VM Tuning Re-visited

[atom4488]

Two consecutive ride days. Reduced the pre-load on the rear springs (Progressive Suspension) by two positions. Vastly improved ride and feels very much balanced with the front forks with stock springs and a 1" pre-load spacer. The other comments above will have to wait to get addressed.

Finally got a decent ride done on my 650 SG. As the bike is still relatively new to me, there are a number of little details that reveal themselves as I get more seat time.

• rear springs too stiff > may have too much preload cranked in
• engine is slow to pull past 4500 rpm in 5th, is also a little flat between 4 and 6000 rpm in lower gears > may be a little lean in that range?
• the seat (or my backsides) need more padding
• need better mirrors that stay in adjustment (cheap bar-ends on it now)
• the big $ GPS speedo/tach is not watertight!!! Got moisture inside it after a pre-ride wash!

Pics to follow tomorrow...

I have been addressing the jetting issue (Mikuni VM 34s) without much joy. When I got the bike it had 6F9 needles, 159P6 needle jets, 2.5 slides and 170 main jets. The pilots were mismatched 15 on the left. 20 on the right. The needle clip was on the second slot. It ran poorly overall with this setup. The PO had tested mains from 170 to 200, pilots from 15 to 32.5, without ever really getting satisfactory performance.

I only have pilot jets and a few main jet sizes to play with:

• 1st re-jet > mains 180, pilots 22,5 and clip in the third slot; not much better
• 2nd re-jet > 170 mains, 20.0 pilots, and clip on the third slot; idles smoothly @1000 rpm and pulls strongly to 4000 rpm, then falls flat. If you hold it wide open until it creeps up to 6000 rpm, it takes off, pulling hard to redline. Very satisfied with performance up to 4000 rpm, after that, not so much. Before doing this re-jet, I inspected the petcocks and found them to be full of swarf, to the extent that they would not flow correctly. I think that the performance improvement here is mostly attributable to cleaning the petcocks.
• 3rd re-jet > 180 mains, clip on the 2nd slot; performance unchanged up to 4000 rpm, but won't pull past that in 5th. Plugs very sooty
• 4th re-jet > 170 mains, clip on the 4th slot; performance unchanged up to 4000 rpm, pulls better beyond 4000 rpm but surges, not pulling cleanly. Surges at steady-state throttle around 3500 rpm and beyond. Left plug still slightly sooty, right plug tending towards light gray / whitish. These are not pure plug-cuts, just a view of the plug after it was run for a couple of hours.

For the time being, I'm trying to get the upper mid-range to behave correctly. I think the next move is to stay with the 170 mains but move the clip to the 3rd slot. Not sure how to explain the mis-matched plug readings. Anybody care to share some insight/guidance/wisdom?

BTW, this is a 700 cc engine with a Shell #1 cam and a Hoos-rebuilt head.

 

[Jim]

I have been addressing the jetting issue (Mikuni VM 34s) without much joy. When I got the bike it had 6F9 needles, 159P6 needle jets, 2.5 slides and 170 main jets. The pilots were mismatched 15 on the left. 20 on the right. The needle clip was on the second slot. It ran poorly overall with this setup. The PO had tested mains from 170 to 200, pilots from 15 to 32.5, without ever really getting satisfactory performance.

I only have pilot jets and a few main jet sizes to play with:

• 1st re-jet > mains 180, pilots 22,5 and clip in the third slot; not much better
• 2nd re-jet > 170 mains, 20.0 pilots, and clip on the third slot; idles smoothly @1000 rpm and pulls strongly to 4000 rpm, then falls flat. If you hold it wide open until it creeps up to 6000 rpm, it takes off, pulling hard to redline. Very satisfied with performance up to 4000 rpm, after that, not so much. Before doing this re-jet, I inspected the petcocks and found them to be full of swarf, to the extent that they would not flow correctly. I think that the performance improvement here is mostly attributable to cleaning the petcocks.
• 3rd re-jet > 180 mains, clip on the 2nd slot; performance unchanged up to 4000 rpm, but won't pull past that in 5th. Plugs very sooty
• 4th re-jet > 170 mains, clip on the 4th slot; performance unchanged up to 4000 rpm, pulls better beyond 4000 rpm but surges, not pulling cleanly. Surges at steady-state throttle around 3500 rpm and beyond. Left plug still slightly sooty, right plug tending towards light gray / whitish. These are not pure plug-cuts, just a view of the plug after it was run for a couple of hours.

