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Discussion Starter · #1 ·
Hey everyone, I feel closer to getting this roadworthy than I've been in 5 years. Pardon some of the silly questions, I'm a new wrencher and have little frame of reference to know what's normal.

I have a 72 CB350, with VM30's on it. It has the generic short pipes on it.

I started with 150 mains and 25 pilot jets in it. While setting the idle, it was coming back rich, so I switched the pilots for 20's. I did the idle test wrong and didn't warm up the engine before swapping plugs. Doing it the wrong way, my plugs were nice and toast coloured after idling for 5 minutes from a COLD engine, with the 20's. (not sure how important this is)

I don't have a place I feel comfortable doing a plug chop and the bike hasn't been reliable enough to go far. But after a ten minute scoot, I'd come back and both plugs would be black, with the bike dying at idle at the stop signs. I stepped the mains down by 5's until I finally got to 125 and thought it might be a bit much. I read that the cheap pod filters restrict too much air, and ordered some uni filters.

Today, I set the tensioner, valves and points, and put on the Uni filters. It still had the 125 mains and 20 pilots, with 6f4 needle on the top clip. (Air screw 1 1/4 turns out) Once it started, it idled nicely at about 1100-1200 rpm. I took it around the neighbourhood a few times and it seemed to be running well. Brought it in the garage, swapped plugs and let it idle for 5 minutes. By the end of the 5 minutes, it was chugging at times and sounding like it was going to die, but kept going.

Right plug is black and sooty and left plug is white like new. Both pipes seem equally hot. Is this an indicator of something? Is it related to the dying at idle?

Any ideas greatly appreciated.

Thanks,
John
 

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While I can't offer any help at the moment, I would be very interested in how this thread develops. I've been working on my 72 350 for nearly 2 years (and haven't ridden it) and am having similar problems as you.
 

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Off topic but where in Ontario are you? I am in St. Catharines.
 

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I know if you do jetting changes that it's best to drive the bike hard and then shut it off without letting it idle;they don't charge while idling and you should make sure your battery is fully charged also.You may need to go a little smaller on the main jets,imo.. Have you considered taking it to a Cycleologist ? lol :)
 

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Discussion Starter · #5 ·
I'm in a little town just outside of Windsor. I seem to have run into a few people who thought they were cycleologists, but none that gave results. In the end, it's been me doing and re-doing and re-doing a process until I unknowingly fix a mistake I've unknowingly made. I could really use some help with diagnosis to save on the number of times I've taken these carbs off the bike.


But that being said, I have it all off the bike again, even the gas tank. Something was pinching the cable and my right slide wouldn't drop all the way. Swapped in a 6f5 needle, cleaned up all the bits, checked the float levels. Gonna button her up again and try again. I keep the battery on the tender when I'm not riding. (so always). How low is the lowest jets people have put into these things? I rarely see anyone mentioning anything below 20 pilots and 135 mains. I know each bike is different, but being on 20 pilots and 125 mains and having to go lower is starting to make me wonder that there's more to it.

I've searched and searched the threads and half of the time, things are either unresolved, or resolved for some magical reason. Here's hoping for some magic.
 

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Discussion Starter · #7 · (Edited)
Hey Yendor,

Yes, I've been through the sticky and it was super helpful for processes about synching and dialing in pilots. I look forward to posting in there when I get my own dialed in. It's also where I got the idea that dropping lower than 125 on mains or 20 on pilots seemed out of the ordinary. No one lists the lower jets unless they're working with vm32's.

So I buttoned everything back up again and took it out for a spin. Once warmed up, it was idling at about 1800, but otherwise everything felt pretty good. I'm just scooting around the neighbourhood, so I find myself shifting at about 5-6000. After a few laps around the block, brought it into the garage and adjusted the air screws then adjusted the idle down to 1200. The air screws don't seem to change anything after 1 1/4 turns out, but start dropping rpms if I turn it to only 1 turn out. So I left it at 1 1/4. I think I did that right?

Swapped in new plugs and let it idle for about 7 minutes. This is what came out:

Auto part
Auto part Hose


I think I remember reading somewhere that you can tune each carb separately? Does that mean I should turn out the air screw 1/2 a turn, just on my right carb? Or is this a symptom of something bigger that I should be looking at?

Thanks,
John
 

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Discussion Starter · #8 ·
Turned the rich carb's air screw out 1/4 turn and took it out for a ride to warm it up. Everything felt really good, so I kept going beyond the neighbourhood. There were no stutters and everything pulled smoothly to 7-8000 rpm where I shifted. Cruising at 50 or 60 mph, it was at about 6000rpm.

