I purchased a Carburator and the builder supposedly build it to 68 400 HO carb # 7028271 specs.
I found the service specs in the service manual but it does not state the jet sizes. Would anyone have a link to the jet sizes in the above carb or know what it was?
Harold, one of the best places that I ave joined for carb advice is at Cliff Ruggles website. There is a page with forums where many people join in to help out with carb questions. Many of these guys are quick to get you answers to your questions and for your jet size question they'll know.
Don't waste your time looking for or attempting to use original specs. They hold no value anymore. Even if you do learn the correct jet sizes and tune-up settings in a service manual or other old book, they would be entirely incorrect/obsolete. No jet spec or other carb setting from 1968 will work as designed when flowing today's fuels, which are drastically different than they were 45 years ago. Same holds true with base engine timing, distributor curve tuning, compression ratio, etc...
With today's fuels, EVERYTHING has changed. So you have to ignore most original tune-up specs and find your own.
Anyone who attempts to build and tune up these cars using 1968 service manual specs is going to leave a LOT of performance/fuel economy/drivability potential on the table...but the car will indeed "run and drive" a least for a while if all you want to do is get it on the road to sell it or something. At best it runs below potential. At worst it slowly self-destructs.
I agree with OHCMarc's suggestion, Cilff Ruggles is a Qjet specialist who knows how to get you pretty close with modern fuels, and for your particular engine's level of performance, from a carb perspective anyway. Then after rebuilding to one of his recommended recipies, in order to get it to run the best it can, you will still need to fine tune it from there and re-tune the timing curve, ideally with the assistance of a dyno and an O2 sensor.
Cliff writes a real nice book with jetting, mods, and initial setup suggestions for quadrajets mounted on various engine power output levels running on modern fuels. "How to rebuild and modify Rochester Quadrajet Carburetors." Available all over the internet, it's a fabulous resource for this type of thing.
Crazecars? IIRC when ethanol was introduced in full force across the nation many started having trouble. I for one had to update my bike (1999 Valkyrie) that has six carbs. I up'ed the slow jets from 35 to 38 just to maintain low idle and so that the slows would not gum up as fast with limited use. While there I went up from 100 to 102's for the mains. Most of this was b/c of 10% ethanol being less efficient and needing more gas for a better burn.
You're absolutely right. Your motorcycle experience is a classic example of factory specs not working well on "older" engines...and that bike is less than 15 years old? Our cars are pushing 45 years and more...
But I'm not just talking about Ethanol...the changes in gasoline over the years go far beyond that recent development. Ethanol introduction is only one of many incremental changes that has occurred in gasoline over the years. Remember factory specs for these 68 Firebirds are also assuming you're running true LEADED fuel, of a different octane, without Ethanol, and without many of the other additives found in today's fuels. Not to mention substantial unknown changes as a result of environmental regulation which has resulted in different fuel refining and formulation methods, and different sources/grades of oil that it is produced from. Gasoline is produced with completely different methods, technology, and base products today, so therefore the end product is different today. Therefore different quantities are needed in order to get as close to stoich mixture as a given engine can handle when mixed with air, which changes jetting and adjustment needs...the flame front travels at a different speed than it used to, which changes ignition timing needs...resistance to detonation has been altered, which changes compression ratio needs...lubrication/cushioning properties are different, which changes valve seat needs...and ethanol is a whole separate topic not only with regards to energy output per unit, but also in the fact that it has changed gasoline into a dessicant that absorbs and suspends water instead of separating from water like it did in the past. Burning off trace amounts of water changes everything yet again (ever hear of water injection as a power adder?). And ethanol and other additives have the potential to react with plastics, rubbers, and even some metals differently from the gasoline of yore...
It's been nearly a half century since these cars had their tunes established by Pontiac. And gasoline of today doesn't even remotely resemble the fuel that Pontiac used to establish that tune...from ANY source. This means the tune that Pontiac established back then, as good as it may have been, is flat-out wrong for today.
What everyone needs to recognize is that if you or your mechanic use 100% factory service manual specs and parts to tune/build a FGF, and you intend to drive it, you'll end up with the wrong jetting, wrong carb mixture, wrong compression ratio, and wrong initial timing/advance curve/vacuum curve. Of course it will start, run, and drive, and it won't just blow up in your face. But it certainly won't be tuned anywhere near its potential, and it could actually cause some engine and fuel system damage over the long term....
