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rebuilding/rejetting 77 750 carbs, HELP!!!

  • Thread starter Thread starter 77GS750inCincy
  • Start date Start date
7

77GS750inCincy

Guest
I am about to tackle the rebuild on these carbs and don't even know where to begin. I have rebuilt hundreds of carbs, but these have no diaphrams? The needle setup is a bit over my head. Will I need to mess with this, I have a kerker 4-1 and pods to go on at the same time... any jetting suggestions to get me in the ballpark would be appreciated.
 
The overwhelming response is going to be "get a DynoJet kit", but I seem to remember some posts about the '77 carbs being a bit different and no jet kit is available. :-k

TheCafeKid knows about these carbs, let's see if we can rattle his chain and get him to offer his suggestions. :D

.
 
seems like sw ohio is taking over thegsr... I have never seen any custom GS's riding around here but it says you all live in this neck of the woods... I think we need a GS ride/ meet ang greet...And to Cafe, please chime in with your infinate wisdom... As I am completely stumpt
 
This might help...

This might help...

A recent conversation that a member of this board, had on that same subject with TCK.

the carbs at the moment, are on my bench being properly cleaned.
It did run in the ball park with 125's installed and the clip in the 4th position. but because of choke issues and a plugged pilot jet in #2, I could not do the plug chops to see how close it really was.

the info was also in the range that Keith Krause had suggested to others on this board.

I will add if $$$ is not an issue I would pick up some 122.5's also.

TheCafeKid said:
The jets needed would be dependent on a few different factors, and you may have to try out a few different sizes, and switch needle possitions a couple of times. But, to give you a good base to start at, i would try a 125 or a 127.5 main, stick with the stock pilot jets, needle clips in the fourth position from the top, air screws 2 turns out, fuel screws 1.5 turns out. Jets arent super expensive, so you might try getting both the 125s and the 127.5s at the same time, just in case, so you dont have to wait for one should the other not work. I dont have an airbox currently for that bike that im willing to part with. They come up from time to time on Ebay, any from 77-79 will work, including the 850, and 1000s from those years. they were all pretty much the same i do believe.

In order to tell if your settings and jets are correct, you'll need to do some plug chops at different throttle positions, and then read the colour of the plugs. This will help you determine if you're running rich or lean or good to go. Do a search on plug chops here on the board, and theres quite a few great threads on the subject.

Hope this helps some.
Click to expand...

also http://www.thegsresources.com/_forum/showpost.php?p=324909&postcount=43


A previous thread I had on the same subject.
http://www.thegsresources.com/_forum/showthread.php?t=139768
 
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do stock needles have the option to adjust clip settings? or should I get a micrometer out and start stacking washers?
 
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picked up some 125s today, anyone got any tips for a DIY carb cleaning, as the site says to pull the carbs apart ( i hear this is a big no no) Is this ultra sonic tank my local shop is pushing me to do worth the $120 he wants to do um? mind you the carbs are taken down to the bare bodies of the carbs ( all jets and floats, springs, needles are out) seems like the leg work is done for him. Can anyone walk me through this or point me in the right direction. Thanks in advance
 
77GS750inCincy said:
picked up some 125s today, anyone got any tips for a DIY carb cleaning, as the site says to pull the carbs apart ( i hear this is a big no no) Is this ultra sonic tank my local shop is pushing me to do worth the $120 he wants to do um? mind you the carbs are taken down to the bare bodies of the carbs ( all jets and floats, springs, needles are out) seems like the leg work is done for him. Can anyone walk me through this or point me in the right direction. Thanks in advance
Click to expand...
Do it yourself. Go to AutoZone and get a can of Berrymans ChemDip. Soak each body and all the metal bits in it for 24 hours each. Keep track of what came from which, though. And YES, you have to take the rack apart to clean them properly. Whomever told you that was a no no was a mor ron, and probably has a bike that runs like crap. Its not hard, you just have to follow the directions on the site here, precisely. To THE LETTER. and you'll do ok. If you get stuck, shoot me a PM. Im not too far from you and I know those carbs like the back of mah hand...
 
