Thanks guys I do appreciate the support and makes this all worthwhile. I need your ideas as much as you need mine and really like how information is shared here.

Flathead thanks for your input to. Thats the kind of info I would really like to see. 7500 on a 20-6? Holly smokes I would like the details on that build up. I can hardly get to 7500 on a 18-8. It would be interesting to compare what you have done to mine. are you getting that rpm with a stock ignition or a electronic conversion? I am starting to wonder is my stock ignition is a limiting factor. I cant seem to get much past 7400 RPM. When you say stock carb are you talking about the stock 7-8 mm venturi carb. If thats the case thats pretty impressive as I have found the biggest gain in power came with a carb increase. I see you are in goodland Kansas. Whats your altitude there? Its 1200 feet above mean sea level here. If your getting those results and are at an altitude between 1200 feet here and the 5280 feet of denver colorado than thats pretty impressive. I may have to come visist you on one of my trips back to denver to see my folks so we can do some side by side comparisons.

Ok back to today testing

I installed the crank and flywheel and coil combination from the new engine. I also refound TDC and remarked the timming marks as I had them before. I then installed the 17-8 prop and ran. I recorded a RPM of 8220. I now know at this RPM I now belive that im not up against an RPM limiter. I also tested the timming again and found that it seemed to be static set at about 29 degrees. It did not have the drop in advanced timming that was very prominent when the rpm was reduced to low speed like the other coil would. I think this made for a very rough idle. In my opinion an electronic ignition that retards the timming at idle will have a greatly improved idle quality and have lot less vibration.

I then installed the 18-8 prop with the stock valve to get the days baseline testing numbers. I recorded 7410 RPM

Next I installed the modified reed valve again and test ran. I recorded an RPM of 7410

I then installed the rubber reed valve again and test ran with a recorded RPM of 7410.

I now am wondering if in my first testing of the reed valve with the old ignition set up if my baseline reading was off somehow. I dont know how or why but i think it may have been as my readings today were all the same no matter which valve was used.

Again in my first testing of test #7 reed valves

stock valve RPM 7050
modified reed valve 7410
rubber reed valve 7410

Second testing of test #7 reed valves

stock valve RPM 7410
modified reed valve 7410
rubber reed valve 7410

I am now thinking there is some other area of the engine holding up flow and not the reed valve. Each modified reed valve offered less restriction to flow and had no effective increase in power. In my opinion this tells me that the reed valve even in stock form can flow way more than the rest of the set up will flow. I dont know weither from here I should try to improve the intake manifold or the exhaust system. I really think my muffler flows real well as a can cover one of the two outlet pipes with no change in power. output. Also changing the port timming may help to.

Test #8

With all this in mind i now decided to try and reduce the thickness of the bar in the middle of the exhaust port and improve flow to see what results I could get. I did not at this time chan.ge the port timming. so after spending some time with my die grinder and narrowing this bar i Cleaned it all up and reassembled and test ran.

Recorded an RPM of 7440. For a net gain of 30 rpm. Not much of a gain but its a step in the right direction.


Ok to recap all this info so far

Test #1 stock engine no performance mods. Using old 1 ring piston and newer engine with the twin intake runners per side. This should give a good baseline for a starting
reference point.

Recoded RPM 6300

Test #2 Again stock engine and no perfomance mods. Using two ring piston and newer engine.

Recorded RPM 6330 For a net gain of 30 RPM I actually expected it to be a little more than this.

Test #3 Stock engine no performance mods. 2 ring piston and the older and looks to be higher compression combustion chamber design. If you look at the old crank case and
look at the new one the new crankcase is just a modified version of the old crankcase ports. Therfore the ports should match the ports on the old cylinder just fine. Again the purpose of this test is to see if the higher compression help.

Recorded RPM 6570 For a net gain of 240 RPM!!!!!!!! At this point I was not convinced this rpm gain was due to the higher compression and could have been a result of better ports.

Test #4 Takeing the best combination of test 1-3 cylinder and piston combination. Removing the cylinder base gasket and try to increase the compression ratio some more. With the combination of piston and cylinder I have, when the gasket is removed it causes a piston to cylinder contact. I measured the height of the piston in relation to the top of the spark plug hole. If you take a dial caliper and extend it you can use it for a depth gauge at the end of it. If you hold it on the spark plug hole and rotate the crank when the piston comes to the top you will have your reading. Ok so my reading with the gasket was .610 Without the gasket it was .585 and agian there was contact. Ok so i measured the gasket and figured it moved the cylinder down .030 this means that i had a .005 interfereance fit. So i cut a gasket out of carboard stock that measured .010 . When I installed the cylinder with the gasket and remeasured it gave me a reading of .590 so I now have moved the cylinder down .020 and should have .005 clearance between the piston and the flange of the combustion chamber.

