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| Bob_S
12-20-2004 23:51:15
12.149.228.109
11994
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Hello, everyone.
I have a '78 K-10 (350/auto/Roch. M4MV Carb) that I use for work around the property. I've rebuilt quite a bit of this truck over the last three years (engine overhaul, trans overhaul, suspension overhaul, etc.) but I've found a new problem.
In cold weather (especially cold and damp, it seems) the carb floods out while starting to the point where gas will spit out the top of the carb and drip off the butterfly rods, bad enough to have caused a fire last year that, luckily, didn't destroy the truck.
I've installed a new carb kit (gaskets, float, needle, etc.) and set the float per the directions. I've also checked the fuel return line (from the pump) in case it was plugged. I've run out of ideas about what the cause is, but I did find this (http://www.tocmp.com/manuals/Carbs/Rochester/QJet/) yesterday. Anyone ever hear of a problem like this before?
The truck runs fine in warm weather.
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| PA_ Bill
12-24-2004 19:12:52
209.247.222.43
12033
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Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob_S, 12-20-2004 23:51:15
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| You've gotten some good advice already so I won't try to confuse you anymore. One thing no one has mentioned though (and you probably don't want to hear) is that valves out of adjustment will cause same thing as could being out of time badly. There are some great manuals out there on the Q-jets. Don't give up and spend $200 + on a new carb. The Q-jets are some of the best available and can be make to out preform most anything out there. Yes, they take some tinkering ( again, a good manual will help )but I'll put a properly tuned Q-jet up against their high dollar aftermarket carbs anyday !
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| RB/CT
12-24-2004 09:35:37
67.163.171.82
12027
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Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob_S, 12-20-2004 23:51:15
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| Sounds like a problem that an Edelbrock Performer carb, (which is really a Carter AFB copy) with a manual choke would solve. (About $200.) Thats what I run with no problems in 6 years on my 71 Chevrolet Custom Camper pickup with 350 motor. Still have an Edelbrock rebuild kit ($35) that I thought I should have in case its needed but hasn't been needed in six years. Good luck. mIght have worn throttle shafts on that Rochester.
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| Bob M
12-21-2004 05:52:17
151.190.254.107
11995
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Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob_S, 12-20-2004 23:51:15
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| | Sounds like you've got an improperly adjusted or a failed vacuum break on the choke. On startup the vacuum break (or "pull off") supposed to partially open the closed choke the moment the engine fires and prevent flooding. (Punch below for a description from the manual...) The vacuum break is kinda fussy to set. Requires an external vacuum source, a gage (drill rod) of the correct diameter, then bending the linkage to make the adjustment. Also if the pull off linkage is binding, or the vacuum break unit is stiff or has a pinhole in the diaphragm it'll cause the carburetor to flood. Good luck - stuff like this can be tricky to get right!
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| Bob S.
12-21-2004 12:58:54
12.149.228.84
11998
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Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob M, 12-21-2004 05:52:17
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| OK, that makes sense. :D Thank you for your post.
One of the problems I'm fighting is that the truck had been worked on by several people in the past. What I don't know is how many corners were cut to get it running (vacc., arms and jets adjusted out of spec., etc.).
When it acted up last year, I parked it until the summer when I stripped and cleaned the carb. The gaskets and float/needle looked good but the "pot" (bellows?) for the choke pull-off was very loose in its bracket. I was able to fix that by drilling some small holes in the bracket (one of the ears had broken off) and using wire to lock the pot into the bracket.The truck ran fine until it got cold again.
I've already installed a new rebuild kit but I didn't replace the pull-off.
BTW, anyone have a diagram that shows what the vacc. lines should look like? The heat riser that bolted to the header has long since rotted away and the lines were changed to by-pass the butterfly in the air cleaner inlet. I think I can live without the hot air ducting (I have found the metal "hose" that attaches between the air cleaner and the riser at LMC, but no luck finding the riser itself), but I'd really like to see what should be plugged in where. ;-)
One last thing- I remember from an old Caddy that a blocked EGR/PCV valve can cause problems. I don't remember if it was choke related.
