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Discussion Starter · #1 · (Edited)
I've been working on trying to get my CB350 with VM30's into reliable shape. (Uni pods, cheap shorty mufflers)

I had it at 20 pilot, 1 1/4 turns out, P5, 6F4 on top clip, 145 main. I took it around the neighbourhood for a few minutes to warm up (burbly at 5000), then put in new plugs to test the pilot circuit. The plugs from the warmup ride were completely carbon fouled black. After letting it run for 5 minutes on idle only, the left plug looks absolutely brand new and the right plug has a light tan to it.

So I turned the left air screw in to 1, leaving the right at 1 1/4 turns, and switched out the 6F4 top clip for a 6F5 bottom clip. After the neighbourhood warm up ride, my plugs look like this:

left Auto part Plant Spark plug
right Spark plug Auto part Automotive engine part Automotive ignition part


New plugs, after a 5 minute idle look like:

left Auto part
right Auto part


The left still looks lean on the pilot circuit, but the right looks pretty good? I was hesitant to have different idle screw settings, but I'm super hesitant to have different jets. Is there a point where a 25 pilot screwed out is similar to a 20 pilot screwed in?

But then the right looks lean from the warmup (when I go around the neighbourhood, throttle is between 1/4 and 1/2). It was black with a 6F4 leanest top clip, and it's going to clean white with 6F5 richest bottom clip. Is there something in between?

Thanks a ton,
John
 

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Discussion Starter · #2 ·
Keeping on efforts, swapped in 25 pilots, 2 turns out on both sides, P5, 6F5 bottom clip, 145 mains.

After warmup ride,
left Auto part Spark plug Automotive engine part Automotive ignition part
right Auto part Automotive engine part


New plugs after 5 minute idle
left Auto part Automotive engine part
right Photography


Focusing on the pilot circuit, I tried 25 pilots, 1 3/4 turn on left, 2 1/4 turn on the right. P5, 6F5 bottom clip, 145 mains. Took it for a longer warmup and it felt great. A little burbly around 5000 in second, but not bad in 3rd or 4th. A couple little pops, but held decently at steady throttle around 6000. Idle was a bit rough, but kept going at 1200.

After warmup ride,
left Auto part Tire Automotive wheel system Wheel
right Auto part


New plugs after 5 minute idle
left Auto part Spark plug
right Auto part


Might try one more quarter turn out on the right, so 1/3/4 on left and 2 1/2 on right, and I'm hoping that settles my pilot circuit.

Hoping for some help with the mid range settings though. Not chops, but 6F4 top clip leads to black, black plugs after 5 minutes between 1/4 and 1/2 throttle. 6F5 bottom clip leads to what you see above. How do you get in between?
 

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I don't have experience with the VM30 carbs. specifically but wondered what other size main jets have you tried? I would install and sort out the main jet size first(my feeling is possibly leaner in the mains,imo)and then work your way down to the midrange and pilot circuits.
 

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First, the suggested settings for the mixture screws are nothing more than a place to start. It is quite normal to end up with slightly different settings.

With the engine warm and idle speed adjusted to spec, adjust one mixture screw to where the idle rpm is highest. Re-set the idle speed to spec and repeat on the other carb. Check/adjust the carb sync. Adjust if necessary then repeat the mixture screw adjustment.

The idle mixture settings have little effect at throttle settings significantly above idle. If you have differences in your plug readings you might have vacuum leaks or unequal float settings. I'm sure others will be along with more ideas.
 

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I would start with checking the slide openings being exactly equal by manually synchronizing both throttle cables visually before running the engine,you want to be sure that each carb. slide is exactly at the same opening as the other one.This will really help by adjusting both carb. slides(cables)at the carb. tops;remove the air filters and hold the throttle almost wide open and reach inside each carb. throat from the back to feel at what height the slide is from left to right side.
I apologize for the things I've written being redundant as I've never tried typing this out for someone else but rather have just done it myself on different multi cylinder bikes I've worked on.
 

