davep_96

Hey guys I just need some tuning advice. What are the basic things that need to be tuned on a rebuilt engine? The carb was set up enough to get the car to run, and the timing was also set so it would run, and the car does run decent but I feel like it should/could be more powerful and I know there is some basic tuning that everyone does to dial in the engine and get it to its peak power capability. I just dont know what that tuning is. Any help would be great.

Thanks,
Dave

cambirdracing

iTrader: (4)

Put about 32 total timing in it. Get a bigger carb, around a 750. What gears in the rear?

davep_96

The bigger carb is comming. Holley 750. Also the gear is a 2:73 which I know is not ideal. I am about to buy either 3:73 or 4:10, not sure which yet tho. How much of a difference will a gear change actuall make though?

All I know is most dyno runs ive seen on tv and such the first run a newly built engine is always way donw on power and they tune some things and by the next few runs the thing gains a ton. I figure mine is low and could probably gain some

STOCKTA

iTrader: (8)

Gears will really pick up a tired feeling combo and also a good holley will always make more power than 90% of your off the shelf carb choice's. Also a good quality stall will make more difference than any of the cahnges your making. You have some nice breathing heads and decent cam so it will really love a 750cfm carb once you have it dialed in.

cambirdracing

iTrader: (4)

700r4, I would go with a 3.73. They had a lower first gear than a th350 and the o/d isn't so much that it would bog the engine at highway speeds.

davep_96

Yeah I really need a carb swap. Just didnt have the funds last fall when I swapped the cam. And the gears also, because I think the car would wake up a bit if it actually got into the powerband quicker.

The car as is, is decently quick. Havent made it to the track thought. But it just seems like it should be pulling harder than it does. Aside from the carb and gears, is there anything else I should take a look at? What about fuel pressure? I dont have a regulator or guage yet but I have a feeling the presure is too high because the fuel pump (mechanical) that is on there I think was rated around 7-9 psi. How much would that be draining power, since Holley Carbs generally want I believe 4-5 psi?

1 FMF

iTrader: (1)

the 4-6 psi fuel pressure is just a rule of thumb for carbs. You can generally run up to around 10 psi with no problems on most carbs, what will happen when the fuel pressure is too high is it overpowers the needle in the bowl resulting in a higher fuel level in the bowl which (depending on the carb and bowl vents) either makes for a richer mixture or dumps raw gas into the intake flooding the engine. If it idles fine at low rpms, and that's when fuel pressure will be highest, then fuel pressure is not too high. For power you actually want as high a fuel pressure as possible because that's what will replenish the bowl and help keep fuel at a constant and correct level especially when you jam the throttle wide open. Having too low a fuel pressure is bad because that will lead to starvation and lean out, and having too low a float level in the bowl (resulting in too low a fuel level in the bowl) also results in a lean mixture which is bad for power so you want to at least make sure the fuel level in the bowl (a.k.a. float setting) is correct, this is always step #1 in carb tuning because everything runs off float level. And when tuning the float level setting spec'd by the carb maker is not a must, and many times adjusting the float so fuel is slightly higher in the bowl (richer mixture) solves a lot of problems especially hesitation when opening the throttle at low/mid rpms.
with a rebuilt engine, i would verify that valve lash / lifter preload was set correctly and that will manifest itself when you check vacuum readings at idle. without knowing what carb you're running i can't go too far into carb tuning but for starters you want float set correctly, then get the engine running and warmed up, set your timing to what you want. And know depending on what ignition system you got, disable any electronic or vacuum advance because that will prevent you from setting the idle mixture, you'll end up turning the mixture screws a whole turn each way and notice no difference because the ignition may be adjusting timing to compensate. Have the base timing set & fixed at idle, should be between 8-16 dBTDC, start with the idle mix screws about 1.25 to 1.5 ccw from seated, slowly turn one cw leaning it out till you notice the engine idle dip/stumble and remember mixture screw position. Now turn ccw to richen, past original position till engine gets too rich and idle stumbles, note this position, then set the mixture screw halfway between the lean most and this rich most position. Do this procedure for both mixture screws, one at a time or do both at same time if possible. The mixture screws should be even- you shouldn't have one side 1.5 turns and the other 2 turns, or one shouldn't be more than 1/4 turn different. And you sometimes can also verify idle mixture with the vacuum gauge monitoring manifold vacuum, you'll want to adjust mixture to give highest vacuum reading but this only works on engines with a mild cam giving a good solid idle. And to help with power and wot stabs, it helps to have idle mixture slightly on the richer side either from the idle mix screws or raising fuel level in the bowl. after that, it's onto accelerator pump cam & squirter sizing, and secondary metering block changes or vacuum secondary springs if applicable.