Holley 6299 on a 256 Merc

Holley 6299 on a 256 Merc

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By Bruce Compton - 12 Years Ago

I have a Holley 6299 (with a power valve) to try on my 256 Merc.. It came with #49 mains and I'm thinking they will be a little lean. Has anyone tried one of these before and found the optimum jet size?? Thanks : Bruce

By Ted - 12 Years Ago

I see two versions of the List #6299 in the Holley Spec Manual with one of them being a model 4150 (full rear metering plate) and the other being a model 4160 (List #6299-1). Both come factory supplied with #50 primary jets while the model 4150 version also comes with #52 secondary jets. As a general rule, always first try the carbs with the original jetting before doing any tinkering with them.

By Bruce Compton - 12 Years Ago

Thanks Ted. Good common sense rsponse so I tried it the way I got it. Really lean, with major hesitation on acceleration with sputtering. I then installed #51 main jets and tried again. This time it was better but still had a hesitation. Took carb further apart and installed a new 8.5 power valve and #54 jets, and verified the fuel levels . It's almost O.K , but still feels a little sluggish on acceleration, but it idles absolutely perfect, almost as smooth as my previous flathead and by far the smoothest Y block I've ever had . Thanks : Bruce

By pegleg - 12 Years Ago

Bruce, Not what you asked but dump the Loadamatic distributor and install a later unit with mechanical advance. That'll solve your sluggish issue. Be careful to use the proper distributor gear when you do, the later ones are (probably) different from your 256.

By Bruce Compton - 12 Years Ago

Thanks Frank, but I have already replaced the loadamatic with one from a '64 Ford FE by changing the lower section of the shaft with the gear and tang type oil pump drive from the original one. I had a nice looking fairly new Holley 8007, 390 cfm on it and it was working O.K., but I found the base plate was cracked so replaced it with the one on it now that is giving the sluggish feeling with a minor hesitation. Bruce

By pegleg - 12 Years Ago

I'd start looking for a 390 baseplate, sounds perfect for you 256. w00t

By Bruce Compton - 12 Years Ago

I think that the 6299 and 8007 are both 390 cfm and I'd like to get the 6299 working right even if only to have it as a spare. Apparently the replacement base plate sold by Holley is generic and not correct for the 8007. You're right though, the 390 is as close to perfect as you could get for a 256" motor. Thanks again: Bruce

By pegleg - 12 Years Ago

i may be all wet on this, but it seems as though I remember the 8007 was intended for V6 and other small engine applications, I know I had one on a 3.8 Buick V6 that worked well.