7.4 Mercruiser Low manifold vacuum?

[thegodown]

My buddy Bob has a 1991 Sea Ray 225 bow rider that he purchased about 5 years ago and has never used because of a long standing mechanical issue. When he purchased the boat it had a rebuilt 454. The previous owner spent about $8k on an engine rebuild. After returning to the mechanic multiple times trying to get it to run properly, he finally ran out of money and needed to let it go. He told Bob that a new carburetor was probably all it needed.

Before making any changes, the engine was hard to start, ran poorly and wouldn't idle worth a damn. The vacuum at the manifold measured about 6" HG at 950 RPM. Compression checked out to be good on all cylinders. First we replaced the Quadrajet, then we pulled the intake (Edelbrock Performance Z-0 dual-plane) and inspected it for leaks, holes or cracks. Neither of these proved to be the problem but several of the lobes on the cam looked suspicious, so we replaced the cam and lifters with a brand new Isky 278 Megahyd. We also replaced the spark plugs and wires with a set from Mercruiser. The cap and rotor look to be brand new.

With the new cam, lifters, carburetor and a myriad of other new parts, the engine runs smooth and idles well at 650 RPM at 8 degrees of advance, but we still are only getting 7-8" HG of vacuum at idle. The low vacuum also makes it impossible to set the idle mixture screws on the carburetor. Vacuum increases to 20" by the time you reach 2000 RPM. Clearly we haven't found the problem that we set out to fix.

Any of you master mechanics have any suggestions on what could be causing our low vacuum?

Thanks in advance.
Dan.

 

[dubs283]

Re: 7.4 Mercruiser Low manifold vacuum?

vacuum should be at it's highest inch/hg. reading at idle. (12-14ish)

as the throttle plates open, allowing air pressure to enter the manifold, vacuum decreases.

your readings as explained seem backwards

 

[thegodown]

Re: 7.4 Mercruiser Low manifold vacuum?

Yes, I believe you are correct. Manifold vacuum should be at its highest at idle. In our case, you can see that it is very low at idle. We just can't figure out why.

If the vacuum at 2000 RPM is 20", then at idle it should be at least 20". But it is only 7".

 

[HT32BSX115]

Re: 7.4 Mercruiser Low manifold vacuum?

Howdy,

EVERY TIME I ever had a problem with low vacuum (I.E. extremely lean idle that couldn't be adjusted) It was a vacuum leak somewhere.

You might even go as far as to look for intake leaks like the intake and carb gasket and or cyl head cracks/imperfections......You might even have the wrong carb on it or a port on the carb might be open that should be capped etc...

 

[dubs283]

Re: 7.4 Mercruiser Low manifold vacuum?

then i would say you are reading your guage incorrectly or it is connected to the manifold incorrectly or the guage is broken/not working properly

the reading should drop as throttle increases, regardless of leaks/problems

 

[Bifflefan]

Re: 7.4 Mercruiser Low manifold vacuum?

Just because im cerious, is the timing chain adjustable?
If so are you sure that its set to 0 deg?
The cam was the first thing i thought of when you had low vac at idle and running bad. But as you changed it and things got a little better, im just wondering if it has more than one slot in the crank gear.
there is also a slight chance that the heads and manifold arent the same, oval port and what ever the other one is (im a ford guy).
thats all i got.

 

[thegodown]

Re: 7.4 Mercruiser Low manifold vacuum?

Carb. We replaced the carb originally thinking this would solve the problem. When the issue remained, we put the carb on another working vehicle and it worked fine with good vacuum.

Vacuum gauges. We have one permanent gauge attached directly to the intake manifold. As a test, we installed another today at the rear of the carburetor and got exactly the same readings.

Cam: We were meticulous in our cam installation and triple checked 0 degrees.

Intake manifold/Carb/gaskets. We went over the intake and carb and gaskets closely, even inside the runners with mirrors. We've even sprayed liquid around each while the engine was running to see if there was any change. None.

If it were heads (which are both oval port), wouldn't the leak show up with a compression or leak down test?

 

[mkast]

Re: 7.4 Mercruiser Low manifold vacuum?

Is the needle on the vacuum gauge steady at idle?
Where on the engine is the gauge connected?

 

[thegodown]

Re: 7.4 Mercruiser Low manifold vacuum?

mkast,

The vacuum gauge needle vibrates rapidly +/- 2" around the 7" mark at idle. Definitely not what I would call steady.

I have a permanent Holley vacuum gauge connected to a direct port on one of the runners on the intake manifold. Yesterday I connected a second vacuum gauge by opening the 3/8" port at the rear of the Quadrajet. The second vacuum gauge gave me the exact same readings as the primary.

This definitely seems to be a vacuum leak that shows up at lower RPM's and then becomes less significant as the engine speed increases. But we just can't find where it is coming from. Does anyone have any experience with vacuum leaks coming from odd places? What's the best way to find a "hard to find" vacuum leak?