Randy82WS7

ok i am working on a 4MV carb(704 series) and i have tried three other power piston assemblies and am having a real problem with the retainer on each one not fitting tight into the hole in the carb bowl casting- to keep it down in place in the casting.

every one of them is sloppy loose

none will fit tight and therefore keep the piston assembly down like its supposed to be

whats the deal ?
was there a retainer size change that i dont know about ?

anyone else ever had this one happen ?

i would keep the original piston assembly in it but it was in there so hard i had to pry it out of the bowl casting, it didnt evne want to pop out when depressing it to get the retainer to come out, even that was not easy,
something was bad wrong with it, was jammed in the casting or something

all the other piston assemblies i have around are fine and slide nicely in the bowl casting but the retainers wont lock into place

never had any kind of trouble like this before- new one on me :O


thanks for anything!

Sonix

iTrader: (1)

hmm, no never had that happen to me.

You mean you're using a 70xxx, rather than a 170xxx carb? I imagine the power piston is different between the two of those, but i've only really dealt with 170xx carbs. hmm, looks like you're stuck fishing around until you find the right power piston...

sofakingdom

iTrader: (1)

You're supposed to stake the edge of the casting over the edge of the little phenolic retainer, with a screwriver or something. Gently. Just enough to hold it in.

Randy82WS7

yeah, i just dont like that idea, hmm

but thats just me :P


i appreciate

i decided to take some sandpaper and clean up the original power piston real good with it and that worked, it slides real nice now, not all jammed up now

how important is it that the piston assembly fits and possibly seals in its bore in the casting ?

what kind of trouble could there be if it was too sloppy, or anything ?

thanks again

Sonix

iTrader: (1)

well it could have a slight vacuum leak, and let the power piston come unseated early, richening it up earlier than normal.

Randy82WS7

oh

hmm well i guess thats a better problem than how it was before when it was stuck and not moving at all

right ?

lol

thanks

five7kid

iTrader: (14)

Agree with Sofa, it's supposed to be staked in place. Doesn't take much, just enough to keep it in place.

As long as you only cleaned up the piston enough to get it slide freely (assuming the real problem wasn't in the bore), it should be fine. The ribs provide the "sealing" function - there is a little flow past it, the ribs break it up somewhat. When you really get into the details of q-jet tuning, you adjust the power enrichment with different springs under the piston - too much leakage, put in a weaker spring.

Randy82WS7

i thought that high vacuum keeps the assembly down in the bore and lean it out,
and that open throttle does the opposite and allows it to come upwards and richen ?


yeah there was a problem in the piston bore,

i dont know what,,


the piston assembly was badly jammed in there, i dont know if it was just dirt or carbon or what, it was in there really good, never had one that wouldnt easily slid eout of the bore before, this is a first,

after the piston travel needed to unlock the retainer,


beyond that -when pulling it out of the bore- it was really a fight to get it out of the bowl casting

after giving up on trying other piston assemblies with retainers that would actually fit tight in the hole i took the original one and sanded it enough to make it slide nice and freely in the bore in the bowl casting


well now the car has a bog down at the top of every gear

i mean it runs and drives and everything but it just doesnt have any real power, real slow now


hum

what would you do.

just never had this problem before, its crazy,,

runs great but no power anymore,


and i cant believe the difference in retainer fits, wow, huge differenc,e i just hate the idea of staking it like that,

maybe if it was just a minor difference, but.,., this is more than just a minor, the retainers are just plain totally sloppy loose, all excet for the original one



so i dunno,

hmm


thanks for any further

Sonix

iTrader: (1)

yep, that's right.

that sounds like too low float level, weak fuel pump, or mis-rodded secondaryies.

you set the power piston depth stop correctly? float level is "in spec"?

you set the mixture screws right, and all that?

Randy82WS7

yeah thats what i did was i cleaned up the piston enough to get it to slide freely

weaker spring, hmmm

problem is that you dont know how strong of a spring that you have, and how strong any others you have around are

hmm

thanks

sofakingdom

iTrader: (1)

The hole in the bottom of the little cylinder there, that feeds it vacuum, is pretty large; the piston would have to have a MAJOR leak to affect its operation.

I don't think I've ever seen one where that was an issue.

Usually I put in a stronger spring, if I ever change it at all; that brings in power enrichment sooner, and helps get rid of the tip-in sag that 704 (early 70s smogger) Q-Jets usually have. Of course I try to avoid 704xxxx carbs entirely, but that's not always possible. In fact I used to buy a certain stronger spring in bags of 10, I used so many of them.

