Running rough under load mystery.
ok so background info. its a 1972 chevy c10. rebuilt 350 with a mild cam, hydraulic lifters, roller rockers, edelbrock aluminum e-street heads, performer intake and a 1406 carb.
it used to run just fine, but slowly over the course of a couple months it ran richer and richer. to the point where it was dieseling every time i turned it off. I even changed the jets and metering rods twice to lean it out, but it still ran really rich. I'm currently at the 13 setting on edelbrocks little jetting chart.
most recently i drove it on a 300mile trip and i couldnt put the gas pedal past a gentle cruise without the motor shaking around and stumbling. If i dropped a gear and revved it up it would accelerate slowly but pop and bang out the exhaust like crazy. gas mileage went down the tube on this trip. i originally did the 300 miles on one tank, the way home with it running like this took almost two full tanks.
upon getting home ive done a few things that have helped lesson the symptoms of my problem, but i think the main issue is still there.
- I changed the weights and springs in the HEI distributer. total timing was coming in at 44, with idle at 8. vacuum advance was on ported.
- after the new weights total timing was a proper 36
- I tightened the T-fitting for the brake booster manifold and sealed it more properly
- i redid the intake manifold gaskets, thinking they were leaking, but they were not, same with carb gaskets and anywhere else that uses vacuum. Im 99% sure vacuum leaks are not my problem, I've checked with carb cleaner and ether. nothing found.
- ive disassembled the whole carb, cleaning it with carb cleaner and the compressor. also double checked the floats and needle and seats. cleaning it single handedly made the biggest improvement, but still didn't entirely fix the problem
- i changed the vacuum advance to manifold. definitely runs better, more low end power, a little less rich running.
- gone through and metered out the spark plug wires, all are fine. all are in the correct order.
i still have to get a reading on my fuel pressure and I also want to get a vacuum gauge to make sure i dont have any leaks.
also to describe the sound more clearly. when i'm accelerating hard in second or third, the truck switches back and forth between running great with strong acceleration and a smooth sounding exhaust to running rough with poorer acceleration and a louder crackling exhaust. it goes back and forth every second or two.
my best guesses are a bad coil,
a bad ground (is a solid number 12 awg good enough to ground the block? worked before this issue came up),
possibly a bad spark plug that comes on and then quits.
perhaps a sticky lifter on an exhaust valve (but would that cause the terrible gas mileage?)
sorry for the huge essay, just want to get you guys as informed as possible so that you can have the easiest time helping me figure this crap out.
Wiped cam lobe or lobes? I ran an 8ga. ground wire straight to the pass. side head then 8ga. from the driver side head to the frame, then a 10ga. Wire from each head to the fire wall. I also grounded the HEI dist. at the base. I thought when coils went bad they just died. This sort of sounds like a dist. wire grounding, you said that it runs great then switches to bad, sounds like a bad connection or a loose wire flopping around some where with a bear spot on it an is grounding. I'm just guessing. I had an old points system an the black wire from the coil to the dist. had a bear spot on it an at speed it would cut out then run then cut out again, took forever to figure out. You might also look at the connectors inside the spark plug boots on both ends.
Hope this helps.
it is an edelbrock carb, so no power valve.
also i've gone through the fuel delivery system so much that the only thing it might be is not enough fuel pressure, which i will check today.
other than that its gotta be ignition im thinking.
True Edelbrock doesn't have a power valve...it wasn't mentioned in the original post and both Holley and Edelbrock have the part number 1406...so it was just a thought...One other thing you could check that could cause this if fuel delivery isn't the problem...check your ground cable from the engine to the frame...run a heavy gauge wire from the block to any area of the truck that has a clean ground...see if the engine doesn't perk up. Often these problems can seem overwhelming when they are something very simple...like a ground wire...I know you mentioned it...Have you tried it?
check the vaccuum on the hei dist. mine was going south and was causing engine to act up, especially at speed.
Check your fuel pressure.
