VM29 S/Bore Jetting?

[srsupertrap]

Last weekend I replaced the VM29 1.5 Sides with VM 29 2.0 Slides in my Smoothbores. Last night I tried to squeeze in synchronization session and found the GS1000 only runs with a combination of half Choke & throttle and won't idle even when warm. The half choking fouled one of the plugs and that cylinder was cold so I replaced the plug and still no better. The only changes made to the previously synched, start right up & idle VM 29 S/B was I replaced the 1.5 Slides with 2.0 replated Slides. The VM 29 Slides are machined to precise tolerances and shouldn't upset the carb synchronization or so I thought. :-s I was able to get the Twin Balance Carb Syncher on Cyl 1 & 4 and they were close.

I will be pulling the Carbs of tonight; what's your opinion on the next step. 1) Bench synch with a Gage Pins slid under the throttle valves or 2) Since the 2.0 Slide Cutaway allows more air than the 1.5 Slide they replaced is the Pilot jet now to small to let my GS1000 idle? Sudco recommends a Pilot Jet of 25 but that's at sea level (See below) The intake boots & O-rings are okay. I believe I have check Step 1 & bump the pilot because it flat refused to run without a liberal dose of choke.


Mikuni VM29 S/B Configuration:
Pilot: 15
Needle: 5DL31-2
Main: 127.5
Air Screw: 1 turn out

79 GS1000: Wiseco 1085 10.25cr Compression sound
Andrews S-1
K&N Pods
Supertrapp 4 into 1

Altitude: Denver, CO. 5150 Feet

 

[49er]

Last weekend I replaced the VM29 1.5 Sides with VM 29 2.0 Slides in my Smoothbores. Last night I tried to squeeze in synchronization session and found the GS1000 only runs with a combination of half Choke & throttle and won't idle even when warm. The half choking fouled one of the plugs and that cylinder was cold so I replaced the plug and still no better. The only changes made to the previously synched, start right up & idle VM 29 S/B was I replaced the 1.5 Slides with 2.0 replated Slides. The VM 29 Slides are machined to precise tolerances and shouldn't upset the carb synchronization or so I thought. :-s I was able to get the Twin Balance Carb Syncher on Cyl 1 & 4 and they were close.

I will be pulling the Carbs of tonight; what's your opinion on the next step. 1) Bench synch with a Gage Pins slid under the throttle valves or 2) Since the 2.0 Slide Cutaway allows more air than the 1.5 Slide they replaced is the Pilot jet now to small to let my GS1000 idle? Sudco recommends a Pilot Jet of 25 but that's at sea level (See below) The intake boots & O-rings are okay. I believe I have check Step 1 & bump the pilot because it flat refused to run without a liberal dose of choke.


Mikuni VM29 S/B Configuration:
Pilot: 15
Needle: 5DL31-2
Main: 127.5
Air Screw: 1 turn out

79 GS1000: Wiseco 1085 10.25cr Compression sound
Andrews S-1
K&N Pods
Supertrapp 4 into 1

Altitude: Denver, CO. 5150 Feet

Why did you change the slides? How did the engine run with the stock 1.5mm cut aways?
Z1 have some tuning specs for the VM29's as fitted to several Kawasaki models. As a guide, the KZ1000's were running the following:

Main Jet 120
Air Jet 0.9
Needle Jet 0-6
Jet Needle 5DL31-3
Throttle Valve 1.5mm (cutaway)
Pilot Jet 17.5 up to 25
Air Screws 1+1/2 turns out

Because you are running pods, cams, and at altitude, you will need to play around quite a bit before getting it right.
What's your mid range response like. I seem to recall that the GS1000's running VM26's were running 5DL36 needles. I think the tappers are steeper on the 36's.

 

[srsupertrap]

Re: 49er

Re: 49er

Why did you change the slides? How did the engine run with the stock 1.5mm cut aways?

At sea level the 1.5 Slides worked great but when I moved to Colorado no amount of air screw adjustments would eliminate a part throttle stumble around 3000 rpm. Last Fall I moved the Pilot Jet from a size 20 down to a 15 (Sudco recommends Pilot: 25). The spark plugs indicated a low speed rich glazed condition which was seen on the bottom threaded ring portion of the plug, not on the electrode. When running above 4000 rpm the VM29's carbuerated cleanly, pulls hards and my plug checks verified both the mid range & WOT were dialed in. I changed spark plugs from NGK B8ES to the hotter from B7ES's and still the symptom persisted. . .