125 mains sound good. Each bike can vary. Test at full throttle, chop off and do what the plugs/performance say.
If you have approx' .022" jet needle spacers, I'd try the e-clip in the bottom position with the spacer on top the clip. That would be position "4 1/2". Some owners claim the 4th position was good but if the pipe and pods flow well then the bottom (5th) position is more likely to work. Kerker made a good flowing pipe. If not enough fuel, aftermarket jet needles will be needed. Be sure you re-install the stock jet needle spacers in factory order. Thicker one on top the e-clip, thinner underneath the clip.
A 17.5 pilot jet should work but you can try the stock 15's with richer pilot fuel screw adjustments (underneath) if you want. Adjusting the fuel screws beyond approx' 3 1/2 turns out from LIGHTLY seated is out of their effective range. Then adjust side air screws for best idle from a base idle of 1,000 rpm's.
Be sure to remove the 2 floatbowl vent lines and leave the ports open when running pods.
Jetting depends on the bike being in tune. Valves clearances must be good, not just assumed. Ignition timing spot on. In fact, after you jet it well, try advancing the timing a few degrees as long as you don't get pinging. Clean carbs are a must. Replace all inner o-rings if any doubt about their condition. Float levels must be correct. The carbs MUST be in synch to accurately test the jetting. Be sure of no intake leaks. Check for proper voltage and be sure all electrical connections are clean/tight including coil connections and plug caps. Be sure exhaust gaskets are good so you don't blame decel' pop strictly on the jetting...
 
I now have THE GS gods (cafe and Keith) input and am in the midst of trying this whole carb cleaning deal. I will be following both of these posts to the T and we'll see how it turns out. I have intentions of going to the track on saturday (1week) and am hoping for some whopping 12.5s outta this dinosaur. Is this out of the question?
 
Also, you usually don't need to completely split the carbs to clean them. Only if you think the fuel passages between carbs could be dirty.
Mentioning this because sometimes the fuel "T" (with coated/built in o-rings) may leak once tampered with. You can find a site that will build a new one if needed, though expensive for what it is. You could be lucky and already have the T that utilizes separate o-rings that are replacable but don't count on it.
 
The settings and results on the gs750 bplubin and I finished working on for the time being,..

valves adjusted, new o-rings, carbs properly cleaned, sync within 1/4 in Hg, 4 into1 exhaust, pods, 125 mains, stock 15 pilots, clips on the needles in the 4th position from the top, fuel 1-1/2 from lightly seated, air 1-1/2.

it idled well, didn't stumble and pulled hard to redline? through 1st and 2nd (tach broken), no sign of stumble and no pop on decel. the plugs showed slightly black at the firing tip end, the middle part of the insulator was brown and close to the very bottom was brown.
but because a storm that blew through with 70 mph winds and the streets that were still wet we couldn't run it hard enough to do plug chops and make sure the 125's were the perfect choice.

when the roads are dry and he can take it out and run it, I asked him to set the air screws at 1-3/4 and do the plug chops to check the mains.

but I believe it will be very, very,close.
 
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Here I am us?ng the search function and digging around in old posts, but then again, who hasn't been down this road themselves? Pods, 4-1 header and needing to re-jet the carbs? I am just a little confused though. Some have said 10% larger jets, another 2 sizes for the pods and 2 for the header, the last theory is 6 sizes up for both pods and header. My manual tells me I have 100's now (I can't verify this cause once I start this job I need to finish it again fast, so that means having all parts in place when I start). I bought the bike with 4-1 marshal pipes on it, it had apparently had pods on it before but they could never get it running right and went back to the orginal air box. 4 sizes or 10% gives me 110's, 6 sizes 115's and this thread is talking about 122.5-127.5. I don't mind ordering a couple of sets to make sure I get it right but I am not sure which ball park I am aiming for...

