Just installed double pumper
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Joined: Oct 2009
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From: Marysville Ohio
Just installed double pumper
1979 with the following setup: 383 with AFR 195 heads, XR270HR cam, 1.65 roller rockers, 700r4 tranny and 3.55 rear end.
Put the 750 DP on and really like the expertise at cruise, but have found 2 areas to work on in terms of tuning:
1 - at hard deceleration, tends to stumble; have read this likely secondary float level, and am working through that as I drive it. Any other issues I should consider for this issue?
2 - on highway at roughly 2000 plus RPM, if I nail it, I get a minimum .5 second delay before u hear and feel secondaries kick in.
Additional info: Throttle response out of the box is good, cruise acceleration feels good/responsive. "stock" jetting/setup for this carb is still in place (from memory so may be off slightly - 71 front jets, 80 rear, 6.5 power valve).
Any suggestions on the secondaries? There is no stumble, it is really best described as a delay in response. Looking for best suggestions to dial this in.
Put the 750 DP on and really like the expertise at cruise, but have found 2 areas to work on in terms of tuning:
1 - at hard deceleration, tends to stumble; have read this likely secondary float level, and am working through that as I drive it. Any other issues I should consider for this issue?
2 - on highway at roughly 2000 plus RPM, if I nail it, I get a minimum .5 second delay before u hear and feel secondaries kick in.
Additional info: Throttle response out of the box is good, cruise acceleration feels good/responsive. "stock" jetting/setup for this carb is still in place (from memory so may be off slightly - 71 front jets, 80 rear, 6.5 power valve).
Any suggestions on the secondaries? There is no stumble, it is really best described as a delay in response. Looking for best suggestions to dial this in.
Melting Slicks
Joined: Feb 2006
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From: Japan
If you're experiencing a "delay" in secondary response with a mechanical-secondary carb it's most likely a secondary linkage adjustment, pump cam, or squirter issue. You can buy different pump cams from places like Summit, etc. or you can start by just removing the existing cam and playing with attachments, flipping it over,etc.-as far as pump adjustment Holley says there should be a remaining .015" of travel remaining in the lever with the throttle wide open, and with the throttle closed the lever should be just contacting the lever with the throttle closed. Enjoy?
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From: Marysville Ohio
Good info and thank you. Had some free time from work, so ran over to to grab a Holley tuning book and a couple odds/ends. Terry on the Sales floor also gave a me a couple ideas. He thought the float adjustment I tried (lowering secondary level because of the hard stop problem) may have created my issue with secondaries kicking in - in effect, starving it for fuel. I bumped float level up in the parking lot - the delay is either gone or imperceptible - but still have a slight stumble on a hard decel or stop. I do have the Holley baffles in both fuel bowls. It accelerates like a raped ape though.....
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From: Marysville Ohio
Gonna bump this up with an update - From a stop/rolling stop or even slow speed, it seems to kick in just as I want it to - very agresive and fast - but cruising at highway speed (observed at around 2200 or so RPM and at 65/70 MPH), still seems to kick in late.
No bogs or stumbles, at all - just a delayed response.
I think Birdsmith's suggestion about linkage is my next step - The linkage seems to move as it should just working it by hand, and my next step is to go over the linkage from carb to gas pedal to ensure I have full travel. If that does not have any effect, I was wondering if I should look at my power valve (it's a 6.5)?
Not sure I have a full grasp of the power circuit just yet, but reading secveral posts leads to me to think it could be a part of the issue. If the PV does not do anything until vacuum drops below 6.5 inches, I may need to see how quickly my vacuum level changes at cruise anyway. At idle, in D, I have between 12 and 13 inches - the motor pulls good vacuum, so any thoughts that it's not droppping below 6.5 as quickly as I would have expected?
Thanks in advance.
Jim
No bogs or stumbles, at all - just a delayed response.
