When you rebuilt the carbs did you use Mikuni parts? I have read somewhere that the diaphram pin on some of the aftermarket kits is longer, which would depress the lever causing excessive fuel delivery. I think the best way to test this is check the pop off pressure of the fully assembled carb to see if the needle holds pressure or is it leaking cuz it is held open by the diaphram?
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96 gsx 800 4 stroke-ing up to 4000 rpm
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Sportster-2001-951C-Stock
Well-Known Member
I was just brainstorming ideas again...
Sonic reflections in the exhaust can have an effect on fuel mixture too, this is the reason the expansion pipe fills with water at low speed to improve resonance matching and there are specific size orifices in the cooling lines to control water flow into the pipe. Wonder if some of these have been changed?
Sonic reflections in the exhaust can have an effect on fuel mixture too, this is the reason the expansion pipe fills with water at low speed to improve resonance matching and there are specific size orifices in the cooling lines to control water flow into the pipe. Wonder if some of these have been changed?
Sportster-2001-951C-Stock
Well-Known Member
"I have read somewhere that the diaphram pin on some of the aftermarket kits is longer"
That might be a good point, and those diaphragms can't be hard and leathery either, ideally they should be nearly as soft as rose pedals.
That might be a good point, and those diaphragms can't be hard and leathery either, ideally they should be nearly as soft as rose pedals.
That's a good point. The kits came with the machine with no markings so I don't know. But now I'll compare new and old to see what the difference is.
I did the pop off test with and without them assembled. Both times it was close to 32 psi. I even tried with both carbs connected together and both seemed to pop at nearly the same time. I'm going to do the leak test to see if I haven't developed a leak in either seat.
That's possible but I wouldn't know how to check that I don't think there are specs anywhere for it. Even if the pipe was overfilled it should still run smoother than that. Even removing the pipe altogether I would think it would still run smoother than that.
I wish I had a second set of carbs to try
They are both soft and pliable, they seem brand new and so were the other ones. Actually all the seals seemed good and not in need of replacement at all.
Even when I was pulling the carbs apart I noticed that the inside was very clean.
Sportster-2001-951C-Stock
Well-Known Member
Yep, definitely measure the nub on those diaphragms to make sure they're the same. Maybe it messes up the metering if the new one nudges the metering lever too quickly. I guess you already have tested the metering valve isn't held open by the diaphragm once the cover is installed, I think you already covered that.
Yeah pop off test would have shown a big leak if that was the case.
Tomorow I'll have the carbs off again. I'll check pop off again and see what its like now.
One thing I just remembered is that the lever was bent a bit. I bent it back to level. But it was bent up to begin with. So by doing that I should have made it leaner.
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Sportster-2001-951C-Stock
Well-Known Member
That can't be ruled out without consideration, but where? The ignition timing is go or no-go, or is it?
Sportster-2001-951C-Stock
Well-Known Member
One electrical gremlin that occasionally is discussed involves disconnecting the regulator/rectifier output, seems to interfere with the ignition maybe a crosstalk thing or AC interfering with timing/ignition?
Sportster-2001-951C-Stock
Well-Known Member
0.015" is the minimum allowable clearance I believe, and I'll guess beyond 0.070" clearance will probably begin impacting low end(idle quality especially)?
Sportster-2001-951C-Stock
Well-Known Member
If the maximum distance is greater than 0.070 and appreciably wide from grinding out a nick, I'd see if I could score another valve. If the nicked area is fairly narrow and 0.072" it's probably no big deal?
0.070" is a pretty big gap, do you think you had that much?
0.070" is a pretty big gap, do you think you had that much?
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Searching for some rave valves at the moment.
I had it out today and no change.
When I bored it .5mm over would that have altered the manifold pressure enough to now require smaller pilot jets ?
Its still rich at idle up to 1/4 throttle and fouls plugs.
While running its rough untill the main jet comes in.
4000 is where it smooths out just before the main comes on and at 4500 it rakes off.
I had it out today and no change.
When I bored it .5mm over would that have altered the manifold pressure enough to now require smaller pilot jets ?
Its still rich at idle up to 1/4 throttle and fouls plugs.
While running its rough untill the main jet comes in.
4000 is where it smooths out just before the main comes on and at 4500 it rakes off.
I just re-read the carb tuning and noticed this part:
My pop off is set at 27 psi which is the low end of the spectrum in the manual. Its what I ended up having with the new mikuni kit. I had to re-use my spring to get the pop off that high. It was at 24 psi with one of the springs in the kit.
However there is a 115 gram spring in the kit as well and that makes pop off go up to mid 40's.
I wonder if that alone would lean it out and not require a new pilot jet?
