73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Steppin Razor on November 09, 2020, 04:55:03 pm
-
Hi all. I bought a 77 C10 w/a 350 back in February. I'm new to these trucks. It has a Holley carb on it that 1, doesn't appear to be the correct fitment, and 2, is leaking fuel. Can anyone tell me what the original equipment carb was? Research suggests it was a Rochester M4MC. Is that right?
The rebuild on the Holley is $270, so it looks like I can get the original for about the same price, and can sell the Holley.
Thanks in advance for the help, looking forward to learning.
-
I would buy an OEM! Yes, I believe it would have had a Rochester on it?
-
The odds are very good that your 77 was originally equipped with a Rochester Quadrajet M4MC. But it is also possible that it originally had a 2-barrel Rochester, the DualJet. Is your goal to restore back to original or just get the engine running well? Is the current Holley a two or four barrel?
If the previous owner did the Quadrajet to Holley conversion properly, the intake manifold is not original as well. Holley uses a "square-bore" design for the throttle plates whereas the Quadrajet uses the superior spread-bore configuration. They are not fully compatible without using kludgy adapter plates.
I'll freely admit I'm a huge fan of the Quadrajet (but many aren't) and if you are mechanically adept I'd lean heavily towards installing one. But unless you're dealing with the original unit from the engine they are NOT very friendly for random swaps. Nearly every model number had a special calibration unique to the GM engine it was to be installed on.
You mentioned shopping for an original which suggests to me you are likely to buy either a remanufactured unit or something off an auction site. The reman units have been butchered and have an awful reputation, and it's highly unlikely that whatever you get off the auction site will have the correct calibration for your engine.
Now that I've talked you out of a Quadrajet because I don't want to see any more unhappy users of them, what you need to determine is what intake manifold your dealing with, either a square or spread bore. If it's a square bore (four equal size holes) I hate to say this but you'd probably be better off either having the Holley properly rebuilt or purchasing a new in the box Edelbrock or Holley. If you have a spread bore with the Holley sitting on an adapter plate (which is likely leaking) then let us know - that will be a little trickier.
-
Thanks. I mainly want it to run well, and a more knowledgeable friend is also a fan of the Rochester (and not a fan of Holley). It is at a shop right now. The mechanic said rebuilding it would cost $270, so I thought I'd check what the original would cost, to replace instead of rebuild the current.
Luckily the P.O. kept the box and receipts so it looks like it is one of these: https://www.holley.com/products/fuel_systems/carburetors/street_warrior/parts/0-1850S Which makes sense that the mechanic said it looked more like a Ford than Chevy carb (I guess it's universal from the other listings on Holley's site). It is a 4 barrel, and there's an Oreilly's receipt for a 'carb adapter' which I'm guessing is the plate. I take it having the plate means likely the IM is not replaced?
Regardless, it sounds like replacing it with an original is not going to be viable if there's special calibration to figure out and I can't trust what is available to buy.
Thanks.
-
Well good - the intake manifold is most likely original! Do you have any photos of how the Holley looked installed? I'm mainly curious if the throttle cable and transmission kickdown (I'm assuming you have an automatic) were mangled to work with the Holley. It's a common story over the decades since these trucks were new - one of the owners down the line got frustrated with the Qjet and had a buddy who told him to buy a Holley.
I don't know where you are located, but junkyards still exist around this great nation where you could yank a rebuildable Qjet core off a similar year squarebody pickup, blazer or suburban. As long as you find a rig in the junkyard between 1975 and 1980 the Qjet will have a similar enough calibration to be of no consequence. Quadrajets are fairly simple to rebuild on your own (as is the Holley, to be honest). If you find a rebuildable core, buy Cliff Ruggle's book as well as his rebuild kits here https://cliffshighperformance.com/. His site also has an excellent forum where you could probably buy a Qjet rebuilt to Cliff's high standards by one of the forum members that is well calibrated to your engine. You could easily sell the Holley to more than fund the book and the kit.
But really, rebuilding the Holley and installing the adapter plate with the proper gaskets is the straightforward path to decent drive-ability. Just because I don't like them doesn't mean Holley's can't work well. Even the adapter plate kludge isn't the end of the world, but they do require careful installation and attention. Consider your budget, abilities, and your desire to mess with things and choose accordingly.
