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1962 Bonny carb tune #771044
04/14/19 8:54 pm
04/14/19 8:54 pm
Joined: Jun 2018
Posts: 28
San Mateo CA
S
scratchedtank Offline OP
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scratchedtank  Offline OP
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Joined: Jun 2018
Posts: 28
San Mateo CA
I was riding out on my freshly built Bonny and made if half the way to
Alice’s on Skyline when she lost power and stopped
Couldn’t restart her so had to get my truck
Would restart on return home but not idle
I checked everything snd think The cause was a tight
Inlet valve tappet
Of course I messed up the carbs and now seem to be increasingly frustrated with them
Where is a good place to start with the mixture/slow running jet screw
And the throttle slide adjusting screw
Anyone got a good method to set them up?


62 Bonny
68 Shooting Star
2000 Enfield Bullet
Honda 250 CRF
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Re: 1962 Bonny carb tune [Re: scratchedtank] #771045
04/14/19 9:26 pm
04/14/19 9:26 pm
Joined: Nov 2012
Posts: 1,005
Isle of Wight, UK
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koan58 Online content
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Isle of Wight, UK
It would help to be more informative regarding how many miles covered, doing what with speed, rpm, gears and hills etc.
And whether it is the first run of new bores/pistons. Also, Monoblocs, magneto etc?

I am doubtful that one tight valve would stop the engine in such a way. Did it gradually lose power over several miles, or was it more sudden? I am obviously wondering about the possibility of a seizure (I hope not).
It's also unlikely that carb synch/idle settings brought this about, I assume it was running fairly well before embarking on the ride?
The procedure for setting twin carbs in any manual will be satisfactory to eliminate carbs as the problem.

Are you confident of your ignition timing? and the oil pressure?

Re: 1962 Bonny carb tune [Re: scratchedtank] #771047
04/14/19 9:38 pm
04/14/19 9:38 pm
Joined: Jun 2007
Posts: 6,330
Maui Hawaii
HawaiianTiger Online content
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HawaiianTiger  Online Content
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Maui Hawaii
First off, go with the recommended set up for your carbs. ie. slide, jets, needle jets, needle and pilot.
Then, set the idle adjustment screw at 2 turns out from bottom. (1 1/2 is for Concentric carb.)
Then turn in the throttle stop screw until it hits the slide, then one full turn in. This may be too fast or too slow, but without the slide off the bottom your bike will probably be hard to start.
Get the settings close by ear, then get the bike warmed up. The technique in the book works well enough. Get the slide as low as it will go and still idle properly. These bike have too much carb for the engine size in my opinion. So, what can happen is that if you grab too much throttle too fast, the engine will spit back and die. Slightly rich idle settings help with this a little.

All this is done after the carbs are properly synced.

I like to check the heat of the pipes, or exhaust gas heat when tuning for low speed jetting. It's a good indication of where you're at. I use a Laser thermometer for this. Exhaust pulse strength is also a good indication. I watched the old timers tune their bikes to perfection is less than a minute back in the 70's using these techniques.

Another tip. If the pilot jet isn't properly seated, there will be no end of grief trying to get the thing tuned.
Monoblocks get blocked pilot jets, too, but you can just drop them and visually inspect the things.

Cheers,
Bill

PS I sure wish I had a '62 Bonnie again.
B.


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: 1962 Bonny carb tune [Re: scratchedtank] #771057
04/14/19 11:15 pm
04/14/19 11:15 pm
Joined: Nov 2012
Posts: 1,005
Isle of Wight, UK
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koan58 Online content
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Posts: 1,005
Isle of Wight, UK
When Bill understatingly drops in "All this is done after the carbs are properly synced." half way through his post, may be confusing.

Synchronising the carbs is the essential 1st thing to do, it cannot be done after the other steps described.
It requires unwinding the stop screws such that they are not involved with the slides (even take them out if in any doubt), and creating slack in both carb throttle cables and the main cable, to ensure that the slides hit the bottom of their travel in the carbs. So wind in all the adjusters and start from there, when you can be sure of the slides clacking at the bottom of their travel.

Only then can you use the adjusters on each individual carb cable to ensure that both slides start to lift at the same time.
Once you've achieved that, make no further adjustments of the individual carb cables.

