Author Topic: 77 C10 350 OEM carburetor question  (Read 557 times)

Offline Steppin Razor

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77 C10 350 OEM carburetor question
« 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.

Offline Mike81K10

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Re: 77 C10 350 OEM carburetor question
« Reply #1 on: November 09, 2020, 08:57:57 PM »
I would buy an OEM! Yes, I believe it would have had a Rochester on it?

Offline SilverMiner

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Re: 77 C10 350 OEM carburetor question
« Reply #2 on: November 09, 2020, 09:27:14 PM »
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.

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #3 on: November 10, 2020, 09:45:35 AM »
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.

Offline SilverMiner

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Re: 77 C10 350 OEM carburetor question
« Reply #4 on: November 10, 2020, 05:35:38 PM »
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.

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #5 on: November 11, 2020, 07:32:57 PM »
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.

Offline SilverMiner

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Re: 77 C10 350 OEM carburetor question
« Reply #6 on: November 12, 2020, 07:14:10 PM »
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.

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #7 on: November 16, 2020, 07:03:24 PM »
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).

Offline TexasRed

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Re: 77 C10 350 OEM carburetor question
« Reply #8 on: November 17, 2020, 04:48:32 PM »
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.

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #9 on: November 19, 2020, 05:41:44 PM »
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.
« Last Edit: November 20, 2020, 12:08:18 PM by Steppin Razor »

Offline SilverMiner

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Re: 77 C10 350 OEM carburetor question
« Reply #10 on: November 20, 2020, 04:09:10 PM »
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.

Offline TexasRed

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Re: 77 C10 350 OEM carburetor question
« Reply #11 on: November 21, 2020, 10:34:40 AM »
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.

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #12 on: November 23, 2020, 05:13:58 PM »
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. 

Offline Steppin Razor

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Re: 77 C10 350 OEM carburetor question
« Reply #13 on: November 24, 2020, 01:25:38 PM »


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

Offline SilverMiner

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Re: 77 C10 350 OEM carburetor question
« Reply #14 on: November 24, 2020, 06:24:36 PM »


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.