big bore gs750 w/gs1000 carbs, won't idle well

[Chuck78]

Alright carb gurus, I need some advice to save me some time scratching my head in trial&error. I rebuilt a 79 gs750 bottom end, bored 850 cylinders out to 920cc, running 10:1 MTC 72mm pistons, rebuilt head with Serdi radiuaed cut seat valve job.
I had the bike idling good, running rich beyond idle. I got an AFR gauge & wideband o2 sensor, got my mains tuned in for WOT, but mids were far too rich even with needle ALL THE WAY DOWN (highest clip).

GS1000 carbs have different main air & pilot air jets than the identical looking gs750 carbs, and came with a leaner o-2 needle jet that would surely fix my rich midrange problem. I was actually hoping for gs850 carbs, same as gs1000 but o-4 needle jets (&different mains and pilots). None to be found, gs1000 carbs locally though.

The perfect 920cc 750 carb main size was about 2 sizes off (cant recall if too big or small) for the 1000 carbs, so I adjusted that and got it running in the mids fairly well with a set of "o-4" needle jets on eBay that were actually in an o-4 package but were o-3's. I struggled with this thing at idle, and have put very few miles on it since the carb swap. Carbs were ultrasonic'd AND chem dipped, blown out, new boots orings etc. Both sets of carbs.

Here is where I'm at now. Once warmed up, on restarting, I have to crack the throttle open as soon as I kickstart the engine in order to get it to start well. Idle is rough. Carbs bench synch'd. Can't get good enough idle tune to even think about putting synch differential manometer on it.

So I think I was running #17.5 pilots, can't recall last season if I swapped back to #15's. Bike has a pair of very large surface area k&n rc-2222 dual plenum oval filters (2 carbs->1 filter). Vance & Hines 4:1. Valves adjusted.

I believe it was at around 3/4-7/8 out of the fuel screws, and it ran best at 1-1/2 turns out on pilot airs.

It would idle rough, but once riding with a good tune on low throttle low%mid rpm operation, I would pull up to a stop & the bike would be revving at 2000rpm. After a minute of that it would slowly start dropping in rpm and running rougher. Then idle would get choppy & die. If I were to mess with the idle speed adjustment to lower the idle rpm, it would just go from revving high to immediately running rough and choppy. No middle ground yet.

No tank on it right now, making paint chip repairs, but I hope to get back together Wednesday to start tuning. I pulled the plugs yesterday after sitting for a month from the typical run it hard until I'm 2 blocks from home and then creep home kind of run. 1&4 were a nice light tan. 2&3 were mostly white with darker colorings in the top of the outer threaded ring that the ground electrode lands in. Not at all what I expected. I thought I was too rich since I had to crack the throttle open to start it when engine was hot.

The air screws don't make much if a difference in running state, only slight. Could it be that I need closer to 1 turn or more on the fuel screws??? Then maybe closer to 2 turns in the air screws??

[Spyderman]

well your describing running lean on the pilot i would up the pilot jet 1 size and leave the fuel screws where they are at 7/8 (tiny screws on the bottom of the carb) turn the air screws in to 3/4 of a turn and see what happens (screws on the side of the carbs)
the ones on the bottom of the carb are (turn em out = richer) the air screws on the side are (turn em out =leaner)

reading your plugs tells you whats happening
tip of the electrode to 1/4 down the electrode and around the bottom area is for the pilot circuit
1/4 to 3/4 down the electrode is for your needle circuit
3/4 to the bottom of the electrode is the main jet circuit
hope that makes sense and helps you out a bit
jetting is a real trial and error thing till you get it right

also each carb will need tuning as each cylinder will run different so no each carb will have it's own settings and 2&3 cylinders run hotter so they need different settings from 1&4

[Agemax]

Turning the fuel screw OUT = RICHER, not leaner!
Buy a colortune and see what it reads at idle. You should be able to get the mixture spot on at idle with a colortune.

[Spyderman]

Turning the fuel screw OUT = RICHER, not leaner!

thanks for the correction to many work hours lately got it mixed up although there is an easier solution for him and he won't have to mess with a fuel screw (buy a set of 29 smoothies)

[Chuck78]

Aaahhhh... I shoulda known better. Here I thought I was running rich, having to crack the throttle open to give it more air on startup, but really it must be sucking so e fuel out of the needle jet to give it enough to run on. Hanging idle= lean, read it over and over in the past, didn't think that was what I had going on. This old dog knew the new trick but wasn't seeing the forest for the trees. Pullingbthe plugs the other day had me confused as I was expecting too rich! Previous problem on same build was #2 carb choke actuator was not pushing choke plunger all tgecway down, & #2 was sooting the plug up. Thought that would cure my problem, but still had rough idle.

Thanks fellas

[Agemax]

thanks for the correction to many work hours lately got it mixed up although there is an easier solution for him and he won't have to mess with a fuel screw (buy a set of 29 smoothies)

that would be a far better option!

[Chuck78]

Although there is an easier solution for him and he won't have to mess with a fuel screw (buy a set of 29 smoothies)

Haha!!! I actually got a set of vm29's that came off an 860cc GS750, but since I didn't have time to tune & jet the entire throttle range before AMA Vintage Motorcycle Days this weekend (3 days, 20 acres of swapmeet, vintage racing on world class road course, bike show, seminars, camping at race track, etc), I opted for just messing with the pilots on these carbs.