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Discussion Starter · #6 ·
I would start with checking the slide openings being exactly equal by manually synchronizing both throttle cables visually before running the engine,you want to be sure that each carb. slide is exactly at the same opening as the other one.This will really help by adjusting both carb. slides(cables)at the carb. tops;remove the air filters and hold the throttle almost wide open and reach inside each carb. throat from the back to feel at what height the slide is from left to right side.
I apologize for the things I've written being redundant as I've never tried typing this out for someone else but rather have just done it myself on different multi cylinder bikes I've worked on.
Hey bilbikek,

Thanks for the input. I've done synching according to the VM30 sticky, I have a drill bit that gets both slides to a point where I'm idling at 1400. A quarter turn out and I get a nice 1200. I use barbecue match sticks to synch the cables visually and they're both moving and closing at the same time.

I'm not sure if this accomplishes the same thing as you're recommending? With the level of precision these things seem to work at, I don't think I have a tool that could measure the height of the slide at WOT as precisely as it needs to be done. I'm hoping the drill bit at idle is enough to set sync.

When it comes to mains, I've seen two schools of thought; work from the bottom up, or work from both ends to the middle. The sticky recommends both ends to the middle (get your pilot set, get your main set, then work on 1/8-3/4 throttle). I realize my riding style is pretty tame, and frankly I don't trust the bike enough to take it somewhere I can do a WOT plug chop, so I've been trying to do bottom up instead (I think this is what the Sudco manual recommends). From my understanding the mains don't kick in until 3/4 throttle. I honestly don't think I've ever cranked it past half.

I'm working purely off research, with no experience, so if I'm missing something please let me know.

Thanks,
John
 

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Discussion Starter · #7 ·
First, the suggested settings for the mixture screws are nothing more than a place to start. It is quite normal to end up with slightly different settings.

With the engine warm and idle speed adjusted to spec, adjust one mixture screw to where the idle rpm is highest. Re-set the idle speed to spec and repeat on the other carb. Check/adjust the carb sync. Adjust if necessary then repeat the mixture screw adjustment.

The idle mixture settings have little effect at throttle settings significantly above idle. If you have differences in your plug readings you might have vacuum leaks or unequal float settings. I'm sure others will be along with more ideas.
Hey J-T,

It's reassuring to know that it's okay to run different settings. I find when I'm trying to use the method you describe, I don't get a response until I change both carbs. For example, if I turn out the left idle screw 1/4, then the idle won't drop until I turn out the right 1/4. The mixed signals from that, along with the mismatched settings on the carbs, showed highest idle at 1 1/4 turns out on 20 pilots. At those settings, I was crazy lean on my left. Because of that, I turned toward plug colour to help me dial in the pilots. If I get my plugs right, (feels like I'm almost there?) will that be good enough to move on?

Boots and gaskets for carbs are new, and spraying carb cleaner shows no signs of vacuum leak. Float arms are set parallel to the carb body when resting on float needle.

More reading suggests I might have to swap slides to fix my 1/8 to 1/2 throttle. Just need to richen it a touch there. Cheaper, faster suggestions always appreciated.

On a weird side note, last night the exhaust from the right side smelled like sour pickles. Just the right side. There's a bit of Seafoam in this tank's gas. If it's cleaning junk, and the right has more junk than the left, could that be it?

Thanks for your help,
John
 

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John,
Raise both slides in your carbs to full throttle(w/ both air filters off)and then do the 'feel test'(my name for it)by allowing both slides to be exactly even when it's at full throttle,lower the slides just a hair until you can literally feel them both at the top of each carb. bore and exactly in the same place by adjusting the cable slack in each carb. separately.Do the main jet choice first and then work your way down to the needle jet height then pilot,etc. This Will work for you.
Don't 'over think' it :)
 

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Discussion Starter · #9 · (Edited)
John,
Raise both slides in your carbs to full throttle(w/ both air filters off)and then do the 'feel test'(my name for it)by allowing both slides to be exactly even when it's at full throttle,lower the slides just a hair until you can literally feel them both at the top of each carb. bore and exactly in the same place by adjusting the cable slack in each carb. separately.Do the main jet choice first and then work your way down to the needle jet height then pilot,etc. This Will work for you.
Don't 'over think' it :)
Am I overthinking it again? Sometimes I get caught up in trying to understand the idea and forget about just getting it done. Right now, I'll take riding over understanding, though both would be nice.