For the time being, I'm trying to get the upper mid-range to behave correctly. I think the next move is to stay with the 170 mains but move the clip to the 3rd slot. Not sure how to explain the mis-matched plug readings. Anybody care to share some insight/guidance/wisdom?

BTW, this is a 700 cc engine with a Shell #1 cam and a Hoos-rebuilt head.

You given thought to possible exhaust reversion? 4 to 6K is in the ballpark for it. I'm basing that on understanding you've never got that range to work properly, no matter the jetting. That correct?

 

[atom4488]

You given thought to possible exhaust reversion? 4 to 6K is in the ballpark for it. I'm basing that on understanding you've never got that range to work properly, no matter the jetting. That correct?

Interesting thought... the pipes are what I would consider very conventional; individual 1.5" head-pipes going into a pair of Cone Engineering reverse-cone mufflers. I'm not sure why they would cause reversion, Lets assume that it is an exhaust reversion issue. Is the fix an exhaust port optimizer? Or possibly a primary length adjustment? My 650D has a MAC 2 into 1 with 1.75" primaries. It behaves well, mind you with BS34s. If anything, I would have thought those large primaries might cause an issue. If I recall correctly, @Mannyroad had some really odd engine behaviors that were ultimately attributed to unconventional custom pipes.

My understanding of the Mikuni theory of operation is that the midrange is primarily controlled by the needle and needle jet, so that is the focus for now, mostly because that's all I've got to play with. I'll fiddle some more and report.

Appreciate any theories on the pipes, though.

 

[eidemaddin]

BTW, this is a 700 cc engine with a Shell #1 cam and a Hoos-rebuilt head.

Is it certain that she has already run well with this setup, including the carbs and pipes? If so, does she still have the mechanical ignition advance and is it working properly? Compression on both sides and valve timing are OK?

What irritates me is the contrasting plug image on both sides despite the same jet setting. The membranes are also OK and there is no secondary air problem? At this point the compression issue could also be interesting again.

 

[atom4488]

Is it certain that she has already run well with this setup, including the carbs and pipes? If so, does she still have the mechanical ignition advance and is it working properly? Compression on both sides and valve timing are OK?

What irritates me is the contrasting plug image on both sides despite the same jet setting. The membranes are also OK and there is no secondary air problem? At this point the compression issue could also be interesting again.

The previous owner sold the bike with the statement that the carbs needed further tuning. He also supplied a lengthy log of tuning setups that he had tried, along with notes of his observations. His favourable comments are on setups that I found very rich (180, 185 mains and 30 pilots, needle clip on the 2nd slot) and unworkable. His observations/ comments include many references to the mid-range being flat. Ignition is Boyer Bransden and advances normally when checked with a timing light. I have checked lash but not cam timing. If the cam was off, it would not run as well as it does, if at all. I have not checked compression but will do so as a sanity check. It's essentially a new engine.

I agree; the fact that one side shows rich and the other lean requires investigation. I'm planning a full carb teardown and micro-inspection. I have had the carbs apart already. They are also new and whistle-clean inside. By membranes, I assume you mean diaphragms. There are no diaphragms, these are Mikuni VM-series round-slide carbs.

Just as a general observation, the bike runs well; it starts immediately with no choke, idles flawlessly and pulls well to about 4000 rpm. That makes it very rideable. Its really the upper mid-range and the top end that needs to be made right.

