VM29 S/Bore Jetting?

[srsupertrap]

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.

[srsupertrap]

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.)

The knocking sound is constant and sounds like it's coming from the lower cases although I have not ruled anything in or out. I looked in the Factory Manual and it details measuring crank pin wear and big end side cleareance but no diagnostic methods.

[srsupertrap]

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.

I think the compression #'s just rule out top end damage. I honed & reringed this GS back in 2005. One of the local shops told me you lose 15% compared to sea level. No I have not been hammering the engine, based on the throttle position I normally ride well within the Needle Jet & Jet Needle operating range.

[49er]

I think the compression #'s just rule out top end damage. I honed & reringed this GS back in 2005. One of the local shops told me you lose 15% compared to sea level. No I have not been hammering the engine, based on the throttle position I normally ride well within the Needle Jet & Jet Needle operating range.

From your description, it sounds like a big end failure.
Use a long screw driver as a stethoscope and place it adjacent to each cylinder against the front of the crankcase, between the headers. You should be able to clearly identify which rod the noise is coming from.
Typically, a big end knock is more prevalent when the engine is under load. When you were nursing the bike home, was the knock quieter than when you were accelerating?
If it is a big end, the question is what caused the failure? Oil starvation and/or over reving are the usual culprits!

[KEITH KRAUSE]

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.

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.

Well, as I said in my example, sometimes it just didn't work out by doing the mains first. Just made more work, though no big deal. It works for me.
I'm not sure about this formula you mention (carb bore x 4). Are you saying at sea level my bike (26's) should run 104's? The factory uses a 95 main. For my pipe/pods/larger pistons, I run a 138 DJ main which is approx' the same as a Mikuni 130. My bike (and many others here that I've helped re-jet) can handle the larger 142 DJ main at full throttle/high rpm's but often it creates a noticable bog/to serious bog during 55-65 mph top gear roll ons after I know the jet needle is jetted correctly. Going with the 138 eliminated the bog and may cost me a couple mph top end but I'll take real world passing power over a couple mpg off top end anytime. No way would I consider leaning the needle in this situation and trying to make the larger main work.
I'm just saying this is my experience with jetting. If it meant I had to change the jetting sequence to get things right then that's how it went.

[49er]

Well, as I said in my example, sometimes it just didn't work out by doing the mains first. Just made more work, though no big deal. It works for me.
I'm not sure about this formula you mention (carb bore x 4). Are you saying at sea level my bike (26's) should run 104's? The factory uses a 95 main. For my pipe/pods/larger pistons, I run a 138 DJ main which is approx' the same as a Mikuni 130. My bike (and many others here that I've helped re-jet) can handle the larger 142 DJ main at full throttle/high rpm's but often it creates a noticable bog/to serious bog during 55-65 mph top gear roll ons after I know the jet needle is jetted correctly. Going with the 138 eliminated the bog and may cost me a couple mph top end but I'll take real world passing power over a couple mpg off top end anytime. No way would I consider leaning the needle in this situation and trying to make the larger main work.
I'm just saying this is my experience with jetting. If it meant I had to change the jetting sequence to get things right then that's how it went.

Keith, the formula gives a ball park starting point. It is erring slightly on the fat side, for obvious reasons. It's only used when running filters. When using low restriction filters or unfiltered velocity stacks, the formula will be too lean.

Mikuni's jetting on the VM's was radically lean anyway. They were tuned to the 4-2 exhaust system, which didn't scavenge as wildly in the lower/ midranges as some 4-1 pipes do. This increased demand is one of the main reasons some tuners struggle to match the right needle shape to their engine's state of tune.

When I dynoed the 850, I also did a series of air/fuel ratio checks at set rpm's. I started at 3000 and went up to 8000, in 1000 rpm increments.
The first runs were with no load on and I noted the AFR's. When a 20% load was applied, most of the AFR's leaned off by one full ratio.


As you say, there are many ways to achieve ultimate performance. In the end, all tuning is a compromise to some degree. The trick is to get as close as possible to your target without greatly compromising performance in the lower or top ranges. You are doing that anyway.

