Setting Idle jet needles & engine balance

[Paul in MN]

I'm working on my 88 W350 w/360 and 727 automatic. My first question is about the basic settings for the idle mixture adjustment screws on the Rochester 4 bbl carb. This carb was rebuilt by PO, and has the idle screw plugs removed so the screws are visible. I read in my Mopar service manual for this truck that I should do some hocus-pocus with propane to set the idle screw adjustment. My thinking is that is just plain nuts! Neither the Mopar manual nor the instructions that came with the carb rebuild kit give any "basic setting" info the idle mix screws. I am familiar with the old method of setting them for max vacuum with the RPMs set to a very low idle (but that might have been 40+ years ago??).

Can anyone give me the basic info needed to set the idle screws? P.S. This truck will never have to get smog tested, so anything close will probably be good enough. As it sets right now, she will start and run but has a very rough idle. I did find some major vacuum leaks the PO did not discover. So he might have adjusted the idle rich to make up for the vacuum leaks. The engine seemed to idle smoother (but not good) before I solved the obvious leaks.

The engine balance might just be related to the idle screw settings, but it definitely has a harsh vibration, very noticable at idle, and less so at driving RPMs, but it still seems to have some vibration that my older 86 360 does not have. I suppose anything is possible, like maybe the converter threw a balance weight. But are there any engine balance issues to be aware of? I did a compression check and found all 8 to be within 5 psi, from 135 to 140. This engine supposedly has less than 50K miles on it, so I doubt it is a cylinder problem. All new plugs, wires, and cap and rotor, and I have pulled all plugs, set them on the exhaust manifold and watched for a good strong spark at every plug while cranking it over. So I doubt I have an ignition miss.

Your thoughts and replies are appreciated!

Thanks,

Paul in MN

[712edf]

Has this truck sat up for a while? Unless its been in continuous service I'd use up the existing tank of gas, then refill with gas that doesnt have ETHANOL, before tuning on the carb further.
Ask someone who is in the lawn or tree trimming business how cruddy ethanol gas is.

Sometimes low rpm vibration is symptom of dead cylinder, but you said you checked that already.

Bucky

[greg rider]

The engine may have the wrong or a bad harmonic balancer. The 360 is balanced differently than the other small blocks, it is externally balanced and the others are internally balanced. A harmonic balancer from any other Mopar will fit but not work correctly. Another thing to think about is the motor mounts. If they are shot or have been replaced with urethane you will feel this as a bad vibration also. Just some more for your brain to chew on. As far as the carb,, turn the screws in,,NOT TIGHT,, then back out 1 and a 1/2 turns and this will put you in the park.

[Paul in MN]

Thanks for your replies

Bucky and Greg, thank you.

I asked a neighbor who is an active drag strip racer with a 1970's GM car to come over and give me some carb advice. He knows the Rochester 4 bbl well, as that is what he uses on his race car. Just as Greg has suggested, he turned in the idle screws 4 full turns and then backed them out 1.5 turns as a start. We got to 2 1/4 turns on each when he was satisfied and the vacuum gauge was happiest. The engine did smooth up somewhat, but still has a rough feel. Greg's advice about the harmonic balancer is what I had a very vague suspicion about, and I think he nailed it down for me. Meanwhile I am going to run out the current half tank of gas and get some non-ethanol brew into it. Maybe it will smooth up some more, but the harmonic balancer is the next suspicious place to go.

Can the harmonic balancer be replaced without messing up the front crank seal, or having to remove a bunch of stuff to change it? I suspect it needs a puller once the center bolt is out?? There is no leak on the front crank seal now, and I do not want to create one.

Thanks guys!

Paul in MN

[712edf]

Balancer usually removed just by loosening belts, removing lower pulley, removing large bolt, then using proper puller. This puller has 3 bolts that go into 3 of the 6 pulley bolt holes.

Of course radiator, fan, hoses & tranny lines all get into your way.

Bucky

[Don Williams]

You can identify the correctness of the harmonic balancer without pulling it. The balancer for a 360 is shaped differently from a 318 etc. balancer. Looking at the balancer straight on it will have what I can best describe as a cam lobe shaped counterweight as it would appear when looking at a cross section of a cam shaft. Hope that makes sense. The 318 etc. balancer is symmetrical.

[greg rider]

If this engine is a rebuild then it may have a different cam causing it to thump a little.