mechanic

[Bullfrog]

The extra holes in the piston below the rectangular one at the top of the piston are required to achieve proper operation when a reed valve is a part of the induction system. However, having those holes in the piston on a "stock" Combat Wombat with piston port induction just won't work.

With piston port induction, the piston MUST close off the intake port to prevent "pumping" the fuel/air charge in the crankcase back out through the carburetor. The Victory port and the small upper window in the piston allows some of the fuel air charge to flow across the bottom of the piston crown and by the wrist pin bearing, through the little window and the victory port up into the combustion area, but the main intake port is closed off to work properly in a piston port engine.

So . . . the little window works on a piston port Combat Wombat . . . but the larger, lower holes simply don't work on a piston port engine.

Ed

[bchappy]

Drawing of what I called a 5 ported Hodaka. This was a piston port before I started playing with reeds. Surely one of you guys has tried this in the past. Of course it would not be recommended today as the ports are all so wide there would not be room for the slots.

The Combat Wombat drawing beside it is just for comparison.

[hodakamax]

Interesting Bill, I never tried that one!

Max

[taber hodaka]

I just looked at one I had done years ago. Good drawing Bill.---------Clarence

[dcooke007]

Vic,
Ed and Bill answered your question about the piston. Bill's drawing shows how a combat wombat piston should look and I was trying to duplicate it, in part, on a non ported two ring piston. The factory combat wombat piston has a single ring.

Bill, thanks for the drawing...very well done. I would assume the top of the groove would be the same height as the transfer ports?

I had some time after church today to get some work done. The engine is ready to pressure test. I have been using a Lexan adapter plate to attach my air pump to the intake but it has proved less than satisfactory. The Lexan material is not rigid enough for a good seal and it does not get along with shop chemicals.....carb cleaner. I had to use rtv sealant between the plate and manifold to get a proper seal.

So I chose some quarter inch aluminum flat stock to make a new and improved adaptor plate. I also wanted to make one adapter that would work on multiple engines. I made the adaptor wide enough in both directions so I could use it for the combat wombat and all of the reed valve engines. The stud spacing is different between the two engine types. By rotating the adapter 90 degress I can use it on the mentioned engine types. Added a sheet of rubber as a gasket and ready for pressure testing tomorrow.

Danny

[dirty_rat]

Those extra ports ground into the side of the cylinder were also done by E.C. Birt on many of his engines. There was a three part article on E.C. Birt in Dirt Bike Magazine back in the mid 1970's and in the article he had a diagram of a Hodaka cylinder ported like that. His Zundapps were also ported with these ports.

Dirty Rat

[Zyx]

If nothing else, the finger ports shown in the left hand side of the drawing would cause a flow of fuel oil mix from crankcase to combustion chamber through the wrist pin area rather than around the skirt. This may boost transfer, I assume it does, but without a bench to test it, only a seat of the pants dyno could tell. But the increased flow upward through the wrist pin area increases both piston crown cooling and lubrication, both good.

[rtboone]

Greetings

I had a 1968 HD Rapido 125 that had those finger ports which made a noticeable gain in performance. That mod along with an expansion chamber and milled head made it the fastest 125 at our local TT/Scrambles track for a short period of time. I also had an ace 100 at the same time that had the same mods which improved it's performance quite a bit. Switching between the two bikes sometimes was interesting as the Rapido shifted on the right(wrong) side. Once I ended up in the weeds when I planned to brake late and hard but only shifted up instead. oops. Forgot that I was on the Harley.

Tom

[hodakamax]

Hey Tom, I can relate to your story about having two bikes with one having of the shifter/brake on the wrong side. I guess it's what you started with that decided what was right or wrong. I had a 350cc BSA and along came the Hodakas. Finally the BSA had to go for many reasons but that control reversal was one. I decided that the Hodaka way was the "right" way.

Max

[dcooke007]

I was able to complete the engine today. Still have some work to complete on the bike before starting.

Danny

[dcooke007]

This is how the air filter looked when removed. You can see the extra holes someone drilled for extra air flow. There is adhesive residue on the inside so I am sure there was some air filter foam in there at some point. Replaced the air filter, trimmed the new filter and glued a piece of air filter foam to the new filter.

I don't have any pics, but the engine is installed in the frame. The bike is not ready to ride yet but I did start the engine, allow it to come up to operating temperature, shut down and allow to cool. Restarted and brought back up to operating temp again. Throttle response was a little sluggish and seemed like a good time to check cranking compression. Cranking compression with a warm engine was 145 psi and lower than desired. This was with a standard .020 inch thick Hodaka head gasket. At some time in this engines past the cylinder head was drilled for a compression release. An E length spark plug with the ground electrode removed was used for a filler. Even with the filler plug there was some additional head volume that was not filled. I installed a .010 copper head gasket I bought from Paul and the cranking compression increased to 155psi and is more in line with what I was shooting for. Throttle response improved markedly but still not quite as it should be.

