KMZ MB-750 ignition problem

Are ya having a problem with your rig? We'll try to help. Share your tech tips and experiences here. Dr. Billy Glaser, author of the "Unofficial 750 Ural Service Manual" site myural.com, is moderating this section.
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brianural
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KMZ MB-750 ignition problem

Post by brianural » Fri Nov 22, 2019 5:01 pm

I got an MB-750 a few months ago, ran into a problem yesterday, battery voltage dropping rapidly (I usually turn switch off when it hits 2-3 volts) when key on (no lights, just ignition). Battery new in Oct. During troubleshooting I replaced voltage regulator and coil, no change. I thought it was the capacitor, I have an M-72 on the bench, so borrowed capacitor from it and at key on problem disappeared, voltage stayed steady (there is a voltmeter mounted on triple trees and I had my tester connected to wiring as well)..Started it up and ran for 45-60 seconds and died, voltage dropping rapidly again. Turn key off and battery goes back to 6-7 volts..M-72 capacitor was smaller in size, but aren't capacitors just that capacitors regardless of size.
Kind of stumped right now, any advice or thoughts welcome. I did check points, they are opening and closing normally and look in good shape.
2011 HD RGU
2003 Honda VTX1800 - sold
2005 Honda VTX1300 - sold
Bunch of other metric bikes for last 45 years
"1966 titled" Dnepr MT-11 (reality is a 80-90's)
1956 IMZ M72 bought in pieces, working on build
1967 KMZ MB-750

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brianural
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Re: KMZ MB-750 ignition problem

Post by brianural » Fri Nov 22, 2019 5:18 pm

SOB, walked back out to shop and its acting normal now, ran it up for 5 minutes no issues, at least not the voltage drops, the annoying back fire from left cylinder is still there..got carb rebuild kits the other day, so I'll clean them up and balance them then look into setting points. Valves were done a few weeks ago.

Still interested if anyone else ever experienced the voltage drop issue and what the solution was.

thanks brian
2011 HD RGU
2003 Honda VTX1800 - sold
2005 Honda VTX1300 - sold
Bunch of other metric bikes for last 45 years
"1966 titled" Dnepr MT-11 (reality is a 80-90's)
1956 IMZ M72 bought in pieces, working on build
1967 KMZ MB-750

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propwash
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Re: KMZ MB-750 ignition problem

Post by propwash » Fri Nov 22, 2019 5:32 pm

For testing purposes, leave the run switch at "OFF".
Run a jumper from the + side of the battery to the + side of the coil.
Start engine, monitor voltage. This test will eliminate any wiring, switch issues.

Hard to believe carbs would be much of a problem. Those are so simple.

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brianural
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Re: KMZ MB-750 ignition problem

Post by brianural » Fri Nov 22, 2019 6:21 pm

Propwash,
I should have mentioned I did use jumpers and all issues pointed to points area. Outside this electrical\ignition issue the left side has a backfire from 1/4 to 1/2 throttle, doing the carb clean and rebuild just as a cover all bases. I did tighten cylinders and heads prior to setting valve gaps a few weeks ago.
2011 HD RGU
2003 Honda VTX1800 - sold
2005 Honda VTX1300 - sold
Bunch of other metric bikes for last 45 years
"1966 titled" Dnepr MT-11 (reality is a 80-90's)
1956 IMZ M72 bought in pieces, working on build
1967 KMZ MB-750

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propwash
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Re: KMZ MB-750 ignition problem

Post by propwash » Fri Nov 22, 2019 7:00 pm

I'm certain you know points are a path to ground.
Could they have been closed during this voltage drop event?
One reason there is an "Accessory" position in ignition systems.

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brianural
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Re: KMZ MB-750 ignition problem

Post by brianural » Fri Nov 22, 2019 10:50 pm

propwash wrote:
Fri Nov 22, 2019 7:00 pm
I'm certain you know points are a path to ground.
Could they have been closed during this voltage drop event?
One reason there is an "Accessory" position in ignition systems.
Yes - path to ground. I pulled points assembly off and tested prior to replacing capacitor, they were closed when exhibiting the voltage drop, soon as I opened points voltage stabilized.
2011 HD RGU
2003 Honda VTX1800 - sold
2005 Honda VTX1300 - sold
Bunch of other metric bikes for last 45 years
"1966 titled" Dnepr MT-11 (reality is a 80-90's)
1956 IMZ M72 bought in pieces, working on build
1967 KMZ MB-750

jeffsaline
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Re: KMZ MB-750 ignition problem

Post by jeffsaline » Sat Nov 23, 2019 10:28 am

On points systems if the ignition is left on for long without running the engine AND the points are closed it can lead to damage to coil(s). I've had to address that a few times on bikes. It is an easy fix if the owners are willing to quit turning on the ignition without starting the engine.

Best,

Jeff
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Re: KMZ MB-750 ignition problem

Post by dneprlover » Sat Nov 23, 2019 4:26 pm

brianural wrote:
Fri Nov 22, 2019 10:50 pm
propwash wrote:
Fri Nov 22, 2019 7:00 pm
I'm certain you know points are a path to ground.
Could they have been closed during this voltage drop event?
One reason there is an "Accessory" position in ignition systems.
Yes - path to ground. I pulled points assembly off and tested prior to replacing capacitor, they were closed when exhibiting the voltage drop, soon as I opened points voltage stabilized.

Do you run an OE coil from a dizzy? What's coil like?
Neval BMW/MT11 800cc hybrid, Neval 2wd MT10-36, Neval standard MT10-36, Neval MT16, another MT11, BMW/MB650 hybrid , K750m and an MB750 with locking diff.

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brianural
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Re: KMZ MB-750 ignition problem

Post by brianural » Sat Nov 23, 2019 10:17 pm

Jeffsaline - understood, I don't let key on for long without starting.

Dneprlover - don't know origin of coil, was on rig when I bought it. No markings on coil that I can find. I swapped new coil I installed (bought it from a tractor supply store) during troubleshooting and went back original coil when I determined a points\capacitor issue.
2011 HD RGU
2003 Honda VTX1800 - sold
2005 Honda VTX1300 - sold
Bunch of other metric bikes for last 45 years
"1966 titled" Dnepr MT-11 (reality is a 80-90's)
1956 IMZ M72 bought in pieces, working on build
1967 KMZ MB-750

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