Frustrated

[faber]

Hey guys,

So I'm very frustrated with this bike. I cannot seem to solve this run away / slow to return to idle issue. I've changed out jetting, restricted airflow, gone through the carbs, verified no leaks, etc, etc.

The only thing I can think of is that the TCI/CDI unit is faulty, have any of you experienced this?

On start up when cold it starts with full choke only, once started it races up to 4500 RPM, if I kill the bike and turn it back on quickly (just a quick blip on the kill switch) the RPM drops as it should, back down to idle and it idles nicely there until you touch the throttle again.

Once the bike warms up it's either slow to rev up, or idle back down. I've gone through 20+ jetting combo's and the only thing I can think at this point is the CDI unit is failing (which I've seen many times on honda's). I'm really getting to the point of where I'm thinking of just ripping the bike (title in hand) apart and parting it out.

Thoughts?


Bike details:
This is the spoked wheel version with the drum front and rear.
1982 xs400 special II with TCI/CDI (not mechanical timing)
BS34 carbs, currently with pods, had the original filters on it, made no difference
Not sure which combo of jetting I've currently got in it, but it's running rich, when it was running right, or lean it still had this run away throttle / slow to idle down problem.

Rebuilt carbs from the ground up, replaced everything except diaphragms which looked fine, and passed leak down test
compression is a little low at ~116 on each side.

Vacuum line is good connecting to rebuilt diaphragm petcock, other side is plugged, carbs are vacuum balanced.

 

[xschris]

Diaphragms could have a leak. Could still have a plugged passage in the carb Any pics of what carbs you have?

 

[Scorpio1963]

Only things I can possibly think of would be your throttle cable is stickingHave you checked the rubber vacume hose going to rubber hpipes to your carbs?Missing orings in the air mixture screws?Is there anyway to test your cdi box for proper voltages or something?

 

[16VGTIDave]

Got a timing light? Check and see if the timing advance is on the curve. That will either verify, or rule out, the TCI module.

I'm thinking that XSChris is on the right track with his suggestions. Also verify that the pistons in the carbs raise and drop smoothly. I've made the mistake of not having the needles seated properly in the pistons, and they were binding in their jets causing the pistons to hang.

Dave

 

[faber]

Yeah going to check the timing tomorrow, the carbs have been gone through over and over, they've been sonic cleaned, the diaphragms passed leak down test which should rule out crack / puncture. Vacuum hose is good to rebuilt petcock, and the other side is sealed. the carbs are balanced.

xschris they're BS34's this is an 82 special II.

 

[faber]

oh, xschris, the throttle cable isn't sticking, the whole movement is nice and fluid, it's a new cable on a completely rebuild throttle.

 

[bentwrench]

I don't know if the angles work on your bike, but they do on mine. Remove the pods so you can see into the carb bodies, and watch what the slides are doing in your carbs. Is one side sticking open? Are they both sticking open? Or are they closing properly, indicating a vaccum leak?

 

[faber]

Thanks for the suggestion, I actually already tried this, the slides fall correctly and concurrently.

Interesting if the bike is running away on throttle I can temporarily kill it (quick off on on the switch) and it drops down to a nice idle. This is what tells me it's electrical more than anything. I've got a few other things to try later and see if it helps.

 

[Scorpio1963]

How much is cdi box?I would try it

 

[Drewcifer]

Check the carb boots are fitted correctly. This will cause a racing/high idle.

 

[faber]

Hi, the carb boots are brand new and seem to be sealed correctly, leak testing didn't yield anything interesting.

A new CDI box is ~180+ on ebay, or I can spend about that on an Ignitech one which, if this is the problem will solve it. I'm just not sure I'm going to do that yet.

I did end up replacing a CDI on my CX500c restoration project and it fixed _all_ of my ignition problems.

 

[Scorpio1963]

Hi, the carb boots are brand new and seem to be sealed correctly, leak testing didn't yield anything interesting.

A new CDI box is ~180+ on ebay, or I can spend about that on an Ignitech one which, if this is the problem will solve it. I'm just not sure I'm going to do that yet.

I did end up replacing a CDI on my CX500c restoration project and it fixed _all_ of my ignition problems.

Have you checked all the connections going to this box also any other connections on your wiring harness?Trace down all the wires make sure nothing is shorted out.If that doesnt work looks like a new cdi is in your future.

 

[16VGTIDave]

Interesting if the bike is running away on throttle I can temporarily kill it (quick off on on the switch) and it drops down to a nice idle. This is what tells me it's electrical more than anything. I've got a few other things to try later and see if it helps.

Keep in mind that hitting the kill switch will drop engine RPM and vacuum. You may have carb problems, but are being misdirected by these results. Keep an open mind, and try the least expensive fixes first, is all I'm trying to say.

Dave

 

[Drewcifer]

The test for bad cdi is remove/replace. So if it's not that....

you'll still have to track down the other gremlins. My only experience buying a cdi proved it wasn't the cdi. ended up needing a carb kit.

So...hang in there.

With the bike running and the idle high, put a lever beneath the carbs and gently tilt them. if they're not sealed correctly the shifting will alter the air that shoudln't be getting sucked in and you'll drop your revs.

 

[xschris]

How much is cdi box?I would try it

These bikes use a tci NOT a cdi (transistor controlled ignition) What do you have your mix screws set at. take some pics of your setup so maybe we can see what it all looks like.

 

[jmd_forest]

Throttle shaft seals?

 

[xschris]

You also said you are using replacement diaphragms. There is a hole in the slide that needs to face the pilot mix screws, do they? Also with a low compression engine there might be an issue with a bad sealing intake valve causing pressure to push back through the carb making it idle go up. I had this happen with a xs650.

 

[faber]

Hi guys,


Thanks for all your suggestions, I'll have to try this, this weekend.

xschris - yeah TCI sorry I was using CDI as in generic, either way the ignitech boxes will work on our bikes (btw, they do rock big time if you've not used one yet).

The throttle shaft seal is an interesting one to test, though I would have thought leak test would have found it but I'll look again.

 

[Flexeril]

sub for reference...

 

[Scorpio1963]

You also said you are using replacement diaphragms. There is a hole in the slide that needs to face the pilot mix screws, do they?


I didnt know this time to take the carbs apart and check this.

Edit theres a tap on the rubber of the diaframs that lines them up so as long as your paying attention they cant be put in wrong.