M94 Transmission issues

[matt glascock]

Hey fellas, here's the scoop. 1972 Wombat currently stripped and set-up for MX work. Piston port induction. Stock ignition. By my reckoning, the gear set was replaced with a close ratio set when compared to my stock M94. I placed it in the hardware department on a hare scramble a month ago. The bike rips. Here's the issue. To preface the problem, not unexpectedly, there is for all intents and purposes, no over-rev. In all gears (well, I'm not sure about 1st), just as the motor begins to fall off the pipe at the upper rev limit, it feels as though the transmission slightly disengages. No strange sounds. The motor doesn't rev out when this occurs. It just seems that the bike ceases putting power to ground. My impression is that the transmission, while loaded during acceleration is engaged properly but when the bike comes off the pipe, the unloading is as sudden as though the bike is "coasting". There is a subtle and brief "twitch" transmitted through the seat and bars when this occurs. This disappears during reacceleration. I rebuilt and blueprinted the clutch using updated components and changed the pinion bushing. There is spec free play at the clutch actuating arm. The sense of losing power to ground is a sudden drop-off not to be confused with the sensation of coming off the pipe at the upper rev limit. Otherwise the bike runs perfectly and the state of tune is dialed. Thanks all.

[Bullfrog]

Matt,

First, would you elaborate a bit on this phrase from your post: "The sense of losing power to ground is a sudden drop-off . . ." Sudden drop off of what? Forward speed? Engine revs? Acceleration?

My first impression is that you have an engine set up which just "runs into a wall" somewhere up there in the rev range. If that is the case, then things are suddenly going to go to "constant state" operation when it "hits the wall". Acceleration may drop off so rapidly that it feels like the clutch fell off. But since engine rpm is apparently constant (and so is ground speed?) - I'm going with a hunch that the power band simply falls like a rock once you go past peak power RPM.

Ed

[matt glascock]

Thanks Captain. Yes, I will elaborate, or in this case "exaggerate". As such, it seems that if what I'm sensing was any more severe, I would slide forward onto the gas tank. So, loss of ground speed with no loss of RPM is accurate. I'm very familiar with the "wall" you've mentioned - especially so with vintage 2-stroke bikes as opposed to my modern 4-stroke SX bike which has "holy crap" scale over rev even with a moderate rev limiter mapping. Mayhaps, in this instance, that's just a peculiarity of this bike. On a side note, did Hodaka make a close ratio gear set for the Wombat, or would, say, a gear set from a CW, SC or painted tank SR be a direct replacement for the stock gearing? If so, would that exaggerate the drop-off I'm describing? Unfortunately, I didn't run the bike before I opened the motor and may not (certainly not ) have been astute enough to examine the gearing beyond the "nothing looks roached" level of sophistication. Many thanks, Captain.

[olddogs]

You can use any of the transmissions you mention. The CW is different only in 5th gear ratios. Everything else is standard Wombat parts. The 97 SC or 98 SR will fit fine, but everything inside the case needs to be swapped, including the kick gear. I never cared for the busy feeling of racing a close ration transmission. I think it would intensify the hitting the wall feeling you are getting. I personally use a CW transmission in my Super Combat. I find is less tiring during a long moto. I also have buckets of trashed close ratio transmission gears. Second gear takes a beating and usually take the main gear set tree with it. The cushioned primary and hardened gear sets were developed to help eliminate this weak spot in the close ratio transmissions. I prefer to customize the "powerband" using sprocket changes, both front and rear. There are dozens of combinations that can be tuned to the track you frequent most.

[matt glascock]

Thanks Old, this is great information you've provided. The gearing is definitely closer than my CW so it must be from a M97 or 98. I totally agree with the sense of "business" you describe is piloting a close ration tranny. My KTM has a close ratio gear set. It's less of a bother because of the giant 450cc motor which lugs so well and has so much over rev that there are a few tracks and courses where I can practically run the entire moto is 2nd gear! Not the case with a 2-stroke. I do change countershaft sprockets to tune the power band - often at track side. Some times the change is more abrupt than I'd like but I can do it much quicker than changing the drive sprocket. There are some folks that bring along entire rear wheel assemblies for track side swapping purposes. How do you approach this. In other words, do you set up the bike for the day at home based on your historical experience with the track or at track side? Thanks Old!

[Hydraulic Jack]

Without riding it myself and running sensations through the internal database, I am going to have to guess here. I can't think of a single transmission related scenario that would produce a free-wheel sensation only at the very top of the revs. So I am assuming it isn't the transmission. My guess is that it is your carburetor. I think you are running out of gas at full throttle. So either your carb is too small for racing, or there isn't enough reserve gas in the float bowl to sustain full throttle, or you have a restricted fuel flow rate that only presents itself at sustained throttle. I would look for clogs in the fuel inlet passage between inlet and float needle as a starter, and then verify that your float level is correct. If set too low you can run out of gas on high demand.

