Carb help please

[BurtyMcSquirt]

Not trying to come across as being aggravated, just trying to be clear about the missing info.

I never assumed you were...No worries. Both coils were replaced and spark is getting to all of them. It idles well when it does so I would think they are firing in the correct sequence.

[allojohn]

How do you mean? That the 29's were put on later in favor of the 26's?

A PO put the 29's on, VM26's were stock on all '77-'79 GS750's.

[BurtyMcSquirt]

A PO put the 29's on, VM26's were stock on all '77-'79 GS750's.

How much of a difference will that make?

[allojohn]

How much of a difference will that make?

Significant would be my guess, it's likely someone here has done it. Finding them might be an issue.

[BurtyMcSquirt]

So do you think my current carbs lack the compatibility needed to run on this bike? Would proper jetting offer compatibility? I see a lot of these carbs being used on gs750's, though after a little research it seems as if they were built with the honda cb750 in mind.

[allojohn]

So do you think my current carbs lack the compatibility needed to run on this bike? Would proper jetting offer compatibility? I see a lot of these carbs being used on gs750's, though after a little research it seems as if they were built with the honda cb750 in mind.

You can probably make it work, but how much time/effort do you want to put in? Try PM'ing Chuck78, he's played around with VM29's.

[BurtyMcSquirt]

You can probably make it work, but how much time/effort do you want to put in? Try PM'ing Chuck78, he's played around with VM29's.

Will do. Thanks a bunch.

[BurtyMcSquirt]

Just to be clear that I know what I'm talking about this is a picture of my carbs. Can you guys verify the model? 450-30 is stamped on the motor side flange.
20171002_162946.jpg

[Chuck78]

I sent you a detailed PM reply on tuning. Hopefully not too jumbled as to confuse you. After reading all of this above, I will reiterate the important parts, and add to it.





I wish for your wallet those were VM29 smoothbores (worth $400-800), but a stamping of:
450
30
on the mounting flange of the carbs indicates stock GS750 78-79 spec VM26 carbs


the smoothbores have a little square casting on one side above the fuel bowl gasket, and take a different shape fuel bowl and have a large hex nut plug on the bottoms for quick change main jets. Also they only have the side air screws, no pilot fuel screw underneath, going from memory. Other than that, they look IDENTICAL as installed, but internally, the slide and venturi are TOTALLY DIFFERENT. Very awesome carbs but long out of production, Keihin CR Special 29mm's are similar and are still made to this day, better bet to buy a fresh set but they are $1,049... or $600-900 used.
ALSO - the VM29 smoothbores DO NOT HAVE A VACUUM PORT TAP! You either can't use the vacuum petcock function, or you have to rig up a carb synch gauge adapter from one of the intake boots to get vacuum.





You've gotten some great advice here so far. but something isn't adding up.

Your plug photos earlier definitely show too rich. BUT... what throttle positions were you riding in IMMEDIATELY before shutting the bike off and pulling the plugs?
You should have a float height of 22-24mm. download the factory service manual (it will be a 1977 with 1977 spec, but still pretty darn relevant other than shorter float height for yours)
How did your float needles look? If they had a ridge worn in them that you can feel with your fingernail and see with your eyes, might be best getting some replacements. Then re-adjust. No bowl gasket on carb, measure from the metal. There is always a big debate over whether you measure while holding the float up (carb upside down) to the oint of where the float just barely touches the tip of the needle, or wether you adjust them with gravity sitting them on the spring loaded tips of the float needles. set it in the middle at 23mm.

If you can find a small piece of poly tubing that you can whittle down to a point, then remove the bowl drain screw and jam the hose in there and hold it in to seal the fuel. you can make an actual fuel level u-tube gauge to check to see the true float adjustment via the fuel level this way. this is seldom done by any, easier to just stick new float needles in, but it is a very very useful bit of info to know about where your fuel level is at. When running.

