hi guys, ive been trying to fine tune the 71 402 in my vette(73, 4spd, 336 gears)

only thing i dont like about how it runs now is its a dog from idle to 3k rpms, then it pulls well.

the PO claims a mild rebuild 5+ years back, i dont doubt him, it doesnt burn oil, smoke etc.i have no idea whats in the bottom end or cam, but it pulls 13" of vacuum at idle(800rpm). the heads are 6272292 71 dated and probably came with the engine originally. intakes a edelbrock oval port performer, NOT an air gap or rpm. carbs a new holley 670SA that came free from holley under warranty. only thing ive changed is the squirter up to a 35, and ive messed w the secondary springs a bit. stock spring is in now. setting up the accelerator pump arm and changing the squirter fixed the off idle stumble last summer. ive been drivin the car this way, but now im lookin to improve the bottom end performance.

i dont plan any cam swaps, new heads etc...if im goin down that road itll be w a 454.

ive just rechecked my timing, im 16deg initial, 36 all in at 2000rpms. ive got an assortment of springs, and the stiffest ones are on now. the distributor was recently rebuilt w new bushings, tach drive etc....yea its points and condenser, im not changing to MSD etc. no detonation

could the early advance curve be hindering low end performance? if so since im using the heaviest springs ive been able to find should i drill a hole in the flyweights to reduce the advance rate?

or should i leave the timing be and start lookin at the carb again? plugs look good and grey, but i may be lean or rich down low. fuel bowl level is set, 6.5 PV and stock jets(65/68 i think) installed. idle screws set for max vacuum

any help appreciated!

 

Are those advance numbers with the vacuum advance disconnected? Are you using the stock distributor weights or the ones that came with the kit, use the stock ones. Are you using the Gold colored distributor springs? I believe you are using the plain spring in the secondaries, try the long yellow spring. Are the float levels correctly set? What kind of air cleaner assembly are you using? Read this thread

http://www.nastyz28.com/forum/archiv.../t-219308.html

 

those 670's are known to be lean. put the 68 in front, 73 in rear. use lighter spring.
drill them yourself if u have bits. .079"

 

vac advance is disconnected, stock weights. springs im usung are the black ones, supposedly the strongest,from the kit. plain sec spring, 2" drop base w 4" filter. float level set and rechecked.

i had the bigger jets, 68/72 i believe in the prev 670sa, stared w them inthe new one, but it ran better w the 65/68s if i rememberright. i need a dry day to test any changes, looks likeitll be friday at the earliest

i also have a speed demon650 dp here, never ran since i rebuilt it. i could try it, but w no choke ive been hesitant to mess w it. also didnt want tointroduce a bigvariable until i was sure the timing is good

 

Pull the vacuum spring cap and look down at the diaphragm while pushing up on the diaphragm rod and be sure the diaphragm and rod unit rise smoothly in the housing. Be sure the diaphragm rubber has no "creases" in it that would restrict smooth operations. If you have a problem there, you'll see immediately what I'm talking about. Be sure the diaphragm has no holes.

 

Are you using vacuum advance? Is it working correctly and the correct can for your vacuum. It is very odd your mechanical is all in @ 2000 with the heaviest springs, should be the rate with the lightest springs. My guess is a timing issue. BBC like more than 36 degrees. 18 initial and38-40 all in @ 2500 is more like it if you can do it with no ping.

 

thx for the info so far guys, but all bets are off. hooked up the timing light, when i revved her up the timings jumpin all over the place....bug jumps 20-30 deg, so im guessin the stock appearing balancer is slipping

more later

 

I suspect that you need to look at the end play in your distributor gear. When the timing is all over the place it is frequently the result of the distributor gear walking on the drive gear of the cam.

 

Timing jumping around is usually a screeched out timing chain.
Full advance @2000rpm is extremely early and the heavy springs should delay the advance until higher rpm.
What's the history on the motor? Miles, year,up grades,ect..

 

distributor rebuilt 1000miles ago by me. motors a 71 402 mystery, no idea other than what the PO stated, mild rebuild 5+ years ago. starts and runs very well, but not alot of power. i think its the balancer, since i see no signs the yimimg is actually changing; just the mark hops around under the timing light. car runs the same, no missing, backfiring or other problems. if the timing chain was toast i imagine the engine would run rough but i dont know never dealt w that before.

 

I had similar response issues with various vehicles, turned out throttle cable/linkage not opening carb/throttle body all the way.

With engine off have somebody put gas pedal to the floor while you look down carburetor and check throttle blades opening all the way.

Just a simple thing that people don't think to check.

 

Try a different timing light.

Timing curve sounds good although I would bump the initial timing up some... Personally I would bump your initial timing up to around 20 degrees, then play with the springs/weights to get the total timing back to around 36* all in my 2000. You can run the vacuum advance as well but the moment you nail it, the vacuum drops and the vac advance backs out so its not gonna give you any more timing/power...(vacuum advance is for idle quality and low speed cruise efficiency)..

As for you carb, make sure your throttle linkage is opening the carb 100% and try putting a lighter spring in the secondaries. Personally, a 670 carb is a little small for a 402" motor but....if it pulls good upstairs with it, then the carb is not the restriction...Its a tuning thing. 6.5 PV should be good. My guess is the secondaries are being lazy.

But some more initial timing will give it some better "snap" off idle. Also it "may" want more then 36*.... Depends on the cam/heads but... try bumping it to 38* and see what it does. Some like it, some dont.

My 496 gains NO power after 34*.... I normally run it at 32*...thats with GM 049 oval port heads. My buddys 498 runs the best mph at the strip at 36*... At 38* he lost 2 mph. He had ProComp aluminum square ports. My point is, they all want something different but too much is definitely detrimental.

 

A little off topic, but any chance you are running a stock 73 hood, and mind posting a pic of your motor with the air cleaner installed? Its always nice to hear that something other than the torker can fit.

 

thx everyone, im gettin another timing light from a friend soon, but tomorrow the car goes in to get the front bumper painted, so i wont have it for a few days. i did up the main jets to 68 and the secondarys to 71, and switched to the long yellow spring. definitely smoothed out the power curve down low. she pulls much better from 2k now. gotta check that balancer timing light issue but itll need to wait. throttle does open fully, and yea im guessin the 670 may be a bit small. but, its new, was free, and i think ive got a bit of valve float or bounce anyways....right at 5k rpm she will cough and backfire. thats an issue for another time however

BB dave, stock hood, my intakes a performer 2.0, NOT an airgap or RPM. aircleaner setup came w the car when i got it, and yep it was pretty tight. so tight that when i replaced my motor mounts the lid hit the hood w a 4" element. i dug through the books at napa and came up w the cleaner shown, its just under 4", ive got 1/4" of clearance

hth

 

Thats a thing of beauty! There is so much discussion on here as to what fits and what doesnt, and to see that the 73 and up hood will clear a decent manifold with a reasonable sized air cleaner is just awesome. Thank you so much for sharing. Any chance you might have the part numbers of the air cleaner base and lid [although Im guessing a 1 3/4" drop based on the ruler]

 

thx! its comin along, and the engine bay is at the bottom of my list.

i dont think theres any numbers on the base, and i know theres nothin on the lid. i wont have the car again till wednesday evening or so, ill check then. i dunno if any of the 2" drop ones would gain much. id like to see measurements like these on one first. other issue is the primary vent tube doesnt clear the lid by much. i cent remember what holley recomends any more, but i dont have it. the extra 1/4" of drop wouldnt help there.