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So my 1970 Cutlass S had some work done before I bought it. They put a mild cam in the original 350 Rocket along with an aluminum intake and Holley 4 barrel. I have been told by several sources that the carb is too big for the motor. This appears evident by the dumping of fuel under acceleration along with the strong smell of gas from the exhaust and robbing of vacuum from other parts of the motor. I've always had Quadrajets in the past and tend to hold to the thought that a return to the QJ is the best option. However, I'm having a whale of a time finding the proper, bolt on ready QJ for a reasonable price. I'm not against using a different aftermarket carb of a proper size since the motor isn't all original anyway but I'm not sure which one would be best for the Olds. Enter you fine gentlemen with your abundance of Olds expertise! What do you think is the best route to go to correct these issues? Dose anyone have a source for a ready to go 7040250 QJ that I don't have to sell my first born to acquire? What are your suggestions for a solid aftermarket carb that will make this Olds run like it should considering the minor engine enhancements its had done? Any and all suggestion and recommendations are welcome and encouraged as I'm at a stand still until I get this figured out. Thanks guys!
What is the number on the air horn of your Holley? I doubt it is "too big" for the engine, but it may be jetted incorrectly. It is possible that the installer just slapped it on with the out-of-the-box jets did not tune it for your engine.
570 cfm vacuum secondary carb is very similar to the 600 cfm Holley that I had on my car back in the 80s. I spent a LOT of time tuning the jets, accelerator pump shot, and secondary diaphragm springs to get it to launch similarly to the factory QJet. You should be able to get yours running well with tuning.
The older Holleys had an issue with the accelerator pump rubber rupturing during a backfire. At some point a check valve was added to prevent this, so maybe check to see if your pump is OK or not. If good, then get a tuning kit and start having fun!
Thanks Joe, yeah I discovered that very thing earlier tonight. I think you’re right on my carb sources. They seem to know less than I do and I know very little about carbs. It was second hand so I was suspicious. That’s why I came to you pros! So let me tell you what’s it’s doing and y’all tell me your thoughts. First it runs super rich. It also seems to dump fuel when getting on it and the motor seems to choke, like it’s getting too much gas. When cruising at a steady speed, say 50 mph, it surges at regular intervals rather than maintaining an even rpm/feel. It starts pretty well but wants to die when you put it into gear until you run it a bit. It’s just not right. I’ve had one of these several years ago and I know how these cars run and this ain’t it. I took it to a guy I trust who’s a really good mechanic and he tried to noodle with the carb a little and didn’t have much luck. Granted, he didn’t get super into it, he just tweeted on it a bit. But I’m kind of stumped. So maybe it’s too small then? Could that be causing this mess? Quadrajets are 750 CFM on the 350 if I’m not mistaken, correct? Should I look for something that size? What Ebrok carb would you recommend until I can get the proper Quadrajet? Thanks for the guidance guys, I REALLY appreciate it!
I had a couple friends who had that 80570 Holley carburetor - consensus was it's a complete piece of junk.
...source for a ready to go 7040250 QJ that I don't have to sell my first born to acquire?
A ready-to-got 250 is going to run you ~$500-$650 - rebuilt, looks pretty, proper jets for the application. They're out there (not many but they're out there), do some research. I might be mistaken, but I believe you could mount a 7042250 (455 cid) on your 350 cid quite nicely. I ran the 1971 455 cid Q-Jet on my 1971 350 cid and it ran superbly.
I wouldn't consider installing anything made by Holley on any Oldsmobile during the 1968 - 1972 timeframe.
I sense you want this done preferably ASAP. Consider lowering your expectations from immediate to several weeks or a month or two out. They don't make them new, they work great, but you need to be patient.
A ready-to-got 250 is going to run you ~$500-$650 - rebuilt...
Call a couple of the vendors and talk with them - they're a wealth of information...
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Thanks Chief! Exactly the pointers I was looking for!
Another thing is to make sure that the throttle blades are closed and it is operating on the transfer slots at idle. So many people overlook this and end up idling on the part throttle circuit. Bottom line: back all the adjustments off and start fresh. Once you get it idling clean, move onto part throttle, then onto full throttle.
If you want to install a QJet, don't get stuck on that exact carb number. Anything from 1970 (that's the first year for the integrated hot air choke) up to whenever the electronic carbs were introduced can be made to work. Many people have posted they prefer the 170- series carbs, and have made them work well on their engines.
August 28th, 2023 | 03:04 PM
