need help with rough running motor at high rpm

[nate2800]

i just put a 1997 115 mercury on my boat today , took it to the lake , and on the lake it idles great , comes out off the hole like never b4 but when you get it up to higher rpms it acts like it thirsting for fuel every once in a while it acts like it clears up for a split second but goes right back to bogging like it does not want to open on up emoBang any help would be appreciated thanks
Nathen

 

[wrechin2]

possible fuel pump?? Can someone pump the bulb when it acts up and see if it clears up.

Could be a stator, switch box, coil plug, wire, ect. Do you have access to a timing light???

 

[nate2800]

probaly could get a timing light so i guess i should start by checking pumping the bulb what about the carbs? ive ha d a couple guys say maybe carbs

 

[Shankopotamus]

Don't overlook a dirty fuel filter! Perhaps the increased flow of fuel at higher speeds sucks garbage to the output of the filter, obstructing the fuel line.

 

[wrechin2]

Shankopotamus - 5/21/2009 6:42 AM Don't overlook a dirty fuel filter! Perhaps the increased flow of fuel at higher speeds sucks garbage to the output of the filter, obstructing the fuel line.

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Agree! Pumping the bulb would help determine if fuel. Using the timing light, not to check timing, but to check to see if you are loosing spark on any cylinder. It will no measure how much, just if voltage is passsing through. Could be in the carbs. That why I said ect. Need to narrow down fuel or electrical. If the engine has been sitting for any time at all. Doing the carbs is a GOOD ideal. A jet may be parially stopped up but they usually show it on a hole shot. You said it had a good hole shot. There are soooooooo many posssibilities.</p>

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I damage a inline 6 because of a fuel pump. Would come out good and run good for a short distance. It would start surging and if you went to half throttle you could feel it staighten up. Hit the throttle and gone again untill it start to miss again. Found pumping the bulb would fix. It was too late. The damage was done. So be carefull! Hope this helps</p>

 

[nate2800]

so are u saying i shouldnt run it or not so hard and what kind of damage can it cause

 

[nate2800]

how would i check the firing just run it with timing light on each plug what rpm? another thing is the rpm limit module could this be tested easily?

 

[wrechin2]

Have someone pump the bulb and see if it runs better. If so, don't run it hard anymore untill the fuel system is gone through. If it doesn't help. Put the timing light on each spark plug and see if one quits flashing at any time. If so you are dropping a cylinder electronicly.

Here is what type of damage from a lack of fuel is here....

This had a bad fuel pump.

http://www.chattanoogafishingforum.com/forums/thread-view.asp?tid=27265&posts=35&start=1

 

[nate2800]

sorry for being so stupid but should i try the timing light when im running it on the lake or at higher rmps with the water hose on it

 

[wrechin2]

You have to run it on the lake. You have to duplicate the problem to troubleshoot it. I have a cable tie on the trigger on mine so I can hook it up and just lay it on the back deck. That way I don't have to hold it then when it acts up I look behind me and see if it is still flashing. That is when I am by myself. If you have someone with you they can watch it. If the light flashes good then stop and switch to another cylinder.

Have you removed and looked at the plugs???? If you have little sprinkles of stuff on the plugs it has started melting a piston. That is not good.

I have one here that had water in the fuel and it melted a piston very badly. Water is heavier than fuel and thicker and can not pass through a jet as fast and leans it out.

Need to make a determination of fuel or ignition quickly. Or this will happen.

This is a pic of the one I mentionion above. This is a phone camera pic so sorry.

 

[nate2800]

going threw the carbs now and gonna go get a kit for fuel pump tomorrow if they have one in stock, and have already replaced the fuel filter so if this dont fix it that means its just about gotta be firing dont it?

 

[wrechin2]

That should do it. Just make sure ALL the passages are clear.

 

[nate2800]

rebuilt and rebuilt and still missing so we put the timing light on it like u said all good but 3rd from top on idle seems to be firing about twice as fast as all the other cylinders and when u start running it seems like it quiets some all together but it only seems to do that on that 3rd from top cylinder but really that gets me a little closer and now we now that it hasnt been running any cylinders dry right

 

[wrechin2]

Sounds like either a trigger or a switch box is double firing is double firing. That is a BAD thing. That will melt a piston as quickly as a lean condition. Are any of the plug wires close or touching? They may be inducing voltage to one another. When the engine rpms raise the engine goes leaner naturally. It take more spark to fire a lean cylinder than a rich one. Voltage is going to take the path of least resistance so it may be jumping to another cylinder.

Here is a quick test. Take all the spark plugs out. Turn the engine by hand clockwise. Use a screw driver and stick in the cylinders. Rotate the engine untill the screwdriver quits coming up. Where the timing pointer is, Take some white paint and put the numer of the cylinder there at the pointer. They are numbered 1-2-3-4 from the top to bottom. Put the plugs back in. Now with the timing light on it. Get it to act up. Using the timing light, look at the pointer. Be CAREFULL. You should only see the number of the cylinder the timing light is attached to. If you are seeing that number and another cylinder's number, it is being induced or the switchbox is double firing. This will help you narrow it down. The most common part is the swithbox. But you are still on a DANGEROUS diagnostic trail, as that double firing also can cause a mechanial failure just like a lean condition.

And of course this is going by what information you have given and MY opinion of what it is. Hope this helps!!

 

[wrechin2]

YOU MAY WANT TO GET IT TO ACT UP ON THE TRAILER!! It is VERY dangerous to run the boat down the water while on the back. I DO NOT SUGGEST YOU DO IT!!!!!!!! Just back it in just enough to get the water inlet covered with water. This will help the engine to get the rpms up and troubleshoot it safely or TAKE it somewhere where they can run it on a dyno and load it up safely. If you do it, USE COMMON SENSE and BE SAFE!!!!!!!

 

[nate2800]

ok went out and did what u said marked all 4 cylinders put the timing light on it at idle speed and the results were #1 looked like it was hittimg about 1/8 to the left of the arrow #2 about 1/8 to the left #3 about 3/4 to the right and #4 about 1/4 to the right i guess on the #3 lastnight i said it was firing about twice as much i think maybe the timing light wire was getting to close to another plug wire so i dont think that is a issue anymore tonight i seen it doing that and just pulled it away from the other wires and it quiet oh yeah did a pressure test today and it was 120 on all 4 cylinders and on the plug wires touching 1&2 are mounted to the block in the same holder and so are 3&4

 

[wrechin2]

That would explain the quicker firing. It is being triggered by 2 cylinders. Sounds like it is ok as far as that goes. To really check the timing on it you would need a dial indicator and mark each cylinder. Were were trying to find out if it was double firing. There is no need to do the other.

But it sounds like #3 is dropping spark and causing your issues. You would need to run it again in the water and verify this. If it is falling off then you may want to switch #3 coil to another cylinder and see if it follows the coil or if #3 still drops. If it does then it can be a stator, switchbox, or trigger. It may be a simple coil. Keep in mind, That is about the year Mercury switched to CDM's. These are the switchbox and coil built together. They can still be switch between cylinders.

Just keep us posted on what the results are and we will try to get you fixed up!!!

 

[nate2800]

where do i get a dial indicator

 

[wrechin2]

You really don't need one at this point. It sounds like you are dropping a cylinder electronicly. If you can verify this. Then we can go from there. IF not then you can run it over and we can check the pointer and mark all of them for you as it takes a special adapter to screw into the plug hole and the indicator goes into it. It would take about 5 minutes. Just let me know what you figue out.

 

[nate2800]

thanks man so ill take it to the lake one day this week and check to see if its dropping fire ill get back