Mikuni VM 24

[ParrotGreenUS90]

I have a Mikuni VM 24 carb. I have cleaned it 2 times. I mounted it today and when it starts it idles high but will not accelerate. I adjusted the needle by 2 notches and get the same. I have adjusted the air and idle and get the same. I have fuel getting in the carb, I opened the drain and it flowed out. Any idea what the problem is?

[Jeckel]

When you accelerate what does the motor sound like? starving for gas? or bogging sounding? Whats your pilot screw turned out at?

[Vealmonkey]

Have you tried giving it gas with the choke on? It could be your main jet is clogged up or if you find it accelerates with the choke on then it could be your main jet is too small. Or you could even have the wrong needle. That is the problem with used carbs, sometimes people rob them for parts and then put the wrong parts back in that carb. Or the carb could just plain be too big for the engine. Or someone screwed with your head as in homemade port and polishing and could have trashed the head even though it looks perfectly fine.

[ParrotGreenUS90]

Swigin mentioned I may have a small stock cam. If I do have a stock cam will the cam from dratv.com work? I recall reading back a ways a st90 cam is larger and was a cheap mod in the day. As for the carb I have on it now, I may just set it to the side and buy a 22mm similar to the one on dratv.com. I just know I will find a VM 24 after I get a new carb.

[Vealmonkey]

An st90, sl90 or cl90 cam has a more aggressive grind than an atc90 cam. The first performance cams were cams from these bikes. The atc90 was set up to run at lower rpms and less compression that the stock bikes due to the auto shift. If you do not have a better air cleaner and a better exhaust system or a head in perfect or modded condition, than a larger carb will kill performance and just flood the engine with too much fuel and air. But people always insist that a larger carb makes more power. But it's also easy to over carb the engine. There was a surprising amount of hand work that went into these engines to run big bore and stroke and to make any decent power than people realize. Everyone thinks you can bolt on a piece or 2 and have a rocket ship. It will never happen. Do lots and lots of research. The info is out there, just not all in one spot. I'm not expert by far, but I remember seeing how pople modded these engines and lots of there little tricks. Some was in the magaizines but most was by word of mouth and still more was considered "top secret" by the people that built the engines. I can't begin to imagine all the info that was lost when these machines were the only trikes around. Berfore 1980, there were no 185 trikes and the 250r or anything like it was just a dream to be built in peoples garages. People or companies were just starting to mod trikes with suspension and more often than not with dismal results. I wish I was around the dunes or the west coast when all the really cool stuff was starting out, but I wasn't. There was no internet and most really cool innovative stuff just wasn't known about. But people were still building big bore stroker 90s. And it wasn't alot of over the counter stuff, not on the east coast anyways. There really wasn't alot of stuff on the east coast until 3 wheelering popped up. Then you could order alot of parts, but there were already people that had been building atc90 engines for years with what parts they could mix and match and modify from existing machines. Some of these engines were pretty crude looking and many hours for just modding the cases since cl90 or sl90 cases were hard to come by. Keep at modding your trike, but you are going to have to do alot of research and think alot more outside the box.

[ParrotGreenUS90]

If I locate cams from the other 90's I could gain some power? I do agree with the bolt on a part and instant power, it rarely happens that way. I should locate a worn atc90 bottom and experiment. Maybe I can figure something out between 90's and the clone parts? We now have the clones everywhere and can purchase parts fairly cheap. I do keep thinking of the cb750 setup as an option. It all depends on what I can locate and not invest any large money into. What do I need for the cb750 set up? I know I will need alot of time but is it a cylinder and head setup from a cb750 or do I just need a cylinder and piston?

I think for now I will go back to a stock carb or take a chance and buy a 22mm and adjust the jets down, Oscar has explained this to me.