Over the winter I disassembled & Berryman dipped each VM29 S/B's, examined all the components paying particular attention to the Needle Valve seats which are good, adjusted the Brass Floats to .95 using a D/Caliper. I replaced all the O-rings. Verified the bypass port ahead of the Needle jet was open. While @ Sudco I saw they recommended a 2.0 Slide which got me thinking this might explain my symptom. I purchased a set of bruised 2.0 Slides from Z-1, replated them. Last night I bench synchronized the carbs and found out why one of the plugs was fouling; one of the Accel 8.8 wire wasn't fully seated . That's the current status. After doing some additional reading on the Forum, I believe my Dyna Coils would also benefit from the Coil Relay modification.

The VM 29 S/B as you know were built in different series; there were initially the A-2 & A-7 (GS750 & GS1000) series both of which specified 1.5 Slides. It was kinda of an eye opener to see the A-12 series on the Sudco website; I have no idea when Mikuni released the A-12 revision for the GS750 & GS1000 2V but they specify 2.0 Slides instead of 1.5's. Since the 2.0 allow more air flow in the pilot circuit I am hoping that addresses my symptom. Further testing will tell . . .

 

[coltrain]

go to 17.5 pilots

go to 17.5 pilots

Definitely need to richen the pilot jet. If not go to a 20 pilot jet with your setup.

 

[49er]

The VM 29 S/B as you know were built in different series; there were initially the A-2 & A-7 (GS750 & GS1000) series both of which specified 1.5 Slides. It was kinda of an eye opener to see the A-12 series on the Sudco website; I have no idea when Mikuni released the A-12 revision for the GS750 & GS1000 2V but they specify 2.0 Slides instead of 1.5's. Since the 2.0 allow more air flow in the pilot circuit I am hoping that addresses my symptom. Further testing will tell . . .[/quote]

You shouldn't be running those B7ES's with your 10.25-1 CR! You're courting detonation!

To me it sounds as though you are lean at the transistion point between the pilots and the needles. I think that your needles are lean at the change over, but are correcting as they are drawn further away from the main jet. I believe that the reason that your plug reads are correct through the mid and upper rev range is that your main jet is too rich. It will be causing compenstion for your needle's shallow tapers throughout their entire metering range. If you can get hold of some 5DL36's and bring the main jet back to 120 or 122.5, you may be surprised at the improvement.

 

[srsupertrap]

Re: 49er

Re: 49er

You shouldn't be running those B7ES's with your 10.25-1 CR! You're courting detonation!

To me it sounds as though you are lean at the transistion point between the pilots and the needles. I think that your needles are lean at the change over, but are correcting as they are drawn further away from the main jet. I believe that the reason that your plug reads are correct through the mid and upper rev range is that your main jet is too rich. It will be causing compenstion for your needle's shallow tapers throughout their entire metering range. If you can get hold of some 5DL36's and bring the main jet back to 120 or 122.5, you may be surprised at the improvement.

49er, I agree with most of your analysis. My GS carb transition between pilot and needle circuit is not clean. I am not sure whether using the 5DL36 Needle cures anything since that Needle works in conjunction with the VM26 0-2 Needle Jet. The VM29 uses a larger 0-6 Needle Jet. I had a good tuning session on Sunday and made progress in the right direction although my neighbors might not have shared my enthusiasm. I bench synchronized the carbs and kept the same set of 15 Pilot Jets installed and the GS would still only run on half choke even when I almost seated all the air screws. I then installed 17.5 Pilot Jets with the air screws 1 full turn out; the GS still needed liberal amounts of choke and would not idle. I turned the air screws to 1/2 turn out which richened things up but still no idle, but better. Next, I installed set of 20.0 Pilot Jets with the air screws 1 full turn out and changed the Air Jet located on the Pod side from 0.9 to a 1.0. The GS started better and it now would run without choke but I found the idle speed increased significantly as I again turned the air screws in from 1 turn to 1/2 turn out. Now that it was idling "properly" again, I synchronized the carbs @ 3000 rpm with the Twin Max which uses pressure transducers to measure the vacuum instead of liquid. The bench synchronization was suprisingly close.