Recorded RPM 6,660 For a net gain of 90 RPM now I am convinced the rpm gain of test three is from the higher compression as is this RPM gain is also.

Test #5 This test takes the best results of test 1-4 and uses the crank from the old engine to see if there is a gain as it looks like this crank has more advance in it. Out of pure curiosity i took the ignition coil from the old engine and used it with the new flywheel. It would hardly run in this combination. Likewise on this test i use the old flywheel. In this configuration it runs great with the old coil but like crap with the new coil. On inspection of all these parts there are two manufatures of thses parts. The newer coil and flywheel is made by walbro. the older coil and flywheel is made by phelan. For some reason they are a matched set and cannot be inerchanged in any combination from new to old. The only way to change the flywheel is to change out cranks. Anyhow so the theory is that the older set up has an advanced timming. When the crank was changed out the seal was left on the outside nezt to the flywheel. I will try and put it on the outside of the inner bearing on test # 9 as this should reduce the crankcase volume.

Recorded RPM 6,670 For a net gain of 10 rpm. At this point i am not sure there is any advance in this setup from the other set up. there isnt enough rpm gain to

Test #6 I took the best results of test 1-5 and used a 12.8 mm venturi carburetor that ws purchased on ebay for 15bucks. After making the necassary modifications to the carburetor which pretty much just consisted of swaping stock parts from other carbs to make the configuration that worked best for my application I test ran the engine and had my biggest gain yet.

Recorded RPM 7170 thats a net gain of 500 RPM !!!!!!!!!!!!!!

first testing of test #7 reed valves

stock valve RPM 7050
modified reed valve 7410
rubber reed valve 7410

Second testing of test #7 reed valves

stock valve RPM 7410
modified reed valve 7410
rubber reed valve 7410

Test #8

With all this in mind i now decided to try and reduce the thickness of the bar in the middle of the exhaust port and improve flow to see what results I could get. I did not at this time chan.ge the port timming. so after spending some time with my die grinder and narrowing this bar i Cleaned it all up and reassembled and test ran.

Recorded an RPM of 7440. For a net gain of 30 rpm. Not much of a gain but its a step in the right direction.


In my opinion the best set up so far would seem to be

An older engine with the one ring piston and ring set up with the port and head configuration of the cylinder on the right. also using the older flywheel and ignition coil set up if your going to stay with the stock ignition as it seems to have some retarding in it at lower throttle and will help lessen the vibration at idle.



Taking out the one ring piston tossing it and purchasing this piston and ring set up with the two rings that already have the zero gap setup.

http://search.cartserver.com/search/...words=753-1207

Next lowering the cylinder to the base by taking out or reducing the thickness of the gasket according to what you engine measures out to be safe to do so.

Next installing a 11-12 mm venturi for what my test indicate to be the biggest increase in power gain.

I dont see any need yet to do anything with the reed valve but reducing the thickness of the cylinder exshaust port will give a very small power increase yeild. it all adds up I guess.

So far this should give about or around a 7410 rpm with what I consider to be the best of all the stock parts with no major machining modification.

From here I am not sure where to go. My test # 9 was gona be to modify the intake manifold and maybe bell mouth it at the reed valve end. I just dont see where this will help. the intake runner is already bigger than the venturi and the limiting factor as far as max flow will be the smallest orfice sive in the system which is the venturi. simply put no matter how much bigger i make the port i cannot stuff any more air through the venturi.

I can change port timming for my next test and may be the direction i go on this. other than flat out removing the middle bar i dont think there is anything else i can do for exshaust flow improvement.

I am wondering if reducing the crankcase volume will improve flow as a smaller volume in the crank case volume might help in aidiing air induction. With less volume there will be less of a pressure change swing in the crank case. There will be less time or degrees of crank rotation needed to reduce the crankcase pressure to start drawing in fuel and air. The max volume will be goverend by the bore and stroke of the engine and atmosheric pressure but crankcase volume, and airflow restriction will all affect this function.

I think My next test will be test #9 change the port timming to 150 degrees and record RPM. Next for test # 10 try and reduce crankcase volume. I need ideas on this as there isnt much room to do this modification. and for the last test I will do the electronic ignition to see what this yeilds.

As it stands now what I have is more than adequate for what plane I am gona put it on. I would still like to see this testing out to the end.

In a way I would very much like to get my hands on one of the smaller crankcase engines witht the recessed reed valve. I have my suspisions that this could be the best canidate for the best performance despite what is written here but till I get one I wont know for sure lol.