tune up / ? ? / question 401eng

[Todd H]

hi, i just reinstalled my motor after cleaning, painting and installing a new water pump. i've set timing, dwell, spark plugs to factory specs, replaced spark plug wires, rebuilt carb and it runs great at idle and light loads... its also preaty good with the pedal to the floor. my problem is in the meadium load range it chokes and falters...in the 2000-3000 rpm range... It sounds like it is missing or carb trouble ??????????any help/ remidies are greatly appreciated... thanks todd

[Guest]

FIRST THING I CAN THINK OF TO CHECK WOULD BE WERE THE VACUUM ADVANCE LINE IS CONNECTED AT THE CARB. IF YOU HAD IT OFF FOR A REBUILD THE VACUUM HOSE MAY BE IN THE WRONG SPOT.

[NTX5467]

There are a couple of dynamics here. It's fine in the idle/low speed mode and fine in the power mode, but not in the "main system" mode that covers normal driving speeds.<P>The additional key would be if the exhaust was visibly smoking during the normal driving mode. Also be aware the a weak igniton (as was Pete's case) can also act similar to a flooded carburetor situation.<P>Also, if a main air bleed for the main system is clogged, it'll shift that system to full rich (depending on the degree of clog), yet that extra richness is what is needed at higher rpm so it's not an issue there as the carb will have the "power system" (i.e., power valve open) in operation anyway.<P>As far as restricted air bleeds, these are things that a normal rebuild does not address. The air bleeds will be in the top of the carb air horn and open to the air coming in through the air cleaner. Which type of carb is on the engine?<P>Now, the other issue is that an ignition problem (as Pete had) can act the same as a carburetor situation. A weak spark will not fully ignite the mixture and make it act like it's too rich. Plus, the spark at idle is easier to ignite than most any other time, therefore less demands are made on the ignition system to produce a spark. <P>As the ignition system is the earlier one to deal with, it seems, taking the distributor out and giving it a really good inspection might be in order. Looking for bad wiring and connections, plus a worn distributor cam that can affect the interaction of the dwell spec with the point gap spec.<P>When the distributor is done, then systematically move to the carb, looking for clogged air bleeds, leaking bowl gaskets that don't seal well, and floats that don't float.<P>Hope this helps . . .<P>NTX5467

[Todd H]

thanks for the replys.... for starters the motor in question i pulled from a car that i found, which had been sitting at a womans house for over 20 years because she lost the keys and had a new car. the motor is 100 % orig. stock ffrom 1964 except for a new condenser and plug wires. every thing else i just cleaned and repainted..the carter afb #3633s i diassembled and rebuilt (floats ok)... one thing i did not reinstall is the stainless plate/gasket?? that was between gasket and carb.... could that be the problem????? or maybe dist.mech advance or vac advance... is there a way to check the proper operation...thanks todd

[NTX5467]

