1978 ducati darmah, low compression?

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78ducati
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1978 ducati darmah, low compression?

Post by 78ducati »

I just bought a 1978 ducati darmah, the guy I bought it from, said the carbs needs to be adjusted, it has the upgraded Dellorto PHM 40, said he overhauled the carbs as well.

The bike will only run on high revs, will not idle unless you are at 5-7k RPM...
I drove it home about 36 miles on the interstate, on 3rd and 4th gear mostly, seemed like it ran strong to me but I was on 3rd & 4th gear...

Do I have a carburator problem, or a compression related issue?

where shell I start??
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BevHevSteve
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Post by BevHevSteve »

Start by putting the stock phf32 carbies back on. Even with those you will want to keep the rpm over 3500.
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78ducati
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Previous owener put on Conti silencers & upgraded cams,

Post by 78ducati »

that's why he upgraded the carbs, don't have original carbies any more...
78ducati
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PHM 40

Post by 78ducati »

So you saying those carbies are race carbs, and meant for high revs??? can't really run them at low RPM?
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Post by BevHevSteve »

40mm carbs are basically too big for these engines especially stock. They are pretty much useless on anything but high rpm.

On engines with flowed heads, large valves, high comp pistons etc then that is a different story.

Since 32s are no longer available new, many people opt for 36mm carbs.
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78ducati
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Post by 78ducati »

Are these the want I want???
do you have parts to rebuild these???

would the 40 lower bowl fit these guys?

Click on Url to see pix of carbies.

http://www.flickr.com/photos/64679126@N06/
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BevHevSteve
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Post by BevHevSteve »

that photos shows 2 AS carbies, A bodies, S meaning SENISTRO or LEFT meaning adjusters are on the left.

You need a set of 1 PHF32AS and 1 PHF32AD carbie or for USA PHF32CD/CS set [the D means DESTRO for you got it - RIGHT. The adjusters are on the right, the D carb goes on the forward cylinder. Remember it this way - D for DOWN on a Ducati]

Yes we stock all the bits and bobs to rebuild dellorto carbs as well as sell new dellorto carbs http://www.shop.bevelheaven.com
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Post by rwhc80 »

I guess seeing this is a forum I am entitled to voice an opinion !

78ducati, firstly,you may well have issues with your current carbs, but I beleive the sizing is a personal issue. I acknowledge that some prefer the 32mm setup over 40's, and others prefer the 40's

I took my 32's off my Darmah and fitted a set of 40mm's .I have no starting issues and I can idle the bike at 1200 rpm. I enjoy the extra response I got from fitting them, and would not change back to the 32s.

As I said, just my humble opinion, but I'm entitled to it !

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Post by Kickaha »

I had no problem with 40mm on my Darmah but low speed was a bit snatchy and fuel economy was pretty crap even after rebuilding and balancing them

I haven't ever ridden another Darmah with 32 to compare though

If I had another stock 900 I reckon I'd try the 36mm or a Mikuni conversion
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Post by machten »

I'm with Steve. I've run 40's and 32's on stock 78 and 81 Darmahs. Unless you're running around a track (with long straights!) I'd pick 32's every time, even (and especially) with Conti's and pod filters . Far more tractable and they idle beautifully at 1000- 1100 RPM. With 32's my Darmahs fire and start on the first compression stroke - every time.

They are not that hard to come across on the second hard market.

Kev
78ducati
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Post by 78ducati »

Thank you much for a quick reply... I just checked my compression on the front Cylinder... I got 50PSI, so going to have to do a leak-down test and see where I'm losing all the compression, I also noticed the guy was running very short reach plugs so compression must have been played with, bigger pistons? higher piston domes :idea: ..., so this could have been around the track at some point, its got hot cams and these big carbs, once I open it up, I be able to tell how big the bore + pistons are... what is the usual hot cam kit setup on these motors?? did it come with pistons cylinders? which cams were available back in the days???

What shell I look for??? valves guide seals? also if this motor has been doing high rev RPM, shell I look into my crank case bushing or washers tent to be destroyed easy on these motors with high RPM???

All I can say is when I drove on the interstate, it felt like the bike had lots of power, once it got down to low RPM the engine will stall and have no throttle respond at all...

I think the total Mileage on this motor is 20k.... what compression is within the norm for this motors?? :idea: :roll:
78ducati
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compression

Post by 78ducati »

compression checked out ok, so has to be the carbies will try to adjust these 40s with some help, will let you know what comes of it,

Anyone care to help? feel free/ jet size? needle position, anything will help


cheers.
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