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Old 01-28-2013, 06:41 AM   #1
RT Caldwell OP
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Resurrecting a /5

Hi y'all,

Newish member, first post, nice to be here.

After a 4-year hiatus, I've dug my '71 R75(90)/5 out of the pile in back of the shop and have it once again running a bit. It starts right up, idles relatively smoothly and accelerates well. Problem is, when it settles in at a steady RPM with no positive throttle applied, it misses and runs very rough. What I mean by no positive throttle is that it runs smoothly going uphill. I've cleaned and gapped the points, set the timing and checked the wires, plugs, etc. Before parking it, I'd replaced the twin coils with a single Harley-type coil with a 3-ohm rating. Carbs are new 34mm Mikunis, so I doubt this is a fueling issue, it feels very strongly like ignition.

The only thing I can think of is maybe the centrifugal advance unit, so I have some new springs on the way. Any other ideas or thoughts would be greatly appreciated.

Again, it's good to be here,and thanks in advance for your wisdom and advice.
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Old 01-28-2013, 07:14 AM   #2
JonnyCash
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I would say that it sounds like lean jetting on the needle/ needle jet. If it ran well before it was parked, I'd guess there is gunk in your carbs, most likely in the needle jets, or emulsion tubes as they're sometimes called. Usually ignition problems would worsen under a load, not improve, as you're indicating.
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Old 01-28-2013, 09:25 AM   #3
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Ooops... I just posted over on the Old's Cool side, so you might have to be reading two threads....

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Old 01-28-2013, 09:36 AM   #4
disston
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Yes I agree it sounds like ignition so my advice is to clean the carbs a bit more.

Not kidding really. But I did notice one statement you have here that I have issue with. I only clean or file points anymore in emergency situations. I always have a spare set of points so I don't even have to do that that often. I'm back to points too because my last Booster Box blew out but I'll be getting another Booster sometime this year. The Boosters are great for making the points last a long time.

Ignition points are still made with a Tungsten coating on the contacts but we are told this coating is much thinner than in years past. Therefor filing the points tips will yield no Tungsten as a wear surface.

Buy a new set of points. Report back here.
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Old 01-28-2013, 11:15 AM   #5
Bill Harris
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Been sitting for Four years? Pair of new Minkuis? Aftermarket coil? "Resurrecting"? Old gas?

New-to-you? How did it run before? Why were the Mikunis and Harley coil installed? Why is it an "R75(90)/5"?

There has to be a story-behind-the-story.

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Old 01-28-2013, 01:36 PM   #6
RT Caldwell OP
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Quote:
Originally Posted by Bill Harris View Post
Been sitting for Four years? Pair of new Minkuis? Aftermarket coil? "Resurrecting"? Old gas?

New-to-you? How did it run before? Why were the Mikunis and Harley coil installed? Why is it an "R75(90)/5"?

There has to be a story-behind-the-story.

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No, it aint new to me, I've had Martha about 14 years. Parked it when both heads went south due to a half-assed valve job (ya gets what ya pays for). It had TM36 Miks on it, but the flatslides were a pain to work with compared to roundslides, so when I dug it out, I scored a pair of new VM34s rather than take the time to clean up a set of carbs I liked even less than the Bings. When one of the original coils cracked a few years back, I went to the single coil to simplify things. The coil does very well. Solid core wires and zero-ohm plug caps ensure plenty of fire.

I'd swapped on a set of 1974 R90 slugs and jugs several years back, so technically it's an R90/5. The "new" heads are from an R75/6, so with the smaller exhaust valves, it should be a bit more torquey.

The above sins are by no means the worst perpetrated upon dear Martha; I'll post a pic when the sun comes back out.

I'll try moving the carb needles around to see if it improves things, but experience with VM roundslides tells me they're pretty forgiving and probably(!) wouldn't cause this much of a problem without some accompanying issue in acceleration. New gas(midgrade). The points looked OK once filed. They're 10+ years old and had always responded well to my cleaning/filing routine, so I'm not quite ready to point (pun) the finger at them. For the record, I went through three Dyna Boosters before giving up on them. Y'all may have better luck than I did.
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Old 01-28-2013, 02:24 PM   #7
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So you're saying that the VM34s went on the bike since you took it out of storage? If thats the case I wouldn't think for a second that you can assume that the jetting is right for your bike, even if someone else has success with that setup. It might be as easy as just raising the needles a notch. If the bike idles well and is good under a load, you're probably not too far off.

I'm sorry to hear you've had such troubles with Dyna boosters. I put one on my bike when I went to dual plugs about 15 years ago, maybe 20K miles, and have never had a problem with it. I've come to think of it as invincible, now you've given me something to worry about in the back of my mind.
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Old 01-29-2013, 04:52 PM   #8
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Originally Posted by JonnyCash View Post
So you're saying that the VM34s went on the bike since you took it out of storage? If thats the case I wouldn't think for a second that you can assume that the jetting is right for your bike, even if someone else has success with that setup. It might be as easy as just raising the needles a notch. If the bike idles well and is good under a load, you're probably not too far off.

I'm sorry to hear you've had such troubles with Dyna boosters. I put one on my bike when I went to dual plugs about 15 years ago, maybe 20K miles, and have never had a problem with it. I've come to think of it as invincible, now you've given me something to worry about in the back of my mind.
Update - I think you're right about the carbs; I put the new advance springs on today and it made some difference, but not much. Richening the needles did a little bit as well, enough to maybe indicate where the next issue lies. As the problem is now most pronounced below 1/4 throttle and it responds only slightly to idle mixture adjustments, the pilot jets are clearly not correct. I'll swap out the 35s for 30s and see if it'll come into something that can be adjusted. If not, I've got some 25s to try as well. I think once the pilots are close, the rest of it can be dealt with in fairly short order.

You obviously got hold of a good Dyna Booster all those miles ago. Knowing a few folks that have used either the boosters or the Dyna III ignition, seems like they either work well forever, or die suddenly. Clearly, I had a string of bad ones and am unwilling to once again roll the dice.
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