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Holley carb tuning

sreidmx

sreidmx

Fortify Offroad
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Hey guys I'm messing with my holley 600 vac secondary on my truck. I have a wideband hooked up to help tune this beast. I have it pretty close and I feel like I'm on the verge of it being good enough, my issue is the carb goes lean past half throttle then richens back up as the revs climb, I have identified this as a combination of a few things. My accel pump circuit consists of 37 squirter, hollow screw and green cam on hole 1. I tried 2 but it seems to be happier on 1.
I think I am fighting the duration of when the power valve opens up 10.5 and when the pump shot runs out. That time is where it goes lean. That time can vary depending on load and gear so it's much worse in high gear than in low.

I decided on my power valve based on the assumption I need the fuel sooner to help compensate for time of pump discharge, I have changed cam sand have gone all the way down to a 25 and it likes more fuel sooner but I think I may have to go to the larger 50cc pump setup to have it cover the transition even longer.

I have 62 jets in the main and the rear plate is oem for the 1850-2
At wot I am getting mid 12-13s on Afr and cruising depending on the throttle position I am anywhere from 11 on really low opening like 1/4 or less and 13-14 1/4 or more but this is steady state cruise.

So basically my main question is should I have to go to a bigger pump or am I missing something? I don't want to jet it way up and kill the possibility of much better mpg and better midrange tq...
 
sreidmx said:
...I have changed cam sand have gone all the way down to a 25 and it likes more fuel sooner but I think I may have to go to the larger 50cc pump setup to have it cover the transition even longer...
Click to expand...


What cam are you using now? The pink cam is the big cam for a 30 cc pump. Also, your statement doesn't make sense, a smaller squirter will give you fuel over a longer period, not sooner. A larger squirter dumps the fuel sooner and then runs out right away.

If you want to change your fuel curve more you need to change the air bleeds, this changes the curve. A larger air bleed will richen it at lower velocities and lean it at higher velocities. But unless you have a carb with screw in air bleeds you need to do permanent modifications to change that, and it's unlikely a 600 vac carb has interchangeable air bleeds.

I would try the pink cam. 50cc pump is stiffer and will give you more consistent results, even if you only use a 30cc cam with the 50cc arm. Always make sure the arm is resting on the cam at idle, I have seen many times where the arm will rest on the torsion spring at idle, and this can give you a flat spot, although that doesn't sound like your problem here.

BTW, how much vacuum does your engine make at idle in gear?
 
I am using the green cam. And I actually have switched to a stiffer pump spring from an older carb and was considering shimming it to make it stiffer.
You are correct on the air bleed situation and we believe the high speed air bleeds are to large and I am having to compensate in other ways.
I tried the pink cam and it doesn't provide enough fuel initially and goes lean right off idle, the green cam is the only one paired with the largest squirter to make the engine reapond well off idle. And my statement about squirter duration is related to the fact that I know I have a large squirter but with the larger pump it extends the duration past what the 30cc can provide. I know based on my afr the engine needs a lot of fuel on the lower end of the spectrum because it's takes so long for it to get into an rpm where the mains and pv come into effect. I think this is happening because it's heavy it has a low stall and large tires and technically is overgeared for the tires.
 
Sounds to me like when you put the pink cam on it let the arm touch the spring and not the cam? I've seen it happen many times with the pink cam because it has so much travel it starts out lower. Move it to a different hole or bend the arm a little so it touches the cam and not the spring. Sometimes you will want to drill another hole in the cam to screw it to the throttle arm to get it to work right. If the pink cam is setup right it will give you more fuel than the green cam and it is more linear, so if you give it half throttle to full it isn't maxed out already like some of the other cams. I like to set the spring so it leaves a little gap at WOT sometimes, then you don't bottom it out but you get the stiffest setting you can without bottoming out. You can also bend the arm and then tighten the spring more to get it to be stiffer without any slop, just make sure it doesn't bottom out, you should have at least .015 additional travel at WOT between the bolt head and the lever.

Some people use small piano wire to restrict the air bleeds by pinching it in the metering block gaskets when you bolt it together. I don't like doing that because it feels like cobbling vs drilling and tapping for adjustable air bleeds, but it is an option.

Have you considering drilling the rear plate to allow more fuel when the vacuum secondaries are open but leave it lean at lower throttle openings? What about a high flow power valve?

I think your real solution is adjustable air bleeds, anything else will be just trying to cover up that problem.
 
