1977 70hp johnson seahorse stalls during acceleration

i just recently bought a 1963 boston whaler 17ft with 1977 70hp johnson seahorse... on my second time out with the boat i had it at wot when i got out of the marina and after about 70 yards the engine sputters down and feel like the boat is running at idle even though i have the throttle all the way open...after a few seconds the engine stalls...i restart the engine and it throttles up fine in nuetral...when i out it in gear and accelerate slowly the engine does not respond...when i put it back in neatral and rev up the engine....the engine throttles up fine....when i check the primer bulb it feels soft so i pump it firm and the motor runs fine again...but after another run the engine does the same thing..i have to pump the primer buld again.... my whaler has the gas tank located about 8 feet before the fuel filter/serperater, then another 2 feet of fuel line and the primer bulb before the engine.... i'm planning on ordering another filter for the separater...the filter on there is a sierra 18-7945... my gas tank also has the vent hose and it was open during all of this...do you guys think it could me the fuel pump too?... when the primer bulb is firm it seems like the engine runs fine...topping off at about 30mph.

Re: 1977 70hp johnson seahorse stalls during acceleration

Yep, once you have established there are no restrictions in the system next thing to check would be the fuel pump operation.....it may be acting like a fuel restriction, supplying some but not enough to cater for a full load condition.

Re: 1977 70hp johnson seahorse stalls during acceleration

I would definitely look to the fuel pump. It sounds like you are running fine as long as the carb bowls are full of fuel, but as you run faster, a weak fuel pump is not able to keep up with the fuel demand.

-Rgds

Re: 1977 70hp johnson seahorse stalls during acceleration

change your primer bulb out. I had the same problem the fuel pump will suck it flat restricting the fuel flow.

Re: 1977 70hp johnson seahorse stalls during acceleration

thanks for the helpful responses, i have recently changed the fuel/separator filter and the boat had run fine the first time out, i primer buld stayed firm and full with fuel. The second trip out my boat ran fine on the way out to the fishing grounds and 5 miles out. but when i headed back in, i throttled forward and the engine wanted to stall so i backed the throttle back down and slowly went back up and it ran fine. on my third trip i was making a run out to the fishing hole and the boat was running fine until i hit a wake and i throttle down, when i throttled back up i ran into the boggle problem again. when i throttled forward in gear the engine would only only go about 5 mph and if i fully throttled it the engine would sputter and die. when i restart the engine and have it in neutral, the throttle revs up fine with no sputtering through all rpm and when i put it into reverse gear it runs perfect through all rpms, but as soon as i put it in forward gear and throttle up it sputters and dies if i fully throttle it and will only run about 5mph. after playing with it for awhile the engine choked out and i wasn't able to start it for about 10-15 mins but when i did get it started the motor ran fine again the rest of the day...any suggestions? i don't believe its the fuel pump cause the motor throttles up perfect in neutral and reverse but only boggles in forward drive...but any and all suggestions would be highly appreciated

Re: 1977 70hp johnson seahorse stalls during acceleration

+2 on the primer bulb had the same problem too and that fixed it.

Re: 1977 70hp johnson seahorse stalls during acceleration

I don't think it's the primer build. It's nice and firm now after I changed the filter. But I think the problem is with carb. Maybe the float is getting stuck? When at idle out of water with ear muffs the motor idles fine. In the water the motor idles for a bit and always dies if u don't put it in neutral with some throttle. Anyone have experience is cleaning the carbs with this motor or maybe a video of u doing it?

Re: 1977 70hp johnson seahorse stalls during acceleration

Make sure the engine is running on all 3 cyls. That engine will run on 2 and when in neutral, it may seem to rev just fine. It's another story when you put it into gear. You need all 3 cyls firing. You can do a cyl drop test, or check for spark with an inductive timing light. Weak ignition components tend to fail once the engine heats up to normal operating temps, so make sure you fully warm it up before testing for spark.

Re: 1977 70hp johnson seahorse stalls during acceleration

emdsapmgr said:

Make sure the engine is running on all 3 cyls. That engine will run on 2 and when in neutral, it may seem to rev just fine. It's another story when you put it into gear. You need all 3 cyls firing. You can do a cyl drop test, or check for spark with an inductive timing light. Weak ignition components tend to fail once the engine heats up to normal operating temps, so make sure you fully warm it up before testing for spark.

Click to expand...

ditto ,...i had a loose plug wire on the top cyl. and it did the same thing .