I fired up the new SBC engine I built this weekend for the first time. I can get it to run so so at 2000 to 2500 rpm by lightly pumping the throttle to keep it running. If you hold the throttle steady, it'll start to die. Give it a little shot of throttle and it sounds great and revs up really quick. It has a mallory unilite, and I set the timing at 34 degrees at 2500 rpm with the vacuum advance disconnected and plugged which should be close enough for now. It starts super easy when you barely crank it. I've been trying to keep it up in the 2000 2400 rpm range going up and down to try to break in the engine. I have great oil pressure. It seems like the engine is fine and it's just having carburetor problems. I did double check my firing order and simple stuff like that. What I have is an Offenhauser intake, with three Rochester 2Gs that came off a supposedly running '58 Pontiac. They still had fuel in the bowls when I got them, so they're not super old. They even have the stickers on them from the place that rebuilt them. I'm using a progressive linkage that doesn't kick in until about 1/2 throttle. The end carbs are modified carbs where someone had plugged off the idle circuits with screws and epoxy I think, and there's no chokes on them, and they have new long throttle shafts. The butterflies look pretty tight in the bores. The center carb has a manual choke and it's open all the way, but starts easy even if it's cold if I pump it a little before cranking. I'm running a mechanical fuel pump and it holds steady at 6 psi on the gauge. When the engine shuts down, the pressure holds steady usually for a while, so I'm thinking the needle and seats are sealing well. I see a small stain of fuel leaking out around the throttle shafts on all three carburetors, which seems kind of strange. Is 6 psi too much for Rochester 2Gs? If gas is leaking slightly through the throttle shaft bores, does that mean that maybe the float chambers are filling up too high in all three carbs? It seems like vacuum should keep fuel from leaking out there. I tried wrapping tape around the air filters on the two end carbs, and put the air cleaner lids back on, and it ran a little better. Maybe I just have a vacuum leak on the end carbs? The exhaust seems a little sooty like maybe it's too rich. I started out with the idle mixture screws at 3 turns out. Then at 4 turns out it ran a little better, then 5 turns out still better, then 5-1/2 turns out and even better. I can almost get it to idle at 1500 rpm now. It sure seems like maybe a vacuum leak on the end carbs? But at the same time it seems like maybe fuel is flooding from somewhere. The exhaust is slightly sooty. The oil is super clean. I have GM EOS mixed in with the oil to save the cam lobes. Oh, the cam is a Crane cam with about 282 advertised duration -- not too wild. Before tearing it all apart and rebuilding the carbs and maybe swapping out base plates on the end carbs or something, is there anything else you could think of that I'm missing? Is it possible I just need a fuel pressure regulator? I've put together a lot of engines, but almost always used Holley 4bbls on practically all of them, so these Rochesters are not so familiar to me, and this is my first tri-power setup. I'd appreciate any help you guys might have. I'm sure some of you have probably run into the same problems with tri powers before. Thanks!
Rusty When I set my 3x2 up I was told not to go over 4 1/2 to 5 pounds on these carbs. This info came from Hot Rod Carburetion and multi carb setup were all they did. I would also try pulling the end carbs and blocking the manifold just to see if the end carbs are your problem.
I had an Edelbrock manifold and had a problem with the manifold seating properly. The problems you described were the same problems I had.
Thanks. Yeah, that's a good idea about blocking off those end carbs. If I could get it to run good on the center carb, then I'd know the end carbs were the problem. Maybe I should get a regulator. I was hoping that 6 psi mechanical pump would let me go without a regulator, but maybe 6 psi is too much. I just pulled a spark plug and the insulator looks black and sooty. Might be too rich, or it might be because I'm having to keep pumping the throttle to keep it running.
By not seating, do you mean you had a vacuum leak around the intake ports? Do you remember where it was leaking? Thanks for the tip. Mine is an Offenhauser, so hopefully that's not the problem though.
It was sucking oil from the lifter valley and pressurizing the intake. With the aircleaners off, you could see the gas droplets blowing out of the carbs.
Try to regulate the fuel pressure down to 2.5 to 3.0 psi, old fart advice passed to me on the same basic intake setup. I may be wrong, I'm using the intake/carb setup from the guy that drove the car it came from. He changed intakes for his look and style, not lack of satifaction of Rochester tri-power on a roller 355.