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| Bob M
12-21-2004 16:15:51
69.201.120.79
11999
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Re: Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob S., 12-21-2004 12:58:54
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| | Good luck finding a vacuum schematic for your truck! The hose routing depends on the model year, engine, transmission, truck GVW, whether it was built to be sold in CA, 49 states or Canada, and sometimes even in what month in the model year it was built. So unless you have the OEM sticker for your particular vehicle you’re pretty much outta luck on that. Note also that even when new and properly plumbed and adjusted,engines of this era didn’t run very well. (I know – I working at the Rochester Carburetor plant at the time…) FWIW we ran into the same flooding problem with my wife’s dad’s ’75 GMC ¾ ton (350 Q’Jet). After much screwing around I finally stripped off and plugged EVERYTHING except distributor advance which I plugged into the intake manifold, then put a manual choke on it. The truck now starts and runs pretty good. (Fortunately it is too old to be subject to emissions testing here in the people’s republic of NY…)
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| Bob S.
12-21-2004 17:44:50
12.149.228.56
12001
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Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob M, 12-21-2004 16:15:51
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| Here's one thing that puzzles me about the pull-off... The rear (top) buttefly is the only thing connected to the arm at the rear of the bellows. Even so, the actuating rod just slides along in a long groove. It (the bellows) doesn't seem to control much of anything.
That's been the frustrating thing, I've learned to strip and rebuild a Q-Jet (to the point where I'm now dangerous- LOL) but that one rod still doesn't make sense.
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| Bob
12-21-2004 20:44:55
66.163.134.177
12003
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Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Ca in reply to Bob S., 12-21-2004 17:44:50
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| | The vacuum diaphragm connected to the upper rear butterfly ("AIR VALVE") keeps the air valve from flopping wide open when you mash the accelerator to the floor. It allows it to open slowly as manifold vacuum drops, and the fuel flow gets up to speed in the secondaries, which prevents "stumble". I can't recall the exact details, but I believe there is an offset or bend in the linkage rod that catches a lever on the choke mechanism, and pulls the choke open the specified amount under manifold vacuum at startup. The slot allows the "air valve" to move open and shut without the interferance of the vacuum diaphragm, at conditions of low vacuum. (Four barrels open.) Under those conditions, the amount the "air valve" opens is a factor of the air flow in the secondaries acting against the air valve, and the amount it opens is determined by the "air valve spring windup" setting.
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| Bob S.
12-21-2004 23:31:03
12.149.224.187
12006
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Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch in reply to Bob, 12-21-2004 20:44:55
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"Under those conditions, the amount the "air valve" opens is a factor of the air flow in the secondaries acting against the air valve, and the amount it opens is determined by the "air valve spring windup" setting." Dang, how many models did they make? ;-)
This carb has the mechanical bi-metal "spring" that's attached to the manifold. I think that's what you were referring to? There is no "wind up" spring assembly, just the vaccume pull off that the air valve rod rides in and the bi-metal spring that (I think, I gotta get under the hood to look at it again) keeps the butterflies from opening completely via the small cams.
BTW, for you Q-Jet gurus, the model number stamped on the carb is 17057213. I'm including the number as a longshot because while the carb kit had the right parts, the model number seems like an oddball.
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| Bob
12-22-2004 08:35:04
66.163.134.177
12010
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Re: Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blues ( in reply to Bob S., 12-21-2004 23:31:03
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| The air valve coil spring is just a little bitty spring on the LH side of the air valve kinda hidden beneath an overhang on the carb top cover. There is a little screw, (Allen or Torx) that allows you to adjust the spring windup by turning a slotted adjustment mechanism a FRACTION of a turn with a slotted screwdriver, and therefore, setting how much restiction there is to the valve opening.
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| Loren
01-04-2005 19:39:17
12.108.31.88
12128
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Re: Re: Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blu in reply to Bob, 12-22-2004 08:35:04
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| Bob, check the specs and the settings on this before loosening the set. There should little or no reason for anyone to mess with this, i.e., it most probably isn't set wrong. Set it by the book if it needs changing.
