" Hi,
I'm no expert,
" Hi,
I'm no expert, but I have done a decent amount of hacking around my outboards (most of which solved more problems than it caused), and own a 1986 merc 115, which I recently gave an entire fuel system overhaul (rebuilt carbs/ pump/ new hoses etc. - 20 hours since and it runs better than my truck). Here's my two cents:
If your RPM's pick up when you hit the choke, or if it stalls/hesitates apon acceleration I would say you have something wrong in the fuel side. Another good sign of a fuel issue is hard starting when cold - had any of this?
When you punch the throttle, the first thing that happens is the butterflies on your carbs open wide up. If there is any fuel deficiency,the mixture is going to be way too lean, and the engine will either stall, or hesitate. Hitting the choke closes those butterflies, richening the mix.
You should pick up a manual before you start any work, but my suggestion is first to check the compression, don't waste your time if powerhead is blown. If all your cylinders are over about 115 psi with no single one more than about 10 psi away from the others, your power head is probably ok.. then go to the fuel system.
Pull the plugs; check to see if there are any that appear different.. I'm not sure if you have an inline 6, or 4 cyl (2 or 3 carbs and 4 or 6 plugs to deal with), but either way - keep track of which cyl they came out of. If you find 2 dry plugs and the rest are oily/wet, it may point you to a particular carb/cyl that has an issue. If the plugs are more than a year or two old, I'd replace them; just because it's probably time, and trouble-shooting two problems at once is confusing.
Start with the easy stuff:
1. disconnect the fuel line that goes from the fuel pump to the top carb, pump the priming bulb. If there is a nice easy flow of fuel, you have no restriction here. If there is a restriction work bottom to top checking all connections/ filters (there may even be one in the tank)/ anti-siphon valves etc..
2. If that checks out, leave the fuel line disconnected at the top carb, pull the spark plug leads, prime the pump, and crank the motor.. this will test the fuel pump, if there is a steady pulse of fuel coming out for about 15 pulses, your pump is ok, and your lines should be ok up to that point. One thing I want to warn you about is the manual on mine claims that you can damage the coils by leaving the spark leads ungrounded while cranking the motor.. I have asked experts about this, and no one has ever heard of coils being damaged this way, I don't ground mine.. do what you think is best.
If the fuel flow is not there, or if it dies out after a few pulses, until you re-prime the pump, your fuel pump may need a rebuild - this is a really EASY job - $26 for the rebuild kit with all parts. Check the diaphram for pinholes.. the check-valves may also be shot, or stuck. Another possible problem is a small leak allowing air into the fuel system between the tank and the pump.
3. if your pump is ok, the next thing to investigate is your carbs. My guess is it probably is your carbs, but you really want to eliminate all other possibilies first.
The carbs on mine have adjustable low speed jets, assuming yours are also adjustable, this leaves the possibility open that they are simply out of adjustment. If your motor was running fine, and now it is not, you can probably rule out bad adjustment. Adjusting things to account for a clogged carb is not a good idea, but it might temporarily fix your problem, and to add, a low speed jet set too lean would cause the exact problem you have givin, although so would a clogged jet. Once again, do what you think is best.
A better solution would be to rebuild the carbs and then go for the adjustment; which is what I did with mine, and it brought quite an improvement in cold starts, and overall perfomance - after I finished adjusting the low speed jets. But, like I said, you may not even have adjustment.. and I'm no expert by any stretch, hope this helps,
Jon "
