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Discussion Starter #1
I have been searching on this forum and ran down a few ideas about what could be causing this, but haven't found the issue yet.

Forgive me, this will be a long diatribe, but want to make sure I give all the info:

71 CB 350 - bought non-running a few months ago. Compression test showed R-cylinder around 130 and L-cylinder at around 145 (not perfect, but not horrible for a bike sitting for about a year at least). I did valve adjust, new condenser, carb rebuild, new petcock, and then gave it a try, bike started 1st or 2nd kick, idled low and smooth, and ran perfect.

A few weeks ago, I started noticing it "breaking up" at around 7500 RPM. Acting like points were arcing or it was fuel starvation. It was minor, but consistent, and only in 2nd gear or higher. I could run through 1st gear to redline with no issues.
Around this time it started being a 4 or 5 kick bike to get it started (it has electric start, but I like kicking it). If I used starter it still spun several times before starting.

So at this point I did swap out the points with a brand new set I had in the box, (old ones were the ones that were in the bike when I got it, and were dull and slightly pitted, but I've seen worse) BUT, that definitely solved the starting issue - back to one kick and then the thing roars to life, and idles perfect, but still breaking up at 7500

So I double-checked cam chain tension, valves, timing - all good and no change in performance. Because it wasn't breaking up in 1st gear, and seemed to be worse at higher speeds at 7500, I started thinking it was fuel starvation, even though both plugs were slightly black (indicating rich) but it still seemed odd that it had good power and revs through 1st, but after that 7500 would cause sputter (I thought that fuel bowls were still full through 1st, but were at lower level past that and could not keep up with consumption).

I pulled off the left carb, just to peek inside (all seemed pretty right) so I put it back on and then problem seemed to be worse. Breaking up as low as 6500 RPM but, again, only at higher speed, always coming off the line fine and up to redline just fine.

So now I was stumped, but because it was worse after removing a carb, I thought I was on the right track (being a carb issue) so I took both carbs off and found float height to be about 29mm on both - AHA! (I thought) that would definitely explain it. Bowls are full at the stoplight, I take off, plenty of fuel up to redline, but by 2nd gear bowls were low and consumption was greater than flow, VOILA! That must be it.

I set floats to 23mm (per video from Common-Motor Collective) and slapped carbs back on. Fired right up, but then ran like dog****. Idles and pulls away ok, but then sputtering and coughing and generally ****ty. In order to rule out the tank not venting, I popped the gas cap open and rode it, no difference.

So I figure I must be in the right place, because messing with the carbs is definitely affecting performance (whereas points, timing and valve adjusting had no effect). So last night I pulled a fuel line off the (new) petcock and it was barely dribbling out into my gas can. I pulled the other line off, and started seeing that thing in the video where fuel was only coming out of one hose at a time, and sucking air in from the other carb supply line (there's a post in this forum with a video of a guy talking about how he blocked off one carb supply line and put t T into the other to "fix" this???)

So aside from the fact that the (new) plugs are slightly black, (but with a nice tan ceramic center) I am still thinking this thing is running out of fuel in the bowls, but I can't believe the fix would be to put a T on a single supply line past the (new) petcock - I mean, there's a half million of these bikes that have duel supply lines, right?

Summary:
Vales/cam chain set and adjusted
new points/condenser
new plugs
new wire end caps
(coils are Dyno coils that were on the bike when I bought it, with new wires)
timing is "balls on"
new petcock
rebuilt carbs
floats adjusted to 23mm height

But still sputtering and breaking up past 7500 in 2nd gear or higher.

Anyone have ANY ideas?
 

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I'd do a plug chop in a higher gear right at the point it starts running poorly. If you don't kill the engine and coast to a stop (proper plug chop), you won't know what the mixture is at the rpm range where it runs poorly. Sounds like it might be a bit rich at that range... too lean and it just runs flat
 

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Set float height to 26mm, I think it's a vibration and too rich issue. Have you fitted 'pods'?
 

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Discussion Starter #4
"Plug Chop"? Huh, I've been around a lot of years but had bever heard of that, will try it - good idea, thanks!

And you say 26mm float level? It was around 29mm before I set to 23mm so maybe I went too far, I will set to 26mm tonight. - and no, I am not running pods, just the stock aircleaners (not new, but not bad at all).

Thanks guys!
 

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Set the floats to 26mm before you explore other possible causes. Have you checked the strainer in the petcock for blockage? Are the carb diaphragms in good shape? How about the advance unit behind the points? Is the battery good and fully charged?
 

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In my experience the 7.5k missfire is due to float level, motor gets too rich to run
 
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The air cleaners may not be as good as they look. Take it for a ride, with them removed, and see if that makes a difference.
 

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You didn't say what version carb you have.

Here are the spec's for the different models:


Carb model 350A eng ser# 1000001- 1045164
Float level 19.mm
Std main jet #60 primary #115 secondary
Std slow jet #38
Idle mixture screw initial setting 3/4 turns from closed
Idle speed 1200rpm.

