The Hodaka Club Forum

What does it mean if the motor will rev with no load but won’t rev up under load? This bike all of a sudden is running like crap. It’s the 125 Wombat/Combat combo with a VM24 carb.

Pilot jet is a 30, main jet is a 180. Floats are level, and the float needle is clean & polished & free of defects. New rubber O ring on the needle jet intake, and I’ve tried the 4E1 needle with the clip at both position 3 & 2 with the air screw out 1.5 turns. Also just did some mild sanding on the flange to make sure it was level & not tweeted. (No reed valve and no ported piston.)

Starts easy, will sit & idle no problem, but it just won’t rev up under load. HELP!!! Victor

Is there good fuel delivery to the bowl? I pull the float bowl off and turn on the petcock to see if there is good flow through the needle valve. It may be delivering just enough fuel to let the bike idle. Does the bike have reeds? I have seen cracked or broken reeds cause those symptoms also. Good luck!

No reeds. Float bowl clean and so is main jet. All openings look good so flow shouldn’t be an issue. No, she doesn’t have reed valves. Just a stock VM24 from a Super Rat. Victor

When you attempt to rev the bike and it won't accelerate does it return to a steady idle when you let off the throttle? If not does it die out after attempting to rev, or conversely does it rev higher when you let off the throttle? These symptoms would help in a long distance attempt at trying to figure out if there is a lean or rich condition off idle. If it returns to a steady idle there is likely some other physical obstruction to flow through the engine. Also it would be helpful to know if it misfires or sputters when you try to rev it which would indicate a possible electrical malfunction.

The bike will return to a steady idle. It will continue to idle fine. When you take off there is a very brief moment of acceleration. Then it just holds at a slightly higher then idle rpm and just kind of Boggs down. It won’t accelerate any further. You can shift to 2nd and there’s another very brief acceleration, for maybe a second, and then it Boggs down again. You can go up & down thru the gears, you just can’t get it to accelerate for more than a second, even holding the throttle wide open. The weird thing is, it ran like this with the carb slide in upside down. I figured retuning the slide to the correct upright position would cure the problem. But it hasn’t.

I think I’m going to inspect the intake and make sure that nothing is in there. I never did find the cable stopper. Hmmm...... Victor

Fuel, air and Spark. Pull the carb. Take out the Needle and Seat assembly and blow the passage out with carb spray. You can run a wire through it too. Better to pull the nipple out first. That`s where I had trouble more than once and it drove me nuttier. Your Jetting is PERFECT!!! Joe.

I will give it a go tomorrow. Thanks! Victor

what is the compression? If you turn the fuel off will it clear out and run before it dies? -----Clarence

The toaster tank Super Rat Owner's Handbook lists a 270 main jet as standard. If you indeed have a toaster tank Rat 24mm carb, it is hard for me to believe that a 180 will deliver enough fuel. It would be too small for the engine it came on . . . and it is now on an engine which is 25% larger. Try a 270 or larger main jet. (NOTE; symptoms described seem to match this diagnosis. When you open the throttle, it is immediately gasping for more fuel and falls on its face.)

Ed
PS: Did you follow the book to tune the pilot jet and pilot air screw? Or just "run what ya got" and set the pilot air screw at 1.5 turns out cuz that is the target? Doing the pilot jet tuning is the number ONE carb tuning procedure . . . check the Official Hodaka Workship Manual.

OK. Background on the carb. The carb came from Greg in Prescott. He sent it to me. It was jetted for his altitude which is pretty close to mine and he runs the Wiseco 125 cylinder on his Super Rat. He told me it should be configured close enough since we are at within 800 feet in elevation.

The carb had been on the bike for over 4 years. All that time, until recently, the bike has run great! Then all of a sudden, it started running the way it is.

When I pulled the carb off and found the slide had been inserted upside down (not by me, possibly my grandson) I figured the angle in the slide was up and not allowing the slide to open the Venturi enough.

Also, the cable stopper was missing! Couldn’t find it, so I figured he dropped it & couldn’t find it.

I’m going to check this morning to make sure that the cable stopper isn’t stuck somehow in the intake manifold, blocking air/fuel flow. Since the carb slide was put in upside down, maybe the carb stopper was inverted and fell through and got pulled toward the intake manifold? Just a guess but I’ll need to look into it and at least confirm if it happened or not. I’ll report back after I check. Victor

Actually, The Main Jet is #220 from the Factory. The Yamaha AT1 125 used the same VM 24 carb and was Jetted as follows: Pilot: #30 Slide: 2.0 Needle: 4D3 Needle Jet: N8 Main Jet:#150. The main difference between the Mikuni VM 24 installed on the Yamaha AT1 is the fuel inlet Nipple is 5/16 and the Hodaka Super Rat is 1/4 inch. These Super Rats were Jetted far too rich from the Factory on the Main Jet.That`s why most guys never got them to FLY! The 4D3 Needle is what`s in my Super Rat at this time. So I`m 3 steps richer on the Main Jet than the Yamaha AT1 and the same on everything else. Reed Valved I use a #35 Pilot Jet and a 4E1 Needle. The only time I could use a #220 Main was when I was mixing Castrol R40 at 20:1 . That was in 1974 in Chino CA. Joe.

