70 Blazer, 350, 300hp crate motor, have replaced fuel pump, installed hei
ignition, 4bbl Rochester carb has been rebuilt.
I've got an occasional "loading up" at lower rpm's, 2000 or below. Engine
will die at idle. This has been occurring before and after a new engine and
later elect. ign. Anyone know what might cause this Rochester 4bbl to do
this? OR, could it be a non vented gas cap causing a pressure build-up to
blow through the fp and past the main jets? I frequently have a pressure
build up when I remove the cap to refuel (which is not too often with a
Regarding " From: firstname.lastname@example.org (LD)
70 Blazer, 350, 300hp crate motor, have replaced fuel pump, installed
hei ignition, 4bbl Rochester carb has been rebuilt.
I've got an occasional "loading up" at lower rpm's, 2000 or below.
Engine will die at idle. This has been occurring before and after a new
engine and later elect. ign. Anyone know what might cause this Rochester
4bbl to do this? OR, could it be a non vented gas cap causing a pressure
build-up to blow through the fp and past the main jets? I frequently
have a pressure build up when I remove the cap to refuel (which is not
too often with a 37gal tank).
had this problem with the 4mv quadrajet and I believe I had to replace
the float with the new style one.Also added a large fuel filter in line
before the regular one cause a tank that old probably has loads of
crap,(mine did)Somehow that crud was just big enough to unseat the float
valve and let the floatbowl level get too high ,and sometimes I actually
had fuel coming out the vent.This was a few years ago so it's kinda
foggy but I think if you got a modern float(they're like a solid block
of plastic )and add a fuel filter you might see a difference,cause if
sometimes it runs fine and suddenly it's all rich to the point of
dying,it would seem like that would be the problem.
I kinda doubt that tank pressure would do this since the fuel-pump
supplies fuel at something like 5-9 lbs,and to have more than that in
your tank doesn't seem too likely to me.Plus the float-valve would not
necessarily let in more gas anyway,up to a point.Doe4s this make
I installed this tank just a few years ago (original was only 21 or so gal,
couldn't go anywhere without stopping for gas). I'll look at the float but
I think they replaced it when rebuilt. Once, when it died, on the side of
the road I took it apart, bent the float down about 1/16" wondering if that
was the problem. My thought on a filter is that the more gas it wants (high
speed) the worse it would be.
Check the float level and also the needle/seat. When you remove the center
section look under the mainwell jets. There should be a seal block in the
carb base. When the mid section and the base are together this base should
seal the bottom of the mainwells. I am assuming this is a quadrajet btw. Let
us know how you do.
See reply to B. O. on the float. Not sure what you mean for the "mainwell"
jets. It's been so long that I had this apart, I don't remember anything
under the main carb body. The top comes off exposing the floats and
removing that, with the needles you see the main jets. I remember leaning
then out .002" some years back. I also remember, (in reply above) when I
adj the float level (after rebuild) I saw they put in viton tip needles.
This block is under the main carb body or "mid section"?? Can you describe
the shape and material of this block?
Thanks to both, LD
It's a foam block and it seals the bottom of the main well. It's about
1x1.25" and about .375" thick. If it's not there the plugs under the main
jets can leak and cause the problem you have. I fixed more of them than I
It appears to be fixed. After I had to have it carried back to my house one
morning, I replaced the gas cap with one that allows venting. I bought a
rebuild kit but only pulled the top, replaced the inlet needle and seat,
accelerator pump, filter, and the float.
What I found--
There were a lot of small particles of trash in the bottom of the bowl;
rust? sand? broken pieces of the rubber gasket that used to be on one end of
the sintered bronze filter? Also, the filter, with a broken and deteriorated
rubber gasket was not filtering, and the vition tip on the inlet needle was
grooved. I don't know which or what combination of bad things caused the
My theory is:
The former gas cap, non vented, allowed pressure to build up in the tank
several (5-10)psi beyond the (10-12?) supplied by the fuel pump. With the
amount of trash and with the groove in the inlet needle I'd say it is a fact
that it was not stopping the fuel flow (which appeared to be flowing out the
top of the carb, even with the engine off, shortly after it flooded and
died, which is why I think the pressure build up in the tank was a factor).
The float may have been OK, I don't know. I wish I'd taken the time to weigh
the old and the new on a re-loading powder scale, accurate to .1 grains,
(which is 1/4375 of an ounce).
One other thing (and this is embarrassing). When I started to work on it, I
also noticed that the primary wire to the coil (on the hei I installed
recently) used a 1/4" female spade connector to a male on the firewall and
had become disconnected. This, as we all know, contributes to "lack of
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.