-
Thanks for the reply. It is an automatic. Trying to find a longbed manual transmission anything in Texas is just a test to find your breaking point. If they did exist (seems like longbeds must have been imported from somewhere), the manual transmissions have long been swapped out. sigh.
I went ahead and had the shop rebuild the Holley. I'm actually trying to do a bit less mechanical work, being more selective as to what I tackle. Although as an artist, I typically am light in the wallet so I'm self-reliant. Thanks for the link, I will do some research on it because I would prefer to go back to the original at some point in the near future.
-
Very good - rebuilding the Holley is likely the shrewd path forward. If you're so inclined, you can take time to get Cliff's book and hunt down a rebuildable core at your leisure and rebuild it without any time pressure. OR, you will be so happy with the Holley you will join the ranks of those who like to dump on the Quadrajet. So many choices, so many opportunities. Ain't America great?
By all means update this thread once you're back on the road. I and likely many others are curious to see how the story ends.
-
Will do. The saga continues at the moment. Fuel leak is fixed, but it won't stay running. Battery is good and alternator is new. I took it back to the shop to see if it's an idle issue, but since it wouldn't start even with pressing the accelerator, I suspect it's an electrical issue. Again, something I'm not familiar with. The last 20 years, I've been driving my 5pd Honda and fixing it on occasion. Bought a 92 F-150 a few years ago, and spent a lot of time hunting down all kinds of stuff on it (including an electrical issue I never found that killed it when it got shifted to Reverse).
-
How did you get it back to the shop if it won't start and stay running?
What part of texas are you in?
The first thing I would do is check the vehicle ground(s). I just went through this. On my 1984 the battery grounded to the alternator bracket, which was not a very good ground over the years due to corrosion in many spots before it got to the spark plugs. bd has an article in the tech section: http://forum.73-87chevytrucks.com/smforum/index.php?topic=35756.0
Don't let it overwhelm you too much. I would make sure you've got some decent grounds and that they are fairly corrosion free. The gauges listed in the tech article are heavier (smaller number) than I believe came stock, so keep that in mind. I would choose a different spot to ground the big wire if it goes to the alternator bracket. The engine block is a better place I would think, but it may require you to buy a new cable. You may get away with cleaning the alternator bracket and some of the other bolts that go into the head from the alternator to make sure rust and paint don't stop conductivity. Right now, you're mostly concerned with grounds for the ignition system.
You may want to run another ground wire, like a 12 to 16 gauge to the distributor hold down to provide ground to the module and coil.
After confirming good grounds, it would be a good idea to put some waterproof grease to keep corrosion at bay on the connections.
Once that is squared away, I would look at pulling the module on the distributor. If it tests bad, replace the coil. The coil a lot of times goes bad and takes out the module. You cannot test the coil reliably with a resistance check. The aftermarket "high output" ones are particularly bad. If it were me, I would build an adapter for an external coil or get a bone stock internal coil. A lot of autoparts store have the module tester, you may need to call ahead though because there are a lot of new employees who do not realise they have the tester, know how to use it or, worse.
-
I had it towed back to the shop. They replaced the ICM and bumped the idle. Still stalls out on occasion, esp in reverse. It's funny, I had the same stall in reverse problem with the old Ford I had and never figured it out there, either. That was a 92, so it had fuel injection, but I must have tested every component on that truck trying to find it.
Thanks for the recs. I did the ground stuff on the Ford, I'll check them all on this one. I'll also check on pricing for an AC Delco replacement distributor.
Where on the block did you relocate the ground?
I'm in Houston.
-
Diagnosing engine problems is difficult, and is even more difficult via the internet. But I'm going to jump in anyway and suggest the idle and stalling problems sounds to me a whole lot like a vacuum leak.
-
I misspoke, I found I believe a 3/8-16 hole on the front of the heads. I had to clean up the surface because there was a little bit of corrosion and paint. I ran a screw in with some cleaner back and forth to help clean the threads. I was told there are rolled threads so a cutting tap to chase the threads wasn't a good idea.
I would still consider replacing the coil, because the ignition module really doesn't take itself out.
At 249 and I believe Perry, the O'Reilly's has a module tester, but I had to inform the guy working there and run it myself.
At Bingle and W. Little York, there's an autozone that has a tester, but it's more of a pain.