I would also check the air filter. If you have a reusable oiled filter, you may have too much oil on the filter.

[olddogs]

I know most of the 10 local tracks. Some hard clay, others deep sand, some have long straightaways, some are nothing but turns. 13 thru 15 sprockets always on hand. You can set up rear sprocket selections by just using the hub that fits into the rubber cushions. They are plentiful and a fast swap this way. I keep a selection of 62, 64, 66 and 72. Gearing makes a HUGH difference when you are a heavier rider like I am. The wrong gear can have you pushing a 125 up some of the steep, longer deep sand hills. I have the pictures to prove it. The more highly modified the engine, the more important this selection must be.

[Larry S]

When was the last tine you "cooked" the pipe? I had a similar experience and it turned out to be the pipe.

Larry S.

[Hydraulic Jack]

I thought about the pipe also, but would expect a clogged pipe to present itself more slowly rather than suddenly. Power with a clogged pipe tends to fade gradually as back pressure rises. But here the OP said the power drop is so sudden he feels like he is going to scoot up on the tank. That's pretty sudden. Of course, if the pipe hasn't been cleaned in this century, cooking it is a good idea regardless.

[olddogs]

If you were racing a 4 stroke I would say you are hitting valve float, the sensation of the engine not pulling but not dropping any rpms either. I tend to agree with HJ that you may be over revving the ability of the carb to supply air and fuel at extreme rpms. Since your transmission is in constant gear mesh, and your clutch is crank mounted, I cant explain any disconnected feeling that could occur. Check your hub cushions. Too much play in the dampers can cause a jerking effect when getting on and off the power, maybe even exaggerated by the close ratio transmission?

[olddogs]

Another thought just came to me. Back in the early days of Rat racing, we would run larger fuel lines and drill out the petcocks to flow enough fuel to keep the bowl full. Since you state the bike really rips, it may have some porting or other mods that keep the bowl almost dry. The Wombat has a tiny carb. This would lean you out after a long wide open run. Since your original problem isn't burnt or seized pistons, this might be a long shot, but the advice is free.

[Hydraulic Jack]

I understand the thought process behind drilling out fuel fittings, but in the end, fuel will flow no faster than the most restrictive point. That would commonly be the float valve. Mikuni does make larger float valves I think, but whether they interchange with the smaller carbs I have no idea. Modifying a float valve for increased flow would, I think, be tedious and not likely to offer much benefit compared to the risk of making it no longer work.

Rich recently posted a photo of a modified float bowl for use on a high speed record attempt bike. While this modification might be a bit extreme for a motocross bike, the concept is the same. If you modify your machine to use lots of fuel, there has to be lots of fuel available to use. Setting the float level becomes critical because float level establishes standing reserve of fuel. Depending on the longest sustained WFO run, the amount of fuel available, consisting of fuel in the bowl plus fuel delivered over time, will eventually limit how long you can hold the throttle open. In stock trim, the answer would be just about infinitely or until something melts. If the engine has been modified but the carb is still a stock 26 or 24, it could be possible to run the bowl dry or dry enough to go very lean very fast. It might be time to consider upping the game to a 28 or 32 if your carb is stock.

[Larry S]

Jack-
I agree with you about how a clogged pipe should act. In this instance, it didn't act as one would expect. I don't have a tach on the engine so I'm just pulling numbers out of my hat; the engine ran perfectly normal up to 4000 RPM, but wouldn't turn 4001 RPM, it would just go flat. If it were a 4 stroke, I would say it was hitting a valve float.

Larry S

[matt glascock]

Wow, I get home from work and there is a plethora of great information to ponder. I'll reply piece by piece. Old, great idea. I have a few cushdrive hubs in my parts stash. I'll set up a few options for drive sprockets. I feel a bit sheepish that that didn't occur to me before this but its a great idea and I'm taking it. Thank you. The previous owner had port work done on the engine, but that's all I know since the bike was sold to me by his son (the owner was noncommunicative from the effects of a stroke). All I could tell from inspecting the jug is that it looked like the exhaust port was raised but there may have been some work on the transfer port timing as well. Certainly nothing radical. I'd need to send it to a pro for specifics, but the bike was raced in the desert so I can only assume it was set up for sustained high rev running. Larry and Jack, the bike is outfitted with an "HJ Powerpipes" after market chamber. Its an old school expansion chamber from a long-defunct California performance house. I cooked the pipe after taking possession of the bike so it has 10 motos and 6 hours of hare scrambles-type race time and probably another 40-50 hours of tiddling, jetting runs and basic farting around. The bike has a VM26 Mikuni mixer on it but I have a VM 28 and 32 so that's a great suggestion. Input on which one would be best and any starting point on the jetting would be appreciated. I completely dismantled and cleaned the carb. Float level was set using the tried and true "eyeball" technique bit I will recheck. Fuel starvation at sustained WOT is a distinct possibility that a larger mixer should cure. The bike has stock tank and fuel delivery system. Lots of great stuff to consider here. Thanks fellas!!

