My 72 K20 rebuild thread. (Page 1 of 9)
Posted: 9/4/2021 8:36:59 PM EDT
[Last Edit: TOTHEMAX][Edited]
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So I just bought a 72 chevy with a 350 chevy motor. I have not owned a carburated engine since HS. The truck will start if you pump it a few times before or while it's turning over. It kicks into high idle as it should. When you shut the motor down and attempt to restart it, you need to give it quite a bit of gas to get it to start. Is there anything I can do to remedy this? |
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Float too low, not holding enuff gas. Could be in idle circuits. Might wanna post what kinda carb. I run Holley street avenger and have tweaked it alot. Do yourself a favor and buy cheap vacuum gauge,,install in engine bay and read up on ole skool tuning the carb using that gauge |
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Thanks guys. I am here to learn! It has an edelbrock carb and intake. No spacer between the two, just a standard gasket. It could have ethanol fuel in it as it came from Indiana. I did top off the tank with 91 octane with 10 % ethanol. |
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By TOTHEMAX: Do you still think the float is adjusted too low? The " float bowl" is like mini gas tanks that keep gas for starting etc. If the float is set too low, the bowl isn't keeping enough gas to start it up after sitting. Id find an online manual and set carb back to factory specs. If the choke is working good, don't fool with that part unless you think choke is hanging up which can cause weird starting issues after warm up. https://motorsocietyusa.com/carburetor-float-bowl-issues/ Before messing with carb....... If it's a "new to you" truck and your gonna keep for long time, id probably do a thorough tune. Unless some of that has been done, but check plugs for proper heat range and gap. Wires, fuel filter ( check filter screens on carb) check timing, belts, hoses , fluids (oil, trans, coolant, differential)etc |
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Originally Posted By die-tryin: The " float bowl" is like mini gas tanks that keep gas for starting etc. If the float is set too low, the bowl isn't keeping enough gas to start it up after sitting. Id find an online manual and set carb back to factory specs. If the choke is working good, don't fool with that part unless you think choke is hanging up which can cause weird starting issues after warm up. https://motorsocietyusa.com/carburetor-float-bowl-issues/ Originally Posted By die-tryin: Originally Posted By TOTHEMAX: Do you still think the float is adjusted too low? The " float bowl" is like mini gas tanks that keep gas for starting etc. If the float is set too low, the bowl isn't keeping enough gas to start it up after sitting. Id find an online manual and set carb back to factory specs. If the choke is working good, don't fool with that part unless you think choke is hanging up which can cause weird starting issues after warm up. https://motorsocietyusa.com/carburetor-float-bowl-issues/ Copy. I will try and find a manual tomorrow. |
3-7-77
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By TOTHEMAX: Copy. I will try and find a manual tomorrow. Good deal. On a properly tuned carb with working choke, theoretically you shouldn't need to "pump" the gas peddle. Usually you turn key , wait a second, push peddle down once and this should set the choke cam to fast idle. But, we all pump em a few times ,,lol. |
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Originally Posted By Stutzmech: Check idle circuits for adjustment or gumming up, make sure timing is set right, check spark plugs for fouling etc., basically a tune up Will do. |
3-7-77
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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The Edelbrock carbs are copies of Carter AFB’s built in Italy. If I remember right they also made a copy of a Quadrjet. But most are AFB copies. On a carburetor engine you many times have to crack the throttle to start the engine even warmed up. Post a picture of the carb to correctly ID it. ETA, does it have points in the distributor? Those were point ignitions. Also may have a choke pull off that vacuum makes the choke open slightly. |
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Try to notice if it smokes when it starts. A lot of times, gas will either boil or siphon into the hot intake causing a temporary rich condition at warm start-up. If the bowl is over full, this can cause the issue. Set the fuel level correctly. Many times this can be helped by installing a non heat conducting spacer under the carb as well. Opening the throttle part way allows enough air in to allow the engine to start and the rich condition will make itself obvious by the temporary smoking and a strong smell of gas fumes from the exhaust. |
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Get pic of back side too. First thing I'd do is clean carb with spray top to bottom and get a real vacuum cap and get rid of the "screw in hose". Also looks like the top half where it connects to bottom half is leaking, look for a gasket and/or rebuild kit for that carb. Maybe see if those screws are just loose |
