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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: akoz on September 29, 2011, 02:07:01 pm
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I just finished up a vortec head conversion on my 1984 K20 truck.
I have a small hesitation/bog (don't know if it's rich or lean at this
point). I only replaced the heads, intake and put on headers with
a dual exhaust. It still has the stock cam and Q-jet carb (which I
rebuilt thinking I might have had a bad accel pump...was a little
wet near the plunger...no change). I've played around with the
timing (12, 10, 8 BTDC) and they all seem to have this problem,
maybe with varying degree, hard to say. I've got over 19 vacuum
at idle and no leaks. The engine only had 25k on it, yes 25k original
miles, so I am starting with a solid bottom end. You can put a glass
of water on the air cleaner and barely see a ripple at idle. Any ideas?
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I see this is your first post... Welcome from NW Indiana. Did it do it prior to the head change? Is it the stock carb for a 350??. If no vacuum leak you may be undersized in your carb jets. Years back I played with a 72 Chevy, 350 2bbl. I went up and down in jet size until I optimized the mpg. I had an ever so slight hesitation but the next size up in jets that got rid of it dropped my mpg some so I lived with it. I kept monitoring the exhaust pipe color and how the spark plugs were. At the time it was a 51 mile commute each way, same traffic, so I could base my results. My $.02
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Yes, it ran fine before I did the conversion. I drove it about 220 miles when I
bought it, so I was pretty sure this issue wasn't there. I kept the original carb.
When I rebuilt it, I saw that it had .049 primary rods and .073 jets. I did a little
searching online and .048 are the richest rods that are available, so I'm close
to max. Jets range (I think) .068 - .079 so I'm about in the middle. I was thinking
about bigger jets, but I'd like to consider a few things before taking the carb
apart again. I tried using the outside hole on the accelerator pump rod and the
condition is worse...not sure exactly what that tells me. I guess if I knew if
I was lean or rich, then I'd know which way to go with the carb rods/jets.
Thanks, Al
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Choke functioning properly? (ie not closing up on you under acceleration.)
You can try a little more pump shot. Move the linkage on accelerator arm to the other hole.
Where it ran fine before, I'm going on the assumption it's not the secondaries bogging you.
What makes the Q-jet good (imo) is it's very adjustable, that's also what gives folks (including myself) so many headaches.
FWIW.
Vortec & cam change on my old truck, all it needed was a power piston spring change. It was a re-man carb and at some point the spring broke. I did not need to change jets or rods, though I did fight a stumble for a while.
wish I still had the old truck.....
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With the engine idling take a screwdriver and depress the accelerator pump all the way down. It should stall the engine. Also play with the timing and hook up the vacuum advance to full vacuum.
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Ok, I appreciate the replies, but everything that I've already mentioned is being thrown
out there. I tried to post and give as many details as I could, but just to recap:
1- stock cam, stock carb (Q-Jet) - only mods were vortec heads, intake, headers
2- carb rebuilt which means it has a new accel pump...I already tried the outside arm
hole (stock was set to the inside hole) and the condition got worse
what does that tell me???
3- tried timing to 12, 8 and 4 BTDC (spec is 4) with vacuum advance disconnected and carb
plugged (yes, the dist gets full manifold vac, not ported...also removed the stock vacuum
delay (made no difference)
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forgot to say that I installed a new electric choke...goes with the territory when
the new intake isn't setup for the heat tube style choke. In any case, it works
as expected and it's not closing during accel. This is an off-idle hestitation/bog
which can be seen/felt by just pumping the throttle with your thumb while the
car is sitting.
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You were jumping on a rod/ jet change, may not be needed. Obviously something's not right.
Could be something as simple as the rods may not be in the jets, I had a small piece of gasket fall in and land between a rod and jet, just enough to make it stumble while accelerating.
What's your timing at now? Advance working? How's the idle mixture set? Plugs burning clean? Vacuum leaks?
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Check the accelerator pump as mentioned. It doens't matter that you replaced it. In fact sometimes the new diaphragms aren't sized correctly.
If there's no difference with the vacuum advance then it sounds like it's not working.
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Timing is set to 4 BTDC, I went back to spec. I am trying to say that the condition doesn't go away
with the changes in timing, but it's hard to say that it's not getting better/worse depending on
where the timing is. The advance is definitely working as I can see the change of rpms at idle on
the tach and I see the timing mark advance as I increase throttle.
