Am I missing something (specifically, a tube)?

[Gbrown]

Please see the pic below. I pulled the carbs because my 78 GS750 is still running like crap after rebuilding the carbs and paying someone to sync them. Upon initial inspection I notice that I may be missing a tube between the #2 and #3 carb, where the throttle return spring is. I see that the other carbs each have a tube here and the #2 and #3 carbs have ports where the tube would go, but it seems the throttle return spring would interefere if there were a tube here. Any insight is greatly appreciated.

As a second point, the bike has a 4 into 1 header on it but retains the stock airbox. I noticed last night that the main jets are 102.5. I believe stock main jet is 100? So 102.5 is (1) size up from stock? Should I consider a different main jet size? Thank you all for your help.

 

[Agemax]

there is nothing missing from there, 1&2 and 3&4 are connected because they share bowl vents. they are not connected between 2&3

 

[chuck hahn]

No tube goes there...dummy nipples for application other than what your bike has..they arent drilled thru. Should be a coupler between 1 and 2..and 3 and 4 venting ports and the fuel tubes only. DO NOT cap off the vent holes. The carbs are vented because they need atmospheric air pressure to equalise the bowls to the outer air pressure.

There were two stock size mains for that year..97.5 and 102.5 depending on what EPA requirement they were set up for

Here are the base settings for the screws..BOTTOM PILOT JETS:: 3/4 to 7/8 out..I like mine at 7/8. Set and leave alone.

SIDE MIXTURE SCREWS::: start at 2 turns out and youll adjust with the side mixture screws only. I find I usually end up at around 1 1/2 out as the final good running spot Turning then screw in RICHENS the mixture and out will LEAN the mixture.

Clip on the needles will be at the center groove.

See these links for more information. Scroll down the page for some awesome tutorials as well.

http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html#A01

And heres the EPA chart showing the different jettings per THE ENGINE NUMBER stamped on the top right flat spot of then tranny case by the cover..

 

[Gbrown]

THANK YOU Chuck Hahn! :cheerful: I have read (and re-read) many of the tutorials and specs on BassCliff's site but you have succinctly summarized the essentials here and it's fabulous! I will check the engine stamping info you provided when I'm home tonight. To ensure I'm understanding the chart you posted... it looks like stock main jet sizes listed for my bike could include 105, 97.5, or 100? Is that right? Because you only reference 97.5 or 102.5?

Assuming my bike had 100 main jets from the factory (again, I intend to verify later based on the chart you provided), isn't it the conventional wisdom that changing the exhaust to a 4 into 1 necessitates a main jet change (increase)? So the 102.5 jets installed are (1) size up? Should I try a bigger jet, or perhaps go back to stock? What's your opinion? Thanks.

 

[posplayr]

Bikes are generally setup to run a little lean, even more so in California. You can safely bump the jetting one step even without a pipe change. With pipe chznge but no other air box changes it might be a little lean. If chrome pipes look for bluing but other wise I would get bike running right before changing jetting too much.

 

[posplayr]

Airbox is much more likely to cause jetting changes. A low flow filter change or removal of the top are going to change things as much or more than 4:1 even if the airbox is still there.

Are carbs now balanced? Not sure who did it but you would hope they are good enough to diagnose the running issue if you are paying them to balance carbs.

 

[Gbrown]

...With pipe chznge but no other air box changes it might be a little lean. If chrome pipes look for bluing...

Oh I've definitely got bluing going on...

...but other wise I would get bike running right before changing jetting too much.

That's what I'm attempting to do and wondered if changing jet size might help. The bike starts very easily but cackles and pops on deceleration. Also, it does not idle well.

 

[Steve]

That's what I'm attempting to do and wondered if changing jet size might help. The bike starts very easily but cackles and pops on deceleration. Also, it does not idle well.

Changing the main jet affects the mixture the most when you are over half-throttle. You could probably remove the main jet and not affect your ability to idle. Also won't do much about the crackling and popping.

The need for a main jet change depends on your pipe and muffler. If you have a somewhat restrictive (quiet) muffler, it might flow about as much air as the stock pipes, which are really quite good at handling exhaust flow. That will not require much, if any, of a jetting change. However, stick on a less-restrictive (louder) baffle, you can flow more air, so you need more fuel, too. Check your spark plugs, they will tell you what is going on inside the combustion chamber. If they are looking rather white after a full-throttle run, increase your main jets.

The crackling and popping on decel could be a lean mixture from the carbs or an air leak in the exhaust (or both). Since you are also having difficulty idling, check the settings on the fuel screws (the ones on the bottom of the carbs). As Chuck suggested, 3/4 to 7/8 turn out from LIGHTLY seated is good. 7/8 will give it just a bit more fuel, which might help the idle and decel problems. When the engine is warm, slowly adjust the air screws (the ones on the sides) to give best idle. I like to then turn them in about 1/8 turn to richen the mixture just a tad. I would much rather run just a bit rich than lean.

