76 mer 500 running rough

[afsvanderveen]

Sorry going to keep this short. My 76 Merc 500 Serial 706 range decided to start running really rough last couple of days. Start up on less than a half crank cold and warm, has poor top end power, feels like throttle is stuck at half and wont rev up to get on plane. Now when at idle it starts to miss and run rough and then smooth out and back and forth. My engine has the distributor and single coil. New plugs made 0 difference. When i check for spark its really hard to tell the difference between cylinders. Almost runs as good with any one plug off and the other three on..... Kinda at a loss of where to go next. Do not think its fuel related but not sure, Marine engines are new to me. Used to a 350..... or cummins
Thanks for the help

 

[GA_Boater]

Welcome aboard.

One of the best things to start with is a compression check, even if it is just an elimination step. Has any work been done it lately?

 

[GA_Boater]

Forgot this. Get a spark gap tester. Spark on all cylinders should jump the same gap, 3/8 - 7/16" and be a good whitish blue.

 

[afsvanderveen]

Thanks, I did a compression check and all 4 cylinders came across at 130psi. good compression. Also did a spark test on the wire as well. All good looks to be a strong spark.
I did find that one plug looked like it may not be firing well or at all. #3 Trying a new plug and see what happens. I had replaced all with new but i guess its not that uncommon to get a dud.
I recently bought this boat and upon bringing it home id did the following, New fuel line from tank to motor inc bulb,old one was really stiff, repaired some poor wires that were loosing there shielding, new plugs, carb clean up on the engine, checked fuel filters and cleaned for good measure, replace a soggy fuel line from the pump to carbs.
Also inspected the water pump and changed the lower unit fluid. So now no leaks and tidy wirring, As I said before starts excellent just seems to stop at half throttle in the lake, wont plane. Now if my plug I found isnt truly firing this would cause some problems for sure. Will check shortly and see if there is any diff in how it runs in a bucket for now.

 

[afsvanderveen]

So same problems. Starts good, smooth the idles rough like its missing bad. Doesnt do it right away but in about 30 seconds of start up. Smooths out does it again gets worse as it warms up I think. I did not run it long, long enough to see the problem is the same. Plug #3 seems to be the one giving me the problem, not 100% though. Had good spark from the lead when testing for spark

 

[afsvanderveen]

i should add all cylinders are getting fuel for sure

 

[afsvanderveen]

Thought I would share a video. Maybe gives a better idea of symptoms. Copy and paste the link into you tube
https://youtu.be/gH2XuPPVvio

Thanks for any help. I just want to get er restored and on the lake again.

 

[afsvanderveen]

any ideas? Please help

 

[afsvanderveen]

No one eh? Hmm must be a stumper. I picked up another engine at a good deal so I have a full parts engine. This one does run also but pretty sure it needs carbs rebuilt. Wont stay running, seems to starve itself outta fuel. Once shes going if you keep the revs up it seems to stay running for a bit. Not sure if i should use the distributer and switch box on my original motor or switch the carbs out as my orginal motor starts right up and will idle, rough mind you due to the missfire. Thoughts?

 

[gm280]

I just finished watching the video. Are you certain the fuel pump and carb are in perfect condition? Sound like you could be having some fuel delivery issue to me.

 

[afsvanderveen]

Pulled the carbs off my parts engine to rebuild. Also just did the pump to be sure, for the cost its a no brainer. The carbs were rather dirty in the parts motor so they are soaking in cleaner overnight and most of the day tomorrow likely. All the parts looked mint other than dirty of course so I just went and orderd the gasket kit. Didnt think I needed the entire rebuild kit. I have new fuel filters to put in also. Hopefully by the weekend I will be able to find out if this solves my problem.
I had also swapped out switch boxes and ran it with a timming light and she seems to fire evenly. So I dont think it is actually a spark problem. While in on it all did the water pump also. Good thing because i noticed when it would start to warm up it wouldnt pump very well, old water pump was ready to let go.

Any tips for syncing the carbs once they go on?