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MR175 (A) 1975 Backfires
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Author:  OLDDIRTRIDRER [ Fri Mar 27, 2009 3:50 pm ]
Post subject:  MR175 (A) 1975 Backfires
It ran good for awhile now it backfires........I suspect the exhauste is getting plugged

What say you

Larry
Author:  rayivers [ Fri Mar 27, 2009 4:21 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

Mine did that too - at one point so violently I was worried it had blown out the case gasket or seals. ;>( Since putting in the CR ignition cured it completely, I thought the points ignition might have been double-sparking at TDC and BDC, as I thought I saw this happen once while I had the plug out checking for spark. It could be caused by crank wobble from the huge flywheel, excessive points gap or other points-related issue, a worn main bearing in an older motor, etc..

Definitely clean out the spark arrestor outlets if you haven't already, and I would drill it out too. I told myself I wouldn't drill out my NOS pipe, but it was just too restrictive in stock form; now it runs a lot better.

Ray
Author:  OLDDIRTRIDRER [ Sat Mar 28, 2009 9:13 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
As you know I am bidding on the CR125 igniton. With that, did you say the coil and condenser would not work from the 175 stock coil/condenser? Just curious as I dumped $78.00 into a brand new one and it would be nice to keep it.

Also ll be boring out the exhaust today.

And yes I am worried that the backfire may have blown out a head gasket and maybe some seals also. Fingers are crossed. I just cannot figure out why all of a sudden it just statered happening. My points have not moved.

I found a brand new Clymer Manual for the MR's 1974-1977 at the bike shop lI got the new tires from. Maybe it will shed some light on what happened.

I am anxious to get this thing on the trails..........I need to remind myself to slow down and do it right the first time.
Author:  rayivers [ Sat Mar 28, 2009 10:34 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry.

I believe the MR175 coil will work, but I know the CR coil works, so that's what I used on my bike. The CR ignition's coil is driven by the stator winding as (electronically) switched by the CDI box, while the MR coil gets its drive from the stator winding as (mechanically) switched by the points. I assume the CR coil is matched to the CR stator winding and CDI box, but I would love to know for sure, if anyone has specs or info. For anyone using the MR coil with the CR CDI box, it would be a good idea to remove the condenser, as it's not needed and the CDI may not like driving it.

I remember now - I was looking in through the flywheel 'windows' at the points while rotating the crank, and I saw the points move apart twice per crank rotation. I recall thinking all my usual stuff (maybe that's normal, maybe it doesn't make a difference, wish I knew for sure, etc. etc.), but after the next ride and the huge backfire I decided it really was a problem whether I knew what was causing it or not. ;>) My left side crank seal had popped out about 1/8", but it was easily pushed back - and the bike has been running fine ever since.

It's great you've got a manual, it's so much better than flying blind.

I just took mine out with the 3.5 throttle valve (Honda P/N 16022-373-405) in it, and I was surprised at the difference it made at small throttle openings - the front wheel was coming off the ground a bit here and there. These carbs are far from perfect - as I went from 1/8 to 3/8 throttle on the street I could feel the bike go from lean to perfect to slightly rich to perfect again - but I've learned to live with the imperfections, which mainly show up on the street, not in the woods. The MR175, MT250, and early CR125M's share the same basic 28mm Keihin carb.

Ray
Author:  OLDDIRTRIDRER [ Mon Mar 30, 2009 5:53 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Ray,

Before I make changes to the jets as you sugest I think it may be a good idea to know your temp and altitude? I live in Iowa and my altitude is 700ft above sea level. The temp this spring has been variable. Yesterday I rode in the snow at 38degrees F.....today its in the 50's. Wont be long and the hot weather will be here.........high 70's to low 90's and high humidity. I hate to get things set up now only to start all over again unless our temps and altitude are the same. Although I will admit it sure is allot of fun for this old guy messing with engines again! :lol:

None-the'-less I still have a sputter.......no time tonight to work on the muffler though. But when I do I do have a question? Can the baffles or any of the exhaust guts be pulled out through the spark arrestor end of the muffler? Just curious as the manual has no exploded veiw of how the exhaust is put together.

