Why is manifold vaccuum so low?

nabeel

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Aug 11, 2007
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Our club has a 2004 Yamaha F40 -- 40 hp, 4 stroke, 3 cylinders, 3 carbs. The engine starts and idles beautifully. With the fast idle lever, we can rev it in neutral. We put it in gear and try to advance the throttle and it simply bogs down. We used the boat weekly, even over the winter, and this behavior has come on gradually.

In our desperation to figure what is going on, we attached a vacuum gauge to the intake manifold. In neutral we see good manifold vacuum. When we put it into gear and try to advance the throttle the vacuum drops to very low levels and does not increase. This does not seem right. What can cause that? Do exhausts get clogged?

We have checked everything we can think of short of taking apart the carbs. We can't believe that fuel is the problem. It is perfectly clean. No water at all in the separator. We can see the accelerator pumps squirting gas.
 

99yam40

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Re: Why is manifold vaccuum so low?

Compression or even better a leak down test will show it motor is still in good shape. then it is spark testing and timing.

If all that is good the look at the fuel/air ratio (carbs plugging or pump failing or sucking air)
 
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Re: Why is manifold vaccuum so low?

You may need to balance the vacuum of the throttle plates with Vacuumate of other balancing device.
 

nabeel

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Re: Why is manifold vaccuum so low?

Thanks guys. I can see that no one is buying into the clogged exhaust theory. I can't see it being a bad fuel pump because the bogging down is immediate as we try to advance the throttle from idle. The perfect starting and idling tells us that spark, fuel, and compression are all there at idle. From experience working on 4-cylinder motorcycles, we think that vacuumate syncing could make the engine run smoother but is not going to solve the complete bogging down problem we are having. But what do we know!

With reluctance, we'll take the carbs off and look at the main jets this Saturday. I'll report back.
 

99yam40

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Re: Why is manifold vaccuum so low?

If you do not test anything then you will not find out what the problem is unless you stumble across it by accident. Testing will eliminate some problems and then you can go from there.
Just because you do not believe this or that is a problem does not mean a whole lot to us, but proper test results will tell us and you what is happening.

What do you think would happen if the timing did not advance as it should?
Good compression, and spark, with the timing doing it's proper thing are all things easy to test and then see what else could be the problem

The only motors I have ever seen with restricted exhaust effecting the running of the motor was a old 70's Chevy PU that had a catalytic converter plug up and a 2 stroke weed eater that carboned up so bad it would not pick up RPM's.
I would have to say it could be possible on an out board motor to be plugged up with carbon, but I have never seen it
 

nabeel

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Re: Why is manifold vaccuum so low?

My previous response was incomplete. We did test the timing in neutral and it advances as it should. However, when we put it into gear it does not appear to advance properly. I say "appear" because it is hard to get the engine rpm to increase smoothly in gear. The engine has electronic ignition which normally is either good or bad, but maybe we can do some checks with a multi-meter to see if the overheat rev limiter might be kicking in when it shouldn't. But shouldn't that work regardless of whether the engine is in neutral or gear?

We did check spark and for good measure we replaced the plugs.

OK, we'll do a compression test this Saturday. You're right. It is simple enough.

We rebuilt the carbs about a year ago and, I believe, just used alcohol to clean parts. At that time the accelerator pumps weren't working properly. It was hard to get at those circuits but we did get them clean finally. The engine was running extremely well afterwards, we have been very careful fueling the engine with 93 octane and keeping water out, and we can see that the accelerator pumps work properly.

Thanks guys for your help.
 

99yam40

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Re: Why is manifold vaccuum so low?

If it is not advancing while in gear then I would be looking in the service manual and testing everything that would keep that from happening( oil pressure and overheat maybe), I have no idea what on that motor.

All I know is that the wiper on my C40 CDI tells it what the throttle is doing through linkage and should advance the timing as it is moved. I had to replace mine as it would not allow the timing to end up where it should be at idle (too far retarded by 7 deg).

Running 93 octane is not what the motor is designed for (87) and the slower burning fuel can burn the exhaust system as it will probably be still burning as it leaves the cylinder
 
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Re: Why is manifold vaccuum so low?

This is the information from the 2000 tech guide introducing the F40B...

ignition.png
 

Silvertip

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Sep 22, 2003
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28,758
Re: Why is manifold vaccuum so low?

Cam timing jumped a cog perhaps. Understand also that once the throttle plates begin to open, a drop in vacuum begins and at wide open throttle you would have very little vaccum. While you may have enough fuel to enable an excellent idle, there may not be enough fuel pressure to enable off-idle to fuell throttle operation. Clogged carbs, ignition timing, cam timing, are all possibilities.
 

nabeel

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Re: Why is manifold vaccuum so low?

We found the problem. The engine is now running beautifully. To review, these were the symptoms:

The engine started and idled beautifully. With the fast idle lever, we could rev it in neutral. But when we put it into gear and tried to advance the throttle and it bogged down. The rpm would not go above about 1400. It felt like fuel starvation to me. I was very wrong.

The problem was a ding in one of the blades of the prop that was pushing water the wrong way. Once we replaced the prop, the engine went back to running normally. Much hair pulling, time, and expenses for what turned out to be a pretty basic problem.
 

robert graham

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Re: Why is manifold vaccuum so low?

A ding in the prop cause the motor to bog down!...That's amazing to me....I'm not questioning this, but it's pretty far out, to me! Good Luck!
 

connemaraisland

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Aug 15, 2010
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Re: Why is manifold vaccuum so low?

F40 Yamaha's are great engines especially if they are used all the time and the qwner has fresh fuel. But there is an issue with them if the above is not the case. The carbs used on this engine have two extra mid-range jets that are very, very small and clog up easily and can be hard to clean. Yamaha has a special tech bulletin out on this as a regular carb cleaning (main and pilot jet clean will not fix it). These jets are internal to the body, you have to make sure they are cleaned out. Once you get this fixed, add a new Yamaha 90 micron filter/water seperator kit to your gas line. Change it yearly and you should not have any problems.

Bob



Our club has a 2004 Yamaha F40 -- 40 hp, 4 stroke, 3 cylinders, 3 carbs. The engine starts and idles beautifully. With the fast idle lever, we can rev it in neutral. We put it in gear and try to advance the throttle and it simply bogs down. We used the boat weekly, even over the winter, and this behavior has come on gradually.

In our desperation to figure what is going on, we attached a vacuum gauge to the intake manifold. In neutral we see good manifold vacuum. When we put it into gear and try to advance the throttle the vacuum drops to very low levels and does not increase. This does not seem right. What can cause that? Do exhausts get clogged?

We have checked everything we can think of short of taking apart the carbs. We can't believe that fuel is the problem. It is perfectly clean. No water at all in the separator. We can see the accelerator pumps squirting gas.
 
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