Hunting at Steadyy Cruise

[gs7_11]

GS1000 engine, Mikuni BS34 CV carbs.

Runs well, but getting a bit of hunting at steady 4000 rpm cruise.

Assuming everything else is ok, lift the needles or bigger pilot jets?

(Standard airbox, Wiseco 185 kit, Kent cams, Eagle 4:1. Have tried pilot screw adjusment, but no difference. I have got 2 sets of carbs and get same problem with both. Online guides talk about float heights, but I reckon that is ok, have checked.)

So, being as CV carbs work more on RPM than throttle opening, are we on the needle by 4000rpm?

Edited April 18, 2018 by gs7_11

[Sheep]

You say both sets of carbs do it.....are both carbs identical in jet size,balance,condition etc.....valve clearances ok too???

[gs7_11]

18 minutes ago, Sheep said:

You say both sets of carbs do it.....are both carbs identical in jet size,balance,condition etc.....valve clearances ok too???

Yes.

Well, near as I can tell. Same part number, jet sizes etc and both sets have been ultrasonically cleaned.

Valve clearances were done a couple of years back, but not many miles since.

[wraith]

At 3000/4000rpm is where it comes off pilot jet and starts to go to the needle then the main jet.

So yes, needle for me  or may be the emulsion tubes are worn.

[gs7_11]

9 hours ago, wraith said:

At 3000/4000rpm is where it comes off pilot jet and starts to go to the needle then the main jet.

So yes, needle for me  or may be the emulsion tubes are worn.

Thanks. I'll try that. Bit of a pain as the height on these is set by thos little plastic blocks so Ill have to improvise.

[KATANAMANGLER]

14 hours ago, gs7_11 said:

So, being as CV carbs work more on RPM than throttle opening, are we on the needle by 4000rp

It's still throttle position. The butterflies control the slide lift via the diaphragms. So as long as your diaphragms are good it's still throttle position. So what is the throttle position?

The idle circuit still plays a significant part in cruise speeds but depending on the throttle position the needles will also be at play.

An air leak would also have an impact on the idle circuit.

This might help.

[CockneyRick]

I was gonna suggest Air leak or Oval emulsion tubes  

[gs7_11]

1 hour ago, KATANAMANGLER said:

It's still throttle position. The butterflies control the slide lift via the diaphragms. So as long as your diaphragms are good it's still throttle position.

Only indirectly. Opening the throttle causes a change in differential pressure that is felt by the diaphragm, I know that. However the slide will only rise as far as the pressures in the choke will allow it, requiring an increase in RPM to change the relative pressures, and complete the demanded operation.

So, yes, in a steady cruise, the slide will equate to throttle position, but unlike with a slide carb where you can state categorically where the slide is, it's not so  clear on a CV carb.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

I guess it's mainly terminology, but on CV carbs I think you're more accurate referring to RPMs.

Edited April 19, 2018 by gs7_11

[gs7_11]

35 minutes ago, CockneyRick said:

I was gonna suggest Air leak or Oval emulsion tubes  

It's not an air leak. I've replaced the o-rings on the intake stubs, and the rubbers themselves are in good nick and well clamped.

Oval emulsion tubes, well, maybe. But both sets of carbs? I've examined them closely and can't see anything. The needles aren't worn, and both sets of carbs are in good condition, imported form the US and don't look like they've had that much use.

[CockneyRick]

Just stare down the carb with top & bowl removed, if they're oval it will show as worn front to back, not side to side. Common with these carbs & brass tubes  

[gs7_11]

Well, I lifted the needles, as that's easiest. I swapped the little plastic spacer for 3 x M3 washers, raising the needle by 1mm.

It was a little better, but mainly at the top end of the original issue...so, pilot jets?

Yes. I had a set of 42.5s (stock is 40) which I fitted, and the improvement is marked. Not 100%, but I've been able to back the pilot screw down to 2 turns out so the idle is much more stable. The hunting is 80% gone, confined to about 50mph, and much less than before, anything above that is clean and steady.

I've bought a set of 45s and will try those, hopefully that will clear it up completely.

