carbs and particles - again

[207WF]

On the first leg to OSH last month, just before leveling for cruise at 9500' I had an engine surge. It was similar to what I had experienced before and had previously dealt with (as Roger suggested) by pulling the carb bowls and cleaning out the debris. (I thought stuff was coming from deteriorating floats the first time, so I replaced the floats.) When it happened again I made a precautionary landing and tried the same trick. On restart I had fuel leaking all over the place, running from the overflow vent tubes into the air box and onto the tarmac. I pulled the bowls again and the vent lines from the carbs to the air box, and blew into where the vent lines enter the carbs to dislodge any crud that might have blocked the valve that shuts off the fuel flow into the bowls. That got me back home, but I decided not to press on for OSH again this year.

 

Now the trouble shooting. The debris looked like tiny grains of sand, but smaller. We found that the inside of the fiberglass air box had some rough areas that you could flake off with your hand. We sanded inside the box and coated the whole thing with some 5 minute epoxy, and replaced the scat tubes between the box and the carbs for good measure. I have only a couple of hours on it sense then, but all is normal except for the "automatic rough" when I go over a mountain. It might take a while for that to stop. WF

[FastEddieB]

Ar you proposing that bits of fiberglass got from the airbox into the float bowls?

 

If so, its hard for me to see how - the fuel comes in via the fuel lines alone. Any particulate matter from the airbox should just blow right through the engine, with no way to find its way to the float bowls.

 

Unless I'm misunderstanding entirely (would not be the first time!).

[Roger Lee]

Hi Wayne,

 

I agree. Anything in the air box shouldn't show up in the bowls, but go right through the engine. I have seen this micro size sand looking stuff in the bowls and I believe it is more from contaminated fuel. It is so small that it could get by the filter, but because it is so small it should not be an issue for the main jet. The only carb issues we have seen after a hose change which can happen anytime and from any body is to small rubber flakes and I also see floats that are starting to flake which looks like rubber. If you have an on going problem and the bowls were clean then there is something in one of the carb passage ways. One or both carbs should be pulled and flushed out with carb cleaner followed by some high pressure air. When you checked the bowls last time something may have been in a passage and not just in the bowl.

 

I just had an LS in and the owner thought he needed a prop adjustment because the engine only seemed to get 5100-5150 rpm at WOT. I put the carbs on a set of gauges to look and found one carb had something in a passage way because the bowl was clean. I stripped it and flushed it. He now gets 5600 WOT.

 

The carbs should be put on a set of gauges to double check.

 

 

[207WF]

I too, thought that nothing from the air intake could get into the carb. bowls, but now I am not so sure and my mechanic is convinced that it happens. There must be a number of orfices that the air flow passes over inside the carb, where particles can fall in. I have seen no change in performance associated with this. Exactly the same cruise rpm and speeds, then suddenly an engine surge as if from fuel starvation. As far as fuel contamination, I use a painter's filter in a funnel and have been buying at the same high volume shell station for 7 years. WF

[207WF]

That said, I am certainly going to try the carb cleaner flush! WF

[Roger Lee]

Your mechanic is wrong. Flow moves up from the bowl with some small force and everything that enters the throat gets sucked through to the valves and cyl. It will even suck screws through the system so a small particle doesn't stand a chance of going in the opposite direction. Both carbs need to come off, get stripped, flushed with carb cleaner and then blow out every orifice on the carb. Large and small.

 

It won't take that long. I would bet sandpiper could do both carbs start to finish in 1.5 hours or less.

[FastEddieB]

Your mechanic is wrong. Flow moves up from the bowl with some small force and everything that enters the throat gets sucked through to the valves and cyl. It will even suck screws through the system so a small particle doesn't stand a chance of going in the opposite direction.

 

Again, I thing Roger is right, thanks to Mr. Bernoulli.

[sandpiper]

 

 

It won't take that long. I would bet sandpiper could do both carbs start to finish in 1.5 hours or less.

 

Roger - you would make a great stand up comic. So, you are saying I could do it in 1.5 or less? Dinner for two? I would make sure it would take 1.6

[Roger Lee]

:lol: :lol: I'm sure you would, but I still have faith in you.

You need to come down this winter. We'll turn you into Flash Rotax.

 

I'll call the Safety Officer to help out, he'll keep you on track.

[sandpiper]

Safety Officer looks mean but loves the old scratch behind the ears trick. Don't mention to the S.O. that he's a real pussy cat.