Great idle, Bogs with Revs

[BadYamaJama]

Hey all,

Was hoping to get the collective's quick thoughts on something interesting.

I've got a mostly-stock '83 that I've just pulled out of the garage for the first time since last fall, and it's got a rather large issue.


It starts and idles fine, but as soon as I roll on the throttle the engine immediately begins to stutter/stumble and otherwise feels bogged. Like it makes practically zero power.

Here's what I did since then:
1. Disassembled the stock BS34 carbs and thoroughly inspected/cleaned all parts. Diaphragms are in great shape, jets were all clear. Right side carb a bit out of sync but I didn't make/build a sync gauge yet to test. Just bench synced. Replaced o-rings/gaskets.

2. Cleaned air filters, oiled lightly
3. Replaced fuel line & in-line filter
* Fuel is flowing great through the clear filter
4. Checked electrical, meaning:
* Charged battery (Can't test voltage b/c the bike won't rev past 1200 RPM)
* Replaced potentially burnt 20A fuse
* Tested the relatively new regulator/rectifier (Good ol Mikes XS unit)
* Replaces spark plugs (Left side a bit black/rich, Right side more brown) & shortened lines
* Recently replaced alternator brushes / slap tested OK
* Checked timing & cam chain approx. 300 miles ago (Before Winter)

Here's the weird stuff:

1. I think the ignition coil is original - Measuring it's resistance is giving me weird readings.
2. Just found out the TCI box under the frame is missing it's cover and is exposed to all the road grimy goodness
3. Wiring harness looks original. I cleaned up all the contact but it's far from perfect.

Is it possible for the bike to run at idle just fine, but get completely out of whack as soon as it receives gas? I know for a fact the carbs are in pristine condition, plastic floats are at the right heights, new o-rings, etc. so I'm thinking the coil is bad?

Any immediate ideas? I'm already planning on going through the TCI box to check the diodes, etc. and possibly replacing the coil while I'm messing with her.

Thanks!

 

[pamcopete]

Bad.....,

As a test, try unplugging the regulator / rectifier.

 

[BadYamaJama]

Thank you - Will unplug and report back asap.

 

[BadYamaJama]

Ok, Reg/Rect tested well at all 0's and ~0.51's across both the positive and negative.

Coil is showing 2.4 Ohms.

Currently extracting the TCI, but even that looks ok. Any ideas?

 

[chizler62]

Mouse nest in your exhaust.Loosen head pipes and break seal. Cover ears.

Cover off TCI box?

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[pamcopete]

Ok, Reg/Rect tested well at all 0's and ~0.51's across both the positive and negative.

Coil is showing 2.4 Ohms.

Currently extracting the TCI, but even that looks ok. Any ideas?

OK. I think that you misunderstood me. Unplug the regulator / rectifier. The plug is under the left side air filter cover.

After you have unplugged the regulator / rectifier, start the engine and see if the problem is still there.

What we are looking for here is either a weak trigger magnet on the rotor or an overly sensitive TCI that is influenced by the magnetic field of the rotor.

 

[XSLeo]

On my 81 with low battery voltage it would act that way, idle great but nothing much above idle. Fully charge your battery. Then try running it with the reg/rec unplugged.
If it runs ok this way your charging system might not be working right. It had a bad rotor. It would charge a bit but not enough to fully charge the battery.
Leo

 

[BadYamaJama]

Thanks for all the great suggestions.

Pete/Leo, ya'll were right on. I unplugged the reg/rect and she runs like a top!

So now the question is...what's next? I remember testing the rotor/stair when I replaced the damaged electric starter 'clip'.

Guess I'm going to have to read through curly's guide once I get back to a desktop.

 

[XSLeo]

On the TCI bikes it has a small magnet in the rotor. The magnet passes by the pick ups on the stator. This sends a signal to the TCI box. The TCI uses this signal to tell the engine position and rpms.
The magnet gets weak as it ages. It can get so weak that with the reg/rec plugged in the magnetic field in the rotor over powers the small magnet.
This is what unplugging the reg/rec indicates. This is an easy fix.
Radio Shack sells a two pack of rare earth magnets. They are 3/16 inch or 4 mm in diameter. They cost about $3 a two pack. Just went to their site to get the number and found, Item #64-1895. On sale online now at .99 per two pack. I may order several, just in case.
Just "glue" one magnet on top of the magnet in your rotor. I like JB Weld for the "glue" You can use any epoxy glue you like.
This is a very strong magnet and being mounted out a bit further will give a very strong signal for the TCI box.
Leo

 

[BadYamaJama]

Fantastic - I went to Radio Shack and found the magnets. Just JB Welded one on, will report back late tomorrow once it's had time to cure.

Thanks again.