Vintage Dirt Bike Q & A

how do other people deal with obsolete carbs on vintage bikes? I've got two bikes with obsolete carbs and am frustrated with how to rebuild or replace them. Are some of the inside part #'s the same across various carbs? Are their adapters to upgrade carbs? I'm not too keen on buying carbs on ebay, as their apt to have a lot of the same problems; worn float valves and floats that don't float.

I love vintage bikes, but I'm almost ready to throw in the towel on them if I can't learn to deal with the carb problems better. I live above 7,000ft elevation in the Colorado Rockies, so that ads to the frustrations.

I posted similar but more specific question in Yamaha, but see that there's about 10 times as many Honda posters here and my question is so general I'd like to here from as many people as possible. and learn as much as possible. If I can get over this hump I'll be doing great with these bikes as there's not much I can't work on, so long as I can get parts.

Thanks

I've rebuilt 3 old Keihins recently, and believe me, I feel your pain. In my opinion, the pros of using the original carbs are that everything fits and the jetting is already pretty close; the cons are parts availabilty, wear issues, and low tech (although the carbs I've worked on perform very well).

Without knowing more about the carbs you're working with, I can't really offer much specific help. I will say that any carb with high hours on it will most likely need a new needle jet to work really well, especially at high elevations. I'm real picky about getting the bike to run crisp on all circuits, though.

If you're dealing with Mikuni carbs, you have it a lot easier. Not only do they still make the VM vintage-type carbs - which in many cases are drop-in replacements for OEM - but more parts are available in general than for the old Keihins, and are easier to find.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I've got an old yamaha gt mx 80 with a tk carb, an oddball. It looks a lot like the mikuni 18, maybe that's what it really is. I'm in process of measuring to see if mikuni 18 will fit.

Also, got an '84 kx 80 with a mikuni 29, which it turns out they don't make anymore. Unfortunately, inlet, outlet, and bore are all different sizes than the VMs mikuni still makes.

Also, have a 97 cr125, but I assume that carb is still available.

Love to just replace the yam and kaw carbs with something new/newer, so I'd be able to rebuild any time.

Even if I got newer bikes I'd still like to get these older bikes running well because I live next to BLM land and can ride out from my house, so I'd like to have some extra play bikes handy for whoever might want to ride along. I'm wanting to get a on/off road bike for myself and am interested in a big xt, xl, or kdx but don't want frustration of parts problems I'm having now. I pretty much know how to fix bikes and like the styling of older bikes, not to mention that power to weight ratio was actually better for a lot of them compared to newer bikes. But you get nickled and dimed to death on shipping and premium for obsolete parts on ebay.

When I got the kx 80 for my son I thought I wouldn't have much problem getting parts for it since it was a popular race model, then the carb turns out to be obsolete. Spending more time with bikes down, than running. Need extra bikes around just to keep one running while others are down.

What about adapting intake / reed boot to newer carb, remove factory air box, and running k&n right off carb? Any ideas on how to do this?
Anyone make such adapters?

It's really pretty easy; pull off the air box, find a K&N or Uni filter that fits, heat up the intake boot with hot water and push the new carb in (I fit a 43mm spigot into a 35mm manifold this way), then hook up the throttle cable and see if it works. I found throttle cables to be the biggest stumbling block, but you may have no problems at all. I think an adaptor would mainly be needed if you were trying to make the new carb fit the stock air box.

If you're replacing a Mikuni with another Mikuni of similar bore, you can start with the original jet sizes and work from there. If not, a jet conversion table may get you sort-of close to start, then the jetting adventure begins.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for the input. Really appreciate it. Was helpful. I dimensioned my TK and comparing it to the modern Mikuni's it turns out that the VM16, Mikuni's smallest, is the exact same dimensions all around. Think it may be intended as a replacement.

I've been working on a '77 GTMX 80 on and off for the past month or so. This bike was in dumpster-ready condition when my friend got it, but now it actually runs OK, considering it's probably about to puke a ring. It had been run without an air filter since around the 90's and the (original) carb was more sand-chewed than I'd ever seen, but a 32 pilot and 90 main jet worked wonders. I think it's a 14mm clamp-mount unit. If this garbage carb could be revived with jetting, damn near any uncracked unit probably can be, provided you can live with a rich condition around 1/4 - 1/2 throttle or don't mind grinding the slide.

On your bikes, I take it you've tried rejetting all of them and had very poor results? If so, did you use an altitude nomograph (percentage-leaner chart) to rejet?

