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1980 350 4barrel carb adjust
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tusseltussel
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Feb 20, 2008, 7:54 AM
Post #1 of 7
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1980 350 4barrel carb adjust
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how do i go about adjusting the choke and 4 barrel carb I just got a new choke coil the old one was nolonger working properly
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 21, 2008, 3:37 AM
Post #2 of 7
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Re: 1980 350 4barrel carb adjust
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There were a lot of changes to chokes on quads back then. Some only set in, in one postion and any adjustment is with the rods to the choke plate. Primary choke pull off adjustment is critical for these - have that right. Make sure if heated that it gets current to the choke coil and the connection is good. You can feel those heat up with key in just "run" position,
T
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tusseltussel
User
Feb 21, 2008, 5:40 AM
Post #3 of 7
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Re: 1980 350 4barrel carb adjust
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hi Tom,1980 gmc p chassis. I have 12 volts to the single wire I bought a new choke coil spring because the old one no longer opened or closed fully im not sure the differences in chokes but mine is round and is screwed in but it looks like they want me to rivet the new one in. so im gonna put it in cold and gently close the choke than tighten it down now after that is the fact that it does not hold the rpms up like its supposed to, I think theres supposed to be 2 steps that hold the idle up but they dont seem to catch and I don't know if the quad opens at all i don't think it is supposed to open much. Oh and just for fun the maintenance crew who used to keep this thing running said it was well taken care of and well it was hard to start the guy says i think its the fuel pump so i look and 1 of the spark plug wires was cut in half and it had a mix of 3 different types of plug wires along with the fact that when they gave it a tuneup they just left the plugs in the left side because they were hard to get to. animals! needles to sayy it starts fine with ac delco plugs and wires that fit and arent cut
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 21, 2008, 7:16 AM
Post #4 of 7
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Re: 1980 350 4barrel carb adjust
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Been a while but I did lots of these. The rivot were used in origs and by now it should be srews and use the old ones if there - replacements used to come with the right self tapping screws - don't mess those up!
Older ones you could turn the housing to adjust the spring force - these don't do that. If you look you'll see some way that the outer part of the bi-metal spring must index to a lever that moves the choke plate - probably a slot where the spring must center in and won't work for both on and off choke if you miss the slot. Could be a hoop and pin - just look.
That black housing probably has an index tooth (real tiny) for the pre-set spot - use it. Then the winged washers and screws hold it in tight of course and plug it in.
The fast idle is a cam on the passenger's side. You'll see the steps. Holding throttle open a bit and close the choke it should be on the highest step. That would usually get you 1,800 rpm or set it to about that. One touch with the choke pull off in first specified crack to open gives you the next step lowering the idle. Next step should be idle conrolled elsewhere (I think other side now) and if an electric idle thing that is set to just about stall with ign off. Those two adjustments are 1/4" hex and slotted bolts with springs to keep their settings.
Watch out when changing any adjustment and count turns in or out so you can go back. 1/2 turns at a time should do. Also watch out that one adjustment can effect other adjustment down the line - enough rods and junk on those to cause complete hair loss.
All rods and things that pivot must be free to do so. Primary choke (front) is much easier to adjust when cold and choke spring is holding choke plate shut - trip throttle and choke shuts. Then will hand vacuum pump open the front pull off and you measure with a drill bit how far it cracks open on front against air horn wall - also done with a funky level but those are not necessary and if you need help I'll convert degrees into a drill bit size for you - a 1/4 bit would behave and be on the lean side in most.
I mentioned that as most weren't right when new and now's a chance to make it right. Pull offs and the choke are the most fussy to get perfect and awesome when they are. Some diaphragms are metered to leak so don't just toss those if they don't hold - that was to make their action slower/less abrupt. If pull offs don't behave the back barrels won't open fully also. Found tons of those set wrong when the vehicles were new.
These quads may look the same for decades but adjustment were unique for assorted vehicles and years. There's a 1708**** or something # embossed on body of it on driver's side that can get you the original specs and settings. Some adjustment are by screw or by bending rods which is the real hair loss way to do it.
Note: When you get close just stop and wait till the next day - customer's didn't like that but it's only true cold once a day if you want it perfect.
If the new cover is still off the vehicle you can test the heater with jumper wires pos and ground and watch it move and feel warm/hot on outside. Many faults were with the wiring for those.
I'm obviously watching this thread so ask away with anything you think of. I should still have good paper specs for this carb - no scanner so I'll just try to explain.
Final note: DO NOT DROP DRILL BITS DOWN CARBS! Need I say more?
