Vapor lock issues

[Mr Mower Man]

Been a while since I've been on here...

I've been having a rash of vapor lock issues lately on single cylinder carbureted engines. (Worked on one last year that was giving me fits as well. There is a discussion thread on that one on this forum.) I just checked the alcohol content of the fuel in the one I'm working on (42" Poulan Pro riding mower, 15.5 HP OHV Briggs), and it's 10%. Are any of you having an uptick in vapor lock issues? I used to never have problems with this, but it's quickly becoming more common it seems.

[tuffnell]

I don't use any gasoline that contains any alcohol and haven't run into the problem you are having.

[bgsengine]

I don't think it is so much the alcohol content as it is the octane rating of the fuel - Alcohol is added to RAISE the octane, which LOWERS volatility - and volatility is what's causing the vapor lock (boiling point of the fuel) I'd be wondering what octane rating of fuel those machines are running .. most people I know buy the cheapest possible gas (lowest octane) but they'd be better with the mid-range (89 octane or better) also it can be a design issue (fuel lines running close to a hot engine, engine running hotter due to overheating?, etc etc..) KE4 had some luck with adding insulator barriers to fuel lines that are running in bad spots (where they get a lot of heat) - I rarely saw that myself, but the few times I did it was usually a heat and air flow issue (hood modifications? missing hood? missing muffler heat shield? fuel line routing?) One I had would have no problems with the hood raised, but have hood closed (It was a John Deere - wrap-around hood) and it would get vapor lock in summer.. finally realized that the owner had REMOVED the original hood air duct work and it changed the air flow under hood.. he would not pay to have it reinstalled but I got creative and fabricated a heat shield for the muffler so the hot air coming up across muffler was not being sucked on over the fuel lines and recirculated into the cooling air intake.. the next summer, the engine finally overheated despite the cooling fins being perfectly clean.. because the engine was pulling air in to the rotating screen that was coming from the hot exhaust and not from the as-designed cooling ducts that were removed from the hood..

[KE4AVB]

+1

Plus it even be the wrong fuel blend. I had problem last year where the distributors didn't switch to Summer blends until late. I had fuel boiling in fuel filters and had had heat shielding the fuel lines.

[Mr Mower Man]

I had thought about the octane. I normally just use 87 octane, as the engine manufacturers don't recommend anything higher than that. But fuel volatility is the problem here, and I'm curious as to what 93 octane will do, since it's more stable. A lot of people would balk at the idea of using expensive gas on their lawnmower when they don't even use it in their car, but in reality, the lawnmower uses such a small amount, it doesn't cost that much.

This is a mower owned by an old lady, no noticeable modifications. It's only 3 years old.

A really strange thing though...

She claims that the mower didn't run like this until after she had it serviced last time. The mower came back to her from the shop (my nearest competitor), with a fuel filter in it (Briggs 691035), and she swears that it never had a fuel filter before. It ran fine before the fuel filter, and now that the filter has been added, it starts running funny when it gets hot. I have never seen a lawn tractor without a fuel filter. It goes against my grain to remove a fuel filter, but then again, how many of the Briggs Quantum engines had a fuel filter?

[KE4AVB]

Depends on the setup. IF fuel pump the 40 micron should be fine but if gravity then it should be more like a 70 micron or looser version. I have ran the 70 micron version personally on my gravity fed 17.5 hp engine 31 series Briggs without problems.

Not many Quantum comes in with a filter but leave with at least a red pancake filter as I find trash in the fuel bowls.

[Mr Mower Man]

This is a gravity feed system, so I put a red pancake filter on it. Now my next step is to put 93 octane in the tank and see what it does. I am not comfortable removing the fuel filter - seems like asking for trouble to me.

[bgsengine]

This is a gravity feed system, so I put a red pancake filter on it. Now my next step is to put 93 octane in the tank and see what it does. I am not comfortable removing the fuel filter - seems like asking for trouble to me.

I agree with that - even walk behind mowers - most don't come with a fuel filter, but I will often add a pancake filter to them during maintenance where they will fit. And KE4 got it - a filter like you spoke of equipped to gravity feed and added in the wrong place would be much more likely to see vapor lock - add in vibration to the equation with temperature and fuel quality, even 89 octane or 93 octane could vapor lock - especially if the filter is oriented in such a way that the outlet of the filter is pointing "up" where fuel vapor will collect.. more likely to get that cushion of vapor that keeps fuel from pushing through the vapors..

[bluemower]

Ke4 has a good thought with left over winter fuel in the fuel tank - or possibly in the spare gas can. Many owners do not realize the difference in summer or winter fuel blends. A low level of fuel in the tank may have greater tendency to vapor lock. I have experienced vapor lock with the 40 micron - corrected with the red filter on gravity feed. These situations occurred when the hot engine was shut off and restart attempted a few minutes later.

In one situation, my only solution was to add an electric fuel pump with a spring loaded off toggle switch. Instructed the owner to press the toggle switch before every start. This occurred on an AYP craftsman with a hood similar to the poulan.

[creia]

Depends on the setup. IF fuel pump the 40 micron should be fine but if gravity then it should be more like a 70 micron or looser version. I have ran the 70 micron version personally on my gravity fed 17.5 hp engine 31 series Briggs without problems.

Not many Quantum comes in with a filter but leave with at least a red pancake filter as I find trash in the fuel bowls.

I have a couple of those Briggs red "pancake" filters. I do not see a fuel flow directional arrow on them anywhere, so they can be mounted either direction?
Michael