Power Tuning

[PMS]

I am trying to get the most performance from a 72 LWB GMC. We have installed a new GM 350 HO w/ 330 HP and 380T. We have been playing with the timing and carb. Should you be able to kill the engine by placing your hands over the carb? Also we changed the jets from 78 pri. to 73 and sec. 78 to 75 and installed a lighter sec. spring. Now when driving the truck you can feel the sec. kick in but from a dead stop we can't get it to leave very hard - only about 10' of tire spin. The truck has a 3.42 rear with a 2800 rpm stall. Shouldn't it destroy the tires from a dead stop? The dist. advance is set-up with the second fastest curve possible. Do I need to change accelerator cams or what? Any suggestions would be helpful. Initail timing at 12 deg. with max. 28 deg. in at 3200 rpm. Am I missing something?

[Stroker]

Most 350s with late model heads like 34-36 deg total centrifical advance. 28 is probably costing you. What type of carb?

[cableguy0]

i agree a little more total advance would help you. and it would be best to have it all in around 2800-3000rpm

[PMS]

Thanks for the suggestions. The carb is a 650 holley vac. sec. We also changed the power valve from a 6.5 to a 8.5. Anyone else on this board use the GM 350 HO motor?

[Harold Shepard]

According to the holley manuals yo should not be able to feel the 4bbl kick in as you would a q-jet. When you feel the kick you are experiencing a lean condition- contrary to what is believed as a rich condition. You may have the springs tool light. Why did you change the power valve- the reason I ask is if you changed w/o checking vac. then it was pointless. The engine vac determines the power valve choice in the carb. Play some more with the timing as well. I assume you are using vac. advance as well, you might loo into a crane adjustable vac. advance to tune to your particular combo.

[PMS]

I changed the sec. spring to see what would happen. I know I need to go one spring heavier since I can feel the sec. come in. I changed the power valve because we drove the truck at different cruising speeds and the vac was around 11". The original power valve was 6.5 so I went up to an 8.5 because that was the only one the parts store had. The distributor vac. advance has a max. of 10 deg. of advance at 15" of vacuum. I feel the truck should have more low end power than what I am feeling now.

[gmc67swb]

The book that comes with the motor tells you you should have 10 BTDC at 700rpm with the vacuum advance disconnected. You should have 32 total timing and I believe it should all be in at or before 3000. I have the same motor and the timing to me seems conservative. I have a speed demon carb with vacuum sec. and I too can feel the secondaries open...bog. I talked to the tech and he said the timing should be at 16-20 BTDC to eliminate the bog. He said the distributor should be recurved to compensate for all the initial timing. I went ahead and tried the timing and it helped but did not eliminate it. I wish I got mechanical secondaries now. Also, the motor didn't ping or clatter with the timing advanced and no hard starting. Good luck.

[crazy longhorn]

I agree with the rest of the guys, you need more total timing. as a rule, I have seen the sb chevs like 34-36 degree total, & usually all in around 2700-2800. Your 330 hp crate should have Vortec heads, & from my understanding, it will run best with around 32 degrees total. you need to recurve the dist (HEI?). most run around 20 degrees mechanical advance, so 12 initial should get you close to where you want to be. On the vac timing, i would go for a Crane adjustable can, & run the hose to full manifold vac @ the carb. set the vac can up for around 16 additional degrees advance, under part throttle(that will get you around 48 degrees under part throttle cruise) .......this will give you that "kick", or ripped up tires that you should have! you will also get much better mileage with the vac advance set up this way I wouldnt worry as much about intial timing.....more concern with (TOTAL timing). Total is with the vac can unhooked & plugged(use a balancer tape, or dial back lite). i like to try to set the initial where the eng "lites off" the best....then adjust the curve around that. That is a little tough on the HEI dist, as the mech advance(amount of advance) is not adjustable without welding or grinding. the curve that my 383 seems to like the best, goes as follows.... 9 initial, 35 total , all in @ 2700....+ an additional 14 degrees on the vac advance (plugged to manifold vac source on my edelbrock). this gives me 23 degrees advance @ idle, with the can hooked, & 49 degrees@ part throttle @ 2800-3000 rpms(old pionts dist converted to pertronix). they all tune a little differently, i would shoot for a little less on the inital, than i run with my Edelbrock heads, but you may go a little more on the vac timing(50-52 degrees part throttle is not uncommon on 9-1 comp ratio). just listen to what that little chev wants......& let her have it! best of luck,crazyL

[Stroker]

Here's a link that might help you tune your Holley.
http://www.bob2000.com/carb.htm
Most of the actual tuning information is toward the end of the page.

[PMS]

Thank you guys for the information. I will try again this weekend and let you know what happens.