[SWIGIN]

An st90, sl90 or cl90 cam has a more aggressive grind than an atc90 cam. The first performance cams were cams from these bikes. The atc90 was set up to run at lower rpms and less compression that the stock bikes due to the auto shift. If you do not have a better air cleaner and a better exhaust system or a head in perfect or modded condition, than a larger carb will kill performance and just flood the engine with too much fuel and air. But people always insist that a larger carb makes more power. But it's also easy to over carb the engine. There was a surprising amount of hand work that went into these engines to run big bore and stroke and to make any decent power than people realize. Everyone thinks you can bolt on a piece or 2 and have a rocket ship. It will never happen. Do lots and lots of research. The info is out there, just not all in one spot. I'm not expert by far, but I remember seeing how pople modded these engines and lots of there little tricks. Some was in the magaizines but most was by word of mouth and still more was considered "top secret" by the people that built the engines. I can't begin to imagine all the info that was lost when these machines were the only trikes around. Berfore 1980, there were no 185 trikes and the 250r or anything like it was just a dream to be built in peoples garages. People or companies were just starting to mod trikes with suspension and more often than not with dismal results. I wish I was around the dunes or the west coast when all the really cool stuff was starting out, but I wasn't. There was no internet and most really cool innovative stuff just wasn't known about. But people were still building big bore stroker 90s. And it wasn't alot of over the counter stuff, not on the east coast anyways. There really wasn't alot of stuff on the east coast until 3 wheelering popped up. Then you could order alot of parts, but there were already people that had been building atc90 engines for years with what parts they could mix and match and modify from existing machines. Some of these engines were pretty crude looking and many hours for just modding the cases since cl90 or sl90 cases were hard to come by. Keep at modding your trike, but you are going to have to do alot of research and think alot more outside the box.

i tried to tell him that he needs every thing mod wise to get that carb to work....even a 22mm is big for a 90

[ParrotGreenUS90]

SWIGIN, does it appear I just have a stock cam? What will the option be with the SL, ST, and CL 90 cams in the way of Carbs. I still have to take the top end off and see what I have inside. For now, I will get the stock carb back on and locate some ideas and parts. I believe Oscar pointed out a few ideas with a Dial a Jet, have to see what he says.

[SWIGIN]

SWIGIN, does it appear I just have a stock cam? What will the option be with the SL, ST, and CL 90 cams in the way of Carbs. I still have to take the top end off and see what I have inside. For now, I will get the stock carb back on and locate some ideas and parts. I believe Oscar pointed out a few ideas with a Dial a Jet, have to see what he says.

like i said its behind the cam gear....pull the cam and look at the hole.

it will be notched if its a large aftermarket cam

[Yamada]

I don't think you cam is the problem...
If I understand it right you are trying to make a atc90 run with a ytm-225dr carburetor. No matter how clean the carb is, the problem is the size of the carb and the jetting inside of it. I'm not a tuner by anyway but your jets are probably to big for this engine.

[ParrotGreenUS90]

I have talked to a few others and have come up with a decent set up. It will get a 22mm with a 78 jet in it. I found a nice set of instructions online last night. It is a 61mm Pauterbuilt big bore. I was reading that a 90 rod with allow 133cc of displacement and a 110 rod would allow 144cc of displacement. It is cool what is out there to read. I know I will not locate one of those setups in the near future but we now know of another set up. I asked on the chat last night about putting a 110 rod in a 90 motor and was told it would send the timing off. I do not understand how but I have not looked closely at the internals of a 90 and 110 in a manual to see. I am looking at the options of a Honda 90 Bike motor to do some experimenting. I have a CM91 in the shed and was looking at the motor it has. I was wondering if the recoil would mount to that motor as it has a plate covering the stator.

[SWIGIN]

the 110 rods have a larger piston pin so any of the 90 big bores will not fit the 110 rod.....just so you know

[Vealmonkey]

