Author Topic: Head shims? (Read 5174 times)

Rich Perry · « **on:** April 09, 2016, 08:23:28 PM »

So from what I have gathered, adding head shims tends to make you run hotter/leaner, so increasing compression would make the engine run richer cooler?

I have some engines I have for combat, on bladders. They are going lean in the loops, and are tricky to needle. Granted, I know there are a bunch of variables, but I would like to focus on head clearance. If I remove the shims, increase compression, would this possibly help with this going lean in the loops issue?

RandySmith · « **Reply #1 on:** April 10, 2016, 12:21:15 PM »

What engine?
how many head shims? thickness?
have you tried double bladders?

randy

Rich Perry · « **Reply #2 on:** April 10, 2016, 01:54:04 PM »

Randy,

It is an ASP .25. 2 head shims at .005" each. The engines came with the 2 installed. I have not run the engine without the head shims. I am running a typical bladder set up. Just one.

It is something with the engine, as I just swapped it out for an Os fp .25, same bladder and remote needle, and it ran perfectly.

Rich

RandySmith · « **Reply #3 on:** April 10, 2016, 08:17:18 PM »

Quote from: Rich Perry on April 10, 2016, 01:54:04 PM

Randy,

It is an ASP .25. 2 head shims at .005" each. The engines came with the 2 installed. I have not run the engine without the head shims. I am running a typical bladder set up. Just one.

It is something with the engine, as I just swapped it out for an Os fp .25, same bladder and remote needle, and it ran perfectly.

Rich

Hi Rich the OS may use a smaller venturie than the ASP, hence the double bladder question... it will give a stronger and more even fuel supply to the engine. The 2 .005 gaskets are not excessive, and I doubt removing 1 will change the problem
, but it won't hurt to try and see
Randy

RknRusty · « **Reply #4 on:** April 10, 2016, 08:26:23 PM »

Randy, does adding shims make an engine run hotter? That's opposite what I would have thought.
Rusty

RandySmith · « **Reply #5 on:** April 10, 2016, 08:52:13 PM »

Quote from: RknRusty on April 10, 2016, 08:26:23 PM

Randy, does adding shims make an engine run hotter? That's opposite what I would have thought.
Rusty

many times YES it does.. if you try to get the same power you have to lean the engine, to do that, a lean engine run hotter than a richer running one.
Adding shims decompresses the engine, Most every time you decompress an engine, you cut the power, and it runs leaner at the same power setting

Randy

RknRusty · « **Reply #6 on:** April 10, 2016, 09:16:20 PM »

Okay, that makes sense. I'd neglected to think of having to compensate for the power loss. Thanks for the reply.

Rich Perry · « **Reply #7 on:** April 11, 2016, 05:22:08 AM »

Randy, what I was thinking is that the fuel jet is too far out on the venturi. Hanging out in disrupted air. As I am sure you are familiar with the FP series, the fuel jet would be pretty far down in the venturi. I will get a picture posted so you can see what my set up looks like. The venturi was made for me by a well known pilot, but I think the design is what is causing the problem.

RandySmith · « **Reply #8 on:** April 11, 2016, 08:24:38 AM »

Quote from: Rich Perry on April 11, 2016, 05:22:08 AM

Randy, what I was thinking is that the fuel jet is too far out on the venturi. Hanging out in disrupted air. As I am sure you are familiar with the FP series, the fuel jet would be pretty far down in the venturi. I will get a picture posted so you can see what my set up looks like. The venturi was made for me by a well known pilot, but I think the design is what is causing the problem.

Hi Rich
I would like to see the picture, but, running a bladder should make that a moot point, your not relying on suction from the engine to feed it fuel, I have experinced exactly what you describe, the double bladder fixed it in my case, yours may or may not work the same

Randy

Rich Perry · « **Reply #9 on:** April 11, 2016, 08:51:09 AM »

I am not familiar with a double bladder set up. Could you please explain how this is configured ? I am curious now.

Rich Perry · « **Reply #10 on:** April 11, 2016, 05:10:59 PM »

The diameter of this venturi is .19", the one installed on the ASP. Tough to get my calipers all the way into my FP without taking it apart, but it looks to be right around .25"

RandySmith · « **Reply #11 on:** April 11, 2016, 07:43:10 PM »

But the OS has a spraybar all the way across the venturie, how far does the bar go in the other engine?

Randy

Rich Perry · « **Reply #12 on:** April 11, 2016, 08:01:59 PM »

In my FP, I am using a Fox fuel jet. Is hs a speed limit screw tapped on the opposite side just like the ASP engine does. I do not have the FP set up stock with the spray bar going through. Another typical speed limit combat set up with the speed limit screw to fine tune lap times.

Steve Helmick · « **Reply #13 on:** April 15, 2016, 01:22:44 PM »

Something I might add, on the Magnum. They're sort of Chinese/Fox in the quality department, meaning that they have potential for tolerance issues, and you "need to get all the screws in the right place". One Magnum XLS .36 that Mike Haverly has/had, came with two head gaskets. With one head gasket, the piston hit the head. So, the assembly crew added a head gasket. It may be their normal way of dealing with this sort of tolerance problem, so if you decide to take one of them out, be sure to check for piston clearance!

