View Full Version : 1978 merc 150 bogging down

04-25-2003, 10:30 AM
I have 1500 merc inline 6 cyl.When you take off its slugish and takes it a bit to get on plane when i get out on the lake it doesnt seem to want to open up to full rpm it will get to about 4200 rpms and after about ten minutes of running it just dies out in the water and i have to idle back to the ramp. When you try to give it gas after that it wants to die. I have checked compresion 110+ on all cylinders. There is not 2# difference on any of them. I checked to see if all plugs are fireing and they are good and hot. I have put in new needles an seats in carbs because they were leaking gas and that fixed that. I have put all new gas lines on it and that didnt help. I found two wires coming out under the fly wheel that the insulation was cracked and were touching and fixed that but no change. I used carb cleaner sprayed in carbs no change. I put new plugs in also. It will idle great.

04-25-2003, 10:38 AM
I would have your electronics checked Bterry. Maybe failing when engine gets up to operating temp. I had similar problems and i changed timer base and sensor assembly.

Jude B. Performance Marine

04-25-2003, 02:52 PM
" The wires with cracked insultation could be a clue.. it does sound electrical, the way it's almost all or nothing. How does the spark look if you test it right after it dies out? What do the plugs look like, as far as deposits & condition after they are used? Do they all look the same, or are some different?

You could have a malfunctioning fuel pump too - test it out by disconnecting the lines at the carb, pull the safety kill switch to prevent ignition, and observe that the pump gives you a steady pulse...

You could have a malfunctioning fuel pump, do to a closed or missing tank vent. In this case, it could take time to build up vacume & then the pump would fail to work correctly, letting it sit would release the vacume eventually & it would work again for a while.

The reason it doesn't sound like carbs is that it is sorta working, and then not at all.. not performing right at high speed sounds like possibly clogged high speed jets, but it also sounds electrical - lack of strong enough spark.

As far as the carb cleaner, never do that. That stuff is nasty for the engine, melts pastic & rubber, and won't help when simply sprayed into the carbs. If you suspect the carbs need service, the only option is to dissassemble, clean, rebuild, re-install and re-adjust. The carb cleaner will work well for cleaning out passages & jets, only when you are rebuilding it though.

What did the engine used to hit for WOT RPM's? 4200 is a little low, if the operating range is equivlant to my 115, which is 5000-5500.

Jon "

04-25-2003, 10:40 PM
Bbterry....Sounds like a fuel problem and as jon said high speed jets sound like the culprit. You have what is called a battery Cd ignition with a distributor so it unlikely that motor will idle if it has an electrical problem. Id remove clean thoroughly and rebuild your carbs.

05-04-2003, 12:01 PM
" I have it at the shop getting the electrical end of it tested so hopefully to rule that part out,I don't have the equipment to test that. If he dosent find any thing wrong then on to the fuel pump check then carbs for rebuilding. The last time i tried it when it warmed up and started to bog down the tac. quit I didnt notice it doing that before. when it is running right WOT is 5200-5600. The tank vent is also open one of the first things i checked. I havent checked the spark after it has warmed up, but have looked at the plugs the top two were wet and the rest looked good. THANKS FOR YOUR HELP!!! "

05-27-2003, 12:11 AM
" Got the problem fixed, it had some blockage in the high speed jet of the middle carb back in action. thank you all for your help!!!!! "

06-01-2003, 07:59 PM
" I made the stupid mistake of putting diesel in my 2 stroke outboard gas tank, not paying attention what I was doing. I have a 140 HP Suzuki. Needless to say, fishing trip never got off the ground. When I returned home, I emptied the tank as best I could, then used the bulb to pump out the rest. I got the engine running by putting the fuel line in an auxillary tank and squirting gasoling in the carb. Tried it on the lake yesterday. Started out o.k. But as the day wore on, engine got very sluggish on take offs and by day's end, never would plane out. Does anyone out there have any suggestions on how to fix this problem without an expensive overhaul? New spark plugs?? I did notice some dark thick matter around the spark plug hole and even cleaned the plugs. Am I the only one that has made this mistake??? "