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| Joelski
12-21-2004 18:11:25
24.31.171.15
12002
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Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Ca in reply to Bob S., 12-21-2004 17:44:50
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| Get a Ford. :D
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| Bob
12-21-2004 12:35:48
66.163.134.177
11997
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Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob M, 12-21-2004 05:52:17
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| | Carburetor icing, or a cold intake manifold will make this worse, too. Is your heat riser present, and operating properly? In addition, the heat riser directs exhaust up through a cross-over in the intake manifold for mixture pre-heat, and also warms the choke thermostat. If the heat riser isn't closing, or the manifold pre-heat passage is carboned up, there won't be manifold pre-heat, PLUS the choke will stay on way too long. Is the pre-heat tube from the exhaust manifold to the air cleaner snout present? Do you have a functioning thermostat, and possibly cover part of the radiator to aid warmup? Like Bob M says, the choke setting on these is critical, and it's almost impossible to get them set where they will work in all seasonal temperatures.
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| Bob S.
12-21-2004 17:29:23
12.149.228.56
12000
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Re: Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob, 12-21-2004 12:35:48
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| | I already covered that. http://www.yesterdaystruck.com/trforum/messages/11998.html ;-)
Any idea where I could find a new one? The old heat riser part that sat on the header rotted out a long time ago.
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| Loren
12-24-2004 18:11:47
12.108.31.11
12030
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Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob S., 12-21-2004 17:29:23
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| Bob S. Do a little test and see if when you start it up and it begins to load up, open the choke a bit. If the choke is the problem that'll tell you. Bob is right on the pull-off connection, it's a little arm that sticks out and catches on the linkage to the rear. The arm points down from the pull-off.
Leave the air cleaner off and plug the hose. If I remember right should be from a port pointing UP from the #8(passenger's side rear)intake runner. Either there or from the pass. side lower rear corner of the carb. This port should have vacumm with throttle closed. The choke pull-off hose will come from directly below the fuel inlet on the carb(pass. side front lower corner of carb). Do ALL final adjustments with a cold engine. If it runs 5 or 10 minutes it's too warm to be accurate. Push the throttle to the floor and check to see that the choke is all the way closed. Pump it 2-3 times and it should start. If it doesn't and the choke is closed you've got other issues. When it starts watch the choke pul-off diaphram, should pull all the way back. If it starts loading up pull back on the pull-off and see if clears up a bit. If it needs help it's bad. If it's in all the way(towards the front of the truck)then it's an adjustment there or another issue. The adjustment is a screwhead facing the front of the truck either below the pull-off and tough to see, or above and in plain sight. For a leaner mix move the screw and watch the adjuster link, needs to move towards the front of the truck more. The choke stat(heat element) acts more of a timer. Don't try to adjust the amount of choke with it. Set it so it closes the choke well, but not hard. Q-Jets are a real good carb, very adjustable and the adjustments WORK. On a 71 there should only be one 3/8ths hose from the bottom of the carb to the PCV valve, a very short small hose from the front pass. side lower to the choke diaphram, and vacumm advance hose from the drivers lower front corner to the dist. It's a simple but reliable system. Email's visible if you need it. Loren
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| Bob S
12-24-2004 18:48:35
12.149.228.70
12031
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Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Ca in reply to Loren, 12-24-2004 18:11:47
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| Well, here's the basic situation: :-)
Its a '78 350 K10 with a TH350 automatic/automatic Xfer case.
In warm weather it starts and idles like a champ. In cold weather, and this just started last winter, it doesn't matter if I pump the gas pedal or not. As soon as the carb gets gas from the pump line, the carb will flood to the point where gas will spit out of the butterflys on top of the carb and drip off the shaft for the rear throttle body ports. Imagine pouring a quart of gas into your carb. ;-) At best, the engine will just begin to catch and then die. I did have a guy help me last year. One thing I wasn't sure of was that it looked like he was adjusting all of the mixture needles (EEK!?). Like I said in an earlier post, I'm dealing with multiple unknowns.
