Hello, We just bought a 1969 396. I know very little about how it was built. It has a Mallory unilite (no vacume advance) dizzy. Edelbrok single plane intake, Very lumpy lumpy cam, At idle 1000 rpm it has about 6-7 " vacume. It has a 750 vacume secondary carb. Hooker headers, 4 speed, 3.08 gears are not helping but will rebuild the 12 bolt this winter and add some 3.55's Now the problem is it has a miss while cruising and breaks up at full throttle. It has new plugs. I think it is running lean on us. Main jets are 73 power valve has no numbers. I think it was bought new and put on with no tuning. Could I get some ideas on where to start tuning. Main jets, power valve? Thanks......
i had the same problem with one of my cars it ended up being the new coil was bad. on acceleration the jets have no part of the circuit it is all the accelerator pump. jets only come in to play while cruising. what is your total timing? i would start there.
Usually the 750CFM Holley,R3310-A is the most common over the counter it has a 10.5 power valve.Based on your vacuum reading I would swap it out for a 4.5 power valve and try it out before you change jetting,the breaking up may be in the secondaries,get the Holley vacuum secondary spring kit # 20-13 and try out different ones,Holley part# 20-12 is a 8 piece pump cam assortment you may want those also,try bumping the jets up to 75's to start after trying the other changes if needed. The Holley website also has a lot of tuning tips you can download and print.
What do you mean by this? You are cruising at steady throttle and you punch it and momentarily misses? If so it could be you have too heavy of a vacume secondary diafram spring causing the secondaries to open too slowly making a temprary lean condition. Holley sells a kit containing a assortment of springs for adjusting. EDIT: too slow, JeffB2 beat me.
It has a miss while cruising, steady throttle. It has a smooth transtion to full throttle but starts missing at about 3000rpm like its running out of fuel..
Where to start tuning? What is your timing set at? Initial, total, and at what rpm? Someone should have asked for the Holley list# on the carb. Gotta know what we are talking about before ya get too far.
huh? jets and power valve are definitely in use during acceleration. Lumpy cam usually requires a bit richer idle settings, more pump shot, maybe bigger jets to run right. I have a thing against unilites, so I'd suspect that too....but that's just me. Tuning a mismatched combo (gears and cam are not working with each other) is difficult.
I agree with a few here . We really need to know more about the car . The 3.55 rear gears are OK but you will be better with a 4.10 with that big cam . Is the carb a single or double pumper ? Yes to the timing and the advance like someone said . Idle a 1000 RPM's seems to high . Should be down around 700 to 800 rpms . On big carbs I don't like vacuum secondaries at all . Sounds like the power valve isn't working right . Also check your float and make sure that's set right too . Also check to see what size pump diaphragm it has . That will be on the front bowl on the bottom . Another place to check is the vacuum to open the secondaries . Make sure it's connected right to the intake . With Holley's , these are a few things to check when you have the problems you have . The big cam doesn't help matters either but can be taken care of with a few things to get the carb set up for the engine set up you have . Retro Jim
First off Does this carb have a secondary metering plate or a jet plate. It sounds like you are lean all around. Changing the power valve will do nothing for cruise miss, It is a power valve - it transitions from the idle circuit to the power or main circuit when vacuume drops below the set rating of the power valve. There could be soooooo many things wrong with this carb it would be really hard to diagnose here. My suggestion is to borrow a carb from a buddy that you know works. Put it on the car and drive it. If every thing works cool then dig deeper into the carb but if it does not then go to the coil or the Dizzy.
I've never had a Holley that took massive jet change from the factory to make it run right. I'd look somewhere else. It does seem like they could use a bigger squirter than the factory provides. I usually end up with somewhere around a 31. Other than that, I'd check to make sure the carb was right (float settings/no plugged passages/idle mixture right/etc.), check for vacuum leaks and then move on to the rest of the tuneup-timing/etc. Larry T
I agree with Larry here.The Jett thing is a fine tuning thing with small changes in most cases unless they were waaay off.I suspect the idle mixture is way off or there is a problem with the floats set too low or a fuel passage blocked .Holleys/Demons are great carbs ,but real touchy and not very forgiving .I would pull it apart and clean it real well ,then put a fresh kit in it ,reset the floats properly ,and go from there.I set the mixture screws by ear first then follow that with a vacume gauge hooked to the manifold vac. port on the carb .I keep backing them out till it has the highest ,steady ,vacume reading .May have to repeat this process a few times to get it dialed in.If its lean missing at all rpm ranges then i doubt the power valve or Jetting is the problem .As said a vacume leak at the carb base or intake will give you fits and act like carb problems as well ...
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