Mr Fufu

I took my '67 coupe for a long run yesterday and the car ran very well.

But today, it took a crap. And, I'm puzzled.

I have the stock base 300 hp motor with a service replacement Holley 4160 which I had rebuilt about a year ago. It has been running excellently, until today.

Today when I went to start the car, I pressed the pedal to the floor once and cranked the engine. No start. So, as often is the case, I pumped it once or twice more. On the third try it started up, but went to high idle, at about 1000 rpm. It's warm today so I didn't expect the electric choke would be activated. I also noticed the idle wasn't smooth - rather rough to be precise.

I backed the car out of the garage but as soon as I let off the gas the idle dropped to about 500 rpm, so low that the engine wanted to stall. So I had to feather the throtle to keep it from stalling as I eased the car out onto the road.

I thought maybe once the engine warmed up the problem might go away. It didn't. The roughness continued, although it grew less noticeable when cruising at a constant speed.

The next thing I noticed was that upon relatively quick application of throttle the engine would "bog" before "catching" and accelerating. Sort of like a turbo-lag effect. I never had this problem before.

The rough running did not smooth out during my 30 minute run. At traffic lights the idle would drop so low that I had to keep my foot on the throttle to keep the engine from stalling out.

So, what could have changed from yesterday when the car ran great, to today when it ran so poorly? Could something be stuck in the carburetor, or might this be caused by something else (e.g. bad coil?).

If anyone can give me tips to troubleshoot my suddenly poor running Corvette, I would be grateful.

Alex

Tooth Doctor

Fuel filter maybe?

firemist1

Sounds like a bad power valve in the carburetor. Pull the primary bowl and metering block, install a new power valve 125-6.5 and you will be in business. Happens to mine about once per year with the exact same symptoms as yours. I just bought some Viton coated power valves that I hope will last longer with today's gasoline.

Clonetrooper334

I had a somewhat similar situation recently...next day it wouldn't start. Turned out to be the accelerator pump (in the carb) had failed. 9 bucks and 15 minutes later all was back to normal. Pull your air cleaner and pull the throttle, if you don't get gas squirt then your pump is bad. Smarter more experienced folks here...I'd follow their advice, but one thing I read while researching the pump was hesitation on acceleration...

DansYellow66

Are you running a fuel filter? Is the sintered bronze Holley filter in the carb inlet? If not it's possible some junk has gotten in the carb and blocked something. If nothing else comes up quickly you might pull the carb off and with a can of carb cleaner shoot all the bleeds, transfer slots, shooter orifices, pull the idle mixture screws and shoot them out, etc.

It doesn't really sound like an electrical issue unless timing has slipped or a condenser is going bad. Do you have points - if so I would check point dwell.

JL66REDCPE

I agree with the power valve theory

MikeM

I believe the first thing I would do is check and see if the choke is hung up closed/mostly closed.

The second thing I would do is not drive it until you fix it. You could be dumping large quantities of gasoline past the rings into the crankcase.

DansYellow66

Looks like MikeM may win the award for best reading comprehension and early morning alertness. Symptoms do sound like a closed choke if the engine is kind of chugging along.

dcamick

Just to rule out "Bad Gas"...You didn't happen to stop and fill up the tank? It would probably be the last thing to consider...

tuxnharley

Others - myself included - have had similar symptoms caused by a fuel pump failure where the pivot rod either displaces partway out one side of the case or wears an eccentric hole in the case. That causes intermittent delivery of adequate fuel. Check the outside of the fuel pump case where the end of the pivot rod mounts.

TCracingCA

If the power valve blew, you would get basically some black smoke out of the exhaust due to the over rich mixture! It would seem to run OK with throttle once you got over the stall RPMS!

I would blow out your air bleeds on the top of your carb also! When they get plugged, you can develop idle problems!

Ya and everything else above! Take off your sight bowl plug and check for gas. Fuel filter and feed scares are stupid when you are operating at lower RPMS and you have float bowls! The sucker would have to be going dry, and it is possible but highly unlikely if it is still running at all! Now higher rpm op, different story having pump and filters good to maintain the bowl fill!

And Check the choke for operation as above was suggested!!

Frankie the Fink

Uh yeah - and will cause zany symptoms...

However, its 10 seconds to pop the air cleaner and see if the choke is stuck...

reno stallion

you fouled a plug. when you pumped the gas it flooded and then fouled a spark plug. That's why its running rough.

Mr Fufu

Thanks everyone for sharing your tips on solving my problem.

Carburetors are mostly a mystery to me, but I'm learning. This morning I'll be checking the choke and power valve (=accelerator pump?) operation today.

I'll report back on what I find...

TCracingCA

That's why they made them so pretty looking. You can still get enjoyment, even when they ain't running!

firemist1

Just to confirm what you are looking for - the power valve is mounted inside on the back side of the primary metering block. The accelerator pump is mounted on the outside of the bottom of the primary fuel bowl. After reading all the responses, I am still convinced that your problem is the power valve, not completely blown, but a crack in the rubber diaphragm leaking enough fuel to make the mixture rich at idle and stall the engine. Usually you cannot tell the power valve is bad by looking at it. The power valve only costs about $10, so just change it and the bowl/metering block gaskets (unless you have the blue non stick kind) and you will be in business.

MikeM

Would you be willing to forward the OP a money order for $10 if you made a bad guess?

firemist1

Yes! I have had my '67 Corvette for 42 years and had this exact same problem with the same symptoms many times over the years. I will stand behind my recommendation.

RatDog

My car has behaved like that twice. Both times I worked the gas pedal too much when starting. Both times I had a fouled plug.

The OP said he pumped the gas when trying to start and then the problem started.

Steve

MikeM

Okay, yer on record to the OP for the $10 bucks! I am glad to see you're a man of conviction.

I ran Holley's for many years and never blew a power valve. Heard a lot about it here, never had the problem myself.