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Discussion Starter · #1 ·
I have a 72 cb350 with vm30's. It was running okay (not great, little puffs and hesitations when maintaining constant rpm around 5K), then a helicoil fell out so it got parked for two weeks. During that time I swapped in a set of Sportster mufflers to replace the no-name shorty mufflers that were on it.

Right now, I have it at:
145 mains
p5 needle jet
6f5 needle - 2nd clip from bottom
20 pilot - 2 turns out on left, 2.25 turns out on right
2.5 slides

When cold, it needs choke to start, but races up to 3000 right away, then slowly drops rpms until it dies about 30 seconds later. If I turn choke off when the rpm come down, it still sputters and dies. But once it's dead, I turn off choke, and hit the starter and starts right back up again. Left side exhaust does a little misstep at first, but right side exhaust is smooth and consistent. Once it's warm, both sides are smooth and consistent.

If I give it a bit of throttle, it makes a sound almost like sneezing, almost dies, then slowly works its way back up to idle. If I stay on the throttle, it dies. If I roll on the throttle slowly when it's warm, it'll rev up.

I've done the searches, and tried taking each spark boot off while it's idling. Taking off either boot causes a dramatic drop in rpm. The effect seems the same regardless of boot, but taking off the right ultimately ends up in the bike dying after a bit, while the bike will maintain a slow chug if taking off the left.

Condensers, coils, boots, plugs, points are all mostly new in the last 3 years or less, and everything has less than 500 miles on them.

Timing is set static and then with strobe. Carb slide heights are matched and both move at exactly the same time. I bought the carbs new. Battery is kept on a tender. Both headers seem equally hot.

When people say it bogs, is it that sneezing sound? From what I've read, that means lean? Shouldn't the increased restriction of Sportster mufflers make it richer? The air screws were dialed in using a colortune, just to the point where orange turned to blue (the edge of rich). And I've moved the needle clip down from 2nd from top to 2nd from bottom with little effect. Should I keep going? Maybe try popping a 6f4 in there?

Or is this not a carb issue?

Sorry for the lengthy post.

Thanks,
John
 

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I'm guessing vm 30s are aftermarket ..
Float levels perhaps??? I have found out first hand that if they are wrong (to high or to low) that your fuel delivery can be effected... I read about it for years in all sorts of car / motorcycle articles, forums etc.. but never (and still learning) learned proper settings..
I do know things are "easier" with factory carbs, as they are designed around that particular motor. "Performance" carbs are made for that... performance, "speed parts", and they require "tuning" or dialed in for that particular motor / setup...
I have seen it plenty of times on old V-8 Chevy and Ford motors.. guys buy speed parts and wonder why the car runs bad. A good mechanic / tuner is worth their weight in gold...they are the human equivilant of a self learning fuel injection system.
 

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Discussion Starter · #4 ·
Float levels are set. I think they're supposed to be 22 or 23mm.

The bike came to me carbless, airboxless, and with the cheap shorties. Back then, I got the vm30's for about the price that people want for used carbs. I'd never worked on a bike before, and I figured new was better. Little did I know.

I put on the Sportster mufflers because the shorties were godawful for noise, and because of a post by outobie that a longer exhaust works better on these bikes.

I think I can make this work. I need a bit of help translating bike-speak into english. Is the sneezing the same as bogging? When the VM manual says the engine sounds "dull" what does that mean? How is a rich "flat performance" different from lean "poor acceleration"? What does "popping back through carburetor" sound like?

With some clues as to rich or lean, I'll keep dialing. From research, I'm guessing lean right now, though the immediacy of the stumble makes me wonder if the needle is even in play yet. But the pilot was set with colortune and is set a bit rich. If I don't hear otherwise, tomorrow I'll try the 6f4 needle in the top clip and see if it makes a difference.

Thanks,
John
 

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When it spits back like that off-idle it's too lean. Richen up the pilot circuit until it picks up smoothly from idle.
 

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Discussion Starter · #6 ·
Id try dropping the slide height a little
Do you mean the idle setting? Right now, it's a thing of beauty at 1200, but I can definitely try tomorrow. How would this effect the stumble when given throttle?

Thanks,
John
 

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Discussion Starter · #7 ·
When it spits back like that off-idle it's too lean. Richen up the pilot circuit until it picks up smoothly from idle.
This makes sense to me, but I guess I was skipping it because I was trusting the colortune. I'll give it a go tomorrow progressively screwing in the air screw. Thanks for the suggestion.
 

