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av8tor1977
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Default Homelite (and other) Hop Up Techniques and How To:

[h=1]HOMELITE 25cc HOP UP[/h]


This article is specifically about the Homelite 25cc, and pertains to the Homelite 30cc engine in most respects as well. However, the port measuring and porting techniques apply equally well to most small two stroke engines. I hope you find it helpful.

First and foremost, it is important that one never attempt to “hop up” a worn engine. The results will be frustrating at best. All important components should be in perfect condition, and I always recommend starting with a new Frank Bowman ring as well. I have seen engines that still ran pretty well gain 800 rpms by only replacing a worn ring.

Second, a coordinated approach is always needed when hopping up any engine. The modifications need to work together to achieve good results. It does no good to bolt on a huge carburetor for example, and leave the stock, restrictive exhaust in place. Common errors such as this create an engine that actually runs worse than stock.

Third, unless you are willing to go through a lot of parts and do a lot of testing, stick to known, proven modifications. Parts cost a lot of money, and if you make a modification that doesn’t work, well, you just created an expensive paper weight. Don’t re-invent the wheel….

And lastly, don’t “polish” the ports. Just a nice finish with a small sanding drum to clean up the casting is all that is necessary. “Polishing the Ports” is old tech, and it has been proven that a slightly rough port actually flows somewhat better than a mirror smooth port. Seems odd, but it’s not if you know some of the intricacies of fluid dynamics.

You will need the proper tools of course. Torx drivers, screwdrivers, etc., etc. as normal, plus a few special tools. Here are some that I use:








Most of them are self explanatory. The small light on the end of a flex cable is my favorite for checking port timing as I can insert it inside the engine and really see when a port opens or closes. The Dremel cutters are all carbide. Don’t try to use grinding stones on aluminum or magnesium! They will immediately clog up, then can over heat and explode! Lastly for the Dremel is a small sanding drum tool. (Good for final dressing up of the ports.) The tool that looks like a spark plug is my Positive Stop for finding TDC, (Top Dead Center), and the last is a length of PVC tubing with a piece of thick, hard emery sand paper securely glued onto the end. This is for removing the squish band. A person should also have some small jeweler’s files in flat and round. They should be heated and have the last 1.5” or so bent at about a 20 to 30 degree angle. These are for dressing and chamfering the edges of the ports to prevent damage to the piston and/or ring. I do it with the small round carbide tool in my Dremel, but, one slip and, well, remember what I said about expensive paper weights?

I don’t need to mention about eye protection and a dust mask for use while porting and grinding on metal do I? Didn’t think so….

The first step, after disassembly, and a thorough cleaning and inspection of parts, is to mock up the engine the first time to take baseline measurements. Hopefully, we already have the Frank Bowman modified piston and special ring in hand. Then we start taking our baseline measurements, but one of the measurements will require the squish band to be removed first, so I do that using the PVC tube with the stiff sand paper glued on it. The PVC tube is a nice smooth slip fit into the cylinder. One could make a special cutter to do this, but unless you do a lot of engines it wouldn’t be worth it. Just use the sand paper tool. Push it up against the squish band and push and turn to remove the band. Check often, and be very careful not to scar or score the cylinder bore with the tool. You are done when you can insert a screwdriver or scribe into the cylinder head area, and no longer feel that “step” around the outer edge of the combustion area. Here is the cylinder with the squish band ground out.



Another thing we are going to be doing is to “stuff the crankcase”, and now is a good time to take a measurement of that to see what will be required. As you can see, there is quite a large gap between the crank pin and the backing plate. We want to reduce that to about .025” for better engine efficiency. (Increased secondary compression ratio.)





Now we can do the first mock up assembly and start taking actual measurements.



First we set up the degree wheel and pointer by finding TDC. This is the only proper way to find Top Dead Center of the piston, and is called the “Positive Stop Method”. You install your positive stop, (seen screwed into the head in the picture above), and set the degree wheel and pointer so that it shows the exact same degree reading whether the piston is up against the stop going in one direction, or the other. In this case the piston hits the stop at 35 degrees when I rotate the engine clockwise, and when I rotate the engine all the way around counterclockwise and the piston hits the stop again, it also shows exactly 35 degrees. You move the degree wheel or the pointer as necessary to achieve this; equal in both directions. Now, and only now, when you take the Positive Stop out and the pointer shows “TDC”, the piston is actually and truly at Top Dead Center. Now we can take the port timing measurements. Here’s what I got for stock readings:

INTAKE OPEN: 60 degrees BTDC
INTAKE CLOSE: 60 degrees ATDC
Total Duration = 120 degrees

EXHAUST OPEN: 106 ATDC
EXHAUST CLOSE: 74 ABDC
Total Duration = 148 degrees

A special note here because I know people will ask. Here’s how you find that exhaust duration:

180 – 106 = 74 degrees. 74 (from ATDC to BDC) + 74 (ABDC) = 148 total degrees the exhaust port is open. If it is easier for you, just count the total open degrees on the degree wheel.

