Replaced main jets with 115s... STILL LEAN

[pasteurized]

So I replaced the main jets with 115s on my gs750 today and with the stock airbox, it still leans out and pops and hits a wall around 5-6 grand. The bike has stock tailpipes with the muffler innards removed (so basically its like running straight pipes) and with the stock airbox it is still way lean at speed. I have individual pod filters for it too but of course it is even leaner with them.

I can't continue to just buy jets in hopes that one size may work. I am at my wits end with this bike and am this close to trying to sell it. Any ideas would be appreciated.

 

[earlfor]

Re: Replaced main jets with 115s... STILL LEAN

Well, lemme see here. At one time my 79 750 was running with a V & H 4 into one, stock airbox and filter. The only jetting change I needed to make was changing the stock 102.5 mains to 115's. I changed my float height setting from 25 mm to 23mm for the 115's. Changed the pilot fuel screw to 1 3/4 turns out. Set the pilot airscrew to 2 turns out.

A precise vacuum carb synch is vital.

You must retain the airfilter element in the airbox and there should be a good seal between the filter and the box.

Frequently, it is a lot of small things that are not quite right that combined, result in your problem. Make sure your ignition timing is not advanced. It should not be greater than 32 deg at full advance. With a 77 750, you have the induction tubes between the carbs and cylinder head that have the two screws and an "O" ring between the boot to cylinder mounting surface. If you have not replaced those with new ones, it is probably the major cause of your lean running condition and I would guess the cause of the deacceleration popping.

Earl

So I replaced the main jets with 115s on my gs750 today and with the stock airbox, it still leans out and pops and hits a wall around 5-6 grand. The bike has stock tailpipes with the muffler innards removed (so basically its like running straight pipes) and with the stock airbox it is still way lean at speed. I have individual pod filters for it too but of course it is even leaner with them.

I can't continue to just buy jets in hopes that one size may work. I am at my wits end with this bike and am this close to trying to sell it. Any ideas would be appreciated.

 

[pasteurized]

I am actually having a hard time trying to find the pilot screw on my carbs. I see the air screw obviously (about halfway up each carb, facing outward) but the only other screw I see is a float bowl drain screw. Not that it will help with the higher rpm ranges, but I was wondering anyway.

I have a Morgan Carbtune on the way, should be here any day. Hopefully that will help.

The original airbox and filter are in place. How many holes is the stock airbox supposed to have? It seems to have a large hole in the back/bottom of it that looks factory.

I do not know how to check timing on a bike, and do not have a timing light.

I know the problem with leaning out seemed to really start occurring when the mufflers were gutted. Thinking back, I wish it had never happened because I wouldn't have to screw with it at all.

I will have to look into the o-ring between the carb and intake. Hopefully somewhere other than the dealer will have this because the nearest dealer is 45 minutes away and I already went there to get the 115s today.

Thanks for the help Earlfor, I hope I can get it sorted out before I go completely out of my mind. I even considered starting to take it apart and part out the bike today but I just can't do that, I think these bikes are great and mine is in good enough shape that I would hate to see it in pieces.

 

[earlfor]

On the underside of your carbs, protruding through the bottom at the front of the float bowl, is the pilot fuel screw. You will need to remove the carbs and turn them upside down to view the screw and set it accurately. Turn them in VERY lightly until you feel them stop. (two greasy fingers on a jewelers screwdriver is sufficient torque) The points of the screws are fine and break easily, jamming themselves into the pilot fuel passageway and can be near impossible to remove if you break one off. Count the original number of turns out it was when you turn it in, so you can reset to that setting later if you wish. All carbs may not be set to the same number of turns out.

The stock airbox has one hole in the rear, bottom center about 1 1/2 x 2".

A timing light is not needed to check and set timing. I assume you have the stock ignition points system?

The "O" rings cost about $1.50 each at the dealer. I would bite the bullet and make the trip. Shipping from an online source will cost you more time and gas than going to the dealer. I would call ahead. My dealer stocked them, but others may or may not. Leaking "O" rings will guarantee you will not get the mixtures right, and you cant synch the carbs either if they leak. Forget creative fixes or siliconecrap, they dont work. Neither does the wrong composition of rubber for the "O" rings.

Are your mufflers now completely hollow/gutted?

I would not consider giving up or parting the bike out. You just have a couple bugs in the ointment to work out. Nothing really major.

