a 355 (had a good shortblock kicking around and couldn't stand the though of not ripping it up this summer). this is the result. I have looked into the .015 head gaskets, but i was told with the AFR-195's (68cc) they would not seal right.
Head Gaskets are Fel Pro #1003.
Yes this is a street car also. Although more of a toy and weekend/lets go for a hop kinda car. not a daily.
The holley is a box stock 4779DP with Pink Cams front and back on the first hole, I don't know the shooter size.
3:08's are on the way out ASAP, when I get some time off work to swap diff's.
yes im going to insulate the entire fuel system from heat. I also have some heat vents and such to get the air moving.
Yes I need a spacer in there, im gonna measure some stuff and see how much it needs.
Im thinkin if I leave the timming, swap the 4:10's, and then hit the dyno to figure the jetting, (after insulating the fuel lines and spacer) that would be a good way to go about doing things.
Please Keep the comments and suggestions comming, might just get this thing working awesome. well atleast till I can get the 383 plan back enroute.
Thanks guys.
Gacckkkkk!! The Pink cams are horrible. Special cams made for Trans-Brake cars. Very lazy profile. Pump shot occurs too late in relation to throttle movement to be effective.
This is a good baseline to start with:
Blue Cam in #2 hole in Primary side with #31 or #35 shooter. I'm thinking with that big cam that you will need a #35. You can also try a Green cam in the #1 hole.
Secondary side. #31 shooter with a blue cam in #2 hole.
Get tube type shooters. They direct the shot more accurately to the center of the venturi.
Pump linkage adjustment. Correct way is to adjust linkage so that there is no clearance between the pump cam and lever at idle. Then add 1\2 to 1 turn of pre-load to the pump spring. There must not be any claerance or slop whatsoever between the pump cam and lever. Pump shot should be quick and sharp as soon as the throttle is moved the smallest amount. If pump shot is weak or dribbles...then you've done something wrong.
Note" the .015" or .020" extra travel check at WOT is NOT how you adjust the pump lever. That is only a MINIMUM amount required after proper adjustment of the pump lever spring as described above. With some cams, after proper adjustment, you can have over .050" clearance at WOT. The .015" figure just ensures that you don't damage the pump diaphram or bend the pump arm if you screw things up.
Idle setting may be too lean as well. 3\4 off seat does not sound like enough.
Warm car up. Attach vacuum guage. Adjust idle mix out ( richen )until you get highest vacuum reading. Then turn in slowly until vaccum reading just starts to waiver or drop. Turn idle screw back out 1\4 turn. This will give you Rich best idle...which is what you need with a big cam.
Check your transition slots. You cannot have more than .040" uncovered on the Primary side. Either open the Secondary side more and cloase the Primary side or drill a couple of 1\16" holes in the throttle plates. Note: I would leave this to last...as richening the idle and adding more timing may enable you to get the idle speed down.
Timing. Either lock out the mechanical advance entirely and set the timing at 36 to 38 degrees static\total or use vacuum advance. Let us know if the distributor has a vacuum advance provison. If it does you can make some mods that really, really help out a big cammed engine...without the starting hassles of locking the curve. Obviously two totally different timing curve setups, so we need to know what you have.
Originally posted by Chickenman35 Gacckkkkk!! The Pink cams are horrible. Special cams made for Trans-Brake cars. Very lazy profile. Pump shot occurs too late in relation to throttle movement to be effective.
This is a good baseline to start with:
Blue Cam in #2 hole in Primary side with #31 or #35 shooter. I'm thinking with that big cam that you will need a #35. You can also try a Green cam in the #1 hole.
Secondary side. #31 shooter with a blue cam in #2 hole.
Get tube type shooters. They direct the shot more accurately to the center of the venturi.
Pump linkage adjustment. Correct way is to adjust linkage so that there is no clearance between the pump cam and lever at idle. Then add 1\2 to 1 turn of pre-load to the pump spring. There must not be any claerance or slop whatsoever between the pump cam and lever. Pump shot should be quick and sharp as soon as the throttle is moved the smallest amount. If pump shot is weak or dribbles...then you've done something wrong.
Note" the .015" or .020" extra travel check at WOT is NOT how you adjust the pump lever. That is only a MINIMUM amount required after proper adjustment of the pump lever spring as described above. With some cams, after proper adjustment, you can have over .050" clearance at WOT. The .015" figure just ensures that you don't damage the pump diaphram or bend the pump arm if you screw things up.
Idle setting may be too lean as well. 3\4 off seat does not sound like enough.
Warm car up. Attach vacuum guage. Adjust idle mix out ( richen )until you get highest vacuum reading. Then turn in slowly until vaccum reading just starts to waiver or drop. Turn idle screw back out 1\4 turn. This will give you Rich best idle...which is what you need with a big cam.
Check your transition slots. You cannot have more than .040" uncovered on the Primary side. Either open the Secondary side more and cloase the Primary side or drill a couple of 1\16" holes in the throttle plates. Note: I would leave this to last...as richening the idle and adding more timing may enable you to get the idle speed down.
Timing. Either lock out the mechanical advance entirely and set the timing at 36 to 38 degrees static\total or use vacuum advance. Let us know if the distributor has a vacuum advance provison. If it does you can make some mods that really, really help out a big cammed engine...without the starting hassles of locking the curve. Obviously two totally different timing curve setups, so we need to know what you have.
Awesome, good to know, I didn't not know that.
Im going to get all the tunning parts and such when I get some time off work .
the Dist is an MSD probillit with no vac advance (race version).
looking at it now, its pretty far outa tune I would say, I think this thing should be able to drive and run pretty good (and hard I hope) once its all sorted.
Thanks again, this is helping me big time. Im stoked about the car again.
Important note on drilling throttle plates. I said to drill a "couple" of 1\16" holes....that would be ONE hole per throttle blade. Just wanted to make that clear. Apply to Primary side only at first. Only drill holes as a last resort.
I'm sure that you understand that...but someone else reading this may not.....Don't want any one screwing things up and then cursing me to high heaven LOL
