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Starts then dies

Jonala

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#1
I'm restoring a 74' Honda MR 50. It now has a new piston, cleaned carb, and crankseals. The bike had been stored for 25 years, after it seized its piston.

The problem is the bike will start when stone cold, and run for about a minute. Then it dies and can't be restarted. I'm certain it is an electrical problem because there is no spark after the engine dies.

The bike uses a AC alternator that sends power thru a half wave recitifer, then into the coil.

The resistance measurements are a little off in both the alternator and coil.

I did a bench tested the coil, and found that the coil would not produce a spark until it got 1.3 volts. The coil then produced a weak spark. At 3 volts it would produce a strong spark.

I put a voltmeter in parallel with the coil to watch the coil input voltage. When I managed to get it to start the coil was getting .9 - 2.5 volts depending on rpm. The spark just died.

When measuring the resistance of the coil and alternator after it died I found no change.

When I bench tested the coil I let it run for 15 minutes at 3000 sparks / min without it shutting down.

I'm I missing something? I'm thinking there is something wrong with the coil despite the bench test.

Thanks for any help.
 
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#2
Any luck with it yet?
 

Jonala

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#3
After posting the question I have taken the carb apart at least six times. Rechecked all my electrical measurements.

This afternoon I got it to idle. I'm now more inclined to think the problem is with the carb. When I got this bike the carb had a very thick layer of curd in all the passages, and jets. The slide was actually stuck because the top of the carb had been removed. The bike had been sitting around like that for at least 20 years.

Despite the multiple cleanings I think I'm still missing some passage that is not operating properly.

My basic problem is that I can't trust any of the components, because I don't know how the was running before it seized its piston.

Thanks for asking. I'll get this bike running even if I have to get a new carb. :think:
 
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#4
I wish I had something like that to toy around with. Guess I'll have to keep my eyes open.
 

Jonala

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#5
This has been a fun project. I'm keeping costs down my only replacing items that are either broken, or missing. Everything else has been bead blasted, sanded, bored, polished, scrubbed, then painted to cover the all the old rust damage.

This bike was a rusted mess when I got it. For example I had to pry off the chain off the sprockets because it was rusted solid around the sprockets.

If you find a project bike, stay away from bikes that are literally basket cases.
The grief never ends. I've been very lucky to find a Yahoo group that is dedicated to the MR 50. They have helped me out a lot. But we had run out of ideas for my particular problem. That's why I posted here in the brain trust know as DRN.
 

Imho

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#6
I'm sure you've already thought of this but ... a friend of mine had the same symptoms that turned out to be rust in fuel filter at the fuel tap. It would let enough fuel through over time to fill the float bowl but not enough to keep it running once the fuel in the bowl was used.
 

Jonala

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#7
Imho,
Thanks for the idea, I have not checked the tank fuel filter. I put a inline filter because of all the rust coming out of the tank.

I will check it tomorrow. The bowl is full when the engine stops running is the reason I haven't checked the tank filter. I even tried taking the gas cap off to see if it would help. It didn't help.

I'm taking the carb into work tomorrow. I'm going to test the carb by setting up a regulated low pressure box to simulate engine vacuum, and watch what is actually going on.

I'm going to use a graduated syringe to mimic the gas tank.


I had to check it out, went back out to shop to check the tank filter. The filter had a bunch of gunk all over it. Clean the filter, inspected the petcock, put the carb back on.

No change, it still dies. :ugg:
 
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#8
Why not just run a fuel line from an auxillary tank and bypass the the oem tank. That should eliminate that
 

Jonala

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#9
I wish I had a spare carb, coil, and recitifier. This shouldn't be this hard to figure out. :confused:
 
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#10
After the minute or so of idle and it begins to die, does activating the choke make a difference? If so, then the problem is fuel starvation, and stems from blockage in the delivery system.
 

Jonala

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#11
Yes, if I choke it, the motor recovers for a few seconds. What I'm wondering is that the motor requires a rich mixture to operate because I have a weak spark.

When I tested the coil on the bench the spark was very weak at idle speed voltage.
 

Imho

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#12
Based on the fact the float bowl is full when it stops, it's probably not a fuel delivery problem, although the system sounds like it needs to be cleaned out.

I think both too lean and too rich would not be good with a weak spark. If the spark is weak at or near idle, does it keep running if you keep the revs up?

My first thoughts when I read your post was that the stator or coil were shorting or going open circuit as they heated up. After it has died, do you get any voltage at the coil input when you kick it over or try to bump start it?
 

Jonala

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#13
I'm reluctant to keep the motor rev'd, because this a new piston and bore.
That said if I keep it rev'd up it does run better, but as soon as I let the revs drop it starts to bog then dies.

I see I didn't post that the spec resistance for the input side of the coil is .4-.5 ohms. My measurement was 1.1 - 1.2 ohms, which is 100% out of spec.

The pickup coil in the Alternator resistance is suppose to be 1.4-1.5 ohms.
Measurements = 1.1 - 1.2 ohms.

The readings at both ends of the circuit are out of spec. At $70 a try for either component, is the reason I'm trying to really make my best guess the right guess.
 

Jonala

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#14
It actually idles. It's slow reving, not sure if that is the way the
bike is or it needs a coil.

As always it turns out to be something that I just kept either
thinking I had already checked or just thought it couldn't be that.

Turns out the major culprit was a passage that was still blocked. I
took home a air nozzle that had a rubber tip. Put the nozzle over
this brass tube that extends down into the float bowl. Blew air and
nothing. Looking at the casting I saw that this tube had something
to do with the air delivery.

Took the carb into work this morning. Using some micro size drills I
was able to break thru the crud that
had accumlated in the top of the tube.