go up 3 jet sizes at a time,,,then dial back 2 sizes when you go too far.

Why did you buy such a small carb?

 

I got the carb because first i went witha 670 cfm and the car really didnt like it, way too much for it and it was on the top of the recommended range for my engine, so i dialed it back 1 to the 570, and ill try that to jet it rich and go from there

 

welcome 81FF

double check the intake manifold bolt torque, the new gaskets will compress some,
it may have a little vacuum leak..

your rear gears are killin your performance..
you wont usually need much faster secondarys or faster timing curve with those gears
unless the tires are small enuff to let them spin and get the rpm up faster..

as said
jet it up till you see good color on the plugs, then fine tune..

 

yeah the manifold has been on for some time, and ive already tested all the seals and gaskets with starter fluid to check for any vacuum leaks, and those 2.76 gears just came in the car i havnt had a chance to change them out yet and plus it will help with highway speeds alot so im not too concerned about them. but ill for sure be trying what was suggested, thanks for the help

 

you might want to check the health of your engine? 570 cfm is maybe good for a 262? or a small 6 cylinder.The jets are quick to change so continue with that until its good then see how it goes

 

Its around what holley and summit suggested man cfm wise so i went with the experts, and the engine shows no signs of it being clapped out, good compression dosnt burn alot of oil and it did burn outs before i changed the carb, its just this tuning, ill keep trying the jets or what the next guy said about the timing

 

I'm betting your initial advance is too high with the low gears and smallish cam. 15inch of vacuum is indication of smaller cam. What rpm is your idle? Back off timing to 10-12 initial and 36 total. Still think you could be lean on throttle application from idle. Try the suggested timing before adjusting the carb.
If it still backfires on applying throttle try adjusting accelerator pump cam for quicker shot.
ssmonty

 

The main reason why i didnt think it was the total advance was because after it got to 2500 to 3000 it really sprung to life, it was really the engine falling on its face from a stand still at wot, still think it could be that?

 

too much initial timing could cause the engine to ping.The accelerator pump shot might also be inadequate. The rear gears will not in anyway affect how the engine runs. Gears do not cause hesitation or back fires. Not enough timing and or too lean can cause hesitation and backfires(so can a low battery charge)

 

I was just talking about the gears because a guy before me was talking about them so i was just talking about what he was saying

 

Popping through the exhaust and through the carb have completely different causes. If it falls flat when you punch it off the line and pops through the carb most likely your pump shot is to small or the pump arm is not adjusted correctly. The pump arm should have no play in it with the throttle closed and at wot you should be able to move the arm that is attached to the actual pump at least .015 past where the arm from the throttle linkage hit it. This will keep you from bending stuff or not being able to go wot. jet changes will certainly change cruise A/F ratios and have a small impact on flooring it off the line but the pump shot is usually the culprit. Do you have access to an A/F meter. Also the idle mixture being to lean can effect the performance off the line. if it is to lean to start with and the pump shot is not correct it will cause a stumble

 

I bought the car with the engine in it and went with it from there, it has some average smog style heads and i have no idea the specs on the cam or the pistons to be honest im just trying to make this motor run good for what time it has left in the car as in the next few years im wanting to part it out and build a pontiac 455 for it, but. As of now i think im going to jet it up by 3's till its running rich then get it fine tuned and maybe bring the inital timing up some, also, look at the accelerator pump cams and adjusment more in depth, and maybe by the a/f gauge because i was kinda looking to get one anyways. And ill let you guys know what that gets me and if it dosnt get any better i guess ill return that one for a 670 again. ( thankfully i live so close to summit)

 

Also the block was just the 3970010 with 4 bolt mains, so from what i hear its a pretty common and pretty good block

 

Can you call a previous owner and find out what's in the engine?

As I said above, I'd start at #64 primary jets and #74 secondaries and get a Holley spring kit and put a stiffer spring in the secondaries vacuum pot to slow it's opening... that should make it work pretty well...

https://www.summitracing.com/parts/hly-20-13

Like Vinnie said, that carb. came sized for and jetted for a little 260" engine... Although it may be OK with proper jetting/springs if your engine isn't putting out that much power (less than 330 HP)...

You might actually enjoy the car more with a daily drivable, light, hot 425 HP Chevy 350 (aluminum heads, intake, water pump) rather than a heavy Pontiac 455" making it front end heavy and killing MPG... also expensive to build...
.

 

what kind of spark plug wires are those??

x2 try a stiffer secondary spring with the hiway gears..

 

They are taylor cable full metal jacket spark plug wires. and ill try the springs out after i jet it up and give it a test run

 

Well took her up and down the road its getting much better, took it to 20 inital and 36 full advanced, id feel good to put it back to 38, it could take a little more at the bottom but now it kinda dies off at mid rpm, so im going to jet both up 2 more and see what that gives me. Also pulled a plug and it was alot darker then it was before but not to where its running rich it can still take more.

