Cyclinder Compression SD Darmah

[ALinHB]

Hi
1981 SD Darmah 900
Can anybody help me. I am needing to know what the permissable PSI compression range is for Darmah 900. Standard bore.
The problem i am having is no matter what i do, i can not get the front cyclinder to stop poping and crackling back throught the exhaust when i back off. I have stripped, cleaned and re-tuned carbys. (32mm PHF) Forget how many times. Replaced manifold rubbers. Checked timing. Removed and refitted exhaust, installed new exhaust caskets (Conti Pipes).
The compression reading i am getting is 165 psi for rear and 173 psi for front cyclinder. Can not find what the range should be in my manual.
I was told it could be to lean a mixture at idle causing it. But i am inclined to think that unburn gases are igniting in exhaut pipe. (perhaps valves).

Bike runs great and sits on constant speed. Any idea's.

[big phil]

hey dude, the valve has probably worn. check the valve seat and guide. if guide worn, no amount of lapping in will cure seat being worn. try lapping valve and then adjusting closing shims to match. or the closers arent big enough and its not closing properly which will wear out the seat as well.

[ALinHB]

Hi Big Phil
Thanks for the info but if the valve or vale seats were worn it would show on the compression test. The front cyclinder is slightly higher than the rear and rear is running perfect. I will check the clearances to see if valve is riding. Thanks

[machten]

Don't know if this helps, but I've had a Darmah start first kick and idle and run perfectly well at 115 psi for many years. Sometime after I did a bore to 1 OS. I had no change in ability to start or idle (it did that anyway - first kick always) but sure increased the power!! I'd have thought 170 was around the top end of compression for standard pistons - as in new rings and bore at those numbers without oil down the pots.

Having said that I think that compression readings will depend on how you take them (kickstart (and how fast you kick), poor battery electric start, good battery electric start) and are more useful for your own comparison that comparing to someone elses.

Kev

[Craig in France]

Hi
<snip> I cannot get the front cyclinder to stop poping and crackling back throught the exhaust when I back off. <snip>

I was told it could be to lean a mixture at idle causing it. But I am inclined to think that unburn gases are igniting in exhaust pipe (perhaps valves).
Any ideas?

Hi Al,

I think you could be barking up the wrong tree looking at cylinder pressures. I've needed to look into the causes - and fixes - of banging back on the over-run and this is what I've found:

1. Classic cause: air leak into the exhaust.
I''ll assume you've eliminated this as a possible cause - ?

2. Too lean an idle jet.
Test this by pulling the choke on (ok, not easy on a Dramah, I know!) and see if the banging stops.

3. Exhaust valve clearance set too tight/leaky exhaust valve.

FWIW, here's one explanation of what causes the banging - not my words, but it struck me as plausible.

Deceleration backfire.
Deceleration backfire is caused by fuel burning in the exhaust manifold or header. No ‘Ifs or Buts’ - that’s what causes it. So how?

When you close the throttle, you also close the carburettor slides. This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. And when the pressure is reduced in this way, the air-fuel mix burns slower.

(Note to self: you get the same result with too lean a mix - lean mixes burn slower.)

This slower-burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, so slowing it down even further.

In addition, as the engine’s rpm drops, the ignition will retard, and this results in the mix being ignited later.

Because the mix is burning slower and later, the exhaust valve can open before all the fuel is consumed, with the result that unburnt fuel is ejected into the exhaust. So, now with both unburnt fuel and late-burning fuel entering the exhaust - and particularly if there is sufficient oxygen in the exhaust from an air leak - the unburnt fuel will light - and bang! Backfire.

One fix:
Add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens.

HTH

Craig

[Craig in France]

As a supplementary to my previous post, I should add that there is a school of thought which argues, in effect, the opposite, i.e. that banging back is caused by too RICH a mix on the over-run.

"Confused? You will be ..."

Again, FWIW, here 'tis:
"Close the throttles and the intake vacuum sky-rockets as the pistons suck against the closed slides.

This raised vacuum increases the fuel delivery so that, even with the needle fully down, more fuel is sucked in than the engine needs, resulting in it running very rich on overrun. Normally, the fuel just passes through the exhaust, too rich to burn. But if there is an exhaust leak, air will get sucked into the exhaust causing the excess fuel to ignite."

