I've got a 78 Malibu with a 305/2bbl rochester and I know this is a Buick site but this place has tons of great info that applies to my car. The last year I've been tinkering with my engine since it's the original and I figure I'll drive it till it dies than get a 350. So I've upgraded the exhaust and replaced the distributor with one that has a bit quicker advance, also put a K&N open element on it. Well I picked up a bit of power but ever since it's been running a bit lean judging by the plugs and I get pinging unless I use premium now. The main thing though is when I first give it gas the engine will hesitate then run again, sometimes the engine even pops through the carb. When I looked over the carb during the rebuild everything seems to be fine and the squirter is good. I've read that smog era carbs like mine are set very lean, especially the idle mixture. Would modifying the idle circuit to give it more fuel help since the main problem is just off idle? The main jet's are #52 but I they seem hard to find since there different than Holley or Quadrajet's. Appreciate any tips you guys might have.
It's no longer possible to judge rich/lean by plug color. I quit trying thirty years ago. Unleaded fuel burns clean enough that it doesn't leave much color behind. White plugs are common. Does the EGR work properly? Heat riser valve (EFE) work properly? Engine coolant temperature where it should be? How about just "adjusting" the idle mix for highest vacuum, then about 1/8 turn lean?
I disconnected the heat riser valve on the manifold because the vacuum switch for it wasn't turning it off when it got warm, but it never seemed to help much anyway. I read on here that smog carbs are calibrated for warm air from the heat stove, that makes sense because the hesitation problem is only when I put my open element air cleaner on it. But I would like to use the cleaner if I can since I notice my top end power is quite a bit better.
I had this same problem on my 2 barrel until I discovered that the accelerator pump valve seal on the outside of the carb would not open up at idle. All I did was tweak the linkage that touches the little metal tab hanging down and it totally corrected the hesitation off idle. The little rubber round valve stopper seal should be open at idle and go into the closed position when you step on the gas. It was an easy fix. Just my experience... Eric :3gears:
I tried posting pics of my carb but the forum says I can't yet but I'm done rebuilding my carb and the pic tutorial on this site was a huge help so thanks to the guy who posted it:TU:. It idles better than I can ever remember and it solved a bunch of other problems I couldn't figure out like a sputtering sound when I accelerated. I still have to tweak the idle mixture a bit but I found when I accelerate from a stop it seems to hesitate for a sec than it kicks in and runs fine. The pump shot looks good, I polished to power piston to make sure it wouldn't stick and the ignition is set right so what could it be? It's been a chronic problem with this car the last year or so and is one of the reasons for the rebuild. I checked and the throttle isn't open to far uncovering the transfer slot much, could it be a fuel pump problem?
I found a place called Vintage Speed that sells all the parts for my carb so I've ordered several different Jet sizes and a new power piston, idle screws. I replaced the fuel pump and the car runs better but still bogs a bit just off idle. I took the carb off to check the transfer slot and I noticed the throttle plates are almost completely closed covering pretty much the whole slot. I guess it's because I've recurved my distributor for more initial advance so I'm getting a stronger vacuum at the carb. So is there anything I could do so I can have the throttle plates open a bit more at idle? I know it's the opposite problem most people complain about o No:
I finally got my news parts so I replaced the #52 main jets with #55 and also replaced the power valve and piston with new ones. It's gonna take some tweaking but it def has more power and the annoying hesitation is totally gone. The power valve they sent me looks a little different though, is there a way to check if the power valve is working right on 2bbl's? I heard for Quadrajets you can stick a paper clip through a hole in the top vent but in mine it's the vacuum that holds the power piston up and keeps the spring from activating the power valve.
