fun with carburetors...

PoweredByDodge

so... i've got a dead spot.

idle (which i've pegged up at 850 as opposed to the mfg specified 600 -- due to the fact that its the 'sweet spot' where i can leave it at so that in the winter i don't have to worry about it coming off fast-idle too quickly and in the summer [when fast idle is pretty much bypassed because it gets so fkin hot under the hood just from the sun] it'll still idle peachy at startup) through full blow (at 4,500 rpm) is nice and smooth so long as you 'moderately' accelerate... that is, you put your foot in it, but you don/t just ram it to the floor in one step.

when i go from idle to "foot to floor" in one step, it bogs down like all hell, rpms drop, and it almost feels as if its a tiny backfire (i think its dumping the excess fuel into the exhaust as i'm likely flooding the hell out of it).

so what i'm asking is...

1- is this 'need to moderately floor it' normal?
2- if not, then is this likely an advancement issue?
3- if not, ignition issue?
4- if not, carb tuning issue? (and if so, what exactlly?)

note -
1- timing is set 1.5 (1 point 5, not 15) degree advanced past factory spec (timed at 600 rpm with car in drive and foot on brake per spec). any less and it pigs out (in my opinion), any more and it misses... this was a result of opening up the exhaust, eliminating the 'air' pump, and opening up the air intake element. everything else is pretty much bone stock.
2- no it doesn't 'just need a tune up'.

any ideas, much appreciated. thanks kiddies...

-vinnie.

exalted512

Probably just needs a tune up
-Cody

dudeinaroom

I know on the newer Chevys the way that they are designed you have to leave the stock y pipe in and go duals after that or you have the same issue. This is info from flowmaster(i used to work in an after market auto accessories shop. Your Dodge could be suffering the same fate as the Chevys. For off idle response they need the back pressure so they don't bog. Jump on a Dodge forum, or an after market exhaust company's site, and see what people are saying about the dodge and how to fix it.

Jstas

Dude, if it's bogging down like that, you could probably have jets that are too fat in it. You also did not mention what kind of carb it is. (Holley, Edelbrock, Carter, Q-jet?)

I'd need to know what kind of carb, model number if possible, what kind of secondaries are on it if it's a 4-barrel and what kind of choke you have on it (mechanical or electrical) and is it a smogger carb?

disneyjoe7

Fat no I think its lean.

PoweredByDodge

sorry for the confusion -- its the caddy, not my truck.

Carb - Rochester Quadrajet M4MV 4 barrel OEM carb (roughly 30-35 mm primaries, and about 65-70 mm secondaries). Electric choke based on free air temp under hood. I believe they're vacuum assisted secondaries. What i mean is that if you have the carb off the car, and you throw the throttle all the way, the secondaries will open about 1/4 of an inch (10 degree angle or so). If you apply vacuum to the diaphragm (mounted on the passenger side of the carburetor), and then throw the throttle all the way, then the secondaries will open up to about a 45 degree angle if not a little more.

To the best of my understanding, the purpose of the diaphragm and lever jumping to to the secondaries is that it keeps the motor from doing just what it IS doing... provided you're off the cold-idle cam (er 'fast idle' er whatever you want to call it), a tab disengages, allowing the secondaries to function past 1/4 of an inch opening. the same 'tab' is also 'locked' (for lack of a better term) by the vacuum diaphragm. you can open up those secondaries all you want, until the motor starts to generate 'less' vaccuum (well, too little to move the diaphragm, which would indicate a stalling condition) at which point it sets that tab -- now as long as you haven't gone past about half throttle, that tab still has room to catch and lock the secondaries out... if you've gone too far, too fast, it'll throw the tab but it won't catch and you'll have effectively 'floored' it. the vacuum diaphragm is supposed to be nothing more than a 'helper' for driving right after the cold-idle stopper has been let go. you're still supposed to be able to stab it to the floor in one quick motion and it should -- 'should' -- respond with very little lag. I may be way off on that though. However, I did check the functionality of the diaphragm and related linkage and they're both working the same as they always have... whether or not that's 'the right way' is up for debate.

the jets may be too fat... but they are stock.

the car was a 'smog' car, in that the heads have air-ports for the exhaust manifolds and it used to have an air pump... the smog holes in the cylinder heads were covered by the header flange and header gaskets (which are copper to aid in the 'hole plugging') as this was something that the header manufacturer took into account when building them... it's done on purpose. however the carb was not a 'smog' carb. just a standard and very common Rochester that was on a pile of GM's... whether or not the jets are different for smog and non-smog is something that i do not know.


