1971 mercury thunderbolt 650 fuel problem?

New to boating and just bought a old 70's runabout 650 mercury. I could not nail the year down exact but believe it to be 1971 thunderbolt has a red stripe.
Issue I have, hoping someone smarter than me knows, is that the engine will stop sometimes when running at either slow or fast speed, in about an hour the motor may stop 4 to 5 times.
When stops can usually restart but sometimes takes 5 min or so.
When stopped the fuel bulb has been both hard and soft.
Sometimes when re-start and get going the engine seems to be have less power but then rev up slightly to more power even without moving throttle

Any ideas on troubleshooting / what it could be / fixes to make sure it's not just bad gas - ie recommendations to add filter/other to fuel to ride water etc...

I know the sparks are good and the cyl compression checks fine, also replaced water pump and impeller so hope is not simply ethanol vapor lock.

Please help as I am new to outboards and am only learning this stuff from reading your threads

PS anyone got the original mercury service manual for 1971 650. 4 cyl? Will buy it if can find it

Andrew

Re: 1971 mercury thunderbolt 650 fuel problem?

You should be able to find the manual in a PDF format online for like 8 bucks. I cant remember the website, I just searched for Mercury Manual download or something. As for your issue, there are a few questions I will ask to help assess the problem:

1. Is the vent on your gas tank open?
2. Have you ever rebuilt the carbs?
3. Have you ever rebuilt the fuel pump?

Re: 1971 mercury thunderbolt 650 fuel problem?

If its a red band 4 cyl (smooth runner) its probably a late 60,s model, in 70 or 71 they changed to blue band and went to 3 cyl. i believe, need s/n to know for sure.

From the symptoms it sounds like fuel, maybe only one carb getting enough fuel to run? check for airleaks and/or plugged/restricted items.

If it happens to be a spark problem you can tell by hooking up a timing light, leave it hooked up and when it quits check to see if the light works. intermittent problems are the worse. To fix you will probably need a factory manual as there were several changes in the late 60,s on ignition, points/no points, solid state trigger distributer system, etc.........I havent had to mess with mine luckily

make sure to grease drive splines once in a while.

Re: 1971 mercury thunderbolt 650 fuel problem?

The motor serial number is 2874681, it never fits the range on websites apart from being close to a 1970. I do not know for sure - please tell me what you think.
The gas vent is open, I have not rebuilt carbs or fuel pump will do when finalize year and get a manual

Thanks to both replies

Andrew

Re: 1971 mercury thunderbolt 650 fuel problem?

Looks to be a 1970, possibly a canadian engine

http://www.maxrules.com/models/Merc_model_files/60-100.html

Re: 1971 mercury thunderbolt 650 fuel problem?

Thanks, I think you are correct