I'm trying to decide between a stock 257 and a 5003 (with the accelerator pump) from WHH. Does anyone have a feel for which one is better, and can the 5003 be modified for better airflow like the 257?

I am not certain which model exactly the WHH pumper carb is, but I have some here that are sold with the Zenoah performance kit (Walbro 813 carb and NGK Irridium plug) and I like them. They seem to respond well to throttle. I will be honest though I need to test them some more, but from what I noticed with the ones I did run, you could keep the low speed a lot leaner than a 257 due to the fact that it pumps gas in the throat of the carb on the start of the stroke. Only drawback is that if you wanted to bore it upon like we do on 257's you would have to do it by hand as the pump nozzle is in the throat...so unless you can remove it and re-install it, I'm afraid it's either leave it alone or try your luck by hand with a dremel. I haven't tried it yet. I have 3 or 4 of those carbs here...I plan on doing some back to back testing on some boats with 257's to see how they compare for heat racing.

Thanks, Marc. I think I'm gonna try the 5003 - Its not like I'm heat racing... I'm sure it'll provide enough power to start with, I can always change to a mod 257 later on!

We just did flow bench testing on a pile of stock carbs. The stock 257 was superior in flow to all the other butterfly type carbs we tested. Though this doesn't say anything about different metering, most racing applications are for full throttle performance. There is a reason the 257 is used by most racers.

Lohring Miller

I agree completely. That is why I want to do on water testing to see how it is for mid-high throttle and also initial low-high as in a race start. Want to see if there can be any advantages to using it in heat racing...will mostly suffer a little on the top-end speed, but if there are advantages in the other areas, it could come in handy in heat racing, as we all know you need good throttle response off the start and also if you have to let off and get back on. That being said though, I have always had great results with the 257 and doubt there is much improvement possible. Only testing will tell. (I also did dyno testing, not flow testing, but comparative testing on dyno between the 2 carbs, and I would stick with the 257 if based only on these results).

Have you guys tested the carbs in heat racing applications Lohring? If so, what were your findings?

Keep in mind these facts ...

ALL of the popular Big Bore walbro butterfly valve carbs are the SAME SIZE !
They measure .625 across the throttle bore and .500 at venturi.

The MAJOR difference lyes in the factory calibration on the metering circuits.
Pending on engine size carb was originally calibrated to be used on, engine rpm range and mixture requirements needed dictate the internal passage sizes, placement and effect on how engine responds threw out the rpm range.
The HIGH & LOW needles only regulate the quantity of fuel for their respective circuits.

As lohring has found the actual flow volume comes from more that just size.
In the case of a WT-257,WT-488 and WT-771, the intake bell design helps feed air into the carb free of turbulence. Carbs like the stock Zenoah with a choke shaft in or removed does not have the bell shape and therefor do not flow as well.

IMO, the 257 while seemingly the most popular is not the best in terms of mixture control and smoothness, the 488 and 771 work even better.

Scott

I agree with Scott the 488 in my testing is by far superior in water and on the dyno.

The bore may be the same, but the flow isn't. A WT-257 flows better than the WT-287 for no reason I can see. An interesting example of small details was that a modified carb with a .535 bore flowed less than one with a .530 bore. The difference was butterfly modifications. The difference between the two carbs shows up on the dyno, but it probably is too small to be noticeable in a heat race. The really big differences come when you go from a modified WYK carb to a modified 257 on a Quickdraw. The 257 is more durable, consistent, and dependable, but it drops the power noticeably.

Lohring Miller

Have any of you actually tested the WT-813 though? That is the one with the accelerator pump...I am sure it will flow less than good modded 257s or 488s, but I am more interested in knowing if the accelerator pump can be a good or bad factor for heat racing. This is one of my first on the "to-do" list soon as I can find soft water. I want to do back to back comparisons with it and the 257 in on water testing. I expected more people would have actually tried this carb?

FYI
Accelerator pumps on these type carbs are typically found in applications where the Idle too Midrange circuits have been calibrated to be on the side of lean. What this lean setting does is allow engine to run cleanly at partial throttle settings and not load up. In a 2 stroke racing engine this is quite an important thing.
The Accel pump aids this lean spot when a quick throttle opening may have engine stumble as the low fuel circuit transitions to the high speed fuel circuit.
Engines with a pump equipped carb gives the shot of fuel independent of the metering circuits.
Under slow throttle position changes, the pump becomes passive.

This LEAN transition mixture is WHATS WRONG with WT-257 carbs !
If you adjust a 257 for a nice crisp idle and transition. It is TOO LEAN once in the water and engine gets a load on it.
This is why 257's must me adjusted so darn rich on low needle setting. Basically a crutch adjustment too band aid the carbs lean spot in the low to high fuel circuit transition.
WT-488 and WT-771 on the other hand can be set leaner on low circuit giving them a MUCH smoother idle and crisper transition, yet still not stumble or go lean once boat is launched and engine gets a load on it.