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Discussion Starter #1
72 cb350. Long journey of chasing carb and electrical/timing settings to get rid of a little balky "pop" that comes at 1/8 throttle, between 5-6K rpm. Having tried different pilots, mains, needles, needle settings, air screw turns, and even slides, with no success, I turned to electrical. Recognizing a wonky advancer and compensating for it, the problem remains. Points, Coils and condenser, caps are newer. (all changed in last few years, with very little miles on them)

From what I've read, I've thought it was a lean pop; either because of jetting or advanced timing. (no leaks, and doesn't give symptoms of leaks.) But having turned things crazy rich in the past, hasn't solved the problem.

So as a stretch, I come to my shady exhaust system. PO seems to have cut an inch or so off the headers. These are connected with expanders to cheapo 12" mufflers. I've noticed when running Seafoam, that some of the smoke show came out of the joint between the headers and mufflers. I've since tried sealing those gaps with red RTV.

People write about the necessary length of the exhaust, and how short pipes wreck performance. I've read about the exhaust pulses, and how if the pulse doesn't clear out the cylinder, it will "8 stroke". Does that sound like what's happening on my bike? It's almost like a little pop and a misfire. Like an intermittent stutter.

Is there anything I can try in the quick short term, to see if this is my problem, and to do a quick fix? I postponed my motorcycle test last year because the bike didn't feel reliable, but it's coming up next week and isn't postponable any further. Was planning on taking off the exhaust right to the gaskets, and making sure it's all sealed up. (I've done it before, gaskets are new.) Any suggestions besides "get a new exhaust" would be awesome. Live in a small town and just checked craigslist for the nearest 100 miles. Nothing in the area.

Thanks a ton,
John
 

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You didn't mention the ******* exhaust system in your original posts. Check this out: Pipe Physics
 

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8 stroking is when a firing stroke is missed due to excess richness, so it's like slowly turning the choke on until it starts to blubber
 

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Discussion Starter #4
You didn't mention the ******* exhaust system in your original posts. Check this out: Pipe Physics
Sorry about that. After all the carb work I put in last year, I really had my eggs in the "bad advancer" basket, and wasn't thinking about the exhaust. Little stupid things I'd dismissed previously, start to pop up as possibly important as I do more research.

Last year, I repacked the baffles with more fiberglass. In a 12" pipe, there's only about 8" of baffle, but hopefully that's enough to deal with the reversion pulse in your article? And then the last guy says that double carbing ISN'T an issue. Not knowing either of those guys, I don't know who to believe. But if either of them had a "hey, go in the garage and try this out" suggestion, at this point, I'd do it.
 

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Discussion Starter #5
8 stroking is when a firing stroke is missed due to excess richness, so it's like slowly turning the choke on until it starts to blubber
Now, is there potential harm in this? Someone put the fear of "holing a piston" in me when I thought it was a lean pop. But now that I'm fairly certain it's not a lean symptom, is it okay to run it like this without damaging the engine?

I've read two different schools of thought on 8 stroking; one being a rich condition, and the other being an exhaust dynamics condition. I've run it lean to rich and it's always done this. So, if it is an exhaust condition, is there something I can try to remedy it?
 

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I can't speak to the exhaust dynamics condition causing 8 stroking, but a rich condition can certainly cause it. Since you mention it has always done what it's doing, then it may well be carburetion (I guess the opposite of "half of carburetion problems are electrical")... that said, the only way it causes harm is to continuously run the engine too rich long term (aside from going through fouled plugs) which can result in advanced piston and ring wear from the excess fuel in every firing stroke helping wash away some lubrication, but it wouldn't be that much and is pretty rare that it causes any real, significant wear. Too lean, holing a piston? Absolutely a possibility
 

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Discussion Starter #7
Ancientdad, I came across a thread where you mention a "breaker cam mod" you've done to increase dwell and avoid a misfire. Can you tell me about that?
 