[ossa95d]

It would be helpful to determine if the bog is rpm related or more affected by sustained wide open throttle regardless of rpm. If you accelerate aggressively but short shift the engine before reaching peak rpm do you experience the same condition or is it at roughly the same rpm regardless of how long the acceleration? This would help in determining if the source of the problem is fuel delivery to the carburetor or flow of air/fuel volume through the engine.

[matt glascock]

Hi Pop, I couldn't say I never short shift the motor, but the power band is significantly narrower and at a higher rev range than stock so with the close ratio gears, I'm probably never below 4800 RPM (guessing, no tach) during MX. The effect is at the upper limit of the rev range on acceleration just as it falls off the pipe. This effects WOT. If I come off the gas then reaccelerate, it will gain revs to the same point, then fall off again. This effect is predictably reproducible. This "falling off" effect is pretty dramatic and not gradual or subtle.

[ossa95d]

I would have to agree with Jack that the bike is not getting enough fuel delivery to satisfy the need of the engine. The piston is an air pump among other things, and the engine's ability to produce power depends on it's ability to provide the correct air/fuel mix to satisfy the volume of air being pumped. If there is a restriction in air flow (such as a restrictive air filter or exhaust pipe) the engine won't rev past a certain rpm, but the inability to rev wouldn't be so abrupt as to pitch the rider forward. It the carb was running dry not only would the bike stop accelerating, but the fuel would not be available to keep the bike running at that rpm. The bike would bog. Letting off the throttle might let the fuel flow catch up to the lower demand. Restrictive air filters and mufflers are easy to identify by simply removing the filter or baffle and seeing if the bike will rev cleanly. Fuel flow to the carburetor is fairly easy to check by pulling the gas line off the carburetor and checking flow to that point. If that is all clear pull the float bowl and see if fuel is flowing cleanly through the needle valve. Clogged or kinked vent hoses can also raise heck with fuel flow! Fuel flow has to keep pace with the highest demand of the engine/air pump and should not be confused with proper jetting. Good luck!

[racerclam]

Is this a new problem? Or has it always been? No one has asked what pipe you are using ? Is the porting stock wombat ? What carb are you running ?

Rich

[Hydraulic Jack]

Something in what you just said Matt has me wondering. You said you can back off the throttle and hit it again, and it falls off predictably at the same spot, and that this is repeatable.

I would think that fuel starvation would be a bit unpredictable when riding in real time because of variables not under control. So if it is starving, and you back off, then get back on the throttle, it apparently revs again until the same spot is found, and there it signs off.

Have to ask, do you have a magneto ignition or CDI? CDI systems are pretty bullet proof as far as rpm goes, timing curves notwithstanding, but I don't think they have rev limiters built in. So assuming you are still using the magneto system it came with, I am wondering if you are getting points float at high speed. These systems were not really intended for use over 8K or 9K and if your bike is ported, geared down, and close ratio, you could be hitting an rpm range it was never intended to see.

The problem I see with this, if it is the points, is that about the only way to overcome floating points would be a stronger spring on the points, or keeping revs down. For moto, keeping the revs down isn't the best option if everything else is working for you. You could mitigate high rpm by gearing up slightly, which will make the engine work harder to reach high rpm, and take a bit more time, which means you might not top out on the long stretches. That might help.

The better solution would be points with a stiffer spring. First thing to check would be the points to see if they are old and weak, and compare what you have to a new set, if you have one handy, to see if there is a difference in spring pressure you can feel. Otherwise, modifying the spring would be an engineering exercise. You might be able to insert a partial spring inside the spring curve that would act something like an overload spring on a truck, exerting most force only at the limit of travel, and this would take some trial and error. Another option would be to reset the curve of the spring to make it stiffer, but rebending the spring will probably shorten its service life.

Or, I suppose, you could convert to the Penton CDI system which I believe will work on your bike, but it isn't exactly cheap. I suppose a model 97/98 CDI could be retro fitted for less money since lots of folks have pulled them in favor of the newer designs. Something to consider.

