1971 Mercury 800 Fuel or Fire?

[roost]

Thanks in advance to anyone who can help. I am perplexed with the problem that I am facing. My motor ran flawlessly all year last year. When starting it this spring on the ear muffs, it started and seemed to run fine. It idles and raps up just like it should. When I take it to the lake I start it then put it in gear and it dies. I can hit the choke briefly and it will pick up some. I then open it up all the way and it will only pick up a little bit. It then slowly peters off until it dies. Since the choke seemed to make a difference I suspected a fuel problem. I rebuilt the fuel pump, and disassembled the tops of the carbs. I replaced floats and needles and seats last year and all looked fine this year. I checked fuel flow just before the primer bulb and it looks good. Is it possible for a switch box to create this problem? It seems to me that if the choke has any effect at all it would be a fuel problem. I'm not sure where else to look in the fuel system. Again thanks in advance for any help on this one.

[Laddies]

You need the remove the carbs and clean the high speed jets and the idle pickup tubes

[roost]

Thanks for the advice!! Duh! I will try that tonight.

[Mnemonic]

Dont forget to check if all cylinders are firing... I had the same problem due to one bad coil one of the cyls were not firing and thus the cause.

[roost]

Update.... I pulled both carbs and cleaned the main jets and idle pickup-tubes. All looked fine. I tried it on the lake anyway and the problem still persists. It starts and runs fine in neutral but when I put it in gear it dies. As the motor is slowly rumbling to a stop I sprayed some fuel in the carbs and it kills it even quicker. I now dont believe that it is a fuel issue. In neutral and idleing I have spark at all cylinders. Compression ranges from 117 to 121 for all four. Could the spark be deteriorating under load? Does the advance of the distributor cause it or is it a load issue? Spark box? What tools are required to test the spark circuit on these? Thanks in advance.

[roost]

My clymer manual does not have a detailed instruction for testing these circuits on my motor. In fact the most explicit information I gained from this book is to make sure that I bring an oar. Are the Seloc manuals better? I searched this forum and have not found any proceedures for this motor exactly. It seems that their are some differences even during the same model year. This is a Thunderbolt ignition. 4th of July is coming and I really need to get this boat running.

[roost]

Heeellllpppp!

[roost]

Update... I found that my prop shaft has a hairline crack in it. Is it possible for this to cause this problem?

[roost]

I finally figured it out!! This was weird. The wire for the electric choke had a small abrasion and was touching the 12v for the switch box sometimes. When it would touch it would ground out the switchbox, when I would hit the choke it would send power to the switch box. That is why I couldnt tell if it was fuel or fire. At least it was an easy fix.

[laker925]

I have the same trouble with my older mec.I had the carbs rebuilt and new fuel pump/fuel line and bulb.I loose all power as I go into gear and it wants to die on me .The dealer who built the carbs said the advance screw may need to be readjusted(The control arm was also a bit tight but not binding),and the throttle cabel reset.Sometimes I get lucky and can"fast shift" and it holds fine then at idle it sometimes dies.I also need to raise the idle a tad.I will look at the choke wire too since I just taped them up.Any thing else that may need adjusting?

[roost]

Not sure what motor you have but you might look at your idle screws or low speed jets on the carbs.