Further carburation questions...

Popester David
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Re: Further carburation questions...

Post by Popester David »

bh.sprint wrote:Hello,

When putting the Mikuni carb on the Arrow it started OK cold and warm but did not start easily when hot ( after about 25 miles ). The problem was the pilot jet which was too large. Dropping down two sizes cured the problem. The engine also had a heavy note to the tick-over with a smokey exhaust from the silencers ).
I have come across problems with Amal carbs where the pilot circuit is blocked or the pilot jet does not set properly.
At what throttle opening does the lack of response appear ?

Brian H.
Hi Brian, I decided to go with the Mikuni carb too. I am told I need to replace the studs on the cylinder block with Allen key bolts, as the original studs don't extend out enough to accommodate the Mikuni adapter. Did you find a supplier for those? I am told allspareskeith from the Yahoo forum would have them but can't seem to get on to that forum. Do you happen to have Keith's contact details? Best regards, David
bh.sprint
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
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Re: Further carburation questions...

Post by bh.sprint »

Hello Davud,

The rubber adaptor for the Mikuni carb is normally attached using two allen screws, with a UNC thread. There is not a lot of clearance for the head of the screw so you may need to take a bit off to clear the side of the adaptor. Most of the traders who supply nuts and bolts for classic bikes have them. Keith may also have them in stock.
I'll put Keiths tel number on the private message board.

If you need a list of the Mikuni carb. jets for the Arrow/Leader post a message and someone will respond.

Brian H.
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Trev Sellars
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Re: Further carburation questions...

Post by Trev Sellars »

I have always used 2 stroke Dellorto carbs with 4 stroke atomizers, personal preference for racing 2 strokes.
The nozzle in the venturi (where the needle rests into), can be flat/angled/flush with floor of the venturi, depending where you want the mid range power. The needle choice can be critical as it controls the mid range, also the atomizer is made in different bore sizes dependent on the needle used and the amount of fuel you wish to pull through the carb. 4 stroke atomizers are drilled along the length, to allow more air into the mix, so more of a volatile mix to ignite.
The slide will usually be available in different angles, again to select where you wish the main fuel/air mix to be operating at, (throttle position).
Different jets in the carb, come into play, on, upto1/4 throttle mainly idle jet and needle, 1/2 throttle needle atomizer tube, WOT (Wide open throttle), main jet. So you have to jet each stage of your throttle differently. Sometimes the selected needle can be tweaked by placing a small washer under the circlip before placing it into the slide, this will allow you to select a position between the marks on the needle, making it richer or leaner.
Symptoms like yours are looking like running too rich, (4 stroking), so try without an airbox to lean the mix a bit. If that improves the running, then alter the parts that effect the throttle position. If your top speed is fine, then look at idle jet, slide, needle choice and atomizer.
Change only one thing at a time and keep a note on how it affects the performance.
The idle jet is not used for 60% of the throttle action but be wary not to go too small , as it does play an important part, when rolling on/off the throttle, like bends, pulling away etc. Too small a jet can cause seizing. Usually 2 strokes seize on the transition stage accelerating of low to high speed
Dellorto, do have a UK website and it has lots of info, that can be used with good effect on other makes. Try Dellorto.co.uk. This might give you an idea or two.
Remember, what may work on someone else's engine, might not work on yours. I have even seen pistons put in backwards and the owner wondering why it wont run correctly. Float position in the carb bowl is also critical, to ensure sufficient fuel is available to feed the atomizer tube.
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