rebuilding/rejetting 77 750 carbs, HELP!!!

[Beaver]

New Zealand originally, but Denmark now. Cursed with vehicle 'afgift' (it is like a fee penalising you for owning something that pollutes, making cars and motorcycles here some of the most expensive in the world) and living in an apartment with no yard of my own, nor garage. It doesn't make embarking on a 'project' particuarily easy

 

[beergood]

Every now and again I get asked what jets I ended up with in mine. I wish I was organized to keep a log.

If I remember correctly, I jumped up to 125 on the main. I tried 17.5 on the pilot, and that was too much, so went back to the 15. And I think I moved the needles up one notch (tried two and it was too much). It pulls pretty strong through the whole band, and I haven't been fouling any plugs this year (so far).

Someone far more organized than I should make a spreadsheet of user, bike, pipe/filter configuration, and main, pilot, needle settings. I think we would probably populate it with a ton of data that would be useful to track what works with what.

 

[TheCafeKid]

Every now and again I get asked what jets I ended up with in mine. I wish I was organized to keep a log.

If I remember correctly, I jumped up to 125 on the main. I tried 17.5 on the pilot, and that was too much, so went back to the 15. And I think I moved the needles up one notch (tried two and it was too much). It pulls pretty strong through the whole band, and I haven't been fouling any plugs this year (so far).

Someone far more organized than I should make a spreadsheet of user, bike, pipe/filter configuration, and main, pilot, needle settings. I think we would probably populate it with a ton of data that would be useful to track what works with what.

Ive thought of doing that BG..but its SOOO dependent on whats going on with the bike at hand...While the chart would get you CLOSE, in the ballpark, final settings would still require testing and retesting... Altitude, state of tune, type of filter etc etc... So many variables..

 

[beergood]

While I agree that it is difficult to get your bike 100% in tune (personally, I think this is pretty much impossible in the average garage), being in the high 90s is within the grasp of a little work. Which is where I would guess most of us are.

By the way, these numbers aren't based on anything concrete, just a gut feeling.

Sure altitude, temperature, barometric pressure, changes in the wind can all effect the final results of your efforts, but I don't think most of us worry about these fairly negligible effects. These bikes are workhorses, not surgical instruments. They were built 30 years ago and sold around the world. The fact that we are all able to keep these things running points to their sturdy and forgiving design.

We know the bid deals: timing, valve clearance, good voltage, keep your carbs synced and clean.

When it came time to change the jetting I found a post by Keith where he threw his best guess at a similar question, so I went with his advice and it got me pretty darn close. Almost but not quite close enough. But without his advice I bet I would have spent a heck of a lot more time shooting in the dark.

Now I am pretty happy with the results. I am sure I could probably dial it in a little more if I felt like putting in more effort, but I think I am at the point of diminishing returns.

I also think it would be kind of nifty to have a running list of members, and what they have going on in their rides. I think I will start a new thread asking just that.

 

[Daffyd]

Jets

Jets

Hi mate, jets are the bain of my life too, so I will tell you what little I have found out......google suzuki carb jets, somewhere on here there is a chart for Mikunis. The setup it recommended was 115 main, 22 pilot.
The advice I"ve been given is upgrade by 10%, so depending on what standard jets you have, (Haynes manual says it depends on your engine number), go up from standard 10%,start from there. Then fiddle around, see my thread "jet sizes AAAAArrrghhhh", sound advice there

 

[KEITH KRAUSE]

Hi mate, jets are the bain of my life too, so I will tell you what little I have found out......google suzuki carb jets, somewhere on here there is a chart for Mikunis. The setup it recommended was 115 main, 22 pilot.
The advice I"ve been given is upgrade by 10%, so depending on what standard jets you have, (Haynes manual says it depends on your engine number), go up from standard 10%,start from there. Then fiddle around, see my thread "jet sizes AAAAArrrghhhh", sound advice there

For a typical pipe/pod set up, 10% will get you nothing but a very lean running bike. That would be saying a typical VM equipped 1000 with a stock 95 main would require a 105?...just won't work. Most 1000's require a 130 or slightly larger main for pipe/pod mod'. 750's/550's won't run well either on that 10% suggestion.
Haynes isn't a reliable source when it comes to jetting.

 

[chuck hahn]

In post 18 Dale posted a link next to pertinent carb info. In that chart is say the mixture screw settings.. i always get confused on the technical names here. The MIXTURE screw is the one found inside the carbs bowls and the AIR screw is the external one on the side of the carbs...right???

 

[rustybronco]

Correct... The fuel/mixture screw is on the bottom and the air screw is on the side.

 

[chuck hahn]

and if i remember it is to the left of the main jet seat inside there?? Was wondering cause when i found all that gunk in the #4 carb on my 77 i took that out and blew some air and carb cleaner thru the passages..i just never really was sure i had reset it correctly. Have to check it out next time i go up north...Thanks Dale.

 

[vladdmd]

Sorry guys... Newbie here.

Sorry guys... Newbie here.