This is just a baseline measurement. This is an older engine, so the exhaust duration is not too bad. Newer engines have been coming out generally with a much lower exhaust port duration. However, the intake duration is rather low. All this will change however, when we lower the cylinder to raise the compression. When the cylinder is lowered, the exhaust port timing is lowered, and the intake port timing is raised. (The exhaust port will open later, and close earlier, and the intake will do the reverse when the cylinder is lowered.) So when we lower the cylinder, we will lose ground with the exhaust port total timing, but gain some with the intake. We’ll compensate when we do our porting.

Next, while working with the degree wheel in this first mockup, I like to use my dial indicator to see how much piston movement affects port timing. This could be figured out mathematically using the piston stroke, rod length, and some tricky math, but it is quick and easy to do it with the dial indicator, and fool proof.



I measure how many degrees of wheel movement it takes to move the dial indicator .100”. (For best accuracy do this test measurement with the piston at about mid travel, and don’t try to figure for just one degree or a smaller dial indicator amount as it won’t be as accurate; use .100”.) In this case, 10 degrees of wheel movement moved the dial indicator .100”. So dividing 10 into .100”, we find that each degree of the wheel moves the piston .010”. That’s a nice easy number to work with. Now, here is an important fact that is absolutely imperative to remember when working with two strokes. Whatever amount you take off a port top or bottom to change the timing DOUBLES the timing change. This is because you are not only changing when the port opens, but also when the port closes.

Example:
20 degree change desired.
20 x .010” = .200”
.200” divided by 2 =.100” (Because both opening AND closing is changed.)
.100” to remove from port opening edge for a 20 degree timing change.

So now we know that in order to make a (for example) 20 degree port timing change, we would remove .100” from the top of the exhaust port or the bottom of the intake port. If we removed .200” it would change the timing 40 degrees because remember, when we make a change to a port, it alters BOTH the opening AND the closing of the port.

The next thing to check/measure, is the quench or squish distance. This is also sometimes called “Deck Height”, though that term is not exactly correct for this application. What we need to know is the distance from the piston to the head at TDC. The way to do that is to bend a piece of soft solder, (not silver solder) so that it will go into the spark plug hole and go over against the cylinder wall. We then rotate the engine several times while holding the solder secure in one position and making sure it is touching the cylinder wall. The piston will smash the solder flat, which we can then measure and this gives us the piston to head clearance. I normally use 1/8” electrical solder for this, but the clearance was so wide in this engine that I had to use larger diameter plumbing solder. The pic is a bit blurry, but you can see from the gap in the calipers that it was a large piston to head clearance. It measured about .080”, while we want a much smaller gap of .020”. (.015” absolute minimum for safe operation.) Lowering the cylinder and closing up this wide gap will give a nice raise in the compression ratio of this engine. Now we know about how much to remove.



So, the crankcase is set up in the milling machine, and we proceed to take off .060” to arrive at the .020” clearance that we want. (I actually took a bit more off, because I like to make a new, slightly thicker cylinder base gasket and so I allowed for that.) Some people make up a spud to mount the cylinder in a lathe, and remove material from the cylinder base. I prefer to take the material off the crankcase when possible, because then if you ever have to replace the cylinder, you won’t have to do the machine work again. The squish clearance can always be fine tuned with different thickness gaskets if you take enough off the crankcase to allow for that.



Next comes the cylinder porting. There are several important items to note here. One is obvious but critical; you can always take more material off, but you can’t add it back on. So I recommend that you take perhaps half of what you calculated you would need to, and then mock the engine up again, and measure your port timings. See if they are what would be expected for the amount you took off. If you go too far, well, there we are with a paper weight again! You might want to do the porting in three stages instead of just two. Grind a little, check, grind again. (Hey, if it were quick and easy anyone could do it right??!!)
Another thing is that you must always clean up and chamfer the port edges after grinding on them and before mocking the engine up again. If not, the raw port edges where you ground them will scar, score, and destroy the piston and ring. (More paper weights!) It is your choice whether you remove the ring or not for these mock up tests. If you do, be careful as it is very, very easy to break a ring while taking it off or putting it back on. If you don’t take it off, just be very careful when assembling the engine that you don’t break the ring. Go slow and easy, if anything needs forced, something is not right.

