Very Tricky Situation... Input would be much appreciated.

[ThunderBaird]

Thanks for looking at my thread..

Okay, so. I have a 1977 GS 750 that I love. However, I sort-of set the airbox on fire after I sprayed starting fluid into it and the bike backfired. Since then I have bought pod filters, which are a good conterpart to the 4-1 V&H exhaust that was on the bike when I bought it. I have also totally rebuilt the carbs with new jets and gaskets. Stock jet sizes are: main 100, pilot 15, needle jet O-6. Without going too far into it, I now have these jets: main 102, pilot 27.5 (or 22.5- there is a 22.5 on the jet that has a stamped line through it, and the 27.5 is clear.) The bike is now running but it seems like I just can't get the mixture right... At idle, it fouls my spark plugs in a matter of hours, yet my carbs start sputtering if I lean out the mixture too much. I have size 105 main and 117.5 pilot jets coming in the mail. Does the pilot jet affect the entire throttle range so much that even with aftermarket pods and exhaust on the bike, it runs rich as a mofo with oversized pilots? With the size 15 pilots in the carbs, the bike would idle in the 2300-2500 range, so I quickly looked at the old jets that I had replaced with stock and lo, the pilot jets were 27.5 (yet the mains were at 97.5... odd.) Also, the exhaust baffle is all black on the inside... could this also be dampening my air flow even though it's an aftermarket? Would a carbon-crusted V&H under-perform compared to a stock exhaust? Is there a way to clean my exhaust out without having to replace it? And instead of buying new sets of plugs, I keep two sets that I switch out for each other after cleaning them with a wire brush and fine sandpaper. I also don't know when the oil was last changed, and my manual doesn't tell me what size oil filter my bike needs.

Things that I do know- The floats are at correct heights, the carbs are clean and have good airflow, the slides are synced (almost totally closed at closed throttle), the air screws and fuel screws are always adjusted in sync, I have a crankcase breather, the battery is new, all of the plugs are getting spark, the petcock is functional, there is minimal rust in the tank.

So basically, am I still looking at a matter of carb adjustment, or am I dealing with something that cannot be fixed with carbs (i.e. burning oil, bad timing, clogged exhaust.)

Any insight would be very much appreciated, this stuff keeps me up at night!
Again thanks for reading, have a super day.

 

[kdo58]

check this out if you haven't already
http://members.dslextreme.com/users/bikecliff/

 

[ThunderBaird]

Oh, right. One more thing.

Oh, right. One more thing.

I do have a Clymer manual for my bike.

 

[kb_air]

Thanks for looking at my thread..

Okay, so. I have a 1977 GS 750 that I love. However, I sort-of set the airbox on fire after I sprayed starting fluid into it and the bike backfired. Since then I have bought pod filters, which are a good conterpart to the 4-1 V&H exhaust that was on the bike when I bought it. I have also totally rebuilt the carbs with new jets and gaskets. Stock jet sizes are: main 100, pilot 15, needle jet O-6. Without going too far into it, I now have these jets: main 102, pilot 27.5 (or 22.5- there is a 22.5 on the jet that has a stamped line through it, and the 27.5 is clear.) The bike is now running but it seems like I just can't get the mixture right... At idle, it fouls my spark plugs in a matter of hours, yet my carbs start sputtering if I lean out the mixture too much. I have size 105 main and 117.5 pilot jets coming in the mail. Does the pilot jet affect the entire throttle range so much that even with aftermarket pods and exhaust on the bike, it runs rich as a mofo with oversized pilots? With the size 15 pilots in the carbs, the bike would idle in the 2300-2500 range, so I quickly looked at the old jets that I had replaced with stock and lo, the pilot jets were 27.5 (yet the mains were at 97.5... odd.) Also, the exhaust baffle is all black on the inside... could this also be dampening my air flow even though it's an aftermarket? Would a carbon-crusted V&H under-perform compared to a stock exhaust? Is there a way to clean my exhaust out without having to replace it? And instead of buying new sets of plugs, I keep two sets that I switch out for each other after cleaning them with a wire brush and fine sandpaper. I also don't know when the oil was last changed, and my manual doesn't tell me what size oil filter my bike needs.

Things that I do know- The floats are at correct heights, the carbs are clean and have good airflow, the slides are synced (almost totally closed at closed throttle), the air screws and fuel screws are always adjusted in sync, I have a crankcase breather, the battery is new, all of the plugs are getting spark, the petcock is functional, there is minimal rust in the tank.

So basically, am I still looking at a matter of carb adjustment, or am I dealing with something that cannot be fixed with carbs (i.e. burning oil, bad timing, clogged exhaust.)

