Day One - New Project

[hodakamax]

Tech question for my expert Hodaka friends. On the project I'm using a VM2400B carb from a 94A wombat. The Model 93 Super Rat came with a VM2400 and since I'm using a 93 cylinder and head, although reed valved, I started with a 220 main, 4El needle, 2.5 slide and 25 pilot jet. Way too rich at full throttle and I'm down to a 200 main which still appears to be rich. The engine stumbles off of idle but has good mid range power. Adjusting the air screw doesn't seem to have any effect and I just rechecked all the passageways in the body which seem to be clear. The carb is mounted on the stock intake manifold to level it out which extends the intake tract before the reed. While I have it apart I thought I'd better run this by my friends on any ideas about the loss of power coming off of idle. I see that the 94A only required a 140 main. I'm headed that way . Hmm.

Max

[Dale]

Max,
If you are happy with your pilot/air screw and it stumbles off idle, I would try a 2.0 slide. A bit richer off idle.

If your question is specific to your intake setup, I don't have a response to that...
Dale

[Bullfrog]

25 pilot jet and 2.5 slide are standard in the Wombat "B" carb . . . for a piston-port engine. Yours is reed-valved - that alone suggests that going richer on pilot jet is needed. Since the pilot air screw is having no effect, I think that is telling us that even with the air screw all the way "in", things are still too lean. Gotta get the pilot jet dialed in BEFORE doing a bunch of main jet tuning. If it were me, I'd put in a 40 and see how it tunes in with pilot air screw. I get a little concerned about statements like "all the passageways in the body which seem to be clear." If you squirt carb cleaner in one place and it comes out where you expect somewhere else you have a result which exceeds "seem to be clear". Yes, I know you know your way around the machine and the carb . . . so the fact that you used that phrase suggests some un-certainty to me.
Ed

[hodakamax]

Thanks Dale. I think I've discovered the problem. The cylinder I repaired needs to go back to the machine shop. It looked and measured flat but it's blowing past the head gasket big time. Grrr. Looks like the file was not accurate enough so I'll have to address that. Thanks for your info and I'll keep you posted. Appreciate it!

Max

[hodakamax]

And thanks Ed, all sounds reasonable. I am a little rusty in the fine tuning the carb department. Hopefully stopping the compression leak will get me back on base. It had me temporarily stuck.

Max

[hodakamax]

Meanwhile in Dr. Science's lab comparison testing has been going on between the vintage Webco head vs the model 93 Super Rat head. The Webco combustion chamber holds about 12 ml and the 93 head 11mm. My method is somewhat crude but shows roughly a 10% difference in the two. Now to compute how much to mill off the Webco while also considering that a new squish will need to be cut also. Fortunately I now have the 93 head to use until all of this is accomplished. First things first, I'm off to the machine shop to true up the cylinder head gasket surface after my failed attempt with a large file. Hey, I'm not perfect. And that's my report.

Max

[Bill2001]

Not a bad way to "cc a head". One way to improve accuracy is to get a piece of acrylic ("plexiglas"), place it over the head/combustion chamber seal it around the edges with grease and fill it through a hole drilled in the plastic. If you true up or "deck" the cylinder, be sure to subtract that volume from the comb ch volume or you might end up with a CR of 16:1.

Is there a list of CC volumes for the various Hodakas online? My freshly top-ended W'bat 94 has a compression of 125psi, which is a tad shy of the nominal 150. The engine has 10 miles and 2.0 hours running time on new rings/hone, so the rings maynot be seated yet. I assume that the head is a std 94, but I'm not sure.

[MWL]

Max
I have the same combination (webco head and SR piston). The project is unfinished and never run so I have no results, however, I did cc the webco head and a Ace head as Bill described with a plexiglass plate. The webco head was 11.8 cc and the ace head was 13.5 cc.
Mike

[hodakamax]

Hey Mike, thanks for the report. My method was a bit primitive. It looks like I was fairly close. It was indeed difficult to tell when the chamber was full even with soap added and the small spaces available on the measuring devices didn't help either. In reality I was probably +/- .5 ml. I measured both several times though and at least got a comparision of the two.

Max

[Bill2001]

The super-accurate way to measure CC volumes is with a chem lab burette. I'd suspect very expensive nowdays. Let me check lab supply houses online and see what is available in the 10-30 mL range.

[RichardMott]

Hey Mike,

Thanks. I have a Webco (since 1974) on my 1971 Ace motor with a SR RV conversion. I always wanted to know the difference in combustion chamber volume.

[hodakamax]

Hey Gang, more on the story. I took the cylinder to my machinist to true up the head gasket surface and early on in the process my silver solder repair peeled out leaving me with the same problem I had in the beginning. I decided to push on and cut out the flaw thinking that I would just make it up on the head by making the squish a little deeper to clear the piston. After the process which probably removed about 1 mm from the cylinder I checked the piston for deck height and it was not above that. I then went back to the Webco head which actually had a bit deeper squish and needed more compression anyway. All fit well and the engine did gain some compression according to my crude gauge.

