lars

Recently, Dave (superdave269) posted a topic thread about a carb that wouldn’t run right (see http://forums.corvetteforum.com/c3-t...ning-lean.html) under the heading, “Where do you start Quadrajet adjustment. running lean.” He was hoping to get suggestions on how and where to start the adjustment on his Q-Jet. After seeing some incorrect suggestions offered regarding the carb, I offered to take the carb, test it, and identify any problems & issues with the carb for the cost of the shipping. Here are the results:

The carb as I received it, just out of the box, and with my test caps in place plugging all the ports – ready to test. There were no exterior visual anomalies with the carb – everything looked good.


Carb running on the test engine, warming up. Choke is on and operating normally. Fast idle was set at 1250 – perfect setup.


Carb running and warming up.


After warm-up, the carb ran really bad. Idle mixture screws as-received were set at 2 turns out. I turned them all the way in, and idle mixture would not go leaner than 11.6:1. This is a problem.


Popping the top off the carb revealed the float level set very high at 1/8”. This is too high, even for a performance application.


Upon removing the float, an incorrect needle/seat assembly could be seen. Bad idea to use incorrect parts.


A high-flow, no-window seat was installed with the correct needle:


Here is the correct setup installed with the needle clip correctly installed on the float arm.


Float level was correctly set to .350”



At first start-up, the engine died and would not run… I then remembered that I had left the idle mixture screws all the way in… so this was a good sign! Backing the screws out 1 turn produced a perfect 14.7:1 idle mixture.


Idle speed was smooth and stable at 800 rpm.


Carb is now very snappy, with excellent throttle response, a glass-smooth idle, and perfect performance - no popping or hesitation, even when slamming hard into the throttle.

But here is my concern:
Dave felt the carb was running lean, and he was getting popping up through the carb to indicate a lean condition. There was no issue on this carb indicating a lean condition, and I was unable to reproduce any popping or any of the other issues described by Dave. The carb was massively rich, which will not produce lean popping. The only thing I can think is that the large vacuum port on the back of the carb body, which is hard to notice, may have been left uncapped, which will produce a lean condition. Dave: Make sure you cap ALL your ports or have them hooked up to a non-leaking accessory…

The carb is going back to Dave in the morning with a massive bill for a needle and a seat… the carb checks out 100% good – a very nice running carb.

Lars

superdave269

Awesome! I can't wait to put her back on and see what happens. I have a dumb question, can I use the same intake gasket? I have never reused a gasket before so I'm asking. I love all the pics. My carb is now a celebrity.. A heart felt "Thank you" to Lars..

Jeff_Keryk

Impressive. Gotta love an easy (at least for Lars) fix.
I wonder if I could can him and sell him....

lars

Dave -
You can use the same carb base gasket as long as it's not damaged: I've been using the same carb base gaskets on the test engine for several years and hundreds of carbs - no problem.

Keep in mind, though, that even though the carb had problems, it did not have any of the problems you described, so you want to make sure you have all the ports capped off (I didn't see any "clean" marks or evidence that there had been a hose or a cap on the rear fitting on the carb body - make sure it's capped off: See the second photo above: At the rear of the carb, above the orange painted pipe cap I have in the throttle plate, you can see a little black cap that is plugging the rear manifold vacuum fitting in the carb body - this is a big-block-only port that often gets overlooked. Make darned sure this port is capped off), so if the carb does not run right when you get it back, it's a positive indicator that there are other issues not related to the carb that need to be fixed.

Lars

cv67

GOod to have guys around that know these old carbs well. Love the Qjet just dont know how to make them work right

superdave269

Thanks for the tip about the carb gasket. I took a pic before I removed the carb for the first time to help me remember where to put the vacuum lines. I did replace some of the line for the PCV I could tell it was old and would leak as I moved things around. I will replace the little piece of it next to the carb before I put it back on. The vacuum port that Lars wanted me to double check is the one that goes to the TCS selenoid. I now hook up the distributor directly to the vacuum port.
I can't wait to get the carb back on to see how it acts. I doubt the carb shop would of ever fixed the incorrect needle and seat if I simply returned it and said it's not running right as they were the ones to put it in there.
Thanks again Lars for all of your help.

superdave269

I forgot to ask, do you adjust the float by bending the little arms?

Jedi926

Awesome diagnosis and explanation!

rogman16

Great job Lars, as always!!! I have a feeling your motor is going to run like a champ, Dave...

Rogman

Sully1882

awesome detail and explanation. Thanks Lars, you are great. Maybe someday soon when I get my car out and driving I can send the carb to you if it's not running correctly or in need of a rebuild.

SUlly

superdave269

I have no doubt it will run great especially since I just came back into the house from the garage. Went out to replace the 1 inch of vacuum line for the pcv and started re-re -rechecking my work and found wires 4 & 8 rolled on my distributor cap. I am really excited now. But remember I had problems before I put on the new wires. I could not floor it without it bucking before I bet it will rev-up great now that Lars tweaked my carb.

AGVI

I hope it runs perfectly now Dave. There's nothing more frustrating than fixing something only to discover that the item fixed did not solve the initial problem.

Burnout video required if all is well.

Les

P.S. Hi Lars

lars

Yes, the float level is adjusted by bending the float arm.

Lars

Hi Les!!!

DRC

I've read many of Lar's posts and writeups, and once again here we see he really is a good man. Nice to have such people around.

On behalf of others I say a big thanks to Lars.

Have a cold one on me..........charge it to my account!

billla

Lars, thanks for posting this - and for the good work you do every day for the Forum!

lars

Thanks for all the kind words and comments - always fun to help out!

There is a guy from one of the other Forums who thinks I'm a jerk, and he's supposed to be on his way over to beat me up. I'm looking for some entertainment, so this should be fun. I'll let you know how he fares...

Lars

DRC

If he hits you with his purse I hear that's a $20 fine in Colorado.

bluedawg

Really? For what? You seem like a genuinely nice guy to me.

billla

Well, I expect the battery in his Prius is low, so it'll take a while.

That seems to be the way of things these days - rather than just disagreeing, folks make it personal.

Paul L

A couple of Molson Canadians will straighten him out.