Carb rebuild help

6

61pala348

New Member
Hey everyone, hoping someone might be able to lead me in the right direction. Only the 2nd time I'm trying to rebuild a carb & this time I cant figure out what went wrong.

Unfortunately, my 348 from my 61 impala had been sitting for about 3 years (used to run just fine) and I finally got around to giving it a go again. I dropped the fuel tank, cleaned, & replaced with clean gas & had to change the fuel pump & some hoses.

I have a stock Holley series 4150 street avenger 670 on there from the original rebuild about 11 years ago. I bought a Holley rebuild kit for it & replaced/cleaned everything I could break down. Still the stock jets, I just cleaned them out. I put everything back together, making sure with a compressed air can that I could feel air coming out through another hole somewhere. I also made sure to keep items that came off the secondary/primary side went back on the same side.

Well, now my engine only wants to run at about 1500rpm. After a day of troubleshooting, setting floats, & trying to tune by idle or mixture screws, i removed all vacuum lines & plugged holes but to no avail. I threw a buddies edelbrock on there from his 383 stroker & with no adjustments besides idle screw, car runs around 850rpm.

So it has to be something on the carb but I dont want to take it apart again without asking if anyone has any ideas on what might be causing it? Maybe someone has had this happen before. Also, if I take it apart, I'll prob change out the jets this time(prob should've done this to begin with). Does anyone have recommendations as to which numbers I should use in the primary & secondary sides? Any pointing in the proper direction would be greatly appreciated! Thanks in advance.
 
6

61pala348

New Member
Hey wristpin, thanks for the reply. New base gasket & used same one when we swapped over the edelbrock, also same shape/design as the one I replaced it with. Float levels I adjusted so that the fuel level is at the bottom of the clear sight gauge on each side of the bowl. The secondaries look closed(with the car off), the vacuum has about 1/8" of play to where I can see the secondaries crack open & then when I release the vacuum arm they move back to the closed position. I have the accelerator pump arm set to approx.. .015" feeler gauge clearance at wide open throttle. Anything jump out as improperly adjusted?
 
wristpin

wristpin

Well Known Member
Why do you think the carb needs different jets or other changes? It was on there before if i understand correctly.
I like foamy water from spray bottle as a leak detector. Spray base and around shafts. Watch for the foam getting sucked in.
If all that doesnt start clearing up the issue consider checking the power valve gasket seal and fuel pressure. Holleys generally dont do well over 8psig.
Good luck. Let us know results.
 
wristpin

wristpin

Well Known Member
Everything looks properly adjusted. Did you take the butterflys apart from the shafts? If so did you adjust them to seat as perfectly within the base as you tightened them.
How far out are the idle fuel screws?
Is the idle are screw adjusted all the wsy out and the linkage have free play so its not hanging up as the culprit?
 
wristpin

wristpin

Well Known Member
BTW Did you know there are 2 different styles power valve and each takes a different type gasket?
One type has 2 rectangular holes and uses a gasket that is simple round shape.
The other has multiple round holes and uses round gasket that has three ears on the inside area.
 
IMBVSUR?

IMBVSUR?

Well Known Member
Supporting Member 2
Wristpin already asked about why you want to change the jets, so I will just ad that as long as the original jets are clear, don't change anything yet. It will just make finding the problem harder. I am reading along because I just went through my Quick Fuel 780 VS ( holley derivative ). The only issue I ran across was for some reason my rear float was not seating and gas was literally overflowing past the secondary boosters into the intake. You said it runs at 1500 rpm. Are you saying that is where the first idle comes in? Does it run above that? I am asking because it sounds like the issue is in the idle circuit and I am trying to understand before trying to give you advice. There are several areas ( idle air correction jet or passage, and the transition slot ) where air is drawn from to support the idle circuit. If you are not getting air from these, then fuel doesn't flow. You crank open the butterflies and now the main kicks in no longer needing those to supply. Do you have fixed orfices or replaceable jets on the air bleeds and air corrections? Also, just my opinion, however those floats are too low. I would think you would want them half way in the site glass.
 
wristpin

wristpin

Well Known Member
IMBUVSUR made good check list. Quick fuel is designed with fixed ports and fuel level from bottom of glass to mid glass. Holleys are designed with brass plug and level at bottom of plug. That plastic plug is a great tool but replace it with brass plug or the plastic eventually melts sticking threads to bowl and plastic discolors till usesless.
If these checks and adjustments dont work you may need to pull carb and check for internal issue.
 
dm62409

dm62409

Well Known Member
Supporting Member 12
It might be something as simple as the fast idle cam is stuck, or out of adjustment , its on the passenger side of carb, usually a plastic arm with a lobe shape to idle faster when the choke is operating, it should drop down when engine warms up and choke opens . After that idle speed is controlled by curb idle screw on the driver side of carb.
 
Last edited:
IMBVSUR?

IMBVSUR?

Well Known Member
Supporting Member 2
wristpin said:
IMBUVSUR made good check list. Quick fuel is designed with fixed ports and fuel level from bottom of glass to mid glass. Holleys are designed with brass plug and level at bottom of plug. That plastic plug is a great tool but replace it with brass plug or the plastic eventually melts sticking threads to bowl and plastic discolors till usesless.
If these checks and adjustments dont work you may need to pull carb and check for internal issue.
Click to expand...


Not sure if I understand the fixed ports thing? I have replaceable jets on everything in my carb.
 
IMBVSUR?

IMBVSUR?

Well Known Member
Supporting Member 2
wristpin said:
IMBUVSUR made good check list. Quick fuel is designed with fixed ports and fuel level from bottom of glass to mid glass. Holleys are designed with brass plug and level at bottom of plug. That plastic plug is a great tool but replace it with brass plug or the plastic eventually melts sticking threads to bowl and plastic discolors till usesless.
If these checks and adjustments dont work you may need to pull carb and check for internal issue.
Click to expand...


You set the float level correct, and Wristpin is also right about the brass plug and float level. I just seen the site plug thing and was thinking about another carb.
 
6

61pala348

New Member
Sorry all, I've been dealing with family stuff & tomorrow I leave on travel for work for 2 weeks. Currently, carb is still a nogo, probably won't have time to check until after I get back but didn't want to not say anything, all the help has been great.
I figured if I had the carb apart, I'd change the jets to smaller size, I remember hearing the stock jets are really just an average setup so if I had it apart again I might as well change them out to something more suited to the 348. But that makes sense to leave them until I figure this out & if I still want to change then would be the time, instead of adding more variables into the mix now.
I did not remove the butterfly from the shaft, if I remember correctly, those were pinged on there so the screw didn't fall out. I thought I put that idle cam back in the same position I removed it from but I'm unsure of how touchy those are so maybe I'll mess with that to see what the effect is. The car will start ok, and after about 5 seconds the choke starts to open & if I don't raise the idle to about 1500rpm it won't run, it'll run above 1500 too, just gotta press the gas more lol. The mixture screws we tried at 2, then 2.5, 1.5 turns out from bottoming out anything more or less causing stalling or backfire through carb. That idle screw is set to full in, trying to give it the most gas but I know it shouldn't need to be.
 
IMBVSUR?

IMBVSUR?

Well Known Member
Supporting Member 2
I have to ask, when you say it backfires through the carb, do you mean it coughs and a mist comes out, or does it bang and possible spit some flame? How did you set the timing?
 
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