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New carbs - where to start...

Posted: Sat Jan 24, 2015 1:09 am
by pilotg
The scenario...

1981 SD 900.
2nd oversized Pistons
Good battery
New cables
Brand new PHF36mm Dellorto carbs

I've got the bike back together fine. The carbs were fitted as delivered from Steve. After allowing the fuel to run into carbs for a minute or so I opened the chokes on both, gave the throttle a twist and hit the button. Amazingly the bike fired and idled immediately. I gave it quite a while to warm up, a good 5 minutes or so, then closed one choke. The bike died instantly. Restart easy with both chokes open again. In fact even after 10 minutes the bike stalls when the chokes are closed. It will rev beautifully with chokes open. I've tried closing the chokes with the bike at 2000rpm and then we have a snarling farting popping beast that still dies.

My question is where do I go from here? I'm reasonably handy. Apart from the high tech parts I've restored 80% of the bike so far. But when it comes to messing with carbs and timing etc I'm a complete novice. I need some help. I'm planning on getting the bike through the Warrant of Fitness inspection this week. It was certified until last month. Everything else is done. So, what to do?

All advice welcome

Many thanks

Re: New carbs - where to start...

Posted: Sat Jan 24, 2015 9:19 am
by BevHevSteve
2 things:

Prime the pump and sync the carbs correctly.

Re: New carbs - where to start...

Posted: Sun Jan 25, 2015 6:55 am
by machten
I haven't tried to use 36mm Dellorto's on a Darmah, so I can't relate specific jet sizes to you, but as a matter of principle, if you have idle with choke on and you turn the choke off and it stops running, then you are most likely not getting enough fuel through your idle circuit. This could be because your idle mixture screws are set too lean or your idle (pilot) jet is too small, or your idle speed is set too low, or any combination of three. The other possibility is a blocked idle circuit passage, but with new carbs, that's unlikely.

I don't think the pumper comes into the equation until you use the throttle. The start point and duration for the pumper depends on the slide profile. I'd leave that until later. There are some good Dellorto manuals available on line that explain PHF workings through progression from idle to full throttle. Steve may well have some on this site., if not, you can find them.


Re: New carbs - where to start...

Posted: Mon Jan 26, 2015 12:18 am
by pilotg
Hi Kev,

Nice to chat again. Firstly thanks so much for your help in the past and for the advice this time.

I had another brief look at it today. When I tried to start it this time only the front cylinder would fire. When I gave it a few revs there was a lot of back firing and flames from the rear cylinder. Who knows.

Re the adjustments...

The two carbs are not identical. The front one has two screws next to each other. The first has a spring and appears to be close to the centreline of the slide. The second looks to be tight and is on the cylinder side of the carb. The rear cylinder carb has the same two screws but on opposite sides of the carbs, i.e. the screws are not together. Which screw is the mixture screw? I probably sound quite ignorant but I'd rather be that and ask the question than really mess it up, especially when I'm this close. The other query is where is the non return valve?

I've read Steve's carb tuning/set up advice several times. I don't have a set of vacuum gauges and can't find a local set to buy so there's no quick remedy to that one. Is there a rough way to do it? Enough to get me riding again? I can put up with poor performance and fuel economy in the short term.

The bike is in Christchurch and I have no local contacts that i can call on to drop round and sort it out. I'm now doing fly in fly out to Brisbane 9 on/7 off. If you know anyone in either place that I can call on that would be a great help.

Many thanks again



Re: New carbs - where to start...

Posted: Mon Jan 26, 2015 5:18 am
by eternally_troubled
Firstly, to get the simple things out of the way:

My 860GT can take more that 5 mins to warm up, so you might want to leave it a little longer before turning the chokes off, just in case it is 'just' that.

Did you replace the inlet manifold to cylinder head rubber mountings? These can perish and let air in which will screw things up good and proper. If you had an air leak
that would explain why it might only run with the choke on.

The carbs are 'right' and 'left' handed, so to speak - so you can access the idle and mixture screws when they are mounted on the bike. Anyway, the idle screw is the one with the big spring on it that it mounted in the centre of the slide - as you screw it in the slide is pushed up very slightly, this is how it sets the idle speed.

If you have a look at Steve's thread here (you might have looked already): viewtopic.php?f=26&t=10

you can see the initial setup positions for the idle screw and mixture screw - if you haven't set these then you should.

If you haven't got any carb gauges you can use a u tube manometer - a u shaped bit of clear tube with some water in! If you google 'diy carb sync' or 'u tube manometer carb sync' you will get an idea.

Re: New carbs - where to start...

