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?

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.

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.

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.

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.

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

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)

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.

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.

cheers,
Michael

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.

I'd go with a small accelerator pump...most carbs probably dump too much fuel.

About the only thing you will find with a carb that will work with out an accelerator pumps a chain saw. You rev them up to speed at almost no load and go to work, unlike a vehicle that has to do a lot of work to accelerate you up to speed.
Another way you can run with out an accelerator pump is to run your carb catastrophically rich.

Years ago I set my Camaro to run a lean idle after I installed a wide band O2 gauge. One day I was changing out accelerator pump jets and forgot to reattach it to the linkage. I started the car, put it in drive and as soon as I touched the gas it died and I figured I forgot to reattach the accelerator pump more or less immediately.
Now a few years before that, I was messing with that car, when it was running really rich before I had a wide band O2 gauge and deluxe carb tuning set and disconnected the accelerator pump to see if it would go and it kind of did. But it was more or less undriveable with no accelerator pump.

You'll be hard pressed to find any 2 stroke out there with an accelerator pump, not just chain saws.

sorry to revamp an old tread but i have rebuilt my carbs 2 times already and i still have a gas leak at the accelerator pump cover and area. im at about wits end with this carb. lol sadly i sold the donor no pump carb. if i went with a new donor bike without a accelerator pump what should i adjust to make it work (carb jetting etc .

Some 4-stroke scooters don't have accelerator pump, I guess design is adjusted to that.

I have one of such carb, it doesn't die when giving throttle, there just is not much happening, not even at WOT.

Slide in that is huge, but throat is much smaller than what slide is, it is like if they would of put twice too big slide to that carb.

When exploring world of cheap chinese scooters, there are no thing that is impossible, no matter how much against logic some design is, some of them has tested it and managed to sell who knows how much of that junk.

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If there is a leak near accelerator pump, is it sure that it is not the shaft that goes to pump which is worn so that it leaks from there and then just run off to area of cover?

the design is its built into the carb float bowl. its leaking when i pump the throttle even when its not running. and its leaking from the cover. i am going to give it one more try and replace the pump again and this time take the float bowl and clean the crap out of it. or maybe ill buy a new float bowl to see if maybe it cracked

If there is no actual accelerator pump, there is an inbuilt system doing the exact same purpose like on CV carbs with the raise of the needle is slowed down when the throttle is opened so that the engine draws more fuel due to being air starved.

Most motorcycle and small stationary engine carburettors I've messed with had no accelerator pump. Anyway, after seeing some older cars getting adapted with motorcycle carburettors, I wouldn't be so worried about not having an accelerator pump.

I always like the Hitachi SU types on the first 1970 Datsun 240z (11-69 first month produced).

Float height check using a small plastic straw down the vent hole in the float chamber.

Choke just dropped the seat where the needle moved, no real choke butterfly.

Throttle response was magnificent when they were set up right.

Different tapered needles for different performance parameters.

Two adjustments, the same seat the choke moved and the viscosity of the air chamber oil, with ATF the lightest.

Plugs ran as clean as fuel injection and mileage was as good or slightly better.

It did have a summer winter flap, manually adjusted only from the engine compartment, that you had to use when it got below 40-50 or it fell flat on it's arse when you hit the gas in winter.

You could set it to run too lean, then just add some choke for more power (redneck lean burn).

regards
mech

Those Hitachi, SU, Stromberg-CD and Keihin-CV carburettors are awesome.

well im doing a new rebuild on it with a new accelerator pump and sock the float bowl to see if that fixes it

yays i finally fixed it. i stupidly had to tighten the carb screw closiest to the accel pump before the otheres if not there would be a small gap and it would bleed out of it. now time to do some mods to get some better mpg in a new thread