T
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tusseltussel
User
Feb 23, 2008, 10:21 AM
Post #5 of 7
(2565 views)
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Re: 1980 350 4barrel carb adjust
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well i got the new choke coil spring on and i had to adjust the high idle up like 1 maybe 1 1\2 turns and all seems well. at the back of the choke flap it is a little wider than a 1/4" drill bit, not much just a little. it looks like the only adjustment on that is to bend the rod so im kinda thinking that it starts fine when its 33 degrees and the choke seems to operate so no adjustment necessary??? as for the quad opening it looks like i will have to wait for the license plate so i can drive it and even then im not sure how it works it looks like you really gotta be on the throttle for it to open or under a pretty big load and it seems like their would be a significant vacum drop which would hopfully release that choke pulloff\rod to the back butterfly and allow it to suck open does that sound right doesnt look like that needs adjustment or can be adjusted so other than driving 100 feet up and down my driveway i wont know how it's running, sounds good
got a few other things to take care of so until i get a tag for it i'll try to make the reverse lights work again....maybe little switch on side of trans? got all the other lights working even the old blinking school buss lights which i may replace with sum sort of high beam, or off road type lights, oh and the mount that holds on the right side sway bar bushing is torn right off on the front end. can't find one in a junk yard around here. oh well engine work is done for noe other than a few hoses tightening a belt and little things like that I'm just gonna hook the too heater hoses together to bypass the heater core because the big old heater in this thing is comming out anyway, I think not sure yet
(This post was edited by tusseltussel on Feb 23, 2008, 10:27 AM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 23, 2008, 5:11 PM
Post #6 of 7
(2560 views)
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Re: 1980 350 4barrel carb adjust
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Ok: The drill bit thing is done at the front of air horn not the back. The pivots for top plates front and larger back one are offset as to allow air speed to open them so measure fronts only. If 1/4 behaves and isn't choking as in too rich just leave it. Err to a tad lean is better as it won't foul up plugs but may take a second start attempt. You can adjust those till the cows come home for the exact spot and with the rod bending way overshoot it several times, either way and don't really know till the next cold start. PITA but can be done if you care enough.
Back plate is sprung to be closed and air speed alone will open it when the chokes are off and back pull off will sense low to zero vacuum indicating load and it will open with air speed gradually - it should open in one snap or it gets a blast to too much air vs fuel so that's a way to smooth out the transition. You can hold the pull offs with your fingers and move the plates with it not running and know what it will do. In many applications the back pull off waits till engine is warm so they are partially defeated until then. It can be done without and best not to wail on cold engines anyway.
These are perfect carbs but so fussy they couldn't make two of them new behave the same without tweaking. There are different ways to do the same things on different years and applications they were used for also so just know one isn't enough - you need to understand what it's trying to do.
Side note Rochester Quadrajets: I've personally owned three of them in new Chev 350s. 74 and 83 Chev Suburbans and an 89 OMC Marine 350 and NONE were correct from brand new! Dozens more in other used vehicles. None right till tweaked.
Also note: They are intolerant of dirt and just replacing the fuel filter has caused many to become scrap! Careful doing those!! For a bus I would get the marine metal screen for there and put an in line fuel filter for service elsewhere if not already like that.
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Your other issues: Sway bar - if a metal thing perhaps a welder could make one up for you. Not sure what is missing or broken. Lighting: Just don't overpower the circuit with high watt lights and that just up to you.
Good luck. I've almost owned a shool bus just for the heck of it - fun project,
T
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tusseltussel
User
Feb 24, 2008, 6:49 AM
Post #7 of 7
(2556 views)
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Re: 1980 350 4barrel carb adjust
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it is a fun project this bus is, but the sway bar mount if i weld it it will probably rip either before or after the weld (the weld is stronger than the metal, the metal is weakened by the heat from welding) so im gonna keep looking for another p chassis to grind it off of, as for the lights other than the original lighting inside and out Im gonna add maybe 3 more 12v lights inside so i can see in the dark when were parked the other off road type lights probably wont get done its just the reverse lights to get working now which hopefully is just the little switch on the trans. after thats all done it should be ready to go on the road for its first test drive once the tags are on it, all the bathroom sink, toilet, holding tanks, pump, fittings will be hear by the end of the week and i'll probably have the roof a\c put on by the guys at camping world, that way i don't have to cut a hole in my roof cause thats scary stuff and then if i did it it would probably leak . this way they will do it professionally and with a warranty. gotta try to weld up a compartment underneath to hold the generator build the walls and bed than it will be ready to be a traveling home. my wife works from home, as do i, her on the computer me on the farm, so in the winter next year we may just start driving she can get a satellite hook up for her company computer and work from the road. and maybe along the way we will find a diesel bus to add to the collection
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