Thanks for the additional information.<P>The stainless shim carb gasket is a necessary item. If it's like the similar Chevrolets, it seals the heat crossover are in the intake manifold. Otherwise, exhaust heat is put directly onto the bsae of the carb--not good. You might have to do some looking, and might fine one in the back of the FelPro gasket catalog, but it's a needed item.<P>As that is a Carter AFB 4bbl, it has a pair of metering rods that attach to a power piston. There is one metering rod and power piston on each side of the primary venturis. As you probably found out, the power piston/metering rod assenblies move up and down in their bores. The metering rod is tapered at the bottom end where it moves up and down in the primary jet. Also in each of those power piston bores, there should be a small diameter spring (similar to what you'd find in a ball point pen) that governs the phasing of the power piston/metering rod assembly. At high engine vacuum, the manifold vacuum will try to pull the power piston downward in the bore, moving the larger diameter of the metering rod into the main jet. This results in a leaner fuel mixture for the low demand operational situations (i.e., idle, cruise). As the engine vacuum decreases with higher load situations from part throttle to full throttle, the spring pressure pushed the power piston/metering rod higher in the bore and lets the smaller diameter areas of the metering rod control the flow of fuel through the main jet (richer mixture). If the spring is not under the power piston, the piston will not move and remain in the "full down" or "economy" metering area. <P>In the later 1970s, a friend took his Chevy pickup into the dealership with a carb problem. In rebuilding the carb (the 1/2 Q-Jet 2bbl they used that year), the technician left out the power piston spring (why was not determined). The pickup idled fine when hot and ran decent as long as the choke was on in the morning, but when hot, any demands placed on the motor were met with no response--period. He took it back to them and it always came back the same. Eventually, he got tired of that and brought it to a friend of mine that knew carbs. It also acted just like my dad's Chevy pickup (with a full Q-Jet 4bbl) when the power piston would stick in the down position. We put a screwdriver through the bowl vent to lay on the power piston. When we loaded the engine against the brake (in gear), the screw driver did not move. Glen pulled the top off the carb and there was no spring under the power piston. I'd sold him a selection of Q-Jet power piston springs prior to that so he chose a middle tension one--at this point, any of them probably would have worked.<P>When we put the spring under the power piston, we loaded the engine against the brake again (in gear) and it was now responsive as the mixture was being enriched under load. When the owner took it around the block, we heard the large off road tires squalling. He came back with a big smile on his face--all because a small spring was left out that first time at the dealership.<P>In your case, it sounds very similar--no enrichment for part throttle as the power pistons are not moving. I doubt they are stuck as they usually jiggle around in their bores.<P>If these springs are lost, find a speed shop that has calibration parts for the current Edlebrock 4-bbl carbs--whose lineage is directly traceable to the previous Carter AFB carbs of the 1960s. You might get an idea of what you're looking for on the Edlebrock website. Pick a spring that has a low vacuum range of about 6" Hg. That should get you in the ball park.<P>What's happening now is that idle is fine and is being metered correctly. Similar with low speed, but when you "step on it", as soon as the accelerator pump shot is used, it will fall on its face from lack of a rich enough mixture. When rpm finally builds enough to let the secondaries start working, the extra fuel from them (fuel metered via a fixed jet) will compensate for the lean condition on the primary side and it'll pickup and run then.<P>I went through similar things on my father's '69 Chevy 1/2 ton with a 350 and a Q-Jet. With the 3.73 rear axle, it didn't take much throttle to get it down the road, so he never throttled into it frequently enough to move the power valve and metering rods up and down, so they'd stick down periodically. I'd come in from college and dump a can of Gulf Tri-Ad fuel system cleaner in the tank and go driving it, making sure I varied the vacuum levels in the process (which could be fun some times with the 3.73 and PosiTrac). By the time I'd driven it an hour or so, the power piston would be freed up again and it ran like it was supposed to.<P>As this was a recurring situation and Gulf pulled Tri-Ad off the market (probably due to EPA issues), I later put a Holley 4360 Q-Jet replacement on it--end of problem as it was a fixed jet carb with no metering rods.<P>So, gently remove the screw on the plate over the power piston bore. Gently pull the power piston and metering rod out of the bore and check for a spring under it. If there's no spring or it has broken, find the speed shop and order up some power piston springs (in the values I mentioned above). I highly suspect that's where your problem is in this situation.<P>Also, get that stainless steel shim gasket on order too. There should be a "heat track" on the front side of the intake manifold (if it's like I think it is) that connects to the exhaust crossover passage in the middle of the intake manifold. It lets hot exhaust come up there to help minimize carburetor icing under certain cooler conditions. The carb base gasket (the composition gasket, FelPro part number 9803/80200 and GM 3829012 goes against the intake manifold and the stainless steel shim plate goes against the base of the carb--it sounds like it ought to have a leak, but it doesn't as Chevrolet used that same configuration up until about 1970 or so on their small block motors. As that particular 9803 gasket also fits '57+ Chevy 327-409 engines, the Chevy hobby vendors might be an alternative source, providing someone like NAPA can't get one. <P>I think that pretty much covers it. If you have any other related concerns, you can e-mail me direct.<P>Enjoy!<BR>NTX5467

[Guest]

When my '58 Special acted like that, it turned out to be a bad set of points and a short in the distributor (364 V-8).<P>Pete Phillips, BCA #7338<BR>Ector, Texas

[Todd H]

thanks for the great and informative reply.... i will recheck monday and repost the results.....todd

[Todd H]

thanks for the great and informative reply.... i will recheck monday and repost the results.....todd

[Todd H]

now i know why they called it a wildcat!!!!!after 3 years, many many late nights i went on my first road trip (400+ miles) and she purred like a kitten and roared like a lion!!!!..... the problem turned out to be the distr. there was a very poor connection from the vac advance plate to the ground .on the distr. and the mech. advance weights were a little corroded and sticking.<BR> thanks again for all of the great help and responces.... todd