If he has cases notched to run that big of a cam, than that was a killer engine at one time. I haven't seen cams that big unless one was running a hand grenade on alcohol. You can finesse a pretty radical aftermarket cam into the stock hole. I would like to see some of his internals myself. That looked like a stock cylinder, not a pauter special cylinder. The big pauter cylinder had huge cooling fins on them and looked different than a stock cylinder. You can put a really huge sleeve in a stock cylinder and unless you look really really close you wouldn't know. If you run a cb750 piston, you have to get a custom sleeve for it unless you find one of the old sleeves which are damn near impossible unless you get lucky. A 90 head has to be way modded out to fit a cb750 intake valve and then lots of hand porting and polishing. Not for the novice. And skirting a cb750 piston is a major pain without a nice lathe and then you still have to balance the piston to get it down to sl90 weight or the thing will shake itself apart in a minute or so. And for a stoker crank, depending on how it's done with either the squished rod or a stroker plate, then you have to make sure the piston is clearanced for the stroker rod which could involve notching the cylinder and you have to clearance the inside of the cases. Another extinct piece is a heavy duty oil pump which I would gladly pay big money to have one or 2 more of. Depending on your cam is how you set your timing. Most hot cams are not set at tdc. Alot of them are set 4 or 5 degrees before tdc to help keep down predetonation. Stronger springs and custom weights for the timing advancer are advised in certain situations. You have to be very careful in your choice of valve guides, as the early leaded fuels did away with some of the problems of modern day valve stem and valve guide wear. 3 angle valve jobs are a must. A cb750 intake valve also has to be cut down to sl90 specs which can be a real nightmare. And if you are using a rather high compression piston, then you usually have to fly cut the piston top to clear the valves. I have a really neat home made valve flycutter for just such occasions. It's all hand built and hand fitted and that's with a really nice aggressively healthy cam. Some guys ran their own top secret grinds which was where alot of the notched cases came in. And it would do you a world of good to think of running alcohol in an engine like that. It takes alot more alcohol then gasoline to run, but the alcohol burn alot more cooler than gasoline and less problems with predetonation. Expect to run double wound springs and titanium keepers and retainers to help lighten the vavle train. Try finding titanium sl90 keepers and reatainers any time soon. Most people just don't realize how much work it is. I have custom lightened rockers too. And it's a fine line between lightened and survive and lightened and snap in 2 with double wound springs. Hardened rocker feet and rocker adjusters are a must. These parts were alot more common in the past and pretty much nonexistant now. It has taken me awhile to stockpile some of the bits and pieces I have. Thank goodness most people that have this stuff have no clue what it is, but you really have to search and hunt and just plain get lucky sometimes. Until you really pull your head and see what piston and stroke you are dealing with and what cam you have, you are shooting in the dark for a carb that might work. Good luck and keep us posted.

[SWIGIN]

veal, the ''small'' megacycle cam they still sell will not fit without clearanceing the head...im running that cam

[ParrotGreenUS90]

I am thinking hard about the cb750 setup. I may have the ability to get a cm91 bottom end soon. I know its not a sl90 bottom end but the cases are similar enough that I could experiment on. For now I will put your post in a word document for rereading when I get all set.

I do not have a Pauter big bore, I just found a set of instructions for them looking at Honda 90 pistons.

I was on the chat last night and mentioned that I have TONS of compression. It will about tear your arm off if you are not prepared. It makes the motor I have on my trx200sx seem weak. I will try and get a PSI this weekend. That made us think there is something inside waiting to make an appearance. I need to get a complete gasket set before I take it apart, it needs a gasket between the head and cylinder.

[bandito90]

Do you have any online links to any pages that have high performance 90 info?

[ParrotGreenUS90]

I have this and luckily found it again and saved it.

http://plan9.dnsalias.org:90/sell/PauterBigBore/Pauter_Big_Bore.pdf

[bandito90]

Thanks,Thats really informational.I think I have a carb similar to yours.Got it off ebay and it's missing the choke and all the pieces to the slide and the needle.Were having trouble finding parts for it though.There was a 20mm vm on ebay yesterday. that is for a hodaka

[ParrotGreenUS90]

You have a VM24 on yours? What type of machine you building with it?

I did the adjusting Oscar. It idled for about 20 seconds and died, figured I would not do anything until it has a minute of idling. It used to have the high idle now it seems somewhat but nothing like it was. I tried to start it a few more times , it popped and idled briefly so I gave it some throttle and it wanted to go but would not. I think I may have my reserve and regular fuel lines mixed up though. I will swap those and see what happens. I know there is fuel in the carb as there is fuel in the line.

[bandito90]

Heres a link to what I have so far <url>http://www.3wheelerworldforums.com/showthread.php?t=89111,</url>

[bandito90]

I have what is supposed to be a vm24.

Heres the link to my build.If it works this time. http://www.3wheelerworldforums.com/showthread.php?t=89111

[ParrotGreenUS90]

I took it out of the garage today. I flipped fuel lines and adjusted the mixture screw out 3/4 turn out along with putting a different plug in it. I pulled it a few times and it barked. Did it again and it idled for atleast a minute. I bump the throttle and it immediately dies or has to regain its idle. I got it running again and it has a high rev, if I were riding it would be in a higher gear its so high. I would estimate 2000 rpms as stock idle is 1500 or so. I had to find to hit the kill switch as it began dripping oil from the o ring between the head a cylinder. I hope the dealer has one or I can find one at the local hardware store. I think the o ring is standard and I am thinking of the rubber gasket for the cam chain, can you verify this Veal? I also think the choke set up will need a cleaning. Any ideas?