Going with intuition there is nothing mechanical or electrical that would cause this. Just plain rich mixture...
My pop off is set at 27 psi which is the low end of the spectrum in the manual. Its what I ended up having with the new mikuni kit. I had to re-use my spring to get the pop off that high. It was at 24 psi with one of the springs in the kit.
However there is a 115 gram spring in the kit as well and that makes pop off go up to mid 40's.
I wonder if that alone would lean it out and not require a new pilot jet?
Going with intuition there is nothing mechanical or electrical that would cause this. Just plain rich mixture...
Sportster-2001-951C-Stock
Well-Known Member
"Searching for some rave valves at the moment. "
If this is the solution, now I've learned a really cool way to fatten one up out of a lean bog without having to tear into the carbs (valuable lesson if true!).
".5mm over would that have altered the manifold pressure enough to now require smaller pilot jets ?"
Never in my life have I ever seen this happen.
"Its still rich at idle up to 1/4 throttle and fouls plugs."
Seems to me something's wrong with those carbs, you're sure someone didn't modify them? Still pig rich? Is there a bird's next stuck inside the flame arrestors (no need to answer!)? The pop-off pressure spec, the units are psi and not kgf/km (I know you know this).
"4000 is where it smooths out just before the main comes on and at 4500 it [CLEARS OUT AND] takes off. "
I think you're at a disadvantage differentiating mains from raves with reasonable certainty until you isolate one of the two, I believe it's impossible to know for sure by seat of the pants feel which one is coming in at 4,000 and either could be a wrong assumption (A bystander observation only, you may have devised a way of isolating raves effect around 4,000 and bystander is unaware).
Other miscellaneous ramblings:
I cannot imagine an exhaust stinger too full of water could cause a pig-rich mixture.
You've confirmed rotary valve timing satisfactorily (pending further notice).
Plugs are fuel fouled at low speed (pig rich low speed mixture).
Your carbs have been modified somehow (why is first "bypass hole" uncovered at throttle plate idle position? I don't like this).
Unproven: Throttle plate idle position is open beyond normally anticipated position because ?_________? (fuel is voluntarily pouring out of bypass holes, thus more air is needed?)
What is the fuel pressure, less than 3 psi?
I tried removing the springs and the raves opened up all the way. Still acted the same way but was "hitting" a little harder. It probably made it slightly richer but didn't show anything else.
On another engine build I did, I increased the bore's 12mm in size. That required a jetting change. Stock was rich which makes sense since thats a huge increase in manifold pressure.
I thought .5 would have had an effect. Good to know its not enough to matter in this case
I honestly don't know what's the deal with these carbs. I'm guessing the PO only knew how to hold it WOT.
Thinking about it. There was 2.0 needles with 16 psi pop off, low speed at 3 turns out, High speed at 1 1/2 out.
So that must have been insanely rich
One theory I have is that having the high speed open so much was allowing pressure to equalize before and after the butterfly.
Or someone used a wired to "clean' out the ports in the carb and enlarged them !
Also now Its better up to 1/4 turn of the low speed. After that its rich enough to flood it out and throttle response suffers.
I do know that 4500 is when the main jet comes on fully. I know that because I watched it happen on the trailer (in the water) with a mirror. Before 4500 there was just a small trickle coming out of the mains, after that I could see the stream coming out.
To distinguish when the raves come in I could set the screws in more and see if the power starts later on in the rpm. I wouldn't mind it coming on sooner than that or later than that. Its set at an awkward spot where its either on the power or off of it. OR its just my issue causing this
Running so rich it needs more air and gets it only when the throttle is opened that much.
Then again I am setting the idle to 3000 rpm or 1500 in the water. Wouldn't that require a significant throttle opening ?
One thing I do notice now is that the first start of the day causes the RPM to jump up and then settle down as soon as the chambers fill.
My theory is: excessive fuel is choking the engine with such a small throttle opening. Adding more air helps to offset it and when the throttle is opened enough it allows enough air to enable combustion.
On another engine build I did, I increased the bore's 12mm in size. That required a jetting change. Stock was rich which makes sense since thats a huge increase in manifold pressure.
I thought .5 would have had an effect. Good to know its not enough to matter in this case
I honestly don't know what's the deal with these carbs. I'm guessing the PO only knew how to hold it WOT.
Thinking about it. There was 2.0 needles with 16 psi pop off, low speed at 3 turns out, High speed at 1 1/2 out.
So that must have been insanely rich
One theory I have is that having the high speed open so much was allowing pressure to equalize before and after the butterfly.
Or someone used a wired to "clean' out the ports in the carb and enlarged them !
