Acceleration problem

[DaddioRick]

That is my thinking as well. Yes she did run fine on the current distributor until she didnt. When the accel problem was first identified I pulled, disassembled and cleaned the carb. I then test drove with a friend who confirmed the secondaries getting fuel. I even removed the secondary bowl while underway and checked float level making a minor adjustment. The carb appeared to be running fine but the problem remained. This is why I have moved on to the distributor. But I do want to verify it is distributor before ordering a new one. Just need to find time now...

[DaddioRick]

Wow has it really been 4 months since I last focused on the Shamrock issues??? Lots of good excuses but nobody needs to hear them. Anyway... I have started the old girl several times over past few months using the driveway hose. Starts easy and runs fine. This partly encourages but also baffles me. I signed off previously thinking that checking secondaries is job one. But I am not sure how to test them. Is it just a matter of looking down the barrels upon acceleration? Is there any way to do this while running in the driveway on the hose? Thanks for testing suggestions...

[Ship]

Depending on the model carb, push accelerator all the way forward(full throttle) and see if the secondary baffles move to the open position when you move them. This can be done without the engine running. Then with the engine running push the throttle all the way open very quickly and let off all in one motion. The baffles should open and close when the vacuum activates them. The linkage on the side of the carb restricts them from opening until the throttle moves forward enough.

[gary s]

A Holley cannot be checked that way- the engine needs to be under a load,running on a hose won't cut it

[Ship]

Mine would??? Why wouldn't it?

[gary s]

Right from Holley's site-
Don’t judge the secondary opening by revving up the engine in the garage. Vacuum secondaries open under engine load, so if they do open by winging the throttle then they will most likely bog the car when it’s accelerating under load.
More info on checking secondaries- https://www.correctcraftfan.com/foru...ndaries-issues

[Ship]

Interesting articles. I appreciate the technical stuff. I was just using the back yard mechanic version.

[DaddioRick]

Here is my carb... but I don't believe I have any vacuum running to it. Never have. Am I missing something basic? Do the secondaries need vacuum to open? That would be a puzzler because not long ago this engine ran great and I have changed nothing on the carb. Wierd.

[tuner]

The secondary diaphragm gets a vacuum signal from the center section of the carburetor. Look where it attaches, you can see a small gasket. If this gasket fails, the secondary butterflies will not open correctly.

[Ship]

The vacuum is created by the intake exhaust cycle. Exhaust Valve opens and the piston pushes gas out. Theexhaust valve closes and the intake valve opens and the piston on ins way down sucks fuel/air mixture in. The suck IS the vacuum.

[crhodes]

The carburetor displayed has the vacuum actuator on the side and works as described above. Three failure modes I can think of off the top of my head.

1. Diaphragm in vacuum actuator has torn preventing back barrels from opening.
2. Port/vacuum path to actuator became plugged.
3. Gas float/needle valve for back barrels is stuck and the back barrels are running dry.

Bottom line, the best and recommended way to check this vacuum actuated back barrel assembly is to be under load on the water and looking directly into the back barrels. There should be gas coming out of the vortex horns/nozzle bar assembly like the front barrels are doing, though if not full throttle you will not see equal amounts of gas. If you are getting/ seeing gas, it is probably electrical. If not, the problem likely lies within your carb.

Best guess, if she runs great until 2000 rpm and then just flattens out, running fairly steady with more throttle, you likely do not have a fuel pump or supply issue (such as a plugged tank or vacuum line). Cant rule it out 100% but not the most probable cause.

[DaddioRick]

Very helpful thoughts all... hopefully temps will improve around here so I can get on the water to test under load. I agree if the secondaries are opening nd getting fuel my problem is probably electrical... will report back...