IIRC, Edelbrock recommends a maximum of about 6psi for their carbs, but mine ran best when I kept the pressure at about 5.5psi. Any higher and it would behave like yours.
how do i check that? and what exactly goes wrong with it?
i'll check fuel pressure and vacuum pressure tomorrow, I just bought a gauge today. I also bought a heavier gauge wire to ground out the block, see how that works too. wish me luck
and thanks for all your help so far.
I had the same thing happen years ago. It would sometimes even drop a cylinder then pick it back up. Plugs black and no power. Chased my tail for a long time. A simple test proved to be weak spark. Pull a wire, put a piece of metal in the end and look at the spark jumping to the plug. If it short and yellow it is either coil or wires. Should be snapping and blue.
Pay attention to the voltage when it happens, do the head lights dim. You can also connect a digital voltage meter to the aux. fuse at the fuse box (hot side) an see if the voltage drops. To find the hot side turn the key to on, one side will have voltage an the other side won't. If you have a voltage drop then you have a bad/loose connection, then check battery,starter, battery connection wire at HEI. If its intermittent an happens quickly it could only be a voltage problem. Bad module in the dist. I bet.
1) Rich mixture
2) Failing ignition
3) Cam falling out of time
Unfortunately all 3 can make the exhaust appear rich in the first case because it is, in the second and third cases because of failed or incomplete burns.
Happening over time shows a progressive change going on which doesn't eliminate any of the three suspects.
1) Rich mixture:
- Excess pump pressure at the carb to where the float controlled shutoff valve can't
- One or more of the floats is becoming fuel logged then sinks some or completely.
- The vacuum controlled piston that operates the primary metering rods is sticking, or the vacuum isn't applying do to a clog somewhere in the carb or a gasket failure or missing or incorrect type for the carb model, metering rod bent or not in the jet.
- Exhaust plugged
2) Failing ignition:
- Can be the trigger, if points it can be the points themselves or the shaft on the distributor is worn so it wobbles giving a constantly changing dwell, or the condenser can be bad or the voltage from the key switch unstable, points use two circuits from the key, battery voltage while cranking and resistor (usually resistive wire on GM) that supplies 6-9 volts to the coil when running. If an HEI this can be the module or wiring, these use battery voltage all the time , one needs to be careful when introducing HEI to a points vehicle to rewire the "run" circuit with no resistor or resistive wire.
- Certainly the coil could be going out, they fail in a host of ways including almost but not quite. When you demand more power the temperature and pressure in the cylinders goes up, this increases the dielectric strength of the mixture (to say it resists the making of the spark) so this requires a greater effort* on the part of the coil, if it is not receiving sufficient voltage or itself is failing that margin will not be there so when the power demand on the engine goes up the spark misfires or late fires, either of these will make the engine appear to be running rich and in the case of a late fire will appear to be out of time.
*actually the coil has no way to increase its effort based on cylinder pressure, the coil is pretty constant in output, but if voltage or switching time is/are insufficient the output voltage will not be high enough or shaped correctly to sustain jumping the gap in the face of higher pressures.
3) Cam getting out of time.
- One thing that struck me was your redo of the springs in the distributor and stating before advance was 44 degrees and after is 36. First the spring should not be depended upon to regulate the maximum advance; that is the job of stops in the plate that limit the movement of the counterweights with a positive stop.
- A characteristic of the cam getting out of synch with the crank is the need to frequently have to readjust the base advance setting. This is also a characteristic of the distributor gear wearing---to that end you indicated that this is running a hydraulic roller cam which would not be native to a 1972 pickup, roller cams being made from billet steel or CGI iron are not compatible with 1972 distributor gears they wear the old gear fast. The racing community uses bronze or carbon/epoxy composite gears but these wear pretty soon if used on the street. The factory makes a Melonized treated steel gear on roller cams which provides the necessary long term wear for the street. So the question here is what gear is on the bottom of the distributor?
