misfire... carb issue??

[Squirell]

ok. I run a msd pro billet distributor with vaccum advance and medium springs.  The man who does my motor work sets my timing just as I have described to you. I am almost sure the springs are to do with the vaccum advance. but take my advice use it or dont. either way i am just trying to help. I am just passing on info that i have watched done on many high hp units. and that is how they set timing.

[Kyle]

ok. I run a msd pro billet distributor with vaccum advance and medium springs.  The man who does my motor work sets my timing just as I have described to you. I am almost sure the springs are to do with the vaccum advance. but take my advice use it or dont. either way i am just trying to help. I am just passing on info that i have watched done on many high hp units. and that is how they set timing.

man i really do appreciate the help from ya! no doubt about that. and i do agree with ya they both work off eachother. the springs that are in mine are the heaviest ones they have, so they are limiting the weights to fully open till 5k. i have the blue stop bushing already installed that limits total advance to 21*, which is where i need it to be for full advance. i just dont know how light i need to go with the springs.... i can mix and match em to achieve full advance at different rpms, i just dont know at what rpm they should be maxing out at

[Kyle]

played with the advance springs some today. put a light silver and the blue spring in and drove it and it seemed like it was having a very very small spark knock, so i put in both blue springs and it didnt have any knock at all, runs alot better and doesnt bog and stumble when it drops below 6in of vacuum, but it is still running VERY lean....

[old school truck dude]

Maybe put next size jets in it n it hit a mean lick, lmao I still aint heard ya say it sounds badazz yet so I dont think its gettin quite enough fuel youll know when its right cuz itll sing yeah buddy

[Kyle]

Maybe put next size jets in it n it hit a mean lick, lmao I still aint heard ya say it sounds badazz yet so I dont think its gettin quite enough fuel youll know when its right cuz itll sing yeah buddy

yea i havent been impressed by it yet for the amount of of money in performance parts ive sank into it..... right now bein under jetted feels like im just drivin round the 350 again but with a meaner lope 

[Squirell]

just a thought but you might have to small of a carb. i just reread this and realized your running a 400....my 355 used a 750 to run right. your 670 might be to small. find a 750 to borrow one time and see if it changes things

[Kyle]

just a thought but you might have to small of a carb. i just reread this and realized your running a 400....my 355 used a 750 to run right. your 670 might be to small. find a 750 to borrow one time and see if it changes things

i was thinkin the same thing as well.... unfortunately i have no friends nearby that run carbs, let alone one that big.... i was lookin towards the 770s when i was gatherin parts but my machine shop just said do the 670 cuz they said thats all id need. i got a jet kit on the way and i guess if when i get it jetted and it dont wanna work any better ill just go up anyways. but i know i can throw the same size jets a 770 runs in my 670 and see how that does, but i just wonder if thatdbe too much gas for the smaller venturis and id have to spend a bunch more time playin with it and tuning the secondaries butterflys open a lil...

[betterbeaters]

I run a 850 on my 355 I also have a 750 we tryed that was too small.... if  you lived closer I would say stop by and we could test them....

[Squirell]

im almost certain you need a 750. if you spin it to 6500. the carb calculator come up with a 770.

[Kyle]

this motor is mostly street driven, rarely ever seeing those kind of rpms. would i still need those kind of cfm's?

[betterbeaters]

to drive it everyday the 670 will work fine but if you want the power you need the 750....  I would run the 750 so I had the power when I wanted it(all the time!)

[old school truck dude]

I had a 400 in a chevelle back in the day and it needed the 750 holley

[Kyle]

well the reason i went that way over the 770 is because the engine builder i was doing my work with said to just got with the 670 and just put the lightest secondary spring in and thats all id need.......


i had already bought a jet kit before christmas and it should be gettin delivered sometime soon.... would i be able to jet it to what a 770 uses or just under it to make up for the lack of fuel this carb is puttin out? or would i start ending up choking out the smaller venturis?

[old school truck dude]

well the reason i went that way over the 770 is because the engine builder i was doing my work with said to just got with the 670 and just put the lightest secondary spring in and thats all id need.......


i had already bought a jet kit before christmas and it should be gettin delivered sometime soon.... would i be able to jet it to what a 770 uses or just under it to make up for the lack of fuel this carb is puttin out? or would i start ending up choking out the smaller venturis?

No. CFM is how much airflow through the carb. Cubic feet per minute. Thats the total flow through the carb, not just the fuel or just the air. This carb might be too small. A 750 is probably about right.. Could possibly go with an 800, I think an 850 might be a bit much. If you change carbs, you start all over on tuning. 

[Kyle]

well heres an update:

got the jets all changed out, first time was over jetted to an 80  popped and backfired in the exhaust like crazy so i dropped it down to a 70 and put a new power enrichment valve valve in. readjusted the power shot diaphram so its not maxed out. runs a helluva lot better! worlds different!! power all the way through the throttle and rpms. now the only issue is when im slowly rolling and put the pedal to the floor it stumbles a little bit then catches on and takes off like a bat outta hell

[4x4xdodgeman]

you need bigger squirter.

[old school truck dude]

72 - 73 jets

[betterbeaters]

 the stumble is squiter may need bigger or a different one short or long depending on what you have..

[old school truck dude]

72 - 73 jets

70 probably isnt quite enough, Id try 72-73 jets or so before I changed the accelerator pump.

[Kyle]

its hard to tell but ya might be right... cuz for 's and giggles to see what itd do i put the lightest vac secondary spring i had cuz i was running the heaviest just to see what itd do. and suprisingly it doesnt do it anymores