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Cantankerous Carburetor


Dave M

Firstly a big cheery thanks and thumbs up to all who have helped with info to my past questions. Particularly 79toy, thanks v much for Lars Tuning tips. Alas, all is not well, I still feel there is horsepower yet to be unleashed. I have completely disconnected the vacuum advance to the HEI distributor and set maximum mechanical advance to as close to 36degrees as possible. My '79 is now going pretty good. A friend of mine has a Holden Commodore which puts out 195 kilowatts (so it says on the rear of his car). My Corvette now keeps up with it easy but has about 250 kilograms on it . It is now revving nicely to 5000rpm. You could be mistaken to think I would have to be happy with that! Not on your nelly!
From a stand still, push the gas pedal hard, my Corvette will not get of the mark well. It'll cough and splutter, it is worse if turning. A steady foot on the gas pedal is needed to get of the mark with any credibility! I mean, I just want to mash that gas pedal and smoke up the rear end big time, and I really think the motor will do it but needs that little extra TLC.
Dave M
It sounds like your secondary air door is opening too soon. There is an adjustment for it on the passenger side of the carb. It should be about 7/8ths of a turn in from just touching the spring. This is a very sensitive adjustment.
Do you have a book that describes the adjustment?

What kind of rear gear do you have? It may be that if it is in the 2.50 range that you won't ever be able to smoke the back tires.

Check that out before you get to deep into retuning the engine.

What kinda carb? Q-jet which one.. factory get the numbers off it. Need more info on the setup.

I assume you do the Bog....VROOM trip

Your accelerator pump might not be pumping enough at inital squirt..

Are you running a stock air cleaner?

Try removing the top and do the same test.

At lets say 10 miles an hour, you stomp on it, same results?

Also check the fuel pump output, and the float level. After that check the idle mix adj. From your descript, sounds like could be fuel starvation... -Dave
By jingoes I wish I had tomorrow off!!
Five more work days till I get to play with the Corvette, I've taken note of your comments and am keen to sort out the 'ol Rochester for good. I'm beginning to think there is nothing that the great people at CAC can't teach me!!.
79toy, You've helped with alot of joy so far! I've got HP Books Rochester Carburetors by Doug Roe. I've tried reading it but am struggling with it. I'm slowly trying to come to grips with it (Ithink I'm kidding myself). No seriously I am beginning to understand some of it. It certainly helps when some one takes the time to explain stuff! The more info I seek the better.

69myway, how do you work out the rear gear ? That almost sounds funny! I've got a factory workshop manual (AIM ?), Is there a code on the diff housing or something? Ithink I've read about it somewhere.
If I'm not mistaken, How is that mighty big block coming along !! ?

Vigman, Bog...Vroom...Baarrrp...Vroom...Bapp...Bapp...Vrooom!! Then a little birds tweets in my ear "Get yer Foot of that 'kn Gas pedal !!!!" Trip.
You guessed it, that's what I got! The sounds are sometimes horrifying, like something inside the engine wants to get out!!
I have also got an after market 3 inch by 14 inch air filter, fairly open breathing I would have thought.
The carby number is 17085213 0575 HPB and I have no idea what it is designed for, it fits though!

75shark, I thought might be fuel starvation as I dont know what car my carby is for. ie I dont know what it is off. 79toy put me onto the idle mixture scews, so I've been messin' with 'em.
2 3/4 turns out I've started with, It seems to have helped a little.

Dave M

It sounds more and more like a carb problem. However, on the gear there are two things you need to do. First, find the list of options (RPO) codes that it came with from the factory. I am not sure where it would be on your year model. Then, look it up here under the specifications link, or a resto guide.

Finally, jack her up and go underneath. Have a friend mark the tire and rotate it exactly one turn while you watch the drive shaft. You also need to mark it and watch how many times it spins in one turn of the wheel. If it turns two in a half time for one revolution of the back tire, that is a 2.50.1 or so ratio.

The big block is at the machine shop this week. I will start bolting it together this weekend!
Check your screws in the top of the carb. Loosen the 2 front mount bolts and tighten the screws in the proper sequence (should be listed in your book) and tighten the mount bolts back down. That should insure that you are not flooding out from fuel sloshing in the bowl to the primaries.
Check this link for more information on tuning the carb. http://personal.clt.bellsouth.net/clt/m/g/mgervin/garage/myqjet.htm
Good luck with it.
By the way, I got this link from http://www.corvetteforum.net/c3/vette73/techtips.htm
Thanks for the tips guys!
Steve, I book marked "My Q Tips" , excellent value, a little bit like "mind overload at the moment" , I will be checking that lot out on the weekend! I feel a carburetor breakthrough on the horizon. Can't hurt to be optimistic can it?
Dave M

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