Posted: Mon Jan 26, 2015 11:43 pm
by machten
Hi George,

As "eternally troubled" mentioned the carbs are a left and right handed set. I think the ones Steve has are "DD" and "DS". The second D stands for "Destra", the S for "Sinistra", Right and Left respectively in Italian.

Assuming your ignition timing is OK, it sounds like you may be running too lean a mixture to me. As per the previous post, you should check you have a good seal around the rubber intake manifold gaskets. This is common source of carb tuning issues on bevels. Any leaks there will cause you issues and backfiring can be a symptom of that.

If you've run with choke on for 5 minutes, I'd check my plugs, too. My '78 Darmah is choke off and idling "hands off" inside 30 seconds (but it has PHF 32's and I'm in a warm climate). If I left it on for 5 minutes the plugs would be pretty sooty.

For the purposes of getting to a starting point for tuning, I'd turn the idle speed screws in for a few full turns after you first see them moving the slides up (this will almost certainly be to too high an idle, but you can wind them back after/if you get it running).

I usually start the mixture screw 3 full turns back from fully wound in. Once again, this is for PHF 32's. I haven't seen the PHF "D" series idle mixture screws either (there are different profiled mixture screws for PHF 32 Ax and Cx carbs, for example. I'm sure Steve could advise.

A vacuum gauge isn't really necessary once you know what to listen for. I never use them for carb tuning but do it by the exhaust note sound and feel. I had the benefit of some hands on teaching from some knowledgeable fellows and 35 years owning the same Darmah though, so if I don't know how it should sound by now... :oops: . I don't know anyone in Christchurch, sorry. There's lots of bevels in and around Brisbane though. Have you tried to contact someone through the Ducati Owners Club of QLD? They have a strong bevel scene and someone there would likely be able to walk you through the process if you crossed their palm with some Bundaburg Rum!


Re: New carbs - where to start...

Posted: Thu Jan 29, 2015 12:38 am
by pilotg
Ok. I'm lost now.

I listened to you all and decided I would follow Steve's tuning guide exactly. I pulled the carbs off, checked the exhaust gaskets, loosened the cable tensioned, wound out the idle screw, wound in and then backed out 1.5 turns the mixture screw and then pressed the button. Nothing. Not a sausage. I've gone from an engine that would start and run with the chokes on to a very pretty boat anchor. I have good spark. I can smell the unburnt fuel coming from the exhausts and very occasionally get a backfire.

So where to I go now. I can't follow the tuning guide anymore because I can't start it. It must be something really simple now. It has the following:

New Pistons, rings, rods, pin, main bearings.
New solenoid,
New digital ignition system
New high spec coils
New HT leads, plugs and caps
Rewired throughout
New 36mm carbs
New cables
Fresh fuel.

Steve, what settings do you send the carbs out with? I'll put it back to the stock settings if you let me know and start from there again..

Any advice is really really welcome now. I've had this bike for 4 years and have not had it running reliably yet.

Hopefully, George

Re: New carbs - where to start...

Posted: Thu Jan 29, 2015 7:38 am
by eternally_troubled
More dumb suggestions (which you might have tried already) :

* now you've adjusted the carbs, have you tried starting with the choke OFF and giving it a bit of throttle and/or with the idle screws turned in a bit extra? I'm thinking
maybe we've gone from one extreme (too lean) to the other (too rich).

* if you take the plugs out after trying to start are they dripping in fuel? If they are then you should dry the plugs out before trying again, possibly leave them out of the engine so some of the fuel can evaporate though the plug hole.

* Does your battery now need a charge (after what must be quite a few repeated starting attempts)? I guess that your new ignition might not like a low battery (the voltage will sag while the starter is running). If you have some jump leads you can replace the existing battery with a charged car battery (as long as you make sure the leads don't short out) which will give you more juice to get things going/more attempts at starting.

It would be great it you weren't the other side of the world (I'm in England) then I could come and take a look! I had all sorts of (stupid, mainly) problems getting the 860 GT I acquired to start after it was layed up for a few years.

Re: New carbs - where to start...

Posted: Fri Jan 30, 2015 12:25 am
by pilotg
It's working...

I'll post the details soon. Simple but very valuable lesson learnt. I knew it was something simple...

I'm now mobile. Many thanks Macten and Eternally Troubled.

Regards G

Re: New carbs - where to start...

Posted: Fri Jan 30, 2015 7:02 am
by eternally_troubled
Glad to hear it was something simple - it usually is!

Re: New carbs - where to start...

Posted: Wed Feb 18, 2015 8:03 pm
by pilotg
Not so fast....I'm still bothered.

