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Old 03-23-2014, 09:25 AM   #1
ericrat OP
Beastly Adventurer
Joined: May 2007
Location: Cincinnati, OH
Oddometer: 1,109
Dellorto carb problem

I have a Dellorto PHM 40 on a Husky 2003 Husky TE610E. Since that carb has been used on a fair number of thumpers over the years, I figured I would come to the whole group rather than the bike specific thread, especially since only a few years of these bikes used the Dells.

It is scrupulously clean with fresh gaskets and orings. The bike has very low miles. Stock exhaust with a Twin Air filter. I The valves were set before I bought by a trustworthy source.

The idle mixture screw doesn't seem to be acting right. You can screw the idle mixture screw all the way in and the bike doesn't stall. With the screw all the way in, to about two turn out it seems little lean and pops and backfires on deceleration.

I pulled the carb back off to triple check the idle circuit. You can just barley see the idle mixture screw through the little port on the engine side of the carb. At about 2 1/2 turn out the end of the mixture screw is in the middle of the port. Based on that I turned mixture screw out to 2 1/2 turns and it was way to rich… I just don't seem to be able to get the idle mixture right. The carb boots are all in good shape and properly seated. Spraying starting fluid around the boots doesn't change the idle speed.

The stock idle jet is a 50. It came to me with a 60 in it. This is a common swap from what I have read, like many bikes it comes for to lean from the factory.

First question is, would a too-big idle jet make ti NOT STALL when closing the mixture screw? Maybe I do need to go back to a 50?

Any other suggestions, I am stumped.

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Old 03-24-2014, 04:19 AM   #2
Beastly Adventurer
Joined: Aug 2008
Location: hunt country virginia
Oddometer: 1,224
what happens when you ride it with the mix screw closed? that's more important than whether the bike stalls while stationary.

there could be multiple bleed orifices from the idle circuit into the carb venturi. in that case the mixture screw is blocking only one of them. too large of a pilot jet size then could overwhelm the mix screw

also if the float level is too high, gas can spill up into the venturi more easily.

also the choke circuit could be leaking a mixture into the throat

all of these scenarios would cause gas to bypass a closed mixture screw

there's a dellorto bible thread in this forum somewhere

if its otherwise running well and is responsive, maybe the backfire is caused by a leaky exhaust manifold
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Old 03-24-2014, 05:12 AM   #3
Studly Adventurer
Joined: Jul 2010
Oddometer: 979
Cant help with jetting etc,but i have had a few ktms etc with the dellorto and had the choke plunger stick and not be seated correctly,the bikes i had had the cable/lever gig to operate the travel which worked correctly but the plunger was stuck,maybe worth a look/check.
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Old 03-24-2014, 01:06 PM   #4
ericrat OP
Beastly Adventurer
Joined: May 2007
Location: Cincinnati, OH
Oddometer: 1,109
I will double check the choke, good idea.

It just seems odd, the idle RPM doesn't really change from all the way out to all the way in on the mixture screw. At about 3 turns out, it gets a bit rich, sound flat and dull. All the way in it is a little crisper and pops on deceleration, but the idle RPM doesn't really change much.

I think I will try a smaller pilot jet.

Have you visited ride reports lately ?. click HERE
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