Weird Possible Carb Problem
 

I know this is a pretty wide open question; but I know some of you are pretty into Walbro carbs. I think I have a carb problem in a new engine (maybe a gallon through it). In a nut shell, the problem is that the engine will run just fine and then one day, like today, it's dead as a door nail, not even a burp. This is the second time this has happened. the 1st time I sent it back to mfg. They put a new carb plate on it and said all was well - it isn't. I'm not new to gas engines, but this is kicking my butt. I've checked the spark, and it's strong. Checked battery and switch, all ok. I've checked the pickup and it's solid. I've checked the fuel system, all is ok. The first time it happened I took the top off and checked the diaphram, all seemed well. The CDI unit is an Rcexl.

So does anyone know what could be going on with the carb or the CDI unit even though I can see a spark? Thanks.

RE: Weird Possible Carb Problem
 

Open the carb and spray some WD-40 down the thing and see if the engine will start.

Maybe the carb is vapor locked?

Or just fully choke it until the engine sputters once

RE: Weird Possible Carb Problem
 

I have to ask if it will run using a direct prime? If so we move forward to something flopping in and out of a pulse port. Pull the carb and check the port underneath if it's not provided with an external pulse tube. I'm aware you know how to check timing to see if it's way, way off. Next comes looking the reeds to determine they are opening and closing with a good seal. The last might be small particulate matter in both fuel circuits. Does the diaphragm have a button that hooks on the metering needle? If so assure that it's connected correctly. That requires carb disassembly and reverse blowing air through all the tiny passages.

RE: Weird Possible Carb Problem
 

The reason I haven't completely eliminated the CDI is because even though it sparks I have gotten fuel into the cylinder and it still didn't pop. I think I have gone between dry and flooded. Something very strange is going on. I don't see how it can be timing because everything is tight and in place and it ran fine yesterday. The carb does not have an external pulse tube. I guess there is no way around it - depending on what mfg says when I call tomarrow, I'm going to have to tear the whole thing down and see if I can see anything and blow it all out, then try again. I hate these doesn't-make-sense kind of problems.

RE: Weird Possible Carb Problem
 

Doesn't sound like a carburetor problem to me, could be spark plug, plug cap, or maybe a spot on the plug wire that may have rubbed through the shielding and into or near the core. If you are using an optical kill switch or battery eliminator, isolate those next time to see if you get any results.

RE: Weird Possible Carb Problem
 

Sounds like a ignition problem,you should have been able to get a pop out of it when you got fuel to the cylinder.
I had one do the same thing (just four flights) change the rcexl and trouble was gone,but make sure you have a good ign. switch & batt. and all connections are tight.

RE: Weird Possible Carb Problem
 

I had a Zues unit on it during Sat & Sundays flights. I took it off. There is no rubbing on the plug wire. I'm using a 5v Duralite digi switch and an 1100mah A123 battery now and I tried it with a 4.8v battery direct hookup, no luck. I did see where the CDI power lead to battery had rubbed on muffler a little and burned through spiral wrap, but the wire covering was not burned through and the CDI still sparks when I flip the prop. I'm going to try pressurizing the tank while flipping this morning and see what happens.

RE: Weird Possible Carb Problem
 

Did you take off the carb and see if there is anything plugging the pulse port? You could have had some gunk or a piece of gasket come loose and plug the port preventing the carb from pumping fuel.

RE: Weird Possible Carb Problem
 

What engine are you having trouble with?

RE: Weird Possible Carb Problem
 

If it won't run on prime it's likely the ignition. having spark, and having enough energy to drive the plug are two different things. I'm guessing you saw spark at the plug tip when removed from the engine. There's a possibility the spark is grounding through the cap when the plug is attached to the engine, or the spark coil may be weak. Try another ignition if you have one.

RE: Weird Possible Carb Problem
 

My bad...I missed his post saying he had primed it and it didn't fire.

