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1964 merc 500 inconsistent idle

T

Timbogarb

Member
I have a 1965 merc 500. It starts and idles fine but after running for a minute or so it starts to bog and sometimes dies. To get it into gear I have to up the idle then once in gear I have to punch it a bit and then it's fine for a while at idle. Runs ruff between idle and half throttle but once it gets past half throttle the thing comes alive. Is it best to set the idle screws under load, could this be the problem. Maby dirty idle jets. Carbs were fully cleaned and inspected last August and was decent but now giving me trouble.Thanks
 
The carb was completely disassembled by me following the service manual. It's a tillison (I think that's the name) has fixed jet size. Floats were to spec, needle on the float were inspected and cleaned along with bowls screens and any small galleries. Cleaned with carb cleaner to the brush, nothing abrasive. No sign of debris or dicolouration after cleaning. Dose have original fuel lines still on it duty I think the screen in the carb would catch that. No visible leaks from between bowl, carb intake etc.
 
Okay. I'm thinking you have a slow flooding condition. If you pump the bulb HARD, does gas dip from the carb(s)?

Jeff
 
Pull the carb, clamp it in a vise and pressure it up with the tank. When it leaks, drop the float level 1/16 inch at a time until it stops.

Jeff
 
Have you adjusted breaker point gap in the magneto?-----Likely should be done at the start of each season .----Set at 0.008 /0.010"----As long as the carburetor does not leak when running you are good.-You can develop a lot of pressure by pumping the bulb and a bit of leakage from carburetor there is OK.
 
That's a bit of a job, do you think the points are to far apart at idle, it dose seem like 2 cylinders drop out then back in at idle. I'll take a look in a week next time I'm at the cottage thanks
 
If you feel it is difficult it will be.-----Very simple to take out 4 wee bolts and drop the bottom half of the magneto.-------Work on it on the bench.----Test it on the bench before re-intalling.------Nothing to it !-----Note---When the rubbing block wears the point gap closes up, it does not go wider.
 
Not that it's difficult, had the hole thing apart last year when trying to find the reason for no spark. Turned out to be a bad condencer, cheep fix. Cleaned the contacts at the same time. Didn't have the gap specs then. Just that I have to pull the boat, top cowling disconnect throttle lynk and pull magneto distributor unit out. Just time consuming. Plus the bakolite casings are fragile, already had to epoxy a tab back on when I first pulled it.
 
So I did set the points and got an improvement but stI'll not great, I ordered the shop manual and found in one small paragraph under the fuel pump that describes my symptoms exactly. If there is a small tear or hole in the fuel pump during the vacuum stroke fuel gets pulled stright through the diaphragm and floods the corresponding cylinder while at idle. Once at high rpm the motor is using enough fuel to compensate. I actually have a diaphragm and will be installing this weekend
 
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