Merc 350 bolt v 2 barrel carb fluctuating RPMS

[wryork25]

So I was finally able to get my boat out on a test run. I am not sure what is going on but there is a hesitation or drop in rpm’s. You can feel it, it is not just the tach. I am not really sure what to check, any clues? Could this be a sign of my ICM starting to go bad? I am posting a video so you can see and hear it.

Recent work done, carb rebuild, plugs, wires, rotor and cap, sensor in dizzy.
When I did the timing there was a jump in the timing very similar to what is being seen in the video.

 

[Scott06]

So I was finally able to get my boat out on a test run. I am not sure what is going on but there is a hesitation or drop in rpm’s. You can feel it, it is not just the tach. I am not really sure what to check, any clues? Could this be a sign of my ICM starting to go bad? I am posting a video so you can see and hear it.

Recent work done, carb rebuild, plugs, wires, rotor and cap, sensor in dizzy.
When I did the timing there was a jump in the timing very similar to what is being seen in the video.

I would disconnect the tach and see if the mis in timing goes away. Is there play in the distributor shaft/rotor? Also verify that voltage is not dropping to positive side of coil. Also may disconnect the shift interrupt for testing if it is an alpha

It could be the module but might look to rule some other things out. Certainly if you have access to a spare module would swap it, just not sure I'd buy one yet

 

[wryork25]

I would disconnect the tach and see if the mis in timing goes away. Is there play in the distributor shaft/rotor? Also verify that voltage is not dropping to positive side of coil. Also may disconnect the shift interrupt for testing if it is an alpha

It could be the module but might look to rule some other things out. Certainly if you have access to a spare module would swap it, just not sure I'd buy one yet

Thank you for the input, I will definitely try these things. I do not have access to a spare module. If it does end up being that I may just change out the dizzy system. I don’t want to just buy parts though so I am trying to figure it out.

 

[Scott06]

Thank you for the input, I will definitely try these things. I do not have access to a spare module. If it does end up being that I may just change out the dizzy system. I don’t want to just buy parts though so I am trying to figure it out.

i have found them used on ebay in the past for $200-250 range

 

[cyclops222]

old watery gasoline ?

 

[wryork25]

old watery gasoline ?

No drained and cleaned tank new water separator as well. Forgot to add that one sorry

 

[wryork25]

I would disconnect the tach and see if the mis in timing goes away. Is there play in the distributor shaft/rotor? Also verify that voltage is not dropping to positive side of coil. Also may disconnect the shift interrupt for testing if it is an alpha

It could be the module but might look to rule some other things out. Certainly if you have access to a spare module would swap it, just not sure I'd buy one yet

Scott I did have a question about the Tach, if I disconnect the tach, doesn’t that wire feed back to the ICm? Will it still run with the tach completely disconnect?

 

[Scott06]

Scott I did have a question about the Tach, if I disconnect the tach, doesn’t that wire feed back to the ICm? Will it still run with the tach completely disconnect?

Yes . Some mercs have two grey wires on the negative coil post. One tach one to module. If you have two disconnect the tach one. Some mercs have the tach signal wire bonded to the single grey off the coil. In this case tach the signal wire off the back of the tach to test

 

[Chris1956]

The classic reason for rpm fluctuation is an air leak into the intake manifold. In your case however, that doesn't sound like an air leak into the manifold. It also does not sound like fuel supply issues or anything else I am familiar with.

Maybe the carb bowl is overflowing and the motor is being choked every once in a while? Other than that, maybe the issue is intermittent spark? How about putting a timing light on each spark plug wire and running her and watching for steady flash?

 

[cyclops222]

I have tested some smaller powered boats while tied to the dock. At up to 30 hp. I always use saved 1/2" by 30' Nylon dock lines. I sit in the drivers position while a helper helps me snug up the boat with full fenders. I feel safer in the stopped boat struggling at WOT next to the dock. I pick a dock that faces open water and the boat is facing outward. NO boat next to me.
Dock cleats are BIG ones with LARGE solid bolts holding them down.

 

[Chris1956]

Gee, if you are going to tie a boat the dock, and go WOT, you had better be right about the boat being secure, or a serious accident will happen.

Wouldn't it be better to get a helper and take the boat out on the water, where you can run WOT, and have the helper drive the boat?

 

[cyclops222]

No

 

[wryork25]

I have tested some smaller powered boats while tied to the dock. At up to 30 hp. I always use saved 1/2" by 30' Nylon dock lines. I sit in the drivers position while a helper helps me snug up the boat with full fenders. I feel safer in the stopped boat struggling at WOT next to the dock. I pick a dock that faces open water and the boat is facing outward. NO boat next to me.
Dock cleats are BIG ones with LARGE solid bolts holding them down.

Yea, I don't think it is a good idea to attempt this with a 350hp motor, sounds like a serious accident waiting to happen. Might work well with small boats but... Thank you for the input though!

 

[wryork25]

The classic reason for rpm fluctuation is an air leak into the intake manifold. In your case however, that doesn't sound like an air leak into the manifold. It also does not sound like fuel supply issues or anything else I am familiar with.

Maybe the carb bowl is overflowing and the motor is being choked every once in a while? Other than that, maybe the issue is intermittent spark? How about putting a timing light on each spark plug wire and running her and watching for steady flash?

I do not think it is the bowl overflowing, I have looked at it while on the muffs and I don't see any signs of that. Plus the fact that I I am seeing it with the timing light as well makes me think it is electrical. I do not currently have a spark tester but I will probably grab one and check each to verify. Thank you for the input!

 

[cyclops222]

Sound is very difficult to classify. Check ALL spark plugs to see if it is 1 or several cylinders having a problem.
Can you post a close up picture of all the spark plug ends ?

 

[Pmt133]

Is it a thunder bolt 5 ignition? also, is it a crab cap. I've had plenty of problems with those out of the box. They suck.

 

[nola mike]

have you put a timing light on the wire(s) while having the problem to verify that you're getting a consistent spark? if you are having a misfire, it's on multiple cylinders...

 

[wryork25]

Is it a thunder bolt 5 ignition? also, is it a crab cap. I've had plenty of problems with those out of the box. They suck.

Yes it is a thunderbolt 5, no crab cab, plugs attach on top of cap.

 

[wryork25]

have you put a timing light on the wire(s) while having the problem to verify that you're getting a consistent spark? if you are having a misfire, it's on multiple cylinders...

I honestly didn't even think of using a timing light to verify consistent spark to all cylinders. I can do that considering I don't have a spark tester currently. when I did the timing setting I noticed it does not hold the timing steady at 10 before TDC in base timing or regular. It fluctuates very similar to the Tach in the video. Fluctuates between 10 and 8ish in base timing mode.