[srsupertrap]

From your description, it sounds like a big end failure.
Use a long screw driver as a stethoscope and place it adjacent to each cylinder against the front of the crankcase, between the headers. You should be able to clearly identify which rod the noise is coming from.
Typically, a big end knock is more prevalent when the engine is under load. When you were nursing the bike home, was the knock quieter than when you were accelerating?
If it is a big end, the question is what caused the failure? Oil starvation and/or over reving are the usual culprits!

49er, the knock was most pronounced @ idle. I ride with a full face helmet on and that muffles everything. I drained the oil on Saturday and examined the inside of the oil filter and there was no chunks, debris or anything else inside The oil on the bottom of the pan was not full of glistening metal shaving. That's good. I recruited my next neighbor to listen since he had a cam bearing failure on his 96 H-D. I just asked him to describe what he heard and he though it was knock as well. He used the screwdriver method but we were not successful in locating the sound because the sound transmission. The noise isn't coming from the starter clutch though.

The noise is most prounced @ idle and diminishes as the rpm increase. I was only reving it up to 3000 rpm in the garage. I can hear the noise quite well at the cam chain tunnel which may mean one of the valves lost it's adjustment? I also know the cam chain tunnel would transmit sound quite well from the bottom end.

I am going to inspect the valve train components and check the lash later today. Should have a better idea by tonight.

[49er]

49er, the knock was most pronounced @ idle. I ride with a full face helmet on and that muffles everything. I drained the oil on Saturday and examined the inside of the oil filter and there was no chunks, debris or anything else inside The oil on the bottom of the pan was not full of glistening metal shaving. That's good. I recruited my next neighbor to listen since he had a cam bearing failure on his 96 H-D. I just asked him to describe what he heard and he though it was knock as well. He used the screwdriver method but we were not successful in locating the sound because the sound transmission. The noise isn't coming from the starter clutch though.

The noise is most prounced @ idle and diminishes as the rpm increase. I was only reving it up to 3000 rpm in the garage. I can hear the noise quite well at the cam chain tunnel which may mean one of the valves lost it's adjustment? I also know the cam chain tunnel would transmit sound quite well from the bottom end.

I am going to inspect the valve train components and check the lash later today. Should have a better idea by tonight.

From your latest description, it sounds like you have developed the infamous excessive "cam endfloat knock". If so, the engine shouldn't knock until it has reached normal running temperature. The engine won't be harmed by this noise, but it will be a huge annoyance. My 850 used to knock that loud that it sounded like a big end knock at idle, but dissappeared at around 1800 rpm. I did some machining to fix it some 8000 kms ago. It's still as quiet as!
Go to Basscliff's site for the details on the mod.

[KEITH KRAUSE]

When I can't hear the noise myself I always suggest checking basics, even if they don't seem like a probable cause.
Are you POSITIVE the cam chain tensioner is working well? Do the factory test and watch carefully. Got the manual?
Also, are you positive that all 4 cylinders are firing well, and the timing/advance is spot on?
If you're worried to start the bike again you can check all the above without starting. The timing and advance timing can be checked statically.

[first timer]

did you check you clutch basket as well, these get lose and make a racket as well

[srsupertrap]

6/15 Findings

When I can't hear the noise myself I always suggest checking basics, even if they don't seem like a probable cause.
Are you POSITIVE the cam chain tensioner is working well? Do the factory test and watch carefully. Got the manual?
Also, are you positive that all 4 cylinders are firing well, and the timing/advance is spot on?
If you're worried to start the bike again you can check all the above without starting. The timing and advance timing can be checked statically.

I have the Factory manual. I pulled the valve cover checked the lash and found the #2 intake was measuring a .006" loose. Andrews Cam Valve lash (.004 - .006" Cold). Replaced the shim and got it back to .004". Checked the slotted cam sprockets bolts and they had not backed off. Checked the Tsubaki Cam Chain & rivet which I replaced long ago; no signs of contact or wear. Yes, I verified the operation of the Cam Chain tensioner. Looks good.

First Timer: over the winter I had Falicon upgrade the Clutch Hub with the thicker backing plate and welded rivets because that thing was rattling like a you know what.

I am going to call Rapidray and get his input. I will reassemble and see what it sounds like.