strange engine issue from 900-1400 rpm

[Bocephus69]

i recovered my C10 from a slow death in a tree-row at my in-laws farm. it was sitting for 10-15 years. it has a 350 engine that was donated from a 76 caprice. it has all new ignition parts and i rebuilt the quadrajet carb with a simple kit from napa and the help of google and youtube .


the engine now runs great except while in the approx rpm range of 900-1400 rpm. at idle, it runs good, at highway speed it runs good, but low rpm it surges. i dont know how to explain it other than it is "the opposite of cutting out." if i am driving 25 mph in town with my foot barely on the go pedal, it surges. it jerks your body a little bit and you can hear the exhaust get louder. my wife jokingly says "well, the old gal just wants to GO!"


even when it is in park it will do it (right when you start it and the choke is on.) it is idling higher, around 1000rpm, and you can hear the engine "burst" for a second, approx once every 5-10 second. it isnt a backfire, it is just a simple burst (almost as if you would lightly burp the throttle).


all i have tinkered with are the 2 A/F mix screws on the front side of the carb. can anyone provide me with some insight as to what could be going on here?

[Mike C]

Have you ever heard the old saying "90% of carb problems are ignition?" I'd start with plugs, wires, cap and rotor and see where that gets you.

[Derek.Matirne]

To me it sounds like the floats are set a little high, forcing fuel into the manifold...

[Bocephus69]

Originally Posted by Mike C

Have you ever heard the old saying "90% of carb problems are ignition?" I'd start with plugs, wires, cap and rotor and see where that gets you.

Read

[Bocephus69]

Originally Posted by Derek.Matirne

To me it sounds like the floats are set a little high, forcing fuel into the manifold...

Is that possible? How does it "force" fuel? I understand your logic though.

[Derek.Matirne]

The carb has over flow ports, for a lack of better words, that will allow fuel to leave the carb when it has too much fuel... The carb isn't designed to be under pressure, that's where the floats come into play... Other float will shut off pressurized fuel from your fuel pump... If the float is adjusted to high, fuel will bypass the needle and seat filling the bowel passed its capacity.... Also if you are running an electrical fuel pump, without a pressure regulator, it can also put more fuel in the bowel than it is able to handle.... I think a regular everyday carb should not have no more than 5 lbs of pressure, but I could be wrong with that....

[geezer#99]

Originally Posted by Mike C

Have you ever heard the old saying "90% of carb problems are ignition?" I'd start with plugs, wires, cap and rotor and see where that gets you.

You're close Mike!!
Old saying is '90% of carb problems are timing problems'!!

Meaning a wrong or mismatched timing curve is the likely problem.

Likely the case here.
Not enough timing until up past 1500 rpm.
Needs lots of initial timing due to elevation.
And an adjusted mechanical curve in the dizzy.

So the age old question is----

What's your initial timing?

[67 chevelle]

Maybe a sticky advance weight or spring under the rotor , or a bad vacuum advance can , or carb issue

[Bocephus69]

i am running this distributor. it is brand new. http://www.summitracing.com/parts/msd-8362 i didnt change anything with the timing advance mechanism. it was installed as is out of the box. i set my idle timing with the vac advance unhooked at 12 degrees at 800-900 rpm.

[Bocephus69]

Originally Posted by Derek.Matirne

The carb has over flow ports, for a lack of better words, that will allow fuel to leave the carb when it has too much fuel... The carb isn't designed to be under pressure, that's where the floats come into play... Other float will shut off pressurized fuel from your fuel pump... If the float is adjusted to high, fuel will bypass the needle and seat filling the bowel passed its capacity.... Also if you are running an electrical fuel pump, without a pressure regulator, it can also put more fuel in the bowel than it is able to handle.... I think a regular everyday carb should not have no more than 5 lbs of pressure, but I could be wrong with that....

maybe a dumb question.........but there is no way to check this without taking the carb apart, right?

[geezer#99]

Turn your idle down to 700 rpm, then set your timing.
Bump it up to 16 degrees.
At 8-900 rpm the mechanical timing is starting to kick in.

[Derek.Matirne]

Sorry, but I don't know if, or how you could...

[geezer#99]

Originally Posted by Derek.Matirne

Sorry, but I don't know if, or how you could...

If you suspect it's flooding, just look inside the carb at idle.
If you see any raw fuel then you need to find and fix the problem.

[im4darush]

The surge you describe at light cruise is a classic sign of a lean condition. Motor is not getting enough fuel under this light throttle situation. Revisit the primary side of your qjet.

[Bocephus69]

i adjusted timing to 16 degrees. i like the smoother throttle response, and it doesnt seam like it wants to die as easy when i drop it into reverse. i think i will leave the timing there. however, this did not solve my issue i originally asked about.


i messed with the two idle screws. one obvious observation i made was that i can turn the passenger side A/F screw all the way in, and nothing changes. turn it out 5-6 turns, nothing changes. no different rpm....nothing. the drivers side a/f screw makes noticeable difference while adjusting it. am i plugged up on the passenger side?

[geezer#99]

Nothing plugged!!
You're idling on the power circuit.
You need to close the throttle enough to cover the transfer slot.
Turn down the idle or add more timing.