 

[gggGary]

Niche's product description conveniently with all the brass ID'd

My fairly similar, hot cam 750 is set like this.
Carb: VM34
MJ: 180
PJ: 25
NJ: 159-P6
JN: 6F9, 3rd slot
Slide: 2.5
Air jet: ?
Float valve 3.3

 

[5twins]

My buddy had similar issues with one of his 4 cyl. Suzukis. It ran fine at low to mid RPMs but wouldn't run much above 5K. He tried all sorts of carb re-jetting to no avail. It turned out his cams were off by a couple teeth. So, I would check your cam timing. If you're off a tooth, that may be the issue. It's easy enough to do with the engine still assembled. Set the motor on the "T" mark (TDC). On the advance end of the cam, the hole for the advance locating pin should point straight up or down.

 

[atom4488]

My buddy had similar issues with one of his 4 cyl. Suzukis. It ran fine at low to mid RPMs but wouldn't run much above 5K. He tried all sorts of carb re-jetting to no avail. It turned out his cams were off by a couple teeth. So, I would check your cam timing. If you're off a tooth, that may be the issue. It's easy enough to do with the engine still assembled. Set the motor on the "T" mark (TDC). On the advance end of the cam, the hole for the advance locating pin should point straight up or down.

Thanks for the suggestion, but the performance would suggest that the cam timing is correct... easy starting, dead smooth idle, strong initial spool-up to 4000 rpm, then the "dead-band" from 4000 rpm to 5500 rpm, then pulls strongly to the redline, 6500 rpm +. I'm looking for a different gremlin, IMO.

 

[atom4488]

I have been addressing the jetting issue (Mikuni VM 34s) without much joy. When I got the bike it had 6F9 needles, 159P6 needle jets, 2.5 slides and 170 main jets. The pilots were mismatched 15 on the left. 20 on the right. The needle clip was on the second slot. It ran poorly overall with this setup. The PO had tested mains from 170 to 200, pilots from 15 to 32.5, without ever really getting satisfactory performance.

I only have pilot jets and a few main jet sizes to play with:

• 1st re-jet > mains 180, pilots 22,5 and clip in the third slot; not much better
• 2nd re-jet > 170 mains, 20.0 pilots, and clip on the third slot; idles smoothly @1000 rpm and pulls strongly to 4000 rpm, then falls flat. If you hold it wide open until it creeps up to 6000 rpm, it takes off, pulling hard to redline. Very satisfied with performance up to 4000 rpm, after that, not so much. Before doing this re-jet, I inspected the petcocks and found them to be full of swarf, to the extent that they would not flow correctly. I think that the performance improvement here is mostly attributable to cleaning the petcocks.
• 3rd re-jet > 180 mains, clip on the 2nd slot; performance unchanged up to 4000 rpm, but won't pull past that in 5th. Plugs very sooty
• 4th re-jet > 170 mains, clip on the 4th slot; performance unchanged up to 4000 rpm, pulls better beyond 4000 rpm but surges, not pulling cleanly. Surges at steady-state throttle around 3500 rpm and beyond. Left plug still slightly sooty, right plug tending towards light gray / whitish. These are not pure plug-cuts, just a view of the plug after it was run for a couple of hours.

For the time being, I'm trying to get the upper mid-range to behave correctly. I think the next move is to stay with the 170 mains but move the clip to the 3rd slot. Not sure how to explain the mis-matched plug readings. Anybody care to share some insight/guidance/wisdom?

BTW, this is a 700 cc engine with a Shell #1 cam and a Hoos-rebuilt head.

So today went with the 5th re-jet, essentially the same as the 4th re-jet except that the clip was raised to the third slot. In fact, this is the same as the 2nd re-jet, but is significantly better for some unidentified reason.

