25 hp mercury bigfoot vibrates and shakes at idle

[Nautigal]

My engine ran very smooth until this year and starting getting excessive vibration at idle. Mechanic cleaned the carb but I am still not happy with performance. My carb does not appear to have an idle mixture adjustment. Any thoughts?

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Mine is doing the same thing. Did you ever find out what was wrong?

 

[Dallenx]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Have a timing light or a spark tester?
Use these to make sure that you have spark to each plug.
Does your carb look like the one in the attachment?
Does your manual say anything about part #9?

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Mine is a 2001, and says there is no timing adjustment. My carb looks similar, but where that screw #9 is, mine has a 1/8" diameter metal cap. I was thinking that there might be a cap with an adjustment screw underneath where you couldn't mess with it, but the diameter of the cap looks too small. In the service manual I downloaded, they show two different carbs, one with an adjustable air screw, and then mine without it. It doesn't show one under a cap like your diagram does.
I bought mine used, but in real good shape. The vibration goes away completely at around 1,500 RPM's, so I am thinking it might just be the nature of a two cylinder motor. The vibration is the same in gear at idle or in neutral. It vibrates enough trolling at idle that it loosens the thumb screw on the side of my compass.

 

[MH Hawker]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

You can try running some sea foam through it for a while it may still be about hafe clogged up.

 

[Dallenx]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Item #8 may be that cap you're talking about that fits over the screw.
When I mentioned the timing light I meant to clip it onto one plug wire at a time and pull the trigger then watch the end of the timing light flash.
Sometimes you can visually see an erratic firing sequence.
Another option is to find someone who has the same motor the next time your out and eavesdrop or strike up a conversation.

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Thanks for the advice. I lost my timing light years ago, and the dealer cleaned the carb before I bought it. If I drill that cap out and there isn't a screw in there, I guess I could just silicone it back up.
The motor runs good, and starts right up. I have pulled each plug wire off while running, and it is very close to the same RPM on each cylinder.
The only other funny thing this does is the oil level on the dipstick is off. When I got it, it showed filled to the top of the range. It did some oil smoke taking off at full throttle, and there was oil in the air intake. I went so far as to disconnect the breather tube and put a baggie over it. Once it got to the middle of the range, it has not blown any more oil for the last 40 hours. The oil is still clean, so it can't have much blow by. Overall a very good motor, it just vibrates like a Harley at idle, and then smooths right out.

 

[Nautigal]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Hi Guys,

What a surprise to see some thoughts about a question I posted a year ago! Thank very much.
I have done extensive work to isolate this problem. Changed carb, changed coils and double checked all connections and searched for vacuum or hose leaks. Nothing has stopped the shaking! My carb does not have an air adjustment screw and I am intrigued about removing the cap and having a look there. One other thought I have had is are the rubbers on the motor mounts worn out? The issue for me is that my engine did run perfect until I had to have the seal under the block changed as I was getting water in the crankcase. Ever since it has not been the same so I have been constantly fighting with what could have occurred. There has to be a solution and the air adjustment sure seems like a logical approach.

Nautigal

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Nautigal,
You said you changed carbs. Do you still have the original to see what is under that plug? The diagram for mine shows nothing in that location, but the other carb for our motor has the air adjustment screw, and it is exposed with the spring on it like a normal carb.
When mine first sucked the oil in, I talked to the dealer who sold it to me, and he said there was a service bulletin on these. This required removing the power head and changing a gasket under it to either make more suction or reduce it, or something. I was going to do it, until I figured out that the oil sucking stopped when the level hit the middle of the range on the dip stick.
My motor is a 2001, and the rubbers may be harder than new, but this motor still looks like it has had very little use, and I can't believe it has worn anything.
We seem to be the only two people with this motor, that has had this problem and look at forums.

 

[Nautigal]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

I do not actually have a spare carb. The dealer had a new one in his parts inventory and after spending as much as I did he let me try it. Since it made no difference I gave it back. My engine is a 2004 and I bought it about 5 years ago mounted on an 11 foot Boston Whaler Sport Tender. Until I had the seal problem repaired this engine was smooth as silk. You could not feel it at idol sitting in the boat. Last winter I complained enough that the guys in the shop took it on as a winter project. They could find nothing wrong. The "what is under the carb cap" is the mystery I am eager to solve". We are currently out cruising now and as soon as we get back to home port I am going to investigate. Cheers

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Nautigal,
I drilled out the plug, and nothing was inside. You could see the shaft of the enricher I think, so I filled it with silicone. I called my dealer, and they said there was an adjustment inside the carb which apparently doesn't show on the diagram. They thought they could fix it, if there wasn't anything non-fuel related causing the problem. I guess I will take it in some day to see what they can do.

 

[Nautigal]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

hwindle,

I drilled out the plug and sure enough found the air adjustment jet. Turned 1 turn out and she is finally purring like a kitten. It amazes me that a dealer would not have found this problem or known about it.

Good luck and thanks for the help.

 

[hwindle]

Re: 25 hp mercury bigfoot vibrates and shakes at idle

Nautigal,
I am glad you were able to fix yours. On my 2001 where your screw is located, mine has a formed hole that necks down as it goes in to the carb, not a plug or cap. Your serial number must be higher than OT409000, and mine is lower. The service manual I downloaded shows two carbs, and yours with the screw doesn't list the number in the parts diagram. It shows in the picture as part number 30, but the list of parts stops at 27. I guess they don't want to sell this.
Somewhere I found a list price of over $400 for the little carb. Mine is going to keep shaking, unless the dealer can do something. I have no idea how anyone could adjust a carb when it is apart like they say, and the diagram of mine shows nothing inside except non-adjustable jets. Yours does have a larger #50 idle jet to my #45. Maybe they replace that and call it an adjustment?