GT750 with CV carb Needle Jet available

[tz375]

I had a set of keyster needles for a 750 and they were really close, but not perfect. They could be used in pinch.

MPG is not important to me, but running right is. I figure I get less than 10 MPG 1/4 mile at a time, but once the throttle is open it stays that way.

With aftermarket pipes we were seeing very lean mixtures at part throttle. A larger air jet will lean out closed throttle running at speeds above idle. Try 1.0 and see if it's better or worse. The air jet makes little or no difference at idle - but makes a difference when the throttle is more or less closed at higher engine speeds where air velocity through the carb is higher.

If larger air jets improve those conditions, a larger needle jet make make things worse, but the air jet mod is really about closed or almost closed throttle. If your bike runs fat at 1/4 to 1/2 throttle or at WOT at low revs, then drop the needle or fit smaller (ie stock new) needle jets. But get the top end high rpm WOT fueling right first.

[Vintageman]

Tz375

that's all good advice. I run Jemco's 3 into 3 other that I am stock. Actually not true I increased the size of the main jets by at least one step (=2.5 per step) if not two: 115 mains in it now I think? Finish restore last season. Maybe of few hundred miles now.

WOT seams like it is tuned well or at least not lean. Mid throttle 1/2 and up seams OK.

The bike does not need much throttle position to cruise 50-60 MPH. I read again last night a period article that the bike should be +/- 40 street MPG (I think that was USA gal i'll recheck), The last ride when I checked mpg I was mostly 1/8-1/4 throttle, so why I question the quality of the needle jet and did I really need to add the pilot air jet and so small?

Well as I said it still bucks when I let off throttle fully. What you said about air jet roll makes sense. I believe the Pilot fuel screw is what it should be (1/4 turn of a 1975 if I recall), I may open that a little more for bucking is only when throttle is closed and seams a lean condition. But yes soon as I open the throttle say up to 1/4 it may be too rich. just a guess at the moment based on MPG.

I'll come back and if I have facts (bad needle jet etc.) I'll reply to this post.


Thanks

[rngdng]

The bucking is caused by the air jet. There is an service bulletin that specifies which air jets to install. You have to drill and tap the carb outboard of the existing air jets and thread it for new, smaller air jets. I think I have a set of them (for 74 model) I never used.

Bulletins are here:
http://www.oldjapanesebikes.com/mraxl_G ... /index.php" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

It's bulletin GT-36, use arrows to open the rest of the pages. When I get home, I'll see if I can find those jets.


Lane

[Vintageman]

rngdng,

Yes that is the one I read back when. And I read it again thank you

I don't have "bucking part throttle cruising" but I do when "deceleration" throttle fully closed even with richest change. It shows 0.8 and 0.9 for jet. I used the smaller one 0.8 (not 0.7 as I said earlier in this post.

It also states that one may need to raise the jet needle since it “richens idle mixture significantly".. ."to eliminate off idle hesitation". It sure makes the original air passage many times smaller than what it was...I don't remember what I did here but, suspect I was worried about being too lean with chambers and left needle clip as is. I don't have any of idle problems, the bike runs great but, that doesn't means it's not too rich and why poor mpg

I suspect air jet mod is the main reason my gas mileage is low

What is dumb if I change more than one thing at once as i try to fix MPG and solve bucking(what i hav efor bucking is not too bad)...I am sure I'll be dumb .

First I'll see how my Needle jet is ... if pitted i'll change and leave clip and air jet were it was
Or, I should try raising needle a clip and leave the 0.7 per bulletin.
Or, I should leave all as is and just pull out air jet to see how bad bucking really gets without mod?
Or then why wouldn't turning the pilot fuel screw out more help not having enough fuel at idle then stop bucking (them dang pilot air of fuel screws never do jack really my experience)

I think new needle jets, raise clip one, and run 0.8 air (I bought both sets at the time)
Told I would change more than one thing at once

[tz375]

If you look at the Mikuni manual you can see how changing the air jet changes airflow at low throttle settings. It has little effect once the throttle is opened and has the greatest effect at closed throttle high vacuum. The higher the engine speed, the greater is the effect of the air jet at closed or almost closed throttle. It won't have much effect at idle or once the throttle is open.

