Did a search but didn't really find what I was looking for. SBC w/ a freshly rebuilt tripower setup (94's) from Vintage Speed. New regulator set to 2 lbs. Electronic distributor w/ vacuum advance. Engine starts right up and idles well. The problem is that it breaks up very badly from about 1500 rpm's up to the point that it will stall if I don't let off. This happens in park or under load. Does this sound like ignition or carb /fuel related? I'm leaning towards ignition myself, but would appreciate any input. Just a little frustrated as each time I solve a problem another one rears it's ugly head.
Sounds like it could be the distributor. What kind of distributor? I don't think fuel would do that at that low of rpm or maybe bump up the pressure just a little. Good luck. Do you come to the Tuesday night cruise at Patz. Jim
It's an aftermarket distibutor installed by the builder of the car. Not sure what brand as I've not pulled it and there are no obvious markings on it, though it sounds as if I'll be pulling it in the near future. I bumped the pressure up to 2.5 lbs. with no change. As I understand it 94's don't need much more than that, correct? I don't get to Pav's often as my coupe was just starting to get dialed in when these new problems popped up.
I am into multiple carbs on INLINES, so I am probably an idiot, but most carb problems are ignition issues. check the ignition to be sure. put a vacuum gauge on it, have a friend check the dizzy. I am out on a limb here - you didnt say whether its a new rebuild or a running engine and you simply swapped intakes. Any more info??
The engine probably has less than 2k miles on it. Previously it was hard starting and ran very rich. Vintage Speed rebuilt the carbs and jetted them down a bit (53 down to 48). I installed a new regulator when I put the freshened carbs on. It fires right up, much better than ever before, and idles great, but it now has this new stumbling problem. Just for shits, where should I be pulling vacuum from for the advance? It is currently pulling from the front carb. This seems wrong to me the more I think about it, but I could just be grabbing at straws.
My gut tells me that it's a shiet distributor/ignition issue.... On the other note I've always pulled vaccum from the middle carb....my justification was that it's the primary carb.....if thats correct or not I dunno.....but it works for me.
That's what I'm thinking as well. The secondary carbs don't begin to open 'til half throttle, so it can't be pulling much if any vacuum until then. I'm gonna try moving it tomorrow and see if it helps. Thanks for the responses guys!
If it was me I would be running manifold vacuum on the distributor, and if you are sourcing it from under the throttle plates it should not make much difference where it is hooked up. Your above stated theory would apply to ported vacuum which I would not think you would want. I am going to guess that since the problem reared it's head after the carb work overs and was not there before, then your problem is either not enough fuel or carbs jetted down to much.
The reasoning i said that I believed that it was distributor/ignition related is that the OP stated that he was having issues (though not the same issues) prior to the carb rebuilds. In my past experience the issue he is now currently having I have experience and in one occassion it was a weak coil and the other was a distributor that was on it's way out.... Not to say that the COMMISH isn't correct, without being there with the engine in your face it's a guessing game really
Couple things I would do to eliminate issues would be to disconnect the progressive linkage and run only on the center carb. Assuming no air is being pulled in by the other 2 carbs(put your hand over the tops and see if ther is any suction) you willnow be running a simple two barrell carb. Run the rpm's up and look into the carb to see visually if anything looks funny like little fuel being provided. If problem remains I would I would change out the center carb to one that is known to run (if feasible) and/or pull that dizzy and coil and use one that is known to work. It is an elimination process....takes time and effort but you will find the issue if you can round up the spare stuff to assist you.
I'd stick the distr on a distr machine for a quick test, but it also sounds like you don't have any fuel thru the main circuit in the carb - when it is 1500rpmish and starts missing and breaking up can you smell unburnt fuel? it should be rich smelling, if there is an odor that kinda sticks to the roof of your mouth then that is from being too lean. I would question whythe carbs got jetted down, the fuel is metered at the proportion of air going thru it, the center carb will do most of the work (and needs jetting) until the other carbs open and you have 2 more sets of jets to provide fuel.
I'd be checking inside the dist. for loose or worn insulation that can short out when the advance moves inside the dist. Ive had it happen twice. Once when the insulation allowed the wire to short only after the advance moved. My truck would fall on its face and then pick up if I kept my foot out of it. It turned out to be a way too heavy wire to the points that would lose contact until the advance advanced enough to make contact again. I it idles well and only comes in above 1500 RPMs I'd look at the ignition first.