[Chuck78]

Where are your slide needles (aka jet needles) set at? That is what dictates most of the riding range of the bike. pilots and mains just blend in to jet needle metering at the very lowest throttle and at the very highest (approaching 3/4 & up throttle and above 5000rpm) range. The slide needle or jet needle is most of the rideability of the bike.

what did your pilot fuel screws look like? these are NEVER to be "TIGHTENED" tight, once they are moving freely, turn them clockwise slowly and easily until they very gently seat. Tightening them beyond that will groove them out and mess up the hole in the carb that they meter fuel through. the tips can also get totally broken off of these very fine pointed pilot screws, and jammed in the carb bodies. Very bad time there. fixable usually, but a real pain.


Get the idle set right, pilot screw adjustment. read the plugs repeatedly. you need to be pulling the spark plugs often.
A Gunson Colortune is a handy tool for getting the pilot circuit dialed in. a clear spark plug so you can watch the flame color as you turn the pilot fuel screw. white-ish to nice blue to yellow, lean to perfect to rich flame.
Going out too far on the fuel screw on the bottom of the carb will make for a lot of stumbling. if idle or off idle transition is stumbling and plugs looking too rich at idle, try the fuel screw at 15/16th of a turn out up to 1-3/16 out. I think around 1 or 1-1/16 will be the range you will find the bike the happiest. then start with the air screws 1-1/2 turns backed out from lightly seated. use the "highest idle" method of tuning. I do a trick using an ignition timing strobe gun where I just squeeze the trigger nonstop pointed at something dark, and turn the screw. you will see the carb/cylinder misfiring when in too far, and misfiring when out too far. somewhere in the middle is the sweet spot, where you get steady flashes of the timing strobe. A Gunson Colortune lets you watch the flame to give you the same info, but more, as you can see the flame color and perfectly tune it.

I use a wideband o2 sensor and air fuel ratio gauge to tune my bikes anymore, when possible. $175 or so. saves a lot of time. but I used to do it the old fashioned way,trial and error and lots of plug chops! Takes days or weeks depending on your skill at present, time available, etc. BUT EXTREMELY WELL WORTH IT when the bike is fully sorted.

Mains should be somewhere in the 107.5 to 120 range, depending on filters, exhaust baffle, engine compression, etc. 112.5 is probably a safe starting point. Start bigger, work your way down in size. Much safer, and easier to read the plugs when you start getting less sooted and see some nice coloration.

you need to get it running good on the 1/8 to 1/2 throttle range before you even start worrying about the main jet size, however. Just don't go full throttle until you're to the point of testing the main jets.

start out by holding the throttle steady at a position where it acts up. do this riding down the road for 30 seconds if possible. do a plug chop. that means hit kill switch, pull in clutch lever, and close throttle all simultaneously. pull over. remove a plug or 2 or all 4. look at them really closely to see if they look rich (dark brown/black) or lean (white-ish) or a nice light tan light brown color with the tip of the insulator in the center still looking a bit white (the good range).
Look up photos of spark plug reading tips. I lost a bunch of good ones on my old phone. Ill see if I can find some more for you.

you need to see if you are rich or lean in these areas. lean sounds different than rich even. more "zingy" where as rich sounds more deep big fat muffled and saturated. and smells rich! Perfect mixture/jetting sounds aggressive and throaty, you will know, it will just sound real good with aftermarket exhaust...

1/4 throttle area is almost solely dictated by the needle jet inside diameter opening, in relation to the diameter of the thicker base part of the slide needle aka jet needle. the thick part of the needle starts to taper at 1/4 throttle and gets thinner towards the tip. The base needle diameter / needle jet opening size, and this taper on the needle is the biggest thing you are adjusting with the needle height. raising the clip position on the needle drops the needle lower, so the tapered portion of it it blocks more of the needle jet's fuel flow. lowering the clip position on the slide needle RAISES the needle, making it richer, as you are effectively raising the needle up so the skinnier portion will be in the needle jet so that the needle jet is less restricted. the slide needle is key to most of the rideability!

