Older GT750 Carbs on Newer GT750

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Vintageman
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Re: Older GT750 Carbs on Newer GT750

Post by Vintageman »

II am thinking the 110 versus 120 mean the edge of the flap has a taper or cut
I extrapolated that thought from here. (“”don’t mess with edge of butterfly””)
https://www.youtube.com/watch?v=141iSNALBv0" onclick="window.open(this.href);return false;

This guy is knowledgeable
https://www.youtube.com/watch?v=ChNRzfxRidk" onclick="window.open(this.href);return false;

His web http://www.oldschoolcarbs.com/" onclick="window.open(this.href);return false; (I sent him and email)

Based on this I don't think the slide lifts much as I hold the butterfly just "off idle" open for the restriction is right under butterfly mostly and here is the low pressure signal pulling

I do think the "120" versus “110” is the best way to go (What's the luck of finding that) Anyone have any they want to sell

I think smaller pilot too but eed to counter idle position bucking need to open pilot fuel screw


Does anyone else like to ride there gt750 accelerating easy just off idle and let the girl climb up a ways into RPM before shifting. And if so, can yours do so smoothly!
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tz375
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Re: Older GT750 Carbs on Newer GT750

Post by tz375 »

You are correct, that at low revs and low throttle openings, the slide is not lifted much at all. At low throttle and high revs - for example slowing down, gas velocity is high and the slide will lift quite a long way. More importantly the pressure differential across the pilot jet is high and that causes over fueling under those circumstances. That is why the fix is a change in air jet and not a change in fuel jet.

Changing the air jet changes the air:fuel ratio at high load /velocity and make almost no different at low loads.

It's hard for people to get their brains around, but the perfect idle jet size at idle is not the perfect size on closed throttle and higher load because the rate of fuel flow increases is not a straight line relationship with load. Air jets make all the difference as revs change at the same throttle opening.
Vintageman
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Posts: 1483
Joined: Tue May 18, 2010 5:38 pm
Country: USA
Suzuki 2-Strokes: Suz, Yam, Honda, Kaw.
Location: New Hampshire

Re: Older GT750 Carbs on Newer GT750

Post by Vintageman »

Tz I read your reply. I think we talked on this before. Not 100% sure why maybe air is compressible and fuel is not. I'll have to dig a lttle on this.
But, I don't think pilot air jet is it (been here done this as well), for Idle too lean and just Off Idle too rich all using sma epilot air jet

Let me see if I can explain what I am thinking is the theory behind my issue with these two positions only

See page 28 diagram - Main Meter System for “xxxx” quotes to follow ->
http://www.amckayltd.com/carbguide.pdf" onclick="window.open(this.href);return false;

Using all stock setting if the Throttle Plate is fully closed (AKA Idle) and the engine is coming down from cruising rpm bike will buck a little. Opening the pilot “Mix Screw” more than factory suggestion helps this. When Idle I am mainly running off the “Mix Screw Outlet”. That is the pilot “Bypass hole” is behind the plate. Some even say this “Bypass hole” at idle actually adds more air to the “Mix Screw Outlet”.

The issue I don’t (why this post!) like is when I open the throttle plate just enough to be Off Idle where I expose pilot “By Pass Hole” At this point I am now too rich: to summarize at Idle too lean and Off Idle too rich. As the throttle plate opens more you start using the slide and these circuits and all is well.

Trying to get a wide range when running Off Idle may be impossible or is it :?: ? For example if I am cruising at +/-5k RPM and turn throttle all the way down to Idle it can buck, but if I go down to only to just Off Idle I’ll slow down and won’t buck. That further justifies that Off Idle is definitely not too lean.

We expect/require our engine to go from a higher rpm down to low RPM at Idle or just Off Idle. Oppositely, I expect the engine to go from low RPM to upper cruise RPM holding steady from Idle or Just Off Idle 8)

To 100% prove rich I suppose I could hold just Off idle, open throttle and snap back to Off Idle to see if rpm drop before the recover back up (too rich). Regardless I believe too rich for other than a stumble it climbs real hard.

I still not 100% the number on the Mikuni BS Throttle plates (e.g. 110, 120, 135 etc.) are analogous to the VM round slide cut number (e.g. 2.0, 2.5, 3.0 etc.). But, I do think they are where the smaller the number the richer. I think this also come into play only at Off Idle: How esle could that two dimensional plate affect anywhere else?

I still think if I had 120 versus 110 that would help my rich Off Idle with ever thing else the same.
In lieu of this, I think I‘ll try a step smaller pilot and open the “Mix Screw” to solve bucking. I may have to richen needle a clip if I lean pilot jet

Here is one pretty decent explanation on these two circuits (Idle to Off Idle)

http://www.factorypro.com/tech/tech_tun ... ruise.html" onclick="window.open(this.href);return false;

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