Vintage Dirt Bike Q & A

All,

Need some tuning experience, from you the experts, I am close to dialing in the now 978 mile "75" MR175-A to its original power of 13.91 HP at 7500.......but its still not quite right :
_______________________________________________________________________________
Today ........I got rid rid of the original points and put in a fresh set.


Summary:
- Fuel system in spotless I mean it, derusting and then Kreeming the inside of the tank, boiling the carb in carb cleaner, putting new carb gasket kit its ....clean clean clean. Float set to exactly 20MM for both floats. (Thanks for the diagram Ray)
- I installed a new OEM coil and condeser
-Spark is excellent, Rotor has no rust on the magnets, stator pickoffs were polished slightly with high grit sand paper.
- I put in a new spark plug in the side hole (easier to get to)
- Next thing was I got rid of the original points and put in a fresh set. I started by setting the gap to 0.016 just to get it started again and see where I was with the timing light. Timing light at ~1500 RPM showed the timing just before the F mark and its associated timimg line. Not satisfied I tweaked the points to ~ 0.015 and now the strobe showed me the mark right at the F mark but not the associated line. Good enough it popped right off and idles just perfect.
- I set the carb at about 2 turns
- Fuel mix is 20:1 using synthetic racing oil
- Today it was `58 degress and humid with some rain
_______________________________________________________________________________

Now you have the history here is my problem..........I let the bike warm up. I try to get high RPMs on it, (low RPMs are smooth as can be) its want to cut lose but it sputters starting at 5000 RPM.......never really wind up

Heres my queestions:
1) I am happy with the timing but maybe my hapiness still needs tweaking. Whats the word from you tuners do I still need to dial in the timimg?
2) Could I have a semi plugged exhaust system. How do I check it, take the exhaust system off and see what happens with it off?
3) Do these engines have reeds? I checked all the micro-fisch show no picys of reeds and none of them list any?
4) The final question..........I have just rode it around the street for the last 3 miles tweaking this and adjusting that........do you think I need to get the new IRC IX05H knobsters on it (100/100-18 rear and 80/100-21 front) and just ride it through a tank or two and see if it will come out of it?

Thanks again

Larry

Larry,

Based on your last post, here's what I would suggest:

a) A 20:1 premix ratio is way too rich for modern synthetic oils. I use Bel-Ray H1R at 36:1 - very little smoke, no pipe ooze, almost no wear - no complaints whatsoever (except maybe price, but scrimping on oil has always seemed like a bad idea to me).

b) I would put the spark plug back in the center hole, and get the correct sealing bolt from Honda (they're still available, P/N 90061-358-000) to seal up the side hole. The combustion chamber is designed for a central spark plug. I've had excellent results with the NGK BPR7EIX projected-nose Iridium plug in this motor. I've had my head machined for a conventional (dome/squish band) combustion chamber, I hope to try it out today (it was already milled .020" for more compression, which made a big difference in low/mid power).

Regarding your questions:

1) No. Your timing sounds like it's spot-on, I would resist the temptation to mess with it. By the way - my bike uses the CR125 ignition, which just transformed the motor; it revs instantly now.

2) Absolutely - in fact, the stock spark arrestor is fairly restrictive even when new (I have a NOS pipe that I put back on the bike last week, while my modified exhaust was being worked on). I drilled out the center of the spark arrestor 13mm (about 1/2"), it made a noticeable difference in power across the whole rpm range without raising the noise level really at all (at least I couldn't tell the difference). To drill it out, you unscrew the two 10mm-head bolts from the removeable baffle and pull it out, then drill out the center cone of the spark arrestor insert at the front of the tailpipe tube; I would use several drill sizes to do it in steps, and you may have to chuck the bit a little shallow to allow max drill length to go all the way through the cone. You can also clean out the spark arrestor outlets with pipe cleaners, but this is less important once the hole's been drilled.

3) No.

4) Street testing and dirt testing are two different things. I use the street to fine-tune the slide and needle circuits as those are being used more then, but in the dirt I find tuning is mostly about the main and pilot circuits. Your experience may be different, though.

I didn't see any mention of the most important thing - carb jetting. I spent 6 months and maybe $150 jetting mine, you should be able to get the same results for about $20 (48 pilot jet, 118 main jet, needle at #3/center clip position should do it - I also use a 3.5 throttle valve in warm weather, but you'll have to search eBay for months to find one). Or you can leave it stock for that sluggish, smoky, sputtery feel. :>)

I think a lot of what I've posted I've mentioned before in other posts, but here it is all in one place (again?).

Ray

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

Ray,

Again thanks for the wealth of info I went scouting on the web last night for a CR125M CDI igniton no luck so far. A Complete OEM CDI (not available) sure is exspensive at 700.00. But Ill continue my search. Any advise on who might make an an aftermarket CDI?

The bike is in for new tires today. I have the flu,crud whatever, so the guy said he would install them for $10.00. I can't beat that considering I dont know how much trouble the bead locks may be.

Tommorow Ill tear the exhaust apart. And do some drilling on the park arrester. Depending on what I find I think its the culprit. Especailly after sitting for 16 years with no running. I imagine the packing is plugged also.