Randy82WS7

neat good info

where do you get the springs and how do you tell which are stronger or weaker

yeah im only running this 4mv carb because i dont have any later carbs that are running right, every one of the later ones i have has a problem with fuel running out the top, unlike this 4MV

i dont get it

but i am eventually gonna get a remanufactured 80s style non ecm one, for such a vehicle as an 84 1 ton

that way it is correct/direct bolt in and is electric choke,
but not ECM enriched


messing around with old non remanufactured carbs gets to be a real PITA usually and tends to raise your BP..



thanks

Sonix

iTrader: (1)

what I did to test my power piston springs:

take an old float bowl. Pull one of the vacuum nipples off the outside. caliper it, grab that size drill bit. Find the orifice right below the power piston bore, and drill it ~3/16" deep with that drill bit. Hammer the nipple into that hole, epoxy it in place. Grab some vacuum line (7/32"?), go from ported vacuum to a 3 way tee. That tee will go to your junk float bowl, and to your vacuum gauge. In your junk float bowl, install a mystery spring, and a power piston.

watch your vacuum gauge. It'll show ~0, and as you open the throttle it'll come online. I found this to be the easiest way. Bring up the vacuum until the piston *just* starts to come downwards. Write that # down. Continue until you find the vacuum # where it's completely seated. write that # down. put it in a baggie with those numbers written, and do that for all the springs you have. Most are like 8-12, or 6-9, or something like that. If you have a few that are identical (rare), you can chop off a coil or so, and remeasure it, so you have more selection.

Then I drive around with my vacuum gauge strung into my car, and see what my vacuum is under all conditions. easing into gear taking off from a light, cruise, tipping in to go up a light hill, steeper hill, etc. Then find the spring "that's right for me". And swap that in. Then go and dial in rods/jets.

Sonix

iTrader: (1)

edelbrock has a pack of 5 different springs for like $7. Was in stock locally. They color them and tell you a vacuum #. But that's pretty meaningless, since the color had all flaked off, and the colors are : orange, yellow, gold, etc. Notice something? Those are VERY similar colors! Like why not, blue, green, red...? anyway, I got most of mine from junkyard q-jets. That gave me a very large variety.

sofakingdom

iTrader: (1)

:barf:

THE ONE to get is 7029202. 69 Chevelle 350. Problem with ANY 702xxxx carb is of course, they're 30-odd years old. Every one I've come across lately (last 15 years or so) has had worn throttle body, dissimilar-metal electrolysis corrosion that has destroyed all the threads, 62 pairs of p****-pullers that have been in them over the years, mismatched parts of all kinds, warped castings from over-tightening the screws, etc. etc. etc.

But the pre-smog units have MUCH BETTER idle and transition circuits, that you can actually make work with a healthy cam. That was the first thing they ruined with the 704 series. The 1705xxxx just got worse.

Randy82WS7

7029202 is a divorce choke carb though isnt it ?

i cant run a div choke carb on this LG4 intake

thats hwhy this div choke carb i am messin with right now is only temporary and why i was saying that later i was gonna get an 80s 1 ton carb, because no choke will work on the LG4 intake except electric one

see my dilemma

on a side note i removed the power pisotn assembly from a 1705 series carb out here and well its the same piston assembly as the 702 carb im messin with, but the 1705 series piston spring is alot longer

which seems real odd

but i swapped it all into the 702 series carb anyways
but that was a bad idea because now it had no PT at all

will cut off and die if you hold the throttle open past idle

lol

so humm

its not goiing very well over here...


these damn old carbs, worn out,

im gonna have to end up buying a new carb sooner than planned it looks like

electric choke/non ECM

dont need a 69 one, or anything liek that, i need one that fits exact same as the E4ME ones do, but without being ECM controlled, im only running an LG4 engine, daily driver, so,


i think that i have done lost and am wasting my time and just hurting my BP even more by continuing to try to get this one going right,


thanks guys

sofakingdom

iTrader: (1)

Then get rid of it.

You can run a divorced choke on a Performer; which is more intake than the LG4/L69 one, but still pretty mild. Or, as the current ZZ4 one seems to be mighty similar to a Performer, maybe you could score one of those on eBay. CARB approved even, both of them.

If you look closely at the bottom of the 702xxx carb's power piston and the 1705xxxx one, you'll see they're different. The casting at the other end of the spring is different too. Also the springs use different stiffness of wire, and different # of turns (spacing). So length alone isn't enough to completely characterize what any given spring will do in any given carb.

But yeah, old worn-out carbs are not going to give good results; no matter how good they were when new. And of course, you have the moron factor in between 69 and now to deal with besides. Although, that can be an issue (and usually is....) on ANY carb that isn't a virgin untouched core off of an old car, or a KNOWN good one to start with (the Edelbrock or Holley or Jet "new" carbs for example).

Another thing you could use, is a Holley 6210. Very very good runner.