For the cam to be getting out of synch with the crank would be a wear or failure of the timing set, gears and chain. Normal operation can show timing with the light drifting 2 maybe 4 degrees, if you rev the engine and suddenly close the throttle that amount of wander should remain, if it gets more than 4-5 degrees the timing set needs replacement. The cam falling out (retarded because how it rotates demands that outcome, the engine will lose power first and most up in the top RPMs and/or under load, fuel mileage will drop as now the cam events are late regardless of ignition timing settings and the engine will appear to be running rich.
I'm adding this to say that once the plugs are fouled it only gets worse for them. Even if you fix the problems, they have developed conductive paths on their insulators that allow coil voltage to bleed to ground rather than jumping the gap, so you may need to go through several spark plug replacements before this is settled or that you can even tell the problems are settled.
a few updates from some testing.
ok so I bought a vacuum/fuel pressure gauge. when I connect the gauge in the fuel line, the needle bounces around so fast its impossible to see what the actual pressure is. is that normal? it seems to bounce between 3 and 9, sometimes falling steady for about a milisecond on 5-5.5.
when i hook the vacuum gauge up to the manifold port on the carburator, or the T fitting on the manifold behind the carb, i get a reading of 15Hg at idle. as i rev it up slowly it climbs to about 17, when i let off it jumps to 20 and then remains at 15. its idling pretty rough at the moment too. I'm thinking 15 is too low. the gauge says where the needle is sitting that my timing is too retarded, so i tried bumping it up a bit and it made no difference. and also isn't manifold vacuum supposed to disappear completely when you rev it up? not get stronger?
funny thing too, if i let the manifold remain uncovered, it almost sounded better at idle, but the engine was shaking like crazy, if i reved it up, it seemed to run better than it has before. its hard to tell without a load though, it just sounded nice for once. maybe because it was sucking extra air into the mix and leaning it out.
I swear if there is a vacuum leak it has to be in the carb itself, i've checked every gasket and hose and i can't find a single spot where its leaking, but the carb sounds loud, like a rushing, whooshing. kind of like sipping through a skinny straw really hard. but is that even possible? i need to find another spare carb to try. anyone know anyone in calgary alberta?
another few things about timing.
I replaced the weights and springs because the old ones were rusty and probably not working right, and they had a different shape that i believe allowed them to spin open more for the purposes of propane, that's why i replaced them with stock ones. but here is where i get confused....
with the vacuum disconnected, im idling at 10 degrees. when i rev it up, the weights only bring it up to about 16. when i connect the vacuum to the manifold port, it brings it up to 26. when i rev it up it climbs to a proper 36. is something weird going on here??? or is this all good?
also the HEI condenser tested fine, the coil tested as ok, but that doesnt mean much when its off and cold. but it is just an original Iron cam gear. maybe it is warn and thats why the weights are making it climb very high? i dunno? the rotor didnt seem to have a lot of play in it....
new plugs are definitely in order though, im sure they've fouled a couple. but i think my underlying problem is not the spark plugs.
I'm lost, someone please help!
It should be more stable than that, you may need a regulator in the line which will smooth the pulsations and allow a tighter adjustment on the pressure which should be 4 to 6 psi.
is a mechanical pump without a regulator normally like that? it goes berserk when i rev it up too. im guessing its not abnormal considering its a diaphragm driven pump.
You said it has a mild cam having the specs would help determine if the 15 inches reflects a wilder mild cam or whether the cam is retarded to the crank. What you're seeing is normal the off idle vacuum comes up with no load on the engine, dropped shut the vacuum will jump up then fall back. Here we are talking about the cam being retarded rather than ignition, but changing that should have an effect on idle speed and vacuum.
k cam spec, they dont mean much to me but here it goes:
cam lift intake .281
valve lift intake .422
SAE duration: intake 278 exhaust 288
SAE Timing: BTC 27
I hope that tells you something. I remember when putting on the cam and timing chain and everything that i set the cam at 4 degrees advance, they gave a little mark on the gear for it which told me which was what. I'm 100% confident i got that right. and i only have about 4000miles on it since the rebuild. is it possible for the chain to stretch that much to put it out of time?
funny thing too, if i let the manifold remain uncovered,
What does it mean the manifold uncovered?
