An idiot and a 77 XS750

[manas]

Re: Re: An idiot and a 77 XS750

l know you've been focused on the carbs, have you looked at or tested the electrics, put in new spark plugs, checked for spark? On this era yamaha bikes one of the biggest problems people encounter is the crappy fuse box. Since its been converted to electronic ignition make sure the coils have been changed also, to the lower resistance later year coils. Just some starting points. Oh, BTW, l'm also on XS650. com, as hmusket, my alter-ego here is frankenrat.

Hey Frankenrat, I haven't done any of those things. I'll go pick up a spark plug tool today and a multimeter. Obviously the starter motor is working, but how do I go about checking if the ignition system is working. I'll do some searching here on DTT but a quick idiots guide or a finger in the right direction will be useful..

Good to know that you're on X650, I'll make a post there as well...

 

[frankenrat]

If you're at the auto store be sure to pick up some new spark plugs, BP7ES in NGK, or the equivalent in other brands. Don't get resistor plugs. I'm guessing the ones in your bike are probably fouled. Use the new plugs when checking for spark, make sure the plug is grounded to the engine when testing, can ruin the CDl if not. The bluer and larger the spark the better You can make or buy a tool for that. Test the coils, if you have the Clymers, around page 160 outlines the procedure. Look at the fuse box and fuses, especially the ignition fuse, check for continuity between the two sides. That's all I can think of right now, besides checking the wiring diagram to make sure the coils are connected to the correct cylinders. You may also want to replace the plug wires and caps.

 

[manas]

Re: Re: An idiot and a 77 XS750

If you're at the auto store be sure to pick up some new spark plugs, BP7ES in NGK, or the equivalent in other brands. Don't get resistor plugs. I'm guessing the ones in your bike are probably fouled. Use the new plugs when checking for spark, make sure the plug is grounded to the engine when testing, can ruin the CDl if not. The bluer and larger the spark the better You can make or buy a tool for that. Test the coils, if you have the Clymers, around page 160 outlines the procedure. Look at the fuse box and fuses, especially the ignition fuse, check for continuity between the two sides. That's all I can think of right now, besides checking the wiring diagram to make sure the coils are connected to the correct cylinders. You may also want to replace the plug wires and caps.

Replaced the spark plugs. Unfortunately the local Canadian Tire did not have the Yamaha specified plugs but they gave me another set of Champion plugs that they said are compatible with the motorcycle.

This is what the old plugs looked like :

I'm guessing the carbon deposit is due to excessive rich mixture. In any case, they were replaced.

Fuse box looks OK:

Also checked the voltage across the battery terminals. Sat at around 12.6 Volts:

However, when I start the motorcycle with the switch the voltage across the terminals drops to below 12 volts. This is a problem, right? I can't imagine why however, since the battery is brand new and has been charged.

Any suggestions?

Oh, and I also tried to see if the motorcycle would start if I connected the battery to my charger and set it yo jump start mode, in order to see if it was actually a battery issue. No success there either.

Sent from my Galaxy Nexus using Tapatalk 2

 

[frankenrat]

Some more thoughts:
Have you turned off the head light before trying to start the bike? Looking at the plug in the picture it looks dry. Are the other two plugs the same? You may not be getting fuel to the engine. Check the float bowls, see if there's gas in them. You can back the screw on the bottom of the float bowl out until some gas starts dribbling, then retighten. Have you tried starting fluid? Even though the fuses and fuse box looks good, there still may be a broken or loose connection, that's why I suggested to check for continuity.

 

[manas]

Re: Re: An idiot and a 77 XS750

Some more thoughts:
Have you turned off the head light before trying to start the bike? Looking at the plug in the picture it looks dry. Are the other two plugs the same? You may not be getting fuel to the engine. Check the float bowls, see if there's gas in them. You can back the screw on the bottom of the float bowl out until some gas starts dribbling, then retighten. Have you tried starting fluid? Even though the fuses and fuse box looks good, there still may be a broken or loose connection, that's why I suggested to check for continuity.

Hi Frankenrat. Was just waiting for the battery to charge and then went back into the shed to try again. Checked the spark plugs and they were dry. What puzzles me is that there definitely was fuel inside the carburetor.

And yes, just to confirm the headlight was off. Fully charged the battery shows about 13+ volts, after a few attempts it shows 12.6ish volts. However while cranking it drops below 12.

I'm guessing my primary problem is getting fuel to the engine. I'm certain a reassembled the carb as best I could but I'll take it apart and see if there's anything else I can do.

