4bangercraig

Hi everyone, ive got an interesting problem thats been plauging my friends truck for a while. Its an 85 pickup with a stock 22r but with the chevy ignition mod, besides that it has standard cookie cutter yota mods (35s duals lockers exo blah blah blah)

The problem: under high load at high rpm it will hiss like a turbocharger wastegate (sometimes) and stumble as if the engine cuts out for a second, you can go past that rpm at half throttle but at WOT this problem occurs, more prevalent on the freeway but i can make it happen in every gear. Upon this stumbling it almost seems like a backfire through the carb as the intake gets oily residue in it after a while.

So far this is what i have done to troubleshoot it:
Did a compression test found the valves in 3 and 4 were bad, put on a new head, problem became different but similar (used to only do it after it heated up and on the freeway and after it backfired it wouldnt go above 30 and you had to baby the clutch out in 1st or it would die, however sometimes it would run fine). It seems like this was part of the previous problem of the burnt valves but now its different...
Disconnected and checked (after the head swap) the EBCV the bvsv the egr and the PAI system, ac modulator, no significant change.
Checked brake booster for possible leaks
Replaced all ignition components and rewired it
new fuel pump and filter
new plugs
new wires
reset timing after head to 8 btdc, no its no the timing please dont say its the timing anything between 0 and 12 runs just fine
Took carb apart sprayed b-12 in holes, reassembled.
ran some mechanic in a can fuel system cleaner through it after i got desperate
adjusted valves again

All i can possibly think is that when the pressure becomes too great a sudden gust of air is wooshing past a diaphragm or a fitting of some kind and creating a sudden lean condition the carb cant compensate for.
Any advice?

rokblok

iTrader: (1)

Is the exhaust in good shape? Could it be to much back pressure from a clogged cat(if equipped) or a muffler came apart internally? Just a thought.

4bangercraig

Good guess i thought so too but i punched it out after this problem started... now a muffler that came apart internally? im confused as to how this would cause this but the muffler is OBNOXIOUSLY loud right now so maybe thats it, could you expound on that idea?

Vang530

did you make sure to jump the connector when setting timing? if you didn't, its way too advanced.

as for a muffler coming apart internally, i'm guessing he meant aftermarket fiberglass type mufflers..

4bangercraig

It is not the timing, the timing was set around 8ish, with the lines pinched, before we started drinking to drown our sorrows. Also without the lines pinched it would be like 5 degrees advanced and it would ping and not backfire, ive only had a vehicle backfire with retarded spark because of a faulty diaphgragm, although ive only built a few of the 20/22r series engines so they could have some strange characteristics i havent seen yet. doing the pinging test (run a hill in 2nd shift directly to 5th and apply full throttle) comes up negative with no marbles in a tin can coming from the engine.
the muffler is a large flowmaster, not a glasspack if thats what he meant. i hope its broken because i really want to replace it, it resonates at the natural frequency of the cab at like 1200 rpms and hurts my brain and chest while i test drive it. Hate loud trucks.

rokblok

iTrader: (1)

Some mufflers have steel panels(baffles) inside to direct exhaust sound waves a certain direction for sound, or to hold 'glass packing in a certain place. These baffles/panels are spot welded and can break loose and move around inside the muffler. They can rattle or become lodged in a bad place causing a lot of restriction. Glass packs are generally a straight through design. Meaning you can see all the way through them. So it'd be difficult for them to break apart and cause a restriction.

4bangercraig

interesting... so your idea is that some of the steel baffling has caused a restriction which only manifests under extreme pressure in the exhaust, the muffler is very loud but i cant remember if it was like this before he "built" it or not. Is there any way to test this other than removing the back half of the exhaust and driving it around and pissing off my neighbors? if i hit it with a rubber mallet will i hear it much the same way you can test a broken apart catalytic converter?

rokblok

iTrader: (1)

I would venture to say that if its loud, then there probably isn't much of a restriction. Since most of the combustion explosion seems to be making it out. But yes you can hit it and you may hear a rattle, but the best way is like you stated, disconnect the exhaust prior to the muffler and run it... Again, I don't know that this is your problem, since the exhaust sounds pretty straight through already, it was just a suggestion early on in the diagnosis.

Have you been able to narrow down the source of the hiss? Engine compartment, tailpipe, etc?

4bangercraig

the hiss almost sounds like its coming from the drivers side engine but its hard to tell so it could be from anywhere. upon smacking it with a rubber mallet the muffler does have a rattle to it, im gonna cut off the bend in the pipe and shove a baseball bat in there just to be extra sure, least it aint my truck.

rokblok

iTrader: (1)

If you have couplers/flanges, you could just disconnect them at different sections to possibly pinpoint a restriction. I personally wouldn't disconnect right from the exhaust manifold since its close to the exhaust valves and they could bend if cool air were sucked back into the manifold. But if there is a long tube header then not such a big deal. Or a 3-4foot downpipe would probably be acceptable. You just don't want to rev it with a short, stock cast manifold. Although I have seen some guys start and rev brand new engines with no manifold at all with no problems.... Your call.