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Originally Posted By die-tryin: Get pic of back side too. First thing I'd do is clean carb with spray top to bottom and get a real vacuum cap and get rid of the "screw in hose". Copy |
3-7-77
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By die-tryin: Float too low, not holding enuff gas. Could be in idle circuits. Might wanna post what kinda carb. I run Holley street avenger and have tweaked it alot. Do yourself a favor and buy cheap vacuum gauge,,install in engine bay and read up on ole skool tuning the carb using that gauge It has a vacuum gauge in the cab |
3-7-77
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By TOTHEMAX: It has a vacuum gauge in the cab Pretty useless in cab unless you like watching swing back and forth, while you can tune with that one, it's better to have it close to motor so you can watch it and make adjustments right there. Mine, lol Attached File Look up "tuning carb w/vacuum gauge". The idea is to get the highest vacuum reading you can, there will be a point where you can't go any further with adjustments (air / fuel), once you get the highest steady reading, you should be good. |
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Ok, I'll tap out on this one, I know nothing about those carbs ( my advice on vacuum gauge and tuning still apply) but have no idea whats what. I know single inlet but looks,like a 2barrel carb ( nothing wrong with that) |
There is no level playing field in life ~ Para069
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Carburetors don’t like a lot of heat. Whenever hood clearance allows, I always like to run an insulator. Spacer w/ longer studs That’s something your local auto parts store can order. |
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Originally Posted By TOTHEMAX: I plan on running over to the performance shop tomorrow and getting a spacer. I will clean the carb when I install it. Unless the vacuum lines and connectors are new, do yourself a favor and get new lines and plastic fittings and caps to properly block the unused ports. Spacer will help too. I'm running a spacer mainly because I fit a Holley carb to factory manifold , lol. |
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First thing I'd do is get a piston stop and verify #1 TDC matches up with your timing marks on the harmonic balancer. Then get yourself a timing light that lets you dial in advance on the light itself. And finally, snag you a vacuum gauge from harbor freight. Make sure there isn't some wonky setup going on with the way the vacuum lines are run. You never know what the car's previous owner has done. Set your timing and dial in your carb with the vacuum gauge. If you really want to get it set up, install an O2 sensor on your exhaust and tweak the carb to get the ratio you're after. Rule out just incorrect timing or too extreme of a carb setting. It is possible to adjust the timing too far to where the engine will run fine but it has a hell of a time starting. One possible issue that can cause what you're seeing is simply the fuel pump allowing fuel to drain back down the fuel line and back into the tank. You can install a cheap 1-way valve in the fuel line near the carb to prevent that. Another issue is incorrect carb settings or stuck valves that can allow a little siphon action to happen and it sucks all the fuel out of the bowls and down into the intake manifold. Something that would help others help you is to know how the vacuum and timing is setup. Its a 6hr discussion as to whether to run the vacuum advance off the manifold or ported vacuum, with guys feeling strongly on both sides of the aisle. But just knowing how your engine is setup and what your initial timing is would help. From the pictures, it looks like they have the advance running off the manifold vacuum port. |
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Originally Posted By ch3no2: @TOTHEMAX How’s it running? Sorry I haven't updated in a few days. I can down with the vids and have been home sick. I did get that spacer plate installed but I didn't get a chance to drive it. |
3-7-77
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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I have that exact carb on my '68. It's about the only thing that works right on that engine. It pops right off cold or hot. No compression and no power, but the carb seems to work well. I'll try and take some pics of how the vacuum is hooked up, maybe the fuel line and anything else that might help. |
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Originally Posted By Striker: pics of the truck or yee be banished.  Lol, based on his "rear brake" thread, I'm gonna say it has 3/4 or 1 ton axle based on the rear spring setup, the rest of truck is mystery |
There is no level playing field in life ~ Para069
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Hopefully he can give us some more but there are a couple here: https://www.ar15.com/forums/general/1972-Chevy-k20/5-2481591/ |
Here is a couple more. I haven't been able to take anymore lately.  Our plan is to eventually replace both axles with 373s. We also have a bb 427 motor and th400 that we want to drop into it. That may be a few years down the road.. |
