Dellorto PHBH Carburettor

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GLscoot
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Fri May 09, 2014 5:47 pm

My recent build has caused me a few teething problems, runs perfect on the open road but I also use it for work and when I do Im not usually out of second gear and keep it running at all times, so it can be on idle on slight hills and uneven ground after an hour or so, the scooter starts to bog down as if its running out of fuel and eventually packs in. Spark plug is a little wet and I probably put to much oil in the premix and so is a little black. I change plug with a new one and it starts no problem. At first I thought I needed a fuel pump as it felt when running it was running out of fuel, but the tank was at least half full.
After speaking with my neighbour, who is not bad with engines of all sorts. Asked the question, do you think it may be flooding and that the needle on the carburettor slide has a small cir clip on it, with 4/5 notches and said it may not be sitting in the housing deep enough and so on idle it may be allowing to much fuel in and not burning it off, thus flooding the engine.
So I checked this and the cir clip was sitting on the second slot from the top so moved it to the first notch. Does anyone agree with this theory. Nearly every morning the scooter starts with no choke and first kick, Ive also went through 3 plugs in 400 miles, using NGK BH7 plugs.
Worb5 gas flowed crank
Polini 180 barrel, casings ported to match
SIP banded Cosa 2 clutch 22 tooth
Primary 67 tooth
T5, 4th gear
Giannelli Exhasut
MMW manifold with RD350 reeds
Dellorto 26mm PHBH carburettor, 110 main jet, 50 idle jet and 70 float jet
Thanks Gary

mick1
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Fri May 09, 2014 5:53 pm

Had this problem on my Avanti. Dan found the fault on the dyno, it was the choke plunger not seating correctly, so the choke was partially on. Visually it looked to be open and closing ok but it wasn't. I changed to a manual lever type and changed the plunger, spring and seals and it worked fine. You also may have problems "hot starting", starts ok when cold but tricky to start when still warm/hot.

GLscoot
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Fri May 09, 2014 6:23 pm

Im using the flip choke, but will take a look at it. The carburettor was the only used part I put on the engine. What I would add, I don't cut the engine when I use it for work. Its constantly on idle. Its either being used or on idle for about an hour.
Yesterday I went on a ride for around 40miles on decent roads and city centre and after a few traffic lights the problem occurred again but when i was back on the decent roads I opened it up and hit the 100kph without a problem and it had some more to give.
Gary

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jonny snatchsniffer
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Fri May 09, 2014 7:10 pm

if it gets richer and richer as you sit there idling then it could be the float valve letting by, wrong float height or crap not letting the valve seat propperly, and also spitback from the reeds collects and can richen things up

Jack221
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Sat May 10, 2014 5:19 am

What size is your atomiser and needle?
If everything else checks out ok then it could be that the atomiser is too big for the needle. There is often a load of splooge splattered around the exhaust manifold area when this is an issue. The other test for the atomiser is ride in 2nd with the throttle just cracked a few mil off the stop. It should be running clean and dry and really want to pick up speed. If it sits there spluttering and you have to open it up more to clear, the atomiser is too big for your needle.
Vespa est ieiunas, quia semper operaturi

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coaster
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Sat May 10, 2014 7:56 am

It might help to understand that your carb operates on 3 different 'phases' dependant on throttle position:

closed to around 1/4 - This uses the cutaway section on the slide for the air and the idle (pilot) jet for the fuel.

1/4 to 3/4 throttle - Air slide, needle and atomiser.

3/4 to Full Throttle - Main jet

There is a bit of overlap from one phase to the next but rom the above you can see that the needle will have NO effect on tickover. I think the choke as already mentioned would be a good place to start, normally I would agree about spitback too but the reed valve SHOULD prevent that. Are you running with an air filter btw?

GLscoot
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Sun May 11, 2014 10:49 am

Jack,
I think you hit the nail on the head, yes there is a lot of fuel/oil on the casings at the exhaust port and there is a lot of play in the needle, its not exactly tight in the metal plate, I don't know if this is normal. Its a standard 26mm PHBH so will find out the size of the atomiser & needle and replace them.

I forgot to add there is sometimes backfire from the exhaust.

Air Filter is just the standard wire mesh one. I have a Ram foam filter, which I will try after changing the atomiser and needle. Do you think ethanol has something to do with the wear & tare of the needle and atomiser, I would say the carb has done around 2000 miles on this set up and on a non reed set up, which I didn't have this problem and no I don't want to go back to the non reed engine.

i was out on it yesterday and it was no where near as bad as the day before, changes made were the needle was sitting at its maximum length (cir clip on top notch) and new plug.

I will also check the choke is sitting properly.

Gary

Jack221
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Sun May 11, 2014 4:22 pm

Glad to help. Carbs rarely come with the right anything in. Normally slides an float types are usable and thats about it :) Dont worry about the wobbly needle. Once you get the atomiser the right size you will need to move the needle clip down a few notches as well.
Vespa est ieiunas, quia semper operaturi

GLscoot
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Sun May 11, 2014 6:08 pm

Ive had a look on beedspeed and the range of needles is unbelievable.
From what I have found on the net the atomiser is as266 and so Im assuming I should go for the as264 atomiser, but needle size I have not got a clue. Yes I should check the one on my engine, but unfortunately I can't turn my fuel off (long story) so will need to wait until the tank is nearly empty. Went out today and ran perfect although it was full throttle more or less in every gear, so wasn't expecting any problems, it just happens when in city traffic or when I use it for work.
gary

Jack221
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Mon May 12, 2014 6:36 am

It can be quite a challenge when setting up a new carb. The usual thing is just to ask people what they use in theirs and copy it. Its often close enough but if you know what to look for and when you can make a better job of it.
As main jets, needles, atomisers and pilot jet all interact it can be really difficult. You change one part and all the rest changes, get something right and scared to touch the rest. The correct sequence still takes a few goes to get right but when it is really right, it’s reliable, economical and easier to ride well.....oh, and faster!
My order to get things right in is; float type/height, pilot jet & air screw, atomiser & needle diameter, slide cut out, main jet & needle tip size, needle taper length & clip height.
The AS264 will probably do you but it may take a few goes to get right :)
Vespa est ieiunas, quia semper operaturi

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