307 Breaking down WOT ONLY (E4MC Experts Needed)

[Scott183]

Hey guys, my '86 stocker 307 is driving me nuts. My funding has left me short on replacing/ rebuilding the car this year w/ some serious horsepower so I'm stuck with my stock 307 for the rest of the year. Car runs decent 3/4 throttle until you kick in the secondaries. As soon as the four barrels open up it starts breaking down and falls FLAT on it's face. As soon as I back down to 3/4 throttle it's fine again. I've watched spark on an individual cylinder when it happens and the spark seems fine. We teed into a vaccum line (small one) and fed fuel directly into the intake when it happens, no change. I do have an intermittent tps code, that I believe is being set because the throttle's wide open and the rpms are down around a thousand when it's breaking up. I've voltage swept the tps, and it's 1to 4.25 volts (open to close). Can't find the spec but know the code should set when the tps is above 2.5 and the RPM's are under a PRESET NUMBER (thanks for telling me what that # is GM) What the hell am I missing? My time is limited on figuring out this prob. I'm lucky enough to have a busy shop that doesn't allow me the time to work on my own car, but this is getting painful now. I'm an ASE Master Tech who's being embarrased by a 25yr old carbureted !@#$. Thanks in advance for anyones time who reads/ responds to this. Any/ all ideas are appreciated.

 

[trog]

Do you have acess to a 4 gas or a wideband o2 setup? I think that you will find its going dead lean when the secondaries open.

 

[Scott183]

Yeah I was pretty convinced of a lean issue myself, however I would have expected some kind of response from the engine when I fed fuel directly into the intake. It feels like the fuel is being cut off when the secondaries open up and then it starts backfiring, straightens out as soon as I go back to 3/4 throttle.. The biggest problem is having the time to work on it. I was hoping someone had seen this before.

 

[88hurstolds]

I've had this problem before but it was a lack of fuel, make sure it's not a float problem.
Another issue could be the ignition module inside the distributor, lack of proper advance during WOT.

 

[86Olds442]

Yeah I was pretty convinced of a lean issue myself, however I would have expected some kind of response from the engine when I fed fuel directly into the intake. It feels like the fuel is being cut off when the secondaries open up and then it starts backfiring, straightens out as soon as I go back to 3/4 throttle.. The biggest problem is having the time to work on it. I was hoping someone had seen this before.

I have about the EXACT same issue. I replaced my whole distributor, which helped a little, and started adjusting the vaccuum modulator for the secondaries on the carb. it's getting better. I have more tinkering to do, but I think it's helping. I'll let you know if I ever get mine dialed in and it was really the problem..

 

[Scott183]

I've had this problem before but it was a lack of fuel, make sure it's not a float problem.
Another issue could be the ignition module inside the distributor, lack of proper advance during WOT.

If it was a lack of fuel, then wouldn't you think that feeding fuel directly into the intake would have helped a little? I know it's not atomized, but still, some kind of a difference. Probably should have used a propane feed instead. As far as ignition timing, I'd play into that however we're not talking a little sluggish here, where' talking cylinders not firing like someone was cutting off the fuel supply. WOT to 3/4 should not change timing by more that a few degrees. Damn I wish I could describe it better.

I have about the EXACT same issue. I replaced my whole distributor, which helped a little, and started adjusting the vaccuum modulator for the secondaries on the carb. it's getting better. I have more tinkering to do, but I think it's helping. I'll let you know if I ever get mine dialed in and it was really the problem..

I had seen your post when searching yesterday. It's hard to look at a distributor problem as this is a WOT problem only.

Thank you both for your input! Tomorrow I go back to basics.I'm gonna watch the 02 when the problem happens and see if she's hanging down around .1 volt when the problem happens. I'm gonna adjust or replace the tps. I keep getting a code 21 for a TPS high signal. I think idle setting is around .5 volts, not really sure how much this code would play into my problem. Last but not least a fuel pressure test. I'll post my findings as I move along. Thanks again to all 3 of you that responded. 😀

 

[billyjack]

Is your vacuum break functioning well? How about the secondary air valve tension? Sounds like air valve may be opening too far and when you back off the throttle to 3/4, it closes enough to recover.
Also, do you have a code 21 or 22? Normal spec on a TPS is .50-.58v at closed throttle. Anything over 5 volts triggers a 21. A TPS with 1 volt at idle would normally advance the timing a bit and richen the mixture slightly, probably not enough to cause a problem.

Bill

 

[Blake442]

Put a new fuel pump on it, they're only like $20...

When I first got my 442, the 307 ran great, but get past 1/2 throttle or so and it would fall on it's face.
Threw a new pump on there and it ran like a top!

 

[Scott183]

Thanks billyjack and blake for your suggestions. Here's the update from the few short hours I've had time to work on it. First off we found that the cam to lift the secondary needles was missing! Had another on a spare carb, insalled it, road tested, same thing. DAMN IT! Yesterday spent some more time on it, found that when setting the secondary cam, the spring for the rod came off the post, no tension (good suggestion billyjack). Fixed it, tested it, same thing!! Finally got a fuel pressure tester to squeeze in there and I have less than 3lbs of running pressure (even when the pump is dead headed). I found a reccomended spec of 7-9 psi. Good suggestion blake. Oh btw billy yes I'm getting a code 21, but I have not seen the tps go past 5 volts yet. Unfortunately we were not able to get the plug out of the tps adjustment block. I'll shave the green peg if I need to, to get the spec right. I have an electric holley fuel pump laying around from another job. If I can ever get more than 15 minutes at a whack to work on this thing, the pump is going in and we'll take it from there. I hate investing time into a motor that I'll only keep for a year (I hope). May be longer now, since I just watched a large chunk of my Cutlass FUN FUND get blown on a new refrigerator. I may be holding on to this 307 for longer than I intended. Thank you all for your suggestions, I will update once I get the pump installed.