Need Suggestions on Carb Issue

[KitV]

First, let me say that I have had 4 piston seizures on my '73 Wombat. After much hand-wringing & considerable guidance from Ed Chesnut, maybe (fingers crossed!) I have this carb issue sorted out. However, last week I took the bike out for some final carb adjustments. Bike seemed to run fine on long, open stretches of back roads. When I got into more stop-and-go traffic in town, the bike started to buck & lurch at all throttle positions greater 1/4 throttle. The problem wouldn't stop until I installed a new plug. The old plug was clearly wet with fuel & the electrodes quite blackened.

I am reluctant to install a leaner main jet or go to a leaner needle setting since both of those might have been associated with prior piston seizures. So I am open to your thoughts. Maybe a hotter plug?

Thank you for any help you can offer.

Kit

[Dale]

Tell us more.
Has it done this more than once?
What plug are you running?
What fuel, premix oil and ratio?
What jetting do you have currently?

[KitV]

After replacing the piston & ring and honing the cylinder (by Dave Rozier), my conversations with Ed Chesnut resulted in the following specs:

Main Jet: 230 (large; but erring on the side of caution)
Pilot Jet: 40
Throttle Slide: 2.0
Jet Needle: 5F21
Needle Jet: 04 190
Float Valve Seat: 2.8
Air Jet: 2.0
Needle Clip Position: 4th notch

After a couple of hours of break-in, I changed the main jet to a 220 because of "heavy" running at greater than 1/2 throttle. Also, I moved the needle clip to the 3rd notch because of similar "heavy" running from 1/4 to 1/2 throttle. These changes seemed to smooth things out.

Regarding your other questions:

Spark Plug: NGK B8HS
Fuel: Clear gas (i.e. no ethanol)
Premix Oil: YamaLube (32:1)
As far as this problem occurring before: Yes, prior to changing the main jet to #220 & leaning out the needle one notch, I experienced the same problem. I assumed that the subsequent jetting changes would resolve that. But...apparently not.

Also, elevation here in Medford, OR is about 1,500 ft.

Thank you for taking the time with this. The past string of piston seizures has made me reluctant to lean out the air/fuel mixture any more. So it's time to throw myself at the mercy of the Hodaka experts.

Kit

[Dale]

Kit,
I went back and found your previous posts regarding this motor. I had forgotten about them... So you have a Wombat but with a ported and reeded model 95 Combat Wombat motor? If correct, that explains the large jetting numbers.

I also read where you had replaced the condenser to resolve an issue with the motor quitting when hot. That issue seems to be resolved, correct?

Does the bike run well at all throttle positions? How about at idle? Does it load up at idle? And what is your air screw set at for your best idle?

The good news is that it appears that you have a rich condition which is always better than lean. Given the past seizures, I get why you are nervous to reduce jetting. Sometimes it just takes a little loading up at idle to foul a plug. I have several Road Toads that are rich on the pilot jets and I never leave the throttle unattended at idle, frequently blipping the throttle. Could this be similar to your issue?
Dale

[KitV]

Kit,
I went back and found your previous posts regarding this motor. I had forgotten about them... So you have a Wombat but with a ported and reeded model 95 Combat Wombat motor? If correct, that explains the large jetting numbers. I also read where you had replaced the condenser to resolve an issue with the motor quitting when hot. That issue seems to be resolved, correct? .

THAT IS CORRECT

Does the bike run well at all throttle positions? How about at idle? Does it load up at idle? And what is your air screw set at for your best idle?

THE BIKE SEEMS TO RUN WELL AT ALL THROTTLE POSITIONS ON OPEN STRETCHES. AT IDLE THE BIKE RUNS SMOOTHLY. UNLESS IT IS COINCIDENTAL, IT SEEMS THAT DURING STOP-AND-GO RIDING THAT THE ISSUE OF "LURCHING" OCCURS. THE AIR SCREW IS SET AT 1 1/2 TURNS OUT.

