B&S V Twin Vanguard Fuel Starvation


Old 04-04-14, 12:10 PM
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Join Date: Apr 2014
Location: UK
Posts: 2
B&S V Twin Vanguard Fuel Starvation

I have a Countax ride on with a 14HP B&S Vanguard V twin engine. Its at our holiday home in France so gets a winter lay up of about 4 months then used at intervals of 4-6 weeks.
From about 5 years ago, come the first start-up after being laid up over winter, the engine would show signs of fuel starvation - spluttering and missing - for a short while then run fine and not show any recurrence all season.
Last year the symptoms persisted on the first start up but I used a neighbour's 'suction bulb' on the outlet of the diaphragm pump to expel air/pull fuel through. Then the engine ran fine all season.
This year I cannot get rid of the lean running irrespective of what I do to remove air/pull fuel through. The engine starts immediately and idles perfectly. Suddenly putting throttle to max causes a stutter then the engine runs at full revs but soon as any load is applied - e.g cutters engaged - the power and revs drop off and the engine stutters and misses.
If I pressurise the fuel tank by blowing into it the engine will then maintain full power/load for about 30 seconds after before stuttering and missing. The same occurs every time I take the pipes off the diaphragm pump to suck air out - it runs smoothly for about 30 seconds and pulls full power. This would indicate to me that the carburettor is working OK but the bowl is not being refilled.
I've taken the diaphragm pump off and tested the functionality by blowing through. Crankcase connection shows no leaks so diaphragm OK, Fuel line non-return valve(s) OK as I can blow in direction of fuel but not suck. So pump seems OK.

Q1 - Could it be there is no crankcase pulse getting to the diaphragm pump? If so is there a breather that may be blocked and if so where is it because I cannot locate it on the B&S manual?
Q2 - Is a pump actually necessary, given that the tank on this mower is above the carburettor so the pressure head varies from around 12" max to 3" minimum? I'll take some fuel line with me on my next visit.
Q3 - Am I missing anything / Any other ideas?
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Old 04-05-14, 08:32 AM
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,728
It does sound like the pump is the problem and probably the diaphragm in particular.

If the fuel supply is above the carburetor there isn't a good reason why a gravity feed won't work - most of the time. When mowing on steep slopes, the gravity feed may cause a few problems.

Probably the diaphragm has gotten stiff and doesn't flutter with the crankcase pulse like it should. The best fix would be to overhaul the pump or replace it.
Old 04-05-14, 11:08 AM
Join Date: Dec 2006
Location: Canada
Posts: 527
You could also try running it with the fuel cap off or very loose so air can enter it. If it runs better then it was a tank venting problem and just clean out the vent hole in the gas tank cap and you would be good to go. If not, at least it was not a difficult test to run.

Also when was the last time the fuel filter was changed?
Old 04-06-14, 08:44 AM
Thread Starter
Join Date: Apr 2014
Location: UK
Posts: 2
Thanks for those thoughts. Will check/try out next time I get over to France.

Re the filter, its relatively new but I did notice it never gets more than 1/4 full which I'm suspicious of. I have tried running without it - no difference.

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