It is definately rich when this problem is occuring, which is why I'm leaning towards an ignition issue. It really seems to me that pulling vacuum for the distributor from the port on the front carb is not right. The secondary carbs don't begin to open until half throttle, so it can't be pulling much, if any, vacuum until that point and this stumbling is occuring well before half throttle. I tried the old trick of sucking on the vacuum advance hose and you can hear it working and it holds vacuum so I think I'll try another vacuum source today. I'll post my results later. Thanks again for the input fellas, it's much appreciated!
Just playing devils advocate here, but could the progressive linkage be misadjusted to the point where the secondary carbs are dumping too much fuel at too low an rpm?
Have you done a search on vintage speed? Our buddys car was running like shit and long story short we had to rebuild his two 94's. One of them was junk. It had mismatched jets a blocked off powervalve and one jet plug was glued in it because it was stripped. he got them from vintage speed. I would check the float levels. Do you have a quality fuel regulator? those cheep ones with the dial are junk. also yes you need manifold vacuum for the dizzy. Did you check your dwell or points gap if it has points? that can throw timming way off and give symptoms close to that.
With all due respect, you haven't really given us a lot to go on. As others have mentioned, rich or lean?? If the engine is running so rich as to break up and stall, as you are suggesting, then someone standing 15 feet behind the car would not be able to see the car for the black smoke! True, or false? Conversely, a really lean mixture will produce an exhaust that basically smells like rotten eggs. As to fuel pressure: not knowing WHICH model 94 or the size of the fuel orifice, or the price of tea currently in China makes a definitive answer difficult; HOWEVER, the specified NORMAL fuel pressure for a 1952 Ford V-8 with Holley 94 is 4 1/2 psi. As to jetting: the NORMAL main jet for the above carb for a 239 CID is 0.052. Don't know the size of your small block Chevy, but probably larger than 239. Why would you install 0.048's for a larger engine? If the carb truly is overfueling, the first place to look is the neopreme economizer (a.k.a. power) valve. These don't like ethanol laced fuel or any type of octane booster. If the valve developes a leak, the fuel simply runs through the leak. Where do you go from here? I would suggest REMOVING completely the two end carbs and installing block-off plates (this eliminates two variables). Now try to tune the engine on the center carb. Keep us posted, but a wee bit more information please. Jon.
I still think you are misunderstanding how your vacuum source works, if it is drawing from below the throttle plates then it is manifold vacuum and will be the highest at idle, and in my opinion you should be running your engine on manifold vacuum. The only way it would be higher when you really have your foot in it, is if it has ported vacuum drawn from above the throttle plates, and in my opinion you don't want that anyway.
Check the secondary carbs & make sure there NOT open a little! Even one not sealing right right will fuck you up. I ran into the same problem on a 440 with a "six pack".Also if the problem came about when you had the carbs done doesn't it make sense that it is probably the source? I never noticed much differance in how a car ran with or without a vac advance! I think they are over rated & not needed.I haven't on my car for the last 40,000 miles. One other thing you can take the tops off of 94's & run them to see if everything looks OK. JimV
Ok fellas, I finally have an update. After further review by myself and a buddy I was thinking fuel problem. I decided to change the jets in the center carb because it was the easiest and quickest thing to try first. I changed them from 52's to 53's and it seems to be fine now. I didn't think that small bump up in jet size would make that big a difference, but it seems to have done the trick. Keep your collective fingers crossed for me. Thanks for all the feedback!
You CAN NOT use the vacuum source from the 94 for your dizzy. Pick up a manifold full vacuum source and your issue will disappear.
I had done that already Dick but the problem persisted. I never had this issue until I got the carbs back from Charlie at Vintage Speed. He rebuilt the two that were giving me fits. It started and idled great, but then this new problem showed up. Changing the jets was just a quick shot in the dark and for now it cleared up. I'm still stumped, but for now it seems to be working. This car seems to have it in for me at times .
If the distributor is still in question ship it to us. Will run adjust etc the day i get it and return. You are one day away. Then at the very least you know the ignition is either good or bad.
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