model engine carberators

[Edgar]

hi fellow modelers;i am wondering what a standard rule of thumb is for carbs for model hit and miss engines??? it seems to me the seat where the needle adjusts fuel flow is anywhere between .020-.035??also what about veturi sizes??it seems if you go from and inlet of .250 to .187 then back to .250 will work.i had one made for me from an old man whom i never met and he used a needle out of a sewing machine for the needle valve.this particular carb has been running a model i had built with a 1.375 bore x 1.500 stroke.i have used this same carb on 2 more models that has a 1.00 bore x 1.500 stroke and another engine that has a 1.500 bore x 1.500 stroke .the problem is none of my models will not run after about 3-5min??i am using white gas and have tried 2 cycle mix??i thought at on time or another the models were vapor locking ???after they stop running you can wait a minute or 2 and they will crank back up???there aint no binding and they are water cooled???sometimes they act like the intake spring is too tight???they also seem to be starving for fuel??i have checked the chec vale in line an is is working properly???if anyone out there has any suggestions please let me know thanks edgar powell

[Buddy Harley]

Is your fuel tank vented? Thats what it sounds like to me.

[Kelly Tytlandsvik]

Originally Posted by Edgar

hi fellow modelers;i am wondering what a standard rule of thumb is for carbs for model hit and miss engines??? it seems to me the seat where the needle adjusts fuel flow is anywhere between .020-.035??also what about veturi sizes??it seems if you go from and inlet of .250 to .187 then back to .250 will work.i had one made for me from an old man whom i never met and he used a needle out of a sewing machine for the needle valve.this particular carb has been running a model i had built with a 1.375 bore x 1.500 stroke.i have used this same carb on 2 more models that has a 1.00 bore x 1.500 stroke and another engine that has a 1.500 bore x 1.500 stroke .the problem is none of my models will not run after about 3-5min??i am using white gas and have tried 2 cycle mix??i thought at on time or another the models were vapor locking ???after they stop running you can wait a minute or 2 and they will crank back up???there aint no binding and they are water cooled???sometimes they act like the intake spring is too tight???they also seem to be starving for fuel??i have checked the chec vale in line an is is working properly???if anyone out there has any suggestions please let me know thanks edgar powell

I was told by fellow model engineer at one time, that as a general rule the hole in the venturi section should be about 10% of the cylinder bore.

Kelly T

[Edgar]

as of right now i dont have no tanks made for my models;i use a baby food jar and the carbs suck up the fuel that they need;just like the old engines do.maybe i might need to try to elevate the fuel so that it goes in on its own????thanks for all and any suggestions.by the way there aint too many people out there that i get to converse with concerning model engines;so all replies will be most helpful!!ep

[Buddy Harley]

Edgar
I would try an elevated container and start a siphon down to the carb. This will eleminate a possable fuel supply problem. If the same problem persists we will have to look at some other possabilitys.
How is the compression on the engines you are having problems with? Are they hit and miss or throttle control? Next time you experience the problem check the coil, is it hot to the touch?
Buddy Harley
harloc-model-engines.com

[Edgar]

concerning compression ;except for one of my models they are all low compression.i wanted them to run as smoothe as possible;which they do when running.i will experiment with the level at which fuel enters the carb.thanks guys for your suggestions and help!!

[David Tsr]

Edgar
The next time on of your engines quit remove the spark plug and check for moisture in the plug, I have had this problem many times, the moisture does not come from the fuel it comes from condensation in the air as it passes through the venture. It usually dries up in a few minuets and you can start it back up again. another thing to try is to put a spark gap in your wire or leave the wire about 1/8 inch from the plug this will intensify the spark. I am assuming your models are plug fired and not igniter fired, If you do find a little moisture try a slightly stronger intake valve spring this will cut down the air flow through the venturi.
Dave

[Tim Litzenberger]

Hi all,
I am having a similar problem as Edgar, with a newly acquired Breisch1/4 scale model of the Associated Hired Man. It needed some work to get everything free, because the old oil had everthing gummed up. When I had it running I took it to a show a few weeks ago and it ran fine four about 4hours. After that I could not keep it running more than a few minutes. I have cleaned and reset the igniter, tried numerous carb settings, rechecked the timing. Even speed up the engine a bit, it runs best at about 700rpm. But the problem still exists, after several minutes the engine just stops like someone disconnected the battery. Frustrating I must say.
As mentioned in an earlier post my fuel tank is about 1" below the carb. with a ball check in the tank, which is working fine.