77gs750, engine number 28855 (quite possibly not the original one, engine that is)
 
Beaver said:
Here I am us?ng the search function and digging around in old posts, but then again, who hasn't been down this road themselves? Pods, 4-1 header and needing to re-jet the carbs? I am just a little confused though. Some have said 10% larger jets, another 2 sizes for the pods and 2 for the header, the last theory is 6 sizes up for both pods and header. My manual tells me I have 100's now (I can't verify this cause once I start this job I need to finish it again fast, so that means having all parts in place when I start). I bought the bike with 4-1 marshal pipes on it, it had apparently had pods on it before but they could never get it running right and went back to the orginal air box. 4 sizes or 10% gives me 110's, 6 sizes 115's and this thread is talking about 122.5-127.5. I don't mind ordering a couple of sets to make sure I get it right but I am not sure which ball park I am aiming for...

77gs750, engine number 28855 (quite possibly not the original one, engine that is)
Click to expand...
The sizes mentioned in this thread are for Pipe AND PODS...dont put those sizes in there if you're running an airbox...generally you SHOULD only have to go about 2 sizes up for just a pipe...
 
Well when I used to run my 750 with pipe and pods, I only ran 115 mains. I think 125's is a little too much. On my 1085 with smoothbores, pipe, pods, cams, I have it running well with 120 mains.

To go from 102.5 to 125 is a little too much I believe, but if your plug chops come out good, so be it.
 
TheCafeKid said:
The sizes mentioned in this thread are for Pipe AND PODS...dont put those sizes in there if you're running an airbox...generally you SHOULD only have to go about 2 sizes up for just a pipe...
Click to expand...

The pods are in the post, so I am needing to know how big the jets will need to be. It may well be that there are 102.5's or 105's in it now because of the pipe I don't know but working out from that the engine had 100 as stock and when I am finished it will have both pods and pipes I am thinking I am needing to order somewhere betwee 110 and 115 size jets...

Next, is a carb rebuild kit nescessary (Float Bowl Gasket, Float Needle & Seat, Carb Top Gasket) $14.95, or just a float bowl gasket for $3 as recommended in the carb cleaning guide? (O-rings are in the post too)

Cheers
 
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Beaver said:
I am thinking I am needing to order somewhere betwee 110 and 115 size jets...
Click to expand...
Just for reference, my post about a '77 gs750... http://www.thegsresources.com/_forum/showthread.php?t=139768

If I was doing it again I would order both the 122.5's and 125's, I would start out by using the 122.5s because of the plug color when jetted with the 125's, but your mileage may vary.
I have stopped by Bplubin's place a couple of times trying to find out what the final plug chops looked like, but he hasn't been home the times I had been there.

the required jets can be sourced from Z1 enterprises.
http://www.thegsresources.com/_forum/showpost.php?p=995444&postcount=6

pertinent carb info... http://www.thegsresources.com/_forum/showpost.php?p=324909&postcount=43

Beaver said:
4 sizes or 10% gives me 110's, 6 sizes 115's and this thread is talking about 122.5-127.5.
Click to expand...
using 100 for an example, 1/2 size=102.5, 1 size=105, 2 sizes=110
5 sizes=125... (someone please correct me if I'm wrong)

Beaver said:
Next, is a carb rebuild kit nescessary (Float Bowl Gasket, Float Needle & Seat, Carb Top Gasket) $14.95, or just a float bowl gasket for $3 as recommended in the carb cleaning guide? (O-rings are in the post too)

Cheers
Click to expand...
The kits are NOT necessary, only the o-rings and "possibly" the bowl gaskets.
 
Legendary!

Thanks :) I had misunderstood the sizing as I hadn't realised that there was such a thing as ? sizes. I couldn't find the jets on Z1 either as they aren't listed under GS750 parts so your link was a big help too. I am ordering them now...

Cheers

Angus
 
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