I think Birdsmith's suggestion about linkage is my next step - The linkage seems to move as it should just working it by hand, and my next step is to go over the linkage from carb to gas pedal to ensure I have full travel. If that does not have any effect, I was wondering if I should look at my power valve (it's a 6.5)?
Not sure I have a full grasp of the power circuit just yet, but reading secveral posts leads to me to think it could be a part of the issue. If the PV does not do anything until vacuum drops below 6.5 inches, I may need to see how quickly my vacuum level changes at cruise anyway. At idle, in D, I have between 12 and 13 inches - the motor pulls good vacuum, so any thoughts that it's not droppping below 6.5 as quickly as I would have expected?
Thanks in advance.
Jim
Le Mans Master
Joined: Oct 2000
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From: Santa Ana CA
Gonna bump this up with an update - From a stop/rolling stop or even slow speed, it seems to kick in just as I want it to - very agresive and fast - but cruising at highway speed (observed at around 2200 or so RPM and at 65/70 MPH), still seems to kick in late.
No bogs or stumbles, at all - just a delayed response.
Thanks in advance.
Jim
No bogs or stumbles, at all - just a delayed response.
Thanks in advance.
Jim
PERHAPS when you are cruising at ~2,200 RPM you have the throttle part way open. So when you nail it from this position, maybe you are only activating a portion of the squirter volume because you've already used a portion of the stroke.
What happens if you set your squirter to start squirting after the throttle is part way open rather than have it start from an idle? This would leave more of an available shot at cruise speed.
ON the other hand, I just reviewed your post and noticed that it said "in D".
Maybe this isn't a carb problem at all. Maybe it's a lag in the torque converter that you are experiencing. My '91 with the automatic does exactly the same thing and has since I bought it brand new. There ain't no carb on that.
Furthermore, you should probably be running a vacuum rather than mechanical secondary carb with an automatic unless everything has been properly "prepped" for it (i.e., set up for drag racing).
Last edited by toddalin; Dec 18, 2011 at 01:29 PM.
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From: Marysville Ohio
toddalin - will certainly review the info you've provided - Although i've talked to several people runnig DPs with an automatic, and they seem happy with the setup.
I never had this delay issue with my Q-jet (which is a mechanical secondary). Main reason I went with the Holley was that the Q-Jet leaked like a sieve - and yes, it was rebuilt by a pro, I ahve absolutely no complaints there. I ran it for about a year, and I loved the reponse and overall performance - but the leaks gradually got worse, and I couldn't live with a leaking carburator - just not safe in my book.
On the other hand, I spent some time looking at my linkage from the pedal to the carb last night, did find some travel limitations; when my gas pedal was all the way down, the secondary flap was probably about 2/3 of the way open - I had a little more travel left. Adjusted last night, but have not had time to get it on the road and play with it.
I never had this delay issue with my Q-jet (which is a mechanical secondary). Main reason I went with the Holley was that the Q-Jet leaked like a sieve - and yes, it was rebuilt by a pro, I ahve absolutely no complaints there. I ran it for about a year, and I loved the reponse and overall performance - but the leaks gradually got worse, and I couldn't live with a leaking carburator - just not safe in my book.
On the other hand, I spent some time looking at my linkage from the pedal to the carb last night, did find some travel limitations; when my gas pedal was all the way down, the secondary flap was probably about 2/3 of the way open - I had a little more travel left. Adjusted last night, but have not had time to get it on the road and play with it.
Pro
Joined: Nov 2007
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From: Northern Florida
1979 with the following setup: 383 with AFR 195 heads, XR270HR cam, 1.65 roller rockers, 700r4 tranny and 3.55 rear end.
Put the 750 DP on and really like the expertise at cruise, but have found 2 areas to work on in terms of tuning:
1 - at hard deceleration, tends to stumble; have read this likely secondary float level, and am working through that as I drive it. Any other issues I should consider for this issue?