idle and off idle transition are pilot circuit, which blend into the needle/needle jet but are no longer much in the picture past 3/8ths throttle. mains don't come into play at all until past half throttle (unless they are way way lean!), and won't affect it much until 60% throttle. 75% throttle and up is almost entirely dictated by the main jet. This is high rpm tuning territory here. you need a long straight desolate road, with a big hill if possible, to do main jet plug chops. tuning for the main jet sizing is hectic, as you always end up near 100mph on a 750cc four or larger, as you are getting closer to the best main jet size. you have to basically drag race through the gears on main jet plug chops, shifting at 9500 rpm, full throttle, for at least 7 seconds. if you can find a big steep uphill, 10 seconds is better. kill switch pull in clutch close throttle simultaneously, pull over. remove ALL plugs, number them with a marker. stick in spare plugs. take them home and literally CHOP the plugs, cut the threaded portion off, so that you can read the colors down to the base of the porcelain insulator in the center of the spark plug. this is what you need to be concerned with for main jet sizing.

[Chuck78]

45030 carbs should have 5F21 needles and o-6 needle jets, according to bass cliff aka bike cliff's website charts (google search "GS850 float height" to find this page). I have the best luck on performance build GS750's with the skinnier pointed 5DL36 (or 5DL35 from GS550) needles. The 550, 750, 850, and 1000 all used these needles. the 35 seems darn near identical to the bigger bikes 36 needles, but I'm not sure what the differences are. I've held them side by side and measured them.

I think if I recall, the 5DL36 is actually slightly fatter at the base, the just above idle area, vs the 5F21, so that it leans the mixture out a bit there. better EPA test results under normal riding conditons were a big motivator for this in 1978-1979 smog era. Then the needle goes into a more aggressive and rapid taper to a fine point tip. the 5F21 will be richer off idle, but the needle taper is mellow and the tip is pretty fat still, so 40% to 70% throttle will be much more lean.

I'm not saying that you have to go out and buy a set of four 5DL36 needles, but this is generally what I run. I trashed cylinder #4 piston/head/int valve on my 920cc top end, and #1 intake valve... so the end of the season was the first time in many years that I actually was riding my own 748cc GS750 (built a few 77-79 8v GS750's for friends in recent years, and a few VM22 GS50's). Going from 920cc down to 748cc was a huge difference in low-mid and midrange torque, but I was still blown away at how fast the bike would rip through the twisty roads when I would rev it up and wind it out. As usual, even with 172cc's less, I was still way out ahead of everyone else just from using the engine's peak torque through peak horsepower rpm range (and riding like a flaming bat out of hell, adrenaline junkie maniac). a DOHC 750 still goes QUITE FAST if you want to rev it up, but every law enforcement officer and concerned citizen within miles will hear what you are up to!

I just set the carbs up once real quick when going back to 748cc, did a test ride and said "yup, she screams!," and went out of state on a road trip to the West Virginia mountains. I think I was at 112.5 mains and 5DL36 3rd clip down from the top / o-6 needle jet if I recall correctly. it absolutely ripped up high in the revs, although as usual, I seldom ever go full throttle, so that main size might need a bit more experimenting. The mains were close enough to let the skinniest parts of the needle jet do their jobs, as a bit above and below half throttle is where I usually find myself when I'm riding in an adrenaline-fueled twisty road binge.
pilot I think was 17.5, didn't spend hardly any time tuning it before hitting the road, idled just okay, but fuel economy was abysmal due to this. had stumble off idle, and if a mix of riding aggressively and highway speeding, I would hit reserve on the fuel petcock at 110 miles. I had gone closer to 140 miles before 3/4 gallon reserve on the finely tuned 920cc setup. all due to too rich of pilot circuit, I will venture to say.

[Chuck78]

Step by step, a little less overwhelming instructions once you're familiar with above

so to sum that all up, get your valves adjusted, bench synch the carbs, make certain your float needles aren't worn, and replace if they are, then meticulously adjust the float height.

THEN...

inspect the pilot fuel screw tips, make sure they are a nice smooth taper, no signs of being cranked real hard into the carb which would mess up the straight taper on the screw visibly, or break off the tip. Make sure your choke levers are all pushing the choke plungers all the way closed!!! I rode a GS1000 carb rack for a summer and it always idled like crap, thought it was just because it was a GS1000 spec carb on a 750. Nope, later realized it would have idled perfectly if the #2 carb choke plunger were pushed down further by hand to snap it all the way shut! had to flip the nylon double top hat piece upside down to fix this.

