70 ys power upgrade solutions - Pontiac GTO Forum
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post #1 of 12 (permalink) Old 07-11-2009, 04:34 AM Thread Starter
 
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70 ys power upgrade solutions

can anyone make any suggestions on power upgrades without ripping motor apart?i have a 70 gto w/ a ys 400 670 holley street avenger 355 gears but something seems to be missing.motor has 44,000 original miles,oil pressure great,runs tight,no vaccum leaks,but theres a little hesitation if i punch it at 20-30 mph.it feels like maybe the mechanical advance isn,t working like it should.i advanced dist.far enough so its hard cranking hot.maybe i need looser springs in dist?its hei with an accel super coil cap also changed plugs,exaust is 2" with thrush muffs.pipes to small?i feel like its a distributor or exaust issue.also, i added the complete ram air set up,although i heard its not that effective at lower speeds.most of you guys seem to know your stuff so any advice would be appreciated.thanks.
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post #2 of 12 (permalink) Old 07-11-2009, 05:47 AM Thread Starter
 
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also it has an edelbrock performer and i heard i need a carb spacer rather than just the thin gasket they supply w/ the 670 holley.is that true?
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post #3 of 12 (permalink) Old 07-11-2009, 05:56 PM Thread Starter
 
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come on,doesn't anybody have input?
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post #4 of 12 (permalink) Old 07-13-2009, 12:11 PM
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You need to back the timing off so it's not hard to crank. Over advanced timing hurts top end performance. It'll also kill your engine. The advance curve for your HEI is not calibrated to your '70. You need to do the math and get it set up correctly, or go back to stock. It had a points distributor originally. Your carburetor is too small. What happened to the original 750 CFM Quadrajet? you need at least a 750 cfm for your combo.
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post #5 of 12 (permalink) Old 07-13-2009, 03:45 PM Thread Starter
 
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i thought the carb might be too small but some idiot told me that a 770 was too big after i bolted it down so took it off but couldn,t return it so i wound up selling it for 150 on craigslist.as far as the original carb ,it was long gone when i got the car but had a leaky old holley on it.i bought a advance spring kit for hei and the springs are much softer than what was on there so i might try them to see if the advance comes in quicker but your probably right on about the carb although ill have to wait because i took my car to my tranny guy because of a clunking noise and it turns out the posi unit is shot .getting it replaced by gevos rears.so this power problem is probably the carb?
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post #6 of 12 (permalink) Old 07-13-2009, 06:37 PM
 
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A 670 is going to have better throttle response than a 750, so any bog issue is not carb size, but carb. Look into your accelerator pump and make sure they are fully actuating, my Holley had the pump cam mounted upside down and had a huge bog. With 3.55 gears and a good tune the car should jump out of the hole. Make sure all your tune up parts are quality and new. Time it by ear, til it pings, then back it off a bit. I put the lightest springs from the advance curve kit on my car back in the day and it was a tire smoker. High compression will make the car ping with less advance, but if the engine is worn, give it all the timing it can take. I put a 750 vac sec Holley on my 70 400 and get 10 MPG, I am going back to the 670. You only need the 750 if you are actually racing it-RPM. Stop light to stop light 670 is fine. Quadrajet will get better gas mileage, but if it's not stock, a Holley looks so much better.
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post #7 of 12 (permalink) Old 07-13-2009, 07:14 PM Thread Starter
 
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okay but shouldn't the carb be set up right out of the box?dont get me wrong,i can break the tires loose for 60-70 feet but theres a hesitation once it grabs.is that normal ?
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post #8 of 12 (permalink) Old 07-13-2009, 07:17 PM Thread Starter
 
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also,what spring should i be using in the secondary diaphragm?i tried the lightest(white) but it didn't feel right.
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post #9 of 12 (permalink) Old 07-13-2009, 08:06 PM
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Jetstang, he's complaining about a 30mph bog, not an off-the-line bog. That tells me he's starving for fuel. Steven, as a mechanic with 30 years of experience, I recommend fixing what's KNOWN to be deficient first, and then go from there. We know that a 670cfm carb is insufficient for a big Piontiac V8. Put a 750 back on, and then we'll have a baseline starting point for you to dial it in. BTW, I get 16-18mpg with my '67 400 Quadrajet GTO going down the hiway with a 3.36 gear.....(with my foot out of it, tho'!) Of course, all of this is just my opinion.....you'll do what you want to do!!!
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post #10 of 12 (permalink) Old 07-13-2009, 08:20 PM Thread Starter
 
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i have a feeling your right.as soon as the posi unit gets rebuilt,im going to change out the carb and see.could be an easy hundred for you,geeteeohguy.i'll be in touch.
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