BTW.........just left E-bay and they have an original BASSANI HONDA MR175 ELSINORE MR-175 MR VINTAGE MX AHRMA Item number: 190297079614 . The starting bid is $9.99. I dont know your age but in the 70's a Bassani tuned pipe was the only way to go for two strokes. I put one on my Susuki 125 when I was 17 and man did it roar. As I am trying to stay 100% stock with my MR 175 I really dont need it. (actually at my age anything over 35 off road is to fast my wife says.....she also said no wheelies :(


THANKS

Larry
Author:  rayivers [ Mon Mar 30, 2009 6:39 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

The mean elevation of CT is 500 feet above sea level, and the area where I live is kind of hilly so I believe I may be slightly above that... so that's pretty close.

I've got pages of comprehensive jetting notes on this carb/motor, but they all point to the same conclusions...

48 pilot jet, from 40 to 90+ degrees F (see throttle valve info below) - tweak air screw at every 10 degree change
3.0 throttle valve below 50F, 3.5 above that
120 main jet below 60 degrees, 118 above that (I leave the 120 in all year and may try a 122 this summer, but my motor's ported)

You'll have to live with the 3.0 TV unless you can find a 3.5, or a spare 3.0 you can modify into a 3.5. I've done this a few times - I hate having to, but it works fine, as does building out a too-large cutaway with solder. If you could find a slightly larger-diameter needle you wouldn't really need the 3.5 TV, but it's very difficult to even determine the specs of the OEM Honda needle, much less try and find a selection of them in different diameters/tapers/lengths.

The cool thing is, you won't have to spend days or weeks "jetting" your bike, just buy one or two chunks of brass and screw 'em in. If you decide to buy a spare carb and modify the throttle valve - as I would recommend doing - sanding down the throttle cutaway should only take a half-hour or so. By the way - I find it much easier to remove the carb and manifold together than try and wrestle that bastard out of the brittle old manifold rubber. It's pretty easy to just slide the manifold sideways and ease the carb mouth out of the air boot.

The MR175 pipe body is only slightly more complex than a soup can. :>) There's a single divider halfway between the middle and rear heat-shield mounting screws, with a 7/8" I.D 4" long tube running through it. Then there's the spark arrestor, one side of which can be seen through the end of the pipe; the other side looks like a rocket nosecone sticking out of a turbine engine. You want to drill out the center of this nosecone.

I'll be bidding on the Bassani pipe. I think I may have bid on it once before - I believe I recall the heat shield popped off in the back. If I get it, I expect it will be real loud and burn my leg like my other aftermarket pipes, but perhaps I'll be pleasantly surprised. The drilled stock pipe's low end is as good as or better than any of the aftermarket ones I've tried, although they do have a bit more midrange; none of them have what I would call good top end, and none of them are nearly as quiet as the stocker.

Ray
Author:  OLDDIRTRIDRER [ Mon Mar 30, 2009 7:34 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Good stuff as always Ray, thanks a ton for being forthright and saving me dollars!

Ill work on getting some brass screws for the carb this week.
48 pilot jet, from 40 to 90+ degrees F (see throttle valve info below) - tweak air screw at every 10 degree change
3.0 throttle valve below 50F, 3.5 above that
120 main jet below 60 degrees, 118 above that (I leave the 120 in all year and may try a 122 this summer, but my motor's ported)

Tommorow if the weather warms a tad (ran out of propane for the shop heater) Ill be working over the 175 stock pipe.

BTW again I am lucky as my carb rubber went back into soft mode after I boiled it in plain old water for 5-7 mins. . But yep its still a pain to pull the carb, just hesitant to break the seal between the rubber and the intake flange.

I would love to hear that Bassina if you get it. They are loud from what I can remember. I will say this, Ill take the smoke and the sound from a two stroker any day for the dirt. Hope I didnt offend anyone I just grew up with that ring, ding,ding ding or in the case of tha Bassina....whop, pop, pop pop pop.............

Shame you arent a little closer would love to ride in the dirt with our 175's side by side.

Later

Larry
Author:  rayivers [ Tue Mar 31, 2009 6:12 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

Hey, it feels good to get some of this info out into the world instead of just leaving it stuck inside my computer. I remember searching the Web back in '07 looking for MR175 info, and not finding a whole lot.

I get most of my vintage jets and needles from Robert at http://www.motorcyclecarbs.com . He sells single items, whereas some others only sell in lots of four.

My "CR175" has a 30mm Keihin carb (stock is 28mm), using the MR175 manifold. I soaked the manifold in boiling water as you did, then forced the carb into it in front of a heat gun. ;>) It was a bit a of a struggle, but I never felt close to splitting the rubber or anything. It's been a while now and still looks fine.

