Jonriley56

Hi,

I picked up a used 0.90 four stroke a couple of months ago and finally finished the plane I was building and got it mounted. I have confirmed that I have fuel, I have checked the glow plug and then replaced the glow plug, but I cant ge it to fire at all.

I have never tried to troubleshoot one of these little guys. Can anyone give me somme suggestions as to where to start ?

jon

Deadeye

I would start by opening the HS needle 4 turns out from fully closed. Give it about half throttle and try it. Do you know where your LS needle is set?

Missileman

It probably wouldn't hurt to remove the valve cover and make sure the valves are not stuck open.

Jonriley56

Hi Randy,

I think I had the needle at 3.5 turns. One thing I just realized as I was thinking about the plane while at work......... is that I think I have the fuel tank exit higher than the carb. The motor is mounted inverted.

Could that be part of my problem ?

jon

Jonriley56

Hi,

I assume that I just pull the allen screws out of the top and lift the cover off ? If they are stuck what do I do to free them.

jon

Missileman

Yes just take the 1 or 2, depending on what model engine, allen srews out and the cover will pop right off.
If they are stuck soak the top of the head in a some fuel for a while and very gently tap the top of the stuck valve.
You may also find a broken spring is the fault. If so you can order replacement springs, I would go ahead and install new valves at the same time if that is the case.

MinnFlyer

My Feedback: (4)

Once you open the valve cover, check the clearance. There should be a TINY bit of play between the rocker arm and the valve.

Once you have checked that, rotate the prop and make sure that the valves are moving properly (Not stuck or sticking)

Then, unplug the tank altogether. Prime the carb and try it. You should get at least a few pops if not a short run. Then you at least know that you're getting combustion.

Now hook up the tank and go from there

Jonriley56

Hi,

What exactly do you mean by "prime the carb", how much fuel shold there be in it ? I dont want to drown it do I ?

jon

opjose

"Priming the carb" forces fuel into the carb.

Usually when you attempt to start an engine there is no fuel in the lines, so it will not turn over and start.

You prime the engine by leaving the ignitor OFF and putting your finger over the exhaust ( or intake ) then turning the prop.
You do this a good 3-8 times depending upon the length of your tubing.

It helps to watch the fuel flow into the carb itself the first times you do this ( cowls off ), so you learn what is required.

I find that I must hold my finger over the exhaust through the compression stroke, then release it so the fuel does not get sucked back...
I repeat this as needed.

If the valves close, and your glow plug is glowing, and there is fuel in the head, you should at the least get a few pops.

It sounds like you have not primed your engine enough however.

It may surprise you how much you may need to do this before fuel reaches the carb.

With an electric starter you rarely think of this, as the starter will crank the engine over quickly many times, before it ignites.

If you are hand flipping, then you may have a long way to go before you have enough fuel in the carb.

ChuckW

The stuck valves like everyone has mentioned is a very common problem on 4-strokes that have sat a while. Carbon and/or castor oil residue gets between valve stem and the guide and it sticks in whatever position it is last left in. I've bought several used engines that people could not get running and found this to be the problem. This is why I also like to store them with the piston at TDC of the compression stroke so both valves are closed.

I've also seen a few bent pushrods on used 4-stroke engines. My guess is that a valve is stuck and the pushrod ends up bending because of it when the engine is turned over.

If you do have a stuck valve, a little fresh fuel might break up the gunk or good penetrating oil will often do the trick. In extreme cases you may have to disassemble it and clean the stem & guide but that's rare.

jimmyjames213

My Feedback: (2)

are you trying to hand start the engine? if so that could be your problem, i would borrow a electric starter to see if you can get it to run after you have checked the valves

KWJ48

Yes.
If the tank outlet is higher than the carb, it could cause fuel to siphon and extinguish the glow plug. Check and see if you are at least getting fuel to the carb, and go from there. You may need a regulator or lower the tank some.

MinnFlyer

My Feedback: (4)

To add to what I said earlier...

If you disconect the tank, and just squirt some fuel in the carb, it's true that you might flood it... but since you don't have the tank connected, it will eventually (within a few flips) spit out any excess fuel and since it can't get anymore, it is bound to start for a few seconds, or at least "pop".

Once you know that you're getting combustion, you can go on to attaching the tank and setting the needle

forestroke

used engines often are sold because they are no longer performing. i wouldn't be surprised if the used engine no longer has any compression. i've come across several engines on ebay that while the seller says "good compression" it really doesn't have any compression and would necessitate the replacing of at least the ring.

i've been trying to get a compression meter but seems like few people sell them these days. i have bought about a half a dozen used but in "good shape" engines on ebay and only two have been bad so there is still hope :-)

Jonriley56

Hello All,

I pulled off the valve cover and one of the valves seemed to be sticking a bit. I did get the engine running last night. Now I have to try and get the idle and high speed settings right.

Any words of wisdom ?

jon

bigtim

My Feedback: (13)

what brand is the engine they all have a little different needle settings, BTW the cleaner the carb is the more responsive it will become to adjustments, if the engine has sit for a while the carb can get fouled by gum deposits making for sluggish results when your trying to get it tuned properly.

another thing is to (sure to be bashed for this) get a tachometer, http://www3.towerhobbies.com/cgi-bin...?&I=LXPT31&P=0 its helpfull for tuning a 4st engine, if you haven't worked with one before, finding the low and high end of the RPM range is important when tuning a 4st and can be tricky if you haven't done it before