In the heights, I hadn't found one that has the equipment.
There was an Oreilly's on hempstead and around gessner that had the equipment, but they built a new building and haven't asked if they have it.
I don't think you need a new distributor just yet because there's really not much that can go wrong with these.
The stalling may be a vacuum leak, a cheap spray bottle of water at the potential leak points would work to tell you. Or some carb cleaner or anything really. Just looking for a change in idle. The adapter can be a source especially.
I'm out of town right now, but I should be back in town after Thanksgiving. I have a vacuum gauge and a tach but we'd have to figure it out how to hook it up.
-
Thank you. That is super generous of you and much appreciated. I'll save taking you up on it until I'm at my wit's end, so I don't take up your time unless necessary.
-
(https://hosting.photobucket.com/images/i/SteppinRazor73/20201124_124052.jpg)
That's a pcv valve right? Where is it supposed to be connected? Also, I keep seeing vacuum diagrams with a line connected to the air cleaner, but I don't have anything connected there. I can't seem to find a specific diagram for a 77 C10 350 though
-
(https://imgur.com/Y0KfdjS)
That's a pcv valve right? Where is it supposed to be connected? Also, I keep seeing vacuum diagrams with a line connected to the air cleaner, but I don't have anything connected there. I can't seem to find a specific diagram for a 77 C10 350 though
Good news - virtually all of the technical documents you might need for your truck are available here. All of the Service Manuals can be downloaded from this thread: http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.0 . The Emissions Diagrams found in Section 6E of the Service Manual document how the spaghetti of vacuum lines were originally intended to be connected by the fine engineers at GM.
The entire Technical Forum is a treasure trove of factory manuals, diagrams, etc. The electrical diagram specific to your year is an invaluable resource you will likely be glad you have at some point in the future.
-
I don't see a picture of anything, so it may be a PCV valve? You might search google for part number FV184 which is the most common PCV valve I believe. The PCV valve should have a 3/8" hose going to the base of the carburetor or perhaps a spacer. Some people "Tee" into the hose for the power brake booster which is at the back of the carburetor (usually). Some people say that's not a good idea.
Some intakes have a port in the intake on I think cylinder #8. I use that one for the brake booster. The problem using that for the PCV valve is because it will cause a lean condition for that cylinder.
A specific diagram may not be truly helpful because you may get to the point that you've got so many modifications that the diagram won't be very close.
-
Fixed (I hope)
thanks @Silverminer . didn't see a vacuum diagram in the manual
-
Yes, that's a PCV valve. That requires a 3/8" hose to a place that provides full time (manifold vacuum). You'll also need a "breather" on the other valve cover that goes to the air cleaner. Is your air cleaner stock?
Edit: is that a manual choke? Do you have it hooked up?
2nd edit: you may only have one 3/8" vacuum connection on that carburetor. It's probably being used by the brake booster. You also appear to be using ported vacuum for the distributor advance, which is probably not the best way to run vacuum advance, but it's probably not a huge problem.
I don't think either thing is causing the issues you're seeing however.
-
Yes, that's a PCV valve. That requires a 3/8" hose to a place that provides full time (manifold vacuum). You'll also need a "breather" on the other valve cover that goes to the air cleaner. Is your air cleaner stock?
I don't know if it's stock, but it doesn't have a connection to a pcv valve. It has a blocked off port on the side that looks like a 5/8" hose barb (bigger than the pcv valve) and what I guess is the flame arrestor. That's it. No pcv valve runs to it. The pcv valve on the other valve cover looks like it runs to the distributor cap, but I'll trace it down tomorrow and get more pics.
Edit: is that a manual choke? Do you have it hooked up?
It is. Hooked up to what? Sorry, this is my first experience with a carburetor. Oldest car I've owned prior to this was an 84 Benz diesel station wagon. Everything else has been fuel injected.
2nd edit: you may only have one 3/8" vacuum connection on that carburetor. It's probably being used by the brake booster. You also appear to be using ported vacuum for the distributor advance, which is probably not the best way to run vacuum advance, but it's probably not a huge problem.
I don't think either thing is causing the issues you're seeing however.
Honestly, given how it's downing gas since the fuel leak repair, I feel like the first thing is to fix air/fuel, but I haven't figured out how. The PO actually gave me an instructional cd that came with the carb, but both that and the online manual for the actual model on Holley's page show a different setup than mine has.
