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Air slide question
#800415 03/08/20 5:33 pm
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Is there a rule of thumb as to how much of a jump between main jet sizes the air slide of a GP carb will affect? Half a jet size, full jet size, ????

I can ride from 5200 ft elevation to 12,000 foot in less than 50 miles, and am wondering how I should size the main jet.

Thanks for any enlightenment.

JR

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Re: Air slide question
Jerry Roy #800419 03/08/20 6:41 pm
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Originally Posted by Jerry Roy
Is there a rule of thumb as to how much of a jump between main jet sizes the air slide of a GP carb will affect? Half a jet size, full jet size, ????

I can ride from 5200 ft elevation to 12,000 foot in less than 50 miles, and am wondering how I should size the main jet.
Two factors are at play, the altitude and the temperature.

As for the altitude, the Mikuni calculator only goes to 8000 ft. But, it has a linear scale so using the calculator for a 7000 ft. increase in altitude, it says to reduce the jet by 5 sizes. However, it also says if the temperature is 30 ℉ cooler you should increase it by 1 size, i.e. for an approx. net decrease in jet by 4 sizes when you're riding around at the top of Loveland Pass.

According to an early Roy Bacon book, back when he was writing technical information, normally a racer should have their bike start 4-stroking with the air slide halfway shut. He says if it 4-strokes when fully open you need to reduce the main jet by 4-5 sizes. That answers your question about the range of adjustment.

Putting this together, if you tune your GP for the correct mixture with the air slide halfway open for riding around Denver, closing it will enrichen the mixture for starting, and fully opening it will lean it by the necessary ~4-5 sizes for riding around at the top of the Pass.

Re: Air slide question
Magnetoman #800435 03/08/20 9:39 pm
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Originally Posted by Magnetoman

Putting this together, if you tune your GP for the correct mixture with the air slide halfway open for riding around Denver, closing it will enrichen the mixture for starting, and fully opening it will lean it by the necessary ~4-5 sizes for riding around at the top of the Pass.




Thanks MM,

Mount Evans is even closer than Loveland or Berthoud, so I can test out the results in an afternoon.
One other thing that you might be able to clear up: The owners manual I have states that the needle position is normally set at the fourth groove.
Do they start counting from the pointy end, or the clip end?
Sure am glad you guys share your knowledge. Thanks.

Now, are there any GS owners in the Denver area that could tell me what exhaust system they use, and what their GP size is and what settings they are using?
Mine is an inch and three eights GP Type 1 carb, with a GS "Twitter" muffler.
Hopefully I can get this thing road worthy for this summer.

JR

Last edited by Jerry Roy; 03/08/20 9:41 pm.
Re: Air slide question
Jerry Roy #800644 03/10/20 3:47 pm
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Standard GP settings should be ok to 3;OOOft At altitudes up to 6, 000ft a reduction in main jet size by 5% and usually a 4% reduction for every 3,000ft increase over 6;000ft

Re: Air slide question
Jerry Roy #800806 03/11/20 10:42 pm
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Thanks John,

I will assume that when you say %, you are referencing the flow rate of the jet. So if the stock suggested size is a 300, a 5% reduction would be a 285.
Do I have that right?
I have a 280 in there, and I guess that is as good a place to start as any.
Now to find a 5% uphill grade road that isn't inhabited by police. ohno

Thanks for your input.


Jerry Roy

Re: Air slide question
Jerry Roy #800882 03/12/20 1:20 pm
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sure thing, five percent

Re: Air slide question
Jerry Roy #800902 03/12/20 6:07 pm
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Thanks John, that helps.

JR

Re: Air slide question
Jerry Roy #801044 03/13/20 3:03 pm
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What model Goldie do you have, if a dbd 500 main jet aize is 350.

