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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Chevygold on July 02, 2022, 02:09:01 PM
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When I got my truck the fuel system was quite dirty, guess it had been sitting a while, the carb which was on it was an
Edelbrock, not sure what model, but it ended up choked with crud from the tanks. Anyway after giving the tanks a good clean out I needed another carb, I had one I'd rebuilt for my Corvette but due to a mismatch with the manifold it wasn't suitable so I fitted a Brawler 670 and now it's happy.
The carb I had from the Corvette was a Holley List 6619-1 which is a 600cfm with electric choke, this has worked OK on my truck but can't help wondering if it might be a bit small?
The truck came to me with a 383 stroker but I have no other info about the motor other than a sticker from Crane Cams saying the cam is CARB compliant with an EO No. D225-21 but don't know if this identifies the cam or not?
Truck is a 76 C10 longbed with a TH400 tranny, it seems to run OK and seems OK on fuel although I've never tried to get a consumption figure.
Googling the cam sticker seems it covers various cams and valve gear so no help there!
Any ideas?
Thanks
Graham
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600cfm would be a bit small for a 383....just about right for a mild 350.
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That's what I thought although I have to say the truck runs well, wish I knew what was in the motor but I'm not about to pull it down just to find out! Might have to get another Brawler, have been very pleased with the one I put on my Corvette, setting up was very straightforward but will have to wait for the exchange rate to improve as it's not good for me just now and with petrol around 2 pounds a litre I'm restricting my driving and spending my time working on making improvements.
Thanks
Graham
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So Shifty, would a 750 cfm be too big for a mild/crate 350? Looking to improve gas mileage for obvious reasons!
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Still a little big. You could pull it off with a vacuum secondary Holley with a heavy spring on the vacuum chamber, but I'd stick around 650, possibly even a 600 with a little tuning.
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The Brawler I have on my 'vette runs sweet, it's a 670 vacuum secondaries and electric choke, seperate idle adjustment in each choke, again I don't know the build of the 350 motor but suspect it has been breathed on a bit before it came to me, car was originally a 350/350 and still has the original heads on a truck block to get the 4 bolt mains, performer inlet, beyond that I'm only guessing.
Graham
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The Brawler would indeed be a decent choice, but because of the weight of the truck, the secondaries would want to open very early, hence the heavier spring mod, delay it a bit.
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The Brawler would indeed be a decent choice, but because of the weight of the truck, the secondaries would want to open very early, hence the heavier spring mod, delay it a bit.
Thanks Shifty if things improve on the currency front I'll get one on order.
Graham
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Latest update, I managed to find a Street Avenger 670cfm over here at a good price, did a rebuild with all new gaskets etc. and fitted it on the truck yesterday, all standard jets, idle screws 1 1/2 turns out, floats set to middle of the float bowl and she started up first turn of the key, good idle on the auto choke and after warm up.
Didn't have time to put the vacuum gauge on it so that's the next step, a quick snap of throttle results in clean accelleration in neutral but if I open the throttle slowly I get a stumble at about 1/2 throttle which feels like weakness, will need to get my gas tester up the exhaust to be sure.
Will let you know how I get on tomorrow, float bowls have the windows to see the fuel level but finding it very difficult to see the level, first attempt at setting the secondary level resulted in fuel pouring into the chokes as too high!
Any suggestions welcome.
Thanks
Graham
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Your 383 in high performance trim has a volumetric efficiency of around 85%, so a 600 CFM carburetor properly tuned and matched to the other engine components is going to be good to about 6,000 RPM. A heavy truck with an automatic transmission, depending on your stall speed is going to like a smaller carb and will accelerate quicker, even though your top end power may drop off a bit. I would say your choice of a 670 CFM Carb isn't bad though, but it might not be as snappy as with the 600.
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OK had a bit of time today to spend on my truck while the wife was looking the other way! Put the vacuum gauge on it and got the idle set 20" vacuum at 900 rpm, smooth acceleration up to about 15" vacuum then a slight stumble which quickly cleared, could be the power valve opening? will get it out on the road and see what it feels like. I have a selection of springs to try in the secondary diaphragm and see how I get on it has the plain spring in there just now but I have the brown and black to try to hold the secondaries back.
Thanks
Graham
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OK finally got time to give it a road test and it coughed and spluttered badly when trying to get moving so guessed it was going weak.
Next step I moved the secondary jets to the primaries and put a pair of 83's in the secondaries and now its worse than before!
Spent today fitting an AFR gauge and now I can see what's going on.
Starts easy, one pump on the gas to set the choke twist the key and immediate start followed by a smooth idle, off choke get an AFR of about 12.6 so my initial tuning was pretty good!
As soon as I try to give it any gas it goes really weak and won't rev at all, it's almost as if it's worse with the bigger jets in the primaries, how can this be?
