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Old 09-20-2013, 04:50 PM
oldbogie oldbogie is offline
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Originally Posted by martyminnesma View Post
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.

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.

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?

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.

funny thing too, if i let the manifold remain uncovered,

What does it mean the manifold 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?

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.

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?

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.

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?

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....

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?

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!
A lot for you to look at winter's coming on even here you can feel it in the air.

Bogie

Last edited by oldbogie; 09-20-2013 at 04:57 PM.
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