Considering you dont know how to time the bike, I guess its safe to assume you have not checked point gaps or checked points for wear or replaced them either?

What color is the center ceramic on your spark plugs?

Earl

I am actually having a hard time trying to find the pilot screw on my carbs. I see the air screw obviously (about halfway up each carb, facing outward) but the only other screw I see is a float bowl drain screw. Not that it will help with the higher rpm ranges, but I was wondering anyway.

I have a Morgan Carbtune on the way, should be here any day. Hopefully that will help.

The original airbox and filter are in place. How many holes is the stock airbox supposed to have? It seems to have a large hole in the back/bottom of it that looks factory.

I do not know how to check timing on a bike, and do not have a timing light.

I know the problem with leaning out seemed to really start occurring when the mufflers were gutted. Thinking back, I wish it had never happened because I wouldn't have to screw with it at all.

I will have to look into the o-ring between the carb and intake. Hopefully somewhere other than the dealer will have this because the nearest dealer is 45 minutes away and I already went there to get the 115s today.

Thanks for the help Earlfor, I hope I can get it sorted out before I go completely out of my mind. I even considered starting to take it apart and part out the bike today but I just can't do that, I think these bikes are great and mine is in good enough shape that I would hate to see it in pieces.

 

[pasteurized]

Still having a hard time finding the pilot fuel screw. Unless it is something that needs to be drilled out, I am not seeing it. Here is a pic of the bottom of my carbs:
http://www.beat-la.net/bike/3.jpg.

I will have to check but it seems to me that the hole in my airbox is bigger than that... but like I said, it looks factory.

No clue on the timing. I know how to time an engine with a timing light but not this bike so I guess I will have to do some searching on here.

The mufflers are completely gutted. When I bought it one of the muffler's innards was just flopping around in there so I removed it. To even it out a buddy of mine went ahead and removed the innards from the other muffler.

I have not checked points or anything like that. I know I should buy a clymer for most of this stuff but I am at the point where I have already spent so much on this bike that my limited college income doesn't allow for much more.

 

[earlfor]

Damn this must be national weird warb week or something.
the pilot fuel screw is normally in the center of the bowl at the forward edge. I see on each bowl a small raised tower that in the pic appears to have a press on cap and an "O' ring or washer? The fuel pilot may be under the cap. Its something that needs to be accesible for adjustment, so I cant imagine putting it inside the float bowl

Do you have access to any kind of shop where you could fabricate baffle tubes? I know you can buy them, but if you can make things, it will be cheaper.

Earl

Still having a hard time finding the pilot fuel screw. Unless it is something that needs to be drilled out, I am not seeing it. Here is a pic of the bottom of my carbs:
http://www.beat-la.net/bike/3.jpg.

I will have to check but it seems to me that the hole in my airbox is bigger than that... but like I said, it looks factory.

No clue on the timing. I know how to time an engine with a timing light but not this bike so I guess I will have to do some searching on here.

The mufflers are completely gutted. When I bought it one of the muffler's innards was just flopping around in there so I removed it. To even it out a buddy of mine went ahead and removed the innards from the other muffler.

I have not checked points or anything like that. I know I should buy a clymer for most of this stuff but I am at the point where I have already spent so much on this bike that my limited college income doesn't allow for much more.

 

[pasteurized]

Actually, that little "tower" sticking out there on the float bowl is just that. There isn't a press on cap or anything like that, the picture is misleading. It is all one piece, part of the float bowl. Not sure what you mean by baffle tubes so I guess that would be a "no" in response to your question ;-)

I will go ahead and try to get it timed as per this post:
http://www.thegsresources.com/_forum/viewtopic.php?t=9513. Where do you suggest I pick up a new set of points? If I head to the dealer again to pick up the o-rings, will I get royally screwed if I purchase the points there?

I am just trying to avoid future frustration, because I am at the point where if I replace the points and set the timing and all that and it makes no difference on how the bike runs (leaning out) I am going to pull my hair out. I just can't see spending any more money on it if I can't get it to run decently.

Thanks again Earlfor ;-)

 

[SqDancerLynn1]

I think we decided from your other post these are not the correct Vm26mm carbs for a 77GS750
They are from a KZ650 ?