Any insight would be very much appreciated, this stuff keeps me up at night!
Again thanks for reading, have a super day.

hey man, i have the same bike and i had this problem for a long time, have you synced your carbs after your rebuild? also have you checked/cleaned your points. I did those two things and it runs 10000000x better.. literally 10000000x.

 

[kdo58]

http://www.thegsresources.com/_forum/showthread.php?t=187952&highlight=jetting

 

[TheCafeKid]

Your mains are way too small. Pilots should be fine. The pilot air screw should be roughly 2.5 turns out, pilot fuel screw 7/8-1turn out. Move the needle clip down to the second or third notch from the point. Your mains will probably need to be 117.5 or 120.

 

[kb_air]

will it completely kill any chance of decent gas millage?

 

[ThunderBaird]

hey man, i have the same bike and i had this problem for a long time, have you synced your carbs after your rebuild? also have you checked/cleaned your points. I did those two things and it runs 10000000x better.. literally 10000000x.

Thanks for the suggestion- the points were apparently redone by the guy who I bought the bike from, but the carbs have not been synced as of yet. In my manual it says that syncing the carbs has to do with making sure all of the slides are in sync with eachother... If that's not the case then I am planning on a sync for sure.

 

[ThunderBaird]

will it completely kill any chance of decent gas millage?

That's one of my fears, but a more pressing matter is the fact that there's the threat of fouling my spark plugs going to and from the grocery store.

 

[Steve]

So many things, I'll just pick a few. :-k

I sort-of set the airbox on fire after I sprayed starting fluid into it and the bike backfired.

We just can't say it enough. If you "need" starting fluid, FIX THE BIKE. What you have just seen is a very MILD case of what can go wrong. What's worse than setting your bike on fire? How about blowing it up?

I have also totally rebuilt the carbs with new jets and gaskets. Stock jet sizes are: main 100, pilot 15, needle jet O-6. Without going too far into it, I now have these jets: main 102, pilot 27.5 (or 22.5- there is a 22.5 on the jet that has a stamped line through it, and the 27.5 is clear.)

You have your jet increases backwards. Your main jets are the ones that should go up multiple sizes (maybe to a 120 or so) and your pilot jet might need to go up just one size, but usually not.

At idle, it fouls my spark plugs in a matter of hours, yet my carbs start sputtering if I lean out the mixture too much.

Why are you idling your bike for hours? With your pilot jets about five sizes too big, it's no wonder that it's idling rather rich.

Does the pilot jet affect the entire throttle range so much that even with aftermarket pods and exhaust on the bike, it runs rich as a mofo with oversized pilots?

The pilot jets affect your idling and up to about 1/4 throttle, which is really a LOT of your basic running, especially around town.

With the size 15 pilots in the carbs, the bike would idle in the 2300-2500 range, so I quickly looked at the old jets that I had replaced with stock and lo, the pilot jets were 27.5 (yet the mains were at 97.5... odd.)

The pilot jets do not determine the idle speed.

Also, the exhaust baffle is all black on the inside... could this also be dampening my air flow even though it's an aftermarket?

Black soot on the inside of your pipe is not going to affect air flow. It merely indicates that your bike is not jetted correctly and that is going to affect how it runs.

Would a carbon-crusted V&H under-perform compared to a stock exhaust?

No, but an engine that is running rich enough to make it carbon-encrusted will.

And instead of buying new sets of plugs, I keep two sets that I switch out for each other after cleaning them with a wire brush and fine sandpaper.

Get it jetted properly, get a new set of plugs and be done with it. Cleaning plugs with a wire brush will leave metallic traces on the porcelain, possibly short-circuiting the plug. That could also be contributing to your poor running.

When you get new plugs, make sure you get the correct ones, NGK B8ES.

I also don't know when the oil was last changed, and my manual doesn't tell me what size oil filter my bike needs.

In that case, it has been in there too long. The filter is a rather generic one that can be gotten at virtually any Suzuki shop, many places on the internet, even eBay. I think they only made one filter that fit all their bikes from about '76 to '86.

Things that I do know- The floats are at correct heights, the carbs are clean and have good airflow, the slides are synced (almost totally closed at closed throttle), the air screws and fuel screws are always adjusted in sync, ...

Were the floats measured from the correct location? What measurement did you use?
Did you dip the carbs or just spray them out?
How do you know they have good airflow?
You only sync the slides visually to get the bike running. Then you must use a set of vacuum gauges.
The pilot fuel screws (the ones on the bottom) should be out about 1 to 1 1/4 turn for your setup. The pilot air screws (the ones on the sides) should be about double that.
Adjusting the idle mixture screws "in sync" is not always good. You have to tune each carb to itself, they might all have slightly different settings when running their best.

