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2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Yes the spring affects the curve. If you are going too rich and then leaning out your accelerator pump is giving you too much fuel. What color cams are you using?
Do Some internet searches on accelerator pumps for double pumpers. You may want to go smaller on the squirters, go to a smaller cam or readjust the arms.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I forgot to add a softer spring will get your mechanical weights all in faster. So you should go slow when you rev it to see what rpm is what advance and have someone keep notes as you call out the rpm and timing or film it so you can call it out and it Would be recorded. Too fast can be bad as well,
Yes the spring affects the curve. If you are going too rich and then leaning out your accelerator pump is giving you too much fuel. What color cams are you using?
Do Some internet searches on accelerator pumps for double pumpers. You may want to go smaller on the squirters, go to a smaller cam or readjust the arms.
You are getting closer
Currently at #29 squirter with orange cam in primary and #29 with pink in the secondary. I'm going to pull more accelerator pump fuel out of the secondary.
Originally Posted by Rescue Rogers
I forgot to add a softer spring will get your mechanical weights all in faster. So you should go slow when you rev it to see what rpm is what advance and have someone keep notes as you call out the rpm and timing or film it so you can call it out and it Would be recorded. Too fast can be bad as well,
Should I do this with vacuum advance connected and document the full picture of the timing?
Okay so dropped from a #67 to #64 in the primaries this morning:
45MPH cruise now at 13.8 - 14
65 MPH cruise now at 14.5
WOT in low 13's and pulls HARD
The bad: light throttle tip in goes way too lean and engine stumbles (16-17) and hangs there. Looking at my vacuum gauge it appears that it's going down to 5-6in/hg so my 6.5PV should be opening... Makes me think they're not and I should go up to 7.5PV to get the fuel in earlier? It didn't have this issue as pronounced with the #67 jets... Is this a function of not enough vacuum to pull enough fuel at this vacuum/rpm with light tip in or is this a power valve issue?
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
If you are just cruising and come to a hill that needs more power and you dont give it enough gas or you do not down shift the vacuum will drop as load incresse and that is when the power valve should be openning. Very light tip in wont give you that squirt so the extra fuel needs to come from somewhere. The general rule of thumb is cruising vacuum reading divided in half and round up to the next half number. Take that number for your power valve. But every motor can act differently so adjusting from there is wise. I would reasonably believe you should have 14 to 16 inches of vacuum at cruise. Try the 7.5. No changes are permanant so you can always go back. All of your other readings seem spot on. Its good that you keep checking after each change so you know what the cause and effect is!!!
Last edited by Rescue Rogers; Jun 12, 2022 at 03:22 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Originally Posted by Arg0413
I'm not sure which springs I have, but the timing is pretty dialed in - do you think I'm not getting enough advance and could be an issue?
I would give it as much as it will take and not ping but still make power. Once you stop making extra power with the timing increae there is no need to go beyound that. Once you change that power valve you can go out for a test run. Then come back and give it 2 more degrees and see if it improves the overall performance. If it doesnt improve go back to where it was. If it does improve go up until you stop gaining and then retreat to the Last improved setting
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
In changing all these settings. Once you get it where its at its best you will want to go back and verify your idle setting for AFR or best vacuum, which ever you were using, then verify your best timing and always keep checking these. If something suddenly goes out of whack you will catch it before it does damage.
If you are just cruising and come to a hill that needs more power and you dont give it enough gas or you do not down shift the vacuum will drop as load incresse and that is when the power valve should be openning. Very light tip in wont give you that squirt so the extra fuel needs to come from somewhere. The general rule of thumb is cruising vacuum reading divided in half and round up to the next half number. Take that number for your power valve. But every motor can act differently so adjusting from there is wise. I would reasonably believe you should have 14 to 16 inches of vacuum at cruise. Try the 7.5. No changes are permanant so you can always go back. All of your other readings seem spot on. Its good that you keep checking after each change so you know what the cause and effect is!!!
It definitely seems like my PV isn't opening up soon enough, so I'm going to change that and see where it gets me. Amazon had an 8.5 PV with next day delivery, so gonna try that out tomorrow when it get's in. On a DP does the primary/secondary need to have matching PV? seem's the primary needs to be more sensitive due to cruising and needing that little extra boost of fuel - but the secondary opening is always going to be a low vacuum event to the 6.5 can stay, right?
