Jetting Q: It was a GS750B now has wiseco BB 850 kit

[GS750]

Hi, I know that I have asked about jetting sizes previously but now I have a whole new perspective on my motor and have to ask again.....

Now that I know what my motor is on my 1977 GS750B:

(it WAS previously 850'd with suzi pistons but I didnt know that)
It is now wiseco 850 BB'd
has a 4-1 with free flow muffler,
K&N Pods
Boyer electronic ignition kit
NGK Iridium plugs
4-1 transac eggsauced (at least I think thats the brand):
Stock 750 carbs other than the 112.5 main jets


Now, ive noticed in the manual there is a difference between the 750's and 850's with the VM26 carbs, shown below:

main jet
750 = 102.5
850 = 102.5

Air jet
750 = 1.1
850 = (not shown in my manual)

Needle jet
750 = 5F21-3
850 = 5DL36-2

Pilot jet
750 = 22.5
850 = 15.0

Pilot air jet
750 = 1.6
850 = 1.2

Cutaway
750 = 2.5
850 = Not shown in my manual


Can anyone please tell me the differences between the needle jet size and why the pilot and air jets would be smaller in the 850?


It is not possible to get to a dyno (the nearest is 600Km away) and I would like to get it up and running with the best possible carb settings that I can.

Can someone please let me know what I should or shouldnt change?

I do have availability to a set of 850 carbs from a later model (1982?) which were playing up even after loads of carby work, so I would prefer to get mine up and running.

So what should I do and what settings should I use to get it so it is reasonable??

 

[Steve]

I can't help you with the jetting question, but unless you put a different head on the engine, you won't be able to use your '82 carbs. At least here, in the U.S., we had CV carbs, and they have bigger throats. The head had larger intake ports to match.

.

 

[GS750]

thanks for that, I didnt even think to measure the size but you are right that they are larger and wont fit my 750B.


since it is almost impossible to find any suitable 850 carbs (1979, 850N seems to be the only ones with the same carbs as my 1977 750B and different jets) that they dont want a mint for, I think that I will just replace the following parts with new parts to suit the 850N:

needle jets & needles,
pilot jets,
pilot air jets

I think I will also keep my 112.5 main jets because of the pods and free flow 4-1 exhaust.

anyone know where I can get the above parts (preferably in Aust) that dont charge a fortune? Or a reasonable priced overseas supplier who wont charge the earth for shipping these parts.

 

[Suzuki_Don]

The sizes you have quoted for the needle jets are actually for the needles not the jets. The last number after the hyphen is the clip position on the needle. Cheers from Sydney. BTW you have some Mikuni dealers in Queensland.

 

[GS750]

Well Ive spoken with a few carb dealers and they all said they have no idea of jetting sizes without seeing the bike (nearest dealer is 500Km away)

Im really not sure what to do now because Im starting to think that my previous problems may have been related to the use of the 750 carbs which seem to have fairly different jetting compared to the same carbs on GS850's.

I could just fire it up as is but would I be risking damage??

 

[49er]

Hi, I know that I have asked about jetting sizes previously but now I have a whole new perspective on my motor and have to ask again.....

Now that I know what my motor is on my 1977 GS750B:

(it WAS previously 850'd with suzi pistons but I didnt know that)
It is now wiseco 850 BB'd
has a 4-1 with free flow muffler,
K&N Pods
Boyer electronic ignition kit
NGK Iridium plugs
4-1 transac eggsauced (at least I think thats the brand):
Stock 750 carbs other than the 112.5 main jets


Now, ive noticed in the manual there is a difference between the 750's and 850's with the VM26 carbs, shown below:

main jet
750 = 102.5
850 = 102.5

Air jet
750 = 1.1
850 = (not shown in my manual)

Needle jet
750 = 5F21-3
850 = 5DL36-2

Pilot jet
750 = 22.5
850 = 15.0

Pilot air jet
750 = 1.6
850 = 1.2

Cutaway
750 = 2.5
850 = Not shown in my manual


Can anyone please tell me the differences between the needle jet size and why the pilot and air jets would be smaller in the 850?


It is not possible to get to a dyno (the nearest is 600Km away) and I would like to get it up and running with the best possible carb settings that I can.

Can someone please let me know what I should or shouldnt change?

I do have availability to a set of 850 carbs from a later model (1982?) which were playing up even after loads of carby work, so I would prefer to get mine up and running.

So what should I do and what settings should I use to get it so it is reasonable??

Gidday from NZ.

I'm still running the stock VM26 carbs and airbox on my 850. My specs are similar to yours, with the exception of you running pods.

