Sachs 505/1a performance
The Sachs 505/1A is the most common of the 505/504 series engine. It's distinguished by the tapered cylinder marked 40kph. A stock horse power rating of 1.8 puts it on par with most Puch E50 engines and thus deserves a second look as a potential platform for tuning. Though there are not many kits for these, there is some potential nonetheless for a little creativity to take the place of bolt on parts and a thick wallet. During the course of this article I will be referring to the 505/1A, though everything mentioned should apply to the 504/1A as well, but for the sake of clarity I shall only be using the 505 designation.
Regarding the B,C, and D Engines
These are the less common variants on this design, and will not be addressed in this article except for in the following:
505/1B If you have one of these heavily restricted engines you can replace the header pipe and carb to convert it to a 505/1A and then proceed to the following sections
505/1C some things may apply to this engine but since it shares it's cylinder with the 505-1D the similarities will be limited. You can install the "A" cylinder on this as well but you may not want to, because it would be a downgrade.
505/1D This engine produces 2.7HP stock! It has nothing in common with the "A" and "B" engines but the case and clutch and should be addressed in its own article.
The stock setup of the 505/1A features a 14mm intake with a "Square" Bing 12mm carburetor. It's different from the Bing used on Puch as it has a shallow square float bowl secured by 2 screws -- much like a Dellorto -- as opposed to a round threaded bowl. The shallow bowl allows it to clear the engine case, and this carb is therefore not easily replaced. Unlike other engines, the intake cannot be flipped toward the front on this engine as the mounting flange is angled.
Boring The Stock Carburetor
The square Bing can be bored out to 14mm, but to allow for some margin of error, I recommend 13.5mm. This carburetor body can be bored with ease as it can be quickly stripped and it will sit nice and flat on the bed of your drill press. It also does not have an idle circuit in the traditional sense, rather it has a notch cut out of the back of the slide allowing fuel to be drawn from the main jet. This means that you won't run the risk of closing off your idle jet, nor will you need to re-open it with picks. Unfortunately, there are few or no jets available for it, so you must drill out your jet. Alternatively, you can use the jet holder from a round Bing and then use standard Bing jets, the end result is a robust and easily serviced 14mm carburetor. For detailed instructions on boring your Bing, see Over Boring Carburetors 101.
Mounting a Dellorto SHA:
The Dellorto SHA is a very close fit, and with some simple modification to the intake it will not only fit on the engine, it will also still fit under the side covers. There are a couple different ways of doing this,but before you start check the fit of the carburetor as due to slight model differences and casting variations it may already fit. The first way requires no skill or really anything other than wrenches. You start by removing the two intake bolts and washers. Next you remove the intake and reinstall it with the two washers, both on the rearmost bolt between the intake and the intake port. This will angle your intake perfectly, allowing the carburetor to slip on easily, clearing the top of the engine. The last step is resolving the problem with this method; that's sealing the big gap left over. This could be filled with plumbers putty or a similar compound so long as it is both heat and gasoline resistant. Plumber's putty may be neither so read your labels well!
The second method will take a bit more time and is more desirable as the likelihood of sealant breaking loose and being sucked down the intake is much less. This involves making a spacer, for this I chose 1/4" Masonite (basically the cardboard stuff that goes on the back of your entertainment center). It's derived from paper pulp so it should compress and behave the same as a paper gasket. You start by tracing around the base of your intake and then cutting it out with a fine toothed hand saw (hack saw works, coping saw works even better). Once it is cut out and trimmed up nicely, get a stamp pad and push your CLEAN intake into it and then "stamp" the whole pattern onto the masonite. Start by cutting out the center whole by drilling in the middle of it then grinding the rest of the material off with a Dremel. Next use a C-clamp to fasten the spacer to the intake and a piece of scrap wood underneath making sure your holes are lined up. You will now use the bolt holes in the intake as drill guides so as to attain the proper angle, choose a close fitting drill bit and drill through the spacer and into the scrap wood. Once you've cleaned up all the little "fuzzies" and bits of peeling paper, your intake spacer will be finished! Now you need only mount up your intake with your new 1/4" thick gasket. Due to the angle of the intake flange, you've moved the carburetor diagonally up and to the left. This will clear the engine case and should clear the frame. In some cases the spacer may require some tapering to achieve the desired angle. This can be done by sanding or any other appropriate grinding method. It is also worth noting that if you are using a 15mm or 16mm carb, you will need to further modify the intake. Simply grind down the diameter of the intake where the carb attaches until the plastic sleeve that came with the carb slides on with no resistance. At that point use a sealer, or glue of your preference to fix the sleeve to the intake creating a perfect sealing mount for your new carb. Having done this on my G3 I found that it still took some fiddling to get it to clear the frame, but it does. In the case of other frames the fit may require a thicker spacer to be made or may have no trouble at all. The only other modification required at this point is to drill a small hole in your side cover to run your throttle cable.
The stock porting of the 1A cylinder is adequate in most ways. The transfers are good sized and the intake port is certainly large enough considering that the largest intake pipe for this engine is the 90 degree 14mm. The exhaust port though can be raised by 2mm. This is probably the most significant difference port wise between the "A" and "D" cylinders and is an easy adjustment to make. Just make sure to take your time. Doing this required a fairly significant jetting change.
This is a stumbling block for this engine as Sachs uses a one-piece cylinder and head. To increase the meager 8:1 compression, one can leave out the base gasket. However, this will retard your port timing, so this if it is done it should be done in conjunction with porting. For any more increase to the compression ratio the cylinder base will require milling.
If you can weld, you can put any exhaust on any bike. For those who can't, the options seem to be rather limited. The good news is that two piece exhausts for Tomos can be used with very little difficulty. Assuming you have the non-restricted exhaust, you simply unbolt and remove the muffler from the header pipe, you then cut down your header pipe to the appropriate length and slide the expansion chamber on to the header. Of course its recommended that you measure diameter first so that you don't chop up your stock exhaust only to find the new pipe won't fit. From here the stock mounting bracket can be replaced with a hook to support the back of your exhaust. A piece of 1/8 inch steel would be more than sufficient.