Mike Mercury

Our recently purchased 1973 L82 purrs like a kitten at idle, and pulls strong up to 4000 rpm.. and that is where is starts stumbling.

Even with the tranny in park, if you rev up the engine up beyond 4000 rpm, it starts to stumble and won't go any higher. Obviously this worsens under load.

The funny part is that it runs so well up to that point. I can't decide what to check out first, the fuel or ignition?

Sure would appreciate any suggestions. It still has the stock distributor with points. A new dist. cap and rotor did not yield any improvement.

Double Aught

I would look at fuel delivery for a start. Perhaps the current/original (?) fuel pump is just not up to higher rpm usage. Check with one of the carb guys for answers if you suspect the Q-jet. Properly set up, Q-jets can run very good. :chevy

BB73Chris

Seems to me that if you just rev it up in park and it stumbles, that wouldn't be fuel (I mean loss of fuel pressure) because that's not really sustaining power, and fuel pressure problems seem to be more related to pulling power for a while, not RPM. and now, for an anecdote: in my 73 L48, when I would get on it at low RPM it ran fine and would pull until it got higher (My tach didn't work but it couldn't have been more than 4k) then it would start to stumble and miss and generally crap out. It did pretty much exactly what you're describing. When I couldn't take it anymore, I changed the points/condenser, cap/rotor and plugs&wires and it ran great. Pulled all the way up until it just ran out of cam, and to my surprise it ran a lot smoother in the normal driving range too and didn't do the rap/pop thing on deceleration as much (yes, my mufflers were blown out, but it sounded cool). A little while later, my fuel pump started leaking. I changed it, and couldn't bring myself to put it back in the old hard lines, so I went over to the parts house got myself some fuel line and as I was installing it I thought "gee, I hope that doesn't kink." It worked fine for a few days, then I noticed the thing would cut out and stumble again when I was pulling power. This was different though, from a standing start I could pull all the way through first and half way through second the thing would basically just stop running--it wouldn't decelerate too much at full throttle, but all it was really doing was poping and coughing as the few drops of fuel came into this engine. I thought it might be ignition, and I thought it might be fuel. If it was fuel, then pulling first gear and half of second was how much it took to overcome the little bit of flow coming through my kinked line. However, if it was ignition then anytime it was at that RPM and load it should crap out. So, to check, I went to this onramp and with very very little throttle cruised on up to around the RPM where I was having problems by holding it in second gear. Once I got to that RPM I nailed it, and it pulled smoothly all the way through second, or untill I hit freeway speeds. So that told me it was fuel. I ordered a new S-hose and it was fine.
Out of all that, what I mean to say is your problem sounds to me like ignition.

Matt Gruber

none of my 3 antiques still have points. Some people love points 'cause it gives them a hobby, but i'm not one of them.
an engine with broken valve springs will run like yours. points that are closing up will do the same thing.
i say join vette owners 75-present and get electronic ignition. i like the pertronix, 1181, 'cause if it ever dies you can pop the points in and get home. 3 cheers for points they are good for that.
Have electronic ignition since 1990 and they all have worked perfect.

71coupe

The motor problem sounds like a pluged exhaust. A vac gauge hooked into the intake will tell you that.

73Ken73

I had the exact same problem. It turned out to be the resistive wire on the ignition.

The Money Pit

My first guess is the carb's secondary circuit.Make sure nothings binding,or locked out.If it's a Q-Jet there's a lockout tang that the choke is supposed to unlock when the choke heats up.If it's locked when it shouldn't be you'll stumble for sure.

Vetterodder

Could be a lot of things but the fact that is does it in park as well as under load makes it rpm related and probably not a fuel supply issue. Some rpm related things that come to mind are valve springs, points, and timing. Weak, or mis-matched, springs can cause valve float at a lower than normal rpm. Weak springs on the points will cause the ignition to crap out at a lower rpm. When Bubba sets the timing by the seat of his pants, it's likely to result in somethig that feels good but kills power once all the total comes in.

73 LS-4

One other possibility here is ignition timing over advancing. I had a similiar problem when my mechanical advance bushing came off. Bottom end performance was great but fell on it's face bad after 3500rpm. Replaced the bushing and problem was solved.

:cheers:
Pat Kunz

Mike Mercury

you C3 guyz are great. There is a lot of good ideas here. I also thought about the choke - secondary - inhibit linkage, but just haven't had the time to look into it.

When I figure it out, I'll be certain to post the results.

THANK-YOU !!!!!! :cheers:

Mike Mercury

Problem Solved :hurray:


the engine had an incorrect ignition coil. The coil that was on the car had the resistance built into the coil (it measured 4.2 ohms). The correct coil uses the external resistance wire.

So, in a sense, there were two resistors... the one built into the incorrect coil, and the external resistance wire.

The correct coil measured 2.1 ohms (no built in resistor) ; and totally cured the problem.

Damn. that was a cheap $27 repair. :)

Matt Gruber

Nice job Mike!
is it safe to assume the previous owner put that coil on? Maybe he sold it because he could never figure it out :yesnod:

bloodvette80

coil breaking down at high rpm