Went out and did the feel test. They're both spot on. Gone at the same and drop at the same time when I turn it a hair at full throttle. I have 150's I can try in there, but can't/won't plug chop them until it lets up on that lean pop enough to let me go further.

I keep coming back to this chart: Diagram Text Drawing Parallel Technical drawing


1/4 to 1/2. Going through the dimensions of jet needles, my head's starting to spin. Logic says it should get leaner as you go 6f3, 6f4, 6f5 6f8, but this chart shows it's not at all linear like that. In some numbers the 6f8 actually looks like it might be in between the 6f4 and 6f5.

Thanks again for your ideas.
 

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I have no experience with mikunis but if you bought them for cb350 applications the jetting should be pretty close. I'd look for threads and see what others are running to confirm. If one slide is high enough to pull good fuel it can power the engine it may make adjustments to the other side can be harder to notice. The closer they are will have less effect too I suppose. I like to set pilots in my 450 the same, I figure it's close enough. I also like to set the throttle with a vacuum gauge. Keep at it, you'll get it. You can judge better from riding it and it may take a long time to get perfect carburation. But that's the fun of messing with old bikes.
 

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Discussion Starter · #12 ·
Bil, 150 mains seem fairly normal according the VM sticky page. Outobie even mentions it in his "80% work" list. I know the stocks run something like 110, which might be what you're thinking of?

Lefty, I read all kinds of craziness looking for info on vm30's on cb350's. Mine came with 50 pilots and 250 mains, but I don't think it was intended for a cb350. I'm very, very close, but apprehensive about running lean for long. I don't even know how long is "long". As I take it out for a test ride, I hear that hollow thumping with little stutters and just wait for some catastrophe with my pistons.

There is a gap in my carb's fuel delivery coverage between 6f4 top and 6f5 bottom. Looking at the chart of needles over and over again, I don't think I can fill that gap with a different needle. I think my answer is going to be either P6 needle jets (from P5) or a 2.0 slide (from 2.5) I guess the next step is to pull the trigger on the P6 needle jet (cheaper switch) and wait for delivery. In the meantime, I'll keep searching. If anyone has any stories about switching p5's for p6's, I'd love to hear them.

Thanks for your responses,
John
 

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Discussion Starter · #13 ·
Found this from a 2015 post by outobie:

Here is a chart that I made to compare the relative flow rates of different jet/needle combinations

Needle Jet
P-0P-1P-2P-3P-4P-5P-6
Jet Needle2.6502.6552.6602.6652.6702.6752.680
6F5A62.3
B38.1
C19.0
D-12.5152.1912.2742.3572.4412.5252.6092.693
D-22.4563.1123.1953.2793.3623.4463.5303.614
D-32.4543.1433.2263.3103.3933.4773.5613.645
D-42.3644.5054.5884.6724.7564.8394.9235.007
D-52.0988.2348.3178.4018.4848.5688.6528.736
D-61.84011.42611.50911.59211.67611.76011.84411.928
6F4A62.3
B32.0
C19.4
D-12.5152.1912.2742.3572.4412.5252.6092.693
D-22.4423.3273.4113.4943.5783.6623.7463.830
D-32.4363.4193.5033.5863.6703.7543.8383.922
D-42.2066.7736.8576.9407.0247.1087.1927.276
D-51.93910.25010.33410.41710.50110.58510.66910.753
D-61.67813.21613.29913.38313.46713.55013.63413.718

As you can see from the chart, a 6F5 needle with a P-5 Needle jet is richer off idle through 1/2 throttle and leaner above 1.2 throttle than a 6F4 Needle with a P-0 Jet.

Needle Jet's go in .005mm size incriments, so a P-5 is .025 RICHER than a P-0. Needle Jet flow rate needs to be compared in conjunction with your jet Needle to understand actual differences.

the chart isn't lining up exactly right...the red cells compare the two combinations I reference in the text
This is awesome info. If I'm reading it right, it looks like the p-6/6F5 will get me between my p5/6F4 and p5/6F5.