[Hydraulic Jack]

As for the carburetor, seeing as you are racing moto, I would opt for at least the 28mm, and would consider the 32mm if it was modified with Venturi vanes. If your bike is quick as it is, a larger carb may well be just what you need to go with the pipe, the porting, and the gearing.

[matt glascock]

Thanks again fellas. I'll address point by point. Pop, I am going to pull the carb and give it a thorough cleaning and inspection. I did rebuild it after acquiring the bike and it runs a VM 26 with a 200 MJ and 25 PJ. I'm probably going to change to a VM 28 or 32 but I still want to know what's happening with this one. The bike has the stock Wombat air box, which I really want to keep since its so cool. I am running without the snorkel. UNI filter with Yamaha filter oil. Its very tacky bordering on goopy, so that might be negatively effecting aspiration. I'll give it a thorough washing and go back to UNI aerosol filter oil. Rich, the pipe is an after market HJ Powerpipe expansion chamber with a nearly useless silencer comprised of a fenestrated pipe and a few layers of glass wool. The engine has had port work done by the PO but he was not able to give me any details. It looks like the exhaust port was raised and the transfer ports timing was altered, but that's an assessment by a guy (me) with a rudimentary at best understanding of port work details. The problem became most apparent when I dropped a tooth on the CS from 14 to 13. With this, I can keep it on the pipe better on most MX courses without soiling my self. I occasionally run a 12T CS sprocket for really hilly/sandy woods courses and for some MX work, and occasionally go back to the 14T CS sprocket for courses with more open terrain and straight runs. The issue becomes more apparent as CS sprocket tooth count drops.Jack, it has the stock magneto ignition. Points float is a great suggestion. Just using my ear tach, I'd bet the farm the bike does nine grand and maybe a bit. I completely rebuilt the stator plate with all new components. In the mean time, I did pick up in a parts lot a few of the "racing" points, which I assume have stiffer springs. Easy to swap. If that's not the case, I will attempt the modification you recommended. If I pooch the job, I have plenty of replacement points sets. I would like to keep the ignition stock if possible. I will give consideration to upgrading the ignition if I can't get this sorted out by modifying the potential aspiration and induction issues and tweeking the stock ignition. I run Penton PVL ignitions on my reeded SR and reeded CW and have nothing but glowing reports on their performance. Again, many thanks to all for sharing your time in helping me sort this out. Its a great dirt bike.

[Hydraulic Jack]

Oiled filters should never be goopy. If it isn't dirty enough yet to require washing, just wrap in paper towels and squeeze. Thoroughly.

Don't know if racing points are really any different from standard, but take a look. Give each a squeeze and see if there is a difference. I would expect points float tomsound somewhere between misfire and no fire for a couple seconds until revs fall.

Somewhat difficult to sympathize with a guy with an engine that won't quit revving until the ignition quits...

[taber hodaka]

I could be wrong but cannot think point float to be a reality here. I would ask what is the point gap. try no air filter, dry run no dirt and try without silencer only to test the change. Is there a adequate fuel to the carburetor. What is the rear sprocket? Rich if you are out there what is the hi rev range you can relate to? --------------Clarence

[matt glascock]

I know Brother. I feel a bit silly bit@hing about this bike. It was really a very lucky score as I was expecting a basket case when I bought it (and for a song at that). The bike had not been raced since the late 70's and ridden gently only intermittently around a hobby farm in western Pennsylvania until the late 80's when it was put up. Fortunately, it was stuffed with cylinder fog and kerosene so the innards were beautiful. I bought it as a play bike, but it rips as hard as my ported to death, reeded CW when you stay in the power band. It really lacks in the low end pull department, but who cares? In fantasy world, what I'd really like to do is extend and beef up the swing arm, weld on a few gussets, put a no shizzle front end and some WP shocks on it and take it for a whirl on our local SX track in open class . I'll check out the points situation. The air cleaner element is not wet per say. The Yamaha oil is like dried honey. I'm going back to UNI aerosol unless you have some filter dope you like better. We are supposed to be in the 50's this weekend so I am going to address the filter and points and run the wheels off it. I'll report. Thanks Jack. Much appreciated as always.

[matt glascock]

Hi Clarence. Just saw your post. Point gap 0.38 mm. Drive sprocket 58T. CS sprocket usually 13T. I'm going to try a different filter oil. I'll try pulling the silencer. Fuel delivery to the carb is stock hodaka. Two pot metal petcocks with the brass filters removed through 1/4" lines through a "T" connector through in line filter into the carb. Fuel flow THROUGH the carb is a topic of ongoing discussion. Incidentally, I'm running 93 octane nonoxygenated gasoline and Redline 2-stroke racing oil at a 32:1 ratio.