3-7-77
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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All mine were Quadrajets. You should press the pedal to the floor and let it up, at least once should do it, to set the choke/fast idle. Once it warms up some you can goose it and it should drop off the fast idle notch to the regular idle speed. Are you sure your carb body gaskets are dry rotted and cracked? If they are your gas in the bowl can evaporate. Idle speed (when warm) should be around 600 to 650 rpm regardless of the carb on the engine. Fast idle? I used to set mine around 1650 to 1700 rpm. If you're going to drive it while it's still cold and on fast idle be sure to put your foot on the brake real good so the rear end doesn't get a bad "shock" when you put it in gear at the fast idle rpm. My wife tore the rear end up in her Olds when I could not get her to understand she was shocking the shit out of the ring/pinion when she was too impatient to let it drop down off fast idle before throwing it in reverse on cold mornings. Never messed with an Edelbrock carb. So no help on rebuliding/adjusting it (where/how). As someone suggested above, try to find a manual on line for that model number. Might look at changing spark plugs, too. Should have the GM HEI ignition system. Wires usually last forever on those but they can deteriorate over time. You can use an ohm meter to check the resistance from one end to the other - but I do not remember what the "good" reading is for those. Might find that on the internet or someone here may know. Might look at the distributor cap, too, and change it and the rotor button if needed. Only advice I can give you here is not to take all the wires off the distributor cap at the same time. First time I did that when swapping out the dist. cap I got some out of place in the rotation around the cap and had a hell of time getting them straight again (still can't tell you the firing order on a SBC and I've got four V8 Camaros in the garage. 82? Always liked that body style. Pretty trucks. Antiques in VA. You could put antique tags on it and a nice free flowing exhaust. |
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Everytime I have known someone to swap in a big block it always seems like they never drive it after. Most times it seems a LS or small block will do the same for less and get better fuel economy. The big block engine gets so built its not fun to DD anymore it seems. Just saying. I know a guy with a very nice 1985 k5. He used to drive it all the time. Swapped in a 454 and th400 and something xfer case. It's a mean engine but not very practical now. It's not the truck you take down to the river and beat on it. Basically a mall crawler or light duty trails. The original 305 wasgreat for just driving around. Mpg on any big block is crap I'm sure. That's a nice truck btw. My dream truck actually. |
Mine looks an awful lot like that: Probably the same carb and intake. I haven't had to do anything to this one though so I can't really offer any tips on tuning or troubleshooting it. My truck doesn't have power brakes so the vacuum plumbing is pretty straight forward. Just one line straight from the port on the front of the carb to the distributor. ETA: I’m a little curious how your fuel line is routed. |
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Originally Posted By BASE: Mine looks an awful lot like that: https://www.ar15.com/media/mediaFiles/196570/carb_01-2087019.jpg Probably the same carb and intake. I haven't had to do anything to this one though so I can't really offer any tips on tuning or troubleshooting it. My truck doesn't have power brakes so the vacuum plumbing is pretty straight forward. Just one line straight from the port on the front of the carb to the distributor. ETA: I’m a little curious how your fuel line is routed. The routing Looks to be the same as yours. |
3-7-77
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By TOTHEMAX: The routing Looks to be the same as yours. Okay. I’ve known quite a few guys to end up with vapor lock issues when swapping engines and carbs on these old trucks. Sometimes having nice metal lines I think makes it worse. Mine isn’t routed very pretty at all but it doesn’t get close to the hot stuff. I think maybe the easiest solution would be to replace it with a big block 
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Originally Posted By TOTHEMAX: The routing Looks to be the same as yours. OP look at how his vacuum advance line is. Yours is T'd off drivers side. Id move the vacuum advance to the plugged off port, on left side of carb, and keep it dedicated. |
There is no level playing field in life ~ Para069
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Originally Posted By TOTHEMAX: Move the entire vacuum setup from the right side to the left? What is that going to do? No, jus take the vacuum advance line off the "T" in front of motor and move it to the plugged line on carb, it'll jis give direct line to distributor and the plastic T is just another fail point. Where is the other side of T going? |
There is no level playing field in life ~ Para069