So the Homelite 25cc comes with a bridged exhaust port. We have had the piston modified with a pin and a special ring for use with a pinned piston, all done by the Ring Master, Frank Bowman, so we can remove that port bridge for better exhaust flow. The pin will keep the ring from rotating and ever getting caught in the exhaust port, so, away with that bridge!





As a quick side note here, that is the quick and easy way to tell a Homelite 25cc from a Homelite 30cc. They are identical on the outside, but the 30cc does not have an exhaust port bridge.

Some of the later model Homelite 25 and 30cc engines have a hole in the cylinder just above the exhaust port that leads into a corresponding hole in the muffler. Opinions vary as to what that hole is for, but the most probable one is that it is for EGR, or Exhaust Gas Recirculation. We don’t want that for our engines, so if yours has that, drill and tap it for a set screw, and plug the hole with a set screw with Loctite. Take care that you don’t penetrate the cylinder with the tap threads, as that may cause a burr in the cylinder.

Now for the actual porting. There are several ways to mark the ports for grinding, and I have used them all at one time or another. One is to use magic marker or machinist’s Dykem to color the area above or below the port so that it will show a scribed mark. You can then very, very carefully position the piston so that it’s top edge will be where you want the new top of the port to be, and reach in through the spark plug hole and scribe the cylinder near the port, using the piston as a guide. Another is to do the same magic marker or Dykem marking, but use a calipers to get your distance, and then use a right angle scribe to make your mark. And lastly, you can also use a piece of pin striping tape or a similar piece of tape cut from electrical tape. Use a long tweezers or medical hemostats, and position the tape the appropriate distance above or below the port. You can then grind to your mark, (or the tape), but as I mentioned I highly recommend going only part way and doing an engine mock up and measuring/checking your results and progress. Remember, when changing port timing we ALWAYS remove only from the TOP of the exhaust port, or the BOTTOM of the intake port. All in all I recommend the tape method as probably being the easiest and most fool proof means of marking your port for grinding. Using the bottom of a vernier calipers or using a dial indicator are ways of gauging the distance; another would be to find or make something of the thickness desired, to gauge the distance you want to grind the port top or bottom. It really is difficult working up in that small diameter cylinder with a head that can’t be removed. So just take your time and as I keep saying; go slow and check your work often. Here are some porting pics.













Resist the urge to grind out that little “nub” in the intake port. It is there to help keep the piston ring from bulging into the intake port and getting damaged. Leave it there. Also, the transfer ports are not addressed in this treatise, because they are very critical and you should only attempt modifying them after you are very adept at porting. They must be kept identical, both in opening times, and ANGLES! If you mess them up, the engine might end up running worse than stock. Just check that the transfer ports align at the cylinder to crankcase interface, and smooth the transfer port entry slots in the crankcase itself.

Now we will address “Stuffing the Crankcase.” I determined that a 1/8” piece of aluminum would be the correct thickness to get the .025” crank pin to backplate clearance I wanted, so I made a plate out of it to the shape of the inner surface of the backplate.





I sanded both the back plate and the aluminum spacer, cleaned them with acetone, and then installed the spacer using JB Weld and a counter sunk screw in the middle. A counter sunk screw is not really needed as the crank pin goes around the middle and won’t hit a bolt head. However, if you use a regular bolt, be sure to check that it doesn’t hit the connecting rod. If you think you will ever need a crankcase pressure tap for a smoke pump, or remote carb pulse, here would be a good place and time to put one. Use the pressure tap to help hold the plate in place instead of a bolt. The JB Weld actually does the holding, the bolt is just there to firmly hold the plate in place while the JB Weld dries, and to help me sleep better at night. It sure would be messy if that plate ever came loose while the engine was running….






Now we have a much more reasonable piston to crankpin clearance and our crankcase is STUFFED! Note that this is another spot where you can fine tune your clearance with gaskets. Using thicker, thinner, and/or multiple gaskets on the backplate will get you where you want to be; .025”.

Now on to final assembly and the last checks. Use a small amount of two stroke oil on all bearings and give them a spin. Don’t load the engine up, just a few drops on each bearing is plenty.



A pic of the thicker cylinder gasket I made. (use notebook cardboard, cereal boxes, file folders, etc. to make gaskets.) Harbor Freight sells a really neat hole punch kit that makes making gaskets fun. (Well, sort of….)

I don’t use any sealer on the base gasket normally, and NEVER use any sealer on the carb adapter or carb gaskets.

Put a nice film of two stroke oil on the piston, ring, and cylinder wall and put ‘er back together for the last time. (Hopefully!)



With the cylinder bolted on, a final check of the squish distance shows a perfect .020”.



This article is continued in the next post by w8ye. The reason being for the continuation is due to the limitations of the RCU editor . . .

Last edited by w8ye; 03-20-2015 at 11:01 AM.
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