Surging when accelerating
Thread Starter
Racer
Joined: May 2001
Posts: 270
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From: Bath, NY USA
Surging when accelerating
My 66 coupe seems to surge below 2000 rpm's when accelerating. Mostly it does it when accelerating slowly but it will do it some when you get on it as well. It runs well and idles well, but when you are accelerating constantly, it acts like it is almost missing then it will smooth out. It keeps going back and forth under acceleration. The engine is supposedly a 350/370 from the 1969-1970 range. It has a Holley 650cfm that was rebuilt recently by Holley, and a 1967 points type tach drive distributor. Points, rotor, cap, and wires all have less than 500 miles on them. Plugs are brand new. Timing is set at 36 degrees total advance. I'm stumped on this one. Any ideas?
Le Mans Master
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From: delmont pa
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Re: Surging when accelerating (bondo478)
if this is happening under light throttle acceration try a higher number power valve as the one you have may be opening too late. post the carb "LIST"number and i will check out the specs :chevy
Race Director
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Re: Surging when accelerating (bondo478)
Light acceleration at low revs where the manifold vacuum does not go low enough to open the power valve is an indicator of a lean off idle mixture. You might be able to tune this out by richening the idle mixture a bit. If not, an increase in main jet size might do the trick.
You might want to rig up a vacuum gage and see what kind of manifold vacuum conditions exist when the problem occurs and use this with the information that Clem gives you on the power valve as a guide.
The vacuum advance could also be an issue. As you say it is a 350/370, this implies the LT-1 cam, and the vacuum advance should be a Delco "236", which is currently available as the NAPA/Echlin VC1810 (16*@ 8"). Insufficient vacuum advance will reduce manifold vacuum, which will lean out the off idle mixuture since fuel flow in the idle and off-idle regime is determined by manifold vacuum, not venturi signal.
Duke
Duke
[Modified by SWCDuke, 11:08 PM 6/4/2003]
You might want to rig up a vacuum gage and see what kind of manifold vacuum conditions exist when the problem occurs and use this with the information that Clem gives you on the power valve as a guide.
The vacuum advance could also be an issue. As you say it is a 350/370, this implies the LT-1 cam, and the vacuum advance should be a Delco "236", which is currently available as the NAPA/Echlin VC1810 (16*@ 8"). Insufficient vacuum advance will reduce manifold vacuum, which will lean out the off idle mixuture since fuel flow in the idle and off-idle regime is determined by manifold vacuum, not venturi signal.
Duke
Duke
[Modified by SWCDuke, 11:08 PM 6/4/2003]
Thread Starter
Racer
Joined: May 2001
Posts: 270
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From: Bath, NY USA
Re: Surging when accelerating (SWCDuke)
Thanks for the info, SWCDuke. It is supposed to be a 350/370, but I don't realy know. It was in the car when I bought it from the original owner's son. The original owner told me what the motor was and he put it in so I'm relatively sure he knew what it was. It was originally a 427 car, and the original motor came with it but needs to be rebuilt. Just trying to get this motor running right for a couple years until I can get the big block rebuilt and back in it. I'll try what you've suggested when I get the info from Clem.
Le Mans Master
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From: delmont pa
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Re: Surging when accelerating (bondo478)
the carb you have is a 585 CFM for a 1966 327 corvette and if the engine is a 350/370 it came with a 780 CFM carb. what is the letter suffix on the number stamped on the block just in front of the passenger side cylinder head as this will tell us what engine you do have. :chevy
Thread Starter
Racer
Joined: May 2001
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From: Bath, NY USA
Re: Surging when accelerating (clem zahrobsky)
Thanks, Clem! I don't know if it's been decked or not, but there aren't any numbers there. It had a Holley 3310 that was in pretty rough shape on it when I got it, but I found this one at Carslile. I was told it was a 650, so I was hoping it was enough carb for the motor. I am still not positive that it is a 350/370. I'm only going on what the guy said. It does run very well except for this problem. Do you think I need to put a larger carb on it, or should I try a power valve and/or jets?
[Modified by bondo478, 9:56 AM 6/5/2003]
[Modified by bondo478, 9:56 AM 6/5/2003]
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Le Mans Master
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From: delmont pa
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Re: Surging when accelerating (bondo478)
try 2 steps richer on the primary side and see what happens. while you are in there see what powervalve you have as a #65 should work. :chevy
Le Mans Master
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From: delmont pa
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Re: Surging when accelerating (bondo478)
if it is a 350/370 it would run stronger with a properly setup 3310 as that is the correct CFM carb for the engine :chevy
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
Re: Surging when accelerating (bondo478)
you found this carb at Carslile, they guy said it was a 650 and ran great. He lied!
Anytime i buy a used carb i buy a rebuild kit.
Buy a gallon of carb cleaner and soak the metal parts overnight. chances are it has some dirt making it run lean and surge. But as the guys said it could have wrong jets or who knows what.
at this point rebuild the 3310 :cheers:
Anytime i buy a used carb i buy a rebuild kit.
Buy a gallon of carb cleaner and soak the metal parts overnight. chances are it has some dirt making it run lean and surge. But as the guys said it could have wrong jets or who knows what.
at this point rebuild the 3310 :cheers:
Thread Starter
Racer
Joined: May 2001
Posts: 270
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From: Bath, NY USA
Re: Surging when accelerating (Matt Gruber)
It wasn't rebuilt when I bought it, I sent it to Holley for a rebuild so I'm sure it's built to spec. I'm going to put a rebuild kit in the 3310 tonight and we'll see how that goes.
Race Director
Joined: Oct 2000
Posts: 15,009
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From: central FL, near the beach.
Re: Surging when accelerating (bondo478)
it didn't sit more than a month without being run, did it? :confused:
what year did holley rebuild it?
how many years did it run good? :confused:
I don't know why, but you made me think of Martha!
[Modified by Matt Gruber, 3:31 PM 6/5/2003]
what year did holley rebuild it?
how many years did it run good? :confused:
I don't know why, but you made me think of Martha!
[Modified by Matt Gruber, 3:31 PM 6/5/2003]
Thread Starter
Racer
Joined: May 2001
Posts: 270
Likes: 66
From: Bath, NY USA
Re: Surging when accelerating (Matt Gruber)
Martha!! :lol: No, the most it sat without being run is about a week. I don't think it ever ran exactly like I wanted it to, but I kept thinking it couldn't be the freshly rebuilt carb that is making it do this :confused: Live and learn I guess! :cheers:
Thread Starter
Racer
Joined: May 2001
Posts: 270
Likes: 66
From: Bath, NY USA
Re: Surging when accelerating (bondo478)
By the way, can I convert a 3310 with a hand choke to the type that has the lever coming up from a spring on the intake manifold? Thats how the current one is set up and I kind've like it.
Thread Starter
Racer
Joined: May 2001
Posts: 270
Likes: 66
From: Bath, NY USA
Re: Surging when accelerating (clem zahrobsky)
I decided to try jets in the carb that is on it before trying the 3310. No one in town had a set of #67 jets but I had a pair of #66 laying around. Put those in and it seems to run quite a bit better. I've got a set of #67's on order so I'll try those when they come in. Going to drive it to work tomorrow and see how it runs.... :chevy