4.3l timing surge problem

[farmer9009]

I have a 89 merc 4.3L with an OC serial number. it has the thunderbolt IV ingnition system. The problem is that it will run great at WOT for about a mile then it starts to surge and pop through the carb. So today i checked the timing and set it to 8deg BTDC. Also set the mixture per the manual. aditionaly I changed out the sensor in the distributor. New fuel and the filters were clean today. Cap rotor wires and plugs are all new. Timing stays at 8dg when at idle but only advavces to around 18 BTDC when WOT. looking for sugestions.

 

[Don S]

Re: 4.3l timing surge problem

Sounds more like a fuel delivery problem (to the fuel pump) than a timing problem, popping and backfiring is caused by a lean condition. I would be checking the boats fuel system as the cause for the problem.
Like this. http://forums.iboats.com/showthread.php?t=283269

 

[farmer9009]

Re: 4.3l timing surge problem

Sounds more like a fuel delivery problem (to the fuel pump) than a timing problem, popping and backfiring is caused by a lean condition. I would be checking the boats fuel system as the cause for the problem.
Like this. http://forums.iboats.com/showthread.php?t=283269

I performed the tests outlined in the above post. In order to do them I backed the boat into the water and left it on the trailer. Unfortunately the boat did not act up while I was testing it so I don't know if the results are good or not but it did pass them all. As most of you know the Ohio River is to high to be out on at the moment. One thing that I found odd is that I had vacuum while the motor was running and when I shut it off pressure built to 5psi on the supply line to the fuel pump.This makes me think the anti-siphon valve is holding pressure back but I am not sure where it is building. I guess maybe from the heat of the motor. So next I will check the anti-siphon valve so I can identify if it is a problem or not.
I am most likely going to rebuild the carburetor so I can eliminate it also since I don't know the history of it. I bought it off EBay claiming to be a rebuilt carburetor but the only way to know it was done right is to do it myself. I had to buy a new one because someone stole mine. I will post an update once I know more.

 

[Bondo]

Re: 4.3l timing surge problem

In order to do them I backed the boat into the water and left it on the trailer. Unfortunately the boat did not act up while I was testing it so I don't know if the results are good or not

Ayuh,... I give ya an A for effort, but that ain't the same as Runnin' it,... Hard...

I Agree, it sounds like a Fuel Delivery issue,..

 

[farmer9009]

Re: 4.3l timing surge problem

Ayuh,... I give ya an A for effort, but that ain't the same as Runnin' it,... Hard...

I Agree, it sounds like a Fuel Delivery issue,..

Thanks for the grade Bond-o, I agree with you 100% about running it hard. I hope to be able to do that this weekend if the river goes back down. For now I checked and cleaned the anti-siphon valve. It is working properly plus it had no corrosion on it so that can be checked off. Tomorrow I am going to get some new fuel line because what I have is old and I feel it is nothing to second guess at. It will be replaced with USCG A1 fuel line. Also I am going to rebuild the carb tomorrow just so I know it was done right. I am leaning towards the float being set wrong or a small amount of dirt in it somewhere.
I will update more after the river calms back down or I find something with the carb.
FWIW: For you inland guys here is a link to see what the water level is in your area. http://water.weather.gov/ahps/

 

[farmer9009]

Re: 4.3l timing surge problem

While rebuilding my carb I found a small piece of gasket behind one of the main jets. This could have fallen in as I was disassembling it but maybe not. The main problem I ran into is the float. I attached a picture of it below. It does not have the mark to measure to as I am used to. Additionally the seam from production runs along the opposite sides as the Mercruiser floats do. I am guessing this may be an aftermarket float? Is anyone familiar with this style float? Either way I an going to purchase a new float so I know the right one is in there and it can be measured properly.
One more question. I am looking for recommendations of using the solid needle valve or should I go with the spring loaded needle valve?

My float:


What it should look like:

 

[farmer9009]

Re: 4.3l timing surge problem

I got a chance to take it out today after rebuilding the carb and it started acting the same after running WOT for a short time. This time I heard an electrical spark jumping somewhere. It sounded like it was coming from the coil. I touched the negative post on the coil and I could feel the ignition pulsing. It wasn't enough to cause me to flinch but it was still a good jolt. I removed the coil and tested the primary and secondary coils. It passed but it was cool by this point. I am thinking that the coil needs changed out. Should I feel the pulse when touching the negative terminal of the coil?

 

[dubs283]

Re: 4.3l timing surge problem

the sound you were hearing was probably the coil tower arcing to the negative side of the coil due to a faulty coil wire boot

once the carbon track is present from the tower to the primary side, you will be constantly having the issue

i see in your original post you changed out the plug wires - what may have happened is the coil wire boot on the tower had air trapped under it and it pushed up causing the arcing, or the track was there previously and was unnoticed

 

[vwfoolwhy]

Re: 4.3l timing surge problem

You shouldn't be getting any pulsing feeling around the coil or from the terminals. When you do or you hear the coil knocking (almost like a pulsing sound) it's a good indication the coil is bad.

The float in the pic you posted looks like one from one of the cheaper rebuild kits offered. The floats have always seemed to be OK to me and I would just transfer the measurement mark over to the new ones per the manual. I how ever don't like the jets offered with those kits. They never seem to be the correct size or matched correctly. I would just invest in a good rebuild kit. It will save you many headaches down the line.

 

[farmer9009]

Re: 4.3l timing surge problem

Replaced the coil today and took her for a ride. It did not act up at all and we were able to go a great distance at WOT just testing her out with no problems what so ever. The only thing that concerned me is when I touch the negative terminal on the coil with one hand and ground the other hand by placing it on the spark arrestor I can still feel the pulse of the spark. It is not a jolt but a tingle. I can no longer hear the spark as I could with the last coil and if I let go of the arrestor I cant feel a thing. I am going to have to check the wiring going to the coil to be sure there is nothing loose or corroded along the line.
When I get my friends boat out on the water I will have to see if I can feel anything on their coil, unless anyone else out there wants to try in the meantime and let me know.
Thanks if you do.