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PostPosted: Fri Sep 25, 2015 10:48 am 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
So my father and I have decided we want more power, that's lead me down the road of doing some porting to really get it going. I've been reading about this, finished 2 Stroke Performance Tuneing and 2 Stroke Tuners Handbook. I've also been reading this place nearly obsessively, I think I've read everything I can find on here about it.

I know the easy way is to throw a CR everything on. but that is $$$$. I will have nothing but time this winter, not so much money. Here's the Plan I have rolling around in my head-

Ring job
Build Pipe
Raise compression a bit, still needs to run on 92.
28 carb, Might use a Mikuni or a PWK 28.
Bit of port work

Is it safe to turn down the flywheel a bit? Just take a little weight off of it on our lathe. We're keeping the External Flywheel/points.

I know that none of the Pro porter's share spec's, so has anyone else done this? I don't want razor edge power bands or ultimate power( as much as I love the sound), just a livelier engine.


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PostPosted: Fri Sep 25, 2015 3:10 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
The MT125 cylinder's porting is nearly identical to the MR175 (the intake ports are different). I've nearly doubled the power of my MR175, with a wider powerband than stock - and many MR175 mods are directly applicable to the MT125 motor. If you search my username here, you'll find a bunch of MR175 info.

If it were my bike and I was on a budget, I'd

1) gut the MT125 pipe (both the center baffle and rear spark arrestor section) and use a clamp-on silencer on the end; this setup made better power on the MR than any aftermarket pipe I tried
2) replace the MT125 head gasket with a thinner copper CR one
3) install a CR125M ignition, for a big low-end power boost, ultra-quick throttle response, more reliability, easier starting - the list goes on

The porting below should work great - use only the transfer and exhaust height measurements, obviously the widths will be smaller on the MT cylinder:

http://s399.photobucket.com/user/rayive ... l.jpg.html

Ray

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


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PostPosted: Fri Sep 25, 2015 6:03 pm 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
Fantastic, thank you.

What sucks is that I had the exhaust cut open once, to clean out a huge mouse nest.

And then welded everything back up. :( Left the baffles and crap in.

Oh well. cutting wheel's and wire is cheap.


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PostPosted: Sat Sep 26, 2015 7:45 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
No problem at all - here's a shot of the pipe internals I took while mine was open:

http://i399.photobucket.com/albums/pp74 ... 0small.jpg

The rear pipe wall near the air filter box should be reinforced a bit, it's real thin and mine blew out. I welded on 3 strips of sheet metal across the back & it's been fine since.

Have you ever tried the 'ring of fire' 2-stroke pipe-cleaning technique? I soaked the inside with gas for a while, poured that out & lit it up with the acetylene torch, then after it was burning real good I switched to pure oxygen and pushed this orange heat ring through the pipe metal, all the way to the stinger. At the end the pipe just had powder ash inside, not a trace of oil or carbon. Unless those mice were using steel wool, that nest would be history. :)

Re the CR cylinder/head - sometimes they show up on eBay for a decent price, especially if they need boring. It would save you a bunch of porting time compared to the MT jug. The '75 is the one you want; it has the boost port and uses the 28mm carb/manifold, and also has the thinner liner that fits the MT cases.

Ray

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


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PostPosted: Sat Sep 26, 2015 11:53 am 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
I tried burning it out, but I only have a Mapp torch at the moment. All it did was stink and smoke.. Been hoping to get a Oxy/Act torch used, but there arn't many around.

How much of the Spark arrestor tube should I leave as a stinger?

I got a smoking deal on a carb from a CR125. So I'll use that if it's in good shape.

Are there any new intake manifolds for sale? Ive seen a few on eBay, but I've lost out on the few I could afford. May just have to keep cobbling one together if I can't find a cheapish one.


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PostPosted: Sat Sep 26, 2015 12:36 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
I left 3.25" of stinger, but added @ .75" of weld bead to it at the stinger/pipe body junction. I ended up with about 2.5" usable engagement for the SuperTrapp silencer I used. I also drilled out the inner stinger tube with a 23mm drill, which ended up grabbing and spinning the tube out leaving the outer tube intact. The silencer is supported by it's own vibration mount, so there's very little load on the tailpipe.

If it were me, I'd keep the 24mm carb/manifold until you can find the proper '74-'75 CR125M intake manifold and open up the intake passage to match the manifold - otherwise, the 28mm carb won't give any more power and won't run right above 3/4 throttle.

