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CarburetorsCarb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
I have a 1988 IROC that I swapped a 1972 k20 350 into. The engine is stock except an aluminum intake and a 650 cfm edelbrock avs2 carb.
When the engine is warm and I hit the throttle fast, the engine begins to bog down and not gain ANY rpm. If I quickly release the throttle it stalls out and smells of gas, and if I hold the throttle halfway it stays bogged until it clears up and then drives normally. It almost seems like an fuel problem, either too much or too little but I'm not sure.
Any suggestions help, I am currently a 20 year old 2nd year apprentice still learning carbs.
If you bought this carburetor new, it should have come with a pretty comprehensive manual to read. I would suggest reading it if you haven't already and setting your mixture screws as it suggests. Has your ignition timing been set with a light?
I have a lot of experience with the Performer series afb, but not the avs 2. Typically these are pretty rich out of the box. The secondaries air flaps are adjustable on your carb, perhaps they are opening too quickly. I second the notion to buy a wideband O2, I like AEM for these. It is also not a bad idea to take the air horn off and check the float level, my 1406 wasn't quite right out of the box.
Update:
Since the original post I have tried all 3 different settings on the accelerator pump with no succession, adjusted the IAM screws, dialed up the fuel pressure, and took off the air horn and checked the float levels (which were already higher than factory 7/16ths and 1" drop). It currently has stock 1906 carb specs. .101 primary, .070x.037 metering rod with orange 5" spring.
I am STILL having problems with bogging at WOT. I had one suggestion of too small of fuel lines but I'd like to try something else before replacing all the fuel lines!
Please help!
14in/hg on idle and 11 vacuum at WOT if it helps any.
My first guess would be accelerator pump, but since you tried different settings to no avail, what's your fuel pressure and how many degrees of advance are you running on your distributor?
What's your fuel pressure and how many degrees of advance are you running on your distributor?
I am running 5.5psi fuel pressure, with a 33psi pump in the tank (so I wouldn't think it's dropping at WOT but I can't monitor it driving)
As for timing it's right around 12° inital and it's ran off the secondary vacuum on the passenger front of the carb.
11in/hg at WOT? That is really high. It tells me you have a restriction somewhere. I would guess the secondaries are not opening all the way or throttle is adjusted wrong so you don't open the primaries fully.
Originally Posted by Tristen_darrell
Update:
Since the original post I have tried all 3 different settings on the accelerator pump with no succession, adjusted the IAM screws, dialed up the fuel pressure, and took off the air horn and checked the float levels (which were already higher than factory 7/16ths and 1" drop). It currently has stock 1906 carb specs. .101 primary, .070x.037 metering rod with orange 5" spring.
I am STILL having problems with bogging at WOT. I had one suggestion of too small of fuel lines but I'd like to try something else before replacing all the fuel lines!
Please help!
14in/hg on idle and 11 vacuum at WOT if it helps any.
11in/hg at WOT? That is really high. It tells me you have a restriction somewhere. I would guess the secondaries are not opening all the way or throttle is adjusted wrong so you don't open the primaries fully.
What intake manifold is that?
If it has a spreadbore flange, then this is the problem. There is no way your secondaries would be able to open.
You would need a square-bore intake, or a carb base adapter. https://static.summitracing.com/glob...e/edl-1100.pdf
Last edited by NoEmissions84TA; 03-14-2021 at 08:11 PM.
What intake manifold is that?
If it has a spreadbore flange, then this is the problem. There is no way your secondaries would be able to open.
You would need a square-bore intake, or a carb base adapter.
aluminum GM intake with a 1" spread bore to square bore adaptor. Otherwise it wouldn't even bolt up.
Okay, let's try something else?
With that adapter in there, hood clearance becomes an issue.
What are you using for an air cleaner? Dropped base with a 2" element?
My guess is that the lid is so close to the top of the carb, that the air can hardly flow into the carb.
Okay, let's try something else?
With that adapter in there, hood clearance becomes an issue.
What are you using for an air cleaner? Dropped base with a 2" element?
My guess is that the lid is so close to the top of the carb, that the air can hardly flow into the carb.
interesting concept. It's a 1 3/4" drop base edelbrock with a top flow filter cover so it shouldn't be choked as it's a larger cleaner than stock. As for hood clearance there is a hole in my hood.
I believe I'm running out of fuel or getting extremely lean, not rich.
