1964 G15 monoblock carbs

Bodger said:

Maybe I'm missing something but the bike in your picture has two handed carbs - which is the correct set up for this bike - so why not use the stock settings? And no the float does not adjust like a concentric. You adjust by raising or lowering the float needle jet with different thickness fiber or other washers. I dont think monoblocs (a superior carb - sort of joking) are as sensitive to float adjustments as concentrics. In my experience usually better to leave well enough alone. If your carbs do not have matching bores I would be tempted to just get two correct sized new concentrics. The cost would be less than getting new Monoblocs or (if you count the value of your time) fiddling with a miss matched pair of monoblocs).

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Picture was from Web, was not my bike.
My bike is a 1964 G15 that comes with 389 monoblock, the right chopped.
The bike is completely original with stock monoblock and i want it remains so.
Monoblock are fine to see and an update is welcom.
Ciao.
Piero

I have made second thoughts .... the float level will not change by increasing the orifice diameter and using the original float needle.

The float level may change by changing float needles .... example: replacing original viton needle with alcohol needle.

Bodger has explained how to adjust fuel level. Add or remove shims under head of needle seat (Item 9 in picture of Reply #13).

Slick

pierodn said:

Picture was from Web, was not my bike.
My bike is a 1964 G15 that comes with 389 monoblock, the right chopped.
The bike is completely original with stock monoblock and i want it remains so.
Monoblock are fine to see and an update is welcom.
Ciao.
Piero

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Apologies for my misunderstanding about which carbs were on your bike. But I respectfully disagree with Slick. Think you are better off sticking as close to stock as possible unless you plan on racing or some other extreme use.

please, what is the starting setting of the high of the sliders?.
like concentrics about 5 mm?
thank you.
Piero

@Piero:
I agree with Bodger .... you should not have mis-matched bores.

@Bodger:
Where do we disagree?

Slick

texasSlick said:

@Piero:
I agree with Bodger .... you should not have mis-matched bores.

@Bodger:
Where do we disagree?

Slick

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Yes, never thought it.
Please, what is the starting height of the sliders?
Thank you.
Piero

texasSlick said:

@Piero:
I agree with Bodger .... you should not have mis-matched bores.

@Bodger:
Where do we disagree?

Slick

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Just disagree about the value of messing with float needles and seats on stock chopped carb set up on bike that is going to see usual "vintage" use. But I'm not expressing a "political opinion" (i.e. in the scheme of things none of this is all that important and this is supposed to be a fun hobby.)

Bodger said:

Just disagree about the value of messing with float needles and seats on stock chopped carb set up on bike that is going to see usual "vintage" use. But I'm not expressing a "political opinion" (i.e. in the scheme of things none of this is all that important and this is supposed to be a fun hobby.)

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In Reply #6, i pointed out the fuel delivery limitation of the chopped set up, and that dual full carbs did not have this limitation. I pointed out a monster needle and seat could overcome, or at least mitigate, this deficiency. Piero then took this thread in that direction. As it is his thread, so be it.

In the "vintage" use, it does not matter ..... running out of gas occurs at high power loading where the vintage user may never, or seldom go. However, some of us love the exhilaration of feeling that strong push right up to 6800 rpm, and vintage or not, like to stretch the legs of these machines.

This discussion of fuel starvation may be useful to those dual Monobloc equipped, Atlas engined, bike owners who have accepted that these machines "poop out", or do not have the "guts:" over 5K rpm, when they are simply leaning out as the needle and seat fails to keep up with demand. This Forum is to fill such an information void, and such is my purpose in explaining the nature of the problem, not necessarily promoting a "cure".

As a further aside, IMO, I think the air flow dynamics, and the internal fuel metering dynamics of the concentric carb, offer no advantage over the Monobloc, as both are essentially the same. It is that each concentric has its own needle valve that provides the user with a performance advantage. There is a space saving advantage to the concentric as well.

BTW ...... can you answer Pedro's question about "slide heights"? I can not.

Slick

I am certain you are correct about the advantages of modifying the float needle and jet, we just disagree about whether it is worth the effort on a standard bike. The main reason I like Monoblocs is ease of access of the float chamber and separate idle jet which I believe concentrics did not have until the Premier model and the ability to access the Needle and main jet without having to go through the float chamber. I do understand that, aside from being less costly to manufacture, the concentric is less subject to fuel starvation when leaned over and less sensitive to being mounted on an angled intake.

I dont understand Piero's question. By slide height does he mean height of slide to achieve idle? Cut away number? The question is not clear to me.

I think Piero wants to know how far to adjust his throttle stop screw, for the starting point of his initial throttle and mix adjustments.

Edit: And for my part, Piero, I just used pencils under the slides to synch the slide action, then

AgentX said:

I think Piero wants to know how far to adjust his throttle stop screw, for the starting point of his initial throttle and mix adjustments.

Edit: And for my part, Piero, I just used pencils under the slides to synch the slide action, then

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I back the screws out until the slides are resting on the bottom of the carb body, not on the screws. Then I use even size drill bits (same principle as pencils) to help insure both slides are lifting off the carb body simulltaneously using the cable adjusters on the tops of the carbs. It's also important to verify that both slides disappear into the top of the carb bodies at the same time. If you get a good view from the rear with the seat off you can pretty much synchronize the slides with your eyes and ears. I only use the throttle stop screws for a final idle adjust after using the air screws. If you need to use the throttle stops turn each one an equal amount and then again verify with pencils or drill bits and eyes and ears that they are lifting off simultaneously. Of course once you reinstall air cleaners and/or when the bike is hot on a hot humid day the idle will be off and you will have to break all rules and tweak the throttle stop screws blindly, hopefully an equal amount on each carb, to get a good idle.

I have never been able to use a vacuum gauge attached to the carb balance hose take offs to adjust the carbs. No matter how much I fool around with the gauge orifices the readings always bounce around too much to be useful. I must be doing something wrong.

AgentX said:

I think Piero wants to know how far to adjust his throttle stop screw, for the starting point of his initial throttle and mix adjustments.

Edit: And for my part, Piero, I just used pencils under the slides to synch the slide action, then

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yes thank you.
i have started with a drill of 5 mm like i do with concentric.

I adjust the throttle stops fully closed, start the bike, get it warm, then hold the engine at idling rpm with the twist grip and screw the throttle stops in until they start to lift the slides.

Then I let go the twist grip and fine tune the throttle stops and mixture screws.

A 389/689 set just came across Facebook Market for $600/pair. YEOUCH!