OK, so the engine is in a boat, not a truck, this should be easy. Last year I checked the manifold vacuum on the engine while doing a tune up. The vacuum was 15 lbs. and steady. Compression was low on two cylinders due to rusty valves so I replaced both heads. Vacuum went up to 17 pounds at idle with the new heads. I checked for leaks around the intake manifold and didn't find one. I set the timing per specs 6 degrees BTDC. The engine starts easily and idles smooth.
As soon as the engine is placed under load, the vacuum drops to 0. When I advanced the timing 12 degrees, the vacuum goes up to 32 pounds at idle, drops to 20 pounds under load. Of course the engine starts and runs like crap.
I pulled the timing chain cover to see if the chain had jumped a link. All appears normal. The marks match on the gears.
So, I have new valves, no manifold leaks, and the timing marks are correct.
What can possibly cause this low vacuum.
The distributor is a Prestolite with mechanical advance. The timing advances about 3 degress under load (apparently this is standard for Ford marine engines, the other engine advances the same amount). The carb is a Holly 750 4 barrel.
There are no vacuum lines on this engine.
The engines are 1984 Ford Marine engines. This is a normal rotation engine, the other engine is a reverse rotation engine, the vacuum on that engine is 35 pounds at idle and around 20 under load.

 

IF your saying when you have the vacuum gauge hooked up, you bump the throttle and the needles drops to 0 or 0-5, then returns to 17 at idle, then that is normal. That is what my compression gauge instructions refers to as a good/healthy engine. So you might wanna put the timing back to spec as everything is fine on this engine, its the engine making 35hg at idle and 20 under load that IMO has a problem. Good luck

 

Since this low vacuum problem was identified, I have run permanent vacuum guages from both engines to the boat helm. I can monitor the vacuum at all times. I did switch the guages to make sure the guage readings were the same. I have a fuel flow meter that tells me the gallons per hour the engines are using. The engine with low vacuum is using 20% more fuel than the other engine (with the gasoline prices being what they are, this is substantial, considering fuel consumption is one mile per gallon). I don't know where to go from here.

 

Well thats odd. What do the plugs look like on the motor with a great deal of timing? Its possible that the carb needs to be rebuilt, there are other possibilities but that would be my initial thought. You mention one motor was reverse rotation, are these motors otherwise identical, as far as, heads-cam-intake-carb-compression ratio? Good luck

 

Yes, the are identical, I bought them new from a Ford Distributor. I have swaped carbs on the engines. There was no change in the vacuum. I don't have enough hours on the new plugs to really develop a wear pattern. Considering what I have done thus far, do you know of anything else that can cause this vacuum problem?
Thanks for your help.

 

You may wanna find out, again this is a shot in the dark, what camshaft came in your crate motors and what vacuum you should expect, some engines can make more or less vacuum than expected with different camshafts (you knew that). From an average engine you can expect the engine to produce 17hg of vacuum and when you hit the throttle it will drop in the 0-5hg range and return to 17hg when you let off the throttle, thats a healthy motor. Is the rpms identical on both engines?

If I am not mistaken the mpg gauge runs off ideal vacuum correct?

So if your vacuum was unusually high, you would get the impression your mpg was better, but that may not be the case unless its supposed to be there, regardless I don't recall hearing of anyone running 30 hg on a ford 351w at idle, but then I could be wrong being I don't know what a marine 351w was supposed to run vacuum wise. Hopefully someone will jump in here and give you a more concrete answer. Good luck

 

Actually the fuel flow meter is a mechanical device (paddle wheel) that measures the fuel flowing to the carb. I was basing my opinion of the vacuum and the gas consumption on the other engine. I ran a vacuum line from the intake manifold to helm so I could watch the vacuum as the engine was running. I put a "T" connection in the line in the engine compartment so I could observe the vacuum during tune up. I did check the vacuum line to see if it was leaking. I really don't know if the cams are the same, if they have the same timing and lift, I have no way of knowing.

 

According to my vacuum gauge info low vacuum can be caused by:
incorrect ignition timing
incorrect valve timing or adjustments
bad idle mixture
worn piston rings
leak in the intake manifold

Do you see the gauge oscillating?

Note the vacuum at idle. Increase the rpms to 1500 RPM. What is the steady state reading? If the reading is lower than at idle then there is something up with the exhaust system. Either exhaust restriction or weak exhaust valve springs.

 

The boat will not be in the water until next week. I'll check the stedy state RPM at that time.
To reply to your list:
The engine came with a nic in on the distributor collar, that sets the timing at 6 degrees BTDC, that is where I set it with 35 degrees dwell (Per specs in the manual that came with the engines).
The valve timing is in question. I did check the marks on the cam gear to crank. The appear to match. (it has been suggested I check valve timing, I don't know how to accurately do this)
I have switched carbs with the matching engine, there was no change in vacuum.
Compression is 135 to 145 on all cylinders, the engine does not use oil.
I used propane to test for leaks in the intake manifold. There don't appear to be any leaks.
The vacuum gauge is steady at idle.
I have tried everything I can think of and what has been suggested by this and other forums. It is beginning to look llike I'm going to have to get a Ford Engineer to look at this engine. (I have this secret fear it is going to turn out to be something really dumb that is causing this vacuum problem)

 

I would think the only thing that would throw off the valve timing, on your engine, would be a stretched timing chain, but I don't think it would make such a huge difference in engine vacuum, but then again I can't be 100%. I would like to know myself what you find out. If your idle rpm was higher on one engine than another your idle vacuum would be higher. Who knows? Let us know what you find out, this is definitely interesting, good luck

 

A few questions. Do you know if the fuel usage is real or just a 20% sensor problem? What are the specs on the paddle sensor? Can you swap them? Do you know if the fuel usage was different before the heads were changed? Are these different heads?

If fuel usage is really that much higher then the spark plugs will tell all and I think we are guessing until we see them. If these engines are the same except for counter-rotation then the throttle positions should be the same under load. If the carb. butterflys are in the same position then the vacuum should be the same. Something to check.

 

I'll have to wait until I get some engine running hours before I can actually evaluate the plugs. There are new plugs in the engine and only a couple hours of engine run time. The boat will be launched next Wednesday so I'll have ti wait a bit for spark plug check.
In the mean time, is there any way to determine if the distributor is in the correct position for timing. Do I set the dampner timing marks at TDC then match the rotor button to the #1 spark plug pin on the distributor cap.

 

Thats what I do, if you know for sure the timing marks and timing pointer are correct then take out number one spark plug (facing front of motor its the first / left spark plug), put your finger over the hole, rotate engine, when you feel pressure against your finger, bring the damper timing mark that you want for initial timing up to the pointer, make a mark on the distributor cap base of number 1 location, remove distributor cap, loosen distributor, rotate until the rotor points to number 1 (your mark), tighten down the distributor and reinstall the cap and you should be good. Later