(https://hosting.photobucket.com/images/i/SteppinRazor73/20201124_124515_HDR.jpg)
(https://hosting.photobucket.com/images/i/SteppinRazor73/20201124_124527.jpg)
-
OK, a little idea on the PCV Valve: It's more of a system of two sides.
One side: The "valve" goes to one valve cover where it sees vacuum from the carburetor/intake. That's the side that provides "vacuum" that sucks blow by gases into the intake to be burned. It also provides a vacuum to the bottom side of the piston rings.
2nd side: A breather goes into the other valve cover. The idea is to separate the two systems so it functions correctly. The breather has a tube that goes into the air cleaner that has a filter or goes through the normal filter. It does NOT go into the valve at all. It's just to provide clean air to the crankcase.
So oily air gets sucked through the valve into the intake and it's replaced by clean air through the breather on the other side.
The manual choke should be connected to the cable in the cab. If the choke is always on, it can cause a lot of gas usage. The choke is a plate on the primary side of the carburetor (the one nearest the radiator). I've only used electric chokes, except on motorcycles, so I'm not sure what the hookup looks like on a holley. I guess the question is, does it work?
The other thing to check is the fuel pressure too, some stock fuel pumps put out over 7 psi which can push past the needle and seat on a holley.
edit: what is the liquid on the intake?
-
Tex is right on.
Fresh air from a separate filter in the air cleaner gets sucked down one bank of push rods, through the oil pan, then up the other bank of push rods to be added to the air/fuel mixture being consumed by the hungry pistons.
The PVC valve, (positive crankcase ventilation) meters the flow so it's not affected too much by the rise and fall of vacuum, i.e. acceleration/deceleration.
Simple enough, right? What a beautiful design!
-
Ok, so the disconnected one is the breather and from further reading, looks like maybe there's been some EGR delete and it went to the EGR valve, which went to the intake(?). There's no cable or anything on the choke, I have no idea what the PO did with that. Maybe it had an electric one which was also deleted. So right now, to adjust the choke requires popping the hood and doing it by hand.
I also figured since I had the fuel leak fixed, the fuel level could be too high, but the Holley CD and Holley manual for the carb give instructions that don't reflect the actual carburetor. This carb has multiple applications, maybe this isn't the GM one.
-
The one in the red circle should be the PCV valve. If you pull it out and shake it, should make a noise.
The EGR valve should have a ported vacuum line, and in fact the quadrajet had a special one that took some throttle opening to start seeing some vacuum. The holley won't have that. So if you do have an EGR valve, the vacuum hose is probably not hooked up to the carburetor, making it nonfunctional unless it stuck open due to carbon.
If the choke is closed when you are driving, it will cause the engine to use a bunch of fuel. I don't think the carburetor was converted to manual from electric because usually once you go electric, you don't go back unless you need it to look somewhat period correct in the cab. It takes different parts to make it electric vs manual. Some people don't use the choke and just feather the throttle until the engine is warmed up.
A lot of the holley float levels can be adjusted externally. Yours cannot. You would need to set it the dry internal way by pulling the bowls. You'll spill the gas out of them when you do, so be aware. https://documents.holley.com/techlibrary_adjusting_your_carburetor.pdf
I would also check the fuel pressure because too much can still push past the needle and seat.
Some other reading guides: http://bob2000.com/carb.htm
If you can pull your list number off the carb, holley may have some information that is specific to your carburetor.
-
In your last pic, it shows a 3/8 hose coming from under the front of the carb.
Where does the other end go? That might be your PVC intake hose.
-
Thanks. There's no cable to the choke, and no knob or anything on the dash. It has new electrical done in the cab, which seems to have some gremlins (wipers randomly go off on start up, intermittent doesn't work, dome light doesn't always work) so I thought maybe there was an electric choke originally. In any case, looks like I'll have to figure out a cable. An aftermarket radio was installed so I can use one of the old spots for a knob.
I believe there was once an EGR valve in between the pcv valve/breather and the intake based on what I saw in the manuals that are linked. I don't see any egr equipment on it, and since TX doesn't require emissions for a vehicle this old, a delete is common.
@Johnny Popper, that is a vacuum line I believe. It goes to some little canister thing mounted on the firewall at the top passenger side corner.