The 850 cutaway is 1.5. That's why your pilot jet sizes are larger than the stock 850's.

You're running 844cc @ 10.25-1 CR.
I'm running 894cc @ 10.5-1 CR. My pilots are #20 and needles are 5DL36-2. My research results quoted the stock needles for the 850's as 5DL31-3.

The stock mains for the 850's are 95's. I am running 102's.
I have dynoed my setup and the air/fuel ratios are spot on.

Running pods and that 2.5 cutaway, your pilots will need to be at least #25's. Your mains should be in the ball park at 112.5.

Fire it up and see. Warm it up and check throttle response once it is warm. It it is hesitant/dies anywhere in the rev range, this will indicate a severely lean condition and the engine shouldn't be pushed hard for any period like that. If it feels responsive, ride it and see how it feels under load. If it misses or bucks under load, you have a lean condition which needs urgent attention. If in doubt, do some plug chops.

Good luck and keep us posted.

 

[GS750]

Gidday from NZ.

I'm still running the stock VM26 carbs and airbox on my 850. My specs are similar to yours, with the exception of you running pods.

The 850 cutaway is 1.5. That's why your pilot jet sizes are larger than the stock 850's.

You're running 844cc @ 10.25-1 CR.
I'm running 894cc @ 10.5-1 CR. My pilots are #20 and needles are 5DL36-2. My research results quoted the stock needles for the 850's as 5DL31-3.

The stock mains for the 850's are 95's. I am running 102's.
I have dynoed my setup and the air/fuel ratios are spot on.

Running pods and that 2.5 cutaway, your pilots will need to be at least #25's. Your mains should be in the ball park at 112.5.

Fire it up and see. Warm it up and check throttle response once it is warm. It it is hesitant/dies anywhere in the rev range, this will indicate a severely lean condition and the engine shouldn't be pushed hard for any period like that. If it feels responsive, ride it and see how it feels under load. If it misses or bucks under load, you have a lean condition which needs urgent attention. If in doubt, do some plug chops.

Good luck and keep us posted.

Ok, ive fired the bike up and ran it for 60Km through town to the coast at up to 110Km/h. It seems to run fine on the highway but a idle it flutters about 150-200rpm and there is a definite bog when throttling from idle when taking off.

When Ive slowed down at traffic lights too and have gone back to around idle in 1st gear and Im still moving, it bogs again when getting back onto the throttle.


I have ordered the morgan carbtune vacume sync guages from http://www.carbtune.com/ so hope they will be here early next week and I can sync the carbs.

When I last had the bike shoppe sync them for me about 2 years ago the motor was starting to get sick and one inlet valve was probably not sealing due to extreme wear.

I also have to re-do the timing since I discovered when stripping the motor that the cams were both 1 tooth retarded, amongst many other problems such as no O rings on the bottoms of the barrels.

At least now I know what the motor actually is and that it is all done how it should have been!


I was fairly concerned when I first fired the motor up because when I looked through the sight glass the oil looked like it was milky.... There shouldnt have been any water in the motor........
Spoke to a mate a local bike wreckers and he said that is normal because of the rings bedding in and that it would go away after a very short time.... He was right and after the 60Km trip it wasnt doing it anymore.

Anyways, now if I can just get the jets sorted I will be a very happy GS BB'er, and if I cant get it sorted I may just have to take it down to Brisbane to have it dyno'ed

 

[GS750]

This was my post before my user got purged from the database

 

[needtoride]

I have a 1978 GS750 that is need of a rebore and wanted to go for the Wiseco K844 kit.

they say it increases the bore from 67 to 69 mm however all the technical spec on the GS750 is that it is 65mm (so is mine having measured it).

The external diameter of the liner as measured at the bottom where it sticks out of the block is only 72.5mm meaning the liner wall thickness would be a little over 1mm thick. Surely this is to thin??

has anyone actually fitted one of these kits to a stock GS750 block. Wiseco tech info says just a rebore is necessary. Tried to email Wiseco several times but no response

 

[KEITH KRAUSE]

Your main jets are way too small. Try 125's. If the bike is otherwise tuned correctly and the motor in good shape then the 125's will be close. Only testing will tell if you need to go up or down a little.
If using the stock jet needle (5 positions) then try position 4 from the top. You will need to richen the needle a minimum of 2 positions. I believe the '77 had the jet needle clip in the 2nd position down from the top ??? but not sure about that. If factory position is position 3 then I suggest trying position 5. If you're close but need a "1/2 position" change, then get some spacers that are approx' .022" thick which represent a 1/2 position change. If you still have problems at 1/5 to 3/4 throttle then I suggest buying the dynojet needles.
I suggest 17.5 pilot jets. Tinkering with the pilot fuel screws underneath to fine tune things will probably be needed. Try 1 full turn out from LIGHTLY seated initially.
Set the side air screws to 1 3/4 turns out initially. Adjust them using the highest rpm method. REMOVE the 2 floatbowl vent lines and leave the ports open. Bench synch.
Test at full throttle and solid 1/2 throttle for the mains and jet needles, respectively. If plug color and performance look good, then vacuum synch. You can of course vacuum synch before the testing to get the most accurate plug reads. But remember the VM carbs must be re-synched every time you disturb/move the jet needles.

