1978 Darmah won't start

eljefe640 · Post by **eljefe640** » Mon Jan 10, 2011 7:42 am

I just bought my first Ducati bevel twin and I am trying to get it running. I have worked on modern Ducati's and many other bikes, so I do have some experience.

The engine cranks over and will occasionally fire. A few times it actually ran well for about a second. It also backfires loudly and shoots flames out the exhaust. Spark is strong and blue on both cylinders. My inclination is that it is a timing issue, but I can't be sure.

The previous owner already cleaned the carbs. I have pulled the bowls and and the jets. They look very clean, but I am certainly not ruling them out until I have gone through them myself. I am running off of a test fuel tank with clean fuel.

Also, I am getting some odd compression readings on the horizontal cylinder. Obviously these are all done cold. The vertical cylinder consistently gets 120psi. The front cylinder sometimes hits 120, and sometimes won't go over 90. Leakdown is ~20% on both cylinders. I would suspect that the valves aren't closing completely while cranking, but the compressed air from the leakdown test is pushing them closed.

Since it has closer springs, I would think they should always seal reliably while cranking. I have pulled the valve covers and all looks well. I haven't measured the closer clearance, but the springs are intact and nothing seems out of place.

Any suggestions?

Jeff
Austin, TX

machten · Post by **machten** » Mon Jan 10, 2011 8:03 am

Some questions,

What bike is it?
What carbs does it have?
Did the previous owner rebuild it?
Have the Bosch pickups been detatched from the cover at any stage?
Has it run since then?
Have you checked the valve shimming?

Kev

eljefe640 · Post by **eljefe640** » Mon Jan 10, 2011 8:14 am

machten wrote:Some questions,

What bike is it?
What carbs does it have?
Did the previous owner rebuild it?
Have the Bosch pickups been detatched from the cover at any stage?
Has it run since then?
Have you checked the valve shimming?

Kev

1978 Darmah (posted in title, but I should have added it in the subject)
stock PHF32
Don't know
Don't know
Don't know
No

The story I got from the guy I bought it from, was that it was sitting in a garage for ~20yrs when he bought it. He went through the carbs and tried to get it running. He said the only way he could get it to start was with ether. After it was warm he said it would run ok. I did not have him demonstrate as obviously there is something wrong.

machten · Post by **machten** » Mon Jan 10, 2011 8:44 am

OK. So a Darmah has Bosch ignition (that's good - it is high quality stuff), 32mm PHF carbs, also good - easy to tune compared to 40mm.

Many possibilities - it is a process of elimination. Most likely, you are correct - it is a timing issue. To check, you need access to a special tool for setting the electronic ignition pickup position, or some one that has one. Search for Bosch pickup on this forum and you should find more info. If you get stuck, post back.

120 psi is at the mid to lower end, but I've run with no problems other than reduced performance on 110-115 on a Darmah. 90 won't work, but it wont make it backfire either.

One more thing...

"Since it has closer springs, I would think they should always seal reliably while cranking."

Darmahs have desmo heads and don't have any springs. You may have a Darmah with 860 GT heads.

Kev

eljefe640 · Post by **eljefe640** » Mon Jan 10, 2011 9:05 am

Thanks for the help Kev. I just took a look and the pickup wiring has definitely been "worked on".

According to the engine numbers it is a later Darmah engine, so it has been changed at some point. It is definitely a desmo because I have pulled the valve covers. Many Desmos do have springs on the closer rocker. Not sure if all have them or not. My understanding is they are to assist in valve closing for starting and very low rpm.

I sure would like to hear this thing run, but I may have to get a few other projects out of the way first

machten · Post by **machten** » Mon Jan 10, 2011 10:09 am

Many Desmos do have springs on the closer rocker. Not sure if all have them or not. My understanding is they are to assist in valve closing for starting and very low rpm.

Yes, correct, sorry - I misread what you said. 0765. 92. 140 - closing assist spring. They all have them, including both of mine!

If you can find someone with the special tool to help you set the pickup position in the LH cover, that would be a very good place to start.

