still problems with rear cylinder carb ?????

[jerry]

This is driving me mad,,, The bike runs but will NOT TICKOVER or run below 3k revs on rear cylinder ,front cylinder and carb are perfect,

I have rebuilt the carb 4 times now with new parts it has been thoroughly cleaned , in ultrasonic bath twice , blasted with air ,, each time I have fitted it the fault is still the same ,,it has new float , new gaskets, needle, airscrews, even the pilot air cutoff valve is new ,,,baffling ???
The right hand front cylinder carb is fine

I have put other spark plugs , HT leads , coils and igniter units they are from one of my other bikes but the fault persists so its not Electrical ,, There are NO airleaks I have now spent some 80 hours on thsi one problem and im run out of ideas ???? totally baffled

[Ron1000]

In times like this I always think it’s best to go back to absolute basics.

First question I’d ask (as a carb Lehman) is what makes a bike run nicely on idle in the first place?

Idle jet needle (if such a thing exists)

Individual idle adjusters somewhere (if such a thing exists)

[Darren69]

Is the choke circuit closing properly on both carbs? IIRC its nylon pins operated up and down by the choke mechanism/lever but my memory is a bit sketchy at best, since it's been 20 odd years since I had a carbie one.

[manwithredbike]

Jerry, I had similar problem with my rear cylinder last year, though i didn't go as far as you in rebuilds etc.
I resolved mine eventually by simply screwing the air screw on that carb out about 4 or 5 full turns (can't remember exactly how many) with the front carb sitting perfect about two turns out.
edit - by air screw, i mean idle mixture screw

[vince53]

i sure you have done it already but Compression test ?

[jerry]

Originally Posted by Darren69

Is the choke circuit closing properly on both carbs? IIRC its nylon pins operated up and down by the choke mechanism/lever but my memory is a bit sketchy at best, since it's been 20 odd years since I had a carbie one.

Yes Darren it is closing properly

[jerry]

Originally Posted by vince53

i sure you have done it already but Compression test ?

all good and valve clearances are also spot on checked twice ha ha

[slob]

microscopic hairline crack in the casting somewhere?

[Darren69]

There's a 600 set for sale £70: -

https://www.ebay.co.uk/itm/Ducati-Mo...4AAOSwwudgHXkM

Because you seem to have tried everything. There's a couple of Yam TDM/TRX 850 sets too which I think are the same basic type.

[jerry]

Originally Posted by Darren69

There's a 600 set for sale £70: -

https://www.ebay.co.uk/itm/Ducati-Mo...4AAOSwwudgHXkM

Because you seem to have tried everything. There's a couple of Yam TDM/TRX 850 sets too which I think are the same basic type.

Yes I saw that 600 set Darren ,, but they are a total mess would need full stripping ultrasonic clean and rebuilding ,,

I am considering getting a set of FCR 39s from USA or Japan as Allens performance Price quoted me of £1470 is silly and ripoff ,, even paying shipping and customs etc I would save £500 on Allens price

Today Im checking valves and Compression on rear pot for 3rd time

then I will take the carbs off the other M750 the 1997 model and put them on the 2000 one with the dodgy carb and see what happens ????????

[Mr Gazza]

Is there a vacuum pipe coming off the rear tract that could be split or connected to a tap or pump that is letting in air?
You would probably have noticed the tap or pump not working properly thought?

If you hadn't already gone through everything meticulously several times, I would suspect the float height differing between carbs. However it would be very difficult to establish if a float valve or float was sticking shut or catching somewhere so as to spoil smooth operation. The pilot circuit relies on fuel level in the bowl.
It's going to be something very simple at the end of the day.. just which day I guess..

[Darren69]

As Gazza suggests, I suspect you may have a sticking float/valve. Tapping can sometimes free them. But you've probably tried that already. Good luck with the carb swap!

[jerry]

Originally Posted by Darren69

As Gazza suggests, I suspect you may have a sticking float/valve. Tapping can sometimes free them. But you've probably tried that already. Good luck with the carb swap!

all float and float valves are new

I rechecked the valves and shims also ran compression test both e cylinders 115psi

bike still not running on rear cylinder below 4000 rpm ? have now removed carbs again and am taking the carbs off the 1997 monster to put on the 2000 one ,,,

im going to dismantle the bad carb again but what to do with it ??? I am at a loss

[Darren69]

It could be a jet issue as Gazza said idle jet relies on the float bowl. Your on the main jet at 4k and below that your not. So idle circuit could be blocked somewhere. I don't know how you can resolve it but if the carb swap is good, then just more cleaning?

[rollo22]

Jerry,
This may help maybe a new idea to investigate
https://www.ducatiforum.co.uk/thread...-way-up.11701/