Thanks
Graham
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How is your fuel pressure?
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Also fuel filter?
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Don't have a gauge on the fuel inlet but it was fine on the previous carb, filter on the inlet clean also filters on each carb inlet clean.
I'll try raising the fuel level and see what that does, windows not very clear so difficult to see the level, any suggestions for cleaning them?
Thanks
Graham
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You could try some 2500 grit paper, then polish, but those things never stay clear for long....I'd just do it olde skool, and take the plug loose, and raise your float until it just barely spills out, then back it down a turn.
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Had another go today and the issue was float level too low so now runs a lot better but still weakening off with load so going to 75 in the primaries and the 82 I fitted in the secondaries and see where we go from there.
Now I know the sight plugs screw out I can take them out to set the float level then screw them back in, anyone know if replacements are available as mine are not in great condition?
Thanks
Graham
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Summit, Jegs, or just about any speed shop should have them...
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Found them here in UK so should have them tomorow along with the 75's
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Got the brass plugs as the plastic ones should only be used for setting then replaced with the brass ones, much safer I imagine.
Graham
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Sorry Graham, I didn't see you were in the UK...I'm glad you got them under control anyway.
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So I now have the 75's in the primaries and 82 in the secondaries, it now seems a little rich but runs better than before so maybe I need to go somewhere between the 68 that I did have and the 75 I have now perhaps 72?
Wish I could afford the Holley selection box :( it would make life a lot simpler!
Any way of knowing when the secondaries kick in as they might be a bit rich too?
Nowhere to go for a good blast as the roads are crammed with holiday traffic just now >:(
Graham
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Yeah with the cost of petrol these days I would lean them out.
Are the secondaries the same physical size as the primaries?
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Square bore 1.536" primary and secondary
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Just ordered 72's for the primaries so will see how it goes when I get them Tuesday.
Graham
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Ahh yes, square.
The jet sizes for the secondaries seem huge, but I guess that's what you want when you crack those babies.
I just got a Qjet from National Carb's out of Florida, can't wait to hear that familiar drop in vacuum, and then "BuuuWaaaaaaahhh"!
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Got the 72's in the primary now and now running much better, should I be reducing the secondary jetting now as well?
Just looked at the standard jetting for the SA 670 and it's primary 65 and secondary 68 a difference of 3 so should I maybe come down to 75 in the secondaries as it seems to be going rich as the revs get up?
Thanks all
Graham
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I would think with that spread you can use the 75 primary jets for the secondaries. Find a long empty straight away for testing. Video if possible. 8)
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I'll swap the 75's into the secondaries and see how I get on.
Graham
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Be a great place to have a stoich meter....
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Where do you find such a meter Shifty?
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It's the $5 word for air/fuel ratio gauge.... ;)
https://www.autometer.com/2-air-fuel-ratio-elec-carbon-fiber.html
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THAT is very cool! Thanks!
At first glance, it seems like it could be a stand alone device. Or perhaps integrated into a electronic control system.
I need to do a deeper dive, but would love to monitor my AFR.
My truck has no silicon chips.
Just spoke to pre-sales, with their sensor and gauge it is stand alone, just need to weld a receiving nut to my exhaust pipe pre-cat. Great site Shifty one!!!
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Cheers Johnny!
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Installed AEM AFR meter earlier, O2 sensor in LH header outlet so am able to monitor my AFR as I drive.
Now running 72 in the primaries and 75 in the secondaries seems a lot happier, haven't had a chance for a road test as spent a good deal of time helping my neighbour get his digger going again after running the fuel tank dry, absolute nightmare after the fuel line from the tank blocked up! lots of backflushing to the tank and finally got it going then had to take the boss shopping!
Graham
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How close are you able to get to that perfect 14.7:1 Graham?
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Getting closer just a bit more juggling between the primaries and secondaries, just when it looks close the secondaries chime in and it all goes to sh1t but we'll get there, I won't be beat!
Graham
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Just a thought, but why go bigger on the secondaries if they're the same bore?
Seems you want the same AFR from both circuits, no?
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Hi Johnny, the standard set up is 3 bigger in jet size in the secondaries than the primaries so guess it's more about flow than just the size of the chokes?
Another thing that always puzzles me is generally your vacuum decreases the more you stomp on the load pedal (I'm old enough to remember vacuum wipers! the faster you went the slower the wipers went so you had to slow down to see where you're going) so how does the vacuum open the secondaries when you go faster if it's dropping when you floor it?
Been taking advantage of the hot weather we're having now to get on with a bit of paint, got the side of the bed stripped and primed ready for colour, man that Chevy paint is tough stripper didn't touch it ended up with a flapper disc in the angle grinder!
Graham
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LOL my '57 Apache Chevy pickup had the same wipers!
