carb question

schultzy said:

First,I want to say this site has been awsome! 2 years ago I started with a total basket case of a machine,E-start failed a day after I bought it,after it did start ,it ran so bad I almost borrowed my neighbors revolver and put it out of its misery!I had no clue of where to start.Thanks to you guys I have a pretty decent running machine and as a backyard mechanic ,kinda proud I did it myself!Well anyway,my question is ,I read about the fuel screw being turned out 1 to 2 turns and my manual says 11/2 turns.I am finding I have to turn it 3 to 4 full turns before I get it purring like I feel it should. Is this ok or am I compensating for something else? E-start isnt a100 percent reliable but when warm it will fire on the first kick.Settings are ,45 start jet,40 pilot,dr272ndljt,k51 and 180 main.I went to a 42 pilot thinking it might help with e-start but so far havent had any luck.I also installed a 192 main,just to see what would happen but will go back to a 180.Would a dr270 (stock)make that much of a difference?How about a richer start jet?Thanks guys!98 FE 600 Werner

Click to expand...

By turning the fuel screw outward three to four turns you are increasing the amount of metered fuel at idle. This would indicate that your pilot or as Dellorto calls the idle jet may be too small. You may want to try that 42 idle jet again. Use this procedure to adjust the idle mixture/fuel screw:

Check that the throttle cable has about 1 mm of free play with the slide closed. Always adjust the idle setting with the engine fully warmed up. Proceed as follows:

Screw in the idle speed screw (4) to get a slightly- higher speed than normal (about 1200 rpm for four-stroke engines and about 1400 rpm for two- stroke engines); then screw the mixture adjusting screw (5) in or out until you obtain the most even running. Then unscrew the throttle-stop screw (4) until you get the desired idle speed again.

To obtain the best engine running, it is worth finally rechecking by carefully readjusting the idle mixture screw (5).

schultzy said:

How about a richer start jet?

Click to expand...

The start or choke jet should only have an effect when the choke is on.

schultzy said:

even with a 42 idle jet my fuel screw is still out at least 3 full turns.From what I have read I was thinking I need to go to a smaller jet to get the sensitivity on the fuel screw,or is my thinking wrong?

Click to expand...

I am not an expert on these carbs but your post remained unanswered for awhile so I thought I would give it a whirl. Having said that I feel that a smaller idle/pilot jet would give you a leaner mixture and reduce the flow a bit exacerbating the problem. If you really even have a problem at all

If we must get technical, the Dellorto tuning guide states the idle mixture screw/fuel screw controls "metered fuel". What does that really mean? I think it means that it controls the amount of mixed air/fuel at idle but doesn't affect the air/fuel mixture itself. The actual A/F mixture would be controlled by the idle/pilot jet and an emulsion tube (if your carb has one it may not).

I believe a similar issue came up awhile back and a idle circuit emulsion tube was added to the carb, as some have them and some do not, which produced good results. The Dellorto part number for the tube may be #9980 and fits directly under the idle jet. Are there different sizes of idle emulsion tubes? You would think so but I don't know.

So to sum it up:
If the fuel screw doesn't really control the A/F mixture itself but volume only and the idle jet only controls the A/F mixture the only other variable to control the volume is the emulsion tube.

The above is only my theory and not to be taken as fact

Redundant and quite possibly offensive (Taffy) I offer modification and tuning of the PHM 40 Dellorto carburetor.

Since said work is performed via a flow bench the carburetor must be sent to me.

The price is $150.00 plus parts, S/H/I.

The turn around is roughly one week.

I do approximately three each month and to date have a 100% satisfaction rating.

Dale

schultzy,

I am also getting ready to start tinkering with my carburetor jetting to try and optimize things a bit. I don't have any good advise for you at this time as this is new "unexplored territory" for me as well. What I do have to offer you, or anyone that may be interested, a matrix sheet that I put together from a few documents and excel spreadsheet that might be helpful to document things as you go. I have included it here in this post. Keep us updated with your results as I am also going to be asking these types of questions in the near future.

Regards,