The good news is that it appears that you have a rich condition which is always better than lean. Given the past seizures, I get why you are nervous to reduce jetting. Sometimes it just takes a little loading up at idle to foul a plug. I have several Road Toads that are rich on the pilot jets and I never leave the throttle unattended at idle, frequently blipping the throttle. Could this be similar to your issue?
Dale

I DON'T KNOW, DALE. I PROBABLY NEED TO PUT MORE "SEAT TIME" ON THE BIKE TO GET A BETTER HANDLE ON WHEN THE SYMPTOMS OCCUR. BUT YOUR THOUGHT ABOUT LOADING UP AT IDLE ENTERED MY MIND; HENCE, MY CONSIDERATION OF A HOTTER PLUG. I WILL BE OUT OF STATE FOR ABOUT 3 WEEKS. WHEN I RETURN, I WILL GET ON THE BIKE AGAIN.

THANKS, DALE!

[Al Harpster]

Other than verifying the condition of the needle jet and jet needle for wear.... I know it's a guess.... I'd go hotter on the plug WAY before dropping main jet OR pilot jet sizes.

You can easily buy plugs in a couple steps higher heat range. There's not an ocean of difference between them. Just like going from a 230 to a 220 main or moving one clip position on the needle.

Try the next up heat range plugs.
You're on top of it enough that you have little concern about burning a joke in a piston. You'll check to be sure they are 'right'

If she's not 4 stroking like crazy and EVERYTHING else looks ok.... Swap to different plugs.

Yes, I've locked my piston & cylinder....a few times. Time to watch out is when it's running clean and lean.

Go Rich Young Man, sneak up on 'running good'.

My two cents. About worth what it cost.

[Bullfrog]

Kit - sorry I didn't get right after your email -- but it is good that the discussion is up here on the forum.

As I recall the last notable change was moving from a 5D5 needle (which was the likely cause of all the previous difficulties - still kickin' myself for not asking/noticing that wayeeeee back when) to the 5F21. The 5F21 is a considerably richer needle than the 5D5. I'm sure that your "around town, stop and go" puttering would be perfectly safe with the needle in the 2nd notch.

Is the Pilot Air Screw set at 1.5 turns out because that's what a specification said it should be, or because you completed the pilot jet/pilot air screw tuning procedure and that is where it ended up? This is important for a couple of reasons:
1) The pilot air screw tuning procedure "finds" the proper fuel air mix when the engine is at zero throttle (and up to about 1/8 throttle). If things tune in properly with the pilot air screw at 1.5 turns out, then you know the pilot jet size is correct. However, if you simply set the pilot air screw at 1.5 turns out . . . you haven't "searched out/found/tuned" to confirm that the pilot jet selection is correct.
2) Since the pilot jet delivers fuel at ALL throttle settings - you need to get the pilot jet right BEFORE tuning other items in the carb.

If you don't have the pilot jet/pilot air screw tuning procedures, let us know. We can come up with the directions.

The symptoms you described for your around town riding (wet and black) certainly suggests richness at low to medium throttle settings . . . and changing spark plug heat range won't correct jetting issues. Note: Harry Taylor did recommend using a B8HS in my Combat Wombat engine - which is one heat range hotter than "stock", and seems to be perfectly usable. What heat range of plug are you using?
Ed

[Al Harpster]

Believe he said he's using the B8HS now.

[KitV]

Yes, the pilot air screw is set at 1.5 turns out. As a recall, I followed the procedure in the Hodaka manual as follows:

Turn idle speed screw in until engine begins to run faster. Then back the screw out slowly until the engine runs slower & just starts to falter. Now turn pilot air screw in or out until engine runs smoothly. (Turning in: richens the mixture; turning out: leans). Repeat this sequence with the idle speed screw & the pilot air screw until the desired idle speed is achieved.

However, I might not have been as diligent as possible in doing so. I need to recheck this for sure. Then I will verify how the bike runs with the current carb settings & adjust as necessary. I would like to do this in the next couple of days since I am leaving for Alaska for 3 weeks. Either way, I will report back. Thank you, everyone!!

Yes, I am using an NGK B8HS.

Kit

[KitV]

I spent a bit o' time today trying to set the pilot air screw as per the Hodaka manual. The procedure in the manual specifies adjusting the idle screw & the pilot air screw until the "desired idle speed is achieved". Does Hodaka recommend a specific idle speed? Or is the desired idle speed whatever seems right to me? Thanks!

