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Discussion Starter · #1 ·
I'm at a loss as to figuring out what would cause this.

On every bike I've owned so far, when I enable the choke, the engine revs higher.

On my 71 CB350, when I enable the choke (closing the choke plates), the engine stalls.

I seem to get the best RPM (lowest, most normal rpm) with the choke plates 1 notch away from totally closed.

If I open the choke plates all the way, I get outstandingly high revs, but amazing fast performance.


What would cause the bike to stall with the choke plates fully closed?
 

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Discussion Starter · #3 ·
is that tongue-in-cheek or serious? :?

on the bright side, I took the clutch-side carb off and reset the float to 26mm instead of 10mm where it was... getting the other side off is a real pain in the ass though, since that stupid choke-sync lever is IMPOSSIBLE to get to without completely removing the tank...

~A
 

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Discussion Starter · #4 ·
I guess my real question is, why does the choke position appear to work the complete opposite of every normal bike? Shouldn't opening the choke plates LOWER the rpms?

if not - what does it mean is mis-adjusted? my idle speeds or my air/fuel screws?
 

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slickware said:
is that tongue-in-cheek or serious? :?

~A

See any smileys??

I don't know about "normal" bikes, just old Hondas, and they're supposed to run with NO CHOKE (call it open/closed, whatever you want).
So a 350 should falter and die when choke is applied (assuming it's warmed up).

If the bike runs better with choke applied, then you have a lean condition - maybe attributable to your low float levels, maybe something else.
On a 350 the choke is not involved with idle speed in any direct fashion. The chokes on these old bikes are just what they appear to be, a simple butterfly flapper thingy, no connection to the throttle at all.
Chokes that simultaneously adjusted the idle speed upwards are a later introduction.
 

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Sensei
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Could be both, but I'd bet the idle screws are to far "in" (throttle plate open too far)...With a CV carb, as you allow more air in (through) you get a higher vacuum differential and that draws the slide up (revs the bike)...The slide will continue to open until the mixture flow through the throttle plate is exactly matched by the incoming flow...... With a slide valve carb, the choke shuts or opens the air relative to open venturi size as well, but the slide opening stays constant.... Modern bikes generally have enricheners, not chokes....effect is the same/what's actually happening isn't...
 

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Discussion Starter · #7 ·
At the moment, here's what I've got:

Throttle cable screws - are almost all the way out, maximum tension on throttle cable. If I loosen these up at all, I can't keep the revs high enough to keep the bike running regardless of any other position.

Air/fuel mix screws - are almost all the way in. Maybe 1/4 turn open.

Choke - full open.

Bike - warm.

Idle screws - these are the idle-stop-screws near the top of the carbs - I play with these and end up adjusting them so they have the idle set almost as high as possible, and the bike still stalls.


Sounds like I might have to go back in and reset that float to around 19mm or so?

~A
 

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Sensei
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There's some of your problems.... Float height for a '71 350 is 26mm....Air Mix screw is one full turn plus or minus 1/8...... Throttle cables should go/be slightly slack, and Must be balanced for "start of pull" (synched)..... It may work better at factory settings than by guesswork.....LOL.....

Sorry, my amusement wasn't aimed directly AT you, and I'm NOT trying to be a hardass or ridicule anyone.... Modern riders tend to have been spoiled by modern equipment and often don't realize how "finicky" the older models can be.....You almost HAVE to start out at factory specs and adjust/tinker from there.....
 

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Discussion Starter · #9 ·
I tried finding factory settings, but the haynes manual is specific by engine number, which I can't find on my engine. I found the VIN (which is like trying to read a magic-eye painting with that damn checkered background) but not the engine #...

also I couldn't find any factory settings for the air screws or the throttle cable / idle screws.

I'll attempt the full turn and slacking the cables out a bit.

Is there ONE idle-adjuster screw on this bike? I'm used to having one screw somewhere down near the center of the crankcase (where it's impossible to reach without burning your whole arm).

~A
 

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Sensei
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The VIN stmped on the engine checkerboard (actually engine serial number) IS the engine number..... (last 7 digits)
Nope, one Idle screw for each carb.....
Adjust cables until both start to pull simultaneously but still go slightly slack
(NO specific number of turns), Idle screws are adjusted slightly until pressure pulses out the exhaust are even in pressure/force at the desired idle RPM's.....NOT high tech, but it works....
 

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Go to the link below and read the section on Carb Setup - it's for a 450, but the same procedure for a 350.
Like Steve said, your mixture screws are way off, and your floats too.....
If you back the idle adjust screws all the way "out", the bike won't idle...

http://home.comcast.net/~tbpmusic6/450site.htm
 

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Discussion Starter · #12 ·
Fantastic... it looks much better now (At least the throttle adjustment). As soon as the little dude wakes up from his nap I'll see if I can get it idling :)
 

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Discussion Starter · #13 ·
Alright. In case anyone is still watching this thread, here's where I got.

