#3 carb not getting fuel or sizzle on exhaust

[Cyg]

I took them apart again today. I have Gs 1978 chain 750 Suzuiki mikuni slide carbs.

I totally dipped em, set the floats too 25mm. They are clean, shot air thru them etc. I have the stock everything on it so no need to change anything. Bike runs strong, and I can even take it out on three cyclinders if I wanted too.


Checked spark, swapped the coils 1 for 4 3 for 2. Its not the plug, I even put in new plugs. Its the gas going too the the number three carb. If memory servers right that is the main carb as well that feeds the other three?

Ok carbs gurus lets hear your suggestions. I will apply them tomorrow if you respond right away.

seats are good idle screws set too 1 1/2 turns. bleed screw backed out two turn on each carb.
Floats are getting gas. the drain plug shows this.

I use water on the pipes too check for sizzle. that way I know all four are firing .

 

[frosty5011]

What's the #3 plug look like when you pull it? Dry sooty black? Wet?

 

[Cyg]

dry. even when I switched the number one for the three it is dry. I wonder can it be in timeing? That is about the only other thing I have not done too this too bike. I have workded on over 25 in the last three years. Sold most of em. This one has my stumped.

 

[ptm]

Check the vent/vacuum hoses that come out of the side of the carbs.

#1 has none
#2 has a vent hose that should be left open to the atmosphere
#3 has a vacuum hose that should go to the petcock to allow fuel to flow only when the engine is running. If you've replaced the petcock with one that doesn't need the vacuum hose, then plug the nipple. If it isn't attached to the petcock or plugged it will suck in a lot of air and #3 will be too lean to fire (ask me how I know...)
#4 has a vent hose that should be left open to the atmosphere.

Normally the vent hoses go up over the airbox and down the back side (toward the right) through a small bracket.

Good luck...

 

[Cyg]

this sounds good .

two has her vent tube on her. it drapes over the stock air box too a clip in the back. Now I can cut her short. instead of having her drape over.

Three has the main gas flowing into it. well not really three but its close there. The petcock vaccum has three attached too it. Now I could undue three and leave it open too the air.

btw you asked me to ask you how you knew. So tell me.

Four has the normal vent tube cliped in the back of the airbox. Now when I attached too the air box. I mean tucked under this one tab.

I will try and disconnect the vent tube too three and plug her up. Shes the only one that wont pull down her gas.

 

[Guest]

What??????

 

[BadBillyB]

Bike runs strong, and I can even take it out on three cyclinders if I wanted too.

If the bike runs strong but the pipe is cold at an idle, try bench synching the carbs. That one slide may be lower than the rest.....BadBillyB

 

[ptm]

this sounds good .

two has her vent tube on her. it drapes over the stock air box too a clip in the back. Now I can cut her short. instead of having her drape over.

Three has the main gas flowing into it. well not really three but its close there. The petcock vaccum has three attached too it. Now I could undue three and leave it open too the air.

btw you asked me to ask you how you knew. So tell me.

Four has the normal vent tube cliped in the back of the airbox. Now when I attached too the air box. I mean tucked under this one tab.

I will try and disconnect the vent tube too three and plug her up. Shes the only one that wont pull down her gas.

See the picture

The only hose that has gas in it is the larger one that goes from the petcock to the 'Tee' between #2 and 3.

The smaller hose running from the petcock is a vacuum hose. The petcock uses the vacuum to open the petcock and let gas run through the larger hose. If you have the stock petcock, do not remove or plug that hose.

The small hoses from #2 and 4 should not be cut short. They run over the airbox and through the clip so they don't get any water in them.

 

[Cyg]

Thats the exact setup I have on this bike. It,s
almost identical. Ok if I don't plug up the third vent tube. I am still at when I started this post. I even benched check all the hieghts. slides etc. I ran into a new problem when I dipped the bleed screws. I had to take off the rubbers. and they didnt soften up enough when I sprayed on armor all. They ended up splitting in two. I found some other o rings. But they are a slight bit oversized. So I squeezed em down till they set just right. I have them on the bike now checking for leaks with the tank on it.

