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1986 merc 260

T

Tyler1602

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ok so my problem is this, after running my boat under load for about ten minutes it will start to drop rpms.. also it will only happen if i get over 2000 rpm, also, i have changed coil, water seperater, fuel filter, problem still reamins also when i changed the inline fuel filter there was brown granuels in it..

so im thinking its the thunderbolt IV ecm..
anyway i can test it? or could it be something else??

please any sugestions would be great
 
Tyler1602 said:
ok so my problem is this, after running my boat under load for about ten minutes it will start to drop rpms.. also it will only happen if i get over 2000 rpm, also, i have changed coil, water seperater, fuel filter, problem still reamins also when i changed the inline fuel filter there was brown granuels in it..

so im thinking its the thunderbolt IV ecm..
anyway i can test it? or could it be something else??

please any sugestions would be great
Click to expand...


open the carb to check for debris in the float bowl ..
 
...when i changed the inline fuel filter there was brown granuels in it..

so im thinking its the thunderbolt IV ecm..
anyway i can test it? or could it be something else??
Click to expand...

Brown granules are from fuel varnish coming from the tank/pump; not a big deal since the screen caught them.

Do a compression test on the 1986 engine while it's warm. Do it dry then wet w/a tablespoon of engine oil squited into the cylinder and compare the readings. If the wet test increases compression then suspect bad rings.

Tests for Thunderbolt Ignition:


W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!


Check the kill switch first!


#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts are present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.
 
ok so i just did a few things today ill list all action to date

1. replaced water seperator
2. replaced inline filter
3. replaced dist cap, and rotor
4. replaced ignition sensor
5. new wires
6. did a recheck of filters today clear of water and or debris
7. had carberator rebuilt today.. mechanic said it looked in great shape but i had it done while it was out anyway..

so this all helped a bit i still cant get over 3200 rpm but the drastic drop in rpm has been reduced to a 500-1000 drop then surges back to 3200 -3400 rpm, backfire in the carb has stoped also

also i noticed to that when i removed the spark arrester it arched from the coil and stalled the engine... the arch came thru the boot?? so im not sure if it needs to be replaced or if its just becus of such high volts??

i will perform the compresion test, and the ignition moduel test and report back with the results

ps, while in the engine compartment i noticed a random clanking sound coming from left exaust manifold closer to the rear of the engine??

thanks for all your help and sugestions
 
it arched from the coil and stalled the engine... the arch came thru the boot?? so im not sure if it needs to be replaced
Click to expand...

Check the coil tower for fine cracks; if found replace the coil. Replace the boot if no cracks in the coil.

i noticed a random clanking sound coming from left exaust manifold
Click to expand...

That sounds like the exhaust shutter is loose. Check both sides of the engine and replace them if they are worn. They are located at the top of the "Y" exhaust pipe.
 
ok so i checked the coi,l and the boot needs to be replaced ....
i was going to retime the motor so when i lifted the dist cap after setting TDC i noticed the rotor was pointing to #6 plug????

lol its been awhile since i did timing but should it not be pointing at #1

im thinking the privious owner may have place dist cap backwards would this WOT rpm of 3200 then sudden loss of power???

also i rechecked the plugs thismorning and found all the tips were snow white... this i belive indicates running lean or preignition??
 
after setting TDC i noticed the rotor was pointing to #6 plug????
Click to expand...

Are you sure it is at TDC on the compression stroke? Just hook the timing light to #1 wire and time the engine. Also watch the light as you increase RPMs to see what the total advance goes to.

... all the tips were snow white...
Click to expand...

Has it been long enough since the carb rebuild to gauge the fuel/air mixture by the plugs' condition?
 
ok so i reset the timing to day and notice after i got the engine running that the clank i mentioned got way wores... i took the dist cap of and again the rotor was pointing to #6 plug... im thinking ther are some teeth missing in the oil pump.. and thats where the clanking is coming from....

any thoughts would really help im at my wits end here!!!!!
 
ok guy's i have the original problem completly fixed it turns out there was a small break in the thunderbolt ignition wire...


thanks so so much for the help Greatl;y appreciate it
 
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