[ALinHB]

Hi Craig in France

Yep you are right, i am confused. I am absolutely positive that there is no exhaust leak or air getting sucked in through any area of the carby manifold. Had these off and on too many times thinking this was my problem from the intial start. I brought this bike not so long ago and since i have it up and running, it has persisted to keep doing this poping and crackling on throttle back off (front cyclinder). I had virutally eliminated all possiblities that i could think of. It was suggested to me, carby tuning only. But after adjusting the carby to about every setting, it still did it. When i scewed the air mixture screw out further it started to do it less and less. So i figured that too lean a mixture at idle was the cause. But as i went out further it started to get worse again. I then thought that it must be a valve. Hence the reason why i did a compression test on it. When doing the compression test (This bike only has electric start) I removed both plugs and wound motor over with throttle fully open. This was the higher reading that i mentioned in my previous message. Then i performed a compression test with throttle closed. The reading was rear 111 psi and front 98 psi. This really got me confused as first reading showed front cyclinder higher in compression than rear, (throttle wide open). I then checked timing because i figured it was firing before valve was fully closed. But you would think it would run like crap if the timing was out, but it was purring like a kitten. Timing good.

I like your information on pulling choke out to see what happens. I have taken the tank and covers off at the moment to get painted so i will need to wait untill i have them back to rerun the bike. Also the info on Decelleration Backfire has got me thinking.
I am checking valve clearances today. I was always under the belief also, that if it were too lean a mixture it should cough back through carby and too rich a mixture it would cough back through exhaust. It is good to get this help.
Thanks for the info Craig.
I have a few new things to try now and will post a message on how it pans out.
Step one check valve clearances
Step two install carby rebuild kits
Step three rerun bike
Step four change jetting.

[wdietz186]

Al, Pay very close attention to the carb bodies.If the passages for the idle circuit are partly restricted it can cause all sorts of popping and mixture conundrums.If the slide[idle screw] is lifted too high much of the fuel mix entering is being pulled through the needle jet.On overrun more fuel will be pulled through and wind up in the pipe to be ignited in the exhaust pipe on the way past the hot valve seat and valve.The reason the slide is too high? Because the idle circuit can't provide sufficent mixture to idle well.Most all 30yr. old Dellorto carbs have those little white blobs of corrosion or the dreaded brown muck hiding in their most intimate places and a long relaxing soak in carb cleaner followed by some delicate probing with a thin piece of wire and compressed air will usually put things right.

[ALinHB]

Thanks Wdietz186

The carbs are PHF 32mm
Starting jet size 75
Pump jet size 40
Needle K16
Idler Jet size 62
Main jet size 122
Unknown stamped BF1 (22mm long) screws into carby body then idler jet.

I have just been pulling both carby's apart again to do some more checking after the excellant feed back i have been getting from all of you.
I may have possibly found my problem but not sure as yet. When i unscrewed the idler jet and looked inside to see if it was clear after giving it another blow out with compressed air i notice another brass jet! This is not shown on exploded views in both manuals I have. Also have checked exploded views on internet and it is not shown on them either. Don't know what it is called but has a number on it (BF1). The idler jet is a no. 62 Standard size. When I removed this BF1 jet or mixer or whatever it is, I noticed heaps of gum build up and could bearly blow through it. This could be my problem. May well be reducing capacity of idler jet which could be why the compression reading was lower when done with throttle closed.

Don't know if a semi blocked (BF1) jet effects compression reading when performed with throttle closed but it sounds good to me. The idler jet could not be working to capacity with this gummed up. Do you know what this jet stamped BF1 is? Real hassle to remove without damage to thread. It is 22mm long with about 4 full threads on one end. Has small holes along the body and a hole through the middle of it.
Cheers Al

[Lumpy]

Every time I`ve had popping in the exhaust it`s been, as Craig said, an exhaust leak. Point being that if it`s popping in the exhaust there has to be an exhaust leak because it can`t ignite fuel in there if there is no air to burn. As is pointed out that the high vacuum during throttle closure causes a shipload of fuel to go through anyway without popping. Provided the exhaust valve is sealing at least and no burnable charge is getting in that way.
Perhaps roll the front pot over to a point where the exhaust valve only is open, remove the spark plug and put an air nozzle in there. Seal it with a bit of rag, block the exhaust with a bit of rag and then feel around for air leakage, or spray soapy water on all joints and look for bubbles. The header gasket is often the culprit on mine but pin holes rusting through underneath or behind the pipe can cause it too.

[Craig in France]

<snip> When i unscrewed the idler jet and looked inside to see if it was clear after giving it another blow out with compressed air i notice another brass jet! Unknown stamped BF1 (22mm long) screws into carby body then idler jet. This is not shown on exploded views in both manuals I have. Do you know what this jet stamped BF1 is? It is 22mm long with about 4 full threads on one end. Has small holes along the body and a hole through the middle of it.