2 - on highway at roughly 2000 plus RPM, if I nail it, I get a minimum .5 second delay before u hear and feel secondaries kick in.
Additional info: Throttle response out of the box is good, cruise acceleration feels good/responsive. "stock" jetting/setup for this carb is still in place (from memory so may be off slightly - 71 front jets, 80 rear, 6.5 power valve).
Any suggestions on the secondaries? There is no stumble, it is really best described as a delay in response. Looking for best suggestions to dial this in.
Put the 750 DP on and really like the expertise at cruise, but have found 2 areas to work on in terms of tuning:
1 - at hard deceleration, tends to stumble; have read this likely secondary float level, and am working through that as I drive it. Any other issues I should consider for this issue?
2 - on highway at roughly 2000 plus RPM, if I nail it, I get a minimum .5 second delay before u hear and feel secondaries kick in.
Additional info: Throttle response out of the box is good, cruise acceleration feels good/responsive. "stock" jetting/setup for this carb is still in place (from memory so may be off slightly - 71 front jets, 80 rear, 6.5 power valve).
Any suggestions on the secondaries? There is no stumble, it is really best described as a delay in response. Looking for best suggestions to dial this in.
Did you go with Holley or Demon for your Carb ? I lean more toward Demon now . Love the Mighty Demon providing you dont really need a choke ; do need to let it warmup a bit more than with a choke before pulling away.
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Joined: Oct 2009
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From: Marysville Ohio
I do agree a vac secondary carb is usually a better "plug and play" option for an automatic.
I thought about both Demon and street avenger carbs. I think my sister's SO (significant other) influenced my thinking. Plus a fair amount of reading on the DP...
I'm willing to spend time tweaking the setup, so any input I get is appreciated.
Have to say again, my take off from a standstill is just scary (in a good way). It's the response at cruise that is what I'm trying to solve for.
I thought about both Demon and street avenger carbs. I think my sister's SO (significant other) influenced my thinking. Plus a fair amount of reading on the DP...
I'm willing to spend time tweaking the setup, so any input I get is appreciated.
Have to say again, my take off from a standstill is just scary (in a good way). It's the response at cruise that is what I'm trying to solve for.
Le Mans Master
Joined: Oct 2000
Posts: 8,897
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From: Santa Ana CA
Another Clue!
Did you check the revised geometry of your carb/transmission throttle valve kick down linkage?
Last edited by toddalin; Dec 19, 2011 at 12:44 PM.
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From: Marysville Ohio
I started down that path after going through the accelerator/throttle linkage lady night - but got late enough that I did not confirm the TV cable was correct. So thanks, agree I need to be sure that is right!
Melting Slicks
Joined: Feb 2006
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From: Japan
Cooper, running a 6.5 PV with 12-13" of idle vacuum could be problematic. Judging by the problem you're describing I think going up to a higher numerical valve could definitely help, as you're not getting that little extra dose of fuel at part-throttle acceleration that the power valve is intended to provide. I'm not exactly sure just how many ratings they can be gotten in, but something around 10.0-10.5 would likely solve the part-throttle acceleration problem. All the Holley books I've read specify a valve rated 1.5-2.0" below idle vacuum, and in my own personal experience this works. My race car runs about 6" idle vacuum and when I switched from a 6.5 PV to a 4.5 it completely transformed it...Merry Christmas!
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From: Marysville Ohio
Birdsmith, thanks for the help. Now that Christmas is over, I am planning experimenting with an 8.5 and 10.5 PV. If the rain passes early enough, I can test drive it as I go back and forth to work on 71 later this week...
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From: Marysville Ohio
just follow up to close this out - ended up keeping 6.5 PV, I figured out it was all on how I was kicking down out of overdrive (700r4).
Played with the TV cable setup, reset the tension (as if it were newly installed) and the delay went away. Best I can figure is that I was just off enough on the TV cable set up to make the tranny downshift slowly - and I think I was so focused on the new carb that I wasn't really thinking about the whole "system".