RE: Weird Possible Carb Problem
 

I took the carb apart and blew it all out, a tiny bit of gunk on the screen; didn't find anything; did no good - not even a pop. The mfg is sending me out a whole new engine, so he can analyze this. Only thing I can't figure is if the plug is making spark and the timing pickup has not moved you would think it would at least pop part of the time. Thanks for all the suggestions; I've said THWI until the new engine arrives.

RE: Weird Possible Carb Problem
 

Did you check the timing? Even though the pickup hasn't been moved, the hub can if the key is sheared or sometimes it isn't there. Doesn't matter though if they have a new one on the way...

Edit: spelling

RE: Weird Possible Carb Problem
 

I ran an engine yestersay that had timing so far off it would have run backwards better than forwards. The point being that if there is a working ignition at all an engine will run. Badly, but it would run. If nothing else they might diesel if the cylinder became hot enough. I've experienced all of those at one time or another.

RE: Weird Possible Carb Problem
 

Splais,
This "It ran great one day and Dead as Door nail the next" is most likely electrical. Like Pat says timing will run the motor even it its out of whack (of which it might whack you pretty hard) The carb doesnt care if the crank runs forward or backward as long as the piston/ring assy provides pressure. The dead syndrom has to be an intermitten short some where.

Ive seen
1. Known good packs have an internal break in the solder/spot weld on the strap and cause the above syndrom but one day the die in the air.
2. Most people equate electrical problems with Carb problems, when in doubt dont tear up the carb until your sure.
3. Rcexel ignitions are great things but sometimes they spark but won't run the engine, however most of the time they fire one time and that's all they do, never seen one totaly dead one day then run the next but you never know.

Good Luck

jds

RE: Weird Possible Carb Problem
 

It can be the plug: tried a new one lately?
It can be the ignition; Tried to check spark with mass electrode of the plug removed? (resistance in the engine is like that!)
It can be either a flooded or a dry engine: checked the plug tip? wet or dry?
It can be the hall sensor wire chafed/burned and making contact with the engine.
It can be the engine crankcase pulse not operating the carb.
It can be the carb main fuel needle sticking. (needs pop-off tester)
It can be some tiny bit of dirt lodged in the idle jet chamber.
It can be....

So run the engine dry first with plug out, ignition off, using a starter motor;  fit a new plug, gapped to 0.020". Apply prime. Does it want to fire? Does it have compression? If it has compression and there is no fire, the ignition system is at fault.

RE: Weird Possible Carb Problem
 

I had a simmilar problem and I found this out when steve51 came over to try my ignition module...... found out it was a brand new plug that was bad. (CM-6) actually a couple of these were bad brand new.

Gunny

RE: Weird Possible Carb Problem
 

There seem to be cheap copies of that plug.

RE: Weird Possible Carb Problem
 

Please let us know where not to get a bad CM6 sparkplug. Thanks Capt,n

RE: Weird Possible Carb Problem
 

I get mine from the MVVS factory. They are not cheap.

RE: Weird Possible Carb Problem
 

No, no, no, take the rod and piston out and check the ........ Seriously, I love this forum, but sometimes the big wide range of opinoins can get out of hand! Newer gas engines are prone to flood on choke. Let it set (30 min) and try again. If not, spin the engine with a starter with engine at low idle and it will clean out and usually start or show some signs of life. Once the engine breaks in better and gets some carbon built up on the piston, it is less prone to flooding. Ha, ha.

RE: Weird Possible Carb Problem
 

They are not prone to flood with choke at all. You just need to follow the proper starting procedure. NEVER try to choke first with ignition off!! There will be no way of telling you primed too much.

Proper procedure:
apply 1/8th throttle.
ignition on
choke on
flip untill the engine fires or pops
choke off
flip untill the engine starts.
let warm up for the time it takes

If the engine dies again, repeat the sequence.

RE: Weird Possible Carb Problem
 

What ifI run high quality oil, tunemy engine correctly, and I don't have any carbon build up? Carbon reduces the possibility of flooding, never heard that one before. http://www.rcuniverse.com/forum/js/f...bout_smile.gif

RE: Weird Possible Carb Problem
 

That's because you're doing EVERYTHING wrong! :D

RE: Weird Possible Carb Problem
 

:)