Most of the time, when you see jet sizes listed for someone's carb, they are 68/72 or within 2 or 4 digits of each other. Since my car runs fine on the primary 68, but seems to starve when the secondaries open, even a little with the 72 jets, is it OK to keep bumping up the secondary jets and leave the primaries alone? Even if I get in to 68/78 or 68/80 territory? Or do I need to keep the primary and secondary jets within a certain range of each other?
Yes, Scott, it is ok. You can up the jet size until it runs right. Engine just needs more fuel at upper rpm.
Yeah, 10 sizes is pretty common. I had a carb, forgot the size, that liked 70/80 on a 306 with F4B intake.
Cool, thanks for the quick responses, guys This new engine gets quite thirsty when the secondaries open up...and that is just fine and dandy with me cuz it runs MUCH better than my old one
I used it briefly right before the engine cratered. I had the 600 edelbrock on there most of the time. I put this on for about a week just to see if it worked before I bought it.
Before you "bump" the jet size find out if it is lean on top or if you are just opening the secondaries before there is enough air flow through them to pick up the fuel. This is a problem with mechanical secondary carbs that are fitted for maximum flow. You might be better off to open up the power valve earlier or open the power valve channel restriction one size at a time rather than have a carb that is running too rich anytime the secondaries are open. You can wash the oil off the cylinders by running too rich and that will destroy all the hard work and money you put into your engine. To be sure a run a chassis dyno will tell you what you should do but you can also just use the Holley guide for mechanical secondary carbs. They will take the CID of the engine and the RPM that you open the secondaries and tell you the CFM of the carb that you should use. Most of the Holley double pumpers are running in the rich zone out of the box.
Mine runs 68s in the front,99s in the rear with jet extensions, 125 thousandths power valve restriction. (with a high flow power valve). So yes you can run any combination that works. But as PaulS stated using the power valve circuit may be the best way for your combo. That way you will have fuel enrichment in three stages instead of two.
I do agree with your post Paul... I just have to knit pick this one statement. My personal experience is that Holleys are lean from the box. Not fact... just my experience. Your engine may experience different results.
I have to add to both of you. You are both right one thing I have noticed is that most people get them for incorrect altitude, they just buy what is on the shelf at the auto parts store and do not realize that they are calibrated for sea level so it can be dependant on if it is jetted from the factory for sea level or your altitude. (if you talk to summit or jegs I have found that sometimes they will ask you about the altitude when you order)
If you don't have a secondary power valve. The bigger rear jets make up for the lack of a rear power valve. Holley says the power valve is worth 9 sizes. So if you have both primary and secondary power valves the front/rear jet sizes will be pretty close to each other.
This brings me to a question... I have always wondered about running a primary block in the rear in order to get a PV back there. Just never got around to it. Is this what I need to do to get it done, or do I need a special block? What do I do with the idle screws? I don't really care to have 4 corner idle... Just seems like excessive tuning. I like the KISS method. Also, if a 4.5 is the front PV, wouldn't the rear be the same? Thanks! Dave
Running a double pumper on a street motor brings with it a bunch of tuning problems that could be alieviated by staying with a well set up vacuum secondary carb that is springed comes in at the right time. 1. finding a pump shot curve and nozzle hole size. 2. jetting. 3. what will the motor take? 4. more specific application oriented. With a double pumper you can't stomp on it at low rpm and expect the motor to jump up and go. It offers to much open area, the airflow goes to near nothing for an instant and that can't draw any fuel no matter what jets or power valves are used. IMO it foolishness to run a DP anywhere but a race application where the RPM is already up off the line as long as the motor dosn't bog, you good to go. That's not a street application. Saw to many try it and took them off.
Not true. Don't know where you got the info that a DP doesn't respond. You mash one of my DPs and they scream! I don't know everything about them, I am still learning after using them for 20+ years, but I know how to make them respond.