I started from scratch like someone suggested:

-throttle cable tension adjusters: slack
-idle adjuster screws just out enough so they aren't touching
-air screws out 1 full turn

Started the bike with choke, refused to start. It would occasionally try to turn over once, cough, and die. If I cranked the throttle it sounded more promising, but I mean really cranking it. So I decided it was the throttle tension adjusters.

I upped the tension on the throttle cable significantly and got the two swivels moving at the same time again with more tension.
(I also changed the disgusting black spark plugs, probably from me running the bike on choke for so long).

This time the bike starts right up. I let it warm up and removed choke, still idles fine. Adjusted the idle speed around 1100-1300rpm, bumped throttle and got decent sounding response. Felt tailpipes, even pressure coming out of both - HOWEVER - one of them is definitely blowing oil smoke-- and it's NOT the one that I just reset the float level on.

Regardless, the throttle response and idle seemed good, so I took it for a ride.

NO power under any sort of torque (up a small hill or under reasonable acceleration). Stalled trying to go up my driveway which is barely enough incline to roll a basketball down. Also, the non-smoking pipe was making a sort of gurgle-gurgle-gurgle-POP-gurgle sound, and the other one (the oil smoke one) sounded exceptionally brassy/rattly. Not sure how to describe it - maybe like a pipe with no baffles or something.

So... now what? I didn't adjust the air screws, they're still about 1 turn open. Can I bring back any power and stop the popping sounds by just messing with these, or do I really just need to open both carbs and reset them and possibly replace some internals?

~Adam
 

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Sensei
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OK... You "cheated" and put some tension on the throttle cables (That's OK, but after you got it to idle at 1100, you should have turned them back enough that they both go slack, but are still in sync as far as opening simultaneously)

NOW is the time to "tweak" the mixture screws to achieve the highest idles...(it should be somewhere in the 1 plus or minus 1/8 turn range)... You may have to adjust the idle screws slightly to compensate and keep the idle in the 1100-1200 range, and the exhaust pressures equal..... Then re-verify the cable/opening sync is still good (adjust if/as necessary)

You say you have good idle and throttle response sitting, but lack of power riding.... That poses two questions:

Did you set the valves?......

WHAT are your compression readings?......

With the odd noise and slight smoking, either or both could be a factor......
 

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Discussion Starter · #15 ·
I don't have a compression tester and I don't really know what "set the valves" means :)

Can I buy a standard one from an autoparts store, or are there motorcycle-specific compression testers?


This is the really annoying/weird part.

What started this whole thing was dropping the bike on the clutch side. After that, the bike wouldn't idle without choke and that-side muffler was smoking, so I took apart and cleaned/reset floats on the clutch-side carb (the side it fell on).

Now, the clutch-side muffler looks fine, but the brake-side muffler is smoking - and it wasn't, prior to the cleaning/refloating of the OTHER carb.

It's like I moved the issue to the other side without touching anything there!
~Adam
 

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Sensei
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Adam..... "Set the valves" means adjusting the cold clearances between valves and rockers (tappets)..... IF you have not done that, we'll ASSUME they are close enough for now, but you'll eventually have to learn how to do that (and current misadjustments will show in the compression tests)......
An inexpensive "push in/on" compression gage will do for now, however, good tools are useful forever and if you intend to have more bikes are a good investment..... I got a set with multiple screw-in (plug size) adapters at Sears for around $50 (years ago)...
SOME auto-parts stores will lend or rent you this tool......
Read, ask, and understand how to do this test as improperly done results are worthless...... Engine should be hot (at least warmed-up), both plugs out, choke open (not choked, the airflow is required), Throttle wide open (airflow again).....Take and record readings..... repeat after adding 1/2 teaspoon of oil in through each plug hole... Take and record readings....The original readings and the differences in the "oiled' readings will allow us to determine the basic condition of your engine... Steve
 

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Discussion Starter · #17 ·
I just moved to Dallas and there is a Harbor Freight 3 miles from me... I'll probably pick up a tester this afternoon if they have any in stock. Looks like they have 2 multiple-adapter sets fro around $25.
 

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Discussion Starter · #18 ·
bad (?) news.

Sides are listed as sitting on the bike.

Left side:

120psi unoiled,
120psi oiled


Right Side:
90-95psi unoiled
120psi oiled

I'd like to run the bike for a bit and get that oil out of the right side and do another test to see if maybe I did something wrong the first time, but assuming these readings are correct, what would next-steps be?

~A
 

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On the left side the oil never made any difference to the compression so this may rule out the rings on this side. Sometimes this is caused by a slightly open valve. Have you checked and adjusted your valve clearances? If not, do it before trying to start again as they need to be checked cold, they may be tight and throwing your readings off.
 

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Perry already specified that you need to check and perhaps adjust the valves...Please follow the manual proceedure exactly, If in doubt, ASK... It is real easy to damage something if you do it incorrectly.......Once you do that and repeat the tests, be advised that 170 PSI is the normal reading, with a rebuild/re-ring due at around 155 PSI... The engine will run below these pressures, but won't produce much usable HP......
 
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