P6's ordered and should be here next week. Will report back once installed. Fingers crossed.
 

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Bil, 150 mains seem fairly normal according the VM sticky page. Outobie even mentions it in his "80% work" list. I know the stocks run something like 110, which might be what you're thinking of?

Lefty, I read all kinds of craziness looking for info on vm30's on cb350's. Mine came with 50 pilots and 250 mains, but I don't think it was intended for a cb350. I'm very, very close, but apprehensive about running lean for long. I don't even know how long is "long". As I take it out for a test ride, I hear that hollow thumping with little stutters and just wait for some catastrophe with my pistons.

Thanks for your responses,
John
The vast majority of VM carbs come pre-jetted from Mikuni for direct fitment to a 1970's two stroke MX bike with huge mains and pilot jets plus a 'high' slide cutaway (2.5 or 3.0)
In my experience, 'Sporting For Less' is one of the worst 'offenders' for telling people carbs are jetted correctly plus the jetting they supply is 'proprietary' (well, I guess they are, if you have a 70's MX bike:rolleyes:)
Check Yamaha RD350/400 two-stroke if you want much closer to correct parts (although I don't think you can use exact numbers as they are 26mm carbs, depends on the body size)
Yamaha engineers decided the reed valves made bike closer to a fourstroke as far as airflow through intake was concerned so carbs got completely re-worked
Mikuni 'reverse flow' jets have completely different numbering system compared to Kei-Hin originals but the information/comparison charts are 'all over' if you get search parameters right
 
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I'm not sure if you can do it on those needles, but you can fine tune some with washers. I use brass washers for a #3 size screw from McMaster Carr When I do old snowmobiles the gap between clip positions is 0.040". The washers are 0.018" thick so you can get intermediate settings made between clip positions.

Sent from my SM-G900P using Tapatalk
 

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Discussion Starter · #16 ·
Haha. Getting search parameters right is definitely the challenge. Funny enough, some of the search terms I used were "crazypj vm30" and "outobie vm30". I think the vm30 sticky might have been more effective if it stuck to some of the info that pops up with those searches.

Todd, I've heard about the washer trick and I'm sure it's possible with these needles. The thing is, I needed an intermediate between needles, rather than an intermediate between clip positions. As black as my plugs were, I'm not sure if a half clip up on my 6F4's would have made the difference.

Thanks for your ideas,
John
 

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Discussion Starter · #17 ·
So, I got the p6's and popped them in.

After a warmup ride:
left Auto part Tire Cable
right Auto part Spark plug Automotive engine part


After a 5 minute idle:
left Auto part Cable Technology Electronic device Spark plug
right Auto part Automotive engine part


The ride was the best I've had so far. No burbling, popping, or hollow drumming when maintaining a constant throttle. Good acceleration without issues.

Idle was a bit weird. The best I can explain is like both cylinders were running slightly different cycles. When their cycles matched, it sounded strong and steady at 12-1300. But when their cycles were off, it struggled and dropped to around 1000. But kept chugging and lasted the 5 minutes. Still has that weird pickle smell out of the right.

So. 6F5 needle, bottom clip, p6 needle jet, left air screw 1 3/4 turns, right air screw 2 1/2 turns. 25 pilots, 150 mains.

Does my pilot circuit look good? I'm hoping my "ride" plugs might look white because I didn't ride long enough to colour them. It feels great, so will try a longer ride soon.

