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I like your on-road fuel psi setup.
Now you can do some real on-road testing.
Is the 3000 rpm soft spot still there?
What is the fuel psi doing at that point?
If it drops to 1 psi that could cause your problem, by letting the bowl go dry or low.
That's what the 454 at the shop did in a prior post.
But if it stays at 4-6psi, with no sudden drops, all the way to 5000, I'd say fuel is OK and look elsewhere.
Did they dyno it with this carb, distrib, ign wires, coil, fuel pump, etc?
What parts changed between the dyno run and now?
I would concentrate on whatever parts changed since then.
You said it is weak above 2800. Can you be more descriptive? Any throttle level past 2800?
Is it weak at part throttle, easy tip-in, fast throttle movement, or just at wide open, or everywhere?
Last edited by leigh1322; Sep 24, 2024 at 06:36 PM.
I was a little bit nervous about doing too much road testing with temporary fuel lines running everywhere. The fuel pressure stayed around 4 to 4.5 psi with no drop off. The lack of acceleration is about the same with slow or quick pedal travel.
The dyno runs (3), where done with the original (yes, 1971) plug wires and the tuners coil and electric fuel pump. Besides the exhaust system (stock) being on now that is the only difference i can think of.
I have tried three sets of plug wires (all new), several sets of plugs and three different coils. I think the distributor went to Lars but I can’t find anything on that. If it didn’t go out to Lars it went out to a reputable builder.
The original exhaust was in very good condition and I doubt the exhaust is block with debris, etc.
KC
My exhaust does have the “valve” on the passengers side of the exhaust. I verified that it is opening and did a test with it wired open just to verify. I also can tell the exhaust flow is the same on both outlets.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Can you do a video of your car idling, then a slow acceleration and then a quick rev past 3000 rpm. If you post it to YouTube you can post the link...easiest way.
Then do the same while driving it
Somehow we are all missing something.
I summarized all of your comments below so we can all review:
4 speed trans.
71 LS5 pretty much stock other than a mild roller cam.
XR-282HR-10 hyd roller cam #11-438-8 230/236* 110LC
ran very well on the dyno 398hp and 470 torque. thru factory exhaust manifolds (no mufflers), original quadrajet, intake manifold 9.5 to 1 CR. Elec fuel pump on dyno
Dist overhauled, timing, dwell set.
Clean stock plugs and new plug harness.
Qjet Carb just back from Lars and installed.
ran it on the dyno with the fresh engine. It had great throttle response and the AFRs were spot on.
Stock exhaust
The engine was built about nine years ago by a very reputable builder
This car only has a few road miles since frame on restoration.
engine is flat and doesn’t pick up rpm as it should.
carb was overhauled by Lars about eight years ago
a fair amount of debris in the filter at the carb fuel inlet
new fuel tank
fairly new fuel pump
does have the original heat riser
36 degrees of timing all in by 2500rpm
dist weights, they are moving properly
the riser is opening by itself, fairly close to 90*
low fuel psi 1psi
replaced my mechanical fuel pump, still low fuel psi
pulled the supply line to the fuel pump and good flow from tank.
Comp Cams bronze tipped fuel pump rod, was bent a full 1/16”, slight worn place from not turning
new Comp Cams fuel pump rod
I unhooked the pump hose, ran the engine and it flowed a healthy amount of fuel into a container.
plug wires replaced twice.
pulled the valve covers and everything is intact and valves are moving.
I started the car at night and didn’t see any arcing.
vacuum only about 8 to 10 at idle 850 idle rpm
it is the original QJet, it operated like fuel injection on the dyno and has very good off idle response now.
I’ll try to run the vacuum test with the Mighty Vac. Yes, it’s looks the weights are moving back to the original position and I can see the timing come back to “initial” timing with the light
Timing It’s currently at 35* all in by 2800 RPM with the vacuum unplugged.
did bypass the CEC vacuum lines for now
doesn’t ever smell like fuel when sitting in the garage, only when driving
but it seems like my engine is not burning fuel on one cylinder
I did check the cylinders right after start up with a IR heat gun and they did seem pretty close to each other.
I noticed the vacuum fitting on top of the #8 cylinder and decided to plug that off and do a quick road test. This made a really big improvement on the low end (off throttle) and mid range. It does make sense that it ran so strong on the dyno though. With this supposedly plugged.
fuel pressure gauge in and did a test. The gauge is showing 3.5 psi at idle and 6 to 6.5 psi at 3000 rpm.
the fuel pressure is about 4 psi at idle and 4 to 4.5 psi while driving. There isn’t any large fluctuations under hard acceleration.
exhaust flow is the same on both exh outlets
Things mentioned but not yet checked:
low vacuum might not be pulling in your vacuum advance when you need it, or at idle
Use Mity-Vac to confirm the vac level #s that the advance starts to move, and finishes at
use a cheap infra-red temp gun on the exhaust manifold ports just outside of the head, cold and hot engine
Based on your comments I would suggest a coupe quick carb checks. I know Lars set it up, but if these got "tweaked", or the choke coil is not correct or hooked up correctly, it will cause exactly what you describe. If the secondaries will not open, a 454 will run out of air very quickly with rpms: It's pretty basic, but maybe something changed here between the dyno and in-car: If the choke is not 100% open, it may hang up the secondaries.