I think next I'll fiddle with the choke, check vacuum, and pull spark plugs to take a look. Fix a cable, then if I still have problems, take a look at the ignition coil.
-
That is most likely the hose for the PVC valve. The Canister on the Firewall is a booster for your AC/Heater controls. It usually is supplied Vacuum from the port behind your carb, same as supplies to your Power brakes if you have them.
Get your PVC circuit squared away, it's not a bad thing.
-
My other car is a 70 Corvette with 350 and I went through a similar experience with it which turned out to be a vacuum leak due to a carb to manifold mismatch, during my search for a solution I rebuilt the Holley which it came to me with but no cure, the problem was solved when I fitted a Brawler carb which went straight on and cured the vacuum leak which I only discovered after removing the carb and seeing the mismatch on the gasket.
The Holley was dead easy to rebuild everything was in the kit including comprehensive instructionsto take you through the process step by step.
This Holley is now on my C10 after dirt in the fuel blocked up the idle system in the Edelbrock which it came to me with and it's finally running good, I had to make up a throttle linkage to get the Holley to run and mount a kick down switch after swapping out the TH350 for a TH400, the TH350 would only drive in reverse but that's another story!
Hope you get yours sorted.
Graham
-
Harbor Freight sells a vacuum tester that you'll need to adapt some to use it for reading fuel pressure. It reads it but you'll need to find a good way to get it in the line. The tester is setup for reading the fuel pressure level that the stock mechanical puts out.
https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html
I'd google for a 20% off coupon. Hopefully there's a HF near you.
I'd check the fuel pressure, vacuum with it. The choke, as long as it's open when the engine is warmed up shouldn't cause an issue. For driving it some, yes, you would want to find an appropriate solution.
It could also be as Chevygold states a mismatch between the carburetor and adapter or adapter to manifold. There is a metal plate that can be purchased if there is a mismatch. Older holleys like the 4010 had that problem.
-
Ok, so I bought the vacuum/fuel pressure tester, about to go test. Yesterday and today, it stalled out at idle so I had to keep my foot on the accelerator and left foot brake to keep it running at red lights. And it stalled twice while turning at speed. It also happens when warmed up, less so when cold. Parked on a hill, it idles fine. I'm also sending half my fuel unburned through the tailpipe. Also, there is another fuel drip. This one is coming from the spot in this pic between the o and b in photobucket
(https://hosting.photobucket.com/images/i/SteppinRazor73/20201124_124052.jpg)
Edit: Ran vacuum test, it's at 18in/Hg at first, but over time it dropped to 15
-
That leak may be the rear float bowl leaking. May want to see if you can snug up those bolts a little but not too tight. It doesn't take much to strip the threads in the body.
It may also be leaking a little because the fuel pump is pushing fuel past the needle. Did you get a chance to figure a way to hook up the pressure gage on the 3/8" line?
Extra fuel pressure may be causing the vacuum drop at idle.
-
https://nationalcarburetors.com/
-
That leak may be the rear float bowl leaking. May want to see if you can snug up those bolts a little but not too tight. It doesn't take much to strip the threads in the body.
It may also be leaking a little because the fuel pump is pushing fuel past the needle. Did you get a chance to figure a way to hook up the pressure gage on the 3/8" line?
Extra fuel pressure may be causing the vacuum drop at idle.
Not yet. Got my vacuum reading. It was around 18"/hg for a few minutes, then dropped to 15, wavering between 15 and 16.
It looks like the booster for the AC/heater is patched with duct tape. I did lose heat/ac a little while ago, I figured it was the blower motor.
What do I do if the fuel pressure is too high? Tester can hook in at the fuel filter? I'll replace that (which is pretty full) after.
Thanks and merry Xmas/Happy Holidays
-
The AC/heater booster? Is that the black ball thing? Or are you talking about the vacuum line that has duct tape? You could cap the port or plug the line.
Are you reading vacuum when it's warmed up?
If the fuel pressure is too high, you can either find a fuel pump that puts out less pressure or easier to do is to put a regulator to lower the pressure to the carburetor. I usually use a tee that connects into the fuel line to check fuel pressure. Just be careful. You don't need to read it for long, and it will jump a little if you tee in because the needle will be opening at idle, but you should get a good number.