When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Hope I'm not breaking any forum rules by asking this, but the question's not about the 'Vette but my Chevy Apache pick-up. It's a '58 straight six 235 Stovebolt, 3 speed column change and it had major engine upgrades before I got it, such as stage 4 head, reground crank etc and twin Rochester carbs. Here's where the trouble is. It starts and runs well and is quite fast for a fifties truck with an original engine, but it always runs too rich. The fuel consumption is about 12 mpg and the plugs are always black and sooty. I've heard that the carbs might need re-jetting to reduce the volume of fuel, but I wouldn't know how to go about that. It also has an Offenhauser intake manifold, and I think it has the original carb plus one more. I'd buy two carbs if I knew which to buy, but I might end up with the same problem. Anybody have any idea how to reduce the fuel without affecting the running?
Hope I'm not breaking any forum rules by asking this, but the question's not about the 'Vette but my Chevy Apache pick-up. It's a '58 straight six 235 Stovebolt, 3 speed column change and it had major engine upgrades before I got it, such as stage 4 head, reground crank etc and twin Rochester carbs. Here's where the trouble is. It starts and runs well and is quite fast for a fifties truck with an original engine, but it always runs too rich. The fuel consumption is about 12 mpg and the plugs are always black and sooty. I've heard that the carbs might need re-jetting to reduce the volume of fuel, but I wouldn't know how to go about that. It also has an Offenhauser intake manifold, and I think it has the original carb plus one more. I'd buy two carbs if I knew which to buy, but I might end up with the same problem. Anybody have any idea how to reduce the fuel without affecting the running?
Ok couple things:
1. are the Rochetser Carbs dualjets or Quadrajets?
2. Lowering the amount of fuel the carb proportions isn't difficult, but you need to know WHERE in the engines RPM range it is running rich. Best way to do this would be to use a Wideband 02 gauge. If you don't want to go that route, how good are you at reading plugs?
Put simply, (if the carbs are quadrajets) you can go down a size or two on the primary jets to lower the overall amount of fuel used (lean it out). Keep in mind, this will lean it out everywhere, so if the engine is only rich at idle or cruise, but lean at WOT or something, it will make the lean condition much worse.
If you can figure out WHERE in the range your truck is running rich it will make things easier. If it's at idle, we may be able to get there by just adjusting the idle mix screws. If it is at cruise (and you have an APT screw equipped quadrajet) you can lower the APT screw to lean out part throttle mixture. If it is at WOT, you can go with a smaller primary jet, or possibly a different size secondary rod or adjust the secondary hanger height.
It's a 235, so it's not a quadrajet. If it's running too rich I would look at:
1) Fuel pressure, should be 3-4 psi. You HAVE to buy a gauge (they're cheap) because it's very critical. 4 or 4.5 psi will push fuel past the floats and make it run rich
2) Slowly and carefully screw in the idle mixture screws, count and record how many turns it takes to bottom them out. They should be equal and should be around 1 1/2 to 2 turns out from all the way in.
3) Float or floats in carb need adjustment (too high) or sunk. You'll need to remove the top of the carbs, and probably buy a couples of carb kits for gaskets and it'll have the correct float level and a gauge in the kit. Shake the floats around in your hand and make sure neither of them have fuel inside, it's kind of common.A float with fuel in it will not float and it will cause your truck to run rich.
4) Maybe it's just too much carb for that engine. Consider either modifying it so the engine is the same as an early Vet, or swap the manifold and carb for an original one carb and see if that's now enough carb for your engine.
Thanks for your replies. They are Rochester B one barrel carbs. I 've already tried carb kits and I bought an exchange carb a few years ago, but neither made any difference to the running. I've also tried turning the mixture down, but the engine starts to stall. I ordered a fuel pressure meter so maybe I can at least see if the pump is delivering too much fuel. When I get the meter I'll write again and let you know the results. Thanks.
I got the fuel pressure gauge and tested the truck today. It gave a constant reading of 4 psi so I assume the pump's okay and the fault is with the carbs. I've already fitted new kits to them but it didn't make any difference. On You Tube I see similar engines with the same or even triple carbs. Here's some advice for someone using a fuel pressure gauge for the first time - watch how you disconnect it after use. I got a jet of petrol right in my face and had to use a shower head to wash out my eye which was stinging badly.
Here is a parts diagram.........#17 is the main jet. You could easily change this out but a few things:
1) Get kits for both. You may need them when you crack them open.
2) Get a few jet sizes.......it is impossible to know what you have now for main jet........so crack them open and inspect......get about three pair of jets....2 number down from the next one. Lean the jet out only 2 numbers at a time......until you hit the sweet spot.
3) Set idle screws before you start. This can be tricky on a dual set up so hook your vacuum gauge up and go until highest vacuum reading, but realize you have two carbs and try to get them as close as possible to number of turns out.
4) When you have them apart....check for proper power piston operation....#12 on the diagram....these must be free to open and spring back.
5) Check your timing too........find the factory setting and add a few degrees.
Thanks Mel & Jebby - the information is excellent, but complicated if you've never done it before. I'll study the exploded carb diagram and the Carb shop website and see if I can make some sense of it. I found this very useful You Tube clip - https://www.youtube.com/watch?v=V2AINKEauVMand - and I'm studying it now. I'd love to get the truck running properly, so thanks for your help. Regards, Barry.
running too rich
