C-161: B&S twin stalls when hot

[Mrahvin 0]

We got a -78 C-161 with the Briggs and Stratton 16 hp twin. It hasn't really run properly at all this summer and I've been working quite a lot on it but I'm running out of patience and ideas now...

 

So the problem since the spring has been that it stalls when hot. No problems with starting it but after 5-15 minutes it stalls and stutters. I feel like I've done all of the standard stuff:

- Gone through the carb (including the fuel pump) numerous times, new gaskets, seals and jets etc.The float is ok.

- New spark plugs (changed from CJ8 to NGK BR6S that should be correct according to cross-reference tables, the engine manual says RJ12)

- New fuel lines, new fuel filter.

- Checked the tank lid for possible vapor lock. Same thing happens if I unscrew the lid.

- Changed the coil (converted to a breakerless system). Did that after that I checked the spark when it was hot and stuttered and the spark was erratic.

- New head gaskets (and cleaned the combustion chambers).

 

It runs better after I changed the coil but it was then still pretty frustrating when it eventually stalled after all (was able to mowe grass for maybe 20 minutes). It happens when on full throttle and especially under load. It starts running bad, stutters, maybe backfires and then dies. If I just let it idle it just runs, even on almost full throttle. It is often possible to stop it from dying by lowering the throttle and give it some choke. It is also possible to start it up immediately after it dies but then it needs some choking and throttle about halfway. This makes me believe it is fuel-related. I've experimented with adjusting the float so it either opens the fuel inlet valve very generously or that it closes it quite early. Pretty much the same thing happened regardless of this, perhaps that it died quicker with the more generous setting. Of course it could be that it gets too little fuel but I've checked the carburetor bowl directly after it has died and there was plenty of fuel there.

 

All ideas are very welcome! Especially if I've missed something obvious/easy. The only thing I've thought about but haven't had the time to actually do something about are the valve clearances but the valves looked quite ok when I changed the head gaskets. I've also been wondering whether it's worth trying another pair of spark plugs from champion or original B&S.

 

 

Edited September 2, 2020 by Mrahvin

[Stepney 2,351]

I wonder if it's vapor lock?

You say spark is erratic. Very possible your new coil is also failing. 
What's your air gap on the coil to flywheel?
Is there any play in the crank bearing?

 

I've had several Big Twin opposed B&S jobs. Not a single one didn't have magneto problems.. I think its simply overworked. One had worn main bearings, and when it heated up things got so bad the flywheel would smack the coil.. and or move away from it. Sounded like an old hit and miss. 

Edited September 3, 2020 by Stepney

[edgro 679]

I had somewhat similar problems with b/s 16 hp, problem turned out to be clogged screen in the fuel tank fitting. Removed screen, installed inline filter, have had no trouble since

[Rob R 970]

Sure sounds like a fuel and/or carb issue to me check fuel flow all the way from the tank to the carb. Then attack or replace the carb. 

[Mrahvin 0]

Thanks for some good suggestions. I'll definitely check the fuel tank and see if there is a screen that might be clogged.
 

On 9/3/2020 at 4:19 AM, Stepney said:

I wonder if it's vapor lock?

You say spark is erratic. Very possible your new coil is also failing. 
What's your air gap on the coil to flywheel?
Is there any play in the crank bearing?

 

I've had several Big Twin opposed B&S jobs. Not a single one didn't have magneto problems.. I think its simply overworked. One had worn main bearings, and when it heated up things got so bad the flywheel would smack the coil.. and or move away from it. Sounded like an old hit and miss. 

I've unscrewed the tank lid and same problem so I guess it's not vapor lock.
Maybe I was unclear, but the erratic spark was only before I put on a new coil.
Followed the instruction for the gap for the coil using a feeler gauge. I think it was 0.01" / 0.25 mm. But I'll check the flywheel for any play. Couldn't feel any when I removed the flywheel but didn't check that closely.