Too much carb?
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Too much carb?
I have a 355 with unknown internals, other than a roller cam. I have an edelbrock RPM Air Gap manifold, and an Edelbrock 750cfm carb.
I think my primary issue is the accelerator pump not functioning properly/needing replaced.
If I accelerate 40% throttle, she drives fine, idles well, etc. If I slam my foot down when I'm under 3500rpm, rpms drop, car hesitates, and about a second later revs up and moves.
Do I have too big a carb? Do I just need to change my accelerator pump? I had the carb rebuilt a year ago, so new jets, etc. Don't think the accelerator pump was changed though.
I think my primary issue is the accelerator pump not functioning properly/needing replaced.
If I accelerate 40% throttle, she drives fine, idles well, etc. If I slam my foot down when I'm under 3500rpm, rpms drop, car hesitates, and about a second later revs up and moves.
Do I have too big a carb? Do I just need to change my accelerator pump? I had the carb rebuilt a year ago, so new jets, etc. Don't think the accelerator pump was changed though.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
If the accelerator pump is bad, the larger the throttle transition the worse the effect will be.
I'm assuming a sharp stab of the accelerator pedal also results in a similar, but less pronounced version of the issue? (say adding a sudden 1/4 throttle or something)
If so, fix the accelerator pump and try again. You need fuel enrichment on throttle transition as there is a delay in the time it takes for the metering circuits to respond to the new inbound air.
I'm assuming a sharp stab of the accelerator pedal also results in a similar, but less pronounced version of the issue? (say adding a sudden 1/4 throttle or something)
If so, fix the accelerator pump and try again. You need fuel enrichment on throttle transition as there is a delay in the time it takes for the metering circuits to respond to the new inbound air.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Worn, damaged, or missing seal on the accelerator pump. The ethanol in modern fuels will cause most synthetic rubber seals to swell and fall off the pump rods. That seal needs to be Viton or other type synthetic rubber that can tolerate ethanol.
Le Mans Master
Joined: Oct 2007
Posts: 7,445
Likes: 971
From: Tennessee/Rhode Island
I agree about the accelerator pump…FWIW my 355 L-82 with a roller cam .525 lift and AFR heads runs pretty darn good with a 650 CFM carb… I would think that a 750 CFM is a little big for a 355 but jetted correctly will run fine.
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
So you think run smaller jets after changing the accelerator pump and seal?
Melting Slicks
Joined: Jul 2006
Posts: 2,286
Likes: 487
From: Salmon Arm, BC
Those carbs (assuming it's a 1407) have a weighted air valve, or a spring adjustment on the secondary air valve just like the Qjet. That means it's a demand type of carb that will only supply the volume of air, and therefore the fuel that the engine wants/needs (If it was jetted properly.)
So your carb size is fine, and as others have said get the accelerator pump that is compatible with modern fuels and your hesitation should disappear.
Jets are easy to change on those carbs.....YouTube is your friend for that.
So your carb size is fine, and as others have said get the accelerator pump that is compatible with modern fuels and your hesitation should disappear.
Jets are easy to change on those carbs.....YouTube is your friend for that.
Last edited by OMF; Apr 7, 2015 at 01:40 PM.
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Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
I have a 355 with unknown internals, other than a roller cam. I have an edelbrock RPM Air Gap manifold, and an Edelbrock 750cfm carb.
I think my primary issue is the accelerator pump not functioning properly/needing replaced.
If I accelerate 40% throttle, she drives fine, idles well, etc. If I slam my foot down when I'm under 3500rpm, rpms drop, car hesitates, and about a second later revs up and moves.
Do I have too big a carb? Do I just need to change my accelerator pump? I had the carb rebuilt a year ago, so new jets, etc. Don't think the accelerator pump was changed though.
I think my primary issue is the accelerator pump not functioning properly/needing replaced.
If I accelerate 40% throttle, she drives fine, idles well, etc. If I slam my foot down when I'm under 3500rpm, rpms drop, car hesitates, and about a second later revs up and moves.
Do I have too big a carb? Do I just need to change my accelerator pump? I had the carb rebuilt a year ago, so new jets, etc. Don't think the accelerator pump was changed though.
I just removed my edelbrock 600cfm carb from my new 355 and swapped in a street avenger 670 myself.... even holley suggests only using 750cfm carbs on hot 350 setups. and those will have less throttle response but better top hp numbers
the heasitation you describe happens when there is either too much or too little feul hitting the chamber when needed... if you accelerator linkage was adjusted wrong this could happen even if everything was working correctly. I kept getting rust particles in my edelbrock from my tank before adding a paper filter causing the accelerator circuit to not work correctly. I also had an issue with the ethanol eating the aluminum surface in the float bowls.
Last edited by augiedoggy; Apr 7, 2015 at 01:41 PM.
Le Mans Master
Joined: Oct 2007
Posts: 7,445
Likes: 971
From: Tennessee/Rhode Island
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Shouldn't the Edelbrock's accelerator pump and seal be made out of this?
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
even my 3 year old edelbrock had constant issues with ethonal sitting in them... last year I have to scrap out the fuel which turned to clear jelly in the float bowls. I used stabilizer and had no issues this year with it sitting over the winter.
Last edited by augiedoggy; Apr 7, 2015 at 01:47 PM.
Le Mans Master
Joined: Oct 2010
Posts: 6,930
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
750 is not too big, i agree about the accelerator pump needing to be checked, it may even have some fuel coming out the squirter but not enough. get a replacemen thats resistant to ethanol as I've seen new ones go bad in a week.
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
http://www.edelbrock.com/automotive/...erformer.shtml
Last edited by augiedoggy; Apr 7, 2015 at 02:17 PM.
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
I got nothing...other than I thought we were discussing an unknown 355 engine here with unknown internals besides knowing its a rollercam and not a 370-400hp LT1 and there were many many 350 engines that gm sold with smaller or "on demand" carb setups compared to the LT1... was the 780cfm really needed?
Last edited by augiedoggy; Apr 7, 2015 at 02:41 PM.
Le Mans Master
Joined: Oct 2007
Posts: 7,445
Likes: 971
From: Tennessee/Rhode Island
http://www.superchevy.com/how-to/148...ey-carbs-test/
Not a whole lot of difference between the 600/650/750 carbs in terms of power…on a 383, not a smaller 350. Like I said earlier, a 750 Cfm carb will work but not necessary.
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
depends on how old it is (or the rebuild kit was).... remember the edelbrock is a weber carb that was later sold as a carter carb until edelbrock bought the rights and molds to make them in china and assemble them here... thats why they still have the weber markings on the side... they are ancient in design...
even my 3 year old edelbrock had constant issues with ethonal sitting in them... last year I have to scrap out the fuel which turned to clear jelly in the float bowls. I used stabilizer and had no issues this year with it sitting over the winter.
even my 3 year old edelbrock had constant issues with ethonal sitting in them... last year I have to scrap out the fuel which turned to clear jelly in the float bowls. I used stabilizer and had no issues this year with it sitting over the winter.
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
I got nothing...other than I thought we were discussing an unknown 355 engine here with unknown internals besides knowing its a rollercam and not a 370-400hp LT1 and there were many many 350 engines that gm sold with smaller or "on demand" carb setups compared to the LT1... was the 780cfm really needed?
Also, weren't the 71's rated gross HP, and by 72 on rated on net?
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
Its likely some sort of chemical reaction. I had aluminum stereo amplifiers under my truck seat melt from salt in the wet carpet they were sitting on and they had the same slimmy ooz all over them were they were corroding...