68 Tri Carb question 435HP

[James N.]

Re: 68 Tri Carb question 435HP

Jim

With all the help here I did determine mine are opening all the way. As I recall, the test you describe did result in the secondaries going open all the way or close to it when I tried it. You're sure your linkage is not restricting the secondaries?

Rich

I'm sure. I get the same results when the linkage is totally disconnected.

[Clem Z.]

Re: 68 Tri Carb question 435HP

Are the throttle plates on the secondaries, activated by the diaphrams, supposed to open all the way with the engine off, under WOT at the primary when vacuum is applied to the secondary diaphrams or are they supposed to just crack open a little bit? I'm not able to get the secondaries to open all the way when I apply vacuum to the secondaries with the engine not running at WOT. I can manually open the front and back throttle plates by pulling the linkage forward. The springs do not seem to be overly strong, I get the same results when I disconnect the linkage. And I get the same results when I test the diaphrams individually, again with the linkage detatched. Diaphrams bad?

there is a vacuum passage into the veturi of the end carbs to add additional vacuum as they start to open so if you just apply vacuum to the diaphragm thru the hoses you have a vacuum leak into the carb venturi

[John H.]

Re: 68 Tri Carb question 435HP

I'm sure. I get the same results when the linkage is totally disconnected.

Jim -

You can't get full secondary opening by revving it in the driveway; the engine has to be under load (on the road) at WOT to generate enough airflow velocity through the primary to create enough venturi vacuum to feed the secondary diaphragms.

[James N.]

Re: 68 Tri Carb question 435HP

there is a vacuum passage into the veturi of the end carbs to add additional vacuum as they start to open so if you just apply vacuum to the diaphragm thru the hoses you have a vacuum leak into the carb venturi

Got it. That explains why the diaphrams won't hold vacuum and the sucking sound coming out of the throat of the secondaries. Then, as John says, I need to go at WOT on the road to fully engage the secondaries. When that's done, how can I tell if the secondaries opened all the way? I've gone through the '69 Chevrolet Service Manual and understand how to properly adjust the linkage. Is there any way to absolutely veryify that the secondaries fully open?

[Clem Z.]

Re: 68 Tri Carb question 435HP

Got it. That explains why the diaphrams won't hold vacuum and the sucking sound coming out of the throat of the secondaries. Then, as John says, I need to go at WOT on the road to fully engage the secondaries. When that's done, how can I tell if the secondaries opened all the way? I've gone through the '69 Chevrolet Service Manual and understand how to properly adjust the linkage. Is there any way to absolutely veryify that the secondaries fully open?

read my post #9 above