Vintage Dirt Bike Q & A

Jess,

Yeah, the screws love to strip out - I've got a carb body here with two Heli-Coils in it. ;>( What I did with mine is to just buy up a number of OEM carbs on eBay and put together one good main one and one good spare. In the end, it probably cost about the same as a brand-new carb, but was also a lot less work (see below).

The problem with replacing the carb is that there's nothing available that drops right in, at least that I could find. The ideal Keihin 28mm replacement would be the PWK28 Air Striker (around $200 new), but it's 15mm too short and the mouth and spigot are both larger than the OEM carb (the Keihin PE28 is pretty much the same deal). This probably seems like a little thing, but after wrestling with the bugger 20 - 50 times during jetting you may feel differently. The Mikuni VM27.7-74 (apparently the VM28 has been discontinued) is 5mm shorter than the OEM carb, but the spigot and mouth diameters are identical. It won't be easy to replace either the intake manifold or air boot unless you go the duct-tape route :>D - and don't forget the throttle cable! Rather than trying to find a (discontinued) Honda cable to fit the 30mm Keihin and OEM throttle on my 'CR175', I went with a Suzuki right-angle throttle and cable instead.

Bear in mind that with a Keihin, you'll at least have the OEM jetting specs (and my specs, which work even better) to start from, whereas with a Mikuni you'll have to use conversion tables, unless you luck out and find specs on the Web, or hunt through the parts diagrams for 175's using 28mm Mikunis (there's probably at least one).

What specific parts on your carb (other than the threads) worry you the most? Other than jets, most replacement parts have been discontinued.

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

Jess,

I was just reading back over the thread postings and the excellent advice mitchie gave you regarding the tank rust. This really screwed up my carb operation, until the bike wouldn't run at all. I then pulled the carb off and found it full of rust chips.

My tank was completely rusted inside. I didn't try the gravel thing, but I scraped, brushed, and power-washed the hell out of it and ended up with only fine flakes of rust in the gas, which I figured an inline filter would take care of.

Wrong. ;>) The petcock got clogged before the gas even made it to the filter. I ended up soldering a brass-screen "teepee" over the petcock snorkels which kept the medium-sized stuff out of the tiny petcock passages and on/off valve, then watched my inline filter fill up with brown powder. The brass petcock screen above the petcock bowl didn't trap much at all, although some fine rust would fall to the bottom of the bowl.

After all that, the petcock started to leak where it joined the tank, and I just couldn't get it to seal - so I bit the bullet and got a Clarke tank.

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

well i found a carb on ebay thats been totally redone, that i'm hoping i can get for under $100 i did what mitchie suggested with the rocks and WD40, and i just have fine rust in it now.
of course all the gaskets are shot on my carb, it wouldn't stop leaking gas out the overflows, and as i mentioned earlier the screws are stripped... i'm hoping i can get this carb off ebay /c its for a 74 cr125 and it should drop right in to my 75 mr i'm just afraid i'm gonna be outbid

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-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

Jess,

I've bought stuff from the guy in Utah before - no problems at all. That '74 CR carb looks almost new.

There's another auction for a '76 MR175 carb (#180348808828) with a Best Offer option. I've had my last two offers accepted, so you might try it and see what happens. The carb looks horrible on the outside, but the inside looks fine.

Just FYI, I bought an MT250 carb for $30, two 30mm '77 CR125M carbs for $52, a 34mm CR250 carb for $21, an MR175 carb for $19, and a '74 CR125M carb for $18 (all prices include shipping). A couple of these carbs were in great conditon or were easily made so, but most were in typical (i.e., not so great) shape. It seemd like top dollar was being asked for the perfect carbs, so I went the "quantity, not quality" route instead.

Also, try using 8-32 USA screws in your carb, and if they're too loose, find some 10-32 self-tapping screws, which will be MUCH stronger than the originals (you may need a 10-32 tap if the holes aren't stripped too badly).

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

what about this one?
http://cgi.ebay.com/ebaymotors/Honda-MR ... ccessories

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

Jess,

That one looks fine! I've bought from that guy before, too (no worries).

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

hmm.. now the decision of, wait and bid on the better looking one, or buy the dirtier one right away... since i'm saving for a car i think i'll go with the dirtier one and save some dough

EDIT: changed my mind. i think i'm gonna try and get the newer one, just to make sure i get something good if i get outbid, i'll buy the old one if its still there

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

does anyone know where i could get a kick start lever?? the thread on the end of mine is torn half way off, and stripped srry the pic is kinda blurry

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

welp, i got a new keihin 28mm PWK for $44 today i'm gonna put it on saturday and see what happens

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

well, after some work on the throttle cable, we put the carb on, and after a few pushes it started right up, idled ok, and we rode it around the yard just fine the only problem is it overflows so we're gonna take the carb off tonight and see whats happining. i'm just happy its running again

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

well we got the overflow problem fixed. (the float just needed adjusting) and now we have the problem of the rust in the tank clogging the petcock especially since the snorkel is broke off of it. got some pics of my $43 D-slide carb. and the airfilter i hope will work BTW ray. which Clarke tank did you get? one for a CR, or something else?

Thanks, Jess

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

Jess,

That looks like a nice carb!

If that air filter doesn't work, Honda P/N 17212-358-000 or UNI P/N NU 4026 definitely will work with the OEM air filter setup. The stock air box uses a foam sealing ring around the bottom and the Honda filter has a stiff plastic ring that buries itself right into the middle of the foam, and every time I've pulled mine out for cleaning I've found zero evidence of dirt leakage.

I use the Clarke 1362X tank, which holds almost 3 gallons and mates up well with the MR seat. A CR tank will look really skinny, and may not be long enough either.

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

Hey thanks Ray! i almost forgot. i got a new kickstart lever from a guy thats got a couple hundred junk bikes in his yard. and its near where my dad works:) so there's a filter i can get that will fit in the stock air box?

Jess

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650

Jess,

Yes, there's actually two filters available that will work.

Ray

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

does the tank fit on with no modification?

Jess

_________________

-----1975 Honda MR175--------------------------------------1972 Suzuki T500 Titan------------------2003 Honda 250 Rebel

Yamaha XS850
Triumph Bonneville 650