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Author Topic: stock carburetor and airbox  (Read 2485 times)

Roy_d

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stock carburetor and airbox
« on: March 21, 2009, 07:35:54 PM »
Hello.  I recently got an '05 Bullett that the previous owner installed a 32mm Amal MK2 carb and K&N air filter, also a shorter, louder muffler.  My problem is the engine seems to gasp for air and has little low speed responsiveness, although it runs OK on full throttle.  I was wondering if the carburetor wasn't a bit too big for the engine, which aside from the carb and muffler is a completely stock 500cc.  Would putting a stock carburetor and airbox on, (rejetted for the muffler) improve the responsiveness of the engine? Enough so it would be worth doing? 
Also, if the 32mm Amal is OK for the engine, can you tell me a good starting point for jetting, with the "classic" muffler?  (Thinking that might very well be the problem.) I'm in Kansas, about 900 ft. alt.

Thanks in advance for any advice/ideas!


scoTTy

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Re: stock carburetor and airbox
« Reply #1 on: March 21, 2009, 11:39:05 PM »
do a search on Thumper's posts ..he's using an Amal..  I'd find it for you but wifey is ready to go to dinner

RE350

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Re: stock carburetor and airbox
« Reply #2 on: March 28, 2009, 06:39:34 AM »
Roy, The best and most economical solution in my opinion is to put on the original 28mm Mikcarb. The Mikarb comes from the factory with a 110 main jet which is too lean for your purposes. Upjet to a 125 main to improve high speed and raise the jet needle by one one groove richer (clip one notch lower on needle) to improve midrange performance. Hanging that big 32 mm Amal carb on a nonported head stock engine was unlikely to improve performance in any meaningful way. The stock 28mm is large enough and will perform well when properly tuned.  I have seen brand new 28mm Mikcarbs on ebay for under $75. Many people like the Amal Concentrics, but actually they are basically the exact same type of carb as a Mickarb with the same type of operational function and design. The secret to success is in the proper jetting, and the stock Mikcarb is already jetted very close to what you need except for a $5 main jet and a needle adjustment.

                         Karl
             
                  1999 Enfield 350 Bullet
                  1975 Norton Commando 850 Mk3

 

ace.cafe

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  • World leaders in racing or performance Bullets
Re: stock carburetor and airbox
« Reply #3 on: March 28, 2009, 01:10:23 PM »
I tend to agree that the 32mm is a performance carb, and not necessarily the best for low speed responsiveness. The bigger carb will show it's worth at the higher rpms, but won't idle as well as a 28mm.

Alot of people like the CMW 30mm flatslide as an "in-between" carb that works nicely. Not terribly expensive, and a good power producer that is manageable at idle and low rpms.

That said, the 28mm Mikarb is currently on my Bullet and doing fine. I could get a little more power at the top with a bigger carb, but it's a good "all around" carb for moderate street use.



RE350

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Re: stock carburetor and airbox
« Reply #4 on: March 28, 2009, 03:11:24 PM »
If you decide to go back to the rejetted stock mikccarb there is no need to go back to the stock airbox. You will have to change the throttle cable back to original however.  With a 125 main and the needle position change coupled with a K+N or similar filter and your more open exhaust, your bike should perform much better than stock. The stock airbox was actually designed for the 350 and is a bottleneck in free breathing with the 500cc when a free flow exhaust is used. On my 350 I installed the original pre-epa approved long header and short muffler. All I had to do was increase the main jet in the 24mm mikcarb to a 95 (stock 90) and richen the needle position one groove. Didn't have to change the airbox as it is adequate with the smaller engine. Bike now performs better all across the rpm band and top speed is about 5 mph higher.(65 mph actual to 70mph).  I may go to a 100 main jet to see if there is a further improvement, although the plug color looks good with the 95.

               Karl
     1999 Bullet 350
     1975 Norton 850 Mk3

Roy_d

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Re: stock carburetor and airbox
« Reply #5 on: April 07, 2009, 02:22:24 AM »
Thanks to all.  I really do appreciate the good information and advise. Karl, I bought a used air box before reading everybody's replies, but at it does have a K&N filter inside it, so I may give it a try, once I get the Mikcarb, but I will reserve judgement until I try with and without.  I didn't get a bracket or anything with the air box anyway, and there appears to be no way to attach it to the bike, so, maybe I'll forget about it.

Roy
('05 Bullet Deluxe)