When engine is cold, the secondary lockout lever in black should prevent the red lever from turning. The lever in red is the secondary throttle lever. The red lever should move when the throttle is pushed all the way open. But the lever in black restricts that when cold. Open the throttle all the way, "black" should block "red" Even just sitting still you can tell if you look close. This pic is not quite clear enough to tell.
Now push the choke lever down, in yellow. Wiggle the throttle a little. Hold yellow down. The choke in blue, should open, And the lockout lever in black should now swing away and NOT RESTRICT the secondary throttle lever in red. It should rotate, red, when you now open the throttle all the way.
You can push the air valve, in green, open at any time with another finger to confirm if the throttle blades below are open or not.
Next I would check all of this when cold, start the engine, and the choke should slowly open over a couple minutes, and you should see the blockout lever, in black, move away from the sec lever, in red. That will confirm that the choke coil works correctly. Even video it as Rescue suggested.
After the car is good and hot, make sure the lever is not in blocking mode.
None of this will affect any carb setting.
Last edited by leigh1322; Sep 25, 2024 at 11:29 PM.
I can say that the stock exhaust is pretty horrible and if you made 398 uncorked, it is probably 50+ down from that.....
Corked exhaust saps power but it also make an engine not rev happy....it will not rev out as well and the higher the RPM the worse it is....
I tossed the exhaust on my 69' 427/400 and it was worth another 700 rpm.....I literally had to lower the idle as well.......Original 1969 exhuast was 2".....even for a 427.....
I think the LS5 was the same way.....
I went 2.5 Chambered and it was a different automobile....it was really that much of a difference....I would say around 60-70hp....this was the 400hp engine with a mild Comp Cam.....before the exhaust change the end carbs would barely pull open....
Jebby
Last edited by Jebbysan; Sep 26, 2024 at 05:09 PM.
Leigh1322, thanks for your detailed response. I think you know my car better than myself and think you are looking at items I haven’t thought about. I’m going to check those items out and report back.
Concerning power loss thru stock mufflers, while I’m not a performance expert I know pretty close how engines from 200 to 500 hp should perform.
KC
Concerning power loss thru stock mufflers, while I’m not a performance expert I know pretty close how engines from 200 to 500 hp should perform.
KC
I am sure you do and respectively that means nothing....any engine is going to perform pourly with a cork in the exhaust....
The LS6 454 would gain 100hp dumping the stock exhaust and adding headers.....the exhaust alone would eat 60.....this is a big reason that "LS6 on the chassis dyno makes 247hp Chevelle" made that power.....
And especially with a cam in the 230+ degree range.....as the overlap increases....it wants more and more scavenging....and having any positive pressure at all on it is bad....having half the potential it should have (like a stock muffler) is murder.
If the exhaust on that engine is not at least 2.5 with a decent muffler....it is going to kill it....
Even if you have a 2.5 exhaust....the stock mufflers are restrictive and have a 2" outlet....
If you think it doesn't make that much difference I am here to tell you it does.
Think about it....you dyno'd it and have checked everything under the moon and it won't rev.....all signs point to exhaust restriction....
Uncork it and see....cost you nothing but your time.....
Jebby
Last edited by Jebbysan; Sep 26, 2024 at 06:23 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I suggested testing it with an electric pump a month ago and he wouldnt have it...bent fuel pump pushrod. But what caused it? If the rest of it is working correctly, Its either not getting enough fuel , air or it cant breathe through the exhaust. I would disconnect the exhaust first to see if it revs. Costs nothing answers the exhaust question.
Buy a really cheap electric fuel pump that either is capped at 6 psi or a cheap regulator and test that theory to see if it fixes it. IF not them you can concentrate on the carb and the distributor but those seemed to be operating
You have checked so many things, there can't be that much left til you find the problem.
Another one:
Full wide open throttle
It can be a big difference between in the car and on the dyno
Have some one hold the throttle on the floor, while you open the upper air valve with your finger and look down at the lower secondary throttle blades. Are they all the way open? Like 90*?
Move the throttle and see if there is more movement left.
To do it by yourself use a zip tie on the cable to see how far it gets pushed. But better with two.
Throttle cable adjustments, bent pedal rod, thick carpets/insulation, floor mats, can all have a huge effect.
have seen some open barely half-way.
Problem solved!
Thanks to everyone for their time sorting thru this. Leigh1322 got me in the right direction with his post, it was the secondary throttle lever.
It now runs like a 390hp car, spinning the tires thru first gear. I think there is still a little to be gotten out of it. I plan to put it on a dyno and fine tune everything. It’s a bit frustrating to overlook simple things but I guess part of the learning process?
I still have to work thru how I’ll address the CEC solenoid. I may end up leaving it by passed, more study in that area.
KC
I love easy fixes!
And those lock-outs can be a PITA sometimes.
You just invented something....a 2 barrel 454 Corvette! LOL
Well since the secondaries are twice as big as the primaries, I can only imagine how much better it runs now!
More seriously, make sure the choke works properly from cold to warm, like 5 min later.
Just let it idle and watch / video the choke movement.
The new choke coils from the aftermarket are not always correct. One guys was even coiled backwards and non-functional. Weird stuff happens with repop parts.
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