So basically, am I still looking at a matter of carb adjustment, or am I dealing with something that cannot be fixed with carbs (i.e. burning oil, bad timing, clogged exhaust.)

It is VERY MUCH a matter of carb adjustment.

First, get the proper jets in there. Mains should be about 120, you say you have 117.5s in the mail. Start with them. Pilots should be 15. Stick with them. Pilot screws set like I suggested above. New set of spark plugs. Change the oil and filter, go ride that thing.

.

 

[ThunderBaird]

Your mains are way too small. Pilots should be fine. The pilot air screw should be roughly 2.5 turns out, pilot fuel screw 7/8-1turn out. Move the needle clip down to the second or third notch from the point. Your mains will probably need to be 117.5 or 120.

Alright I'll definitely try those settings. By turns do you mean full 360 degree turns or 180 degree turns? Also, if the bike is running rich to begin with, wouldn't I want to lean out my needle by moving the clip in the other direction? I'm not trying to disregard your advice, I just want to understand exactly why that would work..

 

[ThunderBaird]

First, get the proper jets in there. Mains should be about 120, you say you have 117.5s in the mail. Start with them. Pilots should be 15. Stick with them. Pilot screws set like I suggested above. New set of spark plugs. Change the oil and filter, go ride that thing.

.

Thanks so much! I asked a ton of questions and you've answered a lot of them. Do you think I need to adjust my jet needle at all?

Also, I assume I'm getting good airflow from my new pod filters, and the floats are at 27 mm from the interior edge of the float bowl without the gasket.

 

[kdo58]

http://www.livefromhawaii.com/geelyscooters/carbs101.pdf
http://www.mikuni.com/fs-tuning_guide.html

 

[TheCafeKid]

You're going to need a basic understanding of how the carburetors work. First and foremost, RPM has nothin to do with which circuit is functioning and when. It's all about throttle position. In the carburetor throat you will see the slides. These are blocking the air flow to the engine, requiring it to draw small portions of air and fuel from the pilot circuit. The pilot circuit only works from idle to about 1/4 throttle. Once you get to that point, the slides, which are lifted as you turn the throttle, allow more air to the engine, and the needle, which is attached to the slide, allows a certain amount of fuel to be drawn up through the needle jet, which is fed by the main jet. The needle is tapered, and as the bike draws more air, the tapered needle lets by more fuel. The needle circuit is basically in effect from 1/4 to roughly 3/4 throttle, and then from there youre pretty much only on the main jet. As you will see, you spend very little time on the main jet by itself. Most riding is done on the pilot and needle circuits. But because the needle jet is fed by the main jet, an increase in size is necessary. The carbs were set up from the factory based around the amount of air that the airbox would provide. When you add pod filters, youre adding a LOT more air. The current jets aren't large enough to provide enough fuel to keep the AFR where it needs to be for a good burn. The window is small, so even a small change in the amount of air or fuel will greatly skew the AFR to one side or the other.

You're absolutely going the wrong way with your jetting at the moment. Because the pilot circuit sucks air and fuel from tiny openings that are not effected by pods or the slide position so much, stock pilots usually work. The main, and the needle clip position MUST be changed. Moving the clip down toward the point makes the needle sit higher in the slide, effectively making the
taper "quicker" to the point. This richens the needle circuit. But the main jet needs to larger by roughly 5-7 sizes. You need to be at 120, perhaps 122.5 (I said 117.5 earlier but didn't realize that you had an aftermarket pipe, which will effect jetting as well) and the clip needs to move down one notch (perhaps two) from where it is.

Your bike is fouling plugs because your pilots are wayyyyy too big. You're not rich anywhere else, in fact you'll be dangerously lean. Running lean will burn the valves and hole pistons and such. So before you attempt to continue riding, you need to re-jet. Not to mention the fact the bike will fall flat on its face around 3500 rpm the way you have it set up now.

 

[MisterCinders]

When I changed to pods and 4-1 on my 78 GS750, I played with jetting for ages to sort it out.

I went one size up on the pilot jet to 17.5, but that kept things way to rich at low throttle/idle. So I went back to the stock 15 pilot jet.

For mid throttle, try bumping the clip down to the 4th slot from the top. That's where I ended up. You will need to re-synch the carbs when you do this. Bench synch first, then do a vacuum synch. This is important. BTW, the bench synch is not based on fully closed slides. Instead, use a feeler gauge, small drill bit or paper clip to measure a small gap. Then synch them to that gap with the bit just sliding out. This is more accurate than synching them to full close.

On the mains, I am STILL picking over the right jets. Currently, I am up to 125s. Plug chops at WOT are tricky for me, though, since I live in the city and have to travel a bit for room to do a WOT chop.

Since you will be re-synching the carbs anyway, this is a good time to check and adjust your valves (since you have to re-synch carbs after doing that too).

