Vacuum Question

[Haut Medoc]

After reading FreeBeeTonys' posts about bogging, I have question about vacuum readings. After the initial waver from throttling up should the vacuum reading return to 17-21# regardless if under load or not? ....J

 

[Buttanic]

Re: Vacuum Question

At WOT vacuum may be less than 5"s. At a part throttle cruise RPM of 2500 to 3500 vacuum should remain fairly high 14" to 19". A 2 barrel carburator may have an inch or two higher vacuum than a 4 barrel at WOT

 

[Haut Medoc]

Re: Vacuum Question

Buttanic, it is a '73 merc. 888 302 Ford. 2 barrell holly. I ask this because under load the vacuum will gradually decrease as rpms. increase. It doesn't do this in the throttle only position. Wot for this engine is 3800-4200. I barely get to 3800, but it never bogs at all. I'm pretty sure I'm propped right: 14 1/4 x 17....I just attributed it to an older engine....Now I'm not so sure. Thanks for the input and please post back with any more info........Thanks.....Jeff

 

[Haut Medoc]

Re: Vacuum Question

 

[Haut Medoc]

Re: Vacuum Question

Don, can you help me out here?

 

[Don S]

Re: Vacuum Question

If you are getting 3800, you may be at your limit with that prop. Has it always been that way, how long have you had the boat? You say it runs fine, try a different prop.

 

[Haut Medoc]

Re: Vacuum Question

Thanks for responding Don. Well,it was worse when I first got it two years ago, and tuned it up. It starts easy enough, (very), will get out of the hole well, even though I think it is under-powered for its weight & size. My question is : should vacuum decrease with increase in rpm under load, is this normal? In the throttle only position it does not re-act that way. I could go and buy a new prop, but I don't want to just mask the symptom if there a better course of action to get those rpm's. Do you think I have an exhaust restriction? Thanks again for your feed-back, Don........Jeff

 

[Don S]

Re: Vacuum Question

My question is : should vacuum decrease with increase in rpm under load, is this normal?

The answer is Yes, NO, Sometimes, Maybe, and It all depends.<br /><br />First you have to understand how, why, and when there is vacuume in an engine. Once you understand that, the answer is obvious. But for me to sit here and try to explain it on a keyboard. Sorry, ain't gonna happen. You got a computer there, get with google and start researching engine vacuum. Or maybe someone else is ready to be a mechanics teacher. But I'm not gonna get involved in teaching engine mechanic basics on every subject.

 

[Haut Medoc]

Re: Vacuum Question

I will take your advice and do that......Again, thanks for your input, Don.........Jeff

 

[Boomyal]

Re: Vacuum Question

What size boat is your 302 pushing??

 

[Haut Medoc]

Re: Vacuum Question

Boomer, its a Sea Ray 220 SRV, '73< model 888 Ford 302. I didn't know what to expect from the vacuum gauge..... I just think that it should get higher rpm. I a'int afraid to google, but I was hoping to get a simple answer for what I thought was a simple question, and hoped there was one..... I'm not an expert, but I can see what the value of reading the vacuum guage properly. Any light you can shed on the subject would be greatly appreciated....I try to help everyone that I think I can in good faith...I guess its just time to shut up and listen...............Jeff

 

[rodbolt]

Re: Vacuum Question

a simple answer is the engine is an air pump. the only reason it needs gasoline is to make the pump pistons move. whith the throttles closed the atmospheric pressure at sealevel is about 14.7 pounds on the top of the plates. once under a load and the throttles are open and an intake valve is open and a piston is going down the air pressure above the carbs is almost the same as the air pressure below the carbs, thats why at WOT and a half to full load you will see less than 5" Hg. if it drops to 0"Hg then slowly starts climbing with not throttle motion look for any excessive exhaust restrictions. also buy the book called auto engines and chassis electrical. its out of print but someplace on iboats I posted a place to buy it. it will explain almost everytthing ya ever wanted to know but did not know the questions. but its all about the airflow past the carb venturies and atmospheric pressure. its the only reason carbs work at all. but in the above mentioned book it will explain it and there are questions to test your understanding at the end of each chapter.

 

[Haut Medoc]

Re: Vacuum Question

Thank you for the lesson, Rodbolt.