Bronco 125 - header pipe red hot?

[mtraikov]

Any tech articles or posts on how to adjust the timing or checking the timing? Can this be done in a week?

I don't have any real experience with the Bronco engine but I would not be trying to play with the timing at this stage. As far as I can tell, altering the timing on a Bronco engine means pulling off the flywheel/rotor which probably requires a special tool. That makes me suspicious that the timing is out - I'm not even sure you can alter the ignition timing as this engine has a very basic set up with no dynamic advance. You should certainly adjust the points which, if they are way out, will have a small impact on timing. I THINK you adjust the points through a cutaway in the flywheel (http://www.oldtimerworkshop.com/files/Ducati_Bronco.pdf).

When you say the header pipe gets red hot, do you actually mean 'red' or just bloody hot (normal).

As for the the engine stopping when you move the throttle, that sounds like it is fuel related (e.g. blocked jet). It would certainly not be unusual for an carburettor that has been sitting around for some time to have a jet gummed up so you might want to pull it apart and inspect.

Ian

Thanks for the info Ian. So at the header near where it inserts into the cylinder it actually gets red hot. I cleaned the carb again, no blockage. Anything else that can affect the throttles? It dies as soon as I adjust the throttle. And adjusting the mixing screw and idle screw does not do anything as far as I can hear after turning in and out the screws when motor is running

[ducwiz]

Mike,

degree wheel -
offered for sale: https://www.amazon.com/degree-wheel/s?k=degree+wheel
DIY: https://www.blocklayer.com/degree-wheel

piston blocker/stop -
You can make one by yourself from an old spark plug and a long bolt M8. But you should add a counter nut and a washer on top of the plug body.

Many different versions are available on sale https://www.amazon.com/14mm-piston-stop/s?k=14mm+piston+stop But you should prefer one with an adjustable reach.

stroboscope for timing -
You chose one of the luxury versions, with a lot of integrated features like advance angle measurement etc. A very simple one with a trigger pushbutton only will do the job. Btw, white LEDs instead of Xenon flash tubes are state-of-the-art now.
General info https://en.wikipedia.org/wiki/Stroboscope

corresponding pointer -
It's a pointer or indicator, self-made from a stiff steel wire or strip of sheet metal and attached to the engine case in a way that it is pointing closely to the degree wheel's graduation.

Just so I’m understanding - adjusting the ignition timing is rotating the entire base plate that is mounted to the engine.

Yes.

points gap -
To measure it you need a feeler gauge https://en.wikipedia.org/wiki/Feeler_gauge

felt lubrication -
I use BOSCH Ft1V4 grease since 45 years. I don't know if it's still available here in D or at your location. As substitute I would apply a small amount (like a rice grain) of MoS2-admixed high temp grease to the felt, and the point's pivot pin as well.

From where do you know that the timing you found on the engine, or that belonging to the marks you came across is the right one? The decision to keep the present or to revert to the marked position can't be done without measuring. That's the reason why the degree wheel and stroboscope are necessary.

Good luck! Hans

[mtraikov]

Just to confirm - in order to check and adjust timing. I need to get it idling correct and have the ability to adjust the throttle? I’m turning in and out the mixing screw as well as the throttle stop “idle screw” and it’s not adjusting the RPMs. Moving the throttle makes the motor die.

Carb is cleaner than a baby bottom. Any tips? The throttle does not snap back (is that normal for these older motos?) but I cleaned it and it operates smoothly. There’s no adjustment for it at the handlebar but some at the carb.

[IanHood]

It’s very difficult to diagnose this sort of thing remotely but if neither the idle or mixture screws are having any impact you may have an air leak around the carb which would cause the bike to be running extra lean (and hot).

You can check for leaks by spraying WD40 (or carb cleaner) around likely leak locations (e.g. where it bolts to the head). If the idle speed changes when you do that, you have found an air leak and you need to fix it.

This is your biggest issue I think, rather than the timing. The fact that the bike is running at all suggests the timing isn’t that far off.

Ian

[mtraikov]

It’s very difficult to diagnose this sort of thing remotely but if neither the idle or mixture screws are having any impact you may have an air leak around the carb which would cause the bike to be running extra lean (and hot).

You can check for leaks by spraying WD40 (or carb cleaner) around likely leak locations (e.g. where it bolts to the head). If the idle speed changes when you do that, you have found an air leak and you need to fix it.

This is your biggest issue I think, rather than the timing. The fact that the bike is running at all suggests the timing isn’t that far off.

Ian

Thanks Ian. That makes sense. Would a bad mate/connection of the exhaust header to the cylinder port also affect this? The reason I mention this is the chrome but doesn’t thread in as much as I expected it to, so I’m not sure if you are suppose to see threads of the chrome nut or not when it’s fully tightened up. I’ll give these a shot today.