Ill put the spark plug back in the center as you suggested. Juts had it there while I am dialing things in. You went out of way to include the part number for the sealing bolt very much appreciated!

The oil I am using is Red Line racing synthetic. I left the it at factory specs 20:1 until I get a full tank through it. Ill want to ensure the upper is lubed and if anything breaks loose (carbon deposits anything) Ill have a little extar protection.

I dont know if this will help anyone but I did fined an aftermarket air filter that seems to fit like the original. UNI makes it and the part number is NU-3225ST Yamaha. Ill post pictures later.

I was holding off on the jets until I was sure the ignition was dialed in.

Never considered dirt vs street as a factor in dialing it in......more good info!

Again thanks......

Larry

Larry,

Hey, no problem - it feels good to share this stuff, like maybe all the mistakes I've made working on this thing weren't for nothing after all. :>)

I looked over my notes and apparently I've gotten 3.5 complete CR125M ignitions off eBay, all in working order. Prices ranged from a low of $68 to a high of $120 for complete ignitions including shipping, but piecing one together from separate components will almost certainly cost more. There's also the PVL aftermarket ignitions (http://www.pentonracingproducts.com/application.htm) which look real nice.

Thanks for the air filter info. The UNI I got from the New Zealand guy is marked "NU 4026 / HONDA MR 175". That's an excellent price for a tire change too!

One thing I'd recommend would be to finalize your ballpark premix ratio before starting jetting, otherwise you might have to go back and do it all over again - and I wouldn't wish that on anybody.

Ray

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

On Ebay right now. Item # 150334650543. Complete 74-74 cr125 ignition system - CDI, stator, rotor, and coil. The seller claims it tested perfectly before storage and will only sell the ignition as a complete system. If you do end up piecing it together, I have a couple of used stators and a rotor that I would give you a deal on - in real good shape. Your timimg is perfect for that Ebay listing.


Mark

Mark,

Thanks for the link - Larry, check it out! Definitely a '74 ignition (flat-face rotor, huge plug cap) in pretty good shape, just a little dirty. The plastic covering on the wires looks great.

Far better to get it all at once, if you can! You'll lose your lighting, but the motor will run like it's had a double espresso.

http://cgi.ebay.com/ebaymotors/ws/eBayI ... &viewitem=

Ray

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

Ray, is the ignition swap just a "bolt on"? I have the parts laying around & was thinking about doing the swap when time permits. (Two 1972 CB350's followed me home last Sat.)

_________________
77 MR175, 89 HD Sportster
74 CR125, 00 Kawasaki ZG1000
79 XR185, 56 Cushman Eagle
03 XR650L, 79 Vespa P200E
72 Vespa Super150,
74 HD Z90

Jim,

Well, kind of... the stator and rotor go right on, but the CR coil uses different mounts, and the CDI box needs a home.

First the flywheel comes off, followed by the MR stator and cable. The CR stator fits the MR cases perfectly, but uses hex bolts instead of flathead Philips screws as it's slotted for timing adjustment. The rotor also goes right on (be sure to use a new Woodruff key if you can, all the ones I've seen have been damn near sheared in half by the flywheel). The MR crank nut works with all years of CR125M rotors. While you have the stator off, it would be a great time to replace the left crank seal and inspect the bearing if you haven't already.

I just looked under the tank with a flashlight, and it appears I mounted the CDI box vertically with the top bolt into the rear MR coil mount and the bottom bolt through a hole drilled in the frame cross support. The coil I also mounted vertically (plug wire down) on a piece of aluminum angle, with a bolt through the angle to the front MR coil mount (top) and another bolt through the angle to a clamp around the cross support (the same clamp used on the rear of the skid plate, I had a spare). It was a little work, but came out great, and everything's hidden deep inside the tank and safe from harm. There's surely a bunch of other ways to do it as well.

On my bike, the timing is exactly 20 degrees (1.87mm) BTDC with the CR stator backing plate line aligned with the MR case arrow - you can also align the CR stator coil lamination line with the CR 'T' rotor mark at TDC.

Ray

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

Well, after scrounging through my big box of "treasures", I came up with everything but a coil. Hafta keep looking. Can the MR coil be used? Thanks Ray.

_________________
77 MR175, 89 HD Sportster
74 CR125, 00 Kawasaki ZG1000
79 XR185, 56 Cushman Eagle
03 XR650L, 79 Vespa P200E
72 Vespa Super150,
74 HD Z90

ALL!!! Thanks I hava bid on the1974-75 HONDA CR 125 COMPLETE IGNITION SYSTEM, NO#150334650543.

Ray here is my current jetting, ........pilot (slow) jet #32, ........main jet is #122 and the needle is on the #3 positon. As for the throttle vavle I think mine said 300??? does that make sense?

I will go to the bike shop and see about the pilot and mains.

Larry

Jim,

I believe the MR coil can be used, which would mean you'd only have to mount the CDI box.

Larry,

I'd double-check that pilot jet value - I'll bet it's the stock 52, which is what I've found in every one of my carbs when I opened them up. A 32 pilot would be so lean you could run the air screw all the way in and still be too lean. The stock throttle valve is 3.0, and I believe the stock main jet was 122 for '75 and '76 and 118 for '77 (I've only found 122's in my carbs). Stock needle position is #3.


Ray

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