Question for john hinkley on manifold vacuum

[Ralph P.]

I see references to changing ported vacuum to full-time vacuum related to the L71 & L72. Can we also assume this is good for the L68,or, really any other ported configuration? Also, can secondary operation on a tri-power be confirmed by any other method than on the road full power runs? THANKS

[John H.]

Re: Question for john hinkley on manifold vacuum

I see references to changing ported vacuum to full-time vacuum related to the L71 & L72. Can we also assume this is good for the L68,or, really any other ported configuration? Also, can secondary operation on a tri-power be confirmed by any other method than on the road full power runs? THANKS

Ralph -

Yes, the L-68 and L-71 use the same center carburetor (LIST 3660), which has a "ported" vacuum source port for the distributor, so the L-68 will respond positively to a change to full manifold vacuum just like the L-71 does. Verifying secondary operation requires that the engine be under load, so it needs to be done either on the road with a paper clip or blob of grease or on a chassis dynamometer.

[Duke W.]

Re: Question for john hinkley on manifold vacuum

I think you could use a Mity-Vac to open the end carbs. I don't know if there is a spec for full open, but someone may know the value range through experience, or you could rig up the Mity Vac to see how much vacuum the diaphragms see at WOT maximum RPM.

Duke

[Stuart F.]

Re: Question for john hinkley on manifold vacuum

Wish that were true, but my experience trying to test the end carb diaphragms with a Mity-Vac found that the springs within were too strong to overcome. You pump once with the Mity-Vac and the spring would force it to release vacuum before you could pump again. The pump will not hold the vacuum against the strong spring pressure. John's test description (with a capital "J") is probably the only way to do it. I gave up on trying to figure a way on my son's 69 L-89 except to listen to the sound and feel it in the seat of my pants (is that called SOMP?). We still have a problem with his rear carb wanting to hang open occasionally. We've tried adjusting the mechanical "safety" closing linkage on the left side several times without success. It seems to be a throttle butterfly problem, but my son doesn't want to mess with it anymore because it is not a serious issue, except as it upsets the idle speed. A couple of throttle blips will usually close it (did I mention I hate Holley's?).

Stu Fox

[Ralph P.]

Re: Question for john hinkley on manifold vacuum

Thanks for the excellent feedback . Also, I see diaphragm spring kits advertised for the Holly 4-bbls, but, they don't specifically say the same springs fit a 2300 2-bbl . Can I assume both are the same? If they are color-coded, which ones "normally" are the best to use ?

[Gary C.]

Re: Question for john hinkley on manifold vacuum

Stu,
On your rear carb, what thickness base plate gasket do you have? On my car, when I used the thicker gasket, the throttle butterfly would bind when tightening the four nuts. With thinner base plate gasket, no tweaking of base plate, no bind of throttle butterfly.

Gary Craig

[Stuart F.]

Re: Question for john hinkley on manifold vacuum

Gary;

Thanks for the tip. My son mounted the carbs in my absence so I'm not sure which gaskets he used. Believe they came with the new carbs, but I will check with him.

Stu Fox

[Gary C.]

Re: Question for john hinkley on manifold vacuum

Stu,
The thicker gaskets that would bind the rear two barrel throttle butterfly was 1/16 thick, actually measures 0.058. Made some thinner ones (0.031 thick) using the thick ones as template.

Gary Craig

[John H.]

Re: Question for john hinkley on manifold vacuum

Gary;

Thanks for the tip. My son mounted the carbs in my absence so I'm not sure which gaskets he used. Believe they came with the new carbs, but I will check with him.

Stu Fox

Stu -

Hope he had them oriented properly - they're marked "This Side Up Center Carburetor" and "This Side Up End Carburetor".


3x2BaseGasketCtr.jpg3x2BaseGasketEnd.jpg

[Stuart F.]

Re: Question for john hinkley on manifold vacuum

John;

My son has done this several times, so I hope he knows by now which way is correct. I will see him tomorrow morning at an iHop breakfast with his jetBlue Corvette pilot buddies. We're going to convoy together to the October Vettefest at Old Town. Hope to see other Central Florida Vette owners there. We're expecting our first real cool front to come through with temps in the high 60's. Love that.

I will check with him about the gaskets then, but don't expect he'll drive the 69.

Thanks again for your valued advice