Bogs at full throttle

[74bigblock]

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[74bigblock]

Note that the old fuel line is a clear green plastic substance, and is a bit melted on teh edges.... looks like something off a bong I had in college . I think the vacuum lines he used were fish tank tubing. Not sure though, it didn't have a Walmart stamp on it anywhere.

 

[cruzer99]

I'm wondering if you might be getting a vapor lock in the fuel line once things warm up.

Try wraping the fuel line coming from the fuel pump (up in front of the engine), to the new fuel line with some insulation. Mainly between the fuel line and the engine. Just one more thing to eliminate........

 

[Lee74C3]

I'm very curious to see what the final resolution here is. I also have a 1974 Big Block with Auto. It is completely stock with the Rochester carb and stock filters, lines, etc.

My car also exhibits this "bog" condition on hard acceleration. I have also noticed that the transmission is reluctant to make the 2-3 shift when this occurs. Up to about 3500 - 4000 RPM (and below) it shifts fine. I don't think it is a transmission problem and will get into this further over the winter. The areas I'm looking at are fuel delivery, vacuum, and ignition timing.

There is an excellent series of articles in the October and Nov/Dec issues in Corvette Enthusiast magazine that cover vacuum and ignition timing extremely well.

74bigblock, please continue to share your experiences. I'll pass along what I have but I won't be getting into this until the weather gets to cold to drive the car; around Thanksgiving.

JohnZ... do you want to chime in here? I know you are more of a small block guy but, the basics are still basic.

 

[cntrhub]

What the heck, try this.......take the gas tank cap off and run it lose. See if the tank is causing a vapor lock? Run the car with very little fuel in the tank......just in case you punch the throttle, gas doesn't slosh out and go all over the place......FIRE!!!

 

[bigvette1]

Seems a common thread is the BB's with Q-Jets run out of oomph around 3500-4000 rpms. Maybe the stock Q-Jet does not have the fuel bowl reserve to supply the needs of a BB without some tuning?? What doe you think??

 

[motorman]

super stock camaros and corvette in drag racing use Q jets and run in the 10s

 

[Vettehead Mikey]

I'm very curious to see what the final resolution here is. I also have a 1974 Big Block with Auto. It is completely stock with the Rochester carb and stock filters, lines, etc.

My car also exhibits this "bog" condition on hard acceleration.

There's a post further up talking about the air valve on the secondaries opening too soon. This is probably the #1 cause of Quadrajet equipped cars 'bogging' with heavy throttle. Took me 2-3 tries to get mine right, runs like a champ now!

 

[74bigblock]

I'm not seeing an adjustment for the air valve on the 2ndaries.

 

[bigvette1]

It is on the passenger side of the carb. Get a mirror and hold it below the secondary shaft and you will see a set screw. This loosened will allow the spring to for the door opening to loosen or tighten. Before you loosen, mark where you started from. Better yet, go to the library and get the book on Rochester Carbs by Doug Roe. Fantasitic for Q-Jets.

 

[cntrhub]

..... "Took her out for a run, and had a glimmer of hope. The first 2 times I got on her... She ran like a champ (OMG it was insane)."
This tells me the carb should be fine if this thing ran as well as it did the ...."first 2 times..." You have a fuel delivery problem somewhere? Now, is the delivery too rich or too lean?
Take it out to a less traveled road. Force the car to fall on it's face. As soon as that happens, turn the ignition off (not locking the steering column of course) and coast to a safe stop. Quickly take off the air cleaner and see if the choke plates are closed? Next, pull out the easiest spark plug and look at the condition? What does it look like? White and clean? << Lean condition. Is it black and sooty? <<< Rich condition. You need to figure which?
I think Sam might be on to something. You might be over-carbed.

 

[Big "O"]

power loss

I experienced the same problem with my truck the other day. Turned out to be the distributor modulated that was weak causing a power decrease at high rpm's. If you haven't checked this, try it, before you start replacing anything else.

 

[74bigblock]

It is on the passenger side of the carb. Get a mirror and hold it below the secondary

Bigvette, are you talking about a Rochester or the Edelbrock Performer 750? I have an Edelbrock, not the Rochester.

 

[74bigblock]

You have a fuel delivery problem somewhere?

Yes, Saturday It stalled in the middle of the road (this is a chevy not a ford right?)... No gas in the fuel line. Shook it a few times, plunk went a wad of rust into the clear fuel filter. Hmmm........!

 

[cntrhub]

I think you found it. Time to remove the complete fuel line system and clean all the tubes... as if you were cleaning the bore of a gun barrel. Take mechanic's wire, guide it through one end, wrap a little wad of cloth from a gun cleaning kit and pull it through as many times as it takes to clean the fuel line. Only other alternative is to buy same diameter bulk steel fuel line, match bends, and flair both ends.
There, you would have to buy a tube bender and a tube flaring kit. I think the easiest way, is to buy a roll of wire and some cotton balls, WD-40 for cutting lube, and have at it.
At this point, I would replace the fuel pump also. Internally, it is a caked packed powdered mess.

 

[74bigblock]

UPDATE

Tank came out last night. Lot's of rust. got a couple questions....

1) There was a tube coming out of the drivers side of the tank, heading up to the front of the car. Was this some sort of emissions control going to the carbon box or something? Is it needed for proper workings of the tank and engine?

2) Why on earth do they call it a "removable" crossmember? You have to slip an open ended wrench into the fraim rail and pray that you can connect with the nut inside. Then bust the bolts loose and go from there.

3) What's the best way and route to run new fuel line up to the front?



:BOW Hopefully it will all go back together and not choke itself.

 

[cntrhub]

If this tube is on top, or off to the upper side of the fuel tank which you are describing, it is the: Fuel Vapor Return Line. Yes, this is emission related, so fuel vapor is closed off from entering the atmosphere. Make sure that hose line breathes, or you will have the tank collapse from vacuum lock/fuel delivery.
Stay away from using the least amount of rubber fuel line as possible. Eventually it will crack and rot from the fuel and cause tiny leaks all over. If you do need to replace any rubber fuel hose, go to a place like West Marine, or local your boat shop, and buy quality marine fuel line. It's worth the extra money to step up to this quality hose. Replace with steel fuel as much as possible.
I should have also told you to change the meter assembly or at least clean it well. Contact or brake cleaner cans work great cutting the rust. They can be bought cheap at Wal-Mart in the auto section.
I would just remove the stock steel fuel lines and spray brake cleaner through them, and reuse the steel lines once they are super clean. Replace the fuels hoses with marine grade rubber. Replace the tank meter assembly (float assy). Replace the fuel pump. And of course, keep any fuel line away from hot (fuel boil in lines....i.e. = vapor lock) areas.
Put it all back together, fill the tank, start it up, and stomp on the gas pedal!!!

 

[74bigblock]

That's what I thought about the vapor line. Can I just slap a breather on the tank, or use a vented tank cap instead of reconnecting all the lines?

 

[cntrhub]

You have to remember that if you keep that line open, it will slosh out fuel if you go around a corner with a full tank of gas. And you know what I mean by taking a corner. So, with that said, the higher the line, the less chance liquid fuel escaping. But it's emission related, so I would have to tell you to re-plumb the line back to emission standards. I think I established my legal obligation by telling you this.

 

[74bigblock]

LOL... she is registered as an Antique Vehicle in the state if Illinois. Emissions is not an issue for her... just want to make sure I don't implode/explode!