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

We had all sorts of bikes when I was a kid in the 70s and 80s. My dad used to buy junk, fix em up and sell them. I helped and got to ride whatever we had around that was running and had my own bikes. We'd usually have 10-20 bikes at a time. Lot of fun. I got away from it and now my son is ten and am getting back into it with him. Hence my pension for the old bikes. Just burned a piston on 84 kx 80 and have to have it bored.

I went ahead and ordered mikuni carb for gt80. You had good advice, except it costs $80 an hour for mechanics and new carb was about same. I've had too many bad experiences in the past with mechanics not really fixing problems properly, charging a few billable hours, and I'm stuck with a couple hundred dollar bill and not much of an improvement. So I'm trying to get back into doing my own repairs. Or at least knowing exactly what I need from a mechanic vs just handing over and saying fix it. The latter is a good way to get taken advantage of.

I do know cars and have worked on a lot of carbs, but no expert or anything. The GT has just been sitting around for a while and since the kx will be down for a few weeks a thought I'd get the gt running again. If the mikuni works out the advantage would be that I can get parts for it easily. I've bought a lot of parts from ebay and disadvantage to that for the old mini bikes is that you get nickled and dimed into spending more than these bikes are worth. $30 here, $10 shipping, do that more than a few times and you've got a couple hundred bucks for used parts. Adds up pretty quick. I've got too much into the gt80, fussed around with it off and on, get frustrated with it, put it away, and it's still not working right. My kids spending too much waiting for me to fix things and not enough time riding. So I feel bad and if I had the money I'd walk into a dealer and buy him a new bike today. But their great little dirt road bikes and go a really long time on a tank of gas. And as I'm starting to get things figured out again I'm perusing craigslist and drooling over some more vintage bikes when they can be had cheap. Not too smart when newer used bikes only go for a grand.

I just bought house on BLM late last summer, then 4 months of snow. Everything's thawing and drying out now and I want to do a lot of riding with my son this summer. My CR125 is fine for riding with my son, but not a lot of range and as he gets better and I'm getting more back into riding myself, I'm finding myself wanting a big on / off road bike, to take up the old logging and mining roads, farther range, fishing, etc... Thinking about fixing up a Yamaha DT / XT for longer rides. I'm kinda partial to the old Yamahas.

The CR was already jetted properly. KX not sure, but will be sorting that out as I rebuild it. And will have to get GT properly tuned as well. I'm just getting to point of needing to deal with that stuff.

Thanks.

Sorry if I gave the impression that I thought you should take the bike to be serviced or jetted by a mechanic - far from it, especially at current hourly rates (in my area it's around $110/hour). In my opinion, jetting is something that practically anyone can do at home. It takes patience and making lots of notes if you want to get it perfect, but improvements can be had for little money and time provided the symptoms are correctly described. As you know, the motorcycle carb tuning situation is quite different from older auto carbs, where you could just go out and buy a new Holley or whatever, drop it in, and have it work OK from the start. A motorcycle replacement carb will almost certainly need tweaking and/or rejetting, unless you're very lucky. Thanks God for the Internet, with sites like this one, ThumperTalk, and many others full of info and (usually helpful) advice.

An old Yamaha isn't a bad choice, as Yamaha still makes lots of parts for the older bikes - I was surprised.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Here is a good chart to convert jet sizes to and from Mikuni and Keihin. The Amal type jet is the same as the Mikuni. Check it out. I've used the chart and it seems to work well. Would like more info on the topic if anyone has any insite. I would like to know if the chart is ledgit or what.

Used it when I converted my 1979 cr125's Keihin with a 32mm Mikuni vm roundslide. They were both 32mm. Went from a 155 stock keihin main to a 300 mikuni and the chart said the same. I wish I had found the chart before I tried 4 or 5 jets.

Just converted an old yami to go from a Mikuni stock to keihin "style" replacement. Used the chart to start my jetting and it's running good first try.

nola_fiddy

I think that's a good chart. A while back I compared the jetting of a bunch of 125cc and 175cc Jap bikes using Keihin and Mikuni carbs, and as I recall that chart corresponded pretty well to the OEM jetting sizes (I had the same info in a Word doc). There are other charts floating around the Web that aren't even close.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for the info Ray. I thought the chart was going to check out ok. I have a few other charts as well, and they are useless. This was a lucky find. I will have to find a pdf to something converter so I can upload the whole article to this site. No pdf's for some reason.

I have only done a couple handfuls of rebuilds and vintage resurections. Some carbs are so hard to get parts for that I like the option of buying a modern keihin or mikuni that mimics the oem closely as possible. I try to use the carb that came on the bike if possible. I sure wasted a good bit of money trying in vain to get a few of these working well.

nola_fiddy