Now follow Bill's technique for idle setting. Note the angled screw is throttle stop (ie speed) and horizontal screw is air (mixture).

Re: 1962 Bonny carb tune [Re: scratchedtank] #771073
04/15/19 1:39 am
04/15/19 1:39 am
Joined: Apr 2014
Posts: 15
Sydney Australia
T
Tiger Shark Offline
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Joined: Apr 2014
Posts: 15
Sydney Australia
Hey Scratchedtank, was the power loss abrupt (like total failure of ignition) or more gradual (like needing to switch to reserve) ?? Who did the engine and how many miles on it and exactly what was done?? When it stopped did the engine seem unusually hot (tinkling noise, smoke burning smell)??

Re: 1962 Bonny carb tune [Re: scratchedtank] #771081
04/15/19 2:38 am
04/15/19 2:38 am
Joined: Dec 2007
Posts: 99
PA
Oil Changer Offline
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Posts: 99
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Scratched Tank,
I get it. Frustrated is probably not the adequate word. Before you get complicated, think simple. Question is, when you left, was the bike running properly? Idled well, started well, pulled away fine? Idled well at stops? did the issue happen gradually? or all of a sudden?
You say it runs but doesn't idle. If it's a first run, might check the tightness of the carb to intake bolts. Maybe sucking air?

Re: 1962 Bonny carb tune [Re: Oil Changer] #771108
04/15/19 10:51 am
04/15/19 10:51 am
Joined: Jun 2018
Posts: 28
San Mateo CA
S
scratchedtank Offline OP
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scratchedtank  Offline OP
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Joined: Jun 2018
Posts: 28
San Mateo CA
The more I think about it the more I believe it was a partial seizure
Luckily as soon as I felt it i rolled off the throttle and pulled over
I will change the oil (I am using Penngrade sae40 )
And work anew on the carbs
Engine has only done 100 miles so I probably should still be pottering around tow


62 Bonny
68 Shooting Star
2000 Enfield Bullet
Honda 250 CRF
Re: 1962 Bonny carb tune [Re: scratchedtank] #771120
04/15/19 12:55 pm
04/15/19 12:55 pm
Joined: Sep 2001
Posts: 776
Great Southern Land
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Great Southern Land
I do one carbie at a time, start the bike on one and then set a fast idle on that carbie, say 1500rpm. Then I move the mixture in and out until I find the 'sweet spot'...Turn in till it almost stalls, then out till it almost stalls then set in between these two points, you will hear it. Then I check that the bike is still idling on one cylinder at say 1500rpm, trim set as needed.

Then I do the same for the other side. Once done start the bike up - it will be revving at say 3000rpm - and back both idle screws off the same number of turns to a nice idle. Job done.

If you are running with EI you will need to set each redundant spark plug lead up with another plug and a jumper lead back to earth on the frame/motor, cos EI doesn't like sparking only on one cylinder so you need to compensate for this.

And when you say a partial seizure, in the piston or maybe a stuck valve? Only asking cos you said a tappet clearance was out? But it should be loose? Probably wont know unless you tear it down.

Re: 1962 Bonny carb tune [Re: scratchedtank] #771155
04/15/19 7:18 pm
04/15/19 7:18 pm
Joined: Nov 2012
Posts: 1,005
Isle of Wight, UK
K
koan58 Online content
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Posts: 1,005
Isle of Wight, UK
As it restarted later on, it may have just been a fuel flow problem, though that doesn't explain why it wouldn't idle (assuming it idled well beforehand).
It is conceivable that 1 or both pilot jets have picked up muck during your efforts to restart it, but it is a stretch.

If it was the beginnings of a seizure, the most likely reason for a change of behaviour afterwards is a ring or rings stuck by a smear of piston alloy, or maybe damage to a ring itself.
This would change engine behaviour by reducing compression.
I would suggest a compression check before tearing down. Of course torque head and set valve clearances first.

Re: 1962 Bonny carb tune [Re: scratchedtank] #771171
04/15/19 9:57 pm
04/15/19 9:57 pm
Joined: Jun 2007
Posts: 6,330
Maui Hawaii
HawaiianTiger Online content
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Posts: 6,330
Maui Hawaii
If you've had a partial seizure, you will likely have some piston slap, and maybe some smoke from one side, most likely the left.

Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.

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