THEN...

play around with pilot air and fuel screw settings. try just a hair over 1 turn out on fuel screws, 1.5 turns out and then fine tune based on best running/highest idle method. the rpm's won't vary much, but it will be a change. the timing strobe gun thing to monitor lean and rich misfires is a good method to find the middle ground. You should end up between 1.5 and 2 turns out or so on the air screws.

THEN... test ride! See how it does. THEN... pull the slide needles (jet needles) and change the clip position on all 4 to a higher or lower position. Try back to stock at clip notch #3 (I think that's counting from the top down, right? Someone correct me here). Then try raising it to the lower clip notch #4. Then try lowering the needles to the higher clip notch #2.
this is a lot of work and time and a pain, but it will show you how it runs different at all these settings. You should be able to feel, hear, smell differences in these positions pretty drastically. one whole clip notch off and it could run like complete crap!

Lastly, there is a very popular Radio Shack tiny washer bag assortment (30 or so washers?), which contains several 0.5mm / .020" thick washers that fit perfectly around these jet needles. This is a widespread thing across all makes of motorcycles/carbs to use these to shim needles, you can use these 0.5mm washers to put under the clip on the needle to raise the needle a half of a clip position to fine tune. yours may need this to run the best. It will just take lots of time testing and tuning. You should find that stock, a half clip position, or 1 full clip position lower than stock will likely be where you need to be.
***after taking the needles out of the slides repeatedly, if you disturbed the rocker arm linkage at all from the throttle shaft, you will need to re-bench synchronize your carbs. It doesn't take much to throw them off. the rockers aren't machined to super tight clearances, so loosening the bolt and tightening it again can allow them to move a tiny bit.

What are you doing Friday and Saturday? Last 2 days of the year perhaps where we will have 50+ degree weather in Ohio/PA!
Keep in mind, cold dry air is far more oxygen-rich than 75 degree slightly humid air, so it will run more lean on these brisk temperature low humidity days.

On to the mains, I'd stick a 115 or even 117.5 in it for now to be on the safe side. I think 115 or 112.5 may be where you will end up at, but it varies with every bike due to altitude, exhaust and baffle type, air filter type, air filter cleanliness, air filter service status (clean and PROPERLY OILED), etc.

I run K&N RC-2222's on mine, they are cheaper than the individual K&N pods, and are slightly more even running in side wind gust scenarios since they are only 2 filters for 4 carbs, they share an intake plenum basically. the individual pods are definitely "THE LOOK" that people are after though. Especially the oval shaped ones, elongated may be a better descriptor.








Try all of this and let us know how you're doing. I've been tinkering obsessively on GS's for almost a decade now, so I hope my experiences and guidance from the rest of the guys and gals here will combine to get you smiling really big when you twist that throttle!

AND POST SOME PICTURES OF YOUR BIKE!!!! We need some more incentive in addition to just helping out a fellow GS owner/addict for the good of helping them out!

[Chuck78]

These are really nice to confirm your pilot settings and settings just off of idle:

https://www.ebay.com/itm/Gunson-Colortune-Complete-G4074-Kit-14mm-PLUS-18mm-adaptor/112647055781

Good mixture as viewed through the glass top tester spark plug


Here's another showing too lean, good mixture, and too rich:











AND.... THIS is very helpful also. When you do a plug chop, you need to be running the engine at a particular throttle position range before killing it to pull the plugs, so that the reading on the plug will tell you what it is doing at that particular throttle position. This is important. It can get too confusing if you are using the full range of throttle up and down, and then pull the plugs to read them when you get home. you need to do one carb circuit at a time when analyzing.
the two noted Jetting areas are of most concern to you, and the igniton timing check at the top.