I guess they want a real dump of fuel when the secondaries open rather than double the volume in total.
I think the vacuum keeps the secondaries closed until it drops...spring loaded I suppose.
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I guess they want a real dump of fuel when the secondaries open rather than double the volume in total.
I think the vacuum keeps the secondaries closed until it drops...spring loaded I suppose.
Think about it for a moment... Is there a choke plate above the secondary throttle plates? Generally, when would you use the secondaries? When would you want them to open?
If engine vacuum keeps the secondaries closed and spring pressure opens them, wouldn't the secondaries be fully open when the engine is shut off? Are they open with the engine shut off?
Ponder it. What do you suppose? :)
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I have no idea how square bore carbs work, never owned one so I can only guess. :D
His comment about the secondaries opening when the vac drops lead to my speculation.
What's the dope bd?
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JP, speaking generalizations while kicking back around a cracker barrel, look at it from a "lay" perspective. A carburetor is a carburetor. They all operate using the same principles of pressure differentials. With the exception of variable venturi carburetors such as the Quadrajet or AVS or VV, secondary bores typically are unobstructed by choke plates or air valves. So even if the primaries and secondaries are square, meaning the same size (not to be confused with square bore, which means the bore spacing is equilateral), the secondaries will flow more air than the primaries. Besides, secondaries typically open only when there is a high load demand such as rapid acceleration or when the vehicle is pulling heavy loads. Hence, secondary fuel metering is enrichened to provide maximum power production from the engine. So your first guess was correct.
As to the vacuum secondary operation, the secondary throttle plates are held shut by spring pressure pushing through the secondary vacuum diaphragm and its link. The secondary diaphragm is ported to a variable vacuum signal that is initially dependent on the airflow past the primary throttle plates. The greater the airflow through the primary bores, the stronger the vacuum signal. Albeit, some carburetor designs augment the primary bore vacuum signal by blending it with a secondary bore vacuum signal for smoother secondary throttle opening and transition. The sensitivity of the secondary throttle opening is effectively controlled by the pressure balance established between the diaphragm spring and the ported vacuum signal. Tuning the secondary throttle opening is accomplished by installing the proper spring for the desired result, opening sooner or later. A well-tuned vacuum secondary carburetor will provide a seamless transition from engine idle to full power production.
Not a perfect explanation, but you get the idea.
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Wow, I am a smarter man today! Sincere thanks bd!
Sounds like Graham needs to look at the spring tension of his carb secondaries to effect the proper balance.
I have only ever worked with Qjets for the last 40 years. You list them with variable venturis and I am not making the connection.
They have a triple venturi that I know of.
The secondary (not chokes) upper plates, to my understanding, respond to the flow the secondary plates create, lifting the metering rods to allow more fuel.
Is that what you are referring to?
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QJ secondaries are mechanically tied to the primary throttle opening through a spring-loaded lever mechanism. The spring loading allows the secondaries to be locked out by the choke until the engine has warmed sufficiently to safely generate high power without imparting engine damage or causing undesirable lean backfires and acceleration bogs.
As you stated, the secondary air valve opening depends on downward airflow through the secondary bores. And, yes, as the air valve opens it lifts the secondary metering rods to increase fuel delivery through the secondary nozzles. The datum you are missing is that the air valve's position or angle of opening effectively modulates the cross-section of the secondary bores, and with that, the venturi effect at the secondary nozzle openings. Consequently, the purpose and function of the secondary metering rods are not directly comparable to the purpose and function of the primary metering rods. Rather, the secondary rods compensate for dynamic changes in the effective cross-section and airflow potential through the secondary bores due to the air valve angle, whereas the primary power piston and metering rods enrichen the fuel mixture delivered through the fixed cross-section of the primary bores based on increased engine load. Do you see that? The secondary metering rods compensate for airflow; the primary metering rods compensate for engine load (power demand). Essentially, the secondary metering rods, following an initial dip, will maintain a flatter AFR response to changes in secondary airflow, while the primary metering rods will decrease the AFR (increase fuel delivery) in response to increased power demand.
Another way to express the QJ concept is that the primaries comprise a fixed-bore two-barrel carburetor and the secondaries comprise a variable-bore two-barrel carburetor. It's the variable venturi attributes of the QJ that made it so adaptable to the entire GM lineup. Truly, it was an engineering marvel.
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So the air valve acts like a VV! I have always loved these carbs (and hated a few) and now with greater understanding my love deepens. Agreed- awesome engineering!
bd, you are always a wealth of information! I would love to hear the backstory of your relationship to GM. Thank you sir!
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Thanks BD for the great explanation regards the opening of the secondaries, sort of suspected it was something like that but always good to know!
I'm trying 68's in the primary now as it was still a bit rich, hope it doesn't go lean as I don't have any 70's!
Graham