Also, and this seems like a stupid question but my multimeter doesn't have the audible continuity test function. How exactly do I go about doing it?

 

[frankenrat]

After you put your gas tank back on and prepared to start the bike did you switch the petcocks to prime for a minute or two to fill the float bowls? Push down the choke lever? At this point l guess would spray some starter fluid into the carbs and see if the engine fires. BTW, have you checked for spark?

Two ways to check for continuity without sound. With ignition key to on, just do a voltage check between two points. With the ignition key OFF, switch the setting on your multimeter to read ohms at the 200 or 2000. lf its similar to mine, on the left side of the read out there will be a 1 or l, which indicates an open circuit. When you touch the probes at the test points you should get some sort of reading indicating resistance and a closed circuit.

 

[manas]

Re: Re: An idiot and a 77 XS750

After you put your gas tank back on and prepared to start the bike did you switch the petcocks to prime for a minute or two to fill the float bowls? Push down the choke lever? At this point l guess would spray some starter fluid into the carbs and see if the engine fires. BTW, have you checked for spark?

Two ways to check for continuity without sound. With ignition key to on, just do a voltage check between two points. With the ignition key OFF, switch the setting on your multimeter to read ohms at the 200 or 2000. lf its similar to mine, on the left side of the read out there will be a 1 or l, which indicates an open circuit. When you touch the probes at the test points you should get some sort of reading indicating resistance and a closed circuit.

I did switch the petcocks to prime and had the choke on. I didn't have any starter fluid or a spray bottle on hand. And yes, after switching to new plugs I've got a blue spark which seems OK to me. I'll try starter fluid tomorrow.

By the way, I really appreciate you taking the time to help troubleshoot. Much appreciated sir.

 

[manas]

Also, aside from a carburetor issue what could be a possible cause of no fuel entering the combustion chamber? The only one I can think of is not enough compression due to bad seals in the engine. I really hope that not the issue but can't think of any other reason.

 

[frankenrat]

Working with you on this is beneficial to me too. It helps me keep my mind open to problems that come up with these old bikes and see how they can be solved. Came across another website put up by a fellow who owns 750 triples, had a nice write up on carb rebuilding along with good pictures. His were the Marklls, yours are Markl's, don't know how much difference there is between them, but anyway here's the link http://www.yamahatriples.com/index.php/how_tos/carb-rebuild-mikuni-mark-2/
Double check your pilot jets, make sure they're clean.
Here's the link to his main page http://www.yamahatriples.com/
I doubt it's a compression problem, my triple can start and run on two cylinders, I'm not sure about only one though. Vacuum lines from the petcocks to the intake manifolds clean and hooked up properly?
Drop in voltage is probably due to old crusty and corroded wire connections, creating higher resistance. Lots of times I just use the kickstart, seem to fire with one or two kicks.

 

[manas]

Re: Re: An idiot and a 77 XS750

Working with you on this is beneficial to me too. It helps me keep my mind open to problems that come up with these old bikes and see how they can be solved. Came across another website put up by a fellow who owns 750 triples, had a nice write up on carb rebuilding along with good pictures. His were the Marklls, yours are Markl's, don't know how much difference there is between them, but anyway here's the link http://www.yamahatriples.com/index.php/how_tos/carb-rebuild-mikuni-mark-2/
Double check your pilot jets, make sure they're clean.
Here's the link to his main page http://www.yamahatriples.com/
I doubt it's a compression problem, my triple can start and run on two cylinders, I'm not sure about only one though. Vacuum lines from the petcocks to the intake manifolds clean and hooked up properly?
Drop in voltage is probably due to old crusty and corroded wire connections, creating higher resistance. Lots of times I just use the kickstart, seem to fire with one or two kicks.

Well thank you regardless.

Mark 1s vs Mark 2s: I wish I had the Mark 2s if only because there seems to be a plethora of information about them online. I haven't been able to find a single Mark 1 tear down/reassembly. Nevertheless I do think I put them back together fine. I applied a little bit of bearing grease in the threads of the pilot jets and will check if the grease has possibly gotten into the wrong places.

Petcocks/Intake Manifolds: I shall double check the petcocks and lines, but the carbs had plenty of fuel throughout the process...so surely there should be enough fuel to at least get a cough out of her

Voltage/Kick start: is it true that hoping for a start when my battery can't keep above 12v is useless? If so, I might have two problems instead of just one. Also, my kickstarter is completely stuck. I'm 160 lbs and I can't get it to move. I hope this doesn't mean something else is wrong. I know for a fact that the cylinders have free travel since I can feel air being sucked through the carbs.