Some of these things happen because of the venturi/restrictor. A prime example of a factory SNAFU was the Rossi .15 venturi. Big "burp" when you hurled your F1C...not good!

Steve

RandySmith · « **Reply #14 on:** April 15, 2016, 06:43:05 PM »

Hi The double bladder is simply, it is one bladder inside of the other, this give a stronger, more consistent flow, and it may solve your problem, what RPM difference are you seeing from the 2 engines?

Randy

RknRusty · « **Reply #15 on:** April 15, 2016, 08:39:48 PM »

I've never run a bladder on anything bigger than an .061, but I have a Thunder Tiger Pro .25 that would never run well on a hard tank. I was thinking of putting a bladder on it and spinning it up fast with a flat prop. Never got a fine NV for it though. I was told a remote OS#1 RC NV worked well. I might have to put that back on my list.

Till I read this thread, I didn't know anybody flew stunt on pressure bladders. All of my 1/2A sport flying fleet is on small surgical tubing with 128tpi NVs, and I try to tell my fellow 1/2A friends that once you go bladder, you'll never go back. I just get dubious stares. You can't even burn an engine up with a bladder. If it goes lean and sags, in-rushing fuel chills it and worst case scenario is a surging engine run. A self limiting system. What's not to love.
Rusty

Rich Perry · « **Reply #16 on:** April 16, 2016, 06:51:20 AM »

Interesting on the double bladder Randy, not sure I could shoehorn one inside of another. It would be a tight fit! The typical combat bladder is pretty thick walled already, so the pressure is pretty high. I have not tached the 2 engines, so that is data I do not have yet.

At this point, never minding the title of this post about the head shims, What would cause an engine to go lean in loops and such, assuming the fuel system is sound? I have come to the conclusion that it has something to do with the configuration of the ports in the liner, the crank timing/duration, or lastly the venturi design. I have had trouble getting the .25 FX doing the same thing, and it is essentially the same engine as the ASP. There is an article out there on converting the OS .25 FX for combat use (bladder fuel system and such) and the author has since abandoned the engines for essentially the same reasons that I am.

With all of that though, some guys are having great luck with them as I flew against them last year, and said the engines were the way to go. I wonder if it could be that I am flying at 5200' altitude?

Paul Smith · « **Reply #17 on:** April 16, 2016, 07:19:21 AM »

You don't just add or remove head shims. You measure the depth of the piston and the distance the head goes into the sleeve and compute the clearance. Once you have a starting (stock) number, you can adjust from there. The starting number will be someplace around .015" to .020" for an RC or stunt engine. Combat and racing engines might be under .010".
The big deal is if you approach zero you destroy the engine.
If it's mile high you lose power and maybe won't start.
In general mufflers restrict the exit of the exhaust and make you need more clearance.
Thus removing a muffler that was supplied with the engine will allow you to lower the head.
It's a trial-and-error process, but you need to take data to know what you are doing.

Steve Helmick · « **Reply #18 on:** April 16, 2016, 01:05:20 PM »

Increased load causes the engine to go lean in loops and such. That's what causes a 4-2-4 run in olde fashioned stunt engines. For your combat use, I still suspect the venturi, but possibly more oil (castor?) or adding a fuel filter (auxiliary fuel tank) between the NV and venturi might improve things? Do you use a "stuff tube" on that venturi? That might also cause a change for the better.

Have you tried the Magnum RNV assy.? They seem to have a nice fit of the threads, and pretty fine thread pitch, tho I haven't checked to see what it is. A lot better than the crap that comes with the OS LA-series engines, for sure.

Steve

Rich Perry · « **Reply #19 on:** April 17, 2016, 07:57:10 AM »

I started out with the Magnum RNVA, and yes, they are nice. Similar to the OS .25 FX. The OS .10 needles come to a sharp point, and the Magnum is a truncated cone. When setting the needle, it is not hard to get it to where it needs to be (or so it would seem) until you get in the air. So the thread pitch would be suitable with with the stock unit.

A stuff tube may be a good idea. Certainly good for keeping dirt out of the engine.
I know with a bladder the venturi should not matter so much, but my feelings are that the jet is too far out and hanging out in disrupted air. Once the forward motion of the plane comes into play (while flying) it is effecting the mixture. If you look at the pictures of the venturis, the FP set up has the jet way down at the bottom, where the ASP set up is close to the outer edge of the venturi itself. I do not have any other appropriately sized venturis to experiment, so for now the FP is the engine for me. I can get them used, cheap, and Tower and Ebay have the venturis readily available. Not to mention, the FP is considerably lighter. So much so, that I will be making slight design changes to keep the balance where it needs to be.

Rich Perry · « **Reply #20 on:** April 18, 2016, 07:22:13 AM »

What I am going to try on the ASP is drill out the cam bolt hole that holds the venturi, in to accommodate a typical spray bar. I will cap one end, essentially making it a fuel jet, in a "true" venturi configuration. Most like a Super Tigre spray bare I have laying around already modified for this exact purpose. Placing the entry of the fuel significantly deeper into the intake than it is now. The speed limit screw should still do it's job as far out as it is, but I still feel the fuel inlet is to close to the outer edge of the venturi. What I will be doing is making a more conventional configuration that we know works.