Warren Seale

I've got a clogged filter/orifice in the Optispark vent hose assembly that I need to replace. The part number for this assembly is 12555323 but after doing a search I only see it that it is used for the F body and full size car LT1 applications. Is this also OK for the Corvette? The discontinued assembly specified for the Corvette is 12558921. What is the difference? Can the 12555323 be modified to work in place of the discontinued part?

Warren

Warren Seale

There is one thing that the LT1/LT4 engines with the vented Optispark need and that is a clear functioning vacuum line. I lost a low mile GM (Delphi) Optispark due to the filter being clogged up and not allowing the distributor to vent properly. A word of warning to those with this engine; periodically check to be sure the vacuum line is functioning down at the distributor.

bob arrowsmith

how do you check for proper vent tube operation on the opti spark

Warren Seale

Pull the hose that goes to the intake manifold off of the distributor housing and measure the vacuum with a gauge. There should be manifold vacuum there. As a last resort you can use your finger to see if there is suction. I don't like to measure it on the intake hose side where it connects to the engine air intake because some air flow can enter the distributor at the electrical connector thus you may not see manifold vacuum at that location.

Whaleman

PM optidoctor on this forum. He can supply everything you need, Dan

Warren Seale

Thanks for the suggestion. I contacted the Optidoctor. He said that the 12555323 will work in place of the 12558921. The difference is minor and is related to the hose lengths being slightly different.

Warren

bob arrowsmith

thank you Warren for the info..

cardo0

Yes I've intended to tee into that vent line upstream of the check valve for a measurement of vacuum. And if you have already please post your results.
Thx.

1964r2r5388

I just put the F body line on my 95. I think the only difference is the length of the hose. The F body attaches on the side of the intake and the Corvette on the front under the throttle body.

Warren Seale

I get between 15 and 20 in which is typical manifold vacuum.

cardo0

Oh, so you are measuring 15 to 20"Hg on the Opti side of the check valve? I guess I meant to ask between the orifice and the Opti. We may not be looking at the same location. But I am very interested to find out the vacuum reading between the Opti and the downstream orifice. If you measured there it would be nice of you to post what you found but I thank you for your help already.

MatthewMiller

"Upstream" would be between the vacuum port on the intake and the orifice. That's what he measured.

cardo0

Upstream is opposite direction of flow. That should be on the Opti side of the orifice.

MatthewMiller

Okay, fair enough. Do your measurements.

Warren Seale

The 15-20 in Hg is between the Optispark and orifice.

cardo0

Thank you very much Warren for your unbiased report. I didn't want to drag you into a heated argument but your independent measurement is very helpful to understand what is going on inside the opti. That should be the same as about 7psi to 10psi differential inside of the opti itself compared to atmosphere. Well I'll leave it at that as I don't want to ruin your thread here.

Thank you again.

MatthewMiller

You understand that he's not measuring vacuum with full open flow through the opti and a tee fitting to the gauge, right? He's measuring with no flow using the gauge at the end of the line instead of the opti. That is, the gauge is a closed termination of the vacuum line, and there is no airflow, unlike when the line is actually connected to the opti.