.

 

[chuck hahn]

Ditto on both Jim and Steves posts. The thing thats gonna tell the story without doubt is the plugs. The decell popping is indicitve of a lean situation...but...exhaust gasket leaks also make for some pretty heavy decel pops. The lesson is this.."just because"..doesnt always mean this absolute solution.

Why it pops at decel with exhaust leaks is that fresh oxygen gets sucked into the pipe thats very hot. Now you have unburnt fuel ( from it actually running RICH )..mixed with lots of oxygen and a heat source to ignite it all. Result is ignition in the pipe and decel popping

So what you gotta do is check everything. First off see what the plugs look like. If its lean they will look chalky whitish grey like the color of burnt out charcoal. Too rich and they will be black and somewhat sooty.

If they are black and sooty and its decell popping...then see the exhaust leak explanation above. heres some good reading to help you decipher plug speaking to you.

http://www.dragstuff.com/techarticles/reading-spark-plugs.html

http://jetsrus.com/FAQs/FAQ_spark_plugs.htm

http://www.ngksparkplugs.com/pdf/DYK_HeatRanges.pdf

 

[Gbrown]

All of these responses have proved tremendously helpful and I'm grateful for the time you've all taken to share your insight. I hope I can coax more out of you all as I try to sort this out. One question that I've yet been able to get an answer on is how do I determine an exhaust leak, is there a more definitive way than 'by ear'? I replaced my exhaust gaskets and ensured all hardware is properly torqued. Anything else I can do/check? Regarding a plug chop, I haven't been able to locate a safe area to do one so I can't say for sure what those results will be. I do know that every time I have pulled a plug to inspect they are black and wet--but it's not oil. I believe they have unburnt fuel on them. Additionally, as I was disassembling the carbs last night I noted the Pilot Fuel Screw settings and Pilot Air Screw settings and was shocked at my discovery:

Pilot Fuel Screws (the ones on the bottom) ranged from 3.5 turns to 7 turns open! Chuck Hahn has previously advised 3/4 - 7/8 turns out. Would settings like I discovered indicate overfueling?
Pilot Air Screws (the ones on the side) ranged from 3.5 to 5 turns open! Again, Chuck Hahn recommends 1.5 - 2 turns out. Would settings like I discovered indicate a lean condition?

Please bear in mind that I am not the one who set these screws like this. When I rebuilt the carbs I set them very closely to what Chuck Hahn recommends (per the rebuild guide on BassCliff's website). I fear that the guy I paid to 'sync and tune' the carbs did this and it's what's led me to the predicament I'm in. Not in his defense, but just thinking out loud, is it possible that he had the carb settings so screwed-up in an attempt to compensate for some other issue (valves, timing, etc.) that needs addressed?

Oh, by the way, I also paid this guy to adjust the valves and his report to me was that they were all in spec. Mind you, this is a bike I know for fact has not had the valves adjusted in 10+ years, per the P.O. I'm starting to believe I got completely fleeced by this guy and I need to start from scratch with the valves. What's your guys' opinion? I'm relying pretty heavily on you all as I took the bike to the only guy I could find locally willing to work on it and I'm not happy with the results.

One last note, for what it's worth, the bike starts super easy/quickly. Like, sometimes it's fired and running before I can release the start button. I mention this because my experience with old cars leads me to believe (but I am wholeheartedly open to correction) that if it starts this easy then the valves/timing are likely pretty close to spot-on. Again, please advise. Thanks all for your help.

 

[chuck hahn]

Well I wasnt the only one with the screws settings numbers. Seeing that I have 4 running GSs right now and they are all set up with my info.....and others have verified them as well...I would say then guy that did your didnt know his azz from a hole in the ground.

And yup the bottom ones that far out will make it rich in the idle to say 2500 to 3000 RPM range and the side ones are so far out because hen was trying to compensate for his flukk up at the bottom end.


So..taking all that into consideration....I wouldnt trust him on the valves either. Your intuition is already screaming as loud as it can at you. It may start easy and USUALLY if the valves are far out it will be harder. But again, dont take anything for granted unless you do them yourself or are watching over the shoulder of whoever is doing them is my advice here.

 

[Gbrown]

I am pulling the valve cover and checking/adjusting the valves for myself this weekend. That will give me the baseline I need to tune the carbs after cleaning and reinstalling them. Thanks all for your advice and I'll re-post here with an update ASAP. One last* question: is there a way to determine the location of the needle clip without complete dis-assembly? I don't have the carbs in front of me and I was curious if I could at least determine what the clip is set at without completely tearing the carbs apart. If so, would it be possible to adjust it as well, or is a total dis-assembly required? Thank you.

*"last question", yeah right!!!