The Hodaka Club Forum

Hey Greg,
Welcome back! We missed you!

I can't help but giggle that the "high-jack" of this thread was getting "high-jacked", but Arizona brought it all back to the original high jack and then to the original thread.

. . . and now I'm going to return to the original high jack. I may have been a bit harsh regarding the rider of the machine Danny is working on . . . after all, I have a piston-port Combat Wombat in the garage which has been sitting since it seized on a looooongggg uphill on a vintage MX track. The machine had been campaigned for a couple of seasons without problems. I always use straight (non ethanol) gas. Cylinder head and compression are stock - still running points. I do keep a pretty short leash on jetting - adjusting main jet (and perhaps needle clip position) as conditions require in the stock 28mm flange mount carb (with half throttle vanes). I have done nothing to suss out the cause of the seizure since it happened. I'm kind of stumped as to what went wrong. I had raced this track in previous seasons with essentially the same climb without problem.

Number ONE on my diagnostic list is to do a pressure test on the engine . . . since it had been in operation at least 2 seasons (possibly more) since the last rebuild. If a seal suddenly failed (or something else) and I had a large, sudden air leak - then I'll know what to fix. After the pressure test, the top end comes off to examine the details of what happened there. I'll report as the project actually gets some time. Gotta get this scoot running for Hodaka Days!!!

NOTE: There is a nagging thought that I might have run out of Yamalube R (it is very hard to source locally) and Silkolene . . . and may not have been running the best oil. Uh-oh . . . mystery (cuz there is NO WAY to be sure what oil was being used).

Ed

Hey Greg, welcome back! Thanks for your input. Even the Hodaka published bulletin "How to Reed Valve" recommends this modification to the skirt. I totally agree on your oil prognosis also. As long as the oil molecules are in the system all is well. Migration only seems to be flushing accumulated oil from the crankcase that has built up from slow velocity and initial separation of the oil from rapid evaporation aided by low pressure and atomization through the venturi. It clears rapidly as revs increase and does not appear to be a lubricating factor at high engine speed. Not too much time spent idling so who cares?

Max

Ok, every thing will be going on hold for a short while. New Wiseco piston received and measured. Cylinder bore to piston clearance measured .0018 inch. Too little clearance for my taste at this point. I will be sending this back to Powerseal to resize the nicasil plating. That is if I cant find some one equipped locally.

Of note, the seized piston measured the same as the new one. The seize pattern is not what I would expect to see with too little clearance, but would explain the excellent condition of the piston crown. So maybe seize pattern is not as it would appear.

Danny

I would agree that 0.0018" is tight for a Wiseco, but what does Wiseco say it should be? They usually print recommended clearance on the box.

Aluminum grows when hot. It grows most where hottest, commonly at or around the crown to exhaust port area. This is where it seized. The scuff on the back side is probably just a reflection of the seize on the front. If the clearance was too small, it would not appreciate excess heat from a long uphill full throttle pull. Combined with an oil that flashes at around 266F, the tight fit could be what caused the problem here.

Another thought: does this engine as now modified rev higher than the original design? If it does, the point based timing mY be unable to keep up with the high rpm. Erratic spark at high rpm might cause a timing shift that could raise heat. The later Hodaka race bikes used CDI, which better tolerates high rpm. I don't know that this is an issue here, but of your client likes to hit 10,000 rpm and stay there, you might suggest installing CDI.

Arizona,
Label says 0.0019(0.048mm). I had requested .0005 over specified. Since my measurement and the label only differ by.0001 inch, I would say they just followed the label and missed my request for increased clearance. With anticipated use and history going for at least .0025 to .003. inch.

Exhaust port height is standard although the engine should be able to breath more and maybe a few more rpms. Not sure we are running above the ability of the point ignition. Stock exhaust also.

Danny

Doug
Very cool bikes. I did some research on the Suzuki. That is some impressive work. Thanks for posting.
Mike

In their installation instructions, Wiseco advises that additional piston clearance may be needed for modified engines. They don't say how much. Whether the engine in question would be considered modified is up to you, but I think the reed induction might qualify, as well as perhaps the swapped head.

My question would be whether the engine is being driven beyond its intended range. Seems as if it might be, and it has seized twice now. Your request for an additional 0.0005" sounds like a good idea to me. Stock Super Combat clearance was supposed to be 0.003" and stock Wombat at 0.004". I know this one is neither of those machines, and you are not using stock Pistons, but I wouldn't be worried about installing the piston with 0.003" clearance considering its use.