I was really planning on a full rebuild & polishing on the vm29's whenever I get the time to upgrade to MegaCycle or Web cams..

[Spyderman]

Haha!!! I actually got a set of vm29's that came off an 860cc GS750, but since I didn't have time to tune & jet the entire throttle range before AMA Vintage Motorcycle Days this weekend (3 days, 20 acres of swapmeet, vintage racing on world class road course, bike show, seminars, camping at race track, etc), I opted for just messing with the pilots on these carbs.

I was really planning on a full rebuild & polishing on the vm29's whenever I get the time to upgrade to MegaCycle or Web cams..

ok you gave a good reason not to have put them on yet hope you had a good time at the show

[Chuck78]

Upped from 15 to 17.5 pilots, still won't idle well. Plugs look slightly rich (brown), but still acts lean - idle gets hung at 2000rpm after coming to stop, but eventually peters out to a loping 900 rpm til it dies). 7/8 turn on fuel screws, 1 to 1-1/2 turns on air screws seemed to work best.

I am thinking I should set air screws at 1-3/4 & start backing out the fuel screws with the colortune on each cylinder until I see yellowish flickering, then turn back in til only rich blue, then fine tune air screws.

Seemed that inner cylinders needed more fuel that 1&4 so far.

[OldVet66]

Are you sure the Pilot circuit is absolutely clean. It sounds like the passages to the Pilot fuel screws are still clogged.

[Chuck78]

I blew through them with aerosol carb cleaner & air compressor pretty well... That was after soaking a few times in Berryman's & a half hour in simple green in a harbor freight ultrasonic. I suppose it wouldn't hurt to try again

[OldVet66]

I think I have mine dialed in correctly now. The other day I backed the Pilot fuel screws out about 1/8 turn, it dropped the idle from 1,200 to 900 RPM. I adjusted the air screws for highest idle and it brought the RPM back up to 1,100 RPM, tweaked that up with the main idle adjust to just under 1,200 RPM and it is rock solid (not that it won't idle reliably lower). This is the best the bike has run since I have had it. Recently I changed my V&H 4-1 out for a Delkevic 4-1 Stainless Steel system and noticed an immediate power increase over the entire power range. The point of all this is that you should see substantial changes to very small Pilot fuel screw adjustments if they are working right. I am also running pods with a Dynojet stage-3 jet kit 142 main jet (Mikuni 133.1 equivalent) and # 15 pilot jet.

[Spyderman]

Upped from 15 to 17.5 pilots, still won't idle well. Plugs look slightly rich (brown), but still acts lean - idle gets hung at 2000rpm after coming to stop, but eventually peters out to a loping 900 rpm til it dies). 7/8 turn on fuel screws, 1 to 1-1/2 turns on air screws seemed to work best.

I am thinking I should set air screws at 1-3/4 & start backing out the fuel screws with the colortune on each cylinder until I see yellowish flickering, then turn back in til only rich blue, then fine tune air screws.

Seemed that inner cylinders needed more fuel that 1&4 so far.

i still think you should try taking the air screws down to 3/4 and work up a bit at a time
i had a similar issue with my 29's (so no mix screw on the bottom) i used a colour tune and thought they were set perfect by the nice blue flame colour i was getting however i still had a little bit of a hang and a very erratic idle
just on a whim i turned the air screws in to a 1/2 turn out from lightly seated and that was it i had a steady idle and no more hanging and the small amount of hesitation that i had right off idle was gone and the bike performs fantastically now my screws are tuned for each cylinder between 5/8 of a turn and 3/4 and my plugs tell it's just right so go figure
try it you might find it'll work for you 1/2 a turn on the air screws can make a huge difference and it's really easy to play with
i hope you get yours sorted out as well as i have gotten mine
for ages i thought 1 to 1 1/2 turns was where it needs to be turns out not to be the case what i have learned from all of this is each bike is different and what most people say you need for setting is only a place to start

[OldVet66]

I think he has already figured it out, yesterday he was looking for sources for the Pilot screw O-rings and choke plunger O-rings.

[Chuck78]

I am going to reassemble the gs750 vm26's with the o-3 needle jets, 5dl36 needles in the 2nd highest clip position, 15 pilots, 112.5 mains, and throw them on there to get by for now while I fiddle with these gs1000 carbs trying to clean the pilot air & fuel passages extensively, and also while I try & rebuild a set of vm29's. The 29's look like someone beat the front of each float bowl with the end of a box wrench about 75 times, and the rim of the carb bodies on the outsides of #1&4 below the gasket surface is bent inward to the float bowls and cracked, so I am not too hopeful that the jet blocks don't have the typical cracks in them from over tightening, as these carbs appear to have been quite abused by a frustrated mechanic. The 29's are far more advanced than the 26's hiwver, hollow brass floats, slides&linkages more like the keihin CR carbs sort of, much better alloy=much lighter, emulsion tube & needle jet are the same piece and different sizes are available... I am keeping my fingers crossed. I am on the fence about bending the rim of the carb bodies back & trying to do some aluminum brazing with that miracle repair rod from http://www.aluminumrepair.com, or just heating & bending & running as is. It'd be a shame if a chunk off each side broke off. Luckily Z1 has replacement jet blocks made now.