 

[kb_air]

So many things, I'll just pick a few. :-k


We just can't say it enough. If you "need" starting fluid, FIX THE BIKE. What you have just seen is a very MILD case of what can go wrong. What's worse than setting your bike on fire? How about blowing it up?


You have your jet increases backwards. Your main jets are the ones that should go up multiple sizes (maybe to a 120 or so) and your pilot jet might need to go up just one size, but usually not.


Why are you idling your bike for hours? With your pilot jets about five sizes too big, it's no wonder that it's idling rather rich.


The pilot jets affect your idling and up to about 1/4 throttle, which is really a LOT of your basic running, especially around town.


The pilot jets do not determine the idle speed.


Black soot on the inside of your pipe is not going to affect air flow. It merely indicates that your bike is not jetted correctly and that is going to affect how it runs.


No, but an engine that is running rich enough to make it carbon-encrusted will.


Get it jetted properly, get a new set of plugs and be done with it. Cleaning plugs with a wire brush will leave metallic traces on the porcelain, possibly short-circuiting the plug. That could also be contributing to your poor running.

When you get new plugs, make sure you get the correct ones, NGK B8ES.


In that case, it has been in there too long. The filter is a rather generic one that can be gotten at virtually any Suzuki shop, many places on the internet, even eBay. I think they only made one filter that fit all their bikes from about '76 to '86.


Were the floats measured from the correct location? What measurement did you use?
Did you dip the carbs or just spray them out?
How do you know they have good airflow?
You only sync the slides visually to get the bike running. Then you must use a set of vacuum gauges.
The pilot fuel screws (the ones on the bottom) should be out about 1 to 1 1/4 turn for your setup. The pilot air screws (the ones on the sides) should be about double that.
Adjusting the idle mixture screws "in sync" is not always good. You have to tune each carb to itself, they might all have slightly different settings when running their best.


It is VERY MUCH a matter of carb adjustment.

First, get the proper jets in there. Mains should be about 120, you say you have 117.5s in the mail. Start with them. Pilots should be 15. Stick with them. Pilot screws set like I suggested above. New set of spark plugs. Change the oil and filter, go ride that thing.

.

what he said

 

[Steve]

Alright I'll definitely try those settings. By turns do you mean full 360 degree turns or 180 degree turns?

"Turns" are full, 360-degree turns.

Do you think I need to adjust my jet needle at all?

No idea. Only plug chops will tell. You will have to get it running on the needle circuit long enough to color the plugs (NEW plugs help this process), then pull them out to see the color.

Also, I assume I'm getting good airflow from my new pod filters, ...

You do know what they say about "assuming", right?
I am presuming that these might be UNI brand filters?
Do they need to be oiled?
Have you oiled them?
If they are oiled too much, air flow will be restricted, possibly severely.
Do NOT assume.

..., and the floats are at 27 mm from the interior edge of the float bowl without the gasket.

Any particular reason you set them to 27 mm? Specs for your year call for 25-27 mm, so 27 will be on the lean side of the range.
You might want to raise the floats (set to a lower number) 1 mm, if possible. Yes, it really does make a difference.

.

 

[MK3Brent]

For what it's worth, also...
Using a wideband O2 sensor makes all this guess work disappear when you're trying to size your jets properly. For less than $150, you can pick up an LM-1 or whatever they're calling it these days (LC-1 with gauge), and add the bung into the manifold collector.

As far as scratching the dielectric on the spark plugs, you don't have to worry so much. We're talking several kV of potential that sparks a .032" or so gap. The inception voltage through air is approximately 3kV/mm. In the extreme (relative to atmosphere) pressure in the combustion chamber this value will increase. If you have an insulation flaw due to scratches or other semi-conductive elements on the surface, you'll puncture the porcelain before it causes other issues like missing.

 

[ThunderBaird]

Thanks everyone for all of your help, I got a hold of the appropriate jets, fixed some leaks, and tuned up the fuel mixture (which was pretty close to how ya'll said it should be.) The bike is now in fully functional form, and I will be riding it very soon.

 

[Steve]

As far as scratching the dielectric on the spark plugs, you don't have to worry so much. We're talking several kV of potential that sparks a .032" or so gap. The inception voltage through air is approximately 3kV/mm. In the extreme (relative to atmosphere) pressure in the combustion chamber this value will increase. If you have an insulation flaw due to scratches or other semi-conductive elements on the surface, you'll puncture the porcelain before it causes other issues like missing.

I wasn't all that concerned about scratching the porcelain, I was more concerned about a steel wire wheel leaving minute metal deposits on the porcelain that would provide an alternate path for electricity, possibly bypassing the spark gap.

.