Thread: tuning and timing questions???

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alright i think its about time to do this, and im hoping i can get the answers im looking for. 283 chevy, 262/468 lunati cam, hei distributer, holley 600 carb.. 700r4 trans, 4.10, 3800 lb. + truck. ever since i put on the holley carb, (nothin special. http://holley.com/0-80457S.asp ) i have always had hesitation issues romping on it from a start. even in park. i knew that was always a tuning issuses, but i just delt with it and got around it. finally tying the knots and fixing the little quirks in the truck i figured its time to tackle this.

after reading up on the problems i had, i found out i needed a bigger accelorator pump squirter * ordered * but not yet installed. and a timing light. So basically all i am wanting is just about ALL of the steps, precausions, and knowledge about timing as i can find out. i know the bare basics of timing. but everything is better than not enough information.

here is the light i bought. http://www.jegs.com/i/Equus/390/3568/10002/-1/962916|10711

if you have anymore questions about anything i will try to be quick to respond.

Go to Amazon - http://www.amazon.com/s/ref=nb_ss_b?...ey+Carburetors

Buy two of the manuals listed.

Holley Carburetors by Dave Emanuel - an SA book.

Holley Carburetor Handbook by Mike Urich.

They're on sale right now.

well to be completely honest, i dont need a book on the carburators, just the whole timing deal. if anyone has a link or a website that i could go to to read up on this that explains things sufficiently, then that would be great.!

There will be many opinions on how to set timing... Usually a good rule of thumb is 36* total mechanical timing all in around 2500 rpm. With an initial (idle) timing somewhere around 8-12-16*. This depends on many variables including static and dynamic CR of the motor, fuel octane, altitude, and on and on....

What kind of ignition system do you use? Vacuum advance? Timing should be set with vac advance disconnected and plugged...

I would start at 36* degrees total (without vac) drive it a bit including some wide open throttle then check the spark plugs. You can read the plugs to tell you whether you need more or less timing based on the heat line on the ground electrode of the plug... you want this line to be at or just past the bend in the electrode... adding more timing moves this line further from the tip...see this page for more tips on reading spark plugs (I'm sure there are more/better resources but this one is pretty good):
http://www.dragstuff.com/techarticle...ark-plugs.html

There are links to other related articles on that same page...

As for your off idle stumble that is usually accelerator pump shot too small or too late... usually bigger pump nozzles cure this problem... could also be an indication of vacuum leak or excessively lean mixture...

There will be other suggestions from more experienced folks here but I hope this helps in the meantime...

-Chris

Great link Chris, thank you.

If you are looking for some excellent information on igintion timing, check out this link. It is one of my favorites of all time and very educational.

http://www.chevelles.com/forums/show...t=ignition+101

- Skids72 - yes it does have vacuum advance. and as far as a lean mixture? im not trying to say ur wrong, but a 600 cfm carb on a 283 shouldnt be lean AT ALL throughout any rpm should it?? if so i guess i could start looking into that problem some more. next week i will have installed that bigger accel. squirter. and i will let you know how that works. and i will definitely read up on those links which are much appreciated!! thank you!


65ny- also thank you for the link! i will also check it out!

any carb can be too lean on any motor if it's not jetted right... another thing the plugs can tell you is if you are too lean or too rich. Also if you have a vacuum leak you'll be too lean no matter how your carb is jetted...

Probably beside the point as your stumble is most likely the accelerator pump shot ... (also a matter of being too lean when the throttle cracks open)

-Chris

alright well i finally got back to my pickup to check things out, this is the info im reading up on and going off of.. so if theres anything you see different would you let me know? not tryin to stray from clubhotrod but this is about the best discription i could find for the problem.

http://forums.corvetteforum.com/show....php?t=1430339

ok base timing is at the "about appropriate" 12 degrees

the next step was to find my total timing and what not. at just above 2500 rpm, the mark wont go any closer to the pointer , and thats when its set at 36 degrees for a starting point. i had to back it down to about 29 degrees. thats when my marks lined up. i went to check to see if i could turn my distributer any further *counterclockwise* is advanced right? well either way, thats the way i needed to go to get my mark closer. but the distributer is maxed out. i guess what im wondering, is if i may have the distributer set down in there a tooth of of something? or if the 29 degrees total timing is going to be fine?

forgot, i do not have a vaccum canister. do i need one? when i checked the vaccum the other day, it wouldnt get above 11. ill recheck this once i get everything timed.

also, i just took it out for a test drive just to see if anything had changed and it ran perfect. (vaccum advance unhooked and pluged) but when i turned around to head back into town, i just tried to powerbrake it a little to see if that was fixed as well but it just poped and died.

any suggestions?