This manifold's a pretty decent price for NOS:

http://www.ebay.com/itm/HONDA-NOS-CARB- ... 92&vxp=mtr

Ray

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


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PostPosted: Sat Sep 26, 2015 1:51 pm 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
When you say open up the intake to 28mm, so you mean the inner porting passages, or just the intake opening on the cyl? When I had the carb off, I measured the intake hole on the cyl at 28mm wide( I was expecting it to be same size as the carb throat).

I'm amazed that I didn't find that before. I almost paid that much for a used one yesterday!


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PostPosted: Sat Sep 26, 2015 3:20 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
The intake tract both upstream and downstream of the carb venturi (slide-tunnel area) has to be at least slightly larger than the venturi itself. A good indicator is the I.D. of the carb throat downstream of the slide; the 28mm Keihin throat is 29mm, the 30mm is 32mm, etc. When I bored out the 28mm venturi on my MR175 motor's carb to 29mm, I also opened up the throat, manifold, and cylinder intake passage (including intake port windows) to 30mm.

So, to answer your question - the intake opening of the MT125 cylinder should be enlarged to at least 29mm O.D. (that NOS manifold will be fine, you could use it for matching if you want) and down the inner porting passage until it opens up wider than 29mm (it may not). If you want to be sure about the port windows, you could measure them and I'll double-check in my engmod 2t design program and tell you their area (it's a complex calculation involving both down and side passage angles). I have exact data already for the CR125 and MR175 motors, but not the MT125 (the MT intake & exhaust angles are roughly identical to the CR, but all MT port windows are smaller).

Ray

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


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PostPosted: Sat Sep 26, 2015 4:27 pm 
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Joined: Thu Sep 17, 2015 11:42 pm
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Thank you again.


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PostPosted: Sun Oct 04, 2015 10:26 pm 
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Joined: Thu Sep 17, 2015 11:42 pm
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We'll I'll have the ability to now test a gutted mt vs a stock cr down pipe. In my post surgery stupor Saturday I bought a used pipe from a 75cr125 online . :lol:


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PostPosted: Mon Oct 05, 2015 5:46 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
That should be an interesting test. The stock CR pipes worked pretty well, but everyone (including myself) replaced them anyway. :)

Ray

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


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PostPosted: Fri Oct 09, 2015 9:14 pm 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
Got fed up with the constant issues and poor running from the 24mm carb, Nothing but fouled plugs and flooding. So I swapped the 28 on. I also modified the CR down pipe spigot to fit the head and wired it on for a test ride.

HOLY **** it's quite a lot faster now. Rev's fast and crisp, and when it comes on the pipe it will spin the tire on the gear changes. Not much bottom end still, but it's a screamer:shock: I love the sound of the CR pipe, lucky none of the VMX races use a sound meter, because it is LOUD!. The PNW offroad race series is pretty strict about sound meter readings, but I don't think this bike will be doing those with the low pipe.

Next up is the CR copper head gasket and jetting properly.

I also discovered that this bike has ether been rebuilt recently, was never abused or has not ran very many hours, the Piston looks brand new and the bore is immaculate.


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PostPosted: Mon Oct 12, 2015 8:11 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
At this point I'd strongly recommend putting in a CR125M complete ignition, which will complement all the other things done so far and really wake the bike up.The CR CDI box adds 12 degrees advance down low which beefs up the low end big time, and the strong, long-duration spark (which can be set super-accurately) results in a quick and complete burn, every time. The points ignition just feels so weak amd characterless by comparison.

Ray

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


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PostPosted: Tue Oct 13, 2015 12:17 am 
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Joined: Thu Sep 17, 2015 11:42 pm
Posts: 15
How's the reliability on the OEM CDI unit at this age? I've been looking at them on ebuy. In some way's I like the points setup, it should just keep on working, while I've had experience with 35 year old UJM electronics not being so hot anymore. I've seen the mew aftermarket setup, but I don't even have $500 total in this bike... not going to put a $500 ignition in it. :shock:


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PostPosted: Tue Oct 13, 2015 3:09 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Well... I ran a CR125M ignition for ten years back in the 70's, I've had two of them running on my 175's since 2008, and a buddy took my suggestion and has been running one in his MR175 for about 4 years now (he's delighted with it, unlike the expensive aftermarket ignition he had before which had no low end, like the points ignition). Not one of the CR ignition setups has ever given a moment's trouble. With a little patience, I've gotten complete ignitions (stator, rotor, CDI box, and coil) on eBay for $120 shipped. The MR coil should work with the rest of the CR ignition, but I've never tried it.

On the other hand, I've tried to make three separate MR175 points ignitions work reliably, with no luck. They work when new, run for about 1,500 - 2,500 miles, then usually start misfiring / double-firing before they die.

Ray

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


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