• 5th re-jet > 170 mains, clip on the 3rd slot; performance unchanged up to 4000 rpm, pulls significantly better beyond 4000 rpm but surges/ stutters still, but still not pulling really cleanly. Steady-state throttle surge /stutter now occurs above 4000 rpm and beyond. Both plugs now cleaner, showing dark brown on the insulator and slightly sooty on the body of the plug. Again, these are not pure plug-cuts, just a view of the plug after it was run at a steady 4000 rpm for the last five minutes of the ride. If the surge/stutter was reduced when going from the 4th to the third clip slot, it is liking a leaner midrange. I could go one more step, clip in the 2nd slot to see if it likes that

Compression checked hot at 145 lbs on both sides. I plan on rechecking ignition advance operation despite it looking good in previous tests. I could retry 180 mains, with the clip on the third slot, but I don't think that's going in the right direction.

@gggGary, thanks for your jetting setup. So you are two sizes up on the mains, two sizes up on the pilots, but otherwise the same. I may just copy that, see how it works out. I assume you are getting smooth performance from idle to redline with that setup.

 

[zrx1100]

are the cone engineering mufflers open core or "the quiter core"? A trip to a dyno with a "sniffer" would give you lots of info. In my experience, only hooking up an AFR sensor was I able to dial in the jetting. Should have done that from the get go.

 

[zrx1100]

i see you have open core. that configuration will flow much better. one thing. you seem to have different types of UNI pods on your carburetors. the one with the end cap flows much less. I would keep the type as the one on the left carb and then see. you are probably running too rich on the right cylinder or too lean on the left. Also, check your carb sync

 

[atom4488]

are the cone engineering mufflers open core or "the quiter core"? A trip to a dyno with a "sniffer" would dive you lots of info. In my experience, only hooking up an AFR sensor was I able to dial in the jetting. Should have done that from the get go.

The Cone Engineering mufflers are the Quiet Core model. I fully agree on the trip to the dyno, but I am not aware of any such facility nearby.

 

[atom4488]

i see you have open core. that configuration will flow much better. one thing. you seem to have different types of UNI pods on your carburetors. the one with the end cap flows much less. I would keep the type as the one on the left carb and then see. you are probably running too rich on the right cylinder or too lean on the left. Also, check your carb syncView attachment 336499

Good catch...the two Uni filters are different only because I had to replace the one on the left as the boot or mounting collar on the original "closed cap" one tore. The last test iteration was done with the configuration as pictured. While empirically you would expect to see differences in plug readings, they looked the same to me. I have the second "open cap" filter which I'll install before doing any further testing.

Carb synch has been checked and re-checked every time that I have done a needle-clip change. Changing mains shouldn't affect synch as the mains are changed without removing the carbs, just the drain plug needs to be removed to access the jets.

As I read more about exhaust reversion and as suggested by @Jim, the seems to be the next area to focus on, especially after having read @Mannyroad's experience. See his thread "Perseverance Pays Off"

 

[zrx1100]

The Cone Engineering mufflers are the Quiet Core model. I fully agree on the trip to the dyno, but I am not aware of any such facility nearby.

I see the baffle now as I zoom in. Indeed this is a quieter core. For what it’s worth, I did have reversion with commando mufflers that have similarly a plate around which exhaust has to go around, but that was with 1.75” headers. I would not expect this issue with the 1.5” headers. Some suggest exhaust port cones. They are cheap enough to try

 

[zrx1100]

I see the baffle now as I zoom in. Indeed this is a quieter core. For what it’s worth, I did have reversion with commando mufflers that have similarly a plate around which exhaust has to go around, but that was with 1.75” headers. I would not expect this issue with the 1.5” headers. Some suggest exhaust port cones. They are cheap enough to try

I fixed my reversion issue with TPOs. But would not expect the issue with the 1.5” headers. https://www.xs650.com/threads/torque-peak-optimizer-tpos-performance-dyno-charts.66469/

 

[Kevin Werner]