The choke seems to be working and will open the front buterfly about half way. To me, it seems like an extream case of flooding. (LOL-sorta)
One thing you did bring up has me wondering. When adjusting on a cold engine, how cold is cold and would I have to redo the adjustments for running during the Summer? I ask that because we some drastic temperature shifts up here. Figure 80's to 90's in Summer and 20's to -30's in Winter. BTW, what should my "rough" settings be for the mixture needles? IE, how many turns out from fully closed?
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| Iron_Felix
10-01-2005 03:43:22
195.14.40.118
16339
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Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch in reply to Bob S, 12-24-2004 18:48:35
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| Hello everybody! Bob, i think, that your carb & fuel tank is filled with water, and with whis float needle is not working propertly. You must clean all your fuel system or add in fuel 0,5 gal of 95% alcohol on 20 gal of gas, but only in warm weather, and make a little driving (approx. 10 miles). send me answer, if this problem is gone on hi_level@pochta.ru
Best regards
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| Loren
12-25-2004 10:42:27
12.108.31.41
12037
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Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blues (Roch in reply to Bob S, 12-24-2004 18:48:35
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| For starters, you do have an engine in proper state of repair, right? If it needs a bunch of repair you could be chasing your tail working on the choke/carb.
Cold means unstarted for the day, not a temp on the thermometer. Should really only be worried about this when getting the settings dialed in later. When you get it set up right it'll be good for pretty much all seasons or temps it is outside.
Ok, it sounds like you're telling me that if, say, you disabled the ignition and didn't pump the throttle, it would flood just cranking it? Or does it flood only after pumpng it? If it floods with just cranking the choke probably is not the culprit.
The mix screws will be close enough to run decent with 1&1/2 to 2 turns out from all the way in. Mix gets set only after fully warmed up and with a proper idle rpm. Say 500 to 750 rpm.
You say the problem started last winter. This fall before the temps dropped much, how did it run when warm? Is it very smooth idling or is there a roughness? I'm wondering about a bad valve. When you crank an engine ALL of the flow will be into the engine. Nothing to make or allow fuel to come up through the choke. Should pull all of it down, along with the gravity flow. Get my drift? Generally a bad intake valve will push fuel up out of the carb as that cylinder comes to compression. Valve leaks, pushing what's supposed to be compressed back out the intake.
Read your post again, realized I missed a bit. Choke about half way open sounds like it will indeed run. A wide open throttle setting will open the choke to clear the fuel. You may try holding the throttle all the way down to start it. Resist that urge to pump some more if it doesn't fire off. LOL If it's real bad you may have fouled the plugs to where it won't start right too. Be aware of that. I've had situations come to me that I had to change the plugs, block the choke open and first get it running, check for flooding with the choke still blocked, and do most of the adjustments, then do a prelim choke setting, let it sit over night and readjust for a final.
That'll give you more to go on for now. My head hurts this morning. Loren
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| Bob S
12-25-2004 21:27:20
12.149.228.90
12041
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Re: Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blues ( in reply to Loren, 12-25-2004 10:42:27
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| The info on mix screw settings is greatly appreciated. :-) I worked in a garage for several years when I was younger and also did small engine work, but I never worked on a 4 bbl before now.
The engine itself was overhaulled three years ago (rings, gaskets, valves, etc.)by a guy who had 20+ years working on GM stuff (Retired GM dealership Mech.). It ran like a champ this Summer and Fall. Normal warm up took about five minutes with the choke doing what its supposed to. The exhaust looked good, no smoke and no signs of running over-rich around the tail pipe.
During this latest "difficult" phase, I haven't been pumping the pedal at all. I've been turning over the engine while standing outside the truck to watch for leaking gas. FWIW, the last time I pulled the carb was right after it puked some gas. The inside of the manifold wasn't soaked, just some gas directly below the carb.