[Vealmonkey]

The o ring you should get from the dealer. It is not a standard o ring but it is special shaped believe it or not. Also one of your oil galleys should have the metal press in ring too. I don't know what size piston you have, but if it is an odd oversize or someone installed a sleeve, you may have a hard time finding a head gasket. But, as long as you have a copper head gasket and it is not creased, you can boil the old copper gasket in water and reuse it. When you take the head off, you might as well pull the cylinder and replace all the top end gaskets. Don't forget you will need new copper rings for the top cover on the head. Inspect your oil galleys in the cylinder and make sure they are clean and free from a bunch of dirt and crap. You may want to pull your oil screen out and clean that real good as well. The oil screen is behind the clutch cover on the righst side of the engine so you will have to remove the entire cover so you willl probably need that side cover gasket as well. Make sure and clean up any major grit and grime in the bottom of your engine since you will have the cover off. As far as the high rev or the engine dieing while revving it, you may want to look for a leaking intake gasket.

[ParrotGreenUS90]

I will locate a gasket set and take care of it all soon, wonder how much dealer would like for that. I did go outside and pulled it a few times and let it idle for a few minutes. I bumped the throttle a few times to hear the bark it wanted to put out. After a few tries of bumping the throttle and restarting it something happened after I put my hand over the air intake side and took it away. It now will rev up but it needs to sputter a few times. I got my ride out of it, more of a fight to keep the front end down. It is not 100 percent ready yet but it has potential. Any ideas what to do. Should I take it off and give it another cleaning and change jets? If so what sizes should I look for, 80 and 85 we mentioned for main jets.

[ParrotGreenUS90]

I need to know if a change in the main jet will need a new needle. I was on the chat and Oscar said I just need to get some other jets but did not mention a needle being part of it. I also need to get the idle jets.

[Vealmonkey]

A change in the main jet does not mean that you will have to change the needle. Also a variety of idle and main jets is a good thing to have. You might want to get one of those "Jet Organizer" boards while you are at it. That way you can keep your jets in a neat and orderly fashion and it makes it much easier when you are trying to jet the trike and you are much less likely to lose your jets. The needles are to help you really fine tune your carb, but most people are just happy getting the jetting close. Guys who were real fanatics about tuning or people who race trikes would often have several different needles since the tuning on a racing trike could mean the difference between first and second place though.

[ParrotGreenUS90]

Here is todays outcome. I went to a dealer that sells Mikuni parts and he would have to see the carb. I get home take it out of the garage and figure I'll look at it and see what happens. It fires and take the choke off and now it runs decent. I will see what happens as time goes on.

[ParrotGreenUS90]

I took it out Sunday and it took a ride on it. It still has some issues when first starting. It will idle then die. Once it starts it is fine but it will sputter sometimes when taking off. Should I look for a different idle jet? I do have a us90 header pipe that is loud. I read on the forums a crf 50 exhaust is a cheap mod for the atc 90, 110, and 125. Anyone try this?

[ParrotGreenUS90]

Been awhile since I last posted. I have been riding it. Starts on starting fluid and seems good now. I have not adjusted any of the jets yet.

[ParrotGreenUS90]

I took it Friday went for a ride and used it as a machine, I pulled my tractor from the backyard to the front yard. It is a Wheel Horse 700 with a plow that I needed to put a new motor on. I managed to pull it to the front yard in 1st gear hi range, I was impressed with the power. Any of my other 90's would have needed low range.
I was looking and thinking. I was looking at the stroke and bore of the atc110 engines. I know it is not worth installing a 110 crank unless there is something to be mounted to the other end of the rod. I plan on locating a kick start 90 engine with a clutch to do some modding to. I was looking at the 54mm kit and did some math to find it will put out about 113cc. I know that does not seem like much but a 23cc increase is like putting a 88cc kit on a 70 plus an extra 7cc. Is the crank of a 110 easy to install?

[shortline10]

Got 5 kits the pk racing is a 54mm106 kit and the big al's one in the green box is a 55mm 108 kit and the other 3 big al's in the yellow boxes are 54mm 106 kits . The big al's all seem to be cut for a stroker already .