Bike revving on startup.

[EDSchmidt]

I've got a 1981 Yamaha xs650 that was my dad's. It was running and driving last fall, till it started running on only one cylinder. In diagnosing that issue (we did get it fixed) we found that the carb diaphragms were torn. After replacing them, the bike will only start at full choke, and it immediately revs way up, if you touch the throttle it dies, if you back the choke off it dies.
This is my first bike and I'm a little lost as how to proceed in even diagnosing the issue. Any help on the matter would be greatly appreciated.

 

[gggGary]

Plugged pilot jets would be my first guess. But your plan should be to remove the carbs and clean all jets and passages using one of the guides found in the tech section. That will also involve a visual sync probably throttle shaft seals and finally an engine running gauge sync.
A wise owner will clean his fuel tank and petcock at the same time. If the fuel lines are old replace them too.

 

[gggGary]

PS welcome aboard!
Pics we gotta see pics!

 

[EDSchmidt]

Thanks a lot! I'll try that and keep ya posted!
Ah ain't much to look at now, I'm hoping that once I get it running I can give it a coat of paint...

 

[gggGary]

Any time you have one of these Mikunis open check that the orifice in the float bowl is clear, it's the first thing dirt or rust in the gas will plug.

 

[EDSchmidt]

Any time you have one of these Mikunis open check that the orifice in the float bowl is clear, it's the first thing dirt or rust in the gas will plug.
View attachment 369881

Thanks, good to know.

 

[JJames]

Welcome. We like pics of crusty bikes too.

 

[EDSchmidt]

Took the carbs apart as far as I could see how, cleaned everything, put everything back together, and it ran on half coke for about a minute, but died when I touched the throttle.

Also noticed it's back to running on one cylinder, I'll have to have a look at the plugs, coil and wires. Maybe if I can get it to run on both then I can better understand what's going on with the carb?

At any rate, it didn't spike the rpms when it started and now we're down to half coke so I'd say my evening was a partial success!

 

[Adamc]

Welcome EDSchmidt,

Good bag of bones to start your adventure.

Normal practice here is to first chase electrical / ignitions faults and cure them.
Then once the bike is running sort fuelling issues.

Trouble is if the thing has been standing with fuel in the carbs you won't get it to run well if at all.
Small jets and orifice will be blocked / varnished over.

I would completely remove the carbs, remove all jets and floats etc; and clinically clean everything.
Blow through with carb cleaner and follow with compressed air.

Check all airways / passages with a bright pocket light source.

Reassemble and reinstall.
Now you have a chance of getting it to run with a clean filtered fuel supply.

Check my build thread here:
https://www.xs650.com/threads/welsh-flat-track-revival.60449/page-13
It may help with cleaning carbs; and details my tribulations chasing mis-fires etc.

Watch this:

Next chase the Iggy gremlins.

 

[EDSchmidt]

So I visually inspected the wires and caps, every thing looks good and tight. When I spin it over with the plugs out and held against the block, I get kinda meh spark outta the right wire and almost non existent outta the left. Makes no difference to swap the plugs side to side. Crossed the wires and it would fire on the left instead of the right like it used to.

So does anyone know if the Mike'sXS version of this style of ignition is any good?
The bike has pointless ignition, and it's a single coil, duel output type.


Also, are these the idle mixture screws? If so does anyone know the adjustment spec for these screws? It's an 1981. My Clymer service manual says there should be no idle mixture adjustment on a 1981 carb, yet I took these screws out and they have springs so I thought they might be adjustable.
Not sure whether these are indeed 1981 carbs.

 

[gggGary]

Yes 81
Foam floats


Testing that coil both plug bodies need to be grounded. Ohm out the plug caps they should be in the neighborhood of 8K ohms Bad caps are pretty common.

 

[EDSchmidt]

Both plugs were grounded...
I'll check the caps like you said.

 

[Jim]

That choke (enricher) lever is bent... most likely not letting the choke close all the way.

 

[EDSchmidt]

Could be, I'll have to take a look.

Dad also had issues just before he parked it, with the bike quitting out on the hiway and not restarting till it had cooled off. From what I've been seeing that sounds like the coil?

 

[Jim]

Easy enough to swap the coil. A Honda MP08 coil is a damn good replacement for these bikes... a used one off Ebay, not the brand new Chinese junk knockoffs they sell. I think there's a writeup in Tech about the MP08's.

https://www.ebay.com/itm/366350767487?_skw=honda+goldwing+coil+mp08&itmmeta=01KPYRPRTWYSH88HCDMB3E1VGR&hash=item554c35157f:g:kyQAAeSwzJ1p4Ou9&itmprp=enc:AQALAAAA8GfYFPkwiKCW4ZNSs2u11xACZW/1Y5ABIgXlRwLqPmPFCki33sHmWvc8KsJWoIhgAlmp56IVAFEEryg85Ja0vsC/M7FM+B90NvL0ONoGyEOIvQ/6LTWOpcnLBFocC6kYzsc4SGuLAwiwFeC/oxkAvw2gtZqAkJofn9EHU2f1pzERA056F+spXHHkHFywr+iP7AqNZZz9UGG4PdzBavFZnaZAeCYv0d9Cz8+rg3jltes8WJp8hgrX6UK2Prd3sm+oBQc/2w7yKYqAYrzcsv6MCrjHYb3KtWIuJJkIDyg1hsKlT3BCxhJ5q8tWCVioaDxWCg==|tkp:BFBM4o3b2Ldn

 

[Jim]

From Tech.
https://www.xs650.com/threads/honda-mp08-coil.55408/

 

[JJames]

In regards to mixture screws. Yes those are them. Come from the factory capped and lean. With caps removed, around 3 turns out. Looks like the guys are getting your ignition troubles sorted. Have fun.

 

[EDSchmidt]

In regards to mixture screws. Yes those are them. Come from the factory capped and lean. With caps removed, around 3 turns out. Looks like the guys are getting your ignition troubles sorted. Have fun.

Thanks!

 

[nj1639]

As far as the ignition system, the TCI that your bike came stock with is a good’un , no need to frustrate yourself any more than you have to.

 

[EDSchmidt]

Yeah the caps were sitting at around 4-6M ohms,(4000-6000k ohms) so really high resistance.

Have ordered a good used MP08 coil and wires, we'll see if the caps are any good. If not I'll order new.