Between us, I think the truck got spoiled after I overhaulled the suspension, installed a new gas tank and sending unit and cleaned/painted the frame. ;-) It now has an attitude that if it throws a tantrum, it knows I'll work on it and not use a sledge hammer or a cutting torch. In other words, the truck is now spoiled. LOL
Anyway, I have new vacc lines and a choke pull off I'm waiting for. Once I get the parts I'll pull the carb again and dis-assemble it just to be sure none of the small tubes, ports, or jets are plugged with crud (I may as well check them again). Likewise, I'll check the number of turns on each mix screw before I take them out for cleaning. At this stage, I'll also pull the plugs and check them for abnormal wear or color but I think the valves and the block are fine. I'll also pull the dipstick and check the oil for signs of gas in the crankcase.
BTW, to all. I truelly appreciate the info you've provided. I'm trying to give as much info about what's been happening so my posts have been a little long. This problem has been frustrating but hopefully it won't be terminal.
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| Loren
12-27-2004 19:23:39
12.108.31.81
12057
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Re: Re: Re: Re: Re: Re: Re: Re: '78 Chevy K-10 blu in reply to Bob S, 12-25-2004 21:27:20
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| Another thought. If you're going to pop the top on the carb, turn the top upside down while it's off and see what angle the floats at. Should REAL close to parallel to the body. While you're at it take the float off and double check that it floats well. And, another. Tap the emulsion tubes in with a very small hammer. The tubes with lots of holes in them, pointing down from the carb top when it's in normal position. They come out sometimes and drop into the well, causing real rich problems. Good luck. Loren
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| Bob S
01-03-2005 21:13:03
12.149.224.74
12121
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Re: Re: Re: Re: Re: Re: Re: Re: Re: '78 Chevy K-10 in reply to Loren, 12-27-2004 19:23:39
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| So, after taking some time off for the holidays, I got back to wrenching on the truck.
I pulled the carb again and tore it down. Cleaned the interior out fully with carbon tet. spray cleaner (again) and rechecked everything I had done so far. Rechecked all the gaskets I had replaced to be sure nothing was upside down or torn, rechecked all the linkage, cleaned and checked all the ports and to be sure they weren't plugged, and replaced the choke pull off and its vacc line with new stuff (I'll also replace the timing advance vacc line once I have the carb rebuilt) . Rechecked the (new) float and fuel inlet needle. Needle seats easily and the float is set at 11/32's of an inch below the main body. I might luck out and find it was a bad vacc line but something else caught my eye while I was going through the instructions of the carb kit. One thing I overlooked was the Pump Discharge Check Ball and Cover Screw (located underneath the float assy). I don't think I got a replacement ball in my kit, but I may as well take it apart carefully and clean the old ball and the cavity it sits in. I'm guessing that the check ball keeps gas from draining back to the fuel line but at this stage anything is possible. ;-)
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| Loren
01-04-2005 19:48:51
12.108.31.88
12130
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Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: '78 Chevy in reply to Bob S, 01-03-2005 21:13:03
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| Bob, click on my name and email me if you want me to continue, I'm loosing track of this thread. Loren
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| Loren
01-04-2005 19:46:39
12.108.31.88
12129
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Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: '78 Chevy in reply to Bob S, 01-03-2005 21:13:03
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| It sounds like you're saying you found nothing more wrong. ???
Do yourself a favor and do a proper compression check and a good check on the spark condition. Also, put a 5/8ths. wrench on the harmonis balancer center bolt(hard to do but it'll work) and turn it clockwise till the timing marks line up to 0 degrees. Then turn the crank backwards till you feel the slack just take up. What's the degree reading? Sounds like we may be chasing a golde goose here. Lets get some facts on condition so we KNOW what not to suspect. Loren
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| tortmort
12-23-2004 20:06:00
209.247.222.46
12022
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Re: Re: Re: Re: '78 Chevy K-10 blues (Roch. Carb) in reply to Bob S., 12-21-2004 17:29:23
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| I got the heat riser connector thing with my Headman Hedders. They can get you their part number and then you can order it from Summit Racing if necessary. I suppose if necessary I could send you mine as I'm done with smog tests here in California as my truck hit 30 years old this year.
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