650 thunderbolt

[olpace]

I have an old 65 hp thunderbolt. I cleaned the carbs and got it to where it will fire right up and idle.... sounds as if it is idling a little high... also it cuts out when throttling wide open. I screwed the idle screws in all the way and backed them out 1 1/2 turns.... Could these not being adjusted correctly be the cause of the missing or cutting out??? I also thought I might have a pinhole or something in the fuel pump if that is not the case??? There is nothing over the front of the carbs and hasnt been since I owned the motor.... is there some sort of air box etc that may be missing..... I was advised that it would run fine w/o it??? Any help and or suggestions would be very appreciated......

 

[Yachtzee]

Re: 650 thunderbolt

Well there is no air box or filter.
You really need a good shop manual.
I have worked on all sorts of motors "without a net" using just my head and past experience for 40 years and nothing made a bigger fool out of me than when I started messing with boats trying to fix an outboard without a manual.

Could be several things. The 650 came with at least three different ignition systems depending on model year. Ignition issues can cause what you describe.
So you need to make sure wtih a timing light that that it sparks on all four with the throttle open. Some of these igntions will cut out under load or at higher rpm
But you really should start with ignition timining and the "link and sync" procedure that you can find if you search the archives here.

Carb and fuel issues could also be a factor. Bad fuel pump, maybe, Does the bulb stay hard with motor running and throttle open? Could be carb floats or adjustments, or clogged main jets or something like that. But you say that you cleaned them?

As the carbs require some time to get off and the only way to clean them and to set the floats is to take them off an dissassemble and clean/rebuild.
I would check spark timing and sync and fuel pump first
By the way, the only correct way to tune one up is in the water or a serious test tank. Really hard to do on muffs.

 

[Bartman01]

Re: 650 thunderbolt

I just went through my 650 (4cyl) 1970 model. I replaced all vacuum hoses and checked all fuel lines for cracks and leaks. I also rebuilt both carbs. 1 1/2 turns out is a little rich and they should be 1 turn out for a starting point. I have one that is one turn out and the other is a little shy of 1 1/8 turns out after adjusting them. The fuel pump diaphram would make it cut out on the mid and high end and a vacuum leak would make it run fast idle too. You have to check everything over good.