So if I don't plug up that vent tube I will be back at trouble one again. What else could cause a carb too fail on delivering its gas too a single cyclinder? All four bowls fill up just fine. The petcock I removed and cleaned up again. Im stumped still

its not a vent tube but a vaccum tube I going to plug to try and draw gas in

 

[ptm]

The problem I had, which sounds like what's going on with yours, is I didn't have the vacuum hose from #3 hooked up to the petcock. That had two effects
1) I didn't get any fuel to flow unless I had the petcock set to PRIme
2) #3 carb was sucking in a lot of extra air through the vacuum hose so even though it had enough fuel, it had way too much air. The resulting mixture was so lean that it wouldn't fire.

Plugging the #3 vacuum hose (or connecting it to the petcock) will take away the extra air and it should run fine (or at least eliminate this issue). If you have the stock petcock, it needs the hose to open the valve to let fuel flow when the engine is running and stop the flow when the engine is not running, OR, you need to have the petcock set to PRIme, which will let the fuel flow whether the engine is running or not. That will work, but there is a safely issue of allowing the fuel to flow when the engine is not running (like after a crash), and if your needle valves aren't sealing well it will overflow the bowls.

The two vent hoses (from #2 and #4) really don't have much effect as long as they're not plugged. Some folks running pods claim better performance is the hoses are removed, but the nipples left open.

Hope this helps.

If none of this makes sense, or doesn't seem to help, then let's get back to the basics...

Are you using the stock airbox, filter and exhaust system?
Are you using a stock petcock?
Was it running before?
Did you do a good bench sync when the carbs were off the bike?
Have the valves been adjusted?
Have you done a compression test?
Is the ignition timing set?
Are you getting spark on the #3 plug?

 

[Cyg]

found out that the vent tubes does need too be hookedup too this number three carb. its funny we must think alot alike. I ran a test with it plugged on prime. it ran on three cyclinder just like before. #3 would get slightly warm but not hot.
Next test. swapped the coils again 1 for 4/ 2 for 3. This time ran it on reserve. Found out that it need the vaccum tube too draw gas into three. This time three got hot(hotter than before) but not yet a sizzle like the other the three does. I noticed too when I swapped the coils out. I got more blue smoke in the rigth side of the bike when riding this time. Before I swapped them out I had no blue smoke. Now we are talking a faint hint of blue.

About the only thing I havent done yet is timing. You would think since I am getting fuel with the swapped coills too three now. It might be just that last thing I need too do??

One other question can you run a bike at full performance without any damage on swapped coils?

So I would deduct shes headed for a timing test then? I could take her out as she is right now. But I am a fussy guy when I want my bikes too run at thier peak performance. I might sell her after I am done painting her too. I dont need three titled bikes. and I am the only rider now do I? lol

 

[NickNoo]

It may be worth double checking the #3 mixture screw. Maybe open it by a half or even a full turn, to richen the mix. Its possible you are running slightly lean on that cylinder.

 

[Cyg]

Hey just took her out on the road tonight. she is finally now running on all four cylinders. Shes got her power back. I can't really pin down what fixed it. But I do want too tell you I appreciate all the immdeiate responses.

here she is before I paint her. now running strong. I still have to get a new set of gauges for her. The tach meter runs backwards I am assuming the pin broke in her. The rest if fine.

She has the following,
New tires front and rear and balanced by mself
Not one lick of rust on the chrome. Poslished that myself too
No rips in the seat
no dents in the tank primed and ready for paint. wet sanded with 600
rebuilt brakes front and rear
new battery
new air hoses.
rewired and heat shrunk wrapped all the wires
connectors match thier original colors.
new bulbs. all lights work
rebuilt petcock
cleaned and dippped carbs.
new plugs.
all rubbers on her still there. including the ones for the side covers.


Now what she needs.
New air filter
new tachometer
paint.
no seat latch too lock it up. had too drill that out too unlock it.

Now imagine this bike being a barn find. and coverd for years in dust and poop and when I got her no tank, no headbucket. no turn signals, no tires. and rusty.

http://netnet.net/~walnutw/Uploads/Gs%20750%20primed%20tank/

 

[ptm]

Glad you got it running. Swapping the coils should have no affect on how well it runs. Yes, you should set the timing. That will have a big affect on performance. I'd also do some plug reads to see if the carbs are set up properly. Do some searches on the forum for detailed info on fine tuning the carbs. There is also some info in the VM Carb Rebuild article in the GSR Garage http://www.thegsresources.com/files/vm_carb_rebuild.pdf

Have fun. It's a great bike.

btw Three is NOT too many...