Topic: 1979 GS425 Lean Condition + Backfiring Even After Carbs Dipped

[campbmic]

Update: Well I took the carb apart and took my time and dipped them. Put them all back together and the bike is doing the eact same thing! What else should I look at?


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 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

[scottly]

Since you mentioned a petcock with a return line, I'm guessing what you have there is an automatic shut-off type petcock, which prevents gas from flowing if the motor is not running. The extra line is a vacuum line, that operates a diaphragm in the petcock when the motor is running. If fuel comes out of that line, it would suggest a ruptured diaphragm. Just guessing, but it does sound like the motor isn't getting fuel....   

[Rampage1967]

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 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?

It's not a return line. It is a vacuum line to open the vacuum valve when the engine is running. To fill dry carbs for starting set it to PRI. No fuel will flow in RES or ON without vacuum to that "return" line.

Thanks!
-Michael

Well, to start, I am relating my own experiences from an '83 GS450L, but I am pretty sure they will apply to your 425 as well.

On the fuel valve:
It's not a return line. It is a vacuum line to open the vacuum valve when the engine is running. To fill dry carbs for starting set it to PRI. No fuel will flow in RES or ON without vacuum to that "return" line.

These bikes are run quite lean stock. Spark plugs come out very white (at least on the 450 they do). Any air leaks on the intake or the exaust side will have a very large effect on performance. My 450 backfired & set the airbox on fire one cold day, the resulting hole in the airbox made the bike so lean it was nearly unrideable.  Please check for this kind of problems.

Make sure that if this is a points ignition bike the timing is set perfectly.

Now: remove the carbs & clean them very well.
Do not separate them from each other unless you have to.
Make sure all of the jets are clear. (This may require carefull use of wire)
Use carb cleaner and compressed air on all the passages inside the carbs.
Make sure the diaphrams are not damaged and the needle clips have not fell off if you have adjustable needles.
Clean the emulsion tube holes in the needle jets & pilot jets too.
when you reassemble, make sure you set the float levels to spec.
make sure the carbs are synced fairly close too (you can do this visually)

Assuming this bike has no non stock holes in the air box, pod filters, aftermarket mufflers etc, it should run good when reassembled. If it has any of the above you will need to rejet to get it to run right (I installed a jet kit on my 450 to fix the leanness).  Those early 80's Suzuki twins were a bit cold blooded stock.

[scottly]

Thanks for reminding me about the PRI (prime) setting on the petcock, Rampage. The first time I dealt with one of these things, I had to suck on the vacuum line to get gas to the carbs. I later found what PRI meant...

[campbmic]

I took the carbs off and throroughly dipped them, did not have to seperate them and blew air through all of the passages and jets. The carbs are in great condition and I put them back on the bike. It is still VERY difficult to get warmed up and still back fires, pings, and doesn't rev past 6k.

It seems like its lurging at 6k like it really wants to go but just can't quite get there.

[w1sa]

Sounds like it could be slow fuel delivery (filters,tap, vent, lines partially blocked, diaphram cutoff malfunction) and/ or fuel levels in the bowls are too low when running.

Could also be a lean idle mixture causing the popping, especially when warming up.

[campbmic]

I took the bowls off and turned the gas on and it ran fine from there. The jets are 100% cleaned up. Could an electrical or timing issue cause this? I'm pretty confident that I've ruled the carbs out 100% and kind of confident the fuel system is ruled out. What else should I look at?

[w1sa]

If the timing is set/working correctly and air leaks are eliminated, and you're getting white plugs at 6Krpm, its not likely the ignition is the main problem. If you weren't getting good spark at those revs, it would probably start fouling, and leave darker deposits as its combustion performance fell off.

I think the carbs you've got are CV carbs. The main jet/needle is not activated by the throttle twist alone. It requires sufficient vacuum to be created in the piston above the needle for it to rise and allow a greater volume of fuel into the throat. If the needle does not rise or only rises partially, the bike will accelerate upto a point where it cant get enough fuel into the cylinders to continue increasing its speed............... That sounds like what's happening here.

So when you check the carbs' jets/tubes /passageways, also pay attention to the upper piston/slide and seals, and the internal upper air feed passageways that control the vacuum to the carb piston etc.
When re-assembled ensure the piston/needle moves smoothly up into its vacuum cylinder and returns freely. Gently blowing compressed air into the forward carb mouth air passageways should cause the piston to move a good part of its travel.

I'd also do a clear tube check of the fuel level in the carbs and verify the correct airscrew settings.

Be careful of running at sustained higher throttle while its burning so lean, you might blow a hole in the piston(s).... .