Spray your intake runners with a water spritser and if it is drawing vacuum it will bubble and girgle. I think this is better than using carb cleaner becasue it does no superficial damage to the carbs or intake coating. Also, if you are having trouble with the primary carb idling without throttling you may have to drill out your idle tubes especially if you have a thumper motor with a lot of cam. But the above procedure is best first. plug off the end 2 carbs and run the center. If it still stumbles at idle are will only run at a high idle you will need to open the idle tubes. While idling and stumbling slowly close the choke and at some point the idle will smooth out and if so you have proven your idle tubes are to lean so they will need drilled starting at about 2 pin drill sizes at first then 1 size at a time until you get it to run smooth at a 1000 RPM or less. Now when you get a decent idle with both end carbs blocked, take 1 end carb at a time and open it up. If it still gives a decent idle and will throttle up, take the other end carb and do the same. But if you have issues with either stop and reblock that one then go to the other. It's all a process of elimination. That will give you a place to start. I don't want to fill your head with everything at once so check it out this far and hopefully one of these procedures will cure your issues. I would not run the engine to long with carb issues as it will not break in properly so keep tabs on your run time and how efficient it is running for your break in period.
Ditto - what they said. Don't run more that 4 - 5 PSI of pressure in the fue line, and WATCH OUT for the end carbs leaking air! They're sneaky about that, and will totally fool you into thinking it's any number of other things messing you up. Block those badboys the F off at the top - when you can run fine off the center, then mess with the others... My thinking is that the dude that blocked the fuel by blocking the idle circuits didn't make sure the air was also blocked at idle. After all, if you block the air good enough, you don't even have to mod the idle circuit at all. ~Jason ~Jason
Thanks! Yeah, maybe that's the easiest thing to try first, and even if I do wind up rebuilding all three carbs, it'll be nice to have a regulator on there. Seeing fuel oozing around all three throttle shafts is making me think that all three carbs are flooding a little bit. The shafts are moist where they stick out both sides of all three carbs. It seems like if it wasn't flooding that those shafts should be bone dry. It sure starts up easy -- maybe because of all the gas leaking in there. The engine can be stone cold, and start instantly when you barely turn the key, and then it seems to get worse as it warms up a little instead of better. I wonder if the end carb butterflies are leaking enough to pretty much kill the vacuum at idle, but at the same time I have a flooding problem to make it run rough. Whenever I'm revving it up it accelerates up in RPMs smooth and quick. If I just barely tap the throttle and shoot it up to 2500 or 3000 rpm it's really responsive and revs up really quick. But then if I try to hold a steady speed, it starts sounding all uneven and splattery, and I get a few pops and bangs out of the headers. If I decelerate from that speed trying to get it to idle, it's like it just dies. Then I can blip the throttle and it zooms back to life. It's like it runs great revving up, and horrible revving down or trying to run steady. Thanks everybody for helping me troubleshoot this thing! Are there any aftermarket needle and seat assemblies that can handle higher fuel pressures than the stock style ones? Thanks!
I have been dealing with similar gremlins on my 3X2 set up. Getting closer and this is good info. I know it sounds simple but you do have a spring to close the secondaries, right? With a progressive linkage that floats the end carbs will float open without a dedicated spring to hold them shut. At least thats how mine is set up. It was one of those D'OH moments when I figured that out.
Thanks. Yeah, I do have springs on both end carbs to pull them shut, but thanks for the tip. I'll keep everybody posted on what I find out. I guess the first thing I'm going to do is order a fuel pressure regulator from somewhere -- so that'll probably kill another week. I appreciate everybody's advice.
Feel free to post what I PMed ya. Turns out that fix is just what the doctor ordered, more often than you think. You'll figure it out. ~Jason
46 Ford is exactly right. You don't need any more than 4 1/2 PSI on your fuel flow. Your air/fuel mix is also a critical issue. I have the exact same set-up on a 283, & after attending to the 2 points I just mentioned, the carbs ran fine. There's no reason why you shouldn't be able to break both those carbs and your engine in at the same time. Good Luck!
Leaking gas through the throttle shaft bores???????If it's the throttle shaft with the throttle plates then you have a problem..The throttle shafts are most likely worn.. Duane.