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It's starving for fuel.. had a similar issue with a S-90 I have.. idle was a thing of beauty .. like a Swiss watch(or Japanese motorcycle).. get it on the street, open it up.... BLAAAAAAAAAH, BLAAAHHHHHH... would fall on its face.. multiple carb cleanings didn't help, to include a ultrasonics bath.. discovered as I quadrupled checked the float level, and re installed the float bowl, I felt the float contacting the bowl sides..basically not allowing it to properly fill the bowl.. once corrected, plenty of power (as much as a 90 has) and will hit 50+ with my fat ass on it.. oh .. idle still baby smooth..
Wait..,valves adjusted?
 

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Do you mean the idle setting? Right now, it's a thing of beauty at 1200, but I can definitely try tomorrow. How would this effect the stumble when given throttle?

Thanks,
John
Your comments about it needing choke and racing up to 3000 and dying makes me think you're compensating for too much air with extra gas, and I then they may not be balanced right after that. And your jetting could be wrong too. If you had manometer ports it'd help. Get you idle right first and go from there. The slides should be barely open just as much as you need. When it races to 3000, lower them a bit and back off the gas a bit too. See if you can get it to start without choke. (While warm) you can get different slide cutouts for these too.
 

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Wait..,valves adjusted?
Good point, if a valve isn't completely closing it'll either start and not idle, or idle and not start based on carb settings. Along with other running issues I'd assume. If you have a infrared thermometer, that could be helpful.
 

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Discussion Starter · #11 ·
Screwed in the air screw to 3/4 with 20 pilots and it was still acting the same, but with much rougher idle. Swapped in 25 pilots and it'll take more throttle more quickly, but still bogs if given a quick twist to 1/4 throttle which goes away the more it's warmed up. Once it's fully warm, a quick twist gives the slightest hesitation but then revs.

Air screws are at 2 1/4 turns out (it's where idle seemed happiest) but idle is not as smooth as it was. Popped in the colortune, and couldn't get it to not be orange. Got a few little snappy backfires off the right side.

Tried to take it for a quick spin, but it felt dead. Sounded like a hollow drum and had no power.

Lefty, I tried to adjust the idle screw, but it really wasn't having it. 1/4 turn out to get the rpm to 1000 and it was chugging. When I mentioned it jumps to 3000 and then drops down and dies, the whole thing takes less than 30 seconds from a cold start. And once it does it once, the next start fires right up with no choke.

From what I know, right now it's showing all kinds of signs of rich. Backfire, plugs came out a bit black, idle dips lower before returning after giving it gas, colortune is orange. Plus, isn't it also more rich when I put the more restrictive muffler on? This richness was to address the lean symptom of not taking throttle, which is better, but I don't think is completely gone.

Rob, I did the valves and points in July and have put on less than 200 miles since. I will check them again tomorrow and also check the floats and float level again.

I feel like it's acting lean while it's rich. Is there something else that could cause that throttle stumble besides simple carb adjustments?

Thanks,
John
 

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Discussion Starter · #12 ·
I'm trying to think up other possibilities. The last ride I had, it lost power about a mile from home, but I assumed that was due to the helicoil coming loose. Since then, I've replaced the helicoil (it came out in one piece) and replaced the exhaust gaskets and swapped in the Sportster mufflers.

Is it possible for the exhaust to be too restrictive? When I checked the mufflers, the Sportster mufflers seem to have a smaller diameter, and a longer length, as well as some kind of steel plate blocking the diameter, with two smaller holes in it. I don't know much about the exhaust end of things, but I'm grasping at straws here.

Thanks for your help,
John
 

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Did you adjust the slides at the cable adjusters? Give the cable some slack so the slides drop down a bit. If they're too far open there isn't enough pressure drop to pull the right amount of fuel from the pilot jet.
It sounds like your slide is too high and you're having to open your pilot screw too much to compensate and getting bad results. It also sounds like your slides may not be opened quite the same. They need to be balanced with the cable adjusters.
 

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Could rich can cause similar symptoms as to lean...
Just to clarify.. it was running FINE and this issue surfaced while riding??
Timing checked... advanced unit working proper?
 

