Cyclinder Compression SD Darmah

Craig in France · Post by **Craig in France** » Sun Sep 20, 2009 4:01 am

BevHevSteve wrote:guys on a darmah that has phf32/CD and phf32/CS carbies, there is a tiny jet located under the idle jet. I have yet to discover it pourpose or a part number nor for that matter, to remove one successfully as it is so tiny and bends the blade of my tiniest driver.........

This is unique to the CS and CD carbies

Ta Steve. Didn't know that, only having AD/AS carbs as a reference. And like you, I've never seen it in any drawing or nuffin' ...

Good to learn something new!

Ciao

Craig

machten · Post by **machten** » Sun Sep 20, 2009 4:32 am

I had a look in some spare PHF "C" series carbs I have today, and sure enough AL and subsequently Steve A are spot on! I never knew that was there and it is a total mystery to me why you have a jet under a jet. I've rebuilt a number of PHFcarbs for my bikes (even a few "C" series) and never even looked down there as the diagrams I have don't indicate there is anythting down there.

I had a loose go at extracting one, but it resisted, so I left it there and pumped some carbie cleaner through it which seemed to verify flow on the idle circuit. But I still have no idea why it is there.

Kev

Craig in France · Post by **Craig in France** » Sun Sep 20, 2009 4:43 am

Hi Kev,

I've just edited my earlier post as I think I've found the little darlin' in the 1979/80 SS parts book.

As to why it's there, since the CD/CS range of carbs were introduced - as I understand it - for emission control reasons, I guess it's presence must be related to that, somehow ...

Ciao

Craig

machten · Post by **machten** » Sun Sep 20, 2009 4:50 am

Thanks Craig. I'll have a look. Good work

I assumed the reason was emission control, I just wondered why the hell you'd take a two step approach to it. There must be a good reason, becuase the approach seems more expensive than changing the idle jet appeture and/or length in the idle jet.

Kev

machten · Post by **machten** » Sun Sep 20, 2009 7:00 am

Just thinking about this some more...

One possiblity is this..( and I'm only running this up as a theory, cause I'm no fluid engineer)

The Dell'Orto PHF is basically a "demand" based system. You can adjust the idle and mixture to deliver whatever it wants, or you want to give it. As much fuel as you like. Maybe the emissions standards required a maximum under idle to be certified,, and that little jet ensures it. That would be good reason to have a two step arangement - the primary restricts supply for certification.

Just a thought anyway

Kev

ALinHB · Post by **ALinHB** » Sun Sep 20, 2009 12:22 pm

Thanks to everybody whom has taken the time and posted info on this topic for me. Carbs, exhaust, compression etc etc. I am new to this web site and don't no what i would have done with out it. After saying that!

Yes Carby's are cs/cd phf 32mm. Sorry for not mentioning that earlier.

Thanks Steve and Craig for work on the item name and part number.
Diffuser Part No 0799.27.98 (BF1)

I have removed and cleaned the carby's several times now and it wasn't untill the third time that i noticed this little jet (stamped BF1). Yes it is delicate and very easy to miss when stripping down your carbs. I have stuffed it getting it out by the way. I think i mentioned that before. Tried ordering one and nobody else has heard of it either and couldn'dt find a part number for it. If any of you try to remove one just make sure that you don't burr the screw driver slot because as it unscrews, it will chew out the thread inside the carby body. Nothing you can do about it. My rear carby jet BF1 was loose and unscrewed no problem. But I needed to drill out the front one because as i said previously, i stuffed it. Probly never been removed. But on the good side it was covered in gum restricting it's ability for what ever purpose it has. I had the carby soaking in carb cleaner on my first clean down but obviously it didn't quite get threw the tiny holes to clean it out thoughly. I have ordered a set of PHF 36mm carby's that will hopefully fix my problem but I am not convinced this restricted jet BF1 is my problem because the bike was idling good. The carbs are nearly 30 years old now i think no matter how many kits and jet changes i do they need retiring. Who ever had the bike before me must of had this problem and looked at the carby's cause they had been fitted with new slides and every thing looked in good clean condition. Just to mention the bike was not a runner when i got it. The question is now, do i really need to fit this jet. As suggested by Kev, perhaps it is a diffuser for exhaust emission controll. My manuals don't show it. Thinking of it, if you change your idler jet to richer one then you would possibly need to change this as well (BF1). If I get my spray job back before the 36mm carbs arrive from the Bevel Heaven, I will fit the 32mm carbs with this jet removed and see what happens!

The exhaust air pressure leak test was border line. I did find a slight leak on the cross over pipe. The valve cleances are within tolerance. I get my spray job back this week and need to run the bike to see if any of the fixes i have carried out have fixed the old girls problem. New carbs should be arriving this week as well. Not much else I can do now except wait until it is on the road and go for a test run.
Great work by all of you.

rwhc80 · Post by **rwhc80** » Fri Sep 25, 2009 6:28 am

Hi Guys. Just curious as to why the compression would be different with the throttle either opened or closed ? When you are checking the compression you are checking it with both inlet and exhaust valves closed. If your valves are leaking so much that there is a difference in compression with the carb slide open or closed (which in either position could not hold even 10 PSI) then your valves must be fairly nackered. A standard compression reading would be about 140 to 150 PSI for a Darmah or SS motor. If you squirt a bit of oil in the spark plug hole and then do the test again, and notice no change then the rings are fine, if a big change then the rings are nackered. If the compression is down, and then after an oil squirt, its still down, the valves are leaking.

ALinHB · Post by **ALinHB** » Fri Sep 25, 2009 11:41 am

When performing a compression test you wind the motor over several times until you reach maximum reading. Valves are opening and closing. You are meant to take reading with throttle fully open but i was curious as to what it would be with throttle closed as my line of thought was if carby slides were worn or air passages in carby blocked it would produce an indictive reading. It dose not matter what your compression readings are on your bike as the alitude, air temperature etc all come in to play regarding the reading you recieve. I was looking for a distinctive difference (comparrison) in reading from one cyclinder to the other as i believe my front cyclinder was crook. Worn valve was my first theory. I am now certain my problem was carby related. Waiting for new carbs to arrive to eliminate fuel altogether.
Al

Wags · Post by **Wags** » Sat Oct 02, 2010 1:43 am

I know this post was last year. I have the same Issue at the moment.

Did the new Carby's fix it?