There is a lot going on in that random mismatch of cobbled engine swap / parts but the >10X more vacuum that you are pulling at WOT should be for first thing to go after. So a couple questions
1. Did you adjust the choke per the manual?
2. Have you confirmed choke operation as in the engine is not being choked when you are driving?
3. Did you properly adjust the secondaries?
4. Did you confirm they open as expected?
5. Did you check throttle cable for full travel?
6.Did you confirm that when someone pressed the gas pedal everything opens as expected
7. What adapter did you use? Open? 4 hole?
8. Did you confirm the adapter is in no way physically blocking your throttle plates?
There is a lot going on in that random mismatch of cobbled engine swap / parts but the >10X more vacuum that you are pulling at WOT should be for first thing to go after. So a couple questions
1. Did you adjust the choke per the manual?
2. Have you confirmed choke operation as in the engine is not being choked when you are driving?
3. Did you properly adjust the secondaries?
4. Did you confirm they open as expected?
5. Did you check throttle cable for full travel?
6.Did you confirm that when someone pressed the gas pedal everything opens as expected
7. What adapter did you use? Open? 4 hole?
8. Did you confirm the adapter is in no way physically blocking your throttle plates?
all of the above I can answer yes, with a 4 hole adaptor. The only thing not working properly is the choke and I must turn it off by hitting the throttle.
So here is something to think about orange springs are 5Hg springs so it only takes 5Hg to compress them and hold the needles in the jets keeping you in cruise mode not power mode. Generally you want the springs to be 1/2 of your idle value but in your case if you are running 11hg at WOT where vacuum should be the lowest you are never getting the rods to raise and are not getting into the power section of fueling.
So here is something to think about orange springs are 5Hg springs so it only takes 5Hg to compress them and hold the needles in the jets keeping you in cruise mode not power mode. Generally you want the springs to be 1/2 of your idle value but in your case if you are running 11hg at WOT where vacuum should be the lowest you are never getting the rods to raise and are not getting into the power section of fueling.
so other than having a possible restriction in the airflow, you believe I may also need stronger springs for the metering rods?
I think when you get to tuning the carb after figuring out the current issue you will want to put in stronger springs for a faster transition to power mode probably 7 or 8Hg
Something else I am noticing is you are running the vacuum advance. If you are getting 11Hg at WOT then you are also getting way more timing than you should
What is timing without vacuum advance at idle and 3K rpm?
Have you tried running without the vac advance to see if anything changes?
Here is what I would do
1. Force the choke to be open at all times and take a drive
If nothing changes
2. Measure timing without vac advance at idle and 3K
3. Try a drive without vac advance.
I would also consider going to manifold vac instead of ported but that is up to you.
Something is wrong. 11inHg at WOT is unacceptable. Where are you taking this reading from? Are you sure your vacuum gauge is trustworthy? This should be plenty of carb for your engine, and if you have verified the primary and secondary throttle blades are opening fully with the throttle cable attached and that the air filter isn't plugged, the choke is off, to me it sounds like your air valve secondary flap up top isn't opening.
I don't think making metering rod, jet, and spring changes are a wise choice at this time given the current WOT vacuum situation, and lack of a wideband o2.
Find you a 79-84 truck Q-Jet without the electrical plugs on top. When you look down the primary venturi where the booster attaches to the venturi it will have a small lump on the casting. Those are the same as the edelbrock performer rpm q-jets and flow up to 850 cfm when dialed in correctly. I just found 3 of them in my shop. I read Cliff Ruggles book and ordered a kit from him with my carbs stamping number. Stripped it all the way down to a bare casting. Will be zinc plating and iridite dying it after drilling the passageways for a starting point, all detailed in Cliffs book. It will look new and run better than new. Basically recalibrating the idle fuel and idle air bypass ports as well as the primary accelerator pump and secondary pump shot. Cliff sent me custom tappered primary metering rods, 75 jets and DA secondary rods. I have run q-jets, edelbrocks and holleys. The Q-Jets are by far the best street carb you can run.
Another update.
I have since opened the choke fully, set timing while the car was loaded up in gear with an old mechanic friend, replaced all the old vacuum lines, left the air cleaner off, have 0 in/hg at WOT. The car STILL falls flat on its face around 3-4k rpm. It sounds like it started to ping when it bogged down and backfired a couple times out of the carb (before setting timing). It still pings but no longer backfires when it dies.
I was planing today to check my spark plug gaps and make sure they are at 0.045" still as per HEI spec.
Sounds like your secondaries are not opening correctly and we need to revisit the list. 1 and 2 seem to have helped you quite a bit and fixed the issue with air flow so we can cross them off lets add some new questions and revisit the old ones
9. What is your timing at idle with no vac advance?
10. What is your RPM when you hit max timing advance?
11. What is your max timing
1. Did you adjust the choke per the manual? 2. Have you confirmed choke operation as in the engine is not being choked when you are driving? 3. Did you properly adjust the secondaries? 4. Did you confirm they open as expected? 5. Did you check throttle cable for full travel? 6.Did you confirm that when someone pressed the gas pedal everything opens as expected 7. What adapter did you use? Open? 4 hole? 8. Did you confirm the adapter is in no way physically blocking your throttle plates?
Its a long shot, but I once had something similar around 4k rpm and it was due to a catalytic converter that had broken apart and was blocking the exhaust. Can you verify there are no obstructions to your exhaust?
03-04-2021, 03:50 PM
