Idle problem

[dlphil10]

If it is set to about 1 -1.5 turns from lightly seated it loads up at idle, but it seems to run better the more I turn the screws out.

[Cliff Ruggles]

Do the same idle test you did earlier by putting your hand over the front of the carb at idle speed, or gently closing the choke flap.  If the engine speeds up or smooths out, it still wants more idle fuel.  Next step would be to open up the DCR"s slightly......Cliff

[dlphil10]

I finally got to where I can work on it more. It runs without spark knock under load or cruising so I'm assuming the slight flub and irregular spark knock at idle is due to the lean carb setting. What idle down tube restriction size would you recommend for my setup?

[Cliff Ruggles]

.046" to start with, if that doesn't bring in enough fuel open them up to .052".....Cliff

[dlphil10]

So I've got it to where it appears to not want any more fuel by the hand over the choke method. However, it still seems to have a slight miss, it was cleared up considerably with the carb adjustment. Could a warped air horn or bad air horn gasket be causing the issue? All cylinders are firing evenly, the plugs all look good and exactly the same. Manifold vac is rock steady at roughly 14 or 15inHg.

[Cliff Ruggles]

The 350 is a small CID engine, so it's seeing that cam as a pretty big camshaft.  In any and all cases, give the engine the fuel it wants in all areas to be happy.  If it's lean at idle, open up the idle system accordingly.

Compression ratio, cam timing, and ignition timing play a HUGE role in idle quality, and idle fuel requirements.....Cliff

[Pat]

I solved a similar problem by accident.  My heater wasn't putting out hot air on a freshly rebuilt 350.  My carb stumbled upon stepping on the gas and did several adjustments to the carb with no sign of improvement.  I was frustated.  So one cold morning I covered the grille with tape and plastic to help the heater.  To my surprise, my heater put out hot air and the stumble miraculously went away.  Just before a full stop, I thought my engine was going to die but it was because it started idling much more smoothly.  Going from a 160 to a 195 degree thermostat was the fix.  Carbs will work better with a hotter engine while keeping the carb itself as cold as possible.  Something to consider.

[Cliff Ruggles]

I'm not even sure why they sell 160 degree thermostats, and yes, heat is your friend with these things.

Just curious if the heat crossover in the intake is also blocked off?......Cliff

[Pat]

It's nonexistent, Cliff.  I have a Vortec engine. 

[Cliff Ruggles]

For a wet flow system it will work best fully warmed up and heat soaked.  That may take a while when there is no heat crossover in the intake, and even longer if the intake has an air gap under it.....Cliff

[Pat]

I have no air gap either (wish I did), but sure enough, it takes longer than average to reach operating temperature. Once there, it performs like a champ. Those q-jets are fantastic.  Thanks for the response, Cliff.

[Cliff Ruggles]

With a wet flow system a cold intake just doesn't work well.  When the heat crossovers aren't open to heat the intake, it takes quite a while for engine heat to get the intake warm enough so fuel doesn't glob up on the walls and require a richer mixture from the carburetor.

To this day I don't know why folks want to block off the crossovers, it simply works poorly for anything street driven.....Cliff

[dlphil10]

Maybe this is an issue for me too. My cross over is intact, but my coolant typically runs like 160-170. If I idle after cruising a bit it can creep up a bit.

I am also wondering two things uncarb related. I am wondering if this carb is too much for my build or if my rings are fully seated.. I did a compression test and all cylinders were 120psi. Seems low... Even why I was diagnosing the failure of that factory muscle 041 comp cam, the compression was 120psi to 150psi. This cam should close the intake more quickly than that 041. Seems to me those heads are far from 72CC.

[Pat]

Your compression is even on all cylinders, and that is great.  I think your next diagnosing step is trying a 195 degree t-stat.  It's cheap and quickly done.  Then, go from there.  Basics should never get overlooked.

[dlphil10]

It is a 195 thermostat.. thats what gets me. I do have a 3 core radiator. I wouldn't have thought it would be over cooling though. Would a weak radiator cap keep it from getting hot?