Thanks,
John
 

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The vast majority of VM carbs come pre-jetted from Mikuni for direct fitment to a 1970's two stroke MX bike with huge mains and pilot jets plus a 'high' slide cutaway (2.5 or 3.0)
In my experience, 'Sporting For Less' is one of the worst 'offenders' for telling people carbs are jetted correctly plus the jetting they supply is 'proprietary' (well, I guess they are, if you have a 70's MX bike:rolleyes:)
Check Yamaha RD350/400 two-stroke if you want much closer to correct parts (although I don't think you can use exact numbers as they are 26mm carbs, depends on the body size)
Yamaha engineers decided the reed valves made bike closer to a fourstroke as far as airflow through intake was concerned so carbs got completely re-worked
Mikuni 'reverse flow' jets have completely different numbering system compared to Kei-Hin originals but the information/comparison charts are 'all over' if you get search parameters right
I like what PJ wrote here especially about the "huge mains".
John,I have experience on jetting which is why I still think your main jets are too large,the plugs look too black,imo.
get some other smaller size main jets and try them(install some 140's and see if it improves even more)and please believe me on this that the main jet is what you need to sort-out first,get the main jet correct before you work on the needle/midrange or pilot/idle mixture screw;A "plug chop" is what's required for the main jets Then you can work your way down to the midrange and pilot circuit on your carbs.
I have operated (and still do)a motorcycle repair service for years and i'm telling you this from experience.I realize you read other things.
Would you take some detailed pics of your intake and exhaust systems,your bike,carbs.,etc.
I mentioned this earlier and you still haven't changed that 150 main jet.

Why hasn't anyone else who has jetted VM30's recently stepped-in on this thread to give useful feedback ?
 

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Discussion Starter · #19 ·
I like what PJ wrote here especially about the "huge mains".
John,I have experience on jetting which is why I still think your main jets are too large,the plugs look too black,imo.
get some other smaller size main jets and try them(install some 140's and see if it improves even more)and please believe me on this that the main jet is what you need to sort-out first,get the main jet correct before you work on the needle/midrange or pilot/idle mixture screw;A "plug chop" is what's required for the main jets Then you can work your way down to the midrange and pilot circuit on your carbs.
I have operated (and still do)a motorcycle repair service for years and i'm telling you this from experience.I realize you read other things.
Would you take some detailed pics of your intake and exhaust systems,your bike,carbs.,etc.
I mentioned this earlier and you still haven't changed that 150 main jet.

Why hasn't anyone else who has jetted VM30's recently stepped-in on this thread to give useful feedback ?
Hey Bill, thanks for the advocacy. So I took it out with the intention of doing a plug chop today. On the way to the route, I tried giving it more throttle than I usually do and got it up to 3/4. There's no place around here that I can sustain that for 5 minutes, let alone WOT. I read most people top out at 85-95 mph. To do the plug chop are they finding a strip of road where they're tearing at 90mph for 5 minutes? I can't for the life of me think of a stretch of road (7.5 miles of dead straight uninterrupted road according to the math) where I could pull that off.

My plug chop route is about 3 miles between two stop signs on an old country road. Going at a steady 50mph, 1/4 throttle, revving at 6000, it took me about 4 minutes. My plugs looked like:

left Auto part Plant
right Spark plug Auto part Tree Automotive engine part


Now Bill, you mention that my plugs look too black, but don't these look lean? I was under the impression that I'm supposed to be looking at the white insulator.

Is there another way to get my main sorted out besides a plug chop? How do the other chicken****s do it?

Thanks,
John
 

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Hi John,
Plug color is the right way to check the jetting.. with the different fuel additives we have in our fuels today it will look more lean then it used to w/ non-Ethanol fuels;the reason I mention this is they have old plug color charts which will get you close but are not 100% accurate w/ today's ethanol+ additives fuel.The principle is the same though.
I always use a magnifying glass under sunlight to see way down into the plugs toward the bottom of the insulators to get a true visual reading.The tips of the porcelain always get the hottest.How is your ignition system? timing 'spot on' ?
The reason I ask about the timing being 'spot on' on both cylinders and had asked before about throttle cables being synchronized/balanced is when i view your plugs,one cylinder looks like it's working harder(lean looking plug)than the other side.. or possibly your carbs. have different float heights? that could mess-up the jetting/plug chops.
A 'plug chop' can be done using only the first three gears,wide open throttle toward redline in 3rd and then hit the kill switch and come to a rolling stop and check your HOT(be careful :eek: ) plug color;install new plugs every time Before you do a plug-chop.
I asked for pics of your bike,etc. so I can view what your intake/exh. systems look like.
John,please consider me a helpful member here on the forum who is trying to help you dial-in your bike 'over a computer'(very difficult compared to being right in front of the bike)who has experience and been doing this for a while;my true intention is to help and to encourage you. ;)
 
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