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 Post subject: Carb Components
PostPosted: Wed Dec 27, 2017 9:26 pm 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
I have a 1974 MT 250 that I've restored but can't get the carb settings correct to where the bike runs smoothly up to and through the power band. I've had 2 mechanics work on it both making changes that improved the low end and maybe the mid range but not the top end. Or just the opposite, getting frustrated and need your help.

During the restoration I ordered a rebuild kit for this carb, cleaned and replaced the needle jet, main and so on. One of the mechanics took the original parts and compared them to the installed new parts and noted there were differences so he put back the original parts. Then fiddled with the adjustments including the needle "e" clip setting. Got better top end but the low end sucked until 4000 RPM. The second guy got the low end better but still not right, the mid range ok with smoother/but still not right power coming on above 4000rpm.

Can someone please tell me what size needle jet should be in there, where should the "e" clip be positioned and what main jet size you're running?

The new points were set per the shop manual and verified a couple of times. The new condenser & electrical system was checked and was told the output was per the shop manual. The spark plug looks fairly consistent with what a two stroke should look like with a good strong spark.

The engine was totally rebuilt but did not need the cylinder bored, just a quick hone, so new stock size piston and rings. Running the oil pump and the stock air cleaner set-up.

I know it's impossible to diagnose this without seeing the bike but any suggestions would be appreciated.

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Thu Dec 28, 2017 9:54 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 999
Location: Connecticut, USA
I've jetted stock '74 and '75 MT250's and found it pretty challenging. The over-oiling mimics carb richness, but jetting leaner - these bikes are wicked lean to begin with - often just makes things worse. The spark plug selection is also very critical, especially at high rpm - and fresh gas is important too.

I'd recommend a 65 or 70 pilot jet (the correct short kind - Honda p/n's 99103-357-0650 & 99103-357-0700, '6434waltermc' on eBay has them both) to improve low-end power & throttle response - the air screw will require careful adjustment with these; a 122 main jet if it doesn't have one already, and an NGK BP7EVX ($10 shipped on eBay right now) or BP6EVX (very hard to find) spark plug to minimize hi-rpm misfire and make for easier starting. The needle (should be either 271002 or 271004) can stay at the center #3 position until the new jets are tested. If the bike starts with no choke at 70F, the needle and/or needle jet is likely worn (p/n's 16012-358-004 / 16012-358-024 / 16012-358-034).

If the bike blubbers at top speed - both of the ones I worked on did - do a few extended WOT hi-speed runs to try and clean out the oil buildup in the crankcase.

Ray

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


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 Post subject: Re: Carb Components
PostPosted: Thu Dec 28, 2017 8:35 pm 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
Thanks Ray.

Next week I'll pull the carb off again and check what's installed against what you've listed and get back to you.

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Sat Dec 30, 2017 1:17 pm 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
Ray, pulled the carb off today here's what I found

What was currently installed versus what was installed before the restoration:

* slide = 186 K 3.0 - 186 K3.5 original
* needle = 271004 - 271002 original
* e clip setting both middle groove
* slow jet (pilot Jet) = 60 but with larger openings than the original jet - 60 original
* Main jet = 120 - 122 original
* spark plug NGK B8ES - Orignal ???

Interesting find, the intake manifold gasket was installed backwards so it was disturbing the air/fuel mixture coming into the engine from the carb. The opening is off-set so correct placement is important.

1) So which slide should I use? The shop manual & parts fiche does not define less the Honda p/n.
2) With which ever slide you recommend which which needle jet should I use?
3) Go back to the original Main jet at 122? I believe you recommended a 122 above.
4) You mentioned above to use a 65 or 70 pilot jet, the parts fiche for the '74 MT does not list either part number, with these parts and set-up would you still recommend installing a 65 or 70? If so I'll go find one.
5) I found an NGK BP7EVX plug, I've gone ahead and ordered two to replace the B8ES

Any other words of experience you can provide would be greatly appreciated. Really hoping to get the MT running smoothly for some great woods riding next spring.

Thank-you,

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Sat Dec 30, 2017 5:50 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 999
Location: Connecticut, USA
Kevin,

By the numbers:

1) The OEM slide was a 3.0, which is what I'd use.
2) I'd also use the OEM 271002 needle, which is slightly richer than the 271004.
3) 122 main is good
4) I'd still suggest the eBay #65 and #70 jets I mentioned in my previous post - these are very rare, if you wait they may vanish
5) I think you'll be very happy with the BP7EVX - it's expensive, but should last for years. The B8ES is stock

If your needle jet is in good shape (if the bore is egg-shaped, it needs replacing), the above 3.0 / 271002 / #122 / #65 or #70 should work pretty well.

70's Honda carburetor-parts numbers are a subject in themselves. For example, the 3.5 slide you have (most likely from an early CR125M) has at least two different part numbers I know about, and possibly others I don't - the same goes for jets etc. There are 5 different slides available for your carb; 2.5, 3.0, 3.5, 4.0, and 5.0.

Ray

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


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 Post subject: Re: Carb Components
PostPosted: Sat Dec 30, 2017 8:17 pm 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
Parts ordered, thank-you for your help.

Happy New Year!

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Mon Jan 01, 2018 6:03 pm 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
Any other suggestions on spark plugs? Both sellers that said they had them have refunded my money and cancelled the order.

I’ll keep looking but is there another one that works as well and may be more available?

Thank - you,

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Mon Jan 01, 2018 6:56 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 999
Location: Connecticut, USA
I've gotten the BP7EVX from boats.net before:

http://www.boats.net/parts/detail/yamah ... 00-00.html

There's also the current-production BPR7EIX, which is the resistor version of the BP7EVX:

https://www.ebay.com/itm/1pc-NGK-4055-I ... -1&vxp=mtr

Ray

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


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 Post subject: Re: Carb Components
PostPosted: Tue May 01, 2018 11:59 am 
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Joined: Wed Aug 26, 2015 7:31 pm
Posts: 42
Location: Westford, MA
Ray, started the MT today for the first time this year, as we’ve both experienced a long cold spring here in New England.

That said, after following your advice from our previous exchanges, I set the carb and replaced the spark plug. The MT started pretty easy, smoky initially but ran and idled. Took it for a short spin around the block which ended up a 3 mile run. The bike ran as I remember mine did back in ‘74 when it was new. Completely smooth excelleration from idle up to 75mph and back down. Pulled cleanly no matter what rpm I started rolling on the throttle, plenty of power through the power band. That’s with using the oil injection system.

Now I just need to gently break-in the completely rebuilt motor before trying to find some off road trails.

This old man can’t thank you enough for your insight and patience’s getting this girl running like new.

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Kevin
'74 MT 250
2010 Honda Gold Wing
2009 Kawasaki Concours 1400


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 Post subject: Re: Carb Components
PostPosted: Tue May 01, 2018 12:12 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 999
Location: Connecticut, USA
That's excellent, Kevin! and I'm very glad to be of help. The fact that it runs great with the oil injection is especially good to hear, as I was beginning to have my doubts it was even possible on these bikes. Your results are the best I've ever heard of - at this point I can heartily recommend changing not a thing. :D

Ray

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


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