I went back to work after several attempts to get the bike to idle properly with no luck. Back now and still not having any luck.

The carbs as supplied by Steve are as follows:

Main 130
Idle 50
Choke 70
Slide 60/3
Needle K18 3rd notch

I've checked all these myself today. I have not adjusted anything on these except the mixture and idle screws.

The bike will start first time every time with the chokes on. After 20 minutes of warming up it is still not happy although it will idle ' of a sorts'. The front cylinder is definitely not happy and will fart and blow back occasionally and has a distinctly different tone to the rear. The rear cylinder is a really healthy Blat Blat. The front a Duh Duh. (15 years of eduction and I'm down to Blat Blat and Duh Duh! Good grief). It doesn't seem to matter what I do with the mixture screw. It makes no difference whatsoever.

Whilst I investigate any other intake leaks can I request a consensus on what jetting there should be and what notch? There is another jet that is operated by the floats and that has 300 marked on it. Is that OK also?

Summer is nearly at an end here and I'm surrounded by some of the finest roads on the planet. I'm still hopeful I'll get a ride in but do need help achieving that dream. I'm back to work for 10 days as of Saturday 21/2 and want to order any parts I need before I head off. I'll order two new inlet manifolds as well just to be sure.

Incidentally I emailed Dellorto in the UK and they were of the opinion the set up was way out but did qualify that they do not sell these particular carbs.

All help welcome...



Re: New carbs - where to start...

Posted: Thu Mar 05, 2015 3:02 pm
by riccorman
Did you ever get this sorted?

Re: New carbs - where to start...

Posted: Thu Mar 05, 2015 4:54 pm
by pilotg
No, still waiting for any suggestions. I did change to a K4 needle, 65 idle and AB265 atomiser. The rear runs perfect. The front is still unhappy. Once warm the revs will not remain constant. Can idle between 1000rpm and 2500 without my intervention. I did take it for a 20 minute spin and it was fine for a while. The front then refused to fire for a couple of minutes before restarting the same sickly way. Nothing I do with the front mixture screw makes any difference. The rear responds appropriately. Frustrated to the point of tears.

Any help welcome.



Re: New carbs - where to start...

Posted: Thu Mar 05, 2015 5:01 pm
by BevHevSteve
Do you have a motion pro carb sync tool or the newer and better carb sync pro?

Are you using it correctly following the instructions I have given on my writeup or are you trying to make it right with some other method?

I do not buy the "you don;t need no fancy vacuum gauges you can do'r by ear" BS, sorry. You can get it CLOSE but these things work so well they can not be beat.

Since you did new EVERYTHING you need to check EVERYTHING - again,..

Are the valves CORRECT? Don't say yes, check them - again.

Is the timing CORRECT? Don't say yes, check them - again.

Have you checked your coils, plugs and wires and verified they are working correctly? Just because they are new doesn't mean squat.

Have you checked all grounds and put star washers between connectors and frame?

Gas; Just because that is new again, doesn;t mean squat. Dump it into your car and get another batch in the tank and while you are at it, make sure BOTH fuel taps flow WELL.

Did you take your gas cap apart and make sure the vent is not clogged? Didn't think so........... :*<: Probably the most widely overlooked bit of trickery out there that will keep things from going right...

I just read all the posts here, yes, 'dellorto UK' meaning the UK's dellorto distributor - EuroCarb - does sell these exact same carbs PHF36DD/DS carbs. Exact. I have sold hundreds of sets of these to bevel drive owners - all flavors of bevel drive twins are running around with them, a couple of my own bikes have them installed. Sure was an improvement over the original 32s and sure are easier to deal with compared to PHM 40s.

Re: New carbs - where to start...

Posted: Thu Mar 05, 2015 8:23 pm
by Vince
Dont give up,there is always a reason.Sounds like an air leak to me or the choke is not working right.As Steve says take a deep breath and start from scratch.Maybe swap carbs and see it the issue stays or follows the carb.I has a weak off idle power issue with my Pantah for a couple of years.Bought a new Vacuum Gauge as the old one had bubbles in the Mercury and found the carbs would be equal at idle and with a strong twist but the front carb would not respond to a slow pull till it hit 1/4 throttle.Now it is balanced with all inputs and is a pleasure to ride,not needing a drag race start all the time as before.
Fixed with 3 1/2 turns out on the front mix and 1 1/2 on the rear.Well working Vacuum Gauge is a very good thing.Have you looked to see the spark is advancing on both, again with a strobe.Some odd stuff can happen after 30 years to these old bikes.