Stihl 4-mix 65cc questions
 

I am working on one. The crank extension on the "correct" end is pretty small. I see somebody else did this conversion but put the prop on the "back" end of the crank and turned the engine around backwards. With this engine that would make mounting it in the airplane easier. It would also utilize the beefier end of the crank. It would put the spark plug away from the firewall instead of toward the prop. I see he used a tractor propeller. He must have made it run backwards. How is that done on a 4-stroke?

http://www.rcuniverse.com/forum/m_44...tm.htm#4478957

RE: Stihl 4-mix 65cc questions
 

I figured it out.

RE: Stihl 4-mix 65cc questions
 

I did the one for Tripp when they first came out. I made it to his spec but think that was the corect setup to run CW .

RE: Stihl 4-mix 65cc questions
 

What are you putting it in? I had mine in a 1/3 cub. Ran great with a 23x8 xoar.

Mark

RE: Stihl 4-mix 65cc questions
 

If you have the prop hub on the end shown in your photo, you have no concerns, it will turn a standard CCW prop just fine and dandy. Are you still planning on EI?

J

Mark, I'd love to see a photo of your Cub with the Stihl!!!

RE: Stihl 4-mix 65cc questions
 

I am not sure yet. I was thinking my 1/4 scale D-7. It seems like too much engine for it. I also have a Bridi Big Bee that seems like a logical choice. It is overbuilt and could stand the power.

It turned up 6700 with a 22x10 MENS today. Before I put it on anything I need to get it running smoothly.

Scot

RE: Stihl 4-mix 65cc questions
 

If you are using the stock carb, it may be difficult to get the mixture rich enough to run smoothly. You could dive into the carb and open things up (like with the Briggs), but I just replaced it.

J

RE: Stihl 4-mix 65cc questions
 

No matter what carb I put on it it spits a lot of gas back out the carb. It is like there is an air pulse coming back out. It was pointed out to me that this engine may use a reed valve. Anyone know if there is a hidden reed valve inside the engine. I already tore it down and cleaned it. I did not see a reed valve inside. Did I miss it? Should it be on the intake manifold? It does not use a reed valve and something else is wrong??

Thanks,
Scot

RE: Stihl 4-mix 65cc questions
 

it has regular valves typical four strokethroaway the carb and rubber boot, make an adapter out of aluminum I little work and amaginationis needed in this area. and don't forget the case needs vented hense the large tube from case to the carb adapter, this may help a little I will look in my archives and find some pics of when I did one.

RE: Stihl 4-mix 65cc questions
 

Here is an animation of the Stihl Four Mix engine. It operates much like a two stroke by pulling the fuel/air mixture into the crankcase first. When the piston goes down, it then pumps it from the crankcase back into the intake track, and when the regular four stroke type valve opens, the mixture enters the combustion chamber.

http://www.youtube.com/watch?v=eWZfRdm9u9c

The animation doesn't show a reed valve in the system, but I would think it would need one to work well. Can anyone verify if there is a reed valve in the Four Mix engines or not???

Also, if any of you have used this engine, can you outline the mods necessary to the carb to get it to work well? (I have a friend that converted one and is trying to get it dialed in. I imagine the passages in the carb will need enlarged to richen the carb up enough. If anyone has done this, do you have the drill sizes for the passage enlargements?)

Thanks in advance,
AV8TOR

RE: Stihl 4-mix 65cc questions
 

Case venting.....The vent on the side of the crankcase operates the carb diaphragm? The stock carb has a little vent that prevents a pressure build up? The "little vent" looks like a tiny brass fuel fitting?

Scot

RE: Stihl 4-mix 65cc questions
 

Scot, that "little vent" is where the primer bulb was hooked up to draw fuel through the carb. It should be blocked off.

AV8TOR

RE: Stihl 4-mix 65cc questions
 

I've completely disassembled (and put back together:)) several 65s and have yet to find a reed valve.

J

RE: Stihl 4-mix 65cc questions
 

There is no reed valve. A velocity stack sufficiently contains the back pulse. I opened up the main jet. Now I can't get the low end lean enough. This carb lacks a check valve. If you let go at high RPM it floods and dies.

MENS 20x10 7000 RPM

RE: Stihl 4-mix 65cc questions
 

Looks great! How about a video??

How much did you open up the "main jet"? Did you open the main orifice itself, the regulator inlet needle orifice, or what?

AV8TOR

RE: Stihl 4-mix 65cc questions
 

There is a brass button with hole in it under the metering lever, not the needle the lever acts upon, but the other end of the teeder todder.

RE: Stihl 4-mix 65cc questions
 

I added an EI unit. It will break 7K with a 22x10 MENS. That is with the advance too far for hand starting. Kinda defeats the reason I added the EI. With less advance I am getting more like 6800 with a straight pipe. That is with the stock carb. That carb does not work well. It does not run smooth or give a nice slow idle. Something like a 13mm pumped carb with a check valve would probably be about right. More messing around is in order.

RE: Stihl 4-mix 65cc questions
 

I used a wt-201. I put it on a 90 degree intake that I made. I have no spitting from the carb. The nipple on the side of the block is the pressure port for the pump. The only problem that i ever had is oil building up in the pump diaphram. I used a tee to form a sump and put a drain plug in and drained it each day.

Mark

RE: Stihl 4-mix 65cc questions
 

Did you use a spring with the fuel pump diaphram on that setup? Have any videos of it running?

I used a return spring in the fuel pump of my Honda four stroke, but I think the design of the Stihl, being more like a two stroke in actual operation might not need a spring. However, the stock carb has a special spring and diaphram setup for the fuel pump.

Thanks,
AV8TOR

RE: Stihl 4-mix 65cc questions
 

I have not. I have several new carbs coming. I also have a new ignition coming. I suspect my current test stand ignition is not advancing properly.

I doubt a spring on the carb diaphragm is needed. The pulse is from the crankcase and is the same as a 2-stroke, positive and negative pressure. There is a passage way from the crankcase to the intake manifold. The pulse is probably not as strong as a regular 2-stroke due to the crankcase charge being free to go in and out of the carb.

There is plenty of experimentation to do.

RE: Stihl 4-mix 65cc questions
 

No spring. Runs great. Flew it for two years. Flew my 1/3 scale cub for an hour on 32 oz. of fuel. Started aero towing so it is sitting on the shelf right now waiting on a new home.

Mark

RE: Stihl 4-mix 65cc questions
 

Well, I think I figured out why it runs crappy. It was bad oil seals on the crank. The crankcase would hold pressure just fine. I could not check it for vacuum though. I ran it again today, frantically changing carbs, and had a thought. I sprayed some solvent on the seal while the engine was running. It quit. Tried it again, same result. The seals were allowing air to be sucked in. That makes total sense. The idle was fast with the butterfly idle stop screw removed. Remember, I mentioned it spitting gas out the carb? Well, if it were sucking in extra air that would cause it to blow out extra air when the piston goes down. New seals are on order.

RE: Stihl 4-mix 65cc questions
 

Well that should make a huge difference!! What did the seals cost?

Keep us posted!

AV8TOR

RE: Stihl 4-mix 65cc questions
 

I did not ask. I buy generic seals from a bearing specialist. My guess is about $20 for both with tax. I did not investigate seals from a Stihl dealer.

RE: Stihl 4-mix 65cc questions
 

Who do you use? I use Boca Bearings for all my bearings, but I don't have a good seal source...

AV8TOR