Carb model 3B & 3C eng ser # 1045165- 1065278
Float level 21.mm
Std main jet #70 primary #110 secondary
Std slow jet #35
Idle mixture screw initial setting 1 turn from closed
Idle speed 1200rpm

Carb model 3D, 722A, 726A, 728A eng ser# 1065279 and subsequent
Float level 26.mm
Std main jet #68 primary (722A,726A) #70 primary (3D,728A)
Scondary #70 (3D,728A), #105 (3D, 722A, 726A)
Std slow jet#35
Idle mixture screw initial setting:
3/4 turn from closed (3D)
1 1/8 turns from closed (722A, 726A, 728A)
Idle speed 1200rpm.

Other trivial data:
NGK: B 8ES/ ND: W24ES Gap .028-.032in
Ign point gap .012-.016in
Ign Advance "F" 10º BTDC
Max advance 37º- 43º BTDC
Advance starts @ 1300rpm
Max advance 3900rpm
Dwell angle 105º (on 2 cylinder scale)
Condenser capacity 0.22 - 0.26 MFD
 

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Discussion Starter #9
Set the floats to 26mm before you explore other possible causes. Have you checked the strainer in the petcock for blockage? Are the carb diaphragms in good shape? How about the advance unit behind the points? Is the battery good and fully charged?
I did just reset the floats to 26mm and It seems to be a LOT better! I haven't had a chance to really get it out on the highway yet (gonna do that this morning) but around the neighborhood the breakup seems to have all but disappeared, maybe just a slight crackaling . . .

I did check the petcock and is new and clear, and the carb diaphrams are in excellent condition (I double-checked). I did disassemble, clean and lubricate the advance unit and the battery is new, and I have recently installed a new regulator/rectifier and have a solid 14.5 volts at the battery.

It looks like I am on the right track with float level (thanks to you all here on this forum) so I will keep tweaking if it's still off after my ride this morning.

Awesome forum, thanks guys!
 

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Discussion Starter #10
The air cleaners may not be as good as they look. Take it for a ride, with them removed, and see if that makes a difference.
Excellent point Mike, I'll do that and I might just go ahead and replace them (but, ouch, they're $65 ea!)

But, things cost what they cost I guess . . .
 

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Worked on a Black Bomber once that ran strong up to 8500 rpm, then went dead on one cylinder above that. The bike was set up for scrambles racing w/straight pipes and stock air cleaners. None of the usual fixes made any difference, finally pulled off the air cleaners, and tried it out. Without the air cleaners it pulled strong to ten grand. Some strange resonance thing, I guess, interconnected air filters and a 180 degree crank. Enough air through the dirty filters to feed one cylinder but not both.
edit: We didn't replace the filters, ripped the paper off and replaced it with oiled foam rubber, the rebuilt filters worked as good(maybe better) as new paper ones.
 

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You can repair them with Uni filter foam or cut up a 1970's large round filter for v-8 or similar. It works about as well as new filters for about 1/10 cost
 

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Discussion Starter #13
You didn't say what version carb you have.

Here are the spec's for the different models:


Carb model 350A eng ser# 1000001- 1045164
Float level 19.mm
Std main jet #60 primary #115 secondary
Std slow jet #38
Idle mixture screw initial setting 3/4 turns from closed
Idle speed 1200rpm.

Carb model 3B & 3C eng ser # 1045165- 1065278
Float level 21.mm
Std main jet #70 primary #110 secondary
Std slow jet #35
Idle mixture screw initial setting 1 turn from closed
Idle speed 1200rpm

Carb model 3D, 722A, 726A, 728A eng ser# 1065279 and subsequent
Float level 26.mm
Std main jet #68 primary (722A,726A) #70 primary (3D,728A)
Scondary #70 (3D,728A), #105 (3D, 722A, 726A)
Std slow jet#35
Idle mixture screw initial setting:
3/4 turn from closed (3D)
1 1/8 turns from closed (722A, 726A, 728A)
Idle speed 1200rpm.

Other trivial data:
NGK: B 8ES/ ND: W24ES Gap .028-.032in
Ign point gap .012-.016in
Ign Advance "F" 10º BTDC
Max advance 37º- 43º BTDC
Advance starts @ 1300rpm
Max advance 3900rpm
Dwell angle 105º (on 2 cylinder scale)
Condenser capacity 0.22 - 0.26 MFD
BOOM!

Yender, thanks for that info.

The secondary jets that were in the carbs were 115's (and when I rebuilt the carbs I replaced the old 115's with new 115's) but my carb kits also had 105's so yesterday I switched them and VOILA!

Totally fixed!

Runs super smooth and strong all the way to redline.

Thanks guys!
 

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it was too rich then?
 

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Discussion Starter #15
it was too rich then?
Yup, I guess I can't tell the different between too lean and too rich (cuz I'm a dummy) butapparently the larger jets were simply allowing too much fuel at higher speeds when the metering rod was lifted up.

It would just sort of strain and gargle at 7500 RPM but now it's clean (and much stronger) all the way to redline.

I do seem to notice a slight decrease in power in the mid-range compared to what it had with the old jets, but I guess that makes sense . . . the larger jets would have been delivering more fuel in the mid-range than the smaller ones do.

I don't think it's a problem though, just something I noticed.

Thanks all!
 
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