Next, I rode the GS around the block for several miles @ 3000 rpm to get an accurate plug indication for the pilot circuit. I have a couple of uphill sections and the GS didn't handle those sections seamlessly. It bogged slightly and required a smidge of throttle which makes me believe the GS could benefit from a further pilot jet size bump from 20 to 22.5 considering the present air screw position of 1/2 turn out. I pulled one plug and it was very brown. I should note that these are the same plugs the GS was fouling when I started the process. I probably should take 49er's advice and start with a new set of NGK B8ES.

Regardless, later in the afternoon I put some more miles on the GS and once out of town I performed a plug chop @ 5000 rpm on a long extended uphill section. At 5000 rpm the GS was surging a bit which I thought indicated a lean condition. I pulled the spark plugs from 1 & 4 and found one plug was white and the other spark plug had some color. So it's running lean on the Needle circuit.

From a rideability standpoint, the GS is not there yet. Rolling on the throttle from full closed ( around town ) position results in slight hesitation and then acceleration. I believe this symptomatic of the Pilot circuit so I think the next step is to remove the 20 Pilot jets and install a set of 22.5 Pilot jets with new NGK's. Hopefully the 22.5's will work with air screws set one full turn out. Heck I might have to even drop the Needle from position #2 to # 3. That's were the GS is but I making systematic progress . . .

Mikuni VM29 S/B Configuration (5/26)
Pilot: 20
Needle: 5DL31-2
Main: 127.5
Air Screw: 1/2 turn out

79 GS1000: Wiseco 1085 10.25cr Compression sound
Andrews S-1
Dyna Coils & Ignition
K&N Pods
Supertrapp 4 into 1

 

[49er]

From a rideability standpoint, the GS is not there yet. Rolling on the throttle from full closed ( around town ) position results in slight hesitation and then acceleration. I believe this symptomatic of the Pilot circuit so I think the next step is to remove the 20 Pilot jets and install a set of 22.5 Pilot jets with new NGK's. Hopefully the 22.5's will work with air screws set one full turn out. Heck I might have to even drop the Needle from position #2 to # 3. That's were the GS is but I making systematic progress . . .

Mikuni VM29 S/B Configuration (5/26)
Pilot: 20
Needle: 5DL31-2
Main: 127.5
Air Screw: 1/2 turn out

79 GS1000: Wiseco 1085 10.25cr Compression sound
Andrews S-1
Dyna Coils & Ignition
K&N Pods
Supertrapp 4 into 1

Srsupertrap, the difference in the oriface size between the 0-2 and 0-6 needle jets is only 0.02mm so don't worry about having to match them (2.610 mm and 2.630 mm respectively). The taper shape difference between the two jet needles will vary greater than that. You can experiment with the clip position until you get the mixtures right. You should recheck your WOT plug reads to confirm your correct main jet size, then concentrate on the needle circuit, leaving the pilots until last.
I have attached a link that may help you some. It confirms that all the circuits are slighly overlapped, contrary to what some others claim.
You need to be aware of this when fine tuning. There are some suggestions for altitude tuning allowances also.

http://www.iwt.com.au/mikunicarb.htm

 

[srsupertrap]

Srsupertrap, the difference in the oriface size between the 0-2 and 0-6 needle jets is only 0.02mm so don't worry about having to match them (2.610 mm and 2.630 mm respectively). The taper shape difference between the two jet needles will vary greater than that. You can experiment with the clip position until you get the mixtures right. You should recheck your WOT plug reads to confirm your correct main jet size, then concentrate on the needle circuit, leaving the pilots until last. .

49er, Good information on the needle jet dimensions dimensions & article. I will have to try your suggestion to use the stock the 5DL36 jet needles if I cannot fine tune the VM29 5DL31 jet needles. Renobruce sent me a set of VM26 5DL36's jet needles back in March but I misplaced them and spent a lot time searching around the garage :-k. Yesterday I did change the pilot jets from a set of 20's to 22.5's and set the air screws at 1 full turn out. I want to get that air screw setting at 1 full turn, so I achieve +/- idle mixture leeway. I will probably have to wait till this weekend to perform WOT plug chops.