Also now Its better up to 1/4 turn of the low speed. After that its rich enough to flood it out and throttle response suffers.
I do know that 4500 is when the main jet comes on fully. I know that because I watched it happen on the trailer (in the water) with a mirror. Before 4500 there was just a small trickle coming out of the mains, after that I could see the stream coming out.
To distinguish when the raves come in I could set the screws in more and see if the power starts later on in the rpm. I wouldn't mind it coming on sooner than that or later than that. Its set at an awkward spot where its either on the power or off of it. OR its just my issue causing this
Running so rich it needs more air and gets it only when the throttle is opened that much.
Then again I am setting the idle to 3000 rpm or 1500 in the water. Wouldn't that require a significant throttle opening ?
One thing I do notice now is that the first start of the day causes the RPM to jump up and then settle down as soon as the chambers fill.
My theory is: excessive fuel is choking the engine with such a small throttle opening. Adding more air helps to offset it and when the throttle is opened enough it allows enough air to enable combustion.
Sportster-2001-951C-Stock
Well-Known Member
I tried removing the springs and the raves opened up all the way. Still acted the same way but was "hitting" a little harder. It probably made it slightly richer but didn't show anything else.
On another engine build I did, I increased the bore's 12mm in size. That required a jetting change. Stock was rich which makes sense since thats a huge increase in manifold pressure.
I thought .5 would have had an effect. Good to know its not enough to matter in this case
I honestly don't know what's the deal with these carbs. I'm guessing the PO only knew how to hold it WOT.
Thinking about it. There was 2.0 needles with 16 psi pop off, low speed at 3 turns out, High speed at 1 1/2 out.
So that must have been insanely rich
One theory I have is that having the high speed open so much was allowing pressure to equalize before and after the butterfly.
Or someone used a wired to "clean' out the ports in the carb and enlarged them !
Also now Its better up to 1/4 turn of the low speed. After that its rich enough to flood it out and throttle response suffers.
I do know that 4500 is when the main jet comes on fully. I know that because I watched it happen on the trailer (in the water) with a mirror. Before 4500 there was just a small trickle coming out of the mains, after that I could see the stream coming out.
To distinguish when the raves come in I could set the screws in more and see if the power starts later on in the rpm. I wouldn't mind it coming on sooner than that or later than that. Its set at an awkward spot where its either on the power or off of it. OR its just my issue causing this
Running so rich it needs more air and gets it only when the throttle is opened that much.
Then again I am setting the idle to 3000 rpm or 1500 in the water. Wouldn't that require a significant throttle opening ?
One thing I do notice now is that the first start of the day causes the RPM to jump up and then settle down as soon as the chambers fill.
My theory is: excessive fuel is choking the engine with such a small throttle opening. Adding more air helps to offset it and when the throttle is opened enough it allows enough air to enable combustion.
"Running so rich it needs more air and gets it only when the throttle is opened that much.
Then again I am setting the idle to 3000 rpm or 1500 in the water. Wouldn't that require a significant throttle opening ?"
On mine, (which is not the same model and is a reed-valve motor) the "bypass holes" are not uncovered at idle position idling at 1500 in water. I know this because when I last took them off a few weeks ago I noted I could not see the "bypass holes" looking into the throttle bore, just the one idle port, and I also noticed there was a smallish air gap between the throttle butterfly and bore. I had to hold the carb up to the light at a narrow angle to see daylight (I should have measured the gap! I'll guess and say maybe 0.020) because there's a step in the air inlet of the carb throat where the choke plate is.
So, if/when you get your excessive fuel issue resolved, I bet you have to adjust the butterfly else your idle will be way more than 3,000
12mm overbore, eh? - That's a big(no, huge!) change. Once contemplated similar on an old flathead. Machine shop told me they could take it .250" (6mm) but I declined and had them take it the minimum amount (0.060) necessary, wasn't building a stroker although the parts were there on the shelf, Lincoln crank/rods and the whole nine yards, for the right price. Still runs 30yrs later, original '49 holly 2bbl carb + jets. 1/4" overbore maybe way down the road if I get a hankering for a t-bucket but I'd go V6 just 'cause nobody I know goes with V6's in t-buckets and I like fuel economy. Maybe 45 MPG?
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Yeah it was a 1981 Suzuki Gs400L that I installed larger sleeves in Bored it to almost 79mm and lots of head and cam work.
I just got it all back together this month after two years in the making. Its next to fine tune after the seadoo is done.
[video=youtube;TiSbHXzl9FA]http://www.youtube.com/watch?v=TiSbHXzl9FA[/video]
http://www.thegsresources.com/_forum/showthread.php?t=172422
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