 

are you disconnecting and plugging the vacuum advance hose to set the initial timing??


if you try the bigger spring in the secondarys first
you may not need to jet up more if your seeing some color now..
if you jet and tune to cover up the bog, then it may be too rich at cruise..


I have heard about problems with the steel braded plug wires, but it was a different brand..
never tried those..

 

Yeah i am disconnecting it, then putting it back on to adjust the total advance, and ill try the jetting to see how it is, but the jets are very close i feel, also i thought id have a problem with them but theyre actually very nice theyre even grounded with hardware from the kit, but also ive seen before with the dual plane intakes that the side with a larger plenum volume that it will need one higher jet in that side of the carb to condensate for the more air, is this true?

 

initial + mechanical = total.. initial and total should be set/checked with out the vacuum..
vacuum advance is just for idle and cruise, it wont make a difference at WOT..

for the larger plenum volume on 1 side, Its possible I guess,,
you just have read the plugs, but I would not worry till its 90% tuned
then do the real fine tuneing..

 

Adjust all timing BEFORE hooking vacuum advance back up... in fact, leave the vacuum advance disconnected and plugged and adjust timing and test run it with the vac advance disconnected... don't reconnect it until you have it running perfectly some day... Vac advance is just for better light throttle cruising MPG... Once it's running perfectly and vac is reconnected, the timing readings will look insane, but if it's not pinging, then no problem....

 

If i remember correctly my timing dosnt change at all if i have the advance blocked off, but when i do have it plugged in then it changed. And i know from the last time i had the cap off when i was changing the vacuum advance module that the little arms with the springs on them under the rotor still moved like they should so theyre not frozen up, so thats odd.

 

X2 with getting the timing and carb dialed in with the vacuum advance unhooked. Once you get rid of your stumble/backfiring, then hook it up to get the cruising timing set correctly.

 

Okay so i unplugged the vacuum advance, and it was alot perkyier off the line, it had some slight pops from the exhaust in the mid range of the rpms, also may try to change out pump cams to see how it goes

 

it had some slight pops from the exhaust in the mid range of the rpms, also may try to change out pump cams to see how it goes

Wouldn't worry about the pump cam for now, sounds like it needs an even stiffer spring... because of the small carb...

Don't need to watch the tach that much... 20 - 30 timing advance isn't enough... hopefully, revving the engine slowly gives something like 20 - 38 or 20 - 40... if it goes over about 38-40, I'd lower the initial timing a bit to bring it back down...
.

 

did you do the spring yet??

 

Yeah i did that when i changed the jets, the carb came with 2 different springs i put in the heavier of the 2 until i can run to summit to pick up a few different ones to test out. So i have the timing set to 20 and its slightly popping out the exhaust which would mean un burnt fuel getting to the exhaust so maybe a little more advance to make sure its all burnt? Also thank you everyone for your help i really appreciate it ive been messing with that for a long time i appreciate all the pointers you have all given me, also i adjusted the air fuel ratio so its at the max rpm/in of mg, i feel like it could use a little more in the secondary jets and maybe a different cam for the pump, ill be doing more testing on it tomorrow

 

Well I yanked the #3 and 4 plugs just because theyre easyest to get to and this is what they look like still looks like it can take a little more in the Jets

 

What jets do you have in now? .. Plugs look plenty dark...

What ignition timing advance range is it going over now? 20 - 40 degrees? (vacuum disconnected)

 

What jets do you have in now?

The 64 and 74 that was suggested above i havnt changed them from those yet, and im not sure what the range is exactly i was having to do it by myself i had no one to watch the tach as i operated the timing light. So far all i was able to test to make sure it was advancing with out the vacuum advance which it is (atleast 10 degrees putting it around 30 from as far as ive seen) and i do appologize for not being able to answer they timing question better then that) trust me im giving you all i can at the moment

 

I can try that once the inital timing is down, right now im leaving it un plugged till i can get it all sorted out on the inital with out the timing and once i do i will try to change ports, and ill let you know how that turns out

 

Got the graph of degree of advance at certain rpms with the initial at 20 degrees

 

Looks good... did the maximum advance stop by 40 degrees?

 

Around 36 - 38 id say it didnt go too much farther then what my graph shows

 

it dosnt need much maybe a little faster to get to the full advance, and new weights, with the distributor all ive done is new msd cap and rotor, new performance coil, and a msd street fire module.

 

Did you put the stiffer vac springs in yet?

 

I didnt have a chance to go out and get them today but im going to be out and about tomorrow running errands out in the area so ill be grabbing them tomorrow and slapping them in on thursday, also got the vacuum from the 15 up to a steady 17 in mg when the engine is at operating temp, so im pretty satisfied with that. Do you think i should do anything with the springs and weights in the distributor or just let it be for now? Im half temped to get them to put a lighter spring in one side like the other guy said