Im very conciese of my pump arm adjustment and not to have the pump arm to tight or loose, and I always check the clearance at wot.
I think you are on the right track with air bleeds, regarding the rear plate I have thought about drilling it for standard jets but I havent gotten that far yet because I keep messing with this.
 
So I did a few things I wouldn't normally do mainly because I had some goals in mind but for he sake of trying it I did this anyway as per your guys advice.
I put in a 4.5 power valve 69 jets and the pink cam with a 35 squirter. This was all done by sequence one at a time and finally ended up here. And it's a lot better now and the lean spot is virtually gone. It still goes lean but I think I need to mess with my governor in my trans because it's really in the wrong gear at lower rpm sometimes which causes a bog. That's besides the point it runs much better. It's still rich but I think I can lean jets some and maybee reduce pump shooter some to compensate.

Here's my conclusion, I had been shooting for mpg and performance, I didn't realize the 600 had large air bleeds and that requires a pretty decent jet size to match that, I got away from stock settings on my main jet so far it was way to lean (62) stock is (66), I tried to compensate with power valve but that's not enough fuel and just had the whole thing outa wack. There is still some adjustments to be made but I am realizing that there is many ways to skin a cat but for the most part in my situation it's not far off the oem settings.
It's now fat in the midrange 11 Afr but that's because I tried a super drastic change the opposite direction I was going. I may lower this to 65-67 and get a std flow 6.5 and go from there.

Regarding power valve selection, there is two schools of thought, the divide vaccum by 2 and use 1-2 inches below idle in gear vaccum. I think it should be based on what your engine needs. A wideband helps this process for sure. Summit racing has some great videos regarding holley tuning.
 
And folkenheath you were right the pink cam was compressing my spring so I had to tighten it some to allow the shot to be more active.
 
I mentioned two springs, the spring you were talking about needing to be tighter, and also, sometimes the arm touches the torsion spring that wraps around the throttle shaft. This causes the arm to not move off of idle until it contacts the cam, creating a flat spot.

I think you seem to be very detailed when it comes to your tuning so you will get it right. And I hear you on the power valve, some people say 1/2 idle vacuum, some people say 2 below, so anywhere in that range should be OK. I tend to stick closer to 1/2 because it leaves more wiggle room if it drops RPM some day in weird weather or something and then the valve will open at idle and the engine may want to flood and die. In theory it should only affect part throttle driving because when you floor it vacuum is at 0 so all power valves should be open then.

Also, the fact that you have 12" of vacuum tells me you must have a decent cam in there (or a vacuum leak which can cause it's own problems), most stock and low overlap cams produce 18" or more, 20, 22, etc. Trying to lug that at low RPM will reduce the signal to the carb so your also on track with the tranny shifting, that needs to be right too.

If you dare you could try to drill and tap, here is a kit. Or just get the tap individually and buy the holley air bleeds so you don't have to drill the bleeds out too.

http://www.summitracing.com/parts/aed-7932
 
That's funny you mentioned it because I stuck the cam on a spare carb to see how the arm would actually move vs what the chart says, and on the second hole I saw exactly that.. It wasn't even close to touching the cam until very late. That told me last time I tried it on hole 2 what was happening. I ordered a quick fuel rebuild for this which is why I got a high flow power valve but it does me no good with stock sized pvcr. I didn't want to introduce mor variables, the truck has 12 inches in gear and 16 inches out of gear at idle. The cam is rv... Whatever that means I don't know because the motor was built in 94 and my dad lost the specs for it. I may go up on the cam at some point but my gearing ad tire says otherwise, I am trying my best to make this the best I can for now.

Heath thanks for all the insight and help it's really been a huge help. Taught me to listen better instead of discarding someone's opinion.
 
No problem, glad I could help. I still think the high flow power valve ups the flow a little, but the PVCR definitely becomes the restriction at that point, it might depend on the carb though. Maybe sometimes the PVCR is already limiting it with the std power valve.

One time I found this fancy graph that showed pump cam travel vs throttle angle, and I took it as gospel. Turned out it was wrong. I think it even said the green cam was more volume than the pink which is not true. I have drilled out cams before with another hole to get more travel out of them. Because sometimes the distance between holes is too large and it leaves some travel out that could be there, but if you use the other hole there is a gap because of the torsion spring.

Here is a good sheet that illustrates what everything does in a metering block, it's worth studying. I don't know where it came from, I found it online a while back.

metering block.jpg
 
That's funny I was looking at the same picture on the Internet when you mentioned air bleeds
 
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