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Topic Review (Newest First)
04-16-2006 02:53 PM
19nova72 i never got a chance to play around with the vacuum advance much. i had an adjustable crane can on it and i had it setup for the 305 in the car prior to the 400.

Unfortunately, i lost the cam in the 400 after about 5 miles. it was comp xe268. about half the lifters went concave. i was pretty pissed since i put basically all the money i earned with my summer job last summer into buying the complete 30k mile motor and doing heads for it, cam, intake, carb, noisy gear drive, high pressure melling oil pump, aluminum water pump, etc. altogether i had about 2000 into the drivetrain.

now im about to take it out (as soon as im home from college for the summer) and drop in a 71 350 for the time being.

ive been reading a lot of articles on excessive flat tappet cam failures as of recently. theres been lots of things contributing to this. i think i had problems with the break in because i used stiff springs that came on the heads, comp cams red lube that came with the cam didnt stick well at all and kind of just ran off the cam (next time im using moly lube or comp cams special lube) i did have the valve adjustment right but i also used cheap oil and now that ive read some articles i realize that flat tappet cams need high zinc content which almost no motor oils have anymore because all new cars have roller cams. ive also heard about comp cams having quality control problems and having a high amount of soft cores. also some lifters are from overseas and are not well made.

either way i now have to disassemble the 400 and see the extent of the damage. i also managed to break a pushrod which was probably due to a backfire from the failing cam and the unforgivingness of the gear drive on pushrods (read that gear drives add up to 70% more stress on pushrods). i will keep posted my progress after i get home from school and start work on the car again.

thanks for the help, i have spent a lot of time playing with carb adjustment and vacuum and mechanical advance and managed to get a very strong running 305. i will now see what i can do with a puny vavle (1.72 intake) 350.

04-13-2006 06:48 AM
hotrodf1 You mention that your vac. adv. is already connected to the manifold port, which is good. Do you know the specs on the can you are using? In order to make the most of the manifold vacuum setup, I had to use Crane Cams' adjustable vacuum advance kit (which also includes a vac. adv. limiting plate that is VERY useful).

The goal here is to get the vac. can set to peg out below the least amount of vacuum (which is at idle in gear of course). I set my 406 to 18 initial and with the vac. advance it idles with about 33 degrees total. The motor runs great like that (and the throttle blades are almost entirely shut). My 355 was the same - Crane Cams adjustable kit, 15 initial, for a total of 32 at idle.

The adjusting plate that comes with the kit is nice because you can limit the amount of vac. adv. that you get, and then you use the screw in the vac. port to adjust the "Hg that allow it to open.

All this will help your fuel economy too. I've found a lot better mileage after setting the vac. adv. up well. Although both motors still run a little rich at idle- but never load up or foul plugs.

Another thing: I drilled holes in the 670 HOlley for the 355 - it actually didn't seem to do that much for me. I had better luck getting the idle correct with the vac. adv., but the holes don't seem to have hurt me. The 406 is much hotter, only has a 750 holley, and I cracked the secondaries just a little. Have the primaries almost totally shut here, too, made possible by vac. adv.

Good luck!
02-11-2006 06:41 PM
StevenC I wouldnt change carb or drill yet. You shouldnt have these problems with your size carb
02-11-2006 06:23 PM
57 Chevy Don't drill anything unless your real sure you know what your doing.
This is an area for pro-tuners or at least their guidance.
02-11-2006 05:47 PM
19nova72 the vacuum advance is hooked up to manifold vacuum and the other port on the metering block is plugged. as for the timing i will prob try bumping it ahead to 15-20 as you guys recommend. i thought about modifying the carb or getting a bigger one because i thought the same thing that the carb is idling on the main metering circuit and i need to let more air by.
i dont know the affect changing the timing will have though. i only have the idle speed screw up this high because it needs it to run because the motor will not idle which still leads me to believe i have a vacuum leak. i still havent got any time to start up the car and run it again though so i havent gotten any further. by drilling the throttle blades i will be able to keep the idle speed screw turned in farther but not draw as much fuel correct?

i will get back to you guys as soon as i get to run the car again.
thanks for all the help so far
02-11-2006 08:56 AM
StevenC 454C10,

If his timing is too low, then the required throttle opening would be affected no? meaning if he bumps up his timing his carb is fine
02-10-2006 03:17 PM
454C10 If you have the idle speed screw in that far then the carb is idling the main circuit (blades open past the transition slots). You either need to get a bigger carb (bigger throttle blades) or you need to drill some holes in the front throttle blades to let air in without opening up the throttle blades past the transition slots. A 0.100" hole in each front blade should do it.

Also a 268 cam should have about 15 to 20 degree initial ignition timing.

Try using paragraphs next time. Your post is very hard to read.
02-10-2006 03:04 PM
StevenC Try this.

Unplug your vac advance and plug it at the carb.
Set your timing to 12 degress advance
Hookup your vac advance to the "MANIFOLD" port on the carb.

Reset your idle and mixture screws
02-10-2006 09:06 AM
19nova72 also i did make the mistake of totally rebuilding a carb to start this motor just cause the carb is fresh and i was confident with my carb tuning skills. the only reason i tore apart the 450 was because i thought it would be too small but it prob would have been fine for break in and tuning. the 600 is set back to just about stock and it is clean and nothing leaks. i will try the carb cleaner/wd 40 method and wil play with idle mixture screws. its definitely possible that its a carb base gasket leak. my friend says that he has 20-25 initial timing in his 357 with the same cam and he says the motor loves it. i thought this was high but i guess i could be wrong. any other suggestions?
02-10-2006 08:59 AM
19nova72 the car has manual brakes so i know that isnt the problem. but what would be a better initial timing to start with?
02-10-2006 07:48 AM
StevenC why is the timing initial set so low?
02-10-2006 07:12 AM
68ss just a thought do you have power brakes?
02-10-2006 12:56 AM
GoneNova/406 yes,i also think it sounds like a vac. leak.sounds like you're going to have a good light to light motor with those heads.i'd be curious to here about it when you get the problem solved.did this carb work before on the old motor?one thing you never ever want to do is start a new motor with a defective carb. but i know how yuo feel at this point.we've all been there.try spraying the wd-40 and see what happens and come back if need be.good luck
02-09-2006 11:11 PM
19nova72 yes ive used that method before, i will probably try that tomorrow. i do have a vacuum gage but have not got to use it on this motor yet. but thanks for helping to steer me towards the problem.
02-09-2006 11:09 PM
57 Chevy That almost sounds like a vacuum leak. Do you have a vacuum gauge? Some guys spray carb cleaner or WD-40 around the base plate and other areas to see if there is a change in RPM.
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