Bigger jets than typical?

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

rgalajda wrote: Thu Mar 23, 2023 10:54 am Your problem is that this carb originally had front and rear power valves. So if you run without the rear power valve then you must compensate with larger rear jets. If original was 80 front and 80 rear , with rear power valve, then aprox equivalent would be 80 front and 86 rear without rear power valve. Of course this is dependant on the size of the rear PVCR and your combo.

I assume you don't use AFR gauge.
No Afr gauge, I normally finalize jetting at the track but with the warm weather here in the northeast I had more time to do some tinkering before the track opens up. Hopefully sunday Ill make some passes I was just surprised that the engine is taking as much jet as it is for the combination. The car part throotle and cruise is pretty close as is.
HQM383
HotPass
HotPass
Posts: 1021
Joined: Sun Sep 12, 2021 7:25 am
Location: Geelong, Vic

Re: Bigger jets than typical?

Post by HQM383 »

Brokejoebuilds wrote: Thu Mar 23, 2023 4:35 pm
rgalajda wrote: Thu Mar 23, 2023 10:54 am Your problem is that this carb originally had front and rear power valves. So if you run without the rear power valve then you must compensate with larger rear jets. If original was 80 front and 80 rear , with rear power valve, then aprox equivalent would be 80 front and 86 rear without rear power valve. Of course this is dependant on the size of the rear PVCR and your combo.

I assume you don't use AFR gauge.
No Afr gauge.....
.... The car part throotle and cruise is pretty close as is.
How is “pretty close as is” validated?
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

HQM383 wrote: Thu Mar 23, 2023 6:39 pm
Brokejoebuilds wrote: Thu Mar 23, 2023 4:35 pm
rgalajda wrote: Thu Mar 23, 2023 10:54 am Your problem is that this carb originally had front and rear power valves. So if you run without the rear power valve then you must compensate with larger rear jets. If original was 80 front and 80 rear , with rear power valve, then aprox equivalent would be 80 front and 86 rear without rear power valve. Of course this is dependant on the size of the rear PVCR and your combo.

I assume you don't use AFR gauge.
No Afr gauge.....
.... The car part throotle and cruise is pretty close as is.
How is “pretty close as is” validated?
Just the overall driving of the car. It doesnt pop or bang or lag when cruising. Part throtte response is good, it was just the wot that had me confused. Just didn't think it would of needed so much jet
HQM383
HotPass
HotPass
Posts: 1021
Joined: Sun Sep 12, 2021 7:25 am
Location: Geelong, Vic

Re: Bigger jets than typical?

Post by HQM383 »

Brokejoebuilds wrote: Thu Mar 23, 2023 7:34 pm
HQM383 wrote: Thu Mar 23, 2023 6:39 pm
Brokejoebuilds wrote: Thu Mar 23, 2023 4:35 pm
No Afr gauge.....
.... The car part throotle and cruise is pretty close as is.
How is “pretty close as is” validated?
Just the overall driving of the car. It doesnt pop or bang or lag when cruising. Part throtte response is good, it was just the wot that had me confused. Just didn't think it would of needed so much jet
With AFR data it might not need so much. Mab should be verified though before drawing conclusions.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
perfconn
Pro
Pro
Posts: 343
Joined: Wed Nov 04, 2009 9:07 am
Location:

Re: Bigger jets than typical?

Post by perfconn »

Put 76 jets in front with 6.5 power valve & 86 jets in rear with no powervalve.It was lean on top because more jet was not added when powervalve was removed.Or if you want both ends the same put 76 jets in rear with 6.5 powervalve.
stealth
Guru
Guru
Posts: 1391
Joined: Fri Jun 30, 2006 9:37 am
Location:

Re: Bigger jets than typical?

Post by stealth »

When is it popping? WOT, Mid, or when you let off the throttle?
Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

:wink:
perfconn wrote: Fri Mar 24, 2023 10:28 am Put 76 jets in front with 6.5 power valve & 86 jets in rear with no powervalve.It was lean on top because more jet was not added when powervalve was removed.Or if you want both ends the same put 76 jets in rear with 6.5 powervalve.
I need a smaller power valve because I only have 5in of vacuum at idle. If I put a 6.5 it would be pulling from the circuit at idle
Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

stealth wrote: Fri Mar 24, 2023 11:41 am When is it popping? WOT, Mid, or when you let off the throttle?
It was under wot load. It went away as I upped the jet.
HQM383
HotPass
HotPass
Posts: 1021
Joined: Sun Sep 12, 2021 7:25 am
Location: Geelong, Vic

Re: Bigger jets than typical?

Post by HQM383 »

Brokejoebuilds wrote: Fri Mar 24, 2023 5:58 pm :wink:
perfconn wrote: Fri Mar 24, 2023 10:28 am Put 76 jets in front with 6.5 power valve & 86 jets in rear with no powervalve.It was lean on top because more jet was not added when powervalve was removed.Or if you want both ends the same put 76 jets in rear with 6.5 powervalve.
I need a smaller power valve because I only have 5in of vacuum at idle. If I put a 6.5 it would be pulling from the circuit at idle
Power valve has no relationship to idle. The idle circuit is drawn from the main well already. Opening the pv to let more fuel into the main well via the does not increase flow through the idle fuel restriction. Put a 10.5” in there and idle won’t change. By design Holley put an easy user adjustable power enrichment feature in their modular carburetors so had to give an easy, safe method of adjustment so came up with the below the idle vacuum method. If your in a situation where the engine is under load and demanding more fuel due to throttle position like maintaining steady speed up a steep hill pv selection at idle won’t help there. It’s unlikely in that situation your vacuum will get below 3.5” of vacuum so we tend to increase jet size if we hear some pinging. Reading engine vacuum in that situation is the best way to know what pv rating your engine wants.

You could potentially lower the primary MJ and raise the pv rating to improve fuel metering at steady state and increase fuel flow on demand in a wide range of situations. Incorporate and use the power enrichment circuit to your advantage.

1. An AFR gauge is the best thing you will ever purchase for your carburetor

2. Hook a vacuum gauge to your engine to read in car while driving and use that to find when your engine could do with the extra fuel. This method combined with the use of an AFR gauge is child’s play to set pv rating.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

I finally pinned the carb today with the pins I have,
Hsab in the front .028
Hsab in the back .024
Currently 82 jets in front and 88 in the back. Im planning on racing the car tm lets see where we end up.
econo racer
Expert
Expert
Posts: 775
Joined: Mon Nov 26, 2007 11:44 pm
Location:

Re: Bigger jets than typical?

Post by econo racer »

Try another known carb and try it.
pamotorman
Guru
Guru
Posts: 2802
Joined: Thu Mar 06, 2014 11:55 pm
Location:

Re: Bigger jets than typical?

Post by pamotorman »

popping in the exhaust can be caused by air leaks in the exhaust system.
Brokejoebuilds
New Member
New Member
Posts: 22
Joined: Thu Jul 28, 2022 8:50 pm
Location: Connecticut

Re: Bigger jets than typical?

Post by Brokejoebuilds »

Brought the car to the track and broke the rear in the first 100ft. Will have a video on my you tube channel on it. Since giving it more jet it really cleared up the popping and woke it up even more.
Post Reply