Quadrajet - bog

I give the pedal a smash,
Oh what a gas!

The Qjet gives me a bog,
What a dog!

Whats a Vette owner to do,
When you got the QJet blues!

I have a 1978 L-48 vette Federal
auto trans
carb #17058206

A quick search gave me this great list from an old Lars post re: bogging...

Unless the airvalve is mechanically blocked and locked closed, there is nothing that can prevent it from opening at wide open throttle: If the throttles themselves are opening, the airvalve will get pushed open. Nothing will stop it from opening. How are you checking it..? You can't observe the airvalve opening by "winging" the throttle in neutral.

If you're getting a bog going into the secondaries, you may have one or more of the following problems:
•Plugged fuel filer
•Low fuel pressure
•Retarded timing
•Excessive plug gap
•Incorrect secondary airvalve spring windup
•Defective choke pulloff
•Improperly adjusted choke pulloff/airvalve rod
•Low float level
•Incorrect secondary metering rods
•Incorect jetting
•Incorrect secondary metering rod hanger height


I need help creating a plan to get rid of the bog, looking at the Lars list above of probable issues above where should I start next?

What I have done...

1. Installed another distributor to make sure I did not have an ignition issue, using the same cap and rotor
2. Replaced plugs and wires
3. Changed fuel filter
4. Blew air, till I was blue in the face, thru the gas line in an attempt to dislodge debris, if any, from the sock in the tank
5. Checked vacuum with a gague, it was between 19 & 20
6. Timing is at 14 degrees, the top of the timing tab is where it is set to but its hard to read

Background

Prior to 2011 the car sat for several years.

I siphoned out the gas summer of 2011, tuned it up, replaced lots of hoses, fluids and belts, calipers, fuel pump, steering ram, steering coupler and calipers

In Nov 2011 I had the car smogged, that required the carb to be rebuilt, it was done by the local Corvette Shop.

It ran great for a while but the distributor bushing were sloppy, it kept chewing up rotors and caps so bought a MSD 8362 Street Fire.

It was hard starting so I replaced the choke therostat

I got it Califoria smogged again in May 2013, i had the bog at that time, maybe not as bad as it is now it passed except for the pressurization of the gas tank test. I pinched the hose going to the charcoal canister and put on a vented gas cap to get it to pass

Since I recently got the headlights and brakes working better, steering coupling and ram replaced and new tires I am more confident to give it some gas instead of slowly putting around town hence the bog is more pronounced. I am real close to having it be a reliable driver!

Questions
Which of these issues shoud I check next and how is i done?
How do I know if the secondaries are opening? I understand they will not open unless under a load?

Can I just suck on the hose going to the choke pull off to test it?

How do I check Incorrect secondary airvalve spring windup,

Can I assume the secondary metering rods, jetting, secondary metering rod hanger height are Ok cause the carb was working ok last year?

Could floats have leaked and sunk?

I have always drove the car very conservativley, that is why I am so vague on how long this issue has pesisted. I do know that when I had the carb rebuilt in 2011 there was no bog, only a slight hesitation when cold.

Pictures






Owning a rubber bumper vette is a humbling experience, especially on the cheap! Sorry for such a long post.

Thank you in advance!!

 

It certainly seems you have done your homework and are looking at all the right things, however, what if it is not the secondaries that are causing the issue.

The first place I would go if there was a bog on "pedal smash" is to look at the accelerator pump. Is there a strong squirt of gas into the primaries when you smash the pedal. You should be able to see that looking down into the primaries.

Probably obvious, but you didnt mention it.

 

Are all of your vacuum lines connected to carb? Lower left front corner of carb pictured... there is a port that does not appear to have a hose connected. I have a small hose that runs from EFE actuator on my '80 L48 to that port.

 

You should make certain that all of the emissions components on your engine are in good condition and functioning properly, before assuming that you have a carb problem. Likewise for the ignition system/distributor/advance can. If any of those components is malfunctioning or improperly adjusted, the engine performance will suffer. EGR valve, all the vacuum switches, etc., etc. can lead to performance problems.