I have the Parts Express Coil Relay & connector on order so in the meantime I will wire that in thus eliminating another GS electrical weakness. Keep watching . . .

 

[srsupertrap]

Update 6/1/08

Update 6/1/08

On Saturday I fine tuned the air screw mixture to accomodate the larger 22.5 pilot jets I installed the previous weekend. My GS1000 still obtains the highest RPM with the air screws turned out 1/2 turns from being seated. I putted round & around the neighborhood block for several miles to check the pilot circuit; the guys sitting on the back of their work truck drinking beer smiled but they probably had a pretty good buzz on by then. I was pretty happy too because the surge on the uphill section was now gone and when I did a plug chop I found the plugs had a medium grey color.

I headed out to Rocky Flats to check the needle circuit; the GS was still surging a bit so I knew I had more work pending. When I performed a plug chop @ 4500 rpm I found the plugs in cylinders 1 & 4 were still too white for my liking. On Sunday I moved the needle clip position to #3, resynched the carbs with the TwinMax Carb Synchronizer and set back out for another road test and plug chop.

The move to the #3 needle clip position eliminated the part throttle hesitation; I can now roll on the throttle and the GS pulls cleanly and transitions smoothly from the pilot to the needle circuit. Next I headed back out to Rocky Flats and maintained throttle @ 5000 rpm for 1 mile. I had no issues with surging but the plugs in cylinders 1 & 4 were actually whitter than the day before. This is probably because I didn't maintain 4500 rpm for 1 mile on the previous day or at least I believe that's plausable. All these plug chops were performed with B7Es's, so since I just received my latest Z-1 order of VM25 pilot jets I will be installing a set of B8ES's and will recheck this week and than hopefully move onto the main. I believe the coil relay modification (thanks guys) bolstered the performance of the green Dyna Coils & Accel 8.8 wires but that just a seat of the pants opinion. More later . . .

Mikuni VM29 S/B Configuration (6/01)
Pilot: 22.5
Needle: 5DL31-3
Main: 127.5
Air Screw: 1/2 turn out
Throttle Cut Away: 2.0

79 GS1000: Wiseco 1085 10.25cr
Andrews S-1
Dyna Coils & Ignition
K&N Pods
Supertrapp 4 into 1

 

[srsupertrap]

6/08

6/08

I installed a set of new NGK B8ES's gapped @ .028", logged some miles around town and set out for another round of plug chops to check the Needle Circuit. I maintained throttle @ 5000 rpm on a long uphill section, chopped it and found the #1 & 4 spark plugs electrodes were still white with little/no color on the inner shell. I repeated the process @ 4000 rpm with the same result except a hair more inner shell color. Once home I pulled the VM's and moved the Needle Clip down to position #4, reassembled and that's the current state.

So what's the consensus out there. . . to check the Needle circuit is it more accurate to perform a plug chop @ 4000 or 5000 rpm? One other fact I have not mentioned is upgraded the entire charging system with a new RM Stator, Rick's R&R and Coil Modification which probably means I getting the most performance out of those Dyna Coils & Wires I have had in years.

From a rideability standpoint the GS feels great once again.

 

[jknappsax]

D@mn, that's one pretty and CLEAN GS 1000E. Glad to hear you got her running right again!

 

[49er]

When it's running as good as it looks you'll have one sweet bike. Changing those needles one more position should richen things slightly, which sounds like you still need to do. 5000-6000rpm ( around 1/2 + throttle) under load should give you the ideal reading point. Good luck.

 

[KEITH KRAUSE]

Supertrap, the bike looks great!
Just reading bits and pieces here. Remember, the cut-away is its own circuit, just like the pilot, jet needle or main. Its purpose is to provide transition from the pilot circuit to the jet needle. Past replies from you suggest you didn't know that.
Also, forget about rpm's. When jetting, focus on throttle position and the jetting circuit that regulates that throttle position.
The sequence is: pilot, cut-away, jet needle, main. Respective throttle position(s) are: closed throttle to approx' 1/5, 1/5 to approx' 1/4, 1/4 to approx' 3/4, and 3/4 to full throttle. Of course, the air jet size factors in too.
Yes, there is some overlapping of circuits but the above is fairly accurate.

