'76-79 GS750 factory jet size variations in relation to engine mods

[Chuck78]

If you have looked up GS750 VM26 carb specs, you will notice the factory jumped all around on the jet size over the 3 or 4 year model run. I have a 77 GS750B which had a MAC 4-1 already but airbox and factory filter. It was running lean at WOT, found it has 105 mains, carbs have needed rebuilt for some time and I am now getting to it. Previous owner didn't know much about what two owners ago did, but I did get out of him "the moter's been board out" - not sure if just .5 or 1.0mm overbore or else GS850 pistons or Wiseco high compression 844 pistons. Despite being lean at WOT, it is really fast and runs very strong (after warmed up) up to 3/4 throttle, so I was thinking the wiseco's were a good possibility.

Anyhow, you will notice the factory really jumped around A LOT on jetting these bikes. It may have been based on emissions standards at the time and the country the bike was built to for export standards, or maybe the engineers were just trying to make the bike run more consistently good or make more power.

1976 GS750: 105 main / 5F21-3 needle jet / P-1 jet needle / 22.5 pilot jet / 1.25 turns pilot screw
1977 GS750B: 105 main / 5F21-3 neede jet / P-1 jet needle / 22.5 pilot jet / 2 turns pilot screw
1977 GS750: 97.5 main / 5F21-3 jet needle / O-6 needle jet / 27.5 pilot jet / 1.75 turns pilot screw
78-79 GS750: 100 main / 5F21-3 needle jet / O-6 jet needle / 15 pilot jet / ?? turns pilot screw
197? GS750: 105 main / ??????? needle jet / ??? jet needle / 15 pilot jet / ??? turns pilot screw

1979 GS850: 102.5 main / 5DL36-2 needle jet / 0-6 jet needle / 15 pilot jet / 1.2 turns pilot screw

77-79 GS1000: 95 main / 5DL36-3 needle jet / O-2 jet needle / 15 pilot jet / "preset pilot screw"
77-79 GS1000: 95 main / 5DL36-3 needle jet / O-4 jet needle / 15 pilot jet / "preset pilot screw"


Anyone have any theories or actual experience with what jets, particularly the pilot jets, will be ideal for good performance? One member here with a 78 GS750 got a spare set of 77 carbs and said that the carbs were actually QUITE DIFFERENT from the 78-79 VM26's. Interesting!


I'm going with a pair of Dual Oval K&N open element filters that have one large oval plenum and filter shared by one inside and one outside carb, thinking that it will eliminate some of the typical pod tuning inconsistencies. I have a 4-1 with very minimal baffle (straight through a perforated/unpacked inner core), and the bike has an unknown size of overbore pistons (gets tricky here, bigger displacement with same carb venturi size needs smaller mains due to more fuel being siphoned due to the higher velocity of a greater amount of air moving through the same size carb).

[Chuck78]

I'm basically trying to figure out why the variations between 15, 22.5, and 27.5 pilot jets from the factory, yet the pilot fuel screw settings are all pretty close. If it weren't for the unknown displacement of my engine, I would be very likely to be dead on with a 117.5 main jet. Knowing it's been bored out to some extent up to 844cc's max (stock GS750 block, not an 850 block), I picked up a set of 115 main jets to start with. If it just has .5 or 1.0mm factory overbore pistons, then I may need to go up still.

I talked with a guy who tuned a custom piston 870-ish cc GS750 or 850, and he said that using an air fuel ratio wideband gauge setup, he determined that the 5DL36 (GS1000/850) was the best needle to use, and I think he was running 110 mains with the larger displacement. I can't remember what pilots he had. I will eventually be upgrading to 850 cylinders and 72mm 920cc pistons, so chatting him up was very helpful. For starters, I am looking for any suggestions on pilot sizing while I have the carbs on the bench, so I can get it in the ballpark a little closer and maybe get lucky and not have to change them.

[Chuck78]

Spent 3 hours cleaning the carbs mostly externally (and scraping gasket material remnants off) and disassembling them to dip. I didn't pull the needles yet, but mine seems to be the oddball with the biggest 105 main jets and the smallest 15 pilot jets, pilot fuel screws only 7/8 of a turn out, look to be never touched and never synched (paint marks as loctite/tamper-proof from factory).

Still unsure of bore diameter, but the K&N filters I got have instructions included that say with a stock exhaust and these dual oval plenum filters, increase the jet size by 15%! Wow, that'd be a 120 or maybe 122.5-125 with a 4-1! more surface area than pods might have something to do with it. I have 4-1 exhaust and these RC2222 filters (fits GS750/850/1000/KZ900/1000), the cc displacement is the only thing uncertain, so maybe my 115 guess was falling way short!?