Hi guys, I am trying to figure out what adjustments to do on my 77 gs 750. I added pods and High compression forged ARIAS pistons, and I retarded the timing about 3 degrees. Stock exhaust. Bike is running, yet at full throttle it bogs down. My first project and learning as I go. I already.. rebuilt the top half with ARIAS high compression pistons, added a copper gasket, had a valve job done, re did the electrical, duneage's RR and 81 GS 850 magneto and RMstator's stator, rebuilt front and rear calipers.... Carbs scare the crap out of me. A shop wants 300 to jet and sync the carbs.... seems like a lot. I was told that I am running stock jets. Plugs do NOT look white. They like they have normal wear and tear. Also in terms of exhaust heating up (I dont know if it matters) number 2 and 3 heat up first, with 4 then 1. Could that be from poorly calibrated carbs? Also since I am breaking in the engine, I should have an even compression +- 15 psi, before I jet right?

...Overwhelmed.

-Vlad

 

[TheCafeKid]

Hi guys, I am trying to figure out what adjustments to do on my 77 gs 750. I added pods and High compression forged ARIAS pistons, and I retarded the timing about 3 degrees. Stock exhaust. Bike is running, yet at full throttle it bogs down. My first project and learning as I go. I already.. rebuilt the top half with ARIAS high compression pistons, added a copper gasket, had a valve job done, re did the electrical, duneage's RR and 81 GS 850 magneto and RMstator's stator, rebuilt front and rear calipers.... Carbs scare the crap out of me. A shop wants 300 to jet and sync the carbs.... seems like a lot. I was told that I am running stock jets. Plugs do NOT look white. They like they have normal wear and tear. Also in terms of exhaust heating up (I dont know if it matters) number 2 and 3 heat up first, with 4 then 1. Could that be from poorly calibrated carbs? Also since I am breaking in the engine, I should have an even compression +- 15 psi, before I jet right?

...Overwhelmed.

-Vlad

HI Vlad, SOunds like youre pretty close. Maybe a move of the needle clip is what you need? Its not as easy as the CV carbs to do that, as you have to pull the throttle arm out to pull the slides, and then you'll have to resync, but its not all that tough either..

 

[vladdmd]

So then, Is it possible to operate with pods and high comp pistons, with standard exhaust on stock jets?. I thought changing jets is a must..

 

[Guest]

Wouldnt wide open throttle as he described be in the main jet and not the needle?

 

[vladdmd]

Actually maybe someone jetted it way back... The bike runs with no pods at all too. Not sure if that means anything.

-Vlad

 

[TheCafeKid]

So then, Is it possible to operate with pods and high comp pistons, with standard exhaust on stock jets?. I thought changing jets is a must..

Well you have High COmp pistons.. but retarded timing. The plugs arent white.. and they will tell you if its lean more than anything other than the bike itself. Im not a motor whiz, I will be the first to admit. The High Comp pistons and timing, I dont know what that equates to, but if the plugs arent lean, Then.. I dunno. But bogging out before WOT, or breaking up around WOT, maybe a main, maybe the needle, maybe both! The slide carbs dont quite work the same as the CVs..

 

[Guest]

VM carbs tend to be a bit more forgiving.

Lets see what CK has to say..

 

[TheCafeKid]

Actually maybe someone jetted it way back... The bike runs with no pods at all too. Not sure if that means anything.

-Vlad

Well, PROPERLY JETTED, you dont need filters. I can take my 1100ES's filters off and blast all over the place..In fact, it might run just a *touch* better. Dont want the dirt in there tho.. The only way to really know what's going on is to crack em open and see what you have in there for mains, pilots and needle settings. The pilot circuit pretty much controls everything up to about 1/4 throttle, the needle from there to about 7/10s , and WOT is all main jet. If it falls flat on its face at WOT then you know its probably the main. You cant relate RPM, speed or anything OTHER than throttle position to the carbs, because they make no difference. At 70MPH, just cruising with a slight bit of maintenance throttle, you're likely only using the pilot circuit, MAYBE the pilot needle transition, but certainly no more than that. So it may run like a champ. But wack the throttle (something you CANT do on VM carbs, no matter how they are jetted BTW, ask if you dont understand why, ill be happy to explain) and get into the higher needle, or main, and it could fall on its face..

 

[vladdmd]

I did notice that when I ease into open throttle the engine eventually revs up. If I want a snappy throttle, am I better off with going with a 80 CV carb and put in a stage 3 dynojet kit?

 

[TheCafeKid]

I did notice that when I ease into open throttle the engine eventually revs up. If I want a snappy throttle, am I better off with going with a 80 CV carb and put in a stage 3 dynojet kit?

Well not necessarily. It will depend on whether or not your carbs, once tuned propperly, still dont perform like you want (VMs you have to ease into the throttle, even when they are jetted right. Snap em open, and you drop all the vacuum in the motor, which pulls the fuel/air mix out of the carbs, and it bogs out for a sec or so) The CV carbs have more of a two stage valve set up. The butterfly valves are operated by the throttle, the slides are lifted by vacuum. Snap those open, it bogs, just for a split second, but recovers quickly. They're a more "streetable" carb for sure. But if you wind it out, run it wide open alot, like on a track, or hot thru the country twisties, and dont do alot of city wacking thru traffic, the mechanical type (VM, RS, etc) might be more your speed.

 

[vladdmd]

Great Info!! Thanks!! One last one... If, in the appropriate order, my exhaust pipes are ALWAYS heating up in 2,3,4,1 can that mean that the carbs are not synced? Does it mean anything? Also on the fins, same temp distribution. Seems that 2 and 3 are also running a bit hotter than 4 and 1. I am going to get a laser temp gauge.

-Vlad

Also, if anyone needs dental advice, I'm a dentist.. I know about dentistry a whole hell of a lot better than about my GS.