troubleshooting fuel

[jdstephwacotx]

76 Merc 85HP<br /><br /><br />How tough is it to rebuild a fuel pump? Also, what characteristics should I see if it needs rebuilding?<br /><br />Right now it idles good but dies when I put it in gear.<br /><br />I'm about to rebuild both carbs, decarb the motor, and replace tank, connections, fuel line, and bulb.<br /><br />That is just about all that there is regarding fuel right?<br /><br />Thanks in advance for the help.

 

[JB]

Re: troubleshooting fuel

Well, yes. That is the most expensive and probably the most ineffective way to troubleshoot, but it does cover the fuel system.<br /><br />What if you have a dead cylinder or two from other causes? You would be pretty upset spending all that time and money and still having a sick engine.<br /><br />That engine will start and idle "good" on one or two cylinders, but croak if you put a load on it.<br /><br />Start her up and idle. Pull the plug wires one at a time and listen. If there is no change in idle speed when you pull a plug wire that cylinder is making no power. Check it for spark (should arc 1/2 to 5/8" to the block). If there is spark you have a bad carb, broken reed, lost compression or lost scavenging.<br /><br />If all cylinders are making power at idle, put the output line from the fuel pump in a jar and turn her over. You should see strong spurts of fuel into the bottle. If no spurts, pump the primer bulb. If you then get spurts you have a bad fuel pump. No spurts or weak spurts you have a bad fuel supply to the pump.<br /><br />Let us know what you find, and good luck.

 

[LubeDude]

Re: troubleshooting fuel

Fuel pump rebuilding is relativly easy, do yourself a favor and get a manual for your engine, you will save the price the first time you use it. Lots of little tidbits in there you wouldnt think about. Im basically a mechanic, and I was surprised what I didnt know about outboards.

 

[jdstephwacotx]

Re: troubleshooting fuel

JB, thanks for the detailed checklist. I will do each of those things.<br /><br />Exactly how do you 'check spark'? I have seen people talk about it but I've never quite understood.<br /><br />Do you remove the plug from the block but then put the top back into the harness and hold it to the block? Or is it just the wire itself that should spark?

 

[jdstephwacotx]

Re: troubleshooting fuel

OK JB, educate me.<br /><br />I took the carbs off....they look clean as a whilstle. I guess the guy I bought it from wasn't lying after all.<br /><br />For what it is worth I also checked the compression in 3 of the 4 cylinders (I can't get the hose and fitting down into the bottom hole due to the bottom housing, any ideas there?) and all 3 were 125.<br /><br />With the carbs off I pumped the bulb and got a whole hose size stream out of the hose that is now disconnected from the tee that split the line to the carbs. Then I bumped the starter a few times and nothing came out. If I let it turn over for 3-5 seconds a dribble started to come out. Probably a tenth of what came out when I squeezed the bulb. How strong should it be? Could this be my problem?<br /><br />If the fuel pump is bad though, why would it idle and rev up using the warm up lever but then die only when it is put into gear?<br /><br /><br />Thanks for all your help. Keep the tips coming!!<br /><br /><br />--J.D.

 

[WillyBWright]

Re: troubleshooting fuel

Worn reed blocks. Check your post back at Johnnyrude. You're kinda all over the place. Maybe the moderator can lock a few out so we can can concentrate on just one thread.

 

[JB]

Re: troubleshooting fuel

Sounds like a fuel pump to me.<br /><br />Checking spark is holding the plug wire 1/2" or so from the block and turning the engine over. If you get an arc from the plug wire to the block you have spark for that cylinder.<br /><br />Good luck.