[im4darush]

The motor should die when you turn the screws in. If it does not, as stated above, you are pulling fuel from somewhere other than the idle circuit in the carb. 900rpm idle is too high. The primary plates should be almost closed at idle. If they are not, you will pull fuel from the transfer slots. You can see these slots in the carb if you turn it upside down and look right where the primary blades close. At idle, only a very small square of the slot should be exposed to vacuum. Any more and you are off the idle circuit. At idle, your mixture screws are there to richen or lean out the idle. If you can not make it idle well with these slots not overly exposed (say 650 rpm warm), there is something else going on. Make sure your timing is correct as well. In terms of your surge at cruise, you are lean at that particular moment.

How was this carb rebuilt? Was the body soaked in cleaner and all the orifices confirmed clear?

[Redline novdog]

X2 w/ geezer#99

[bw46]

Maybe a vacuum leak around the intake manifold where it meets a head. Could explain the unbalanced idle a/f screw reaction.
I agree this sounds like an intermittent lean condition.
You should not need 16 degrees at idle. Remember that the mechanical advance from the MSD factory will add a bunch more, often well over 20, which would bring total to too much. When an engine needs a lot of initial it's because something else is off, or it has a big cam, which doesn't seem to be the case.
A big imbalanced vacuum leak could lead you to adjust idle up on the power circuit as well, to compensate. the guys in the thread that mention this are spot on, but I suspect if you close your throttle plate your engine won't run due to something else.
To look for a vacuum leak, you can spray small amounts of starter fluid around the suspected location. If the RPM jump up, you've found it.

All just a guess, but maybe...

[Bocephus69]

i purposely turned the idle up to show how it runs in the specific rpm range. this is in park, high idle, not touching a thing. https://www.youtube.com/watch?v=dDCYShlPNHY


when i turn the idle down to approx 600rpm, i still have no changes when making adjustments to the passenger side a/f screw. the drivers side adjustments are very noticeable.

[Bocephus69]

Originally Posted by geezer#99

If you suspect it's flooding, just look inside the carb at idle.
If you see any raw fuel then you need to find and fix the problem.

Originally Posted by im4darush

The motor should die when you turn the screws in. If it does not, as stated above, you are pulling fuel from somewhere other than the idle circuit in the carb. 900rpm idle is too high. The primary plates should be almost closed at idle. If they are not, you will pull fuel from the transfer slots. You can see these slots in the carb if you turn it upside down and look right where the primary blades close. At idle, only a very small square of the slot should be exposed to vacuum. Any more and you are off the idle circuit. At idle, your mixture screws are there to richen or lean out the idle. If you can not make it idle well with these slots not overly exposed (say 650 rpm warm), there is something else going on. Make sure your timing is correct as well. In terms of your surge at cruise, you are lean at that particular moment.

How was this carb rebuilt? Was the body soaked in cleaner and all the orifices confirmed clear?

i took the carb apart and followed some instructions i found online. i used about 6 cans of carb cleaner and compressed air.

[geezer#99]

Experiment!!
Get it warmed right up.
Loosen the dizzy a touch.
Just enough that it is barely easy to move.
Go for a test drive, find a slight hill to drive up in high gear at 30 mph.
Advance your timing until you hear pinging.
Turn your idle back down to 600 rpm.
Then put your timing light on it to see what it reads.
That will be your maximum initial timing.

You might be suprised what number you get.

[67 chevelle]

After watching the video , I would not rule out a valvetrain issue

[Bocephus69]

well, i am convinced that my issue is not timing related. even though i did make an adjustment that was recommended and my issue seems to be less "obvious", the problem still exists. it seems as if i need alot of initial timing to get it to idle nice (just by sound). at 600-700 rpm, if i unhook the vacuum advance and time it, it idles best at upwards of 20 degrees what it "sounds good". i know thats not the correct way.....so here are some numbers for you guys. it idles really nice by sound at 20-26 degrees, decent at 16 degrees, and 12 is about as low as i can go before the engine starts bogging down, losing RPM, and sounding like it wants to die. i loosened up the distributor and tested in multiple positions with no luck.


i asked for the help of one of my dads buddies over the weekend, and he agreed with you guys in saying "i had the idle too high, and it was perhaps not running on the idle circuit of the carb." so what we did was lower the idle until the engine wouldnt even hardly run on its own and tested out both a/f screws. this thing was idling well below 500 rpm. the drivers side screw made immediate changes for the good/better of the way the engine idled. the passenger side a/f screw still did nothing. all the way out, all the way in.....it made no difference in the way the engine idled or even sounded.


we took it for a drive, and he seen how nice it ran at cruising speed (or anything over 1/4 throttle for that matter). he is convinced i have issues with the idle to 1/4 throttle portion of the carb.


i did check for vacuum leaks around the carb. i did find one on the back side where the brake booster vac line hooks up to. i got that fixed, but it didnt change anything in regards to the issues i am asking about.


and getting back to the a/f screw on the passenger side. at this point, not matter what the idle speed/timing/after small vac leak fixed............that a/f screw does nothing. all the way out, all the way in....no changes. should my next step be to remove the carb and attempt to clean out that side of the carb?

[Redline novdog]

If it were me would pull the carb off and run some tag wire through all the passages and blow air through afterwards. Check your primary throttle shaft for play(worn) also.