Any throttle causes a shot of fuel from the accel pump so it seems to work and changing which hole
the linkage is in also causes a bad hesitation (outer hole), so something is causing the accel pump
to change also the old accel pump did exactly the same thing.
The primary rods are seated in the jets correctly. I double-checked when the carb was put back
together.
For vacuum leaks, that's a good question. I put on a gauge and have between 19-20 at idle and
see no changes (steady) unless I give it throttle. I heard about spraying carb cleaner around the
manifold/carb or propane and listen for the idle to go up, but I thought that the gauge was a
better way to know for sure.
I can't adjust the idle mixture on this carb because they are plugged and I couldn't get the plugs
out. Since I have no idle problems, then I didn't think this was important to cut up the bottom
section of the carb body to access them.
This is an off-idle hesitation. Once you get past that initial spot, the truck runs smooth and goes
up to 4000 rpms with crisp shifts and plenty of power. If you really hit the throttle by hand...I
mean you "pop" it hard with your thumb, you can get a loud bang from what sounds like the fuel
exploding up near or right in the carb. You really have to quickly bury the throttle to cause that.
I think it's important to know that I didn't have this problem until I changed the heads and intake.
You would think that if my timing wasn't advancing or my accel pump wasn't working, then you
would think this would have been a problem before I did the mods since I have the original dist,
carb and cam.
I definitely opened the door for a vacuum leak because of the mods, so that is worth a second
look for sure. As far as anything else goes with the carb or the timing, then there is definitely
some tuning that needs to happen to compensate for the increased flow in the heads.
I'll update this post later today after I do more testing. I'll try the propane trick to look for vacuum
leaks and I'll pull a few plugs and see how they look.
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Why the gauge wont tell you about a lot of leaks is because they are constant leaks. I spray carb cleaner on mine every now and then, it cleans the outside which is the main objective, but leak tests everything all at the same time. I've caught leaks that way that I wouldnt have found because the engine was running really good (or so it sounded)
aside from possible leaks, just changing the intake and mostly the heads is going to screw up the settings in your carb. your carb was setup for the stock heads and intake, which dont flow well compared to the new intake and heads.
you will need to get to the idle mixture screws. you need them to do much of anything tuining-wise. then if they dont fix it, play with the primary jets a bit.
usually if the heads and intake suddenly flow a lot better, you'll have a lean condition because extra air is coming through even though the carb setting are still set for lower flow.
The carb has 3 part. Idle circuits, cruise circuits, and WOT circuits. The accelerator pump only eases the change from one to the other.
if the accelerator pump is OK, then either the primary jets/rods need to go richer/leaner, or the idle mixture needs to be set richer/leaner, though to get a really smooth transition, you'll have to adjust both. mixture screws are usually easier but yours are plugged, so id try a rod change to something a little richer and go from there.
I don't know what's available for Q-jets, but for my edelbrock I have a box kit with lots of rods, jets, springs and such in it for fine tuning. it was about $45 8 years or so ago.
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Thanks.
I just did the propane test and there are no vaccum leaks. I definitely hear a noticable
idle change when I get the gas nozzle anywhere near the primary butterfly so it's a valid
test.
I randomly pulled 2 plugs and not only are they clean/dry, but I can't even smell a hint of
gas on them.
I also pushed down the accel pump rod with a screwdriver and I needed to bottom it out
before I noticed the idle getting slightly rough. Based on one of the replies to my post, I
expected the truck to stall and it didn't come close to that.
Based on those last 2 points, I am more likely to believe that it is too lean. That also makes
sense considering that the vortec heads flow better (i.e. more air in the system) so you
would think that it would be too lean after the mod.
Since I'm .100 from the richest primary rods, I'm going to try a couple of larger jet sizes.
I'm at .073 now, so I'll pick up .074 - .079 since I can get a decent deal on a 6 pair kit for
around $35 plus shipping.
Any more thoughts before I pull the trigger on jets and tearing the carb down again? I
am willing to expose the plugged idle mixture screws, but I would really like to avoid that
if I can because there's a chance that I will damage something.
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oops, I meant to say that I'm .001 (not .100) from the richest primary rods.
I'm at .049 now and the richest rods that I can find are .048.