 

[srsupertrap]

Re:

Re:

Thanks for the advice 49er & KEITH KRAUSE. To be systematic I think I will mark the the throttle grip before the next round of testing with chalk marks to make sure I am @ 1/2 throttle. Stay tuned.

 

[49er]

Carb circuit tuning

Carb circuit tuning

Supertrap, you appear to be attempting to get the needle circuit sorted before the mains. It is important to work backwards from the mains doing plug chops or dyno comparison. The accuracy of needle metering is dependant on the correct main jet size. As stated previously, the needle works in the 1/4 to 3/4 throttle range and is designed to keep the air/fuel ratio at an optimum ratio, during that period of operation. It is really a restrictor of flow and a percentage of full flow through the main jet oriface at a given throttle setting. From 3/4 onwards, there is such a small part of the needle inside the needle jet that maximum flow is achieved that matches the capacity of the main jet.
It is possible to have a fat or lean main jet and spend untold hours trying to tune in the needle without much joy. This frustration is mostly eliminated by having the main jet sorted first.
In my experience, the factory slide cutaway is ideal for most tuning applications and is rarely changed, unless serious transistion problems are encountered.
The fact that the circuits do overlap, causes some difficulties in fine tuning. It is very rewarding when you have finally got it sorted though!

 

[srsupertrap]

Friday 13th . . . Disaster Strikes

Friday 13th . . . Disaster Strikes

I used a bottom up approach because my initial Carb ridability problem was a part throttle burble/hesitation that occurred at around 3000 rpm. I knew my jetting problem was associated with interaction of the pilot jet, pilot air screw, Throttle Valve Cutaway and perhaps the jet needle clip position. As you pointed in Post #5 "sounds as though you are lean at the transistion point between the pilots and the needles" Well, I wasn't sure which way but as you know I replaced the 1.5 Throttle Valves with those replated 2.0's and Sudco recommends using a much larger Pilot Jet to deliver more fuel in the pilot circuit since a larger Cutaway makes things leaner @ 1/5 to 1/4 throttle opening. I could have gone directly to Sudco's recommended #25 pilot jet but I decided to keep the mixture lean and bump the size of the pilot jet incrementally and make sure the pilot circuit plug chops were correct. Heck it wouldn't even idle until I installed # 20 Pilot Jet. That's one reason for the bottom up approach. Like the article you posted from Ian Williams states, "Carburetor troubleshotting is simple once the basic principles are known. The first step is to find where the engine is running poorly."

Second, I made no mechanical changes to my GS engine, although I will be. Same old Wiseco Kit, cams, pipe & carbs with the exception of the 2.0 T/V's. I did all of those engine modifications I listed over twenty years and performed plugs chops & readings @ sealevel. This would infer the 127.5 Mains should be rich @ 5100 feet.

I took the day off: Removed one K&N and marked the kill switch housing with tape to denote both 1/2 & 7/8 throttle position. Reinstalled the K&N and head off for another round of testing with hopes of checking both the Needles & Mains. I did a plug chop on the needles first, the #1 had a blotch of color on the insulator while # 4 electrode was pretty white but both plugs had brown residue/color on the inner shell. Looks like #23.
www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Started Big Blue back up on the side of the road and was greeted by a Loud Knock emenating from deep within. Limped home with the throttle barely open expecting it to seize any second and highside me. Came back a few hours later pulled the plugs and performed a compression check with the throttle wide open. I recorded the following values 1) 135 psi 2) 130 psi 3) 125 psi (last one tested) 4) 135 psi. So I didn't hole a piston? It sounds like a rod knock . . . I am admittely over my head and I haven't rebuilt a bottom end and would humbly ask for HELP to diagnose.

I was so close to being done.