[mtraikov]

Mike,

degree wheel -
offered for sale: https://www.amazon.com/degree-wheel/s?k=degree+wheel
DIY: https://www.blocklayer.com/degree-wheel

piston blocker/stop -
You can make one by yourself from an old spark plug and a long bolt M8. But you should add a counter nut and a washer on top of the plug body.

Many different versions are available on sale https://www.amazon.com/14mm-piston-stop/s?k=14mm+piston+stop But you should prefer one with an adjustable reach.

stroboscope for timing -
You chose one of the luxury versions, with a lot of integrated features like advance angle measurement etc. A very simple one with a trigger pushbutton only will do the job. Btw, white LEDs instead of Xenon flash tubes are state-of-the-art now.
General info https://en.wikipedia.org/wiki/Stroboscope

corresponding pointer -
It's a pointer or indicator, self-made from a stiff steel wire or strip of sheet metal and attached to the engine case in a way that it is pointing closely to the degree wheel's graduation.

Just so I’m understanding - adjusting the ignition timing is rotating the entire base plate that is mounted to the engine.

Yes.

points gap -
To measure it you need a feeler gauge https://en.wikipedia.org/wiki/Feeler_gauge

felt lubrication -
I use BOSCH Ft1V4 grease since 45 years. I don't know if it's still available here in D or at your location. As substitute I would apply a small amount (like a rice grain) of MoS2-admixed high temp grease to the felt, and the point's pivot pin as well.

From where do you know that the timing you found on the engine, or that belonging to the marks you came across is the right one? The decision to keep the present or to revert to the marked position can't be done without measuring. That's the reason why the degree wheel and stroboscope are necessary.

Good luck! Hans

Hans. This is immensely helpful, thank you so much. I’m getting this stuff delivered sometime today.

I have feeler gauges to measure the points gap. Is the points gap from the fiber block to the crankshaft? Any certain position it must be?


Is the degree wheel held in place with the nut that typically holds the flywheel down? Having a hard time visualizing how to mount it. Will it flush and touching with the flywheel or is there a small gap between the two?

Mike

[mtraikov]

Everyone!! Good news! The bronco now idles, shifts, and the throttle revs the motor!! I adjusted the points to ~.33mm. Looking back now that it runs smoothly, the points gap was extremely tight for some reason. After I adjusted, everything operated way better. I am going to try to check the timing now. The header pipe also does not get red hot anymore, thank god.

I am having a problem with the throttle not snapping back. I cleaned the mechanism and got all the all gunk out of there, but still won’t snap back. When I pull on the throttle cable it snaps back no problem, leading me to believe the mechanism is not working correctly. Any tips?

[IanHood]

If the throttle doesn't 'snap back' properly there usually four possible causes:

- the return spring in the carburettor slide is weak. Note that this spring is typically the only thing responsible for return the throttle to the idle position.

- the throttle side is binding within the carb body

(If you can pull the cable directly from the top of the carb and it returns properly, both the spring and the slide are probably OK)

- the throttle mechanism (the bit you turn with your had) is binding. To check that, disconnect it from the cable to see if it turns freely (you will likely have to disconnect the other end of the cable from the carb first).

- the cable itself is binding either because it is kinked, frayed or rusty (quite common on bikes that have been sitting for some time). To check that, disconnect the cable from the carb and the throttle and see if the inner cable moves freely in the outer sheath. It may simply need lubricating but could require replacement.

Ian

[mtraikov]

If the throttle doesn't 'snap back' properly there usually four possible causes:

- the return spring in the carburettor slide is weak. Note that this spring is typically the only thing responsible for return the throttle to the idle position.

- the throttle side is binding within the carb body

(If you can pull the cable directly from the top of the carb and it returns properly, both the spring and the slide are probably OK)

- the throttle mechanism (the bit you turn with your had) is binding. To check that, disconnect it from the cable to see if it turns freely (you will likely have to disconnect the other end of the cable from the carb first).

- the cable itself is binding either because it is kinked, frayed or rusty (quite common on bikes that have been sitting for some time). To check that, disconnect the cable from the carb and the throttle and see if the inner cable moves freely in the outer sheath. It may simply need lubricating but could require replacement.

Ian

Thanks Ian. I’ll have to go through the cleaning process again. But everything operates smoothly but the throttle mechanism, and I cleaned it very thoroughly.

I am having a new problem however, when the bike is running, idling etc and I go to shut it off - I remove the key and it’s still running. I haven’t changed any wiring at all. What are the causes of this?!

[ducwiz]

Unfortunately, now you mixed up this thread with your other one. Go to http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=3599#p28545 and look into the downloadable user manual. It contains an electric scheme for the Bronco, where a "4-position key" switch is scetched in. As you can see, the upper pole or contact pair shorts the alternator's output for the ignition power to ground, when the key is in the OFF position. This action prohibts sparking, stops the engine immediately. You should check the function of this pole in the key switch with a multimeter for continuity/interruption, and you can also try to shut down the engine by pulling the green wire which connects points and key switch, and then connect it to ground.

Hans