Now what's wronmg with my Robin engine?

[MuncyBob]

I have about 200 hours since the valve job. She's been running great. Today I was about 20 minutes into what I was hoping would be the last grass cutting of the year and suddenly I was losing power and it started backfiring. Tried to coax it to the garage but no go, could not even make it up a small hill. Left it sit for a bout 30 minutes and fired up but same thing again. Limped to the garage. Waited a few hours and started up no problem again but then started backfiring again. I was actually hoping the problem would be vapor lock but the last time I started it she was fairly cold. Any ideas?

 

[bobmisi]

If it was mine, the first thing I would check is the coil. I've had similar problems and it was the coil both times on different engine, as well as vehicles, especially when the engine is hot.

 

[MossRoad]

When mine would lose power, I'd find the fuel level in the tank down to about the level of the carb. I'd fill up the tank, and it had full power. Run a couple hours till the fuel level dropped, and it would lose power again. Turned out to be the fuel pump.

However, I had no backfiring issues like you. Just loss of power.

Backfiring is usually due to timing and timing means proper closing of the valves and firing the spark at the correct time.... if the exhaust valve isn't closing at the right time(or not closing completely) the fuel/air mixture will slip through and ignite from the heat of the exhaust manifold.

Since its unlikely you've lost timing... its more likely on these types of engines to have a sticking exhaust vavle staying open, or a bent pushrod keeping it lifted and open. Hate to say it, but its most likely.

It's only one side that's backfiring, correct? If it was both, that would be timing most likely. If its only one side, suspect the exhaust vavle first.

Just my opinion. Hope its something simple like loose rocker. Pull the valve cover and look. Have a spare gasket on hand or gasket maker RTV stuff to close it up again.

 

[MossRoad]

But I think a loose rocker on an exhaust valve would keep the valve closed. It could be exhaust valve retainer....

I'll shut up now, don't want to give you indigestion... good luck diagnosing it.

 

[CADplans]

The last time I had that occur was with a Kohler K engine,,, it was the recently replaced ignition condenser.

So, I vote for either ignition, or a fuel pump.

If it were the fuel pump, filling the gas tank should show that to be the issue,,
a full fuel tank would act completely different.

Good luck with the hunt!! :thumbsup:

 

[travlitt]

I think a bent push rod would cause the valve to not open completely.

 

[MossRoad]

If it were the fuel pump, filling the gas tank should show that to be the issue,,
a full fuel tank would act completely different.

You need a fuel pump to push fuel up hill and keep the bowl pressurized. As I recall, about 3 gallons of my 5 gallon capacity fuel tank is below the level of the carb. When my fuel pump died, I could only use about the first 1.5-1.75 gallons of fuel from gravity feed.

Fuel injected units would not run at all on gravity alone, I'd guess, as it needs pressure and return line to tank, correct?

 

[MossRoad]

I think a bent push rod would cause the valve to not open completely.

Yep, because a bent rod is shorter. However, if its bent enough, the cam can push it up and it can stay up, the valve spring not being strong enough to push it back down, keeping the valve partially open.

 

[Egon]

But I think a loose rocker on an exhaust valve would keep the valve closed. It could be exhaust valve retainer....

I'll shut up now, don't want to give you indigestion... good luck diagnosing it.

Did you by chance learn about timming back when young and loud bangs were sought after?

 

[MossRoad]

A litttle. But once you blow a muffler off the car and have to pay for it yourself, you rethink your priorities. :laughing: