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Ignition Coil and Idle Speed

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sign216

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I've got a 1965 Riverside/Benelli single cylinder 350cc motorcycle that I just got running.
The idle speed is a little high, approx. 1,300 rpm, but that's as low as I can get the carburetor to go
.
I ask on a forum about it, and one of the most experience people there agreed with the issue,
and said he couldn't reduce the idle speed on his, until he converted to a fully electronic ignition and charging system ((Powerdynamo out of Germany ) I think)).

I don't understand. Battery and coil ignition shouldn't be stressed at idle speed at all. I've heard they might be at their limit at max rpm, but idle speed should be easy for a battery+coil (as opposed to a magneto system).

Any ideas what's going on?

Joe


P.S. At idle speed the generator probably isn't charging, but the battery should still have plenty to fire the coil. Right?
 

Ylli

Active Member
"1,300 rpm, but that's as low as I can get the carburetor to go"

Do you mean that is the lower limit that the carb can be adjusted (throttle plates all the way closed) or do you mean trying to adjust to less than 1300 RPM causes the motor to die?
 

unclejed613

Well-Known Member
Most Helpful Member
have you checked the timing? you might want to have a look, not just at the spark timing, but the valve timing too.
 

sign216

Member
1,200 to 1,300 is as far down as the carb and engine will go. I don't know if it's a fuel+air issue or electrical.
That's simply the limit, and the other guy, with a complete electronic ignition was able to go much lower.

It doesn't make any sense, as the elec. ignition should not be limited by the engine rpm.
With a battery + coil system, idle rpm is an easy answer.
 

crutschow

Well-Known Member
Most Helpful Member
I agree.
I don't see how changing to an electronic ignition would allow a lower idle speed.
What is limiting the closing of the throttle valve if the adjustment screw is adjusted all the way to minimum?
 

alec_t

Well-Known Member
Most Helpful Member
Idle speed is limited by the fuel/air mixture; not by the electrical stuff. Sounds like the throttle valve is worn and letting too much fuel into the carb.
 
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sign216

Member
Uncle, yes, I should look at the valves and timing. Any error there makes the engine run rough.

Crutschow and Alec,
Let me ask the question in a different manner. At idle speed the generator isn't making enough current to replace what's being used. The engine is running off battery, and it's just a 6v 8 amp battery at that.
Is is possible that the battery can't provide what's needed for ignition, and a higher idle speed (1,300 rpm) allows the generator to kick in more current?
 

alec_t

Well-Known Member
Most Helpful Member
That's possible, but in that case you'd get no, or erratic, ignition below 1300 rpm. Is that what's happening? Does the engine actually cut out below that speed? If so, and the generator (or regulator) is deficient, then simply swapping transistor-switched ignition for the points-switched ignition isn't likely to help.
 
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sign216

Member
It's variable. The engine does not cut out like an on/off switch. It's just rough at lower idle rpms, and isn't reliable. It stops, hesitates, just barely runs, unless I adjust for a higher speed.
 

crutschow

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Most Helpful Member
The engine does not cut out like an on/off switch. It's just rough at lower idle rpms, and isn't reliable. It stops, hesitates, just barely runs, unless I adjust for a higher speed.
Okay, now we have the complete story.
In that case, measure the battery voltage when it starts to act up at idle and at when it runs well.
 
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shortbus=

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If you can't get it to idle down using carb settings it sounds like you have a vacuum leak. Between the carb and motor. Ignition timing or type doesn't have much of an effect on idle, if any. I'm not familiar with your model of bike, does it have a slide type carb? Does it have a rubber tube type intake? Years ago I put a Mikuni carb on one of my Harley's, After about a year I had the same problem, no idle. Ended up being a crack in the accordion style rubber tube. The Mikuni/slide type carbs are bad about vacuum leaks in the manifolds, since it lets the gas in from the main jets instead of the idle passages.
 

sign216

Member
Crutschow, okay, I'll get a reading. It might take while, as I'm just getting it out of winter storage.

Shortbus, I've had a vacuum leak on two other motorcycles, but this doesn't feel like it. The idle is high, 1,300 rpm, but that's not crazy high for a single cylinder motorcycle. It's just that if the engine were running in excellent tune, it should be less.

Joe
 

Diver300

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Most Helpful Member
I've got a 1965 Riverside/Benelli single cylinder 350cc motorcycle that I just got running.
I don't understand. Battery and coil ignition shouldn't be stressed at idle speed at all. I've heard they might be at their limit at max rpm, but idle speed should be easy for a battery+coil (as opposed to a magneto system).