Any suggestions as to what the problem might be?

Tim

[Kelly Tytlandsvik]

I have much the same problem with my Breisch Lil brother. But 9 out of 10 times it quits I can remove the plug and find a small drop of water on the Plug (surface gap). Clean it off and the engine will start up again and run fine until the next drop of water forms.

Kelly T

[David Tsr]

Kelly that is one of the problems I mentioned that Edgar might be having. I have found a slightly stronger intake valve spring sometimes helps espicaly if the compression is a little too high. These small models are like the full size engines,they should have a low compression ratio about 4 or 5 to 1.
Tim how is the compression on your engine? Your prolblem sounds like low compression, that little engine should run nice down to 250 to 300 RPM if valve timing and ignition timing is correct. Put some oil on the piston and see if it helps. I assume it is an ignitor fired model and you have a good ignitor coil.
Dave

[Tim Litzenberger]

To David TSR,
The compression seems fine to me. I judge that on when starting it it seems to have nice resistance when turnng it over. Since this is my first model engine I am comparing it to my larger full size 2 1/2 associated compression.
I had the engine to Jacktown show this past saturday and once agian it ran fine for about 2 hours, then all my troubles started again. Just would not keep running. This was even after several igniter cleanings. I oil my piston manually when running because I can not find any oil that will go thru my drip oiler. Even tried the old stand by 3 in 1 oil.
I talked to several fellows at the show and they all run Colemen fuel, so today I bought some and will try it latter this week.
Some other comments are as follows: I do have a hot blue spark at the igniter with it out of the engine, engine seems to be flooded when it won't run, It seems to run best very rich??? There is some play at the carb adjust screw and I find it hard to get a good adjustment.
I will keep everyone posted on my progress so we can all learn together. First Iam going to try the coleman fuel than I am going to try more tension on the intake valve spring. The valve seems to flutter alot on intake

[Kelly Tytlandsvik]

What about Marvel Mystery oil. Would that work?

Kelly T

[BDMelon]

Originally Posted by Tim Litzenberger

To David TSR,
The compression seems fine to me. I judge that on when starting it it seems to have nice resistance when turnng it over. Since this is my first model engine I am comparing it to my larger full size 2 1/2 associated compression.
I had the engine to Jacktown show this past saturday and once agian it ran fine for about 2 hours, then all my troubles started again. Just would not keep running. This was even after several igniter cleanings. I oil my piston manually when running because I can not find any oil that will go thru my drip oiler. Even tried the old stand by 3 in 1 oil.
I talked to several fellows at the show and they all run Colemen fuel, so today I bought some and will try it latter this week.
Some other comments are as follows: I do have a hot blue spark at the igniter with it out of the engine, engine seems to be flooded when it won't run, It seems to run best very rich??? There is some play at the carb adjust screw and I find it hard to get a good adjustment.
I will keep everyone posted on my progress so we can all learn together. First Iam going to try the coleman fuel than I am going to try more tension on the intake valve spring. The valve seems to flutter alot on intake

I would us the cheaper fuel from walmart,Ozark trail fuel, then add 100 to 1
2 cycle oil smokeless i have use this set up on the bigger h&M & have great luck with it, coleman fuel is about $2.00 more a gallon, you will have to play with mixer a little more, then reg. gas, but at lease this stuff won't make you good luck melon

[Kelly Tytlandsvik]

They tell me that aviation fuel will not varnish things up like Regular gas. I went out to the local municipal airport and bought a gallon right at the pump.

Kelly T

[BDMelon]

kelly that would be 100 octane, correct, why run a fuel that is going to burn hotter in the model, latern fuel is 70 octane, i think, a will run cooler,
i have run the other stuff, & muffler on my h&M it very hot, when i run latern fuel , i can lay my hand on the muffler for a long time & not get burned
,, just i could be wrong here, but don't you want those model engine to stay some what cooler ?