4bangercraig

i ended up wising up and unbolted it past where the cat used to be, no change, just louder.
couldnt hear the hiss that time but everything cut out for a second at what seemed like a very high rpm although without a tach im wondering if its like 3 grand and i just think its high because its loud. problems still there.

rokblok

iTrader: (1)

Ok, do we know if this is a pressure hiss or a vacuum hiss? Could it be starving for air coming into the engine?

4bangercraig

No idea but i dont see how it could be starving for either air or fuel since air or fuel starvation is usually gradual but this is sudden and instantaneous. Thats why i thought it was some diaphgram or egr system giving away all of a sudden causing a lean or rich situation. I didnt use to think it was ignition related but now im revisiting everything, just wish i knew more tests i could run. It sounds like air is being suddenly bled from the engine like a turbocharger wastegate, after the new head all the cylinders pressure check excellent though so it must be something accessory? is there anything in the carb that opens up only under WOT high load circumstances?

rokblok

iTrader: (1)

I would venture to say secondary opens at WOT or close to it.allows the primary to do a lot of the work for better mileage. But if secondary system isn't getting fuel to mix with all that air, it will create a funky noise. Like its choking out.

rokblok

iTrader: (1)

I would venture to say secondary opens at WOT or close to it.allows the primary to do a lot of the work for better mileage. But if secondary system isn't getting fuel to mix with all that air, it will create a funky noise. Like its choking out.

Can some mod delete this duplicate post please?

4bangercraig

i took the carb apart and cleaned it just to see if there was junk, got rid of some dirt in passageways put it back on and now i swear it runs the same or worse.

at half throttle i can make the vehicle go up in rpms past this stumble but it doesnt sound happy about it, so if im to understand it right i could have a partial blockage in whatever the aisin version of the secondary barrels main jet/venturi/emulsion tube is?

also i really appreciate your help troubleshooting

rokblok

iTrader: (1)

No problem with helping, we all do what we can. Thats what makes this a great forum.

Im not very familiar with the pickup Aisins. Cruisers, yes. Trucks, no.

Can you rev it from under the hood by hand, and possibly squirt some carb cleaner into the carb throat while you're doing this? The goal, and maybe you can improvise something else, is to add some additional fuel when it starts the stumbling stuff. Just trying to see if it is strarving for fuel, which may be causing the stumble.

If its the same, then maybe we can head another direction.

79pickupian

I've got to switch over to my actual account since 4bangercraig went back to his hometown.

I beleive the carb on this 85 is very similar to a 32 36 weber with progressive barrels. It could be a lack of fuel getting to the secondary barrel?

The stumbling is instant, there is no loss of power directly leading up to it. I cannot replicate the situation unless the engine is under load, i also cant replicate it in 1st gear although im probably just too scared to rev this thing without a tach high enough to make it happen. Maybe if it is a fuel delivery issue it might be an actual problem that mechanic in a can (like seafoam) solves? that would be a first for me!

I think i may take the entire carb apart and hot tank it, Im kinda suspicious of the stock carb and the pulse air system at the moment... this is why all my trucks end up with no smog equipment and webers, cant wait until i go fuel injected though.

rokblok

iTrader: (1)

Im confused... This is your carb we are dealing with and trying to diagnose? I feel used... I dont even know you anymore... Not like I used to... Who are you...?

I am kidding bro. A good disassembly and soak would do good, and proper size jetting for the altitude is a great way to go also. Let us know what your next step is.

And just so I understand, you cant duplicate the issue unless its under load? Just sitting there revving, it acts fine at any RPM?

79pickupian

Yah just sitting there revving itll do fine at any rpm's. It'll only do it under engine load, its more prominent uphill i feel like.

As far as jetting i beleive stock should be set right since we are at sea level, unless someone poped in a smaller jet for some reason but i like to think that this truck has been relatively unmolested.

Haha no even worse, me and 4bangercraig were working on yet another persons truck! It's my brothers and i told him id get it running right before i leave for basic. Me and 4bangercraig both have webers and they both run as good as webers ever do.

im running out to run last years king of the hammers course in my truck tomorrow so i wont get any work done until sunday, i beleive im going to take apart the carb and possibly rebuild it, as well as make sure i dont have a blockage in the rail that the vacuum lines run to because i had a similar problem once on a prerunner that ended up being that, individual items tested fine but they werent being activated because of a blockage. Im also going to try retarding the timing to -5 and advancing it to around 15, just to make sure someone didnt put a 20r distributor or one off a turbo on here since they have different advances.