Repsol is supposed to be really good oil. Perhaps it is. But Wiseco pistons and top end overhauls aren't cheap. I would suggest something known to work in air cooled vintage machines, and would suggest a richer oil mix than 32:1, especially until well broken in. Maybe in the 24:1 range, and rejet accordingly.

BTW I checked the Wiseco website. They don't offer current production pistons for Hodakas or for air cooled dirt bikes of the era, so I assume these are a custom run to Hodaka spec. If so, I have to wonder how they came up with the clearance recommendation. It is certainly tighter than stock specs, and might be based on an assumption that we don't know about. Could it be that the Wiseco spec is based on water cooled machines of modern make?

Greg!!! It's nice to see you back! I hope it's for a while! ; D Victor

All of us are here only for a while...

I talked to Powerseal yesterday and they were more than happy to resize the cylinder but I was not looking forward to the wait. Went through some of my parts and found a Strictly Hodaka / Wiseco .040 combat wombat non reed valve piston. Measured it up and using it gave me a cylinder to piston clearance of .0034 inch.....close enough. I had to cut in the reed valve slots so made a template from the other piston and roughed out in my drill press. Completed cutting in the ports with my grinder. Assembled the engine, let it come up to temp and rode at modest throttle openings. Shut down and while it cooled I removed the carb and replaced the jet needle...went from a o-6 to a o-8. Kept the 5dp7 needle in the middle position. Operated the bike and the larger needle jet made the mid range a lot stronger. Made a short burst at wide open throttle and noticed it was running rich on top. Replaced the 310 main jet going down one jet size at a time and ended up at 290 and still slightly rich on top. Started raising the jet needle to see where it gets to rich in the mid-range. Finally started to blubber just above mid throttle with jet needle clip in the lowest / richest position. Raised the jet needle clip to the 4th position and changed the main jet to a 280. Bike pulled hard from just above idle all the way to wide open throttle.Still a good bit of testing to do. I did not want to make any extended wide open runs until a little more operating time on the new piston and ring. Temps today were in the 70's.

I was concerned the original needle jet / jet needle max flow capacity might be less than the larger main jet capacity. And that is the reason I chose to go larger with the needle jet....engine responded very well in the mid range to that change. Jury still out as to if it limited main jet flow.

Exhaust flow through the spark arrestor seemed to be sufficient. Again more testing needs to be done.

Danny

You can always pull the silencer core to see if it makes any difference, other than noise, but it also affects jetting, so if pulling the core makes no real difference, it isn't the silencer.

Danny since you said it was too rich on top with a 310 main jet then clearly the needle jet is not to small it at least flows what your main jet is and probably able to flow more .

Rich

Just looking at a needle jet yesterday for a 24 Mikuni ( just happened to be working on one is all), my impression was that the "jet" part of the needle jet was huge compared to your average main jet, which makes sense as the needle part is generally pretty big. In theory, a needle jet could be the restrictive part of the formula but it would have to be a carb with a really impressive main jet.

Ah, but the needle always fills part of the hole in needle jet . . . and sometimes it does work out that the combination of the two is a "choke point" on delivering the fuel. It is often noticed when a tuner can't find a "rich" main jet no matter how large a jet gets installed.

In this situation Danny has shown that the O-8 needle jet/needle combo can indeed flow enough (since it was rich at WOT with the 310) . . . but the O-6 he removed could have been too small. (just an unconfirmed possibility at this point).

Ed

Ah, but the needle does not block in any way passage through the needle jet at full throttle. The drilled body of the jet is quite large at the top of the jet. Even though the needle doesn't fully retract, it fully clears the metering part of the jet on full throttle. At full throttle you could run with no needle at all. If the needle jet was a restriction compared to the main, you could remove the main jet and see no change in jetting. But that's not how they work.

Arizona,

I appreciate that you may not have personally experienced the situation under discussion . . . and that you have done a detailed examination of a needle jet and have reasoned your way through how things must be . . . but none of that changes the fact that the second sentence in my last post is true. The needle and needle jet can be a choke point which does not allow enough fuel flow at WOT.