[Vintageman]

Is there a Mikuni manual for that CV carb. I think I saw one but forget where it was please

[tz375]

There is a Suzuki manual for carbs on the triples dated 1974 IIRC. I cannot remember where I saw it but I am pretty sure it came from the GT750 field guide or at least from Ian's site with all the old MrAxl files

[GT750Battleship]

hi,Google this for carb info "GT750- Carbs Pink Possum"
Cheers,
Roger

[Suzukidave]

http://www.oldjapanesebikes.com/index.html" onclick="window.open(this.href);return false;

[tz375]

Or to go straight to the carb manual: http://www.oldjapanesebikes.com/mraxl_G ... /index.php" onclick="window.open(this.href);return false;

[Vintageman]

Yes, thanks, read it again briefly.

What is interesting it says pilot air jet is already 1.0. Don't recall seeing it when i did mode to add pilot air jet... seeing it meaning a brass jet.. Maybe 1.0 size is fixed in by casting or I just don’t see the brass jet?

Point being it was a 1.0 and the 0.8 I added is not as great of a change as I thought. And, I still have some bucking ( Am I the only one that has bucking?).

Also the pilot fuel screw only adjusts one of the two pilot passages. The hole nearest the flap is just pilot jet + air jet and ahead screw so screw does nothing for that hole.

Once I get carbs off I'll first see how the needle jet is. 75% of my old toys needed Needle Jet changed some not as bad as others. maybe Oring. The pilot air jet I can change just by removing air box later. The clip position I should be able to do without taking carbs off engine?

Maybe if it buck close throttle still try turning out pilot screw some more.

It is best to try one thing at a time. It would be nice to squeeze 35 mpg out of it, bucking not that bad, but if it can go away. Funny on my Yams I just up pilot jet size to fix bucking. I thinking bucking is the finite chain slack plus clutch dampeners so it too lean that cause those springy things to oscillate. The more springy things are the more sensitive it is. The faster you are revving the more fuel it needs when throttle closed the carbs on my CB900F for example have a vacuum based enriching circuit for that reason. My gt750 only bucks, throttle closed ,when revs are higher for as the revs comes down so does the bucking.

[tz375]

The air jets are 0.5mm for the main air jet and 1.0mm for the pilot air jet. 0.8mm is a 36% reduction in area and air flow.

At idle the smaller air jet makes little or no difference because the vacuum is low. At higher revs and closed throttle, the smaller air jet causes the carb to "suck" in more fuel. The higher the revs, the higher the effect of the air jet change - as you said.

Your could try a 0.65 or o.60mm air jet and see if that is any better. They are stock Mikuni air jets and are easy to find on line.

[Vintageman]

At idle the smaller air jet makes little or no difference because the vacuum is low. At higher revs and closed throttle, the smaller air jet causes the carb to "suck" in more fuel. The higher the revs, the higher the effect of the air jet change

TZ,

You said this I think more than once in this post, and just sinking in. Thanks for your patients.

I was assuming that ration of pilot air contributing air would be the same as the pilot jet pulling fuel regardless high vacuum. I don't understand, yet but, that does make reason as to why Suz engineers choose to change pilot air jet size not size of pilot jet.

So with that said it also makes sense when I am cruising middle RPMs at small throttle this mod is adding more fuel in conjunction with the jet needle to needle jet fuel supply. And, that is why the Suz bulletin says you may want to raise the jet needle.

So I were to too try two things at once: fix bucking and improve MPG I should

Use even smaller air jet and raise jet needle.

Fun stuff!

[tz375]

You got it. We tend to think only in terms of the fuel jets were obviously larger means more fuel and we expect that relationship to be roughly proportional to flow rate size.

We forget about air jets partly because we don't understand them and partly because we think that the fuel jet is the main event and anything else is not important.

The way to think about is is that say at 1000 rpm closed throttle, the engine uses a certain amount of fuel. When the revs rise to say 2,000 rpm still at closed throttle - say slowing down, the engine is creating a stronger pressure differential (aka vacuum) so it pulls in more fuel. So far so good, but with an air jet in the system, at 2,000 rpm, the engine will pull in more air (proportionately) that fuel. Air is lighter than fuel and as such it reacts faster to changes in signal. That extra vacuum might pull in say 3 times as much air and twice as much fuel (numbers are illustrative only) and the air: fuel ratio becomes leaner.

Go with a smaller air jet and it changes that relationship.

Going back to basics, we learned in Fuel 101 that fuel flow is a function of throttle position and not of RPM, well it turns out that is only partly true. At a given (fixed) throttle position, fuel: air ratios change also with RPMs and at steady throttle position (closed in this case) that's what we are seeing here.

It's shown in Figure 27

[Vintageman]

Tz375,

Which manual did you get that page from?