OMC 460 vacuum test/reading

[Trevthefox]

Hey everyone,

I have a '89 7.5 L OMC 460 with a 650 CFM holley carb. I just recently rebuilt the carb and was going to do my due diligence on tuning the carb/engine to the right settings. I've set dwell and have the timing where it needs to be. I was thinking of doing a vacuum test coming off the intake manifold and using the idle mixture needles to get the highest possible vacuum. Any idea on how much Hg this engine should pull.

And any tips on performing this test? I've never done this so after some digging around I found that you need to get the engine up to normal operating temps and then play around with the needles to get the highest vacuum.

Any info would be greatly appreciated.

 

[bruceb58]

Here Is The Correct Procedure For Setting An Optimized Idle Mixture And Speed

Setting the idle mixture and speed on a carburetor isn't the most exciting or elegant subject, but it is a necessary adjustment that often is overlooked or worse, set up incorrectly. Here's the proper way to handle this basic task.

www.holley.com

 

[kenny nunez]

Do not try to readjust the carburetor if you check the vacuum with the boat in the water. The back pressure of the water against the exhaust always shows a lower reading.

 

[Trevthefox]

Here Is The Correct Procedure For Setting An Optimized Idle Mixture And Speed

Setting the idle mixture and speed on a carburetor isn't the most exciting or elegant subject, but it is a necessary adjustment that often is overlooked or worse, set up incorrectly. Here's the proper way to handle this basic task.

www.holley.com

Thanks for this.

 

[Trevthefox]

Do not try to readjust the carburetor if you check the vacuum with the boat in the water. The back pressure of the water against the exhaust always shows a lower reading.

Good to know. I had no idea about the exhaust part. Should I perform the test in water vs. muffs? Or just expect lower vacuum readings? I figured it would be better to put the engine under load to get a true reading.

 

[Scott Danforth]

Use an unlit propane torch to set idle. If you introduce propane and the idle goes up, you are running lean. If you add propane and the idle drops, you are a bit rich