I exist in Idiotville USA. I have VM34s. 4000 rpm is where you are rolling on the throttle. MY recent experience suggests your pilot/main over lap may be rich. I had 25 pilots and 180 mains and switched mains up and down until the crows came home. When I stepped down on pilots I finally saw improvements. That tells me, maybe incorrectly, that the pilots were too rich. I think someone here posted that you need to start with pilots on VMs. My riding is rather sedate and rich pilots respond pretty well. Many of my local rides have me not exceeding 3500-4000 rpm. So I didn't see the burbble/flat spot beyond 4000rpm. AND I ignored what my plugs were telling me colorwise. Ride gently, 1/4 thottle, to town, to pay bills, to visit friends to the hardware store to see the new building and old. Then after 10 or 20 miles read your plugs. That 1/4 throttle is all on your pilots. Cheers

 

[atom4488]

I see the baffle now as I zoom in. Indeed this is a quieter core. For what it’s worth, I did have reversion with commando mufflers that have similarly a plate around which exhaust has to go around, but that was with 1.75” headers. I would not expect this issue with the 1.5” headers. Some suggest exhaust port cones. They are cheap enough to try

I agree; I would not expect that the 1.5" pipes to be the cause of reversion issues. As it is an upgraded engine (700 cc, Shell #1, the VM's), I would expect that it would want bigger diameter head pipes. Also agree that EPO's are worth a try. I'll probably try to fab a set to try before my season ends up here !

 

[atom4488]

I exist in Idiotville USA. I have VM34s. 4000 rpm is where you are rolling on the throttle. MY recent experience suggests your pilot/main over lap may be rich. I had 25 pilots and 180 mains and switched mains up and down until the crows came home. When I stepped down on pilots I finally saw improvements. That tells me, maybe incorrectly, that the pilots were too rich. I think someone here posted that you need to start with pilots on VMs. My riding is rather sedate and rich pilots respond pretty well. Many of my local rides have me not exceeding 3500-4000 rpm. So I didn't see the burbble/flat spot beyond 4000rpm. AND I ignored what my plugs were telling me colorwise. Ride gently, 1/4 thottle, to town, to pay bills, to visit friends to the hardware store to see the new building and old. Then after 10 or 20 miles read your plugs. That 1/4 throttle is all on your pilots. Cheers

Why Idiotville? Sounds like you have a perfectly dialed-in setup. I agree with the gist of what you are saying; performance improved when it was leaned out further, so I'm heading in the right direction. My pilots are 20's, mains 170, 3rd clip. In general, my riding sounds like yours; with most riding being done in the 3500 to 4000 rpm range. From idle to half-throttle, all is good. My goal and expectation is to be able to accelerate smoothly up to redline in lower gears. Not giving up yet!

 

[zrx1100]

I agree; I would not expect that the 1.5" pipes to be the cause of reversion issues. As it is an upgraded engine (700 cc, Shell #1, the VM's), I would expect that it would want bigger diameter head pipes. Also agree that EPO's are worth a try. I'll probably try to fab a set to try before my season ends up here !

I did not realize that your motor is very much like mine. It was transformative for me to switch to open core mufflers. I would try to see if those baffles can be removed on your cone engineering megaphones. I am using their open core mufflers. They are not that loud at all and work very well.

 

[zrx1100]

Atom, here are some charts that I think you will find useful. Below are dyno and AFR curves on my bike (700cc, Shell #1, “street port”) with UNI pods and two types of mufflers: the Commandos and cones from Cone Engineering with open cores. The Commandos have a plate at the end around which the exhaust gasses have to move around. The plate also reflects exhaust waves which boost torque at low rpm, but suppress torque in the middle 3-5 rpm. With the open core mufflers from Cone Engineering, the power is excellent, except for the dip at 2500 rpm. I think that is a reflection from the end of my headers where gasses expand from the Torque Peak Optimizers which help me make the 1.75” headers work. I would expect to not have this dip in your setup if you have the header and muffler inlet diameters matched and the core on the muffler opened. ( I am still using the stock BS38 carbs, so that info is not too relevant for you.)