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TOPIC: 1966 122S Idle Speed

1966 122S Idle Speed 2 months 1 week ago #1377

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Well I need some technical help.

Question: Should I be able to achieve a smooth 6-800 rpm idle on my B18?
Best I can do is 1000 RPM and even that is not as silk smooth as I imagine it should be.

Background:
Rebuilt B-18. .020 overbore, new camshaft-VP 'C' cam
.020 off head, block decked. Valves refurbed. Standard thineer head gasket.

SU HS6's rebuilt; new bushings on throttle shafts; floats checked, new ZH needles; centered properly; good 'drop'.
Distributor rebuilt by Glen Ring, NY; new points, condenser, etc.
Dwell set at 60 degrees; timing = 22 BTDC
Compression all 4 cylinders after breakin = 150 psig (older pg, could be slightly higher; not sure.)

Valves checked; in adjustment.

Adjusted SU's per procedures. Best I can achieve is around 1000 rpm for idle. If I go lower, the engine runs too rough/shakes. I've looked for vacuum leaks. PCV valve is suited for 65 chevy 6 cylinder - AC CV-723C. Propane test around intake manifold gaskets and carb/heat shield reveal nothing. Don't want to use carb cleaner or brake fluid. I feel the gaskets are good and tight; even thought this screams vacuum leak.

Originally, I wanted to boost compression somewhat with the rebuild to achieve more HP, but my engine mechanic was leery of this with today's gas. He didn't want me to have problems; so I think I ended up with pretty much a "D" engine again; albeit rebuilt. The 150 psig across all cylinders indicates this to me.

Bottom line:
I can't get a smooth idle below around 1000 rpm. I was expecting to get 6-800 rpm and have viewed some nice smooth, slow idles on YouTube.

Any thoughts/ideas on my expectations? Any help much appreciated.
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1966 122S Idle Speed 2 months 1 week ago #1378

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Other than high idle is engine performance ok?

Describe the valve adjustment method used.

Describe the hoses arrangement on the PCV system and brake booster if any.

Does the oil filer cap have a filter medium inside?

Pull the vacuum hose at the intake manifold - is there an insert in the manifold connection with a very small hole?

Hook up a timing light and watch the timing mark move as your range up/down the rpm range. Is the mark steady at 3,000rpm? At what degrees BTDC?

Remove the air filters and turn/count the mixture nuts flats to full lockup (lean). How many flats? Return the nuts to the original position.

Confirm that the choke mechanism is not acting on the carbs during adjustments.

Confirm that the linkage action on both carbs is/are simultaneously.

Pull out the choke fully then push in firmly. Immediately press the jet(s) upward with one finger. If you feel any movement at all then the jets are not returning home and idle will be erratic. Be sure the jets are properly centered.

What color is the fine-wire spring inside the carb body?

What oil is in the carb spindle chambers? How much.

Whew - you still here?

George Dill
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1966 122S Idle Speed 2 months 1 week ago #1379

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Afterthought...

The needles in the carb pistons must be properly installed.

The shoulder of the needle at the beginning of the taper should be flush with the adjacent surface of the piston.

George Dill
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1966 122S Idle Speed 2 months 1 week ago #1392

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Sorry for the delay getting back; caught a summer cold from my wife.
Thanks George for the laundry list of things to check. Here's what I found:

I installed the new needles properly with shoulder even with piston.
Other engine performance is good/strong/smooth.
Valve adjustment was go-no-go on specified gaps. Actually, I did it hot and many were slightly more loose on the no-go, but not excessive. I don't think valves are at play.
PCV is from engine flame trap box to valve to intake manifold. Inlet to intake is not restricted (no orifice). When I pull PCV hose from breather box on side of engine and cover inlet to PCV, I get strong vacuum. When I uncover inlet to PCV, idle drops down, but rough. Oil filler cap is new and have K&N filter on top. Cap is filled with clean bronze/brass mesh material.
No other connections to vacuum. (no brake booster)
Timing steady at 3000 RPM.
Choke not in play at idle.
Carb springs are purple.
Jets are centered well.
Oil in damper is ATF. filled to 1/4" below shoulder of neckdown.

I was able to get down to ~800 after more carb adjusting nut effort. I think I might purchase a colortune device to really nail the mixture. Do you have any experience/thoughts on this. My prior efforts (although not significant) seem to result in carboned plugs. Will the plugs foul up if all you do is idle?

Also, and this may be key; the car is up on jack stands. Didn't think about this but I suspect idle might be smoother with wheels on the ground as the suspension may help dampen things. Any thoughts on that as well?

My current plan is to wrap up remaining items and run the car some around the block. I may be that down on wheels will alleviate the situation; that and possibly using colortune for mixure.
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1966 122S Idle Speed 2 months 1 week ago #1393

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Many B18/20 tuners now use the "rule of nines" to adjust valves along with a modified 0.017" tapered feeler gauge adjusted to a "tight" drag on the gauge and skip the "go no-go" step.

The PCV routing looks good. The PCV valve has a fine-wire spring inside that keeps the valve closed until a certain amount of crankcase pressure develops then the valve opens until the pressure drops to some value to cause the valve to close. This cyclical action is irregular and can sometimes affect idle quality especially at low rpm.

Colortune may be unnecessary if the carbs are healthy and the mixture set to run smoothly. A light coating of grey carbon on the plugs is expected.

Note - avoid fine-tuning the carbs when the engine is very hot.
Last Edit: 2 months 1 week ago by gdill3. Reason: Sig
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1966 122S Idle Speed 2 months 6 days ago #1394

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Thanks for the followup review.

Do you have specific PCV Valve numbers that NAPA or others may carry. I wouldn't mind changing mine out with another; cheap try. The one I have I bought off EBAY. It's old and original and I think it's correct, but might be worth a try.
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