mh82camaro

carb guys,
hello i have the gmpp 350/290hp motor with headman shortys,k&n,msd ignition, hei dist with mallory coil,e3 diamond fire spark plugs the cam in this motor s 222 intake and 222 exhaust at .050 .450 intake and .460 exhaust

i also have a 700r4 trans with a b&m 2400 stall with shift kit and corvetteservo and 4.10 gears
this motor shakes like crazy at idle and just doesn't seem to want to go when i step on it and just seems to bogg. i checked vacuum at carb on idle at it was at 4-5 so i did some research in the owners manual and found witha larger cam they rec that you change the step up springs right now on the stock setup it has .098 nmain .095 secondary jet 0.75x0.47 meter rod and a yellow 4" step up spring

i bought a jet/rod/spring kit today and was wondering if you guys have ahad a similar setup and knew which springs to use etc or have a ny suggestions for me. these carbs are hard to work on and are like a science to understand and to get to work on your motor right.
any help is appriciated

mh82camaro

my options are .0.95 / .098/ .101 jets
rods .070 x .037
.073 x .042
.073 x .047
springs blue 3in, yellow 4in, orange 5in, pink 7, silver 8

85T/A350

I have an Edelbrock on my engine. I have a bigger cam and longtube headers. It runs fine and I haven't done anything to the carb except adjust idle. You might want to look a little more to see if it is something else before changing parts.

Jonathan Dyer

iTrader: (5)

Do you have a wide band o2 to check things with? My guess would be .98 jets (probably what's in there out of the box) , .73x.42 rods and maybe pink springs.

For sure, making AFR adjustments with metering rod/jet changes takes more thought than changing jets on a Holley. On the other hand, since you can change both rods and springs in under 3 minutes these carbs lend themselves to experimentation.

Btw, I like to put a rag into the carb in case I drop one of the step-up springs as I lift out the rod.

Jon

shinzon1

iTrader: (1)

Sounds like ur running really lean. Go up twosizes on the mains and look at what rods the book would call for. Will probably say something like 4% richer in both cruise and power. Maybe go one to two higher on ur secondaries. First ull need to get ur idle mixture screws set. U can use ur vacuum gauge to get the highest vacuum or a tach to get the highest idle. Once u achieve highest idle or vacuum turn the screws in about an eighth to a quarter of a turn. Which carb do u have? By the options u have listed I'm guessing its the 1406.

mh82camaro

thanks guys, well so far i took the top of the carb apart and read the book. i found that the stock primaries were.98 and secondarys were .95/ stock rodss were 075 x 047 yellow springs...by the way this is the 1406 carb.

so i took some advise..went up according to the book to step 23richer in power and cruise so that makes it the .98 main jet and 073x.042 rod and i put in the 101 2ndary main jet which is 8%richer and the pink springs which is 7".
got it up to operating temp..it already had a better idle vacuum at like 8-9 instead of 5. then i hooked up a vacuum guage up to the carb and the adjusted the air fuel screws like you guys sadito get the highest level of vacuum at idle...so with this i got it to i believe it ws 15. when you crack the throttle it goes up to 20+..keeps climbing. so with all this crappy weather here in ohio i just backed it out of the garage to go around the block. put the car in 3rd gear and stepped on it..it light up the tires and took off light crazy and sounded great..heck when i got to the stop sign i could smell the aroma of rubber..so i must be in the right direction. i am awaiting a good day to drive it and see what happenms

i don't thinki i need to up the primary jet anymore than it is, but was wondering if i should go to the 070 x037 rod? what do you guys think..may be to much,but these carbs really arent too bad to work oun once you understand them. i know you guys know these carbs and tuning way better than i do ..so that is why i am asking. i think that the primary is good i use enough fuel right now so a bigger jet would be to much...i like when you step on it go go pass someone or to move it does finally.the only thinkg i am debating is the larger metering rod and if you guys think so should i stay with the same spring? thanks for al lyour hlelp i am happy for once this car is moving to its potential, i always thought i had heat soak issues when all it was was choke not set right, wrong jets, to lean, not enough vacuum. putting all these parts on doesn;t matter if you cant get them all to work together...lol

mh82camaro

....

Jonathan Dyer

iTrader: (5)

Well, with a .98 jet and 73X42 rod the effective size of the area that gas can come through is .336 at cruise and .616 at the power step. With a .70 cruise step and the same jet you'd have increased that to .391. Probably too rich.

If the car is not surging (lean), the Cat is not smelling like it's overheating (lean), your not getting a rich smell when you step on the gas and the initial tip-in to part-throttle response is good I'd say leave the economy step alone.

You could experiment with a decreased power step (increased fuel flow) by finding a rod which still has a .73 economy step and a lower number power step. If that doesn't exist you can get another pair of .73X.42 rods and slowly sand away the end to wind up with .73X.37 or something.

Alternatively, moving up to a .101 jet and a .75X.47 yields effective areas of .359 and .628. Again, probably too rich but you'd have to see.

By what you've described about how well the car runs I would expect that you've already found the best combo and now is just a matter of fine tuning how fast the power step comes in by experimenting with the different step up springs.

Hope you get some good weather for testing. It's been rainy here for days.

Jon

mh82camaro

thanks for all the very helpful info man ...i greatly appriciate it. i looked on summitracing website and they sell a jet like you said that came in the 1405 carb kit not in my 1406 kit.it is the number 1454 073x037 for like 9.50...thats not to bad...so maybe next time i am up in tat area i will buy them and try them out. that is also good to know about the springs..i didn't realize thats is what helps kick in the secodaries..i just thought that was something to do with the acuum. well with your help i have come a long way with understanding these carbs so again i appriciate it . i was checking online and gmpp recommeds using a holley 650 street advenger for this mottor to get the numbers it was based on also popular hotrodding used and edelbrock air gap intake and demon carb and got like 40 more horse out of the same motor on a engine dyno. so will drive it when weather premits and see how it does with this metering rods like you said...i will not change the jets anymore i thnk i am in a good setup...but like you said maybe i will play with the 73x37 metering rods and the step up springs. right now i have the pink on installed

shinzon1

iTrader: (1)

The springs control when the rods pull up from the main jets to get into the power mode. The secondaries are control through the linkage and begin to open when u reach 50% throttle.