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PostPosted: Sat Aug 18, 2018 4:57 am 
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I never had very good succes with the old carb on my '74 CR125M. Clean it many times, tried different jetting. So my next option is to replace it.

What can work fine on those bike with stock engine, Mikuni 30mm? Is there a good place to buy a new carb and intake manifold?

I found those items:

Manifold: https://hotfootmoto.net/store/product.p ... 256&page=1

Carb: https://fortnine.ca/en/mikuni-vm-series ... mm-vm30-83

The place for the carb is local near home.

If the Mikuni 30mm is good, what jetting should be use?

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Wed Aug 22, 2018 6:48 am 
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Location: Connecticut, USA
IME the stock '74 CR125M carb works very well UNLESS the needle jet is worn (nearly all of them are, unless they've been replaced or the bike barely used), then the bike will run rich below 3/4 throttle regardless of the other jets, within reason. Compression also has a big effect on jetting (low compression = runs richer).

The OEM '74 intake port only has enough flow for a 29.5mm carb, so a 30mm won't give you more flow and won't pull fuel well through the main jet at WOT.

If you can find a 16012-358-xxx / 16012-360-xxx / 16012-373-xxx needle jet set, that's what I'd use. The Keyster KH-0137 carb rebuild kit has a slightly leaner needle (which might help a bit), but no needle jet.

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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


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PostPosted: Wed Aug 22, 2018 7:33 am 
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I didn't have lot of free time to tune it but I tried many jetting and nothing work. I did this rebuild a few years ago and I never really had a chance to ride it. When I tried to make it work 2 years ago, the bike started and idle fine, thanks to your good advice, but as soon as I try to ride it, it die. Since it run fine at idle, I supposed the problem is with the main jet or needle set. Is it right?

The carb is a 005A and I used the kit below to rebuild it. It doesn't have the details info of each component.


Attachments:
carbKit.JPG
carbKit.JPG [ 52.68 KiB | Viewed 101 times ]

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i
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PostPosted: Wed Aug 22, 2018 8:50 am 
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Location: Connecticut, USA
What do you mean by "die"? Does it blubber and smoke, then stop? Or does it make a 'snoring' sound and stop? Can you rev the engine in neutral? How many psi compression do you have? Sorry for all the questions.

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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


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PostPosted: Wed Aug 22, 2018 9:44 am 
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It's more like a 'snoring' sound and stop. Yes I can rev the engine in neutral.

I have new oversize piston and rings and have it borred at a very reputated machine shop around here. New bearing and seals.

My compression gauge is not very good, but it indicate around 100 psi. From the kick start, there's a very good resistance when kicked.

That's ok, for the questions, any help is really appreciated.

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Wed Aug 22, 2018 10:19 am 
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Location: Connecticut, USA
100psi compression is very low (your bike was probably 150+ when new, and IMO high compression for a vintage engine is around 170-180psi; my MR175 motors are both 175 psi). An engine with very low compression will start easily and idle, but it won't pull much (if any) of a load, it will just stall.

If it were my bike, I'd remove the head and see if you can move the piston sideways in the bore (in a correctly-sized new bore there should be basically no movement at all). I'd also check for evidence of head-gasket leakage. I gave up on the Honda copper head gaskets and had my head machined for the MT125 composite gasket, I could never get the copper ones to seal well at all. You might also want to make sure the piston wasn't installed backwards (a friend of mine did this recently- it's easy to make a mistake).

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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


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PostPosted: Wed Aug 22, 2018 12:05 pm 
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There's no sideway slack on the piston, it is really tight every where in the cylinder, and it is not installed backward.

But you maybe right about the copper gasket. I removed it and can see some oil underside of it, betwen that gasket and the cylinder. I'll try to find a new copper gasket. I don't remember if I replaced it when I rebuild the engine but this one doesn't look like a new one.


Attachments:
IMG_1965.JPG
IMG_1965.JPG [ 91.15 KiB | Viewed 96 times ]
IMG_1966.JPG
IMG_1966.JPG [ 100.19 KiB | Viewed 96 times ]

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i
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PostPosted: Wed Aug 22, 2018 12:43 pm 
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I'd definitely replace that gasket if possible, but if you can't find a new one - you can clean that one really well, then anneal it (heat it very hot with a torch all over, then drop it into cold water). This will make it noticeably softer and it should seal better (I always use gasket sealant, and torque the head bolts to the max 2.3 kg/m, 16.6 ft/lbs). It's also VERY important to re-torque every ride for the first 3 or 4 heat cycles; I've had ones that were barely more than finger-tight.

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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


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PostPosted: Wed Aug 22, 2018 1:33 pm 
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I tried your trick on the copper gasket, Wow it became really softer than before.

I'll make sure the carb is really clean and when I'll have a chance I'll get the CR out of the basement and I'll try it.

I live in a city, so I have to put it in the pickup and go to a field where I can try it. If I try it in the street at home, you can be sure there will be cops here a few minutes later :(

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Wed Aug 22, 2018 1:34 pm 
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Also I took the compression and even with my really cheap gauge, I saw a difference. It indicate now 125 psi.

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Wed Aug 22, 2018 2:22 pm 
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I'm sure you know this already, but compression testing should be done at WOT, using at least 5 kicks (I also short the spark plug lead to ground). The WOT reading can be significantly higher than the idle one.

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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


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PostPosted: Wed Aug 22, 2018 3:21 pm 
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Yes, that's what I did. But the connectors on the gauge doesn't look from good quality. I think they are not very well sealed. There's maybe small leaks at the connectors.

I wish I was in the countryside where I could try it at anytime.

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Tue Aug 28, 2018 4:07 pm 
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I went through this same dilemma a couple years ago when the stock Keihin was giving me problems. It would constantly load up the engine and no matter how many times I re-jetted I just could not get it to run consistently day to day. There were all sorts of recommendations, from 30 to 34, on the size of Mikuni to run. I ended up with a VM32 Mikuni which fit the original intake and air box boots, but I did buy new Honda-style boot clamps. I used jetting recommended by Holly Equip out of Japan.

280 Main
40 Pilot
2.5 Slide
6F5 Needle
159Q2 Needle Jet
2.0 Air Jet


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PostPosted: Tue Aug 28, 2018 4:37 pm 
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Thanks for the info. If I can't make the stock carb to work fine, I'll maybe try that solution.

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1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i


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PostPosted: Wed Aug 29, 2018 3:13 pm 
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Best of luck. I never could get any of my bikes with the old Keihin round slides to run properly. I guess there's a reason Mikunis were a popular upgrade back when these bikes were new.


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