1995 5.7 bogs/pops at 4200 rpm. Thunderbolt IV ignition, non-vortec.

[kbait]

Newer rebuilt long block installed 2017 and has run normally until this spring. Runs flawlessly until 4200 rpm, then seems to cut out partially and a few pops. Advancing throttle to wide open makes symptoms decrease, but still not right and rpm gets to about 4500.
My mechanic buddy assumed a lean issue..so we kitted the 2 barrel mercarb. Test = same issue. It doesn’t seem to be emptying the bowl, so we ruled out fuel, and assume an ignition issue. We checked timing, swapped out cap/rotor and spark plugs mr43t gapped to .035”. Tested same. Replaced coil w/proper Sierra, no change. Disconnected shift limiter switch..no change. Any ideas? Maybe someone has chased this gremlin before..
Pickup in distributor looks new, and not sure if replacing that would solve issue. Perhaps the ignition module on side of distributor is faulty, and advancing spark too far at that rpm? Any help/clues appreciated? Thanks.

 

[Bondo]

then seems to cut out partially and a few pops. My mechanic buddy assumed a lean issue..so we kitted the 2 barrel mercarb.

Any ideas?

Ayuh,..... Go back to fuel delivery, it's goin' lean,......

 

[alldodge]

Test fuel pressure, should be between 3 to 7 psi from idle to WOT. If your loosing pressure, your leaning out

 

[kbait]

Will test fuel pressure and report back.. Thanks!

 

[QBhoy]

Sounds like also could be water contamination in fuel at some point. Most times...even after you get rid of it by the usual means....the carb still needs to come off anyway. Doesn’t often get better just by getting rid of water. All the best.

 

[kbait]

Since fuel was from last spring (stabil), I bypassed fuel tank and hooked up a portable w/non-oxy fuel. Tested the same, then suddenly worse with a obvious miss at all rpms. Ran dry comp test, and all good except cyl 8.. zero comp.
Hoping broken valve spring caused the initial issue, and shifted to leave valve open when noticeable miss/no comp occurred. Pulling valve cover today, and will report back..Thanks!

 

[kbait]

Found a chunk out of exhaust valve. No piston/cylinder damage at all..WHEW. Think it kept losing pieces until total compression loss. Hopefully a straightforward repair.. hope to post back w/good news soon!

 

[Bondo]

Found a chunk out of exhaust valve. No piston/cylinder damage at all..WHEW. Think it kept losing pieces until total compression loss. Hopefully a straightforward repair.. hope to post back w/good news soon!

Ayuh,..... Detonation damage from the lean condition, is my guess,......

 

[kbait]

Probably due to old gas. I have totally drained fuel tank and will use non ethanol fuel when fixed. I’’ll check total timing advance too. Thx

 

[Scott06]

In addition to checking initial and total advance read your plugs when you get it up and running. This will be the proof that carb and delivery system is working ok

 

[kbait]

Replaced valve and motor runs great.. until 4000 rpm’s, then same bog/pop. Little history.. this is a reman 5.7 engine that was swapped when original 5.0 cracked block due to not winterizing by previous owner. Ignition was swapped from 5.0 to 5.7. I believe everything’s the same, except ignition module. 5.7 takes v8-22a, 5.0 takes v8-22. So, module from 5.0 should work fine as both advance 22degrees past base setting of 8 (30 degree total). This module achieves max advance at 3500 rpm and the v8-22a achieves total advance at 4400 rpm. My mechanic and I both believe that this module is faulty. Seems to advance timing normally until max at 3500 rpm, then at 4000 rpms, timing jumps more advanced, firing the previous cylinder post in distributor cap = bog/pop and likely the wrecked valve. I found a good used v8-22 on eBay at a good price and it’s on the way.
We always believed a faulty module would add no advance with rpm increase. I searched, but couldn’t find a case of one failing by advancing too far. Any thoughts?
I haven’t checked total advance yet, as I was alone on test run after replaced valve. If the new module doesn’t fix, I’ll check for any advance past 30 degrees total above 3500 rpm. (Again, all ignition components besides ignition module and sensor under cap have been replaced with same result, firing order cap to plugs is correct, engine runs absolutely perfect until 4000rpm).

 

[kbait]

Replaced ignition module..same issue.
I read that a weak/deformed valve spring can cause a valve to float at higher rpm’s, causing loss of compression/misfire. Replacing all valve springs tomorrow. Seems plausible..hope it works!

 

[alldodge]

Post 2 and 3 say your running lean
Post 4 says you will test pressure (have not seen any pressure readings)
Post 8 says probably detonation (I agree)

Putting a kit in the carb does not fix a lean condition if the carb will not supply enough fuel to begin with. The motor has been rebuilt and probably bored, so it needs more fuel. Don't know what carb model or other info about the motor rebuild, but I'm still thinking lean

Your not turning enough rpm's to have spring problems yet, but replace the springs if you want, its your money, we can talk lean again later