Yeah, it's the throttle shafts with the throttle plates. They're all a little wet where they come out through the sides of the carburetor. Like maybe one drop of fuel oozed through there between the shaft and the casting. Fuel must be splattering on top of the throttle blades and oozing out around the loose fitting shafts. Probably a sign that these carburetors need some new shafts? Either that or I have flooding in all three carbs, and it just weeps out because everything is soaked down in there. I ordered a Holley 12-804 regulator that has a range of 1-4 psi. I'll try that first and let you guys know what happens. Thanks.
You sir, are a victim of flooding, there. Try running the carbs with an IV bag of gas and see if that cures it. If it stops seeping on a gravity feed system, you know that was a big problem. ~Jason
There is some smart thinking going on here. Couple more things to check. Just because someone says the carbs have been rebuilt doesn't mean everything is "kosher". Fuel pressure, float height, tight float seats, clean needle-and-seats and floats that float (not pin-holed or partially saturated) are of paramount importance. End carbs: be sure the throttle plates are centered in their bores when closed; center carb: all idle cicuit passages must be clean, as well check the gasket between the cast iron throttle body and the potmetel mainbody - all holes must line up along with no stray air leaking into any of the idle-lowspeed passageways. (ran into this on a 350cfm Holly - mechanic did not realise he had installed a 500cfm gasket - screwed up idle-lowspeed mixture big time -leaking stray air into idle circuit) Got that sorted out, the 454bbc tri-power, mild street cam idle came right down to 650rpm with a nice steady rumpety, rumpety at the pipes. Don't let the Rochesters scare you . Aside from the accelerator pump and power valve, the idle circuits work on the same principle. Take care of the fine details, stay persistent, ask the well thought-out questions and you will win!
I do gotta agree there, man. If some yahoo says they're rebuilt, taking his word is like firing a weapon in combat you didn't load yourself, you know? Only take their word if you really, really, really know said yahoo, and know them well. I'd buy 'em as a 'core' and just build 'em. After reading that Rochester book I told you about, it really won't seem like a big deal... and it's really not. Besides, after building 'em yourself, you really do have a much better "feel" for what's going on inside, and how to fix/modify/troubleshoot it. ~Jason
the first time i put my flatty from one to 2 carbs (progressivly working to 3, which it is on now). it would run right. it ran fine on one carb, but not two. ended up one of the power valves was sticking open, very hard to keep it running (even harder to get it started). i would suggest going back to one carb, set everything up there, then get 2 carbs running, and readjust, then get 3 running. it just a failsafe to make sure everything is working properly. always best to start an engine as stock as you can get it, or with proven parts (ie run on your friends engine, i swaped dissys with a friend to see if my dist wasnt working for example). onceyou know one carb, and the dissy work, get the second one working, if it quits working, you know its the second carb, then the same for the third. takes a bit more time to do, but if something doesnt work right, its alot easier to track it down this way. hope this helps
Sometimes a touch of crocus cloth or emery paper will make those power valves smooth as silk, both in the bore and on the piston. No carb build passes my bench without it. ~Jason
Update: I just remembered I left this thread kind of hanging in air. I just wanted to thank all of you who gave me a lot of good pointers. After a lot of troubleshooting, I came to the conclusion that the end carbs were acting like huge vacuum leaks, and all three carbs were flooding occasionally. I wound up rebuilding all three carbs using the kit from Speedway that comes with new bases for the end carbs, and new jets, modified power valve, etc. It runs like a champ now! I can idle at 700 rpm no problem. I have the progressive linkage hooked up and working like it's supposed to. I'm using a regulator that keeps it at about 4 psi. The kit from Speedway is great. The instructions that come with it are a little hard to follow, but make sense once you figure out what they're talking about. Thanks again guys, and I appreciate all your comments and help.
Happy to hear this is working well for you. I'm getting used to three deuces right now too. Question - wouldn't a vacuum leak lean out a motor? Seems to me you would be sucking in air when you shouldn't be with this type of problem. I'm interested too in the fuel pressure deal and the return springs on the front and rear carbs. Hadn't thought of the springs and my fuel pressure is up around 10 PSI. Looks like I need a regultor too. Thanx for the info.
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