The single BEST way to isolate that the carb and distributor are in great condition would be to box them up and send them to Lars Grimsrud for rebuilding/repair. He is very affordable, is one of the best at his craft, he has all the equipment needed to execute the work, and he engine-tests the items he rebuilds to set them up and fine-tune them. You can contact him via e-mail at V8FastCars@msn.com to see if he would be interested in working on them, what his charges would be, and what the turn-around time would be. Then you can decide if that works for you or not.

Along with that, Lars will offer his wisdom on other things you need to check on your engine to assure that all is in order.

I can tell you have read up and analyzed what considerations there are with the carb; and I think you can probably do a good job of rebuilding it and making needed adjustments. But, do you have a way to test the carb to know that it is working as it should? Or, when you get done and possibly find that there are still concerns, will you feel comfortable that the problems will NOT lie with the carb that was just rebuilt?

This is just some discussion for your consideration, so that you can feel good about whatever direction you go. If you decide to do the work yourself, you need to get a copy of the Chevrolet Chassis Service Manual for your model year. It has a section on 'Fuel System' which will detail the carb, it's disassembly/rebuilding, and all the details on how to set-up and adjust the carb and the choke for your year's Q-Jet. You cannot find that info anywhere else, to my knowledge. I have the Ruggles and Roe books; and they are both very informative and educational. But, they do not have setup details for the Q-Jets on Chevy vehicles for each specific model year.

Wish you luck. Just make sure the "other stuff" is working properly before focusing on the Q-Jet.

 

When you say top of the tab, what exactly do you mean? Normally the TDC/0 mark is just off center and on a dirty tab is nigh on impossible to see.....ask me how I know! lol. A bit of cleaning will show the timing marks clearly.

 

Make sure the air valve spring has enough preload. You may need to tighten it up a bit. There is a factory spec for your carb; probably 3/4 of a turn, or something like that.

 

I am getting ready to rebuild my Quadrajet because I am fairly certain the Accelerator pump is worn out on mine it bogs at take off then run fine afterwards.

I would suggest this book. Every Chevy forum I have read all the GM guys swear it is the bible for the carb. Cliff knows the carb the best out of all the books for sale.

http://www.cliffshighperformance.com/buy_book_2.html

 

Just follow the instructions in my paper. If you don't have the paper and need a copy, just send me an e-mail request. If you'd like me to fix, test, and set up the carb, just send me a request for the Services Info Sheet.

Lars
V8FastCars@msn.com

 

wajulia -
I checked the squirt!

tedh - checked the front for disconnected hoses and found none

7T1vette - I tested the dist by installing another
I have the Chevrolet Chassis Service Manual, but reading it makes my eyes glaze over lol

Lars - I requested that paper some time ago, I have Rev. BP 5-27-12, it looks more user friendly than the Chevrolet Chassis Service Manual. I will give it a whirl, if it does not turn out well I will be sending you the carb. Us nonballer, rubber bumper 10 footer Vette owners seem to attempt repairs ourselves first! lol.

 

DIY is the reason I bought the car in the first place. Great hobby...work on the car, then drive the car.

But, as Clint Eastwood said as 'Dirty Harry'.... "We all gotta know our limitations..."

When I know I'm over my head and I've not been able to solve my problem, that's when the "specialist" gets called in.

 

Be aware you have an obsolete and outdated paper - you might want to request a current version...

Lars

 

If you haven't richened up the main jets or changed rods, you may be suffering our crappy California gas.

 

I pulled the carb, read Lars papers and decided to send it in for repair. A man has got to know his limitations!
Lars you will be getting an email shortly

 

I'll just add something else to look at since I havn't seen anyone else mention it yet. Repeated 'smashing' of the pedal can lead to the bracket bending slightly, and pressing of the pedal to the floor not fully opening the throttle.

Have someone get in the car and press the pedal all the way to the floor, and when there, check to see if there is still room left in the throttle.

 

OMG that carb is so dirty looking, just for a piece of mind I would clean that carb up, check accelerator pump function and have a look at the e.g.r. valve .