A larger engine may not necessarily need a larger main jet to use the same carb as a smaller engine. It's true that an engine is an air pump and a larger engine will pump more air, therefore more fuel is necessary to maintain the optimum fuel to air ratio... However a carburetor is a venturi. When the engine draws air through the venturi the air accelerates at the narrow point creating a pressure drop. This pressure drop is what draws the fuel through the jets into the venturi. A larger pump will draw more air through the venturi which has to accelerate even more causing a greater pressure drop than a smaller pump at the same throttle opening and rpm. This greater pressure differential will draw more fuel into the venturi even through a smaller jet. This explains why obscenely large main jets are required when we put an oversized carburetor on a 100. For example a 100 will require a much larger main jet than a 250 when a 38mm carb is used. This also explains why a bike loads up while going down a hill with the throttle closed. The engine is pumping a lot of air through a closed throttle which accelerates the air way more than an idling engine. This draws way too much fuel through the pilot jet (even though it's a small jet) causing a rich condition. It also explains why opening the throttle wide open on a flooded engine at low rpm will introduce a lot of air with very little pressure drop, therefore drawing very little fuel, creating a lean condition that will clear out the flooded condition. The VM 24 is a fairly small carburetor so it probably wouldn't exhibit this phenomenon to an extreme. The only way to know for sure is to correctly jet a carburetor as described many times on this forum. Sorry! Probably way too much information, but I hope my point is made.

Your point is well taken. That`s why a VM20 carb has a #90 or #95 main. The VM 28 on the Combat Wombat came with a #190 Main jet . Four mm larger Venturi than the VM24 Carb installed on the Model 93s and the VM 24 came with a # 220 Main Jet? Hodaka got it wrong . Sorry. Hodaka did get a few things wrong. Number one is only offering the Super Combat for 1 Year. Would you rather have a Super Combat or a Road Toad to race your buddies with a Honda CR125 Elsinore? Sorry Hodaka fans. Joe Ormonde.

As far as the main jet is concerned, on a Wombat (which also ran the 24mm carb) it came with a 140 main jet.

Did you also check the fuel flow from the tank? The carb can be spotless and jetted perfect, but if fuel isn't getting to the carb., it won't matter. Check your fuel flow and make sure the gas cap vent is not blocked.

OK. Fuel flow out of the petcock is fine as far as I can tell. When I empty the bowl for carb disassembly, you can see the fuel flowing quickly thru the fuel filter, filling the bowl back up.

There was no throttle cable stopper, or anything else, in the intake manifold (see photo). I will pull the bowl again and inspect/clean the needle jet, one more time. Victor

Needle jet looks really clean to me. I sprayed with carb cleaner again, but it didn’t seem to change anything. It’s very clean. Anything else I should check? I’m really at a loss.

The top end has a new set of rings & cylinder, & less than 5 hours of easy riding on it. No hard riding and a mixture of 32:1 has been consistently used. Victor

I would check the compression? Clarence

Yup, "Pop" all of what you wrote is correct - but beyond the sort of "coaching" I tend to offer to inexperienced carb tuners.

- Yup, the toaster tank Rat may have been jetted wayeeeee too rich. We used to joke that factory jetting specs were settled upon on a zero degree day (F), at sea level with low humidity and high barometric pressure. And yup, the Wombat used a 140 (or 160) with a 24mm carb. And yup, I forgot about the elevation where Vic is doing his stuff. STILL, two things are important 1) Why not try something significantly richer (which is "safe" for an inexperienced tuner) to find out whether things get better or worse? And 2) follow the procedures in the Official Workshop Manual for tuning the carb. Read the book. Understand the book. Don't rush it. Follow the procedures in the book step-by-step.

Dare I say it? Slow down, you'll go faster.

Ed

Ed. I use the carb tuning section of the manual explicitly. Yet I can’t figure out why the motor is running different with no changes from when it was working fine! That’s the answer I can’t seem to find. I’ve been working on single carb motors for 6 years now. There just seems to be no proof discoverable for the change in the way it’s running.

As a far fetched question, is it possible the problem is being caused by a cracked ring??? It would seem like a cracked ring would result in a high rev’ing no power situation. But, it’s about the only thing I haven’t checked ..... Victor

I would check the compression. Clarence

Crappy old gas? Oxygenated gas that's loaded with condensed water? Lawnboy gas? Try fresh BRANDED 91+ octane non-ethanol gas with your favorite fresh premix oil at 32:1. With the slide in upside down and the cable stopper missing, the tank may have been filled by the same mechanic so who knows what you are trying to run it on. Also, make sure your air cleaner isn't glopped up with a bunch of goopy old oil and thus too restrictive. If that's the case, clean or change it.

Let's see if we can figure this out. Your comment that "you can see the fuel flowing quickly thru the fuel filter, filling the bowl back up." tells me that most likely there is good flow through the needle valve and seat.

You also said that "The carb had been on the bike for over 4 years. All that time, until recently, the bike has run great! Then all of a sudden, it started running the way it is." Did that happen before or after disassembly? Was there anything done differently before the bike started running like this? I know the slide was upside down. Could it be backwards still? I know some of these questions might be perceived as ridiculous or even aggravating, but I'm just trying to stimulate thought.

Did you do anything different with the exhaust pipe? I know from previous posts that you barbeque all your exhausts, but it might be helpful in eliminating possibilities to simply disconnect the exhaust pipe and see if it revs. something might have moved in the pipe that is now restricting flow. We'll find the problem.

And yes, what Matt said too!! He answered while I was typing.

Also if it starts easily and idles smoothly I doubt that it has a broken ring or low compression, but as Clarence says it wouldn't hurt to check. As Ed says do things systematically so there's a logical path to see what effect each change may or may not accomplish. You know your way around these engines pretty well so I'm confident you will figure it out.