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quick update: I finally got the new jets and went right for the .079 and the truck runs a lot
better, but I can stall the motor now trying to stomp on the gas from a dead stop. I think
I'm a little on the rich side now after jumping up 6 jet sizes, so I am going to try .077...I
also got the plugs out of the idle mixture screws so now I have some adjustability there,
but I haven't touched that yet. I've heard conflicting stories about whether these ONLY
affect idle or if they also help in the transition from idle to primary (I thought that was
what the accelerator pump was for?). In any case, any advice on that would be helpful.
This thing really hauls ass now, so I'm real happy...I think I spent around $1000 and that
included headers and a full length dual exhaust system...I might have gained 100hp, maybe
more, but the only way to really know is to get on a dyno.
Oh, and that 700r4 is dying for a shift kit or maybe there is a way to tweak that "magic"
vaccum to electrical setup that goes to the trans so that the response is better?
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quick update: I finally got the new jets and went right for the .079 and the truck runs a lot
better, but I can stall the motor now trying to stomp on the gas from a dead stop.
Your secondaries flaps may be opening to soon causing that condition. There is an adjustment to tighten that spring up, sounds like you are almost there...
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tighten up the tv cable on the carb it will firm up the shift and increase the shift points
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ok, so 0.77 jets seems to have set me back some, meaning that now it doesn't take a
full "stomp" on the gas to bog/hesitate so I seem to have take a step back. I only have
another set of smaller jets and I am not convinced that going higher (over 0.79) will help.
The last time I did the jet change I got the plugs off the idle mixture screws (actually,
they are not screws) and did not touch them but at least I can now. I don't have any
idle problems and right now at 750 rpm in park, engine warmed up, it's very steady.
There is definitely a problem in the transition from idle to the primary.
I tried moving the accelerator pump linkage to the outside hole and it still hesitates
and now it pops (backfires?) a little depending on how quickly I put my foot in it. I am
not talking about jamming my foot to the floor, but something more than feathering
the gas, so closer to normal driving. I also noticed that this raises my idle to 850 rpm.
I did this test right after the rebuild with the original jets (0.073) and it did about the
same thing, maybe a little worse on the hesitation. The current setup is 0.077 jets.
My secondaries are definitely not opening.
I'm resisting the urge to start changing more things (jets, idle mixture, accel pump)
until I have a better idea of which way I should be going with this. Any other ideas
would be appreciate (oh, and thanks for that advice on the tv cable adjustment).
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sorry, 2 big typos in my post...0.77 and 0.79 should be 0.077 and 0.079.
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It turns out that my secondary air valve is not opening up and I can't figure out why.
I thought for sure that when I removed the primary and secondary vacumm breaks
that this was the problem, but all I can see that they do is prevent the choke from
closing, pull back on the secondary lock-out and hold back the secondary air valve
from opening. Someone told me that the secondary air valve opens by manifold
vacuum (not linkage), so I can't figure out why they don't open. I am definitely
running out of gas when I hit the gas pedal in any kind of agressive way and it
seems to be due to the secondary air valve not opening. I picked up a Rochester
carb book by Doug Roe and the problem is that the M4ME carb that they show
doesn't exactly match my setup. I have the dual vaccum break setup, but the
primary has no linkage to the seconday air valve, but this is what shows up in the
exploded parts view and in the parts view that came in my carb rebuild kit.
Looks like I'll be replacing the carb with an aftermarket one unless there is a
Qjet expert out that that understands my situation.
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You physically opened it wide open and the secondaries won't open at all?
http://73-87.com/7387garage/drivetrain/damon.htm
http://73-87.com/7387garage/drivetrain/damon2.htm
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Thanks, but I read this article before and the key is that the primary vacuum break (the one in the front) that they keep talking about is NOT connected to the secondary air valve in my case. Also, the "finger opening test" works fine and my air valve opens very wide and has no restrictions. I'll try and attach a picture of my setup so this will be a little more clear. Either way, Doug Roe's book says that the secondary air valve is opened by the force of vacuum pulling on it, so this difference in linkage doesn't seem to be the issue.
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quick update: it is a secondary fuel metering problem...took another carb and swapping the secondary hanger/rods to figure it out, but that seems to be it. I'll do a final post and try and give people the right combination of rods/jets/hangers, but of course, every build is going to be different.
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Good news you found the issue, now you have some great experience with Rochester Qjet.