5.5-6k stumble...

[yanagawt]

I have a 425 L that is finally back on the road!

Just got it out this weekend and everything seemed good until I get up around 5500, the motor seems to stumble. At first it was almost like it was hitting a rev limit, pulled it back in and re-adjusted the points checked the intake for leaks and ran some seafoam through it.

The problem still persists although it is slightly higher in the rpm range now and after about 7500 it seems to taper off (although it doesnt rev much higher, haha).

The problem seems apparent in all gears and only when the bike is under load (neutral shows no symptoms).

Other misc. symptoms: idles sometimes hovers 2-2.5k flatens out around 1.75k. Throttle response is not too exciting, but I'm not sure what to expect. Checked the plugs and 1 was rich and 1 was good. Not sure if that will give any indications... One last thing, there was no airbox lid on the bike when I got it, so there is a makeshift lid to limit the airflow. Since I'm unaware of the required size, its about 2.5"x2" area.

Few questions, what kind of power should I expect out of this 425? I can get up to 45-50 in 3rd at around 6k but it seems like the bike struggles and takes quite some time to get there.

Could I get some help in the proper steps to begin troubleshooting this issue? Start with the airbox and work backward?

Thanks!

[tlepac]

I would start by getting the OEM airbox first--Ebay might be the way to go--- and then see where you are at. Also have you checked the valve clearances and cleaned--by the Bikecliff tutorial---the Carbs? And don't forget those O rings between the cylinders and the heads! When I checked mine they were squashed flat and fell apart in my hand!
Those three or four things will get you on the way

Good luck!

[tkent02]

Do all the maintenance, Clean the carburetors, seal the airbox, all of that. You should expect a real strong increase in power at around 7,000, and should pull hard through about 10,000 or so.
You have a lot of stuff to sort out.
They are supposed to be quick little bikes.

[Steve]

Fortunately, the 425 was a one-year wonder.

Any of the magazines that bothered to test it complained that it was probably the worst-carbureted bike, EVER. That was during the time that everyone was leaning the carbs down to meet upcoming EPA regulations, and they just did not get it right enough to leave it rideable.

As suggested, the best thing to do is to start with a good stock airbox, then tweak the carbs a bit to richen them up. How long have you had the bike? Has it been like this the entire time or is this something new? If the bike is relatively new to you, I would suggest taking the carbs completely apart and dipping them to clean them, then re-assembling them with new o-rings. Contact GSR member Robert Barr at cycleorings.com to see if he has o-rings that fit your carbs.

It was one of the first of the GS series to get the CV-type BS-series carbs, and the jetting left a LOT to be desired. You can start by making sure the idle mixture adjustment screws are about 2.5 turns out from lightly seated. Stock settings were usually rather close to just 1 turn, so the extra fuel at low-throttle settings makes a world of difference in how the bike runs.

.

[1979GS750E]

Stumble is likely a leaning out

You really need the airbox tight. I've been through it. It will never run without a stumble without the filter in place and the airbox tight.

[yanagawt]

Thanks for the info guys!

The bike is new to me, I've had it for about a month. I had to sort through the wiring and work on some smaller things before I could actually get it on the road. Wanted to make sure it was a runner before I started putting more into it.

I'll be on the hunt for an airbox, if anyone has one for sale please let me know. Or measurements of the cover...?

Another interesting thing I just noticed, the petcock appears to be leaking (carb bowl drains are dripping) so I put a catch can and the fuel seems oily... Its not discolored or anything but has an oil viscocity to it like a 5w30 but smells of gas. Any ideas??

Valves are the plan for this week and looks like carbs are up when I return from my trip to Japan!