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Discussion Starter · #15 ·
Just a quick update before I run out. Decided to start from the beginning. Adjusted my valves. 0.002 can fit in the intakes, 0.004 can't. 0.004 can fit in the exhaust, 0.006 can't. Lost my happy tapping. Going to go back in tomorrow and loosen them a smidge to get the tapping back. Wasn't a huge adjustment though, so nothing was terribly off here.

Carbs were removed and floats checked. All 4 slide smoothly, and the height was set at 24mm. I believe that's the high end of spec, so I adjusted them down to 22mm.

Went back to 20 pilots because 25 is just too rich, even if it's doing the lean wheeze at throttle. 1 1/2 turns out on both. Before, carbs were synched using the drill bit method and a 1/4 drill bit. Then dialed out about 1/2 a turn to drop idle from 1400 to 1200. They were still in sync, but repeated the process and brought it out 3/4 turn out to get the idle around 1100. Anything more and it's chugging.

Cables were adjusted and cable adjusters locked down. Both were moving at exactly the same time. Cables were nestled in their pockets. There's about an 1/8th turn of free play in the throttle before they start pulling. No change was necessary on these at all. Though I did pull on the cables today to try and make some extra room, and now it's binding a bit somewhere.

Popped in the 6f4 needle on the top clip to make a different attempt at richening the mix when giving throttle. Fully warmed up, it still does the slightest little sneeze when giving a quick twist, but is pretty responsive with a steady twist.

There's a Mikuni diagram that shows the effects of each component at each point in throttle. On it, between 0 and 1/8 throttle, the most important item is the air screw and pilot jet. But it also mentions an "I.M.S." being super influential at 0, dropping to no effect by 1/8 throttle.

Any clue what the I.M.S. might be?

Thanks,
John
 

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Idle metering system?
One thing to keep in mind is that opening the throttle too fast on a slide carb will slow the air speed to the point that it's pressure will not be low enough to pull fuel from the jets. The engine will then stumble. You can only roll the throttle as fast as the motor will let you.
 

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That's something I was going to mention... modern stuff or bikes with CV carbs all respond quickly and since standard slide carbs don't have accelerator pumps, I was curious as to how far and how quickly the throttle was being turned when it hesitates for him with respect to "...it still does the slightest little sneeze when giving a quick twist, but is pretty responsive with a steady twist"
 

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Discussion Starter · #18 ·
Idle metering system?
One thing to keep in mind is that opening the throttle too fast on a slide carb will slow the air speed to the point that it's pressure will not be low enough to pull fuel from the jets. The engine will then stumble. You can only roll the throttle as fast as the motor will let you.
Thanks, Lefty. I remember reading about that. When I talk about a quick twist, it's usually only to about 1/8 or 1/4. I guess it might count as a "blip". It used to be able to take it before.

I'm going to take a look at timing again tomorrow. I don't think it's carbs anymore. 6f4 needle was leaving the plugs black and sometimes wet. I ran through all the clip levels on a 6f5 and got it to pop from pretty much every part of the bike while accelerating; exhaust (2nd from bottom), carbs (2nd from top) and in the middle it sounded like it was popping from the headers, which I assume is pre-detonation? As a cold comfort, it still idles nicely.

Moving on to the electrical bits now. Will confirm timing tomorrow, just to see if it's out of whack, then take off points plate to check advancer and start over from the beginning.

Thanks everyone for your input,
John
 

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Discussion Starter · #19 ·
Checked timing, LF is dead on with strobe and moves with advancer, F is firing about 3mm to the left of the stator line. I'll get to that today. Does that mean it's advanced? Doesn't advanced timing give lean symptoms even if it's not?

New item popped up today. I've always had a three drop oil leak when I parked. I assumed it was from the gasket in the top half of the engine. There's a distinct line where it's dry above and wet below. The allen bolt under the spark plug had been loose as well. But while working with the points today, I noticed that the wires coming off the points were shiny with oil. I think the oil might be leaking from behind the points plate and everything seems to have a light skim of oil.

Would this mess up something enough to jack my timing?

Thanks,
John
 

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Probably not unless it gets on the points, but if it's coming from behind the advancer it's the seal on the end of the cam. Not a big deal to change. Eventually you'll want to get the timing correct on the right cylinder but for now, a small amount advanced like that isn't going to cause a problem
 
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