Hello all,<br /><br />I've been having a problem with my 1977 Mercury 1500 150HP inline 6. When I first take it out and after warmed up, I go to pick up speed and all I can get out of it is around 2100 RPM, even at full throttle. After about 5 - 10 minutes of this, it clears up and runs great. It runs good after that even if I kill it and let it sit for a while. At one point, after it was running good, it bogged down and almost died, but finally recovered (similar to problem that has happened before, see below).<br /><br />This is a rebuilt powerhead and didn't have these problems with it before (I do not believe this is a powerhead issue, but rather fuel or electrical). However, this spring when I took it out (before rebuilt), It ran perfect for around 30 minutes. Then, it started to cut out and loose speed. My brother was driving then, so I went back and pumped the bulb and no improvement. Choke didn't help either. It ended up dying. I messed with it for a while on the water (long trip with trolling motor back to the ramp). Never did start. I even used a squirt bottle to squirt gas through the carbs and nothing. So, that eliminated the the fuel issue. Must have been electrical. I ended up pulling the powerhead because 2 pistons had slight detonation and later found out the block was cracked. So I got another rebuilt powerhead with new block.<br /><br />Notes:<br />Compression is 120 - 130 psi on all cylinders<br />Carbs are thourouly cleaned and rebuilt with new needle/seats<br />Float levels all set correctly per manual<br />Fuel Pump is rebuilt<br />All fuel lines are new<br />New fuel filter<br />New primer bulb<br />New portable fuel tank (yes vent is open)<br />Fuel Mixture 25:1 (for break-in)<br />Good spark on all cylinders (jumps 3/8" gap)<br />Carbs all open together and fully<br />Max spark advance is set at 20 deg. BTDC<br />Primary Pickup is at 2 deg. BTDC<br />Secondary Pickup starts as soon as max spark advance is reached<br />Spark plugs are new<br />All carbs are getting fuel<br /><br />Also I just did this test. I used a timming light and adjusted the throttle/advance up until the timming pointer showed at TDC for #1 cylinder. Now I removed spark plugs and used a dial indicator to take #2 cylinder to TDC then I marked a line on the flywheel (even with the timming pointer) and marked the number 2 beside it. I did this for each cylinder. Now my thought here is I can use the timing light and it should hit the line for each cylinder. Cylinders #1 and #2 hit the line perfect, but cylinders #3 - 6 hit around 4 deg. ATDC (after top dead center). I double checked all of my marks per dial indicator and all were correct. I think they should all hit at the same point, in this case TDC.<br /><br />I think my problem is somehow electrical/ignition related. Now, all electrical was replaced with new items prior to me buying this boat. New stator, new coil, new switchbox, new wiring. I don't know about the electrical items in the distributor or the rectifier. Could something electrical, when after good and hot, start working correctly?<br /><br />Any thoughts to what may be causing this would be greatly appreciated. Sorry about this long post, but I figured a detailed description would help out in diagnosis.<br /><br />Thanks
<br /><br />Suspect the plug as faulty. I have had a few plugs over the years that show good in every test, except the 'engine' test. The first thing I'd do is swap the plug with another in the engine and see if the problem stays with the plug or the cylinder.<br /><br />If it follows the plug, fantastic, one for the good guys! If it stays with the cylinder, start thinking about something different. You are right about the middle carb feeding both centre cylinders, so if 4 is running ok, then it's a good bet the problem is not the carb. Have you done a compression test?<br /><br />Chris.............