So since im on break from school i figured i poke atmy 81corvette thats sitting in the garage. So i got to thinking about a few minor running problems i had with it over the summer. First of all... its a crate engine. Previous owner installed it. I got a less detailed build sheet but it has a 800 cfm eldebrock carb (too big in my opionion) on a 350 hooked up to an automatic. I think it is a zz4 crate.. thats what a local corvette shoped thought it was when i had it in there last spring. Anyway, the problem that i have noticed wiht it is that when i give it a full quick throttle from a stand still or from a cruising speed, it bogs down like it is going to flood out, and it will if i dont pull back on the throttle a bit. My first thought is that the carb is just way to big and the car cant handle all the fuel at once. The local corvette specialist shop has tuned the carb and timing for me and it made the car perform a whole lot better but i still find that bog in it. What ideas do you guys have and how can i go about troubleshooting this... keep in mind that i do not know a whole lot about the terminology of a carb. Also ive noticed one more thing and im not sure it is related in someway or not... On the highway when im driving my car runs about normal temp and once i get into the city for stop and go traffic is warms up some to about 220 or so, kind of on the point where it makes me nervous but yet its not tooo dangerous. Well when the temp is warmer like that, i notice that when i have to slam on the brake or make a pretty fast and quick turn, the car wants to die out... Now i'm thinking that this might all come down to an oversized carb (besides the getting on thehot side) that or maybe my idle is set too low? i would say the idle is right around 500 rpms while in drive and just slightly higher, maybe 600 in park. Kind of a long post, i hope i gave you guys enough details to help me out here... I'm not sure if im leaning in the right direction and im not sure how i can go about troubleshooting this without having to buy a different carb. Thanks guys!

 

800 CFM is a bit large, but not too bad. I've posted before about the math of carb sizing, it is probably under search somewhere under C3 if you look for CFM and my name.

600 RPM in park isn't bad, though I usually shoot for 700-800. Make sure that your "kick down" is already there when you set the idle. The carb has two idle settings, one for cold and one for warm. When you first start it, it should be on the cold stop. If you pump the gas once when it warms a bit, you will hear it drop down.

As for the temp... If it is *really* 220, then you might be running a bit hot. Check your electric fan if you have one. If you don't, then I would add one (especially on a zz4). You might want to check the real temp with an external thermometer though to see if the sensor is giving you a good reading, and your guage is correct.

As for bogging down... <sigh>... that is one of the tougher things to track down. Usually not over-fuel related, so over-sizing a carb is unlikely here. Dirty fuel filter, gunk in the fuel lines, air filter needs replaced, funky internal stuff in the carb not giving a good mixture... That's it for "breathe" as I can think of offhand. As for the "burn" side... vacuum advance, timing, shorts/breaks in the ignition paths to the plugs (seen this one several times), weak stuff in the distributor...

Good winter project for you tracking that one down!

bill.

 

would you think that the bog and the stalling on hard braking or hard corner be related? and yes, i have an electric fan.. i'll have to check into if that is kicking on. And when you said distributor, it made me recall that twice last spring and summer, that went to start the car and it would turn over but not fire. So i used a spark tester and found that there was no spark at the plugs, so then i took of the distributor cap to inspect and then after i put it back on i decided that i might as try it again just for kicks... and it fired right up... i had the exact same thing happen twice both times after removing the cap and putting it back on, it fired up. I figured the first time that it might have been lose or some kind of mosture problem... the second time i took out the ignition module and had it checked and it checked out alright (was a tip from local corvette shop, i like to call them and pick their brains, very nice guys) So you think distrubitor would be a good place to start? if so, what kind of things/tests should i be looking for? Thanks so much for your help wrwalke. I did happen to check out your carb sizing post and it helped me out with that. thanks!

 

Does the carb have an air valve secondary or vacuum operated secondaries? The reason I ask is cuz WOT bog is usually caused by too much air too quick. Air valves usually have an adjust on the spring tension that determines how fast it can open.

I agree w/ Bill on idle, about 700 in Drive w/ a warmed up engine. The fast idle should also be set w/ a fully warmed up engine and is usually around 1,800 to 2,200 on the "fast" step of the auto choke. A slightly higher idle might correct your braking/turning bog.

 

ill have to look into that 73shark, i really dont know. I'll check it out tomorrow. I hope all that it is, is a miss adjustment somewhere. Does anyone know a good website or some info on steps to setting the idles? I've never played much with carbs so this stuff is new to me. Thanks guys for the help again, I appreciate it all.

 

If you plan on doing tune-ups and maintenance yourself, I highly recommend getting the GM Corvette Service Manual. An Assembly Manual also comes in handy.

If you can't find what you need, I'm sure everyone here can get you thru it.

 

Setting the idle is pretty easy. After the car warms up and "steps down" the idle, use the linkage on the carb to go "vroom vroom". You will see the funky linkages on the side of the carb rotate back and stop against a set screw. Adjust it out/in as needed until your idle is where you want it. Same goes for the cold idle setting... Take a peek at what set screw is holding the linkage while it is idling cold and adjust as necessary.

Ditto with shark. Chiltons, Haynes, and of course, the GM service manual. They all have sections that I like and sections that I hate. With all three on the shelf, I can find good diagrams and info on just about everything.

bill.

 

what am i looking for to determine if it is air valve secondary or vacuum operated secondary?

 

On an air valve secondary e.g. Q-jet, the secondary butterflies open mechanically but at the top of the secondary bores is a plate like a choke that is held closed by manifold vacuum. As you open the throttle and the vacuum decreases, the air valve is allowed to open. This is good as it allows a large carb to be put on engines that don't need that much cfm.

A vacuum operated secondary e.g. some Holleys, the secondary butterflies are actually opened by a vacuum dashpot.

A mechanically operated secondary e.g. some Holleys, the secondary butterflies are actually connected to and open w/ the primary butterflies.

 

The back throttle plates on a Q-Jet open as soon as you floor the gas pedal but this air valve opens slowly with the air going into the carb and prevents a bog. See if you can get the model of the eldebrock carb that you have from their website, once you get the carb # you can figure out if it is mechanical or not.

[IMG]local://upfiles/2883/9D49551250AF4511850721702F63E252.jpg[/IMG]