Re: Air slide question
Jerry Roy #801050 03/13/20 4:39 pm
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Howdy John,

It is a DB34 GS, '56 model, 1 3/8 GP Type 1 carb, repo "Twitter" muffler, shipped in clubman configuration to Hap Alzina, according to Ian.
It has the Western style bars, and some Scrambler parts, according to the part numbers in the BMS parts list, so it isn't what I would call a Clubman.
I can't find a source for the jet size that lists the exact configuration of this engine, so some practical experience is needed. Which is why I hope you guys can shed some light.
Thanks for any tidbits.

JR

Re: Air slide question
Jerry Roy #801056 03/13/20 5:05 pm
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Originally Posted by Jerry Roy
It is a DB34 GS, '56 model, 1 3/8 GP Type 1 carb, repo "Twitter" muffler, shipped in clubman configuration
An AMAL settings list shows for a 1955 B34 Gold Star, not a scrambler, (which, for that year, could be either a CB or a DB, but it shouldn't matter for present purposes):

5GP 1 3/8"
330 main
#7 slide
.109 needle jet
1/8 air jet

The Mikuni slide rule shows that at 5000 ft. the main jet should be dropped to 305 (i.e. between a 300 and 310).

Re: Air slide question
Jerry Roy #801124 03/13/20 10:11 pm
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Thanks MM,

Oop's, my mistake. On closer inspection, after donning my bifocals, the carb is indeed a T5GP1. I thought the trailing 1 was the type number.
When I took the carb apart after its 50 year hiatus, it had a 300 main, and a #7 slide along with the 109 needle jet.
So I will assume that it was a standard set up for this altitude and exhaust system.
If, as you suggest, I tune the carb with the air slide set at half mast, would a drop to a 280 main be a reasonable starting point? I have a 280 and a 290 jet, plus the 300 that I soldered up and drilled out to a #56 drill size, and figured that it would do as a starting point to arrive at an approximation of the correct jet size.
Your information makes that a moot point.
I bought a new #5 slide, since the old #7 was worn out, and I figured I could trim the #5 to get to #7, if necessary.
I am going to order the Viton tipped float needle kit from AMAL, and can order some jets to extend the tuning range, if you think that would be advisable.

Once again, thank you for your expertise and willingness to share it.

JR

Last edited by Jerry Roy; 03/13/20 10:13 pm.
Re: Air slide question
Jerry Roy #801154 03/14/20 1:46 am
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Originally Posted by Magnetoman
An AMAL settings list
5GP 1 3/8"
330 main
#7 slide
.109 needle jet
1/8 air jet

Looking further, the 1955 Gold Star Maintenance manual confirms that for the DB34.GS:
1-3/8" GP
330 main
#7 slide
.109 needle jet

Further, it lists the needle as being on the #2 slot (i.e. 1 down from the top).


Originally Posted by Jerry Roy
If, as you suggest, I tune the carb with the air slide set at half mast, would a drop to a 280 main be a reasonable starting point?
No, what you're trying to do is have the carburetor jetting correct for 5000 ft. that you can then rejet on the fly to lean it in order to be correct at 10,000 ft. Assuming the values given by AMAL and BSA would be perfect today for your bike at low altitude you'd want a "305" to be perfect at 5000 ft. So, using a 310, since it's better to err on the side of being rich, it should be perfect-ish on your GP at 5000 ft. with the air slide set at half mast. Closing the air slide all the way would make it richer to help get it started at 5000 ft, which you'd then move back to half staff as the engine warmed up and you rode around Denver. As you then headed west and gained altitude you'd slowly raise the lever until it was fully open, and thus ~4 sizes leaner, when you reached the crest.

If you modify a #5 slide keep in mind that on the "racing" TT and GP carburetors the steps are half the size as on Monoblocs and Concentrics. This makes a #7 equivalent to a #3.5 on an ordinary carburetor. On your GP the steps are in 1/16" increments.

Re: Air slide question
Jerry Roy #801331 03/15/20 2:31 am
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Thanks MM,

Now to order jets and hope for some spring like weather.

JR


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