If you're interested, there's a '74 CR125M 28mm carb on eBay right now (item #130297209475) that looks to be in great shape. I've bought from the guy before with no problems at all. It's rare to find a rebuilt carb since a lot of the rebuild parts are discontinued (none of mine are fully rebuilt, I just mixed-and-matched from a pile of parts carbs).

I'll let you know how I make out with the pipe.

Ray
Author:  OLDDIRTRIDRER [ Thu Apr 02, 2009 8:10 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Ray,

As you knew and I suspected allot of the issues was the pipe. The minute I took it off and fired the bike up I think I gained 3000 RPM at idle.

So I drilled it out as you said, re-installed it and tweaked the idle air and idle screw. Yes it improved it allot. Throttle repsonse under no load is very good. High RPM under no load is also good.

However under load on the concrete its still flat (sputters) when opend up. Time to spend some dollars on a some jets is the next step.

But my idle is perfect. Doesn't shake, rattle or even cough!

I just need some power at high RPMs

I am hoping I dont have any major carbon issues causing high compression or even a cracked ring. If so Ill be pinging you on where I can get a cylinder gasket set.

........keep you fingers crossed for me and lets hope its the jets!

Larry
Author:  rayivers [ Fri Apr 03, 2009 6:48 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

If your exhaust was so completely clogged with crap that it affected the idle, in my opinion you're not done with that pipe just yet. A clogged exhaust pipe usually has the greatest effect at high rpm (where your motor is still weak, by the way), so when it has an effect down low in the rev range, it's a big red flag.

I didn't read in any of your posts that you had cleaned out the pipe passages surrounding the newly-drilled hole with pipe cleaners, so I'm assuming that hasn't been done (I did it with mine) - so practically all the exhaust that's leaving your motor is doing so through the new hole.

If you're not going to clean out the passages, you're gonna need a HUGE hole.

I saw an improvement in my motor every time I enlarged the center hole (I went from 3/8" to 9/16", and I may go all the way to 11/16" now that my drills have arrived), and that was with the passages cleaned out. With clogged passages, I wouldn't hesitate to drill out the center hole to 3/4", maybe more.

Or you could pick up some pipe cleaners at the local smoke shop (tell the guy what you're doing, he may have some real industrial-strength pipe cleaners available like my guy did) and clean out those passages. Highly recommended.

To really pile it on :>) - if this were my bike I'd assume that years of operation with a clogged pipe had probably carboned up the motor and exhaust port real good, so I'd go ahead and pop off the head and see what you find - I can pretty much guarantee it won't be a waste of time. The head gaskets can usually be re-used if you're careful in removing them.

There's a complete gasket set right now on eBay, and I would also look for a set of rings when you find out what bore the cylinder's on. I re-used the original set of rings in mine, and it ran fine until one of them snapped in half and tried to take out the top of my exhaust port - luckily the piston got destroyed instead.

Ray
Author:  OLDDIRTRIDRER [ Sat Apr 04, 2009 7:19 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Ray,

I have two issues. One is the exhaust and the second is the carb.

Exhaust- I plan on cutting the the back of the exhaust open with my angle grinder. Taking care so I just the skin. Ill clean it that way. Then Ill weld the flap I cut off back on with a wire welder.

Carb - I need to rejet. Its running rich I can see it on the plug. I went to the bike shop to get jets today but I didnt have my old jets. The salesmen suggested to ensure I get the exact ones I want he would prefer I have my old ones to match the up. Once he knows the match then he'll sell me the sizes I want.

Or do you think the exhaust is the cause of the rich conditon?

Thanks,

Larry
Author:  rayivers [ Sat Apr 04, 2009 10:04 pm ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

Unless your exhaust is completely different from the three MR175 pipes I have here, there's absolutely no reason to cut the pipe - everything that needs to be done can be done from the ass end of the pipe. Just remove the two small bolts and pull the baffle insert out, and the spark arrestor is right there at the front of the tailpipe tube. I hate to sound like a broken record, but the pipe cleaners work really well... and you've already drilled it... you should be able to get it working fine in about 20 minutes. If you have a torch handy, hit those spark arrestor passages for a while - there should be nothing but powdered soot left for the pipe cleaners to scrape out. I've been told that Easy Off oven cleaner works well too, but have never tried it myself.