Removed the shift cover and checked ignition timing with a timing light. Timing was a little retarded and I increased point gap .002 inch. That did the trick and timing marks aligned properly with a slight increase in throttle response.

Proper carb adjustments can not be made before other base line adjustments are completed such as timing and cranking compression. I had roughly adjusted the carb air screw when first started and now it was time for final adjustment of the idle mixture. With the stock #25 Combat Wombat pilot jet, best/ highest idle was with the air screw turned out 1\4 turn from seated. Throttle response was ok but I thought it could be better. Removed the carb and installed a #30 pilot jet and best idle was achieved with air screw turned out a little more than 1/2 turn. Throttle response was much better but I would like more air flow to help atomize the fuel in the idle circuit. So at this point ordered a #35 pilot jet and that should put me about a turn out....I can live with that.

Even though I have not been able to ride the bike yet, all these little things notably improved throttle response. So...pay attention to the little things and reap the rewards.

Danny

[viclioce]

I didn't send you any pilot jets? Thought I did. Maybe not. But next time ask me! ; D Victor

[dcooke007]

Ok, thanks Vic.

Danny

[junker2k]

Bill, on your 5 port diagram you have the ports going around the intake port, is there a reason for this? I have seen them go straight up the cylinder or at a slight angle. I have thought about doing this on my trials bike for years but the bike pulls very hard now and I don't want to take a chance on messing that up.
Jack K

[bchappy]

Jack K, The "5 port design" goes around the intake port because it was run as a "piston Port" and not a reed. When the piston comes down and makes pressure in the crankshaft area the fuel mixture is forced through the holes in the piston and up the slots into the combustion area. Ports straight up from the intake require a reed to stop the fuel mixture from backing up through the carb as the piston comes down and makes pressure in the crankshaft area. The victory ports on the Combat Wombat do not come out into the intake port but use the same technique to force mixture through the hole in the piston up to the combustion area. Also the Combat Wombat used a single ring giving room for the hole where we were using the 5 ports before the single ring came out.
Clear as mud??

[racerclam]

Jack here is what I do to the iron wompat piston port cylinder , I have done several of these Including for Jay Lael for his trials bike and Leo Stinnett as well as Bill Cook it make for a real Responsive and torquey engine . Having these boost ports at the rear of the cylinder also allows for redirecting the rear of the transfer ports toward the center of the cylinder since the boos port takes care of the scavenging allowing the transfer ports to do thier job better.

Rich

[racerclam]

here it is I always have trouble loading pictures

Rich

[viclioce]

Is there a difference in the photos or did it just post twice? ; D Victor

[racerclam]

just twice since it looked like the first one didnt make it

Rich

[dcooke007]

Ok finished going through the wheels, replaced front bearings, rear brake shoes, cables, chain sprockets and just going over the bike very thoroughly. Ended up using the #30 pilot jet with the air screw out one turn and the jet needle clip in the second groove from the top. With the stock 190 main jet the engine ran very well and easily lifts the front wheel in second gear. I still have to determine if a richer main jet is required. That will be a task for later in the week.

I did experience one difficulty that I did not expect. I noticed some minor jumping in and out of gear in second and third. Only happened when shifting up thru the gears and never when down shifting. Considered adjusting the shifter arm in but decided to re-examine the shifter assembly. What I found was excess clearance / wear between the shifter case and the shifter arm pin on one end. I could pull and push on the shifter arm and see the pin move about .005 to .010 inch on the outer end. The shifter case was in great shape other wise and finding good usable shifter cases is not cheap or easy. I guess I could have made an over size pin and reamed the shifter case and arm but decided a simpler solution existed with the small amount of wear noted.

As you can see from the picture I drilled and tapped a hole that intersects the shifter arm pin bore.....4 / 40 thread size. The set screw secures the pin against the opposite side of the bore and eliminates the small amount of play. Re- adjusted the shifter cover and no more jumping out of gear.
Danny

[rough rider]

Nice job on correcting the shifter Danny.

[hodakamax]

Clever Danny, Thinking outside the box, keep up the good work.

Maxie

[Bullfrog]

Nifty and clever solution!

Ed

[dcooke007]

Finished Dan's bike today. New fenders, number plates and final jetting completed. Ended up with a 210 main jet with temps in the 50's. Bike runs really well even with the modified pipe and has a lot of low end torque for a piston port engine. Final gearing 14 front sprocket and 58 rear. The 19 tooth wombat 5th gear works well also for a little more top end on any long straights that might be encountered.

Danny

[hodakamax]

Good job Danny, I do like your reports and your methods. Keep up the good work.

Max