It made little sounds like somebody tapping a bongo at a rhythmic pace while cruising, but it wasn't noticeable while accelerating. Like a faint hollow "pop", which didn't create any noticeable jerk in momentum. I don't think this counts as a "backfire", does it? I've had this thing backfire, and it was a pretty pronounced "bang".

There was also a little metallic rattling which I was hoping was something cosmetic. I got home and noticed the cap/washer on top of the frame neck were loose. I'm hoping it was the washer rattling which made the noise. As I was cleaning and tightening them, I noticed this:

Auto part Metal Steel Machine Engine


The triple tree is cracked! Thank god for parts bikes, but I've been spending the day cleaning and painting parts to get this mended. Now that I know it will go some distance without dying, looking forward to doing some plug chops soon.

Any input or reassurance about the popping sound or different air screw turns on the carburetors in the meantime would be appreciated.

Thanks,
John
 

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Discussion Starter · #10 ·
Ah yes, #8. Missing on both sides of my bike, but present on both sides of the parts bike. Damn PO's. Nice part about the parts bike is that it was so unloved it's fairly untouched.

All buttoned up, with an extra washer above the rubber. It solved an annoying little click coming from handlebar movement. (Thanks Mydlyfkryzis? I think that was one of his suggestions.) That, plus the tightened cap fixes up the triple tree and hopefully gets rid of the metallic rattling.

A day of distraction. Hope to get back to the carbs tomorrow.
 

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That Metallic Rattle MIGHT be Pinging - Pre-Ignition - Detonation typically caused by over advanced timing.

Be aware that will cause a LEAN burn frequently resulting in a Burned and Holed Piston.

Check your timing before your next ride.
Some of the posts from people who suffered motor failures from this issue, talk about very short time frames for the failure to occur.
 

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Discussion Starter · #12 ·
Thanks Yendor. I checked and rechecked static timing, but have never checked with a timing light. The advancer springs also look good and move smoothly when I have it in my hands.

But new problem. Got on today and kickstarted the bike. It made a weird metallic sound on the kick, but started up easily and idled at 1300. I put my hands behind each tailpipe and notice the one is nice and even, but the other does bigger poofs irregularly. I rolled on the gas and it revved to 2500 smoothly. I gave it quick gas and it made an awful metal snapping sound as it revved, but as soon as I let go, it went back to a smooth idle.

Scared to start it again, hoping for ideas.

Thanks,
John
 

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Discussion Starter · #13 ·
Looking through some videos, the snapping sound might be a backfire? I've had it do deceleration backfires on me, which make that gunshotty kind of sound, but this is the first time it's done an acceleration backfire. Do they sound different? Is the acceleration backfire more snappy?
 

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Discussion Starter · #14 ·
Saw this video on dotheton, where the guy is getting backfires. He complains about the kicker "binding up", which mine does too. ( I just thought it was a normal thing every now and then.) Says it's a symptom of advanced timing. So with that symptom, and Yendor's suggestion, I went back and checked the timing again. It is *spot* on. Line on line. Does it ever have to be set a bit retarded if the symptoms show advanced?
 

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Discussion Starter · #15 ·
For future newbs and to close this thread out,

I put two pieces of tape; one on my throttle and one on the fixed portion of my handlebar to mark off 0, 1/4, 1/2 and WOT. Taking it around the neighbourhood and up the street, I found that I barely ever left the neighbourhood of 1/4. Which explains why swapping my mains had no effect, but leaning out my needle to the 6f5 made everything nice. My plugs were a little too white when I had it in the middle clip, so I moved it to the lowest and it was stuttering all over the place around 5000. Moved it to the second from the bottom and it seems to be doing well. A couple very minor stutters if I try to keep it steady between 5 and 6000, but I read that might be because of the short, cheap mufflers.

So, with 20 pilots, 140 mains, 6f5 needle on the 2nd bottom clip, air screw 1 1/2 turns out, this is what the plugs look like. (New plugs, 5 minutes idling after engine was already warmed up)

Auto part

The left are still leaner, so I've turned that air screw to 1 1/4. Took it out again and it's running fine. But I'm out of plugs for now. Will get more, then try some chops and post on the sticky page. Though after finding out I definitely don't ride like I stole it, I'm not sure if I'll ever get to the WOT chop.

Also, for those as new as me, who've only heard distant backfires, or your own with your helmet on going down the street, they sound crazy different with no helmet and no wind in the confines of your garage. I swore metal shards were going to be involved.
 
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