 

[KEITH KRAUSE]

Supertrap, you appear to be attempting to get the needle circuit sorted before the mains. It is important to work backwards from the mains doing plug chops or dyno comparison. The accuracy of needle metering is dependant on the correct main jet size. As stated previously, the needle works in the 1/4 to 3/4 throttle range and is designed to keep the air/fuel ratio at an optimum ratio, during that period of operation. It is really a restrictor of flow and a percentage of full flow through the main jet oriface at a given throttle setting. From 3/4 onwards, there is such a small part of the needle inside the needle jet that maximum flow is achieved that matches the capacity of the main jet.
It is possible to have a fat or lean main jet and spend untold hours trying to tune in the needle without much joy. This frustration is mostly eliminated by having the main jet sorted first.
In my experience, the factory slide cutaway is ideal for most tuning applications and is rarely changed, unless serious transistion problems are encountered.
The fact that the circuits do overlap, causes some difficulties in fine tuning. It is very rewarding when you have finally got it sorted though!

I generally try to get the main right first but sometimes it doesn't work out that way. Many times I thought I had the right main only to find out that once the jet needle was right I now had a certain amount of bog during roll ons that was created by the overlap effect of a too large main. The bog may clear up soon/not be real serious but when this happens I always consider the jet needle more important and if it's right, then it's the main that will have to be decreased to get rid of the bog.
As for how the fuel flows in each of the jet needle and main circuits, it's always the SMALLEST restriction in the entire circuit that regulates flow.
The tapered jet needle rises out of the needle jet and at approx' 3/4 throttle, the opening between the jet needle and needle jet becomes larger than the main jet opening. At that point the main, now the smallest restrictor, takes over completely. Before the 3/4 point, the opening between the jet needle and needle jet is smaller than the main so the jet needle regulates up to that point.

 

[KEITH KRAUSE]

So the loud knock was continuing as it ran or did you just hear it at start up only? (I'm hoping the loud noise was just the starter clutch not operating right which will result in a loud noise but most people refer to it as a "CLACK" sound.)

 

[49er]

I generally try to get the main right first but sometimes it doesn't work out that way. Many times I thought I had the right main only to find out that once the jet needle was right I now had a certain amount of bog during roll ons that was created by the overlap effect of a too large main. The bog may clear up soon/not be real serious but when this happens I always consider the jet needle more important and if it's right, then it's the main that will have to be decreased to get rid of the bog.
As for how the fuel flows in each of the jet needle and main circuits, it's always the SMALLEST restriction in the entire circuit that regulates flow.
The tapered jet needle rises out of the needle jet and at approx' 3/4 throttle, the opening between the jet needle and needle jet becomes larger than the main jet opening. At that point the main, now the smallest restrictor, takes over completely. Before the 3/4 point, the opening between the jet needle and needle jet is smaller than the main so the jet needle regulates up to that point.

You should always work backwards from the main jet. Regardless of your engine's state of tune, the engine's main jet demand will always remain constant in the 3/4 to WOT positions. The only exceptions to this are slight variations in altitude, temperature, humidity or air density.

The size/shape of your needle and the needle jet size will vary according to the state of tune of the rest of the engines modifications. For example, if you wildly port and fit radically timed and/or high lift cams along with a high performance 4-1 exhaust, a stock needle configuration will not allow the engine to operate to it's potential. Under these circumstances, you need to change your needle's configuration and possibly your slide cutaway size, as well.

Plug chops are a reasonable indicator, but the ideal feedback is obtained through a dyno session. Maximum torque is achieved around 13 to 13.5 -1 air/fuel ratio. This is somewhat below the stoichiometric ratio of 14.7-1.

Supertrap, your compression figures are relatively consistent, but on the lowish side. They don't indicate that you have done any top end damage. That knock is of concern though. Have you been hammering the engine at high rpms for a lenghtly period? It sounds as though you may have damaged the bottom end.

A rule of thumb starting point for main jet sizing at sea level is, multiply the size of the carbuettor's bore by 4.
Example, if you have a 26mm carb then the maths are 26 x 4 = 104. This is equivelant to a main jet orafice size of 1.04mm.
I am running a 102 or 1.02mm main jet in my GS850 at present. With my engines present configuration, the air/fuel ratio at WOT from 7000 to 10000rpm is 13.3-1.

Hope this helps. Good luck with that knock!

 

[Pete's GS]

Great job on that original GS! If I may say so, you might want to trashcan those geeky case savers, though.