You need a vacuum advance distributor with vacuum source selected to manifold vacuum and not ported vacuum.
If not, your car will most likely overheat in low speed traffic and at idle.


I've had several people argue that they're doing fine using ported vacuum, but what most people don't realize is that GM developed the ported vacuum system so the engine would run hot at idle and low speeds thereby improving the nox and hydrocarbon factor.
Moh bettah for the smog situation in other words.

GM did have a thermostatic switch to swap over to manifold vacuum when the engine got hot enough and the theory was it would cool down.
True, it did, but sometimes the thermo switches let things get too hot and occasionally engines suffered heat damage.

And . . . some guys are bad about admitting things.
Friend of mine stuck a mechanical advance only distributor on the 327 engine in his roadster.

He claimed it didn't overheat in traffic, but later on I found that the reason he didn't want to go anywhere except Saturday night rod runs and car shows was that the engine overheated.

alright well which one is which.. for vaccum hookups on the carb that is? its a holley, with one on the front mixutre plate and one underneath the front fuel bowl. ?? and with my knowledge i can tell that it is a vaccum advance distributer. hei, little chrome vaccume jobby with a vaccum line hookup on it. ???

does anyone think i could be a tooth off on my distributer? at about 650 rpm idle, the vaccum reads aobut 9. i know its low, and i cant turn my distributer any more because the vacc. advance hits on the intake

I would say 29* is not enough timing and will cause you to run hot (and poorly, including hesitation off of idle). If you physically can't rotate the distributor to get where you need to be, you can rotate the position of the plug wires on the distributor cap to get in the range of where you need to be...

mark the position of the number 1 cap terminal on the intake manifold with a marker, then rotate the distributor (clockwise I presume) to line up the next terminal with your mark, now shift the connections of the plug wires so your number one wire goes to this new terminal and so forth... now you should still be at ~29* but will have room to advance your distributor further if you need to. Alternatively, you can pull out the distributor and try to get it on the next tooth...

Manifold (full) vacuum is on the port below the front fuel bowl and there should also be a larger one in the middle of the baseplate at the back. The one in the metering block is ported vacuum.

Have you checked for vacuum leaks? Do you know if your a/f mix is right? Are all your unused vacuum ports capped?

Also, do you know how your advance curve is? Do you advance more at higher rpm's or did you only check at 2500? You may need to change some springs in the distributor (I don't know the HEI's so I can't help here)

You haven't mentioned much about your motor... compression, cam specs (262/468? does this mean 262 duration .468 lift? 262 advertised or at .050"?), etc... for any kind of "hot" street or performance cam I think 650rpm is quite low for the idle speed. If it's more of a factory cam then 650rpm sounds in the ballpark.

Also, what is the stall of your torque converter? If you're having these hesitation problems in park, then this probably doesn't matter for now...

Hope some of this helps... please excuse the rambling

-Chris

dont worry about the rambling, im glad you were specific in ur descriptions! this kind of stuff helps, im just a young buck tryin to catch up to my problems.

alright engine, short block is a stock rebuild 283 with 4 valve relief flat tops. other than that for comp, i couldnt help you, *oops*

gm performance vortech iron heads, proform 1.5 self align rockers, 64cc. headers, performer rpm manifold

carb, holley 600 http://holley.com/0-80457S.asp
thats it if you want to know any more about the carb, i just installed a .35 squirter in it *as reccomended by holley and jegs guys*

newley installed cam is lunati voodoo, and yes, sorry advertesed duration is 262/268 and the lift is 468/489.

air to fuel mixture, havent messed with it that much, ive done the basic deal as in turn all the way in, * i didnt tighten them, just till i felt pressure then i stopped* then i turned each side a turn and a half out. all three vaccum ports are used, under the fuel bowl, for advance. one on the metering block used for trans. big one used on brake booster.

**(Also, do you know how your advance curve is? Do you advance more at higher rpm's or did you only check at 2500? You may need to change some springs in the distributor (I don't know the HEI's so I can't help here) )****

this is where i get lost, i read up on a little bit today for the total timing and went from there. i had the idle set all the way up to 3300ish rpm and the mark still didnt go up any further from 2400ish.

and vaccum leaks i havent checked for. i suppose it wouldnt hurt seeing how it is only pulling 11" if i would bring up my idle a little more would that also increase?

trans is a tci streetfighter 700r4 with a stock lockup type converter

umm, anything else you want to know, please pry!!! i want to know! but thank you skids for being so informative!!!

also, it is running alot better after messing around with it all day, but its still not where it should be.