1979 GS425 Lean Condition

[campbmic]

Hi! My roomate has a 1979 GS425. He told me that its bogging dowon on him, back firing, and really hard to start. I went out there today and looked at his bike.

It was about 70 outside today. I pushed down on the choke lever and tried to start it. Its VERY touchy. I finally got it started after draining the battery and resorting to the kick start. I held the choke down for about 2 minutes and didn't notice any real change in the RPMs. So I slowly started to rev it trying to get the RPMs to come higher. After a couple of attempts at releasing off the choke and rolling the throttle slightly I was able to get it started and running without holding down the choke lever.

While it was idling I tried revving it up but it was backfiring and popping like crazy. So I got on the bike and took it up to about 5k-7k (he doesn't have a tech bet it felt like 5k-7k range). Right in that range it starts to bog down like its starved for fuel or drowning in fuel. I went through all the gears and right in that range it starts to bog down and surge, like its hunting for energy.

So I hit the kill switch and pulled the clutch in and checked the plugs. They are super white. The bike is all stock with no modifications to intake or exhaust. The air filter was recently cleaned. I pulled the bowls and they appear to be pretty clean. I could also see through the main jets and they were clean. I left the drain plugs out of the bowls and turned the petcock to on and nothing comes out. When the petcock is on reserve it flows freely. I ran the bike on both the on and reserve and in each position it bogs down like this. I also ran the bike with the gas cap off and still bogged down.

I'm not to familiar with these petcocks that have a return line. It seems like its getting gas any help guys?

Thanks!
-Michael (by the way I have a 1975 CB550K1 I hope that isnt used against me )

[Steve]

Back in the day when the 425 was a new bike, most of the magazine testers agreed on one thing: they felt it was the WORST-carbureted bike in the history of carbureted bikes. Note that I didn't say it was the worst bike, just that it had the worst carburetion.

Just because the bowls and the main jets look clean does not mean that the carbs are clean. How long has it been since the bike is KNOWN to have been working well? If it is new to him, it is chock-full of unknowns, so start with a bunch of knowns.

1. Adjust the valves. The valves on these bikes tend to get tighter with age. Tight valves make it hard to start when cold. Even if the carburetion were perfect, tight valves make it hard to start when cold.

2. Clean the carbs. Don't just spray some cleaner, you have to remove the carbs, take them apart and dip them. The dip time will range from several hours to overnight, based on how long the bike has not been running. When you put the carbs together, use new o-rings from cycleorings.com. Do not waste your money buying "carb rebuild kits". The jets they supply are often of doubtful quality and you don't get nearly enough o-rings and gaskets to do the job.

When you re-assemble the carbs, set the idle mixture adjustment screws to about 3 turns out as a starting point. When the bike is warmed up, tweak them slowly to find the highest idle speed.

3. Clean ALL the electrical connections. Start inside the headlight bucket, end at the tail light. Take each connection apart, clean it, apply a bit of dielectric grease, put it back together, move to the next connection.

Overall, it might take you a week or so, but it will be well worth the effort.

.

[campbmic]

Thanks for the help I just picked up some carb dip today but had some questions.

1) I searched for a database of manuals but couldn't find a workshop manual for 79 GS425 any good links?

2) When I was at the local shop I told them the symptoms and they didn't think it was a carb issue. When I took off the bowls they were pretty clean except for a little bit of sediment. The shop suggested the valves might cause this but how could they cause a lean condition to only appear a 5k RPM?

Thanks and I'm ready to break into the carbs once I find a good manual online!

[Steve]

Looking at the bowls only tells you that the bowls are clean.

What REALLY needs to be cleaned are all the passages inside the carb body. They can only be cleaned by a proper 'dip'.

.

[campbmic]

I dipped the carbs yesterday and all of the metal parts. After the dip I blew air through all the passages. I'm very confident that the carbs are properly cleaned. The condition seemed to be a little better but still around 6K RPM the bike surges like its hunting for power. And won't go past that RPM. It also pops and pings a lot out of the exhuast.

[1_v8_merc]

Is the bike running any kind of secondary fuel filters?

Is the airbox/boots sealed up properly?

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I had a weird lean issue like this with my 650.

When I had the fuel filter attached, the bike Idled okay, but rode like crap, no power, and around 5k RPM's the bike would just shake and lose tons of power like it was only sucking in air, and fall flat on its face.

I ditched the fuel filter, problem gone instantly.....turns out the filter was WAY smaller in diameter than the fuel hose, and I guess it choked my fuel supply and forced my bike to run extra lean.
----

But if you're running lean, I would check all air passages for leaks, carb boots and check to see if the boots have O-rings where they mount to the motor, sometimes those look perfectly good, but are completely flat and leaking.

Then clean the carbs(which you said you've done a couple times now?)

Then re-try to set your Fuel/Air mixture screws.


Also, check to make sure your mixture screws are actually working properly, not warped or damaged threads, shot O-rings or springs,etc....

[campbmic]

No secondary fuel filter. One airbox-carb boot has a tear in it but it doesn't seem to be affecting the suction (I sprayed with WD40 and nothing happened) Also, both plugs seem to be the same color

[campbmic]

bump anyone?