[Kelly Tytlandsvik]

Wouldn't hotter be better to try to keep the moisture off of the plug? That was my thinking.

I have hit and miss engines that never warm up because they have no load and the warmer I get them the better they run. The AV gas seemed to work very well for me. But I usually run regular gas.......but I should make the change permanent.

Lots of guys use propane and realy like it. No carbon build up!

Kelly T

[gbritnell]

Gentlemen, I'd like to add my 2 cent here. It will probably come out like 20 cents but here goes. First off lets start with compression. An engine that has a low compression pressure, you'll notice I didn't say low compression ratio, only needs low octane fuel. Low compression pressure can be accomplished with initial compression ratio and valve timing. Being that a hit and miss engine generally has an atmospheric intake valve the amount of intake volume will be much smaller than a cam operated intake where the timing can be controlled. The bottom line for this is most low octane fuels can be used. By using a high octane fuel and especially an aircraft fuel in a low compression engine will only produce deposits from unburned fuel because you don't have the compression to help out.
Next lets try and figure out why an engine will run well for 2 hours and then refuse to start or run. There are 3 elements that are required for engine operation, fuel, compression, and ignition. One would have to assume that all 3 of these elements were present when the engine was initially started because it did start and it ran well for 2 hours. If I were to try and diagnose the problem I would start with the ignition when the engine quit. Do you still have a good spark? If so then that's not the problem. Next is compression. If the engine has run good for 15 minutes it has probably built up all of the heat that it's going to so if it continues to run for another hour and 45 minutes you would have to figure that the compression is still there. Finally there's the fuel. Here again if the engine has run good for the 2 hours with the simplest carburetor that there is I doubt that there's a problem. If the fuel supply was questionable all one would have to do is choke the engine and you would have plenty of fuel. Now we have eliminated all of the elements that make an engine run but it still stalled and refuses to start. Here's my best guess. The intake valve has a fairly light spring on it to facilitate opening by atmospheric pressure (cylinder vacuum if you will). The valve guide has no way to receive lubrication unless you would put a drop of oil on it while it's running and I have never seen anyone do this so as the valve quide dries out and there is more friction against the valve movement I would say that the engine is not recieving an adequate amount of fuel to run. It's not that there isn't fuel available, it's just that the valve isn't opening enough to get the proper amount of fuel into the cylinder.
All of the things that have been said up to this point are valid comments, but most only affect the initial starting or running of the engine. Any engine that runs for 2 hours and then quits has to be looked at a little deeper.
I hope my discourse didn't come across as condescending, it certainly wasn't meant to be.
gbritnell

[BDMelon]

well that just about says it all in a SMALL letter Ah , just kidding,,, good response GB
melon

[Tim Litzenberger]

GB, Great response!! I have on occasion found the valves to be sticking, and now I oil them on a regular bases when the engine is running. I check for spark by taking the ground wire off its connection, holding the points closed and I get good spark when I rub the wire against the engine. The pboblem with that is I don't know what is happening in the engine with the ignitor. With the ignitor out of the engine i do get good spark at the ignitor. I still have not run the engine with coleman fuel to see if that changes performance. Most likely will do that this weekend. Thanks for all great suggestions
Tim

[Anthony J. Wiegand]

Originally Posted by Tim Litzenberger

GB, Great response!! I have on occasion found the valves to be sticking, and now I oil them on a regular bases when the engine is running. I check for spark by taking the ground wire off its connection, holding the points closed and I get good spark when I rub the wire against the engine. The pboblem with that is I don't know what is happening in the engine with the ignitor. With the ignitor out of the engine i do get good spark at the ignitor. I still have not run the engine with coleman fuel to see if that changes performance. Most likely will do that this weekend. Thanks for all great suggestions
Tim

If your testing Spark when the pts. are inside the engine. Simply place a radio tuned to a AM (I think) station..when the pts do their thing, you will hear a static interference...