 

[pasteurized]

Good call Lynn, that looks right!

http://cgi.ebay.com/ebaymotors/ws/e...tem=4535821517&category=35574&sspagename=WDVW

So... where does that put me? I mean, are these carbs set too lean all around for a gs750?

 

[SqDancerLynn1]

I would look on ebay for another set from a gs750 maybe even an 850/1000 You may never get it to run properly with those carbs

 

[earlfor]

Oh Hell, I was thinking it was "ptm" with the KZ carbs, but in going back and checking, he only made a reply. I have to agree with Lynn on going to ebay and getting the correct set of carbs as the best solution. I dont know if the KZ carbs can be made to work satisfactorily or if they can, have any idea of jetting and set up for them. I would not buy or replace anything for/on the bike until I had the correct carbs on it.

Earl

I would look on ebay for another set from a gs750 maybe even an 850/1000 You may never get it to run properly with those carbs

 

[pasteurized]

Ok Earl, gotcha. Any preference on carbs? Like what years to look for? If I am going to purchase carbs I might as well get the better year range if I can. SHould I go for the 77-79 or a newer model?

 

[SqDancerLynn1]

You are limited to the 77-79 because of the head design for intake manifold

 

[earlfor]

Also, 1980 and after used 32mm CV carbs, so you couldnt fit a 32mm into a 26mm boot anyhow. 77-78-79 models used the VM 26 mm carbs.

Earl

You are limited to the 77-79 because of the head design for intake manifold

 

[hoyterb]

My 79 GS850G had the same problem and I chased my arse off trying jets etc.What I finally did was replace one ign condensor at a time.One condensor had gone "south" and was causing a weird mis-fire.Hope this helps.
Brian H

 

[Anonymous]

i feel your pain

i feel your pain

i have a 1980 gs 850 L, also straight piped with pod filters. bike starts and runs well but when i ride it the damn thing is obviously not getting enough fuel, as it hits wall aroudn 4,ooo RPM's also spits and backfires allot. I'm thinking of trying a dynojet kit and seeing what happens there. Stage 3 maybe? with pods and straight pipes i think this might be the route. anybody agree?

 

[pasteurized]

I really hope I can find an answer in the form of jets that will work. I am searching to find a set of stock carbs so I at least know I have what came as original equipment on the bike. After that I know I will need to mess with jets all over again. loghomeartist if you find anything will you please share it with the group or send me a message, I just want to get this thing running right.

 

[pasteurized]

I really hope I can find an answer in the form of jets that will work. I am searching to find a set of stock carbs so I at least know I have what came as original equipment on the bike. After that I know I will need to mess with jets all over again. loghomeartist if you find anything will you please share it with the group or send me a message, I just want to get this thing running right.

 

[duaneage]

I had a CB 550 years ago with ignition points. After setting the gap on new points according to the manual I manually timed the bike without a timing light, and here is how I did it.

I used a circuit tester, basically a 12 volt lightbulb with two wires on it, connected to the + wire of the coil and ground. I brought the #1 piston up on the compression stroke and positioned it right where the engine should fire, there was a little timing mark that lined up.

I then carefully moved the point plate until the light bulb just came on. This is where the engine would fire. Tightened the point plate, and checked it again to be sure. THis also timed the rest of the engine so I only had to do it for #1

BTW,, this is how every engine on earth was timed before someone invented the strobe light so if our grandfathers could do it this way so can we.

 

[earlfor]

Duane, for a GS your proceedure only works to time the 1,4 cylinders. timing of the 2,3 cylinders is accomplished by moving the half breaker plate independently of the full breaker plate and setting the 2,3 pointset to break at |F2 which is 180 deg from the |F1 that you used to set up the timing for the 1,2 side.


Earl

I had a CB 550 years ago with ignition points. After setting the gap on new points according to the manual I manually timed the bike without a timing light, and here is how I did it.

I used a circuit tester, basically a 12 volt lightbulb with two wires on it, connected to the + wire of the coil and ground. I brought the #1 piston up on the compression stroke and positioned it right where the engine should fire, there was a little timing mark that lined up.

I then carefully moved the point plate until the light bulb just came on. This is where the engine would fire. Tightened the point plate, and checked it again to be sure. THis also timed the rest of the engine so I only had to do it for #1

BTW,, this is how every engine on earth was timed before someone invented the strobe light so if our grandfathers could do it this way so can we.