All of this did nothing to cure my coming off of idle bog. The next step is to follow Ed's suggestion to increase the #25 pilot jet to a # 40.

Question: where does one locate a #40 pilot jet for a VM2400 carb?

Max

[Dale]

Max,
Don't tell anyone, but here is a link to one...
http://stores.ebay.com/Niche-Cycle-Supp ... banshee+40

Back to my comment last week, I meant to say, "once" you are happy with the pilot and air screw, then if you still bog off idle, look to go one step richer on the slide.

[hodakamax]

Hey Dale, thanks! I'd been all over the main Niche site earlier trying to find that one with no luck.. On its way!

Max

[viclioce]

Max. I think I have at least 2 40 pilot jets. Shoot me your addy & I'll throw one in the mail to you. It will even be sent clean! ; D Victor

[hodakamax]

Hey Victor, thanks so much! Got one coming, again your heart is in the right place!

Max

[Bullfrog]

Max:
Uh-oh, I may have steered you wayeee too rich with the #40 pilot jet recommendation. I got to thinking that maaaaybe I was remembering what was needed in a stock 28mm carb on a Combat Wombat following installation of a reed valve . . . so I went to check some notes. I found that my Bullfrog observed trials machine (24mm "B" type carb, reed valved 125) uses a 25 or 30 pilot jet. 30 is right most of the year . . . but I have to crank the pilot air screw wayeeee out on 95-105 degree summer days in Athena (about 1,200 ft elevation). SO, it seems that a 25 or 30 might be about right for your scooter. Although . . . the smaller volume of air being drawn through the carb for your 100cc reeded engine will be at lower velocity . . . so maaaaybe 40 or 35 will right for your set up. Anyway, I'm sorry if I've given you a bum steer. I'll watch for what you find when you try the #40.

In the meantime, if you'll provide a photo of the pilot jet you've got, I'll check to see if I have what you might need in the basement parts mine (the ore is rich there sometimes.) There are two styles of pilot jets which were common in Hodies and I've forgotten which one is in the "B" type carb.

Ed

[hodakamax]

Hey Ed, Things happen! Another thing of note is this VM2400B came off of a reeded 94A which ran well at least years ago. Thats why I was surprised that the jetting wouldn't come in.--------WOW! a Kansas earthquake just occurred and flushed me from the keyboard. 7:03 AM local. All is well (I think) but what a start for the Day!

Anyway, back to the jetting, we'll get it resolved eventually and thanks for your help.

Maxie

Update: Magnitude 5.6 at Pawnee Ok. Felt over several states, biggest ever for me. Yikes!

[michael_perrett]

Ed, could you briefly explain the two different types of pilot jets? I always assumed there were only one style used.

Mike Perrett

[Dale]

I happen to have the dimensions for the smaller style of pilot jet in front of me, so I will provide a partial answer.
The Model 94 Wombats 24mm type B carburetor uses the smaller "Banshee" style pilot jet. Some other models do as well, but I am not going to try to identify them...
The dimensions of this smaller style jet is:
9.5mm total height (4mm lower portion+5.5mm threaded portion)
Bottom end 2.8mm wide
Top end 3.9mm wide

[Bullfrog]

Woweee, notable earthquakes in your neck of the woods! Glad to hear things are OK for you!

Your carb having come off a reeded Wombat provides more information that the 25 may well be right for the application . . . but if that is so, we are back to the question of why did pilot air screw adjustments do nothing? Which leads me to recommend a detailed check of carb passages - gotta successfully squirt carb cleaner through them (not just "in" them - "through" them).

I think I can find a photo of the two styles of pilot jets - will get back to you.
Ed

[Bullfrog]

Here is a photo of the two pilot jet styles which were commonly used in Mikuni round slide carbs back in the day. I don't mean to suggest that Wombat 94 carbs used both . . . I just don't remember which one they used. So it is always best to know which style is needed before ordering one or searching for one.

[michael_perrett]

Ed, I have only had experience with the longer jets. Never seen the shorter one in any Hodaka carbs I have.

Mike Perrett

[viclioce]

Me too! ; D Victor

[hodakamax]

Another update. The passageway between the pilot jet and the air screw has two right turns. When I squirted carb cleaner in either end it came out several places making it look like it was clear but it wasn't. Several squirts of carb cleaner and air along with some difficult threading of monofilament finally cleared it. I then was finally able to get a response from the air screw. Unfortunately it still has poor response coming off of idle. I even removed my longer intake tract to see if that had any effect and it didn't. I now think I'll try the #2 slide as soon as I locate one. As always I'm open for ideas.

Max

BTW this pilot jet is short.