Started out with rebuilding my engine, 64, 300ci with al heads and 4 barrel all stock. I had the engine done at a machine shop where he went to lower compression pistons (so I could run reg.). I purchased a TA Performance, TA 112 cam to make up for loss of power of reducing the compression. 1st problem was I ran the car 600 miles and broke one of the brand new valve springs! While recovering from that I had the original Rochester 4 barrel rebuilt at a parts store here in GR that specializes in old car parts etc.. Ken the guy that did the work has been doing this stuff forever; he found an old kit that had the leather seals for the pump etc did a good job. I put it all back together and it stalls at start and if you get it going it starts stalling and stumbling at mid range at higher speeds runs good. Pulled the carb off asked Ken to check the floats see if we were starving it everything good. I am still having same problem. I took the car to a local dealership to a mechanic they have that specializes in these old cars to go over the timing, ck vac leak, tuning etc just to see if I was missing something. He set every thing said he worked on it extensively and could not get the flat spot out, no difference. Does any one have any ideas? The mechanic cited the cam that it may be messing with the vac. I am at a loss, I am almost at the point the first $50 takes her
There's not a lot of info out there on these. It very well could be that the lower vac characteristics of the cam are giving the carb fits. The power piston operates much like th 2GC...the stock spring may be too strong for part throttle vacuum to keep the piston closed, so you may get too much enrichment too early. Devon
Try putting the distributor vacuum advance hose to full manifold vacuum instead of ported vacuum. Very easy to do & no $$$$$.
Choke seems to be working properly, secondary air valve...the butterflys in the bottom? they seem ok Setting up to full manifold vac? where do I get that? Sorry I have been reading all that I can just have not messed with this stuff much.
the 4gc i keep tripping over on the floor, only has one vacuum port for the dist. that i can see. it has a port in the back for pvc i believe. of course i could be wrong. jim
Doesn't matter where you get it from. On the carb, on the intake, tee off an existing line (Power Brake, PCV, choke pull off, transmission modulator line) With the engine running, a manifold source will hiss when you unplug it.
Since the PCV valve is a controlled vacuum leak, you shouldn't tee into the PCV hose. You won't get full vacuum.
While it wouldn't be my first choice to tee into, the hose between the carb and PCV does have manifold vacuum. That controlled vacuum leak contributes to the overall manifold vacuum. Manifold vacuum will be the same from wherever you measure it.
"Borrowed" a buddy's carb, works great just a very little hesitation at start some times but runs great. Took the carb back to the guy that rebuilt it will let you all know what we find or dont find. Thank you for all your support!
The 4GC is a nightmare to rebuild; - I did a couple of Oldsmobles that have them, and I had to go into them at least twice before I got it to run right. Cheap old kits usually cause more problems than they're worth. First thing; - Leather pumps are no good. Modern gas attacks them and swells them. Second thing; the check ball has to be installed in the correct place. Sometimes they don't seat right, and therefore leak. If your power piston isn't seated correctly, it will leak and not work right. The spring on it may need to be replaced as well. And lastly; float adjustment is crucial on these things. They have to be exact. I triple check mine now that I've had to go into them countless times. Follow the directions as to how to seat the floats, and measure them exactly, no short cuts. The only modification I had to do was to toss those little retaining clips that hold the needles to the float away, because they always seem to bind up. There is no choke unloader for the secondaries on the 4GC. It's all mechanical. Here's where another problem could lie; you could be actually dumping far too much fuel than the engine can handle with that cam profile. If you can't make the 4GC work, try a Carter AFB instead, they're easier to work on, lots more of them around, and on that Buick it should be pretty close to a bolt-up with no real modifications. Good luck!
The most common problem I've found with the 4GC is clogged enrichment/power jets. These jets are located behind the lead plugs on the front of the carb, and you CANNOT be sure they are unclogged without removing the lead plugs. I have used 175#'s of air pressure on these jets and it would not blow the crud out of them. Only way to know they are clean is to remove the plugs and look at the jets. These jets are located in the lowest part of the carb where the worst of the dirt settles. Don't think you can properly rebuild a 55+ year old Rochester without checking these holes. Access holes can be replugged with solder or small, round fishing weights. Put some Epoxy over the lead if you don't trust your repair.
+2 on the old kits and leather pumps. Those pumps aren't worth two squirts of duck poop. We have modern style pumps for those with good Viton seals on them. I do very few of those carburetors here, just not much demand for them these days. When we do get involved with one, it gets modern internals, which includes the latest production Viton needle/seat assemblies, and modern accl pump assemblies with new springs and Viton seal on them. Anything else is just going to dry up, swell up, shrink or just give LOTS of problems in long term use in this new fuel.......FWIW.....Cliff
Question regarding gasket between base and body. Does the gasket have a built in vacuum leak? On both sides behind the secondary butterfly's there are cut outs that clearly are a leak path. Any how my kit has this same gasket and another one that would eliminate this leak. However if it's suppose to be there I guess I could understand. this is the second Rochester I've taken apart with this strange gasket in place. Thanks Gary
It's not a vacuum leak being above the throttle plates, but I still hate to see any path for dust and dirt to get into the engine. Early 2GC's had the same deal. I build them here and use the gaskets that cover that leak path, or seal it up when the base is installed. Some very early q-jets even had drilled holes to provide a leak path above the throttle plates, they are not needed and I always block them off.....Cliff
Thanks Cliff, This makes sense regarding dirt, pulling ambient unfiltered air in under secondary throttle opening is a strange way to try a balance air inlet. I will seal this up with the supplied gasket.
