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No high speed 1981 Merc 70hp

guido2

guido2

Regular Contributor
Customers motor seems to miss and lack of power at mid to high throttle position. Can assure you I have been through carbs and fuel system. The motor had sat outside for years and wouldn't run at all for him. Now I'm thinking spark, as I had a similar problem with an '89 115 (loosing 2 cyls.) when it warmed up and under load. I'm wondering what the Merc sharpies can suggest for me. I haven't run the motor but the customer's video showed as I've described. Thanks for your help. I'm going to be very happy to get this fixed for him.
 
Re: No high speed 81 Merc 70

If it's missing (intermittent lack of spark) I would suspect the trigger but you still need to check out the entire ignition.

Resistance tests on the plug coils, DVA test of the stator and the trigger (however, a trigger with "phantom" problems will always test good when you have the meter on it - it only misses when you aren't looking at it).

Unfortunately, if they all check out ok then the switchbox is the culprit according to the troubleshooting guidelines - then if it is an intermittent trigger issue you end up replacing the switchbox for nothing (I've done that more than once, but since there is no test for the switchbox you kinda have no choice if the trigger "checks out")...
 
Re: No high speed 81 Merc 70

Thanks and will continue with testing. Sometimes a hair dryer will help when bench testing.......I know what you mean. Triggers more than modules seem to stray outside parameters when the are hot or cold.
 
Re: No high speed 81 Merc 70

Hi, Tim here. Stator failed test.....no continuity on blue wire. Trigger failed too with high continuity on white/black to violet. Wires were dried and cracked too. Got all buttoned up and great cranking spark. Owner tested today and still the exact same symptoms. I'm baffled. He says runs great at low speed but no power when throttled up. Could it be a bad coil under loaded conditions? Switchbox is my last resort as they are expensive. Note: customer says still no charging. Thanks, Tim
 
Re: No high speed 81 Merc 70

Charging is not required for it to run right, 'long as the battery is charged. I think it's switch box time. Sorry.

Do you know someone who could lend you one for a trial? As I understand, the in-line 6 boxes are similar (though marked different) and will work. Plenty of both available on eBay, used.

Jeff
 
Re: No high speed 81 Merc 70

Ok,Jeff. I have a late 70's 90h.p. will check it out. I'll show this thread to my customer so he feels ok about my actions. Its pretty comforting to have the advice of a veteran like yourself. Also have another local outboard repairman who may have a 3 cyl box to try. As for charging, must be a rectifier problem. Thanks, Tim
 
Re: No high speed 81 Merc 70

Oh, another note......the previous owner had to connect up the wiring which for "some reason" had been removed from switch box and rectifier. Flywheel had been marked and removed but the hub had never been off. Maybe someone was trying to fix the problem then and gave up. My customer then bought it off Craig's List or Ebay and it has never run right. He had to put on a starter motor before he could even try it and the fuel tank he had connected to it had infected everything with water. I've had a couple of those water issues already this season. Thanks Tim
 
Re: No high speed 81 Merc 70

Uh-oh! That flywheel can go on a zillion different ways (about 8, actually). The magnetic pulses generated when the magnets go by the coils have to come at the right time for the switch box. If they are off by 'X' degrees, that could be the problem--and I have no idea on how to correctly align it!

Sorry,

Jeff

PS: A 90 hp is an entirely different animal. That's not saying the black box won't work, but...
 
Re: No high speed 81 Merc 70

Ok, I closely examined the plate between the hub and flywheel and followed the little imprints on each side to line things up to their original holes without following the marks. Then it was in the same position as I had removed it. The sharpie marks were in line with the key flat in the splined hub. Seems the guy had enough sense to mark it but who's to say it wasn't a mistake. There has to be a way to check this flywheel using the timing marks configured to the crankshaft. Another thought, the trigger is excited by the hub, which is keyed of course to the shaft. It can't be messed up. The stator is excited by the flywheel, which may still be incorrectly positioned. Could the result be no high speed?
 
Re: No high speed 81 Merc 70

Customer brought back the boat......said the problem is not like a miss but more like no power. Maybe I'll have to take it out myself....... Could be its running on just 2 cyls. Any more suggestions? Thanks, Tim P.S. Ha! we got 4" of snow here yesterday! Fishing opens in one week.
 
Re: No high speed 81 Merc 70

Checked my 90 ser.#5032893 and takes switch# 18-5779 and the 70 1981 ser.#5651696 calls for switch# 18-5775......guess it won't work. Customer found a used one on Ebay for $70 or so. Probably a gamble, myself I'd buy new. What are they, about $200?
 
Re: No high speed 81 Merc 70

Customer chose Ebay unit. New were only $120.00 from Sierra.??????? Not me! Will post results if anyone cares.
 
Re: No high speed 81 Merc 70

A handy way to do the fly wheel is pull the plugs, bring the #1 cyl to TDC,use a pencil or something to check, and have a look at where the timing mark is on the flywheel,if it's on the pointer so much the better
 
Re: No high speed 81 Merc 70

Ok......thanks, I'll check it out. Should get the switch box tomorrow. Owner is chomping at the bit to get his vintage Lone Star on step. Regards, Tim
 
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