[rustybronco]

120 or maybe 122.5-125 with a 4-1! more surface area than pods might have something to do with it. I have 4-1 exhaust and these RC2222 filters (fits GS750/850/1000/KZ900/1000), the cc displacement is the only thing uncertain, so maybe my 115 guess was falling way short!?

125'~127.5's are not out of range on a bike with pods and a 4 into 1. If it were me, I'd start with 122.5's or 125's

[Chuck78]

That image shows the filters that I have. Two open element filters, each filter has two carb inlets. So cyl's 1&2 share the left filter & cyl's 3&4 share the right filter. MUCH more surface area than pods! Sounds like to ride it on the 4th of July I will probably need to put the factory air box and filter back on after the carbs are back together, unless I can find a place that is open today that sells mikuni jets in town. The local vintage Japanese motorcycle place is closed on Wednesdays and on the holiday, crap!

[rustybronco]

http://www.advrider.com/forums/showpost.php?p=18600094&postcount=4

[Chuck78]

Found a place south of town that has them in stock! 122.5 & 125, now I just need to get off work on time to make it there before they close early for the holiday. I'm wondering if the 15 pilot jets are going to be too small considering the wide variations of pilot sizes that the 750 came with. May pick up some #20 pilots if they have them. Thoughts on that???

[rustybronco]

If your bike uses a #15 pilot you 'may' want to pick up some 17.5's but I've never needed to have to use anything other than the stock pilots.

If there isn't enough range of adjustment (the need for addition fuel at idle and slightly above) when using the stock pilots, then you go to the next size up.

What is the plan for midrange? Are you planning on raising the needles one notch?

[Chuck78]

definitely raising the needles one notch. from the paint applied at the factory at a sort of Loctite, it's obvious that the needle have never been touched nor have the pilot fuel screws.

the original intent of my post was to ask about pilot jet sizing since they used 15, 22.5, and 27.5 all on the same bike with vm26 carbs.it seems alarming at the wide range of sizes that they played with from the factory

[Chuck78]

when doing the 920cc conversion I might try the 5dl36 needles that the 850 and 1000 used

[rustybronco]

the original intent of my post was to ask about pilot jet sizing since they used 15, 22.5, and 27.5 all on the same bike with vm26 carbs.it seems alarming at the wide range of sizes that they played with from the factory

I don't know if I'd say alarming as the did run well with them installed. It's all about the amount of fuel necessary for idle and off idle.

[rustybronco]

Set the clip on the needle one notch lower. Balance the carbs (synchronize) at 2'ish K. Get the bike to idle and transition correctly by adjusting the 'fuel' and 'air' mixture screws (usually 2 turns air to 1 turn fuel). Take her for a ride and dial in the main jet. After that is correct and she runs well, then you can think about playing with the needle tapers.

Reading material... Pay particular attention to what Keith Krause has to say.

http://thegsresources.com/_forum/showthread.php?p=994610#post994610

[chuck hahn]

It wasnt so much the model years as it was the engines themselves. Thus this listing for jetting PER engine numbers.

http://free-pu.htnet.hr/medo/carbs.jpg

[rustybronco]

It wasnt so much the model years as it was the engines themselves. Thus this listing for jetting PER engine numbers.

Also, emission standards came into play for the 1978 model year...

[Chuck78]

It wasnt so much the model years as it was the engines themselves. Thus this listing for jetting PER engine numbers.


http://free-pu.htnet.hr/medo/carbs.jpg

Strange, my carbs had never had the fuel screws adjusted, never been synched (paint marks on both still from factory), non-stock intake boot bolts so carbs had been off and drain screws had marring as if the fuel levels had been checked and floats maybe adjusted. Seems that they were pretty original, and had the 105 main jet as a 1977 GS750B should, but had 15 pilot jets... 77 GS750B shows 105 main 22.5 pilots in that link and also on the same list on BikeCliff's site which shows several listings of carb specs, and one does show 105 main and 15 pilot on a VM26 GS750 of unknown year, but no other info on that. Maybe that was a very early model, as in a late 1976 early intro GS750???? ASK PowerSports only had three 17.5 pilot jets in stock otherwise I would have upped them to that without even running the bike yet. Soaking the carbs a third time to be certain they are spotless, motivated by the presence of rain in the forecast pretty heavily the next few days.

Also, I wish I new what the P-1 and other designations meant for the needle jet, and what the numbers meant on the jet needle sizes. Anyone have a chart that shows this info, as far as size and taper so that I could relate these figures to a leaner or richer running setup?