I've finally got my 56 back on the road after about 4 years, and let me say, it sure is nice to be back in an old set of wheels. Trouble is I have a drastic throttle surge at crusing speed. My cruising rpm is about 1500-3000 rpm, due to 4.11 gears and 4 speed. Engine is a 460 cu.in big block with 1407 Edelbrock 750cfm carburetor. The interesting thing is that I pretty much had this thing dialed in for crusing, but then when I got into the gas a little deeper it seemed to lean out up top, only revving to about 4000-4500. I found out my secondaries weren't fully opening and fixed that. Went for a cruise afterward and it revved out to about 5300 rpm. It still seemed lean up top, not really wanting to pull past that when I would think it should pull to 6000 with the xe274h and 4.11 gears. After I set the secondaries to open fully I noticed the surge at cruising rpm. I also noticed it after I nailed the throttle at about 2000 in second. It barked the tires and then it backfired and started surging until it caught back up. I figured it had to be a lean condition, so I checked my filter and fuel pressure (5psi) and it all looked good. So my thought was to try a new set of step up springs, but surprisingly no parts store around me stocks them. Beings that I bought this motor from carb to oil pan and it was once in someones 57 Chevy, I was inclined to think that they probably had tuned this carb to the engine and that they weren't using the 2" offy open plenum spacer that I was. I figured that probably changed the vacuum signal to the carb, so I removed it. The step up springs were the orange ones, with 71 47 metering rods. Tried it again today without the spacer and a new set of plugs but it was just the same. It even actually died today while it was surging. It started bucking and I pushed the clutch in and tried cleaning it out and it absolutely didnt respond to any throttle and sputtered out. Let the clutch out and it fired right back up. I'm not really too experienced with these edlebrocks so it's a big learning curve, but I'm trying. Am I overlooking something simple here? Let me know what you suggest.
Does your carb have the internal sintered bronze filter at the inlet fitting ! Had a quadra-jet on a 427 do that, bad filter in the carb.
Just because the fuel pressure was OK, that doesn't mean the fuel flow is OK, you have to check both. I would also take a good look at your steel fuel lines for cracks and bend restrictions, and look really close at any rubber hose in the fuel line system. The rubber hoses deteriorate pretty quickly with this modern stuff they sell us as gas. If the hoses in the fuel line are more than a few years old (or you don't know how old they are), I would replace them all, new hose could be a pretty cheap fix. Gene
Might be your vacuum advance. Look up Lars Grimsrud’s timing papers (google is your friend) he is (almost) literally a rocket scientist and a hot rodder. Master carb and ignition man.
What do you have for a fuel pump? Pull the line at the carb it should flow a good stream. What do you have for a ignition system running out of spark can feel like running out of fuel.
Ignition is a Mallory Double-Life "Dual Point" distributor and Super Stock yellow coil. No vacuum advance, only mechanical advance with weights and springs. Timing is set to 20* initial and 36* total. All hoses are less than 6 months old.
edelbrock has very good tuning and trouble shooting support platform. google edelbrock surge and im sure youll find all sorts of info from edelbrock and gear heads,
I’ve heard it called “trailer hitching “ imagine a trailer bouncing back and forth on the hitch. It’s just a minor surge or maybe even a slight drag on the power you feel at light throttle. I experienced it before I heard of it so it made sense to me.
Try taking 6 or 8 degrees of initial timing out of it and see if the problem goes away, lessens, or occurs at a different speed. A quick, cheap and easy thing to check. How much vacuum does it make at idle and steady cruising speeds? A vacuum advance distributor might make a lot of improvement in street driveability.
In my case it's all throttle dropping off and then returning suddenly. Trailer hitching is a good description for a light surge, but this feels like letting off and hitting the throttle continuously in an aggressive manner.
I think I'm going to try and take some initial out like you've said and see where that puts me at. It's worth a look
Timing problems often feel like carb problems for sure. Maybe experement with different advance springs? Or lower initial as suggested. Edelbrock carbs don't have that built in filter someone earlier suggested at least not stock ones.
Thinking back. I had a cracked fuel line near the rear axle of one of my cars that drove me nut's. Vacuum side doesn't know air from liquid. It'll just keep suck'n.
The carb comes out of the box set to run at tinch lean on a 350 cube engine. Did you up the jetting ?
What’s your fuel pressure during cruise rpm and these happenings. Put a fuel gauge under the wiper blade and go for a drive.
If you're ever trying to detect a surge condition, even a fairly subtle one, try this on your next test drive. Once your vehicle is at a steady cruising speed, lean slightly forward in your seat. Get your back off of the seat back cushion a few inches. Relax and let your shoulders slump a bit. Stay loosened up. Keep your arms bent and keep just a light grip on the steering wheel. Balance yourself from the waist and try to keep your back vertical but don't tense up. You don't want to feel like you're tilting either forward or backward. If there's any kind of surge, or slight increase and decrease in speed, you'll begin to feel yourself slowly rocking back and forth in your seat. And if it's a light surge you'll never feel it if you're leaning back into the seat and straight-arming the steering wheel. I can't take the credit for devising this method. That goes to Sam Shinabery, an auto tech instructor of mine from years ago. Actually I first heard of it when he was teaching an auto trans class I was in. He used this trick to check the shift quality of transmissions upshifting in Drive. A good, firm upshift needed to be felt as slightly pushing you back from the relaxed vertical seating position. If you felt yourself falling forward during an upshift it meant there was some overlap in the apply and release of the clutches and/or bands involved in the shift event and it was momentarily slowing the vehicle by being partially in two gears at the same time.
Long shot. I had a '57 Bird that had a surge problem. Turned out to be a rupture in the vacuum advance diaphragm that was varying the timing. Charlie Stephens
^^^^^ Reply #7 by OP reads "Ignition is a Mallory Double-Life "Dual Point" distributor and Super Stock yellow coil. No vacuum advance, only mechanical advance with weights and springs. Timing is set to 20* initial and 36* total. All hoses are less than 6 months old."
So I came back to finish off this thread with an answer, unlike many do. It all comes back to the idea that ignition problems are usually changed by carburation and carburation problems are fixed by ignition. I went back and checked my initial ignition timing again at 900rpm and it read 30°! I thought maybe I had left the distributor loose and it advanced to cause the surge. I tried it and it was tight. The next thing to cross my mind was maybe the mechanical advance was coming in really early, so I thought to check the initial with the idle down to around 600rpm, it dropped it down to about 24° intial. This really got me thinking that I should set the timing with the idle this low all the time to eliminate the weaker springs allowing the timing curve to come in and mess with my timing settings. So I reset the timing to 18° initial at 600rpm and then reset my idle to 900. Checked the total and it was 36°. Locked the distributor down, went for a ride and the car has never ran better.
Glad to hear it's running good. Most don't post what they found. This way maybe we can all learn something.
Glad it's fixed... my dad used to tell me all the time "90% of carburetor issues are ignition". Took me a long time let that sink in... but, I can pull a carb in about 2 min for no apparent reason...lol Chappy
Yea. The timing issues can really feel like a carb and fuel issue. This has caused many people to chase their tails trying to figure it out. Glad you got it dialed in. Good job.
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