Kit

[Bullfrog]

The specific idle speed you end up with is indeed up to you.

Perhaps the following comments will help on this tuning procedure. Actually, the "tuning" of the pilot air screw is searching for the setting which results in the fastest idle speed at any given setting of the idle speed adjustment screw. The principle involved is - we are leaving the idle speed adjustment screw alone . . . and we are varying the fuel/air mix by adjusting the pilot air screw. The engine will idle "best" (fastest) when the mixture is just right. It will idle more slowly on the "rich" AND the "lean" side of that "best" adjustment.

If you find that "best" idle (fastest) when the pilot air screw is some where between one turn out and two turns out - then the pilot jet is the right size. If you find that "best" (fastest) idle speed with pilot air screw less than one turn out OR more than two turns out - then the pilot jet is the wrong size.

If things are "best" at less than one turn out, the existing pilot jet is too small. If things are "best" at more than two turns out, the existing pilot jet is too large.
Ed
PS: The book procedure keeps having you reduce the idle speed with idle speed adjustment screw and re-adjusting the pilot air screw until you are satisfied, to assure that you don't do the test with too high an idle speed which comes from the slide being lifted far enough "up" that we are getting into the slide/throttle valve cut-away range of tuning in the Mikuni.

[KitV]

Ed:

Well-said! So, first I can set my desired idle speed (with the idle screw). And then turn the pilot air screw both left & right until I identify the fasting idle setting. Correct?

Thank you.

Kit

[matt glascock]

Great explanation Captain. That really clarifies a procedure I've also wondered about. Thank you!

[bobwhitman]

Yes, way clearer, suitable for theRes Captain!
Bob

[Bullfrog]

Kit,

Yup, you've got it. Just be sure to allow a bit of time for engine speed to stabilize after each "try" on a new pilot air screw setting.

Ed

[KitV]

I just finished adjusting the pilot air screw using El Capitan's technique. Now I must admit that I found this to be somewhat of an exercise in subtlety. Within a range of about 1 1/2 turns, I found it difficult to discern any change in engine speed. Having a chainsaw tachometer designed for 2-stroke, 1 cylinder engines, I connected it to the plug's wire. Although the tach readout bounced around within a 1,000 rpm range at any pilot air screw setting (I suspect that this might be a characteristic of 2-stroke engines?), it did give me a bit more confidence in my ability to detect subtle changes in engine speed.

Bottom line: the fastest idle speed seems to be at 3 1/4 to 3 1/2 complete turns out (i.e. "flats"). So I assume that this indicates that I need a smaller pilot air screw. Given the carb specs that I listed above, does a smaller air screw seem reasonable? And if so, what size is recommended?

As usual.....thank you!

Kit

[Bullfrog]

Awwwlriiiight.

Sometimes language gets in the way of communicating. To me a "flat" would be half a turn with a flat blade screw driver (which matches with pilot air screw) or 1/6 of a turn on a hex-head bolt (which has nothing to do with this discussion). Ennneywayeeeee, using that particular word has me wondering - did you count a "turn" out as one complete revolution (360 degrees) . . . or as I define a "flat" (1/2 revolution/180 degrees)? NOTE: All of my communications on this topic intend "one turn" to be equal to one revolution(360 degrees).

While it is fiddly to hear/feel (and even measure with a tach) the difference in engine operation with 1/2 turn changes, I'd have to guess there is a truly noticeable difference between a setting of 1.5 turns out and 3.0 or 3.5 turns out. Further, if 3.0 or 3.5 turns out IS noticeably better than 1.5 turns out . . . then, yes, we have a strong indication that your 40 pilot jet is too rich.

While others might opine to give you a specific size of pilot jet to try, I am only comfortable in recommending that you next try the next normal "1 step" leaner pilot jet - which would be a 35. Then repeat the adjustment to see where the pilot air screw ends up to achieve the best (fastest/cleanest) idle. This honors the two-stroke tuning tradition - start from a known "rich" condition, then "sneak" leaner till richness just barely disappears.

Ed

[KitV]

Thanks, Ed. We are on the same page: to me, a "turn" is a 360-degree revolution & a "flat" is 180 degrees. So, looks like I will try a #35 pilot jet. However, I am leaving for Alaska tomorrow so further work will likely be delayed until mid-September.

