MHR900 V's 900SS jetting difference??

[Adrian]

Hi all,

I'm a virgin to this forum so please be gentle with me .

My 1982 MHR900 seems to be running rich through most the throttle range though I've yet to doubly confirm this.

The factory settings for the bike's 40mm PHM BS & BD Delortos are supposed to be 140 main jet for rear and 135 for front and K5 needles.
However, the previous owner put in 152 mains and K4 needles which is the spec for the PHM AS & AD carbs on the 900ss 40/conti.

Is this jet/needle upsizing for the BS & BD's common practice? Should I leave it in place?
I'm a bit puzzled about the jetting difference between the two types of carb which outwardly look identical. Emmission control perhaps?

Also does the main jet size difference between front and back cylinders really make any difference?

I'm going to muck around a bit using one of those Colortune gadgets (has anyone used them? Comments?) in the meantime any advice would be greately appreciated.

BTW bike has Conti's and K&N's.
Also the bike tends to cough and splutter a bit just off idle.

Adrian

[BevHevSteve]

Hi Adrian, It seems that you simply should start by do a rebuild of your carbs, and then properly sync them. If you *still* have a problem, then lets consider jetting changes. Carbi sync is in my opinion, THE most important step in getting these bevel drives running *right*

Fortunately, I have outlined the proper steps elsewhere on this forum. Click the link below....

http://www.bevelheaven.com/~BB/phpBB2/v ... c.php?t=10

[Adrian]

Hi Steve,

I rebuilt the carbies as soon as I got the bike but used same sizings as what I found. I continuosly synch them (As you do with bevelheads)
But as explained above, the 'as found' sizings do not conform to the Ducati spec for PHM B's (which the bike has) rather they are for the PHM A's.

I did some reading last night and may have answered my own questions:
Mich Walker in his book 'Ducati Twins Restoration' reckons that earlier
900ss (and MHR900) models had their Delorto's tuned on the rich side to optimise power but later models were re-jetted for reasons of economy and emmission control.

This would explain the difference between the A's and B's - looks like the previous owner might have restored the jetting and needle size to the A settings for reasons of power.....

I mean the bike runs well (except for above mentioned hesitation just off idle), its just that I noticed that it seems to run quite rich through the rev range. This also might explain why the original owner might have dropped the needle a notch. I set them at the centre groove per the spec but might put them back on the lower groove and see what happens.

I also noticed that the factory spec for the 900ss has the same sized main jets for both front and back cylinders but the MHR has slightly larger rear main jet. Cooling issues created by full fairing perhaps?

Would be greatful if anyone could add their experiences to the above stories.

Adrian