I mean the T fitting behind the carb, I leave one side of it unplugged to open air, and the truck seemed to sound nicer when i revved it up. but it maybe idled worse.
A vacuum leak would drop idle manifold vaccum but certainly would not cause the engine to run rich unless it was so large as to pull the carbs power enrichment circuit on but that would be a lot lower than 15 inches at idle. Another way of "forcing" a rich mixture is a plugged air filter. I had earlier mentioned a plugged exhaust this keeps burnt gases in the cylinders which slows combustion making the engine appear to be running rich on the carb and retarded in timing. Take a look at the exahust system for crushed pipes, mufflers that collapsed internally, and look at the heat riser damper if it has one to see that it is open (or closed) this thing forces all the exhaust through the exhaust crossover of the intake when the weather is super cold, like what you get in the Great White North.
the exhaust seems to be in fine shape. and these heads don't have the port for the exhaust cross over. so that cant be it. i think it would run noticeably worse if the exhaust was blocked on one side. both pipes sound pretty even.
Propane back to gas OK this makes sense! OK base timing is set without vacuum advance, the centrifugal should be inactive at idle but become active about 1000-1200 RPM and should be all in by 2000 to 3000 depending on the spring choice and where the weights are physically stopped if such stops are on the plate they swing from. If there is no physical stop and the springs are preventing further advance then a softer spring should be used as with 10 degrees base there sould be about 26 to 30 degrees in the centrifugal. The vacuum advance is there to provide advance sensitve to load which affects manifold vacuum. This will be higher with low throttle opening which create high manifold vacuums and will decrease as the throttle is opened, the assumption here is the engine RPMs are going up so the centrifugal will take over. The problem is in mountains where the RPMs may be fairly low and the throttle quite open so the mainfold vacuum is low, this can be a hole in the need for advance, the assumption made by the designers is that in this situation the mixture density will be high which adds speed to the burn naturally. Speed of the burn is what these advance systems are all about. The burn is proportional to the compressed density of the mixture in the cylinder. At low RPMs with a closed or nearly closed throttle the density is low requireing a lot of advance. As the throttle is opened and RPMs rise the cylinder density increases as does the burn speed but under 3000 RPM it is not proportional so systems need to be applied to start the burn early enough for the best cylinder pressure to occur from 20 degrees after top center to about 65 degrees. From 3000 RPM up for some reason mother nature takes over and the burn speed increases on it's own from there on up the RPM band for quite a long time.
k that all makes good sense actually, had to read it about 5 times haha. yeah the weights are really not opening that much. they basically only add about 8 degrees. it doesnt feel sticky though, maybe i'll try a lighter spring and see what that does, but i would think the medium spring would open eventually.
also the HEI condenser tested fine, the coil tested as ok, but that doesnt mean much when its off and cold.
Is HEI original to this truck? If not, was it wired properly to feed it full battery voltage all the time?
yeah im pretty sure they got it right, its been running on hei for 30 years now, im pretty sure i checked when i rebuilt the motor, but i'll double check again.
If you put in a roller cam the original HEI gear if it is GM of that era would not be compatible with the cam's material. Usually the distributor gear wears in this case but sometimes it takes the distributor driving gear of the camshaft with it. Timing would be erratic and lean toward retarded if this is the case. I'd pull the distributor and take a look at its gear. Also how many miles are on the timing set between crankshaft and camshaft?
i had to look up roller vs flat tappet cam, I have a flat tappet cam, so i'm hoping that wont mess with the gears? i had the distributor out a few days ago and didn't notice anything odd about the teeth on the gear, but i'll double check.
thanks so much for all your help, this is helping me narrow it down big time. tomorrow I'm going to;
try lighter springs in the distributor to get a proper advance out of it.
check the spark plugs for a good hot spark
check the gears on the distributor
check voltage at the hei
possibly just replace the plugs and wires
possibly try a fuel pressure regulator.
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