[Bullfrog]

Got your weighted boots so the mosquitos can't fly away with you? Have a good trip!
Ed

[KitV]

Greetings, All

I just returned from several weeks in Alaska. I had a chance to spend some time at the various locales that were familiar when I lived there. Had some incredible fishing & spent some time cavorting with the brown bears at the fishing holes.

But back to the business at hand. As I was preparing to replace the #40 pilot jet with a #35 as recommended above, I discovered that a #45 jet was installed much to my surprise. Try as I might, I couldn't find any explanation other than my stupidity.

So, I installed a #40 & rechecked the pilot air screw for the fastest idle. This time the fastest idle speed was at about 2 1/2 turns out (at least as best by ears could detect). As I understand, this is a bit out of the ideal 1-to-2 turns out range.

Being "gun shy" of yet another piston seizure, before I proceed further, I have a question. Rather than installing a #35 pilot jet, should I test the bike with the #40. But this time lower the clip on the 5F21 needle from the 3rd to the 2nd position?

Thank you!

Kit

[Bullfrog]

Generally speaking, no you shouldn't do that. I recognize you are "gun shy" about going leaner, but leaning out a carb adjustment which primarily effects mixture between 1/2 and full throttle to try to "fix" a carb adjustment which is "rich" at 0 to 1/4 throttle isn't a good plan.

How 'bout this? Leave the needle clip setting alone, put a couple of extra plugs in your pocket and go riding with the #40 pilot jet. Get comfortable that the pilot jet is indeed rich by hearing, feeling and fouling plugs . . . and then try the 35 pilot jet.
Ed

[KitV]

Sounds reasonable. Besides, do you really think that a guy who installed the wrong pilot jet is gonna argue with a guy who has more experience with tuning Hodakas than most people on the planet!!!! Duh!

Kit

[KitV]

Well, I just got home from running the bike with the #40 pilot jet. Acceleration seemed "heavy" from 1/4/ to 1/2 throttle. From 1/2 to full throttle, acceleration was better, but seemed to slightly surge. Within a couple of miles, the engine simply bogged down at all throttle settings & died. The plug was wet & black.

As per Ed's advice, I then installed a #35 pilot jet. HOWEVER... I forgot to reset the pilot air screw! (It might have had something to do with the fact that it started raining & I was wet). Nevertheless, I ran the bike. The engine exhibited the same symptoms as with the #40 pilot jet; but a bit less so. However, the engine did not die this time. But the plug will wet & black once again.

For the sake of curiosity (since I was already dripping wet), I lowered the needle from the 3rd notch to the 2nd (i.e. leaner). From 1/4 to 1/2 throttle, the bike ran better. But the engine still surged at a STEADY 1/2 throttle. From 1/2 throttle to full throttle, the bike seemed to ran OK. But, again, the plug was wet & black.

So, since I failed to reset the pilot air screw after installing the #35 pilot jet, I am not sure how valid these results are. I will reset the pilot air screw & run the bike again. Results to follow.

[KitV]

OK, I might be able to wrap up this thread (fingers crossed). I reinstalled the #40 pilot jet & reset the pilot air screw which seemed "best" at 1 1/2 turns out. So far, so good. With the needle clip on the 3rd notch, I ran the bike. After about 20 minutes of running at various throttle settings, the bike seemed to run fine. No surging, no "heavy" running, no dying. So I am cautiously optimistic. Now I just need to ride the bike normally; not just on my 1/4-mile secret test strip.

One other question for future reference: Can I get a ballpark idea on the size of both the main jet & the pilot jet if I decide to run the bike near sea level? Currently the bike is run at an elevation of about 1,500 feet with a #220 main & a #40 pilot.

Thanks!

Kit

[matt glascock]

I'd be surprised if any jetting changes are needed at all. Since your bike, as discussed on this thread, historically has run from prohibitively rich to richish, at sea level it may be perfecto. Remember, the closer to sea level you get, the denser the air is which leans out the mix based on the jetting at altitude. More air requires more fuel. I doubt that a 1500' altitude difference would have a significant effect on a mildly ported/reeded fun bike. Highly tuned race bike - maybe one step richer on MJ +/- one notch richer on JN.