If you do cut it and hit the spark arrestor insert, the flimsy spot welds that hold it together will likely break and the thing will fall apart every which way. How do I know? Because my Honda MT125 pipe uses the same insert buried deep inside the pipe, and I drilled it out (from the side, not the back like the MR pipe) last week and all hell broke loose. :>( It had to be done, though - and at least I didn't have to cut the pipe apart to do it, the MT pipe has a small access opening. The MT pipe's been modified to fit the MR and I'm going to test it tomorrow, weather permitting - I'll report back with the results.

Your clogged pipe isn't really causing a true "rich" (too much fuel) condition - it's causing a "too much carbon/motor choking on it's own exhaust" condition that seems like richness, and surely looks like richness on the plug. The carb jetting may be too rich - the pilot jet is guaranteed to be too rich - but I wouldn't change any jets until the pipe is clear, the air filter is clean (I did mine yesterday, it was mega filthy), no air leaks, clean plug, new gas, etc. Jetting is all about making comparisons, and you need a baseline setting to start from, otherwise you'll never know if the changes you make are better, or just different.

When the pipe is clear, though, here are the Honda jet part numbers - the guy at the shop should be able to get them with these:

Main jet: 99101-357-XXX0 (a #118 main jet would be 99101-357-1180)
99101-357-1200 (#120 - good to have around)
Pilot jet: 99103-373-0480 (#48 - this should be the only one you'll need)

I think it's great that you want to get this thing running well rather than just moving under its own power. It may take a bit of work, but you'll get there. Mine always scares me at least twice a ride, and I'm not easily scared. ;>)

Ray
Author:  OLDDIRTRIDRER [ Sun Apr 05, 2009 8:39 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
Ray,

The more I thought about it last night the more I think the exhaust is minor. Maybe you missed this but after I drilled a 3/4 inch hole and reinstalled my RPM s did come up. They were not quite as high as with the pipe removed but you wouldnt expect it as the pipe does have engineered in restrictions.

However, I will play with the pipe again today. Since I belive in clean when it comes to two stokes my plan is to invest a whole $1.50 and go to the car wash and hit it hard with some high pressure engine cleaner and then flush it out good. I like the idea of Easy Off also.

Thanks for part numbers on the jets. That will help especially since all I can find is OEM part numbers.

I cant say enough for your support. I will say this in repsonse to you liking me going al the way and making this bike a runner. I am 53 and I can remember a friend of mine having a brand new 1975 MR175 when I was 18. It was January in San Diego when I was going Navy diver training getting ready for Vietnam. I jumped on that bike and wow. I was so impressed I wanted one right now. The power seemed just right but most of all the handling was superb. I never did get one back then............................but I have one now!

I am lucky to have one with such low mileage. I think Ill keep it until my legs are to weak to kick it over.


BTW.........heres a website I found that you may not have. They claim to have lots of parts for the vintage bikes http://www.mrcycles.com/. I plan on calling them Monday and see if their claims are true.

Larry
Author:  rayivers [ Sun Apr 05, 2009 10:28 am ]
Post subject:  Re: MR175 (A) 1975 Backfires
Larry,

I didn't realize you had drilled out the pipe a full 3/4" - that's great. If you're leaving the motor stock, you should be good to go. The more flow you have, the better your top end should be. The next time you have the pipe off, you might want to check inside the inlet flange at the manifold - mine had a couple of weld boogers hanging down, and also about a 1/32" mismatch between the flange and the pipe tubing. There might also be a bit of carbon buildup inside the header tube, although mine was pretty clear. The pressure washer should work well.

I'm 51, and this bike matches me well for the kind of riding I'm doing now (woods/fire roads/power-line trails, along with a little street time). I really like the way vintage bikes love to slide, whereas the newer/taller bikes tend to bite in and resist sliding (and try to throw me off at times).

I've used MR Cycles a number of times - they're great, especially if you need to return anything. They also came up with a set of standard-bore MR175 rings when nobody else could, although that may have been a fluke.

Hopefully you'll get your jets before too long, and get this thing running even better. Just FYI - a couple of the MR carbs I got off eBay had jets and needles that had been eaten away by corrosion and ran rich, and I wasted weeks trying to jet one carb that had a drilled-out needle jet and ran SUPER rich. With these carbs, trusting nothing is the way to go, at least at first (although it sounds like you've gone through yours pretty carefully).

Ray
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