P.S. At idle speed the generator probably isn't charging, but the battery should still have plenty to fire the coil. Right?
With a standard contact breaker + coil ignition system, the current taken by the system is largest at lowest engine speed, so the coil will get hottest at low speeds. If the generator isn't charging the battery should keep the voltage up for some time but there will be a significant load.

When the contacts are closed, the current in the coil will rise at a rate limited by the inductance, and if the contacts stay closed, the current will eventually get to a value limited by the resistance. The current is only on when the contacts are closed. The proportion of the time that the contacts are closed won't change much with engine speed, but at higher engine speed, the inductance of the coil will be limiting the current for a larger proportion of the time, so the average current will be less. There's a good drawing here:-http://au.rrforums.net/forum/messages/17001/3480.html?1131066195

Also with a standard contact breaker, the contacts may be opening slowly at low speeds, and the coil current will be large, so the current may not drop quickly, resulting in arcing and a poor spark.
 

Ylli

Active Member
Really curious now if the transistor ignition helps your problem. If not, have you tried a new coil?

At idle speed the generator isn't making enough current to replace what's being used. The engine is running off battery, and it's just a 6v 8 amp battery at that.
Is is possible that the battery can't provide what's needed for ignition, and a higher idle speed (1,300 rpm) allows the generator to kick in more current?
What happens if you hang a charger across the battery when setting the idle?
 

sign216

Member
Diver,
I thought at low speeds, the demands on the coil+battery are low. The coil has plenty of time between sparks to charge up.
Regarding possible poor performance at low speeds because the point open slowly; I just fitted a transistor assisted ignition unit, which should solve that issue. Although some people think the points open fast enough that there won't be any advantage using a transistor switch. Your thoughts?

Ylli,
that's an interesting idea. I'll try it and see if a charger on the battery affects the idle speed.

Cruschow,
I'm still going to get a battery reading at low and high idle, to see if there's a difference. I'm fearful that my digital multi-meter will fluctuate too much, and I'll have to get an analog meter to get good readings.

Joe
 

Diver300

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Most Helpful Member
You're right that the coil has plenty of time at low engine speeds to get to it's full current, but if the generator can't keep up, the supply voltage will be lower.

I don't know whether the points opening slowly would make a significant difference, but a transistorised system would get over that issue whatever.
 

sign216

Member
You're right that the coil has plenty of time at low engine speeds to get to it's full current, but if the generator can't keep up, the supply voltage will be lower.

I don't know whether the points opening slowly would make a significant difference, but a transistorised system would get over that issue whatever.
Diver,
I agree, I think the transistor would be way faster than the points and the faster "break" should increase the field collapse i.e. increase the spark, but...people smarter than me say it doesn't matter. If I remember correctly they argue the condenser slows the field collapse, so that points don't end up any slower than a transistor.

I think the transistor has got to be much faster, but I am not skilled in electronics enough to intuit the effects of the condenser.
Joe
 

alec_t

Well-Known Member
Most Helpful Member
Vehicle faults associated with the electrics are often due to a contaminated/corroded/loose earth connection somewhere. Have you double-checked all earth connections are good?
 

sign216

Member
I tested the bike.
At 2,000 rpm the battery is 6.15 - 6.45 volt.
At 1,250 - 1,350 rpm the battery is at 5.72 volt (tail light is on).
It would not reliably idle below 1,250, and connecting a trickle charger to the battery did not smooth out the idle or let me turn it lower.
At lower idle speeds it ran rough and stopped.

A bright spot is that the new Velleman transistor asssited ignition is helping. Last fall the lowest reliable idle was 1,400-1,600 rpm, but w the transistor unit it's reliable down to 1,250-1,350 rpm! This is a normal range for a single cylinder motorcycle. I'd rather get better than normal, but sometimes you have to be happy with "normal performance."

Crutschow and Ylli, thanks for the idea about connecting the charger. That let me see if the small battery and weak generator was limiting performance, which it isn't, at least not at low speeds.

I started this thread because another rider reported he could tune for a low idle speed once he switched from the antique electrics to a modern charging and ignition system. Looks like he's right, because a transistor ignition got me a partially lowered idle speed. I'm not sure what the issue is, as in theory the electrics shouldn't be stressed at low speeds.

I'm open to ideas and willing to experiment, but this may be an unsolvable mystery of vintage engines and generators.

Joe
 

crutschow

Well-Known Member
Most Helpful Member
What is the amperage of the trickle charger?
It might be too low to make a difference.
 
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