PS -- this may or may not matter, but... it only happens under load. If you nail it in park or neutral, it'll just spin up until the ignition system can't keep up with it or until the motor blows... in drive, whether on the highway at 50 mph or on the roads at 30.. or even at a stop light from 0 ... it'll do the boggy thing if you nail it. otherwise, you go 'moderate' (and by moderate i by no means intend to say 'slow' or 'easy'... if you count seconds from when your foot hits the pedal to when the pedal hits the floor board, it would be about 6-7 seconds) on it and it's happier than a pig in **** all day long.

disneyjoe7

The secondaries are opening to fast, maybe the vacuum disk is bad? This keep the secondaries close until the engine can eat that air. Also the vacuum hose maybe in the wrong port!

PoweredByDodge

hmm... there is wisdom in your words disney-yoda. i'll investigate and post more tomorrow. i highly doubt the vac line is wrong as i went insane being meticulous about getting all that **** the way it was supposed to be, but i will double check.

my vac curve is kinda funny.... its 15 psi at the factory idle (600 -- which for the record is 'stumbly' and 'lopey' sounding)... 21 psi at the "vinnie idle" (850) and stays at that up through about 1,500 rpm (if you let it stabilize) and then falls off slightly to about 17-18 above that through 3,500. that goofyness might be affecting the 'effectiveness' of the diaphragm. it didn't generate this much vac before i fracked around with it. it used to float at 14- 15 and never jumped above 17.

i know... i know... "that much vac indicates either a perfect build with perfect flow, or a small restriction" --- its a restriction. the factory intake manifold doesn't rise up like a hump... it sinks down a little bit. an aftermarket manifold would make a world of difference, but i'm not even going to consider it until the motor needs a true rebuild. i finally have had it running 'very very good' for the last year and a half, and i'm not **** around anymore. just wanna work on the 'little bugs' here and there that i'm experiencing... 'fine tuning' if you will. i would say my true vac reading is in the 17-18 range from what i've observed.

i remember John said something like "my dad told me you can work on a car till it doesn't run" -- he said that after i ripped apart the carb for the first time like 3 or 4 years ago. it wasn't very amusing at that time, but i find it utterly hilarious now.. because its true.

disneyjoe7

That vacuum breaker for the secondaries work best if off the manifold vacuum, there are many vacuum ports of a Q Jet carb... I would push down on the secondaries top plate this should move like no more then a 1/4" at the back. Then while opening the throttle quickly and placing finger on plate see if they would open wide to fast. If so the vacuum breaker is in the wrong port.

BTW where did you get the Q jet, many rebuilds I seen are bad VERY VERY BAD NFG... What is happening is they take Q jets to rebuild take them all apart then reassemble them all. Problem was GM made what a Million of them so the body of one is not the top of another, where the bottom belong to something else... In short you will never get that Q Jet to do anything correctly if that the case.

PoweredByDodge

15 years later, just wanted to follow up with the outcome... put an Edelbrock 800 on the car - runs like a scalded dog ever since. Had some issues at idle, which I figured out just this summer. Factory fuel pump will run 6, 7, 8 or more PSI. Added a pressure regulator to keep it around 4 PSI. Edelbrock carbs don't like anything over 5 PSI (they'll tell you 6 is the 'tippy toppy' - but you'd have to read the manual to know that - took me like a decade to change the carb, and another 5 years to read the manual). Happy as a pig in slop now.

pitdogg2

At least you're determined 😉😉

motorhead43026

Slow and steady can win the race as long as you don’t die first.