Kev

DaleMclean · Post by **DaleMclean** » Mon Jan 10, 2011 11:50 am

Where are your mixture screws set on the carbs??
My old girl ('78 Darmah) did similar after mucking about with the carbs.
A good starting point is 2and 1/2 turns out from closed

eljefe640 · Post by **eljefe640** » Mon Jan 10, 2011 12:28 pm

I have the mixture screws at 1 1/2 turns out and the slides at 1 turn up from the bottom. I'll try backing out the mixture screws a bit more.

It's good to hear you had similar symptoms that were caused by carb issues. That seems the most likely to me given that they have been messed with. I'll give them a thorough going over and try again.

wdietz186 · Post by **wdietz186** » Mon Jan 10, 2011 6:35 pm

You probably have a combination of problems and I would start with the carbs.The idle circuit[s] are prone the gumming up if the bike has been sitting.You need to remove the carbs and strip them completely to make sure the passages in the body are clear. You will also probably need to check the pickup resistances with an ohmmeter to see if the insulation has failed, if the pickup leads short to one another you can get an early/late spark leading to bruised feet,ruined starter sprag,smoke and big blue flames. The pickups should read around 220 ohms and if there is crosstalk between the pickups you will see around half that.

eljefe640 · Post by **eljefe640** » Mon Jan 10, 2011 7:29 pm

So I got home tonight and decided I was going to try and check the carbs out a little closer. I put the rocker covers back on and plugs back in. Gave it a couple of twists of the throttle and hit the button. I'll be damned if it didn't just fire right up like it was ready to ride!

After closer inspection, I realized I had inadvertently changed the spark plugs. I had another pair that I was using to test spark sitting next to it, and I accidentally put them in instead. I tuned the idle mix a bit and man those conti's sound good! I have a feeling this is the beginning of a long affair with these bikes...

So, it sounded good, the engine sounded tight and it seemed to run pretty well. But, it did seem to smoke a good bit out of the horizontal cylinder. I probably ran it for a total of 5 minutes, so I didn't really get it very hot. And, I suspect there has been all manner of fuel and other concoctions poured in the intakes by the previous owner in an attempt to try and get it running so it could have been that stuff burning off.

After dinner I decided to go back out and do some compression testing again. I was a bit rushed last night and not sure my initial leakdown tests were done right at TDC.

I started with the horizontal and got ~38% leakdown. Not so great. I can hear air coming out of the crankcase breather. I do regular compression check and get ~100psi. Hmm... So I decided to check the vertical cylinder and I get ~150psi, but it also shows nearly 40% leakdown with air coming out of the crank breather.

Strange to me that the leakdown is the same, but the compression tests are so different. I'm guessing it probably needs a top end rebuild.

I will probably go ahead and rebuild the brake system, clean it up a bit and see if I can get it riding. Then I should be able to better assess the overall condition.

Does anyone have any suggestions for a prudent course of action given this information?

Thanks,
Jeff

rwhc80 · Post by **rwhc80** » Thu Jan 13, 2011 4:49 pm

Jeff,

Sounding to me like it's time top pop the heads off and have a look. Worth the while, as who knows how long it's been since the shimming was done . also gives you the opportunity to check valves, seats and guides.

Not a massive job, ( "although a bit time consuming" )and if you own one of these things, it will have to be done at some stage anyway, so good practice.

Upside is, you will be rewarded with a beautiful crisp running engine once sorted .

Good luck
Rick

wdietz186 · Post by **wdietz186** » Thu Jan 13, 2011 6:04 pm

You may want to run it a bit and recheck the compression and leakdown.If it was sitting a long time the rings could be sticking in the pistons and the valves may has crud on the seats.
But by all means check the valve clearances and also have a look at the manifold rubbers, they leak if old/cracked.

eljefe640 · Post by **eljefe640** » Thu Jan 13, 2011 8:42 pm

I have run it a bit more, but still haven't ridden it. It starts quickly and easily now and sounds very good. However, after about 15-20 seconds it does start to smoke a good bit.

I think I will get the brakes working next, then take it for a quick ride and see how it does.

Are there any recommendations for machine shops to do a valve job and guides if needed? Ideally in the Austin, TX area.

Craig in France · Post by **Craig in France** » Fri Jan 14, 2011 12:14 am

The front cylinder can suffer from corrosion in the bore if the bike is left standing for a long period of time.

Could explain your lack of compression, air loss thru into the crankcase on leak-down, smoke ...

Ciao

Craig