Ed

I have recently experienced exactly that. Needle Jet and Needle too restrictive. Stock Combat Wombat with 28mm carburetor, 169 O4 Needle Jet and 5D6 Needle. Stock Main Jet is 190 and I could put a 260 in there and not be too rich. I have it all sorted out now, but it gave me fits figuring it all out!

Dale. To what size needle and/or needle jet did you change to remedy this issue? ; D Victor

Victor,
Well... the remedy was actually burning out a plugged exhaust pipe. And believe me, there was no visible signs of it being plugged. I was chasing an extremely rich condition and I inspected the pipe early on and was satisfied that it was fine. I spent much time working on the carburetor, ignition, checking compression, etc... and eventually burnt the pipe just because and sure enough, it was the total culprit. For 1500 ft elevation, I am currently running a 35 pilot, 180 main, O2 Nozzle and 5EJ13 on the 3rd notch. Standard Combat Wombat components would be 25 pilot, 190 main O4 Nozzle and the 5EJ13 on the 3rd notch, however, my 28mm carb is not OEM...
Dale

Just goes to show how much a clogged pipe can effect the air/fuel flow! Glad you got it all figured out!

ddvorak wrote: For 1500 ft elevation, I am currently running a 35 pilot, 180 main, O2 Nozzle and 5EJ13 on the 3rd notch. Standard Combat Wombat components would be 25 pilot, 190 main O4 Nozzle and the 5EJ13 on the 3rd notch, however, my 28mm carb is not OEM...
Dale

Thanks for this, Dale! This really helped me dial in my jetting on my (94) Wombat.

I purchased a 28mm carb last year in the HD15 auction and I finally installed it. I was having trouble dialing it in with the OEM pipe. Doug encouraged me to put on a better pipe so I found a used, (Yamaha?) pipe at the boneyard and modified it to fit. Made a huge difference but I found that I was exceptionally lean. Your info came a the right time to give me a baseline to consider. Starting with a 30/140, I ended up with 40/170 for 5400 ft. I am fairly confident that the 40 Pilot is right or very close, it idles very smooth, steady, and has a nice fat sound to it, compared to a more "tinny" sound before. Still going to try a larger Main until I find the one that's too rich. But overall I think I finally got it. Thanks!

Just curious, is the O2 larger or smaller than the O4?

Roger

Roger,
That is awesome. Glad that my learning the hard way has helped someone.

The O2 nozzle is one step smaller than the O4. The O2 was required to get the 5EJ13 to the 3rd notch.

I am clearly too rich at WOT with a 190 main jet with either the O2 or the O4 nozzle and also with the pilot at either 25 or 35, so settled on the 180 main jet.

Regarding the 35 pilot, the idle is perfect at 1 1/2 turns out. It starts easily and the biggest factor is the sound that the motor makes on a hard deceleration. With the stock 25 it has the "tinny" sound. I tested 27.5, 30 and 32.5 and found that it needs the 35 to sound and run great.
Dale

Did a little more testing on the bike today. Temps are a little higher today and seemed like jetting was a little rich on top again. Dropped main down from a 280 to 270 and felt a little better.....but not quite right. Time to investigate the exhaust. Removed the spark arrestor, test drove and big improvement. More power in the mid-range and reved out nicely....but very loud. So modifications are in order.

Spark arrestor assembly. There is a triangular shaped divider in the end of the arrestor and only two openings into the first chamber. The next chamber only has one opening into the next section of the arrestor. I have some ideas how to address this and will post when completed.
Danny

dcooke007 wrote:Did a little more testing on the bike today. Temps are a little higher today and seemed like jetting was a little rich on top again. Dropped main down from a 280 to 270 and felt a little better.....but not quite right. Time to investigate the exhaust. Removed the spark arrestor, test drove and big improvement. More power in the mid-range and reved out nicely....but very loud. So modifications are in order.

Spark arrestor assembly.
DSCN1985[1].JPG
There is a triangular shaped divider in the end of the arrestor and only two openings into the first chamber.
DSCN1986[1].JPG
The next chamber only has one opening into the next section of the arrestor.
DSCN1987[1].JPG

I have some ideas how to address this and will post when completed.
Danny

Danny.. that arrestor looks pretty restrictive IMO... Prob not helping power or heat..

Can't wait to see how it runs once you get some flow thru it..

Here is another tech article I wrote regarding pipe pressures.. Maybe worth a look?
http://www.2strokeheads.com/techpipe.htm