 

[GS750]

I have a 1978 GS750 that is need of a rebore and wanted to go for the Wiseco K844 kit.

they say it increases the bore from 67 to 69 mm however all the technical spec on the GS750 is that it is 65mm (so is mine having measured it).

The external diameter of the liner as measured at the bottom where it sticks out of the block is only 72.5mm meaning the liner wall thickness would be a little over 1mm thick. Surely this is to thin??

has anyone actually fitted one of these kits to a stock GS750 block. Wiseco tech info says just a rebore is necessary. Tried to email Wiseco several times but no response

Need to ride, you really should have posted a seperate (new) thread with your question, but I will answer you anyways because as I said, I do have the K844 fitted to my 1977 GS750B.
You should have noticed my signature just below the picture of my bike.


OK:
Mine was NOT standard 750 pistons before I started with the 850BB (K844) kit, it was already 850'd but the barrels were rooted so I replaced them with stock (second hand) 750 barrels. I did measure them but cant remember exactly what they were, they were not even 1st oversize 750's.


Ok, Im still not sure exactly what you are asking (or saying) but my Clymer book says the standard pistons should be 64.945 - 94.960 and the cylinder inner diameter 65.000 - 65.015

My factory Suzuki service manual says the same.




You also said "The external diameter of the liner as measured at the bottom where it sticks out of the block is only 72.5mm meaning the liner wall thickness would be a little over 1mm thick. Surely this is to thin??" Well I can tell you that the K844 did fit up to my replacement 750 barrels, I took a piston and ring set (and the spec's sheet that came with them) to my local engine shop, had him bore them out.

I have been running the bike now for 12000Km and had no issues at all and didnt bother to look at the liner wall after it was bored out to take the K844 but I can tell you for fact that the factory GS750 barrels are identical to the factory GS850 barrels, other than the liner sleeve being thicker on the 750 and thinner on the 850, and the casting on the barrels which specifies the engine size (both my sets of barrels said "750cc").

I love my "GS750" and it goes extremely well, is reliable etc etc, and the wiseco kit has had no problems.

You can send me a PM if you have any more questions or just post a new thread



.

 

[Guest]

I have been running the bike now for 12000Km and had no issues at all and didnt bother to look at the liner wall after it was bored out to take the K844 but I can tell you for fact that the factory GS750 barrels are identical to the factory GS850 barrels, other than the liner sleeve being thicker on the 750 and thinner on the 850, and the casting on the barrels which specifies the engine size (both my sets of barrels said "750cc").

.

If both barrels said 750cc and they were identical then your old ones were bored out 750's not factory 850's- if you've still got them I'd be curious to know their thickness.

The sleeves on my 79 750 were the same thickness as the 850 cylinders I replaced them with - about 3mm and I too wandered how they'd hold up with the Wiseco over bore? Were earlier 750 sleeves thicker, or is this 1mm OK?

Nice paintjob by the way Andrew.

 

[GS750]

If both barrels said 750cc and they were identical then your old ones were bored out 750's not factory 850's- if you've still got them I'd be curious to know their thickness.

The sleeves on my 79 750 were the same thickness as the 850 cylinders I replaced them with - about 3mm and I too wandered how they'd hold up with the Wiseco over bore? Were earlier 750 sleeves thicker, or is this 1mm OK?

Nice paintjob by the way Andrew.

Hey Coady, yeah both were 750 barrels, I do actually still have the original 750 barrels that were 850'd as well as the genuine 850 suzi pistons, will dig them out tomorrow and let you know the inside barrel measurement.

Im pretty certain that the '79 barrels were the same as the '77's. They may have changed late in '79 though.



Thanks for the comment about the paint, its done in 2 pak with the badges removed, then the vinyl stickers which were made up the same as the badges, then 2 pak clear over.

It is the GT falcon pearl blue from about 2004 or 2005 Best part was that it only "cost me" 5 hours of my time to strip the bits off and remove the old paint, then 2 more hours of my time to build a computer for the panel beater