Hi Al,

What you've discovered is the atomizer jet. Part 54 in the diagram below. It's spec. should be a 265AB - and yup, it should be clean :-)

The carbs are PHF 32mm
Needle K16
Idler Jet size 62
Main jet size 122

All good (there was another set-up, but I won't digress). The slide should be a 60/3, btw.

Also, be aware that jets do wear. They're not expensive so, if in doubt, fit new.

Ciao

Craig

[Craig in France]

Yep you are right, i am confused. I am absolutely positive that there is no exhaust leak or air getting sucked in through any area of the carby manifold.

Hi Alan,

Have you checked:
1. The seal of the exhaust pipe into the head - a) 'cos the gasket behind the clamp ring doesn't last for ever ; b) 'cos the exhaust ring itself can loosen. (P.s I'm sure you know, but just in case; only do 'em up with the proper tool and always lock-wire 'em in place).

2. The joint between the silencer to the pipe. Sometimes, this can be a tad tricky because doing the clamp up tighter can, in certain circumstances, make the leak bigger. But first, take the can off and see if you've you got any carbon on the pipe. If you have, you've most likely got a leak.

Finally, another trick that a friend who's BMW Meister mechanic told me (I haven't tried it myself, I should add): The best way to find the leak in an exhaust system is to blow air back UP the system with the exhaust valve closed - and then check for noise or use soapy water etc.

I then thought that it must be a valve. Hence the reason why i did a compression test on it.

Yup, I follow your line of thought. However, I don't think a compression test is going to be sensitive enuf to tell you if the valve is staying open a very small amount. A leak-down test with the valve shut would, but I'm not confident that a compression test will.

Also, a compression test is really a comparative rather than an absolute test - one cylinder against another, or the same cylinder over time. As to value, it should be atmospheric pressure (which varies with both weather and altitude, of course) x the compression ratio - which itself is kinda hard to calculate to real accuracy. So you can see why you won't find a value in the manual . FWIW, I think the last time I did mine, I ran out at around 125 - 130, but don't quote me on that .

Ciao

Craig

[ALinHB]

Hi Craig
Sorry but it is not atomiser jet. The atmosier jet screws into carby body then jet holder then main jet.
The little bugger i have removed, screws into carby body then idler jet. It is shown on the diagram you have provided as item 64. But it is not shown in my manual's exploded view. What is it called in your manual please. Is there any info on it and what it is meant to do.
Thanks Lumpy aslo for the info.
With regards to the exhaust leak theory, when i got the bike it had a set of stay tuned exhaust pipes on it and were in poor condition. The bike had problem then when backing off. Instead of mucking around with them i purchased a new set of Conti exhaust replica's. New headers and installed new caskets. As i was believing myself at first that it was because of air leaking in somewhere on the stay tuned set. I paid attention to detail and yes were wired to stop coming loose. I had removed and refitted them again after going for a ride because problem was still there. I am absolutely sure that there is no air leak. I will do the air pressure test just to satisfy my curiousity. I think the problem is with this little jet that is shown on the diagram provided by Craig. I have pretty well stuffed one trying to get it out. The alloy thread is very fragile in the carby body. This jet on the rear carby came out easy. I might bite the bullet and get a new set of 36mm carbs and put these PHF 32mm carbs in the press to destroy them forever so nobody can ever have this frustration like me.

Al

[BevHevSteve]

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

[Craig in France]

Hi Craig
Sorry but it is not atomiser jet. The atomiser jet screws into carby body then jet holder then main jet. The little bugger i have removed, screws into carby body then idler jet. It is shown on the diagram you have provided as item 64. But it is not shown in my manual's exploded view. What is it called in your manual please. Is there any info on it and what it is meant to do.

Understood, Al. Hmmm ... That drawing is from the 1980 workshop manual which, unfortunately doesn't have an accompanying parts list. And I don't have a copy of the 1980 parts book, i.e. the one that might be able to help ...

But I think Steve may have sussed it. I've only got a 32 AD to use as a base reference, not a CD/CS unit. And to judge by the jetting spec you gave, I think that you have CD/CS carbs ?

Edit: Think I may have found it - in the 1979/80 SS parts book (these models also used CD/CS carbs). There's a (albeit faint) copy of the parts book on Steve's site - tavola 9.

It's reference on the drawing is 47, the Ducati part no. 0799.27.980. And it's called a 'Diffuser' - Emulsionatore minimo. No more detail, tho' ...

Ciao

Craig