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460 not performing as is should. carb?

7K views 13 replies 5 participants last post by  jr4spd 
#1 ·
Hello, im new to this forum and I need some advise from a few tried and true engine builders. I'vebeen building and helping friends and family build engines and hotrods since I was 13, so i only has 14years but im more familiar with sbc and sbf.anyway... I've been helping a buddy build a 77 f-100 for his father inlaw. We built a 72 460 all forged rotating assembly, flat top pistons, a set of 1968 429 thunderjet head which have been reworked for bigger cam, and the have screw in studs and guide plates, all comp cam valve train including, big mutha thumpr hyd flat tappet cam Kit, a set ofcomp1.72 roller tip rockers,weiland stealth intakes 750 Holley, vacuum secondary carb, hedman headers, 3in xipe w/ flowmasters, truck has a c6 with shiftkit a 4.10 gear with 32x10.50-15.

Ok so the problem is I think the carb maybe too small on the jetting or cfm.the truck just doesn't seem to making power it should (v8 mountaineer launches better) and the thing could hardly do a burnout, and didn't start pulling till about 3,000-3,500 rpm. So I did the first thing I learned about modified engines, add i checked the plugs. They were all the same color as one other, but were really light , from being lean. I bumped up the jet from 72 to a 76, helped alil bit (it would do a little burnout but wouldn't rev past 3-000rpm in first gear. The timing is pretty mich spot on and didn't change performance for the better I have it about 10 degrees right now so I can get it to start. The carb idles good revs up great in park, it was brand new right out of the box, . Any input would be appreciated, (going broke buying carb gaskets and advil)
 
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#2 ·
10 degrees timing what is the total timing? try 14 degrees,also what size fuel line is going to the carb? dont really care for the weind intake a stock intake will outperform it up to 3700,it should kill the tires it sounds like its running out of fuel,or not enough total timing
 
#5 · (Edited)
3/8 line from tank to carb, the fuel pump is a 100 gph pump. I re set timing to 14 after I originally post the question. How do you get overall timing reading on this the balancer starts 10II can't see any other numbers just the dashes. I tried scrapping all the paint off to bare metal and they are still hard to see thank you
 
#3 ·
68 Thunderjet heads with flat top pistons must be quite high compression. You'll have to run way retarded to run pump gas.
 
#4 · (Edited)
Not a "tried and true" engine builder here but I'll try to lend a hand. That cam is really a car show grind. It has a ton of overlap for the "thumper" sound but tends to lack in performance to really back it up. Either way, the engine will want 4 to 6 degrees more initial timing and be sure it's got 32 plus for total. You may be on the edge here with your compression and pump gas as stated and not be able to run the total it really needs to perform to its potential. Your cam may be actually helping here as it bleeds some off but again it don't do anything for ripping power..

It's also likely your fuel system is inadequate. What size lines? What pump? If stock they are too puny definitely. Been there, done that. It rev's in park but with a load it don't pull past 3k in gear? Sounds more and more like fuel starvation. Feed it!
 
#7 ·
Thank you! These are answers I was looking for. I've never tuned a bbf so I was unfamiliar where the carb should be. The owner bought that cam I would've done way better, and he used the carb calculation to get 750cfm. I was told by a few bbchevy and 440 mopar guys to put a 850 on it.. I figured it wasn't getting enough fuel from primary cause after 3500 rpm it starts pulling as if it weree only half throttle. And did have the timing set at 14, after I posted this not much change. So we bumped it more and it spark knockedlike hell and could barely turn over
 
#8 · (Edited)
The 750 has 76 jet in the front and the rear plate is equal to a 75 jet. I might try the the 780 that is on my truck but I was debating on putting a rear metering block on instead of the plate so I can change the rear jets. Also the secondary spring is the plain one, think maybe I should try a lighter spring before switchin carbs?
 
#10 ·
Yea, you'd have to recurve the dizzy so it take some mechanical out of it. Then you can run more initial and still have the total beneath the knocking.

I'd bet it needs a bit more jet, but I'm thinking the fuel line feeding it is too small. I fought this in my ride as I built it up. With a 393ci Cleveland, the stock 1969 fuel lines killed the top end. Replaced it with a 1/2" aluminum. Solved. Then I swapped in a 545ci BBF. Again, it seemed to starve when I sprayed it (not good!). 10 AN and an A1000 fixed it.

What type electric pump?

I'd think a 1/2 inch line, a good fuel pump and a recurve.
 
#11 ·
Thank you for the help. I ended up borrowing a friends 850 and it pretty much solved the problem. Although his carb was calibrated for his bbc 454, so I couldn't play with jetting . ButI got a new 850 vac sec and I'll play with that one when it warms up, it's 0 degrees in Detroit right now
 
#13 ·
What are you talking about? Never said any thing about Jets changing cfm. And no t we I engines are the same every engine rubs different, Andi said it don't start pulling(as in picking up speed till 3500. The only reason for the pos t was to get an opinion on which carb should work best.Either way the 850 on it rubs great lots of power.
 
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