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Old 01-15-2015, 08:57 PM   #1 (permalink)
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accelerator pump or none on carbs

So I have a 1978 cb 400t with a dual carb with an accelerator pump set up. I found a cb 400a part bike which I took the carbs off the bike. I have to rebuild my carb bc its currently leaking on the seals of the carbs. I was going to rebuild the cb 400a carbs since they are both the same but the a doesnt use an accelerator pump on the carb so I was wondering I know what the pump does but which one is more efficient and without the accel pump how bad would the bike accelerate?

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Old 01-16-2015, 04:17 AM   #2 (permalink)
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With no accelerator pump or one that does not work, the engine will stumble or stall when you try to accelerate. When you give the engine more throttle in order to accelerate, the mixture will lean out causing the engine to stumble. The accelerator pump gives a shot of gas at this time so the engine accelerates and does not stumble.
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Old 01-16-2015, 05:13 AM   #3 (permalink)
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I don't know if this is correct in practice, but here's my theory. The accelerator pump is used like a bandaid for when the air velocity through the carb is insufficient for desired fuel flow. So my theory is the carb w/o the accelerator pump has better designed fuel circuits, or is just smaller diameter carb giving it higher velocity. So I would think the one without an accelerator pump would be better for fuel economy. the carb without the accelerator pump could be just barely good enough to not require one, and was maybe just cheaper to build, but has no real beneifit.

... If the carb w/o the pump has a smaller bore, I think it would be better. If they are the same size, I would just try to find out how the 2 motorcycles were suposed to be different, if one would be better suited to lower speed riding, or if one was supposed to be higher peak hp, higher speed bike.
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Old 01-16-2015, 05:55 AM   #4 (permalink)
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Like I said I know what the accel pump does and i know people that have older bikes with out accel pumps and they have never stalled out accelerating I believe thats more for cars with carbs. The cb 400t and cb400a are the same besides the trans which the cb400a is an automatic as for the bore size I thought they were the same but I do know the jets are different.
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Old 01-16-2015, 06:13 AM   #5 (permalink)
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It's all going to depend on the type of carb, carb bore, jetting, etc etc.

As an example -
My bike stock uses a 40 mm CV carburetor. Throttle response is rather sluggish, and jetting for best mpg reveals a bog just off idle.

Many guys swap to a 39, 40, or 42 mm carb with an accelerator pump. The biggest reason if for quicker throttle response, but what I've been finding in my research is that mpg increases are made as well.

Stock, the bike is good for about 55 mpg
Tweaking jetting for noticeable gains in throttle response knocks that down to the high 40's.
Jetting for best throttle response means a very overly rich mixture to compensate for the lack of accelerator pump, and many guys are stuck struggling to even get 35 mpg.
In comes the pumper carbs, and now you can jet for a steady cruise, and tweak on the accelerator pump for the throttle response you are looking for. Many report being right back up over 50 mpg with great throttle response once dialed in.
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Old 01-16-2015, 08:32 AM   #6 (permalink)
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I just keep em clean and they run fine. If you thoroughly clean the proper orfices you have probably increased the diameter of the jet orfices when cleaning them mechanically.

The auto trans on the donor bike is why they ran no pumps. You could try them on the manual bike and see if you prefer the difference.

Original 240Z 11-69, no accelerator pumps, great throttle response. In fact I learned to keep my clutch heel on the floor in order to drive one decently.

regards
mech
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Old 01-16-2015, 11:29 AM   #7 (permalink)
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I've owned both versions in the past and I remember reading somewhere that the 400A has a smaller diameter bore carb in order to improve the fuel consumption, should be easy to check. If this is the case you may need or want to use the matching manifolds.

Just checked the Haynes manual CB 400T - VB31A (32 mm)
CB 400A - VB24A (28 mm)
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Old 01-16-2015, 12:01 PM   #8 (permalink)
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My old suzuki manuals talked about an accelerator well, even though the carbs had no pump. I think they manage acceleration enrichment anyway, otherwise it would bog.
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Old 01-16-2015, 08:19 PM   #9 (permalink)
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If you learn to not snap the throttle wide open at zero RPM where there's little air flow to pull fuel through the jets, you won't bog it or need the accel pump.

If you look at Dell'Orto carbs with pumps on them you'll see that every time the slide closes and is opened up again the pump shoots a stream of neat fuel down the intake tract, which doesn't sound to me like the ideal situation for good economy.

I've always disabled the pumps on my bikes that had D'O carbs, and they worked fine that way on both street and track.

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Old 01-16-2015, 09:19 PM   #10 (permalink)
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So my best bet without playing around alot on the bike is to rebuild the current carb set up. And yeah the bore is smaller on the cb 400a after looking at my book.

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