This motor has sorely tried my patience this year....is the vagary of buying unknown and 2nd hand . The Ivy Lake do was supposed to be the 'there I fixed it' moment, but it coughed and snorted, even with fiddling of carb knobs (low and high speed needles) and vibrated and did nothing exciting when the throttle was opened fully, while promptly drinking about 3 litres of fuel during the 2 runs of the lake I made. So, after a quick compression (122 psi and top of 128psi, which is good for these old bangers) and a spark check - both plugs were fouled, the top being the worst - I took the carb apart, cleaned it fully then fitted a new kit, including the core plugs. It now seems much nicer, idles properly and settles down quicker. Here's the vid before adjusting the trottle...exhaust bypass good for making tea too. Here's hoping....
haventaclue wrote:I'm afraid that the screaming Merc does it for me,
+ 1
T15 that's a given...I blame you for a Merc epiphany, when we met at the last blast - I still have my one owner '73 200 from the Broom Gemini.
Haventaclue, my 400 is the 'crankbuster' version (with under flywheel disty) were the insulation's peeled off the cores - you've given me a great idea as the internal wiring is readily available in many 500 scrappers..the starter motor I have is from a '64 30cu twin 350, but will just add my own loom, without the cobrahead..is just to much to go wrong with the internal loom I have (not to mention the squeeze to get it in and the unknown qty of the external looms I've hoarded.
And T 15 still has its as original 1976 Merc 500 including it spark plugs.
Got lent an old 'fixed' hi-speed jet carb, with the water activated choke, which I removed. Was extremely smelly inside and most brass bits covered in verdigris. Today have spent time out the rain, in the garage (for a change) fitting a full carb kit - £26 from South Coast Marine. Hopefully I won't need it now.
New coils, condensers and points fitted today.....just too many unknowns, odd readings..etc to put up with it anymore. Hope to do a sea run soon, but emptied a number of drums with a test prop today - at least the weather improved here in the South.
Took advantage of amazing Sunny Skies (and Chevrolet...Braaivleis to follow now.) to head into So'ton water. Hamble's slipway is most unpleasant, looks like effluent running through it; it was the thought of a barney at Crosshouse Lane that put me off going there.
Engine still playing up, but at one point cleared it's throat and took off like the clappers, then died; can't think what was going on that was different. Emptyish tank, had been opening and closing thottle then found it revved properly in neutral. Mystery ..remotes are an unknown. Will swap out the carb with Mike's fixed jet version. Played around the pontoon for awhile, then had to leave it moored for the afternoon as the tide was too far out.
Is a common enough problem with these motors, but am getting 138-ish psi on both pots, plus it idles fine with the slow speed needle not far off setup specs. I would expect a crank seal problem to show both in the compression test and in poor idle?
Flushed it in a drum when I got back yesterday, so would expect some fouling of plugs, but this what they look like now...top pot is left, bottom, right. Bottom doesn't appear to be doing much work but is oily rather than clean. Maybe I need a close look at at that coil wire, as was getting poor rpm reading on my Merc-O-Tronic.
Leaking crank will cause poor idle yes,but wouldn't affect compression.Cylinder and crankcase different entities in that the piston creates the compression in the cylinder on the upward stroke when both inlet and exhaust ports are sealed so nowhere for the compression go except to the gauge.I'm teaching granny to suck eggs,amn't I?
Maybe there is a hairline break in the wire somewhere that opens as the engine and wiring heats up
no matter how bad it is,it can always get worse,I'm an optimist
As the throttle is opened the coil base rotates forcing the plug leads to bend / kink. Could it be that you have a break in the plug lead which at slow speed has continuity but opens out as it gets bent?
Thanks both, am happy to be taught as am an overly enthusiastic amateur with not much mechanical knowledge. Got a long suffering friend to listen to my theories and we came to the same conclusion re the plug wire - either it's making intermittent contact through a break like Floater proposes, or shorting out earlier in the cycle. Will try everything at this stage Hopefully soon, so that I can get it off the boat when I know is working properly..
Interestingly enough I had a problem with the Mercury 350 last week, was not getting any reading on the ohmmeter despite a good earth and the probe squished down the side of the plug cap...turned out the plug holder innards of the gap had rotted away bar the few twists at the open end. Opened my eyes...Another was where the '71 Mercury 400 crankbuster would start on land, pulled over on the water it refused to run. After about 3 launches and recoveries I found the brown kill button wire had fallen apart and each time the boat rocked on the water while pull starting it shorted on the cowling...
. The Ivy Lake do was supposed to be the 'there I fixed it' moment, but it coughed and snorted, even with fiddling of carb knobs (low and high speed needles) and vibrated and did nothing exciting when the throttle was opened fully, while promptly drinking about 3 litres of fuel during the 2 runs of the lake I made. So, after a quick compression (122 psi and top of 128psi, which is good for these old bangers) and a spark check - both plugs were fouled, the top being the worst - I took the carb apart, cleaned it fully then fitted a new kit, including the core plugs. It now seems much nicer, idles properly and settles down quicker. Here's the vid before adjusting the trottle...exhaust bypass good for making tea too. Here's hoping.... 