I would give it as much as it will take and not ping but still make power. Once you stop making extra power with the timing increae there is no need to go beyound that. Once you change that power valve you can go out for a test run. Then come back and give it 2 more degrees and see if it improves the overall performance. If it doesnt improve go back to where it was. If it does improve go up until you stop gaining and then retreat to the Last improved setting
Okay I'm at 34 all in, so going to turn it up to 36 and see how she likes it!
In changing all these settings. Once you get it where its at its best you will want to go back and verify your idle setting for AFR or best vacuum, which ever you were using, then verify your best timing and always keep checking these. If something suddenly goes out of whack you will catch it before it does damage.
I've tuned my idle to AFR at around 14, which is around 1/2-3/4 turn out on the screws.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Usually the secondaires dont have a power valve, but a plug that resembles one. IT shouldnt see much vacuum as you stated its only open at 3/4 to wide open throttle so it doesnt need one.
Usually the secondaires dont have a power valve, but a plug that resembles one. IT shouldnt see much vacuum as you stated its only open at 3/4 to wide open throttle so it doesnt need one.
Geez it just struck me like lightning: I'm getting two big of an initial hit on my secondary accelerator pump, because I have a power valve in the secondary dumping fuel when I stomp it!!!!! There was a block off on the secondary when I got the carb, but the Holley specs said there was a PV in the secondary. I'm going to swap the block off and see what that does when I stomp on it!
Well I went to block off the PV in my secondary, and stripped the metering block when putting the jets in! I had to order a 1/2-32 tap fix it, and once it came in I was able to do a quick fix:
Once this was resolved, my next steps were:
Okay so dropped from a #67 to #64 in the primaries this morning:
45MPH cruise now at 13.8 - 14
65 MPH cruise now at 14.5
WOT in low 13's and pulls HARD
The bad: light throttle tip in goes way too lean and engine stumbles (16-17) and hangs there.
So the big changes were the 8.5 PV in the primary, and blocking off the PV in the secondary with a big jet up. I knew #86 would be rich, but it's all I could get locally so I wanted to get a look at what it came in at. The 8.5 PV is helping the throttle tip in and the engine likes it. Blocking the PV in the secondary definitely helped with my rich bog, and the car really takes off now when I stomp it! When cruising at 55 MPH I was in the 13's , but still when I lightly tip in the throttle I'm having a lean spot..
I decided to put the #67 back in the primary because throttle tip in was much better, then the chinesium fuel like leaked again! It was storming raining, so I'll warranty it out tomorrow and throw it on there! I'm thinking an #82 will get me closer on the secondary - do more testing. I ordered a nice fuel line kit from Jegs, as I'm fatigued by this crap that leaks when I do jet changes! I also found they sell full jet kits 60-99 for $55! Wish I would of done that from the start!
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Good for you. Its fun learning what makes a Holley tick. Just remember not to change more than one thing at a time so you can quantify What changed what
Good for you. Its fun learning what makes a Holley tick. Just remember not to change more than one thing at a time so you can quantify What changed what
It is fun, and I'm enjoying learning about it all through this process! I'm getting ahead of myself sometimes and need to keep it to one change at a time!
Having the gauges is awesome, gaining the knowledge is great and finding the ease to be able to fine tune your carb is worthwhile.
However it can be a double edged sword if you let it.
Back when I had my drag boats, there was a group of us who would camp in different areas in California, Arizona and Nevada.
Some coastal locations, some desert and some in higher elevations.
During family waterskiing weekends, a couple of my friends were so possessed about having the perfect fuel mixture, they would spend most of the first day dialing in their boats while their family sat at the waters edged and watched everyone else’s skiing and having fun.
Once you get it set up, you should be able to forget it and enjoy your time driving.
Having the gauges is awesome, gaining the knowledge is great and finding the ease to be able to fine tune your carb is worthwhile.
However it can be a double edged sword if you let it.
Back when I had my drag boats, there was a group of us who would camp in different areas in California, Arizona and Nevada.
Some coastal locations, some desert and some in higher elevations.
During family waterskiing weekends, a couple of my friends were so possessed about having the perfect fuel mixture, they would spend most of the first day dialing in their boats while their family sat at the waters edged and watched everyone else’s skiing and having fun.
Once you get it set up, you should be able to forget it and enjoy your time driving.
Oh man you must of been laughing at those guys on the sidelines while you were having fun! I agree: once i'm done dialing it in, I don't want to touch it anymore! Time to cruise and enjoy it!
