Follow up to post regarding AV gas problems

[Joseph E.]

I performed the quick test suggested by Jim Trekell. Blocking the primary vent tube did not cause the engine to die within a few seconds nor did it do anything to improve the idle quality or sound. Noxious gas emissions were still high and stung my eyes. However, blocking the secondary vent tube immediately smoothed out the quality of the idle to the point that the sound was normal. The noxious gas emissions seemed to go away as well but it was somewhat difficult to tell with the garage already filled with fumes from the engine running while I blocked the primary vent tube. Is the fact that the blocking of the secondary vent tube improved the engine idle and sound an indicator that the secondary power valve is defective or is there something else wrong? Previous responses were greatly appreciated and those to be forthcoming will be as well. Thank you.

Joe Evans (45420)

[Gene M.]

Re: Follow up to post regarding AV gas problems

With aviation fuel you should be able to read the plugs. I will guess they are very black. Something is causing the rich condition or possible exhaust valve leaking. Leaking exhaust valve will allow the fuel to "burn" in the pipes which causes the "burning eyes". Also an over rich condition will do the same. Check those plugs first.

[John H.]

Re: Follow up to post regarding AV gas problems

However, blocking the secondary vent tube immediately smoothed out the quality of the idle to the point that the sound was normal. Is the fact that the blocking of the secondary vent tube improved the engine idle and sound an indicator that the secondary power valve is defective or is there something else wrong?

Assuming you have a Holley, your secondary throttle plate may be open too far, causing added idle flow through the secondary transfer slots; the secondary throttle plate adjustment calls for 1/4-to-1/2 turn of the screw from fully closed plates. When you blocked the secondary vent tube, it stopped fuel flow to the secondary circuits.

I don't think any OEM Corvette Holleys have secondary power valves, but I don't know which engine you have.

[Joseph E.]

Re: Follow up to post regarding AV gas problems

I checked the secondary throttle plate stop and it was properly adjusted at the lower end of what you quoted, one quarter turn. With regard to the secondary power valves, you may be thinking of some of the Holley carbs that have a secondary metering plate and not a metering block. The metering plate on the secondaries of these carbs is thin, maybe 3/16 inch thick whereas the metering block is thick at just over 5/8 inch. If I'm not mistaken, most of the SHP big and small block four barrel carbs had primary and secondary metering blocks which means that they would have primary and secondary power valves. Thanks again for the input and suggestions and I'll keep you posted if (hopefully) I get the situation resolved.

Joe (45420)

[Clem Z.]

Re: Follow up to post regarding AV gas problems

I checked the secondary throttle plate stop and it was properly adjusted at the lower end of what you quoted, one quarter turn. With regard to the secondary power valves, you may be thinking of some of the Holley carbs that have a secondary metering plate and not a metering block. The metering plate on the secondaries of these carbs is thin, maybe 3/16 inch thick whereas the metering block is thick at just over 5/8 inch. If I'm not mistaken, most of the SHP big and small block four barrel carbs had primary and secondary metering blocks which means that they would have primary and secondary power valves. Thanks again for the input and suggestions and I'll keep you posted if (hopefully) I get the situation resolved.

Joe (45420)

some holley carbs with a secondary metering body,the thick one,have no power valve in the secondary. you can tell by the jetting because the ones without a power valve will have much larger jets in the secondary to make up for not have power valve feed channel in the metering block.

[John H.]

Re: Follow up to post regarding AV gas problems

If I'm not mistaken, most of the SHP big and small block four barrel carbs had primary and secondary metering blocks which means that they would have primary and secondary power valves.

There were LOTS of Holley 4150's (two metering blocks, not a primary block and secondary plate) that didn't have power valves in the secondary metering block. The original 4053 on my '69 Z/28 has a power valve in the secondary metering block, but that configuration is unusual.

What year/engine do you have, and what LIST number Holley?

[Joseph E.]

Re: Follow up to post regarding AV gas problems

All of the Holleys I've ever tinkered around with have always had secondary power valves in the thick secondary metering block. I wasn't aware that some with the thick metering block didn't have power valves. You learn something new every day! Thanks Clem.

Joe (45420)

[Terry M.]

Re: Follow up to post regarding AV gas problems

Me too on the learning. Like John H, the Holley on my 1970 LT1 has a power valve in both the primary and secondary. I was unaware that configuration was unusual. It is all to easy to think that the product we are familiar with is the same product everyone has.

[William C.]

Re: Follow up to post regarding AV gas problems

In general, the single feed carbs had no power valve in the secondary, and dual feed did, not hard and fast, but a good general guide.

[Gene M.]

Re: Follow up to post regarding AV gas problems

In general, the single feed carbs had no power valve in the secondary, and dual feed did, not hard and fast, but a good general guide.

Does the '66 425hp 3310 Holley have a secondary power valve?

What is the purpose of a power valve in the secondaries?
To allow use of smaller secondary jetting?

[Joseph E.]

Re: Follow up to post regarding AV gas problems

Gene,

The Holley on the '66 L72 is a 3247 unit and it does have a secondary power valve. The power valves are rated based upon the amount of manifold vacuum required to keep them closed. When manifold vacuum falls below the rating of the valve, the valve will open and allow additional fuel to be fed into the engine. I think smaller jets can be run because of the presence of the power valve. That's my understanding of how things work with the power valves.