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Poor (wierd) running characteristics

  • Thread starter Thread starter Smokinapankake
  • Start date Start date
S

Smokinapankake

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So I've had my 84 GSX 750SE (pop up headlight Katana) up and running for the past month or two, and for the last few days its been displaying some odd running characteristics. I would do a search but don't really know what keywords to search under.

Anyway, its been running very well up until a few days ago. In 5th gear, about 50 - 60 MPH, steady throttle opening, it starts bucking and jerking like its running out of gas. This is intermittent, not constant buck n jerk, mind you. First time I thought it could be water in the gas as it sat out in a day long rainstorm; the second time I thought it was running out of gas based on mileage since last fill up so I filled the tank. Took four gallons and some change..... A few minutes after leaving the gas station, same thing starts. Off n on buck n jerk, only this time I'm thinking it could be cutting out on a cylinder or two. Open the throttle a little and it accelerates as normal. I've checked the fuel valve (petcock) and it is operating properly. I've ensured the tank is venting properly (I learned this lesson the painful way on my 83 GPz).

Here's what has been done: Carbs are spotless. I just overhauled them this winter using the guide posted on GSR. New o-rings. New intake boot o-rings. New airbox boots. New Uni air filter, oiled using Uni oil in direct compliance with the directions. Factory airbox.

I'm guessing three things:

Coils
Wires/plugs
Ignition/igniter.

The coils and wires are the factory original, 25 year old units. Wires are integrated so non replaceable (I don't see how anyway). I have 11.8 volts at the coil wire (orange) so no voltage loss between there and battery.

Plugs could stand to be replaced - they are old but not the originals.

How would I test the igniter? Seems to me that if the bike starts, idles, runs, accelerates as normal the igniter is not necessarily suspect....

I have a set of Dyna coils and new(er) wires I can install off my 82 Katana 1000 to test the coils/wires theory, and may do that as an upgrade.

Anything else I should check?
 
How are you measuring that 11.8 volts at the coil?
If the bike is running, you should see 13-14 volts there, so 11.8 would be quite low.
If you just turned the key on without starting the bike, it's not too bad.

When the bike starts bucking again, try adding some 'choke' to see if it clears a bit. To me, it sounds like a fuel delivery issue. Yes, you said the carbs are "spotless", but have you examined every drop of gas that has gone through them? I get the impression that something is parly clogging one or more of your pilot jets. When you add some throttle, you move to the needle circuit, so it clears up.

.
 
Wires are integrated so non replaceable (I don't see how anyway).
Click to expand...

NGK make a plug wire splice for this purpose. Cut your old wire off splice in the new wires.

It doesn't take much to clog up your pilot jets. Dirty gas, crud from your tank, a petcock sceen that lets fine particles into your carbs. Are you using an inline filter?

As a maintenance note, the plug wires and resistor caps should be replaced at least once every 25 years. Both are relatively inexpensive and easy to replace. If you want to upgrade to dyna or accel coils it certainly wouldn't hurt. New plugs would also be a good idea.

It sounds to me like this should be a relatively easy fix. Make sure your tank and carbs are clean. Replace the coils, wires and plugs. If the bike is running your ignitor is probably OK.
 
Steve,

Key on, bike not running.
I'll try the choke idea tomorrow.

Yes, coils, wires, plugs should be replaced. I was just so stoked to have decent enough weather to be riding again! Didn't know about the splice of which you speak. Could you show me a pic, please?

Thanks for the suggestions!
 
Did the choke on make a difference? My feeling is that since the issue just started on it own... something had to change on its own.

Clip/Carb settings dont change, jet sizes dont change, I am thinking it HAS to be fuel related issue, since its a SPEED related (read fuel/air flow) related issue.

As good and clean as the carbs are, did you check everywhere? It can take the smallest piece of crud to mess everything up. I would blow them out with compressed air again, and adjust evertything in and out and then back to the old setting.

Same thing happened to be on another bike, the filter was clogged... in the tank. This bike has a screen filter, in the tank.

Steve
 
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