Carb Tuning Question

[Guest]

Hey guys,

I havent posted much on here but this site has been super helpful! I've downloaded most of the carb tuning and rebuild how to stuff on here.

Anyway, I have a 1978 GS750 that I've been working on all winter. I finally got her running pretty good overall but am having some final tuning issues mainly at idle. I'm hoping someone might be able to help me dial in the idle on here. I finally got it to idle strong at about 1200=1400rpm but it is coating me with fumes.

For some reason it seems to to be running rich at idle or somehow fumes backing up through the airfilters. I cant seem to pin point where the fumes are eminating from but it definitely smells of unburnt exhaust. There are visible fumes/smoke but I can't pin point where its actually coming from.

I bought this bike as a project (towing company repoed it.) and have definitely found that it has been tinkered with so I have no starting point.

Here is what I have going on with the bike.

-Boulder, CO Altitude 5300'+ (I don't drive a hybrid or do yoga.)
-All O-Rings replaced including intake boots as recommended.
-Real K&N Oval Pod Filters. (Tried the cheapo ones and they starved the bike at half-full throttle).
-Vance&Hines 4-1 Exhaust (Brand new)
-Main Jet 125 (These definitely are not stock but thats what was there when I bought it)
-Needle Jet raised to richest setting. (Seemed to fix my mid throttle response. Was at middle setting before)
-Unsure of pilot jet size.
-Air screw - 2 Turns
-Fuel Screw - 1.75 turns

It's idling well but I the bike is coating me in fumes and you can see some sort of smoke coming off of the bike.

Is it possibly improper pilot jet sizing or something else?

Thanks for your help/guidance.

[Steve]

Turn the fuel screws (the ones on the bottom of the carb) in about half a turn to 1 1/4 turns out from lightly-seated.

Turn the air screws (the ones on the side of the carb) out about half a turn to 2 1/2 turns out from lightly-seated.

That should give you a decent starting point, adjust the air screws for best idle when the bike is warm.

Stock main jets are 100, so your 115s should be right in the ballpark.

.

[vroomvroom]

Thanks, Steve! I will hopefully give that a shot this week. Themain jets are actually 125 not 115. The bike pulls really strong on the top end though! I havent had a chance to visually check the plugs to see if full throttle is running too rich.

Turn the fuel screws (the ones on the bottom of the carb) in about half a turn to 1 1/4 turns out from lightly-seated.

Turn the air screws (the ones on the side of the carb) out about half a turn to 2 1/2 turns out from lightly-seated.

That should give you a decent starting point, adjust the air screws for best idle when the bike is warm.

Stock main jets are 100, so your 115s should be right in the ballpark.

.

[Steve]

Not sure where I saw "115".

General 'rule of thumb' is to go up 3-5 sizes for a pipe and 4-6 sizes for pods, leading you to believe that 7-11 sizes would be good for your setup. However, since you are up a bit in altitude, I think you knock off a size or two.

Going from 100 to 115 is 6 sizes, so I thought that might be good. You say you have 125s, that would be TEN sizes, which still fits in with the 'rule of thumb', but might be a tad rich for your altitude.

Your spark plugs will be much more accurate than me, though, so go by what THEY tell you.

.

[chuck hahn]

Actually the bottom pilot screw NEVER ( regardless of pipe OR pods ) Needs to be open more than 1 turn from lightly seated.

Secondly...the stock mains for that bike are 100. See the chart below. The stock pilot should be a 15. If after adjusting as I suggest it is still a tad sluggish at the lower RPMS ( below the 3,000ish range ) then go to 17.5 pilots.

Third.... If the needle clip is on the very bottom groove your way too fat..its stumbling at lower RPMS because it cant burn that much fuel as quickly. Bet your plugs are black and sooty as all helll.

Forth...Once the bottom pilot screws are set you adjust the mixture with the SIDE mixture screws and leave the bottom ones alone. Turning them IN reduces the amount of air in the ratio and RICHENS the cylinder....out obviously adds more air and leans the cylinder. Set the side MIXTURE screws at 2 out as the baseline.
Youre 125 mains are probably actually real close.

NOTE: If it runs good but you have an occasional poppping from the exhaust you eliminate that by using the mixture screws. 9 out of 10 times its because a cylinder is a tad lean. So, seeing you have a 4 into 1 pipe, start with carb 1 and turn the mixture screw in 1/8 turn and see if the popping goes away. If not, then turn it back out the 1/8 turn and do carb 2 etc etc. What your trying to do is richen each cylinder to find out which one it is thats lean. This is why i absolutely LOVE my Colortune. Put it in and observe the flame color in the cylinder and adjust itbaccordingly. adjust

http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html#A01

[Steve]

This is why i absolutely LOVE my Colortune. Put it in and observe the flame color in the cylinder and adjust itbaccordingly. adjust

If I had VM carbs, I would have one, too.

Didn't work so well on the BS carbs i tried it on.

.

[chuck hahn]

Yeah its not a CV carb tool for sure...LOL

[vroomvroom]

Actually the bottom pilot screw NEVER ( regardless of pipe OR pods ) Needs to be open more than 1 turn from lightly seated.

Secondly...the stock mains for that bike are 100. See the chart below. The stock pilot should be a 15. If after adjusting as I suggest it is still a tad sluggish at the lower RPMS ( below the 3,000ish range ) then go to 17.5 pilots.

Third.... If the needle clip is on the very bottom groove your way too fat..its stumbling at lower RPMS because it cant burn that much fuel as quickly. Bet your plugs are black and sooty as all helll.

Forth...Once the bottom pilot screws are set you adjust the mixture with the SIDE mixture screws and leave the bottom ones alone. Turning them IN reduces the amount of air in the ratio and RICHENS the cylinder....out obviously adds more air and leans the cylinder. Set the side MIXTURE screws at 2 out as the baseline.
Youre 125 mains are probably actually real close.

NOTE: If it runs good but you have an occasional poppping from the exhaust you eliminate that by using the mixture screws. 9 out of 10 times its because a cylinder is a tad lean. So, seeing you have a 4 into 1 pipe, start with carb 1 and turn the mixture screw in 1/8 turn and see if the popping goes away. If not, then turn it back out the 1/8 turn and do carb 2 etc etc. What your trying to do is richen each cylinder to find out which one it is thats lean. This is why i absolutely LOVE my Colortune. Put it in and observe the flame color in the cylinder and adjust itbaccordingly. adjust

http://members.dslextreme.com/users/...eight.html#A01

Very much appreciated! When I finally get some time, I will make these changes. Thanks for the notes about popping on decel also.

[duaneage]

I wouldn't assume you need to go big on the mains just because it has "a pipe"
If the baffle is in and it's a regular Vh street pipe like the 12500 model I would go just 1 or two sizes up. I have the VH and found stock jets work fine, as VH recommended. Going big just threw a lot of black smoke out there. I went up 6 and then slowly ended up back at stock.

[KEITH KRAUSE]

Smell of raw fuel at an idle of 1200-1400 rpm's? Poor combustion. Plugs fouling.
Try adjusting the pilot fuel screws to 1 turn out. That may help some but your problem is more than that.
Have you verified the ignition timing and spark quality? Do you still run points? Do all plugs show a good bluish spark? If so, did you check the float levels in each carb? Are the float needle valves sealing/any sign of overflow from the bowl overflow lines? Are you sure the pilot jets are #15 or 17.5? Did you at least do a good bench synch of the throttle valves? Could you have over-oiled the pod filters?