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Originally Posted By die-tryin: No, jus take the vacuum advance line off the "T" in front of motor and move it to the plugged line on carb, it'll jis give direct line to distributor and the plastic T is just another fail point. Where is the other side of T going? Originally Posted By die-tryin: Originally Posted By TOTHEMAX: Move the entire vacuum setup from the right side to the left? What is that going to do? No, jus take the vacuum advance line off the "T" in front of motor and move it to the plugged line on carb, it'll jis give direct line to distributor and the plastic T is just another fail point. Where is the other side of T going? I'll have to go look. Been down with covid so it might take me a bit. |
3-7-77
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
Good channel  HOW TO: EASY & CHEAP SBC CAM UPGRADE-DO THEY WORK? |
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I will check it out. Thanks |
3-7-77
Proud Member of the Leather Head Mafia
“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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“In my opinion, the M1 Rifle is the greatest battle implement ever devised” - George S. Patton
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Originally Posted By TOTHEMAX: I will check it out. Thanks Originally Posted By TOTHEMAX: I will check it out. Thanks Another guy that talks about Cams. Its long but in depth and if you watch you will definitely know more than 90% of the folks out there.  How to properly select a Camshaft |
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So ... this fellow is old, cranky, literally had brain surgery that messed with his ability to remember stuff, ... and has forgotten more than most people will ever know about engines. https://www.youtube.com/user/marvingvx1/videos It's worth the effort and time to see what he has to say on the topic. He's also got lots of published books, and I believe some will cover the SBC. |
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I have one with a bad accelerator pump right now. It dribbles instead of squirts. I can't pump it enough to get it to start. I run several Edelbrock AVS series carbs. The accelerator pump piston fails after a couple years. A $40 rebuild kit, a can of brake cleaner, and 30 minutes later, refreshed carb. A ten second glance at your photo says it is rebuild time for that carb. I would be happy to rebuild it for you, but shipping both ways is probably cost prohibitive. Also, remove your PCV valve, spray it out with brake cleaner, reinstall. Put a hand vacuum tester pump on your distributor vacuum advance. I bet the diaphragm is long gone. Mine is, and it isn't anywhere near that crusty. I have a new one sitting on my bar, along with a rebuild kit as we type.  The symptom for the blown diaphragm is intermittent unstable idle and acting wonky, with a fairly fresh carb.This is the last Holley I ever ran. Edelbrock AVS is on everything now, and this particular truck will get a Holley Sniper injection system soon. 
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Very cosmopolitan.
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Originally Posted By wildearp: I have one with a bad accelerator pump right now. It dribbles instead of squirts. I can't pump it enough to get it to start. I run several Edelbrock AVS series carbs. The accelerator pump piston fails after a couple years. A $40 rebuild kit, a can of brake cleaner, and 30 minutes later, refreshed carb. A ten second glance at your photo says it is rebuild time for that carb. I would be happy to rebuild it for you, but shipping both ways is probably cost prohibitive. Also, remove your PCV valve, spray it out with brake cleaner, reinstall. Put a hand vacuum tester pump on your distributor vacuum advance. I bet the diaphragm is long gone. Mine is, and it isn't anywhere near that crusty. I have a new one sitting on my bar, along with a rebuild kit as we type. The symptom for the blown diaphragm is intermittent unstable idle and acting wonky, with a fairly fresh carb.This is the last Holley I ever ran. Edelbrock AVS is on everything now, and this particular truck will get a Holley Sniper injection system soon. https://i.imgur.com/MZQgcJP.jpg @wildearp Ping me if you’re ever in the Phoenix area. Your choice of beverage is on me 
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Also, that it a GM HEI ignition, no points.
Don't over think it OP. It is a simple carb and doesn't require much tuning on a stock motor. Just to be clear, some people's idea of 'cleaning' is just spraying it off from the outside. That carb needs to be pulled apart and a new kit installed. I do this on a clean bench over a white towel. A small ball in the accelerator pump area will probably fall out, but the rebuild kit should have a new one. I have used several of these Allstate kits. They have a brass accelerator pump diaphragm shaft and are better kits in my opinion, than the Edelbrock kits. Here is the AC Delco vacuum advance assembly. These G15 fuel filters are safer than those craptastic plastic ones. Put a date on it with a sharpie. Replace all of those hoses, including the fuel line.
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Very cosmopolitan.