You HAVE verified your ignition timing is good, right??? Have you inspected the points contacts, to make sure they don;t look pitted? a failing condensor will cause a lot of tuning frustrations. a fresh set of points and condensers will help simplify the troubleshooting. Or a Dyna-S ignition powered off of an automotive relay, which also should feed direct battery voltage to the coils (using the old coil feed wire just to switch on the relay, look up "Wired George Coil Relay Mod" or similar search terms.




And here are 2 plug chops from many years ago on my early 748cc version of my GS750, left two are same plug in different lighting, right two are another plug, same in both right photos, but different lighting. I think I settled somewhere around 110 or 112.5? I have since lost the valuable file attachment that had EVERY jet size that I tried, plug chops all lined up left to right from 125 down to 107.5. It was very helpful, but got purged from the server. For reference, going down 1 to 4 jet sizes lower than the plug in the left 2 photos made much better power and response up high in the rev range at 3/4 to WOT. That was a 120 there in the 2 pics on the left, in different lighting. Every bike is slightly different, though. Someone else's seemingly identical 38 year old bike may require slightly different jetting/tuning than yours, so use others' jetting size successes as a baseline only! and test all 4 cylinders when you have reached what you think is the best, make sure they;re all running the same. Compression and fuel bowl fuel level can change things...




To be clear, for Wide Open Throttle (WOT) main jet plug chops, you are looking for a minimal tan/brown ring at the very base of the porcelain insulator that is exposed after you literally CHOP OFF the threaded portion of the spark plugs. you can read it from several sizes too large without chopping the threads off of the plug, but when you get close to the right size going from bigger to smaller jets, it gets really hard to get a good visual on the base of the insulator without chopping off the threads. a small flashlight and a real good set of eyes can see down in there to get a rough idea. but getting the proper exact best jet for max power and efficiency will be best to do an actual chop. especially because slightly too lean will make AWESOME POWER but will put you at very high risk for preignition knock / detonation, which will destroy your piston and head!!!


Here's some other persons' plug chops, for reference:

[Chuck78]

Sorry, have I totally overwhelmed you yet?

Here is yet another thing that you really need to understand in order to be able to properly jet and tune carburetors on non-stock bikes. Throttle position and which carburetor circuits come into play at each throttle position. Notice there is overlap on all of them, where one will fade out when the other kicks in. This is very necessary to provide smooth throttle response. Study this A LOT and correlate to what throttle positions you are having difficulty at.
Throttle Valve = carb slide
Throttle Valve Cutaway = amount of raised arched cutaway in the front of the slide's bottom
Straight Diameter = jet needle thickness near the top, non-tapered portion
Primary Choke in 1st drawing = I'm not really sure what this refers to, not likely VM type carbs though!




and two other versions of the same info:






Lastly, please take note that you can;'t just whack the throttle wide open from idle and expect an awesome and smooth flawless power delivery... with direct linkage carbs like this, you have to "roll on the throttle" when accelerating, and use your ears and the feel of the power delivery to determine the rate of twisting the throttle.
Contrary to what some people think, you can't jet a carb to run wide open throttle well directly off of idle or 2700rpm etc... you will suffer from what is called "losing the jet signal" when you cause too abrupt of a drop in air pressure in the carb venturi.
half throttle jetting should be done at lower mid and mid range on the tachometer. 3/4 and full throttle jetting testing and tuning should be done above half throttle, you will have people argue this point, but this is the best for maximum performance.
If you want to be able to what your throttle wide open from idle, too large of main jets will make it deliver a bit more power in the meantime until the air velocity gets back up, but will not build peak power nor efficiency ever.

CV carbs are more user-friendly to non-skilled (non-mechanically minded) riders, as you can whack the throttle wide open and get fantastic response, BECAUSE.... the vacuum diaphragm raises the slide based on air velocity and pressure differential, and always keeps it in a fairly ideal scenario... better performance can be had with direct linkage carbs, but they require better user input with the right wrist / throttle grip usage.

I've confused you enough. That's a lot to digest, but stick with it, it will be worth it. Best of luck. Come ride Southeast Ohio twisty roads with us sometime. Bring good tires and brakes!!!! A lot tighter than most twisty Pennsylvania roads!!!

[BurtyMcSquirt]

O........m........g!