Revisited the linkage/TV cable, and problem went away. Now just playing with timing, maybe will play with jetting, to finish dialing it in.
Played with the TV cable setup, reset the tension (as if it were newly installed) and the delay went away. Best I can figure is that I was just off enough on the TV cable set up to make the tranny downshift slowly - and I think I was so focused on the new carb that I wasn't really thinking about the whole "system".
Revisited the linkage/TV cable, and problem went away. Now just playing with timing, maybe will play with jetting, to finish dialing it in.
Team Owner
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From: altered state
St. Jude Donor '05
Sounds like its real close.
See if you can borrow someones LM1 this saves SO much time and what ifs..
Check your AF ratio at cruise (steady speed) and WOT (idle, whynot) before you start changing jets this way you know where youre truly at.
See if you can borrow someones LM1 this saves SO much time and what ifs..
Check your AF ratio at cruise (steady speed) and WOT (idle, whynot) before you start changing jets this way you know where youre truly at.
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From: Marysville Ohio
Would love to get my hands on one - I don't know anyone close who has one. Agree that's the best way to go, will start asking around just in case - Haven't had my hands on any real test equipment since I left the USAF in '97...
Everything I did when I was dialing in my old carb (the QJet) was with basic garage tools and trying to teach myself to read plugs (awful subjective, IMO, so I know I never really nailed it).
Everything I did when I was dialing in my old carb (the QJet) was with basic garage tools and trying to teach myself to read plugs (awful subjective, IMO, so I know I never really nailed it).
Le Mans Master
Joined: Sep 2004
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From: Livonia MI
Doesn't sound like whoever did your Q-Jet is a pro to me. These do have a mechanical secondary choke lock outs, but are actually vacuum secondaries at opereating temperature. Dont toss the Q-Jet, as basically the best street carb availible, when set up properly.
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From: Marysville Ohio
Cuisenartvette - good point, and I agree that I need to track down some testing hardware. I've pretty well decided I'm illiterate when it coems to plug reading anyway.
Vette5.5 - I actually love my QJet, and still have it - in truth, it was pretty well set up, just needed minor tweaking on the mixure - The primary reason I wanted to try a differnt carb reason is because the QJet's air horn was leaking.
The leak gradually kept getting worse, until was bad enough that I couldn't live with it - just didn't feel safe (and yeah, I know what typically causes the air horn to leak - I probably did it to myself).
I switched over to the 750 DP because I got it from a relative for cheap. The Holley was an attractive deal, so I jumped on it - And it does run well. Besides I guess I couldnt resist trying to learn something new - that's the fun of these cars, lots of options, lots of good products to play with (I can rationalize anything if it gives me soemthing to play with).
SO I'm keeping my Q until I can find another one off a 79 or same model number, and would like to replace the top. I may switch back then, but who knows? Until then, I've got a pretty good carb.
Vette5.5 - I actually love my QJet, and still have it - in truth, it was pretty well set up, just needed minor tweaking on the mixure - The primary reason I wanted to try a differnt carb reason is because the QJet's air horn was leaking.
The leak gradually kept getting worse, until was bad enough that I couldn't live with it - just didn't feel safe (and yeah, I know what typically causes the air horn to leak - I probably did it to myself).
I switched over to the 750 DP because I got it from a relative for cheap. The Holley was an attractive deal, so I jumped on it - And it does run well. Besides I guess I couldnt resist trying to learn something new - that's the fun of these cars, lots of options, lots of good products to play with (I can rationalize anything if it gives me soemthing to play with).
SO I'm keeping my Q until I can find another one off a 79 or same model number, and would like to replace the top. I may switch back then, but who knows? Until then, I've got a pretty good carb.
195, adjustment, automatic, c3, corvette, double, heads, jegs, linkage, mechanical, pumer, pumper, secondaries, transmission, tricks