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It's basically trimming off some of the lobe that opens the points, taking the material off the closing side of the lobe so the points close sooner and are closed longer during the rotation. The opening point of the lobe doesn't change so you still set the timing the same, but the lobe isn't as long (from well over 180° - in fact, close to 270° - to not a lot more than about 90° of rotation) so the points stay closed longer and build a stronger spark in the coils. It's really only necessary at high revs, and since my engine has aftermarket cams and turns 1000+ rpm higher than stock redline, the points can float at rpms higher than stock redline and it also helps make a stronger spark at the high end of the rev range. I don't expect something like that to be a solution to what your engine is doing, though
 

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Discussion Starter #9
Okay, thanks. I'm thinking about my other thread, and still trying to figure out how to open one points gap while closing the other. Every time I changed the gaps, they moved in tandem somewhat. If the opening point is the same, then you're right, this probably isn't where I should be looking.
 

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Take a look at your spark plugs. I agree with Tom about the 8 stroking cause. If the plugs are black, then it's certainly a possibility.
 

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When the bike is popping, is it with a warm engine?
Choke flap (on both sides) up or down?
Carb pistons are sliding freely on both sides?
Doesn't happen at lower rpm?
 

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I'm having a hard time understanding "1/8 throttle, between 5-6K RPM". What does this mean?

Also, you didn't mention if you have synchronized the carbs after all of the work that you did to them. Was this done also?
 

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Discussion Starter #13
Take a look at your spark plugs. I agree with Tom about the 8 stroking cause. If the plugs are black, then it's certainly a possibility.
20190514_202143.jpg

Took it out for a 20 mile/40 minute ride through town, starting with fresh plugs. These are how they look at the end. Shows a bit rich. I'm finding if I keep the rpm low (5th gear doing 40mph - about 4k rpm) I can avoid most of the popping. As soon as I bring it up 5-6K, it's popping again. The fact that it's rpm dependent, and not throttle dependant makes me think it's more electrical than carbs?
 

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Discussion Starter #14
When the bike is popping, is it with a warm engine?
Choke flap (on both sides) up or down?
Carb pistons are sliding freely on both sides?
Doesn't happen at lower rpm?
The popping happens when fully warm. I can usually get away with the first few miles with no popping.
Both choke flaps are up. (vm30's)
Both carb slides move freely and are synced together.
On this last ride, I tried to keep the rpm around 3-4000 and managed to avoid most of the popping. Still a few, but nothing like what happens when I keep it between 5-6K.
 

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Discussion Starter #15
I'm having a hard time understanding "1/8 throttle, between 5-6K RPM". What does this mean?

Also, you didn't mention if you have synchronized the carbs after all of the work that you did to them. Was this done also?
I have pieces of tape on my throttle to let me know where I'm at, since carb circuits work on throttle position. I was surprised to find I'm a pansy and live most of my life between 0 and 1/4 throttle. When I'm cruising at 50mph, I find the throttle is usually about 1/8 open (or a little less), and the engine is spinning about 5-6000 rpm.

For synchronizing, two wooden sticks to make sure that both carbs are sliding at exactly the same time, and the drill bit to make sure they're at the same opening, then I think a half turn out or so, to get the idle around 1200.
 

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VM30's don't have chokes, They have enricheners...... IF the rubber shut-off seals in the enrichener leak past, that alone could be your problem
 

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Discussion Starter #17
Got it to run well enough to take the class and pass the test. Almost 6 hours of running yesterday. Took the plugs off after 5 hours and they looked like this:
20190520_080356.jpg

Had to lug it at 3500-4000 rpm when cruising through town to avoid the popping. Seems to get progressively better as the gears go up; awful in 2nd, almost bearable in 4th at 5000, and doesn't seem to pop at all in 5th.

But now that the test is passed, and it's proven itself to be somewhat rideable, the pressure feels like it's gone. I'll ride it as is and save up my nickels for a real exhaust. Thanks everyone for your help.
 
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