Help! Rochester tuning issues

[74 454]

Have a 74 big 454 all original. Rochester carb original also, no changes to factory set up.

Like clockwork, after running 1/2 hour or so and engine compartment getting fully heat sinked, I get poor hesitations and even back fires out of the intake when opening the secondaries during excelleration! Doesn't matter if I open them slow or fast, low or high RPM's. Even if I keep them open a little for 3 to 5 seconds to try to get fuel purged, it still doesn't get any better. I'm pretty sure it's lack of fuel to the secondaries. BUT, this never happens the first 1/2 hour of running. I can hit the secondaries fast or slow and takes right off, no hesitations. Engine water temp is at thermostat setting. It stays right there also after the 1/2 hour and has the issue. No rise in water temp when issue occurs. I do get it a little on the primaries also when accelerating hard from a stop. Again, only after the 1/2 hour when the secondaries are doing it.

I'm thinking carb body is heat sinking? Maybe fuel pump heat sinking or starting to go bad? It's the original 1974 pump, 83,000 miles. But issue was there all this summer and last. Just got the car early last year. Wouldn't it have gone dead or worse by now?

I attached a picture of a left over part from my carb rebuild kit when I rebuilt it last year. It's a foam piece. Does anyone know what it's for? Could this be an insulation piece somewhere to help the float bowl from getting so hot? Could this be the secondary issue?

 

[Vettehead Mikey]

The air valve on the secondaries is probably opening too soon, too fast. Try tightening up the spring 1/8 turn and test again.

My SBC used to do exacty the same thing when it got up to full temp. Not a big deal.

 

[MMM]

First, the little black piece of foam was to go under the secondary metering rod wells, in the throttle plate of the carb. If you took the throttle plate off you would see a identically shaped depression in the middle of the throttle plate where it is intended for the foam to be placed. This was a method some carb kits used to seal the secondary wells from leaking (the foam is compressed as you assemble the carb).

As to the hesitation, it does sound as if the engine is too lean, but that may note be a carb problem (unless the float is set too low). Other sources a lean condition can be retarded timing, a vacuum leak or the timing advance system.

So, first make sure all of your vacuum hoses are hooked up correctly and you do not find any potential leaks, including the vacuum to the headlights, HVAC system and vacuum advance. Remove the distributor cap and make sure the centrifugal advance plate moves. Start the engine and check how long it takes for the choke plate to fully open. It should fully open somewhere between 5 to 10 miutes. Also use a vacuum gauge to monitor the manifold vacuum through the choke cycle and let us know what you read. After the engine is warm, check the timing as per the instructions on the tune-up placard over the power brake booster. If the problem seems to start after the choke is opened, let us know, that will narrow range of potential causes.

Last but not least, was this a problem before you rebuilt the carb?

Mike

 

[MMM]

And Mikey is right, it could be nothing more than the secondary air valve spring tension. Your rebuild kit instructions tell you how much wind up to put on the spring. And the secondary air valves are locked out as long as the choke is closed. This would explain the problem after warm up. But I am still concerned how long it takes for the choke to cycle from closed to fully open.

Mike

 

[6880 Mike]

Excellent Q-Jet reference.

 

[74 454]

And Mikey is right, it could be nothing more than the secondary air valve spring tension. Your rebuild kit instructions tell you how much wind up to put on the spring. And the secondary air valves are locked out as long as the choke is closed. This would explain the problem after warm up. But I am still concerned how long it takes for the choke to cycle from closed to fully open.

Mike

I've been adjusting the secondary spring. I've had it at a 1/2 turn in to 1 1/4 turns in. Carb kit said 7/16 turn in. That was too loose. 3/4 or more works best. This seems to be a separate issue. Too loose on the spring and I get a boog until the RPM's catch up, then it takes off. When heat soaked I'm getting a sudden stop in hesitation. I tighten up the spring and get rid of the boog. All runs great, until after the engine runs at full thermostat temp for 1/2 to 3/4 of an hour. Yes, choke is fully open as soon as it approaches thermostat temp. I always let the engine warm up to thermostat temp in the garage before taking it out. That only takes about 5 minutes, so secondary lock out should out of the picture, and time for choke to open.

 

[74 454]

First, the little black piece of foam was to go under the secondary metering rod wells, in the throttle plate of the carb. If you took the throttle plate off you would see a identically shaped depression in the middle of the throttle plate where it is intended for the foam to be placed. This was a method some carb kits used to seal the secondary wells from leaking (the foam is compressed as you assemble the carb).

I did take everthing apart, but didn't notice the match of the foam piece. I should take throttle plate back off and see about installing it. The directions did call the piece out at all. Could this foam piece help with thermo heat of the secondary fuel boiling out and starving?

As to the hesitation, it does sound as if the engine is too lean, but that may note be a carb problem (unless the float is set too low). Other sources a lean condition can be retarded timing, a vacuum leak or the timing advance system.

My thoughts were lean also, but it's only lean, or hesitating, after running a 1/2 hour or so. The first 1/2 hour runs strong and good. If the carb was set lean, wouldn't it be lean the first 1/2 hour also? I do have the float set low, but to the rebuilt kit spec. Says for the big block 3/8" down from the top. I thought that sounded low to me at the time so I set it at 3/16". But this caused another issue with the fuel boiling over into the primaries giving me a low rough idle, and even stalling at a light. Again, only after the 1/2 hour or so. When I set it 3/8", the idle problem went away.

Interesting how these 2 issues are happening during the same 1/2 hour or so of running. It really sounds like a heat issue to the carb. Could this foam piece in the throttle plate help as an insulator?

So, first make sure all of your vacuum hoses are hooked up correctly and you do not find any potential leaks, including the vacuum to the headlights, HVAC system and vacuum advance. Remove the distributor cap and make sure the centrifugal advance plate moves. Start the engine and check how long it takes for the choke plate to fully open. It should fully open somewhere between 5 to 10 miutes. Also use a vacuum gauge to monitor the manifold vacuum through the choke cycle and let us know what you read. After the engine is warm, check the timing as per the instructions on the tune-up placard over the power brake booster. If the problem seems to start after the choke is opened, let us know, that will narrow range of potential causes.

Vacuum hoses everywhere look very good. Completely new distributor was installed by previous owner, matching original equipment. Has points and separate coil, also new. Had the cap and rotor off about a month ago checking the advanced timing springs and wieghts. All look good, easily move, and clean like new. Here are my advanced timing numbers, with the vacuum advance off of course, and at thermostate temp.

600 rpms = 14 degrees advance, 1000 = 22, 1500 = 29, 2000 = 30, 2500 = 30, 3000 = 32, 3500 = 35, 4000 = 36

Looks like the lightest springs were used with the light swing weights. I think it's advancing slightly at 600 rpm's because of this that's why you don't see a full 24 degrees of advance, 36 - 14 = 22.

Choke fully opens in 5 to 10 minutes easy, approaching thermostate temp. Again, this problem only happens after running at thermostate temp for an 1/2 hour or more.

Don't have a vacuum gauge. I could see about getting one. But when hot, and choke fully open, the vacuum advance seems to be working just fine. I'm getting the full 15 degrees advance.

Last but not least, was this a problem before you rebuilt the carb?

Got the car spring of 2007. Had a holley vacuum secondary on it. Previous owner put it on, but kept the original rochester. Numbers match up, he's had the car since 1977, and is a long time friend of the family. Can trust him. So the only thing I can say is the holly didn't do it. But I don't think that says much. Holley's are very different.

Mike

I could try also try plugging up the vacuum advance when the problem starts to see if that changes anything? If it doesn't, that might tell us the timing is not the issue?