These carbs were set up very lean as stock. Emissions laws in US?

Edited April 25, 2018 by gs7_11

[nlovien]

or maybe a smaller airjet for the pilot circuit ( I think with these carbs - not sure - the idle needle is a volume control and the mixture is via the air and fuel jet )

I see CV,s more as load condition versus rpm / throttle position - steady cruse = you could get an idea if its needle versus idle - if its cruze on light load  - idle, same but now climbing a hill say = needle - i.e. watch your throttle position as you hold the cruze relative to the load needed, if you note your having to open up to hold the cruze and this is where its fluffy - needle - if you find your backing off to hold the cruze and its fluffy = idle

also - you maybe hovering around the point of ignition advance - dam't hassle if its a fixed ignition curve - maybe an ignitech 3D system with a TPS is what you need

Edited April 26, 2018 by nlovien

[wraith]

Carbs, always fun to set up  pilot jet for under 3000rpm needle 3000/5000rpm, main jet above that + float which can change the hole lot and that's just to start. 

[gs7_11]

33 minutes ago, wraith said:

Carbs, always fun to set up  pilot jet for under 3000rpm needle 3000/5000rpm, main jet above that + float which can change the hole lot and that's just to start. 

Yep. Keeps the brain ticking over!

 

[gs7_11]

Well, here's an update, might be useful for anyone who has a similar issue.

Float bowl vent pipes.

I still had this slight hesitancy with 45 pilot jets. The bike was as good as it had ever been, but not quite right, and I began thinking needle and taper.

That leads you to Dynojet, but it really irks me to pay £130 for what amounts to a needle. However I got a kit for  £80 from Germany after a bit of searching.

So, to get a baseline, went back to standard first, then fitted the slightly bigger main jet, drilled the slide and fitted the needle.

All seemed good at first, low rpm throttle response much better, and the bike definitely is more perky. Still got a slight hesitancy at about 70 though. Aaaargh!

Read the trouble shooting guide.......make sure float bowl vent pipes are removed it says. I was sceptical......they are as standard, sitting in still air at the back of the Airbox, what harm could they do? Anyway, takes them off, problem solved! 

Went for 100 mile ride to celebrate!

The Dynojet kit is money well spent, as now the jetting is bob on in the mid range, and I'm not trying to compensate for leanness there by using bigger pilots. The bottom end can be leaner, so the idle is perfect and the low end throttle response is great.

The trouble shooting guide has a great diagram showing the overlaps of each part of the circuit, and how much effect each part has, very handy.

[Blubber]

37 minutes ago, gs7_11 said:

Well, here's an update, might be useful for anyone who has a similar issue.

Float bowl vent pipes.

The trouble shooting guide has a great diagram showing the overlaps of each part of the circuit, and how much effect each part has, very handy.

Best post a scan or link to that then 

 

Great find BTW

 

[gs7_11]

1 hour ago, Blubber said:

Best post a scan or link to that then 

 

Great find BTW

 

Here you go! 

[Swiss Toni]

Good, informative read! Pleased to hear you got on top of the problem! 

[gs7_11]

20 hours ago, Swiss Toni said:

Good, informative read! Pleased to hear you got on top of the problem! 

Thanks! You probably enjoyed reading it more than I did doing it 

Anyway, I learnt summat. 

[gs7_11]

I thought I'd resurrect this to finish off.

Having made a schoolboy error - changed two things at once - I was assuming the dynojet needles were part of the fix, besides removing the float bowl vent pipes.

I was wrong. I've since done the obvious test and refitted the standard needles with undrilled slides (I've got 2 sets of carbs) and guess what? The bikes runs better than ever. I think the drilled slides, while giving more instant throttle response, were making things a bit snatchy, and with the standard slides and needles back in, the carburation is perfect and nice and smooth in the transition between off and on.

So, net result, nothing of the Dynojet kit remains!  (Apart from the main jet, but that will be going soon as it is smaller than the 115 I took out.)

It seems all the hesitancy was caused by the float bowl vent pipes doing vacuum stuff in funny air pockets.

Anyone want to buy a GS1000G Dynojet kit, half price?