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Problems with '94 DB2 Carb Diaphragm Operation

 
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wallaman



Joined: 01 May 2012
Posts: 58
Location: USA

PostPosted: Sun Jul 07, 2019 2:27 am    Post subject: Problems with '94 DB2 Carb Diaphragm Operation Reply with quote

Haven't been around for a while, but bringing a 1994 Bimota DB2 out of hibernation. I've been through the carbs with new gaskets, O-rings, and diaphragms (stock Mikuni 38mm CV carbs). If I blow through the diaphragm vent hoses, both slide/needle assemblies lift as they should so I'm reasonably certain the diaphragms are seated correctly. No leftover parts, no leaks, no drips, etc.

Engine starts, idles and revs on the stand, but one slide in one carb doesn't lift as it should when you rev the engine. Also, when the engine is running, the side with the non-operational slide has a lot of "pulsing" in the diaphragm vent hose while there is what feels like no airflow or pulsing through the other vent (the side that is working). What am I missing here? Any help here appreciated.

Thanks.
Chuck S.
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Sun Jul 07, 2019 2:01 pm    Post subject: Reply with quote

Only thing I can think is that the vacuum gallery to the top of the diaphragm is blocked.
_________________
'99 DB4 #104, '96 SB6 #1165, '94 DB2 J #652, '99 DB4 #088, '08 VTX1800, '93 ZXR750R M1, '95 ZXR750, '95 ZXR750 Race Bike, '94 CBR400rr NC29 Race Bike, '94 CB250, '49 BSA C10 250, '61 BSA A10 650, '89 ZXR750, '91 Ducati 851
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wallaman



Joined: 01 May 2012
Posts: 58
Location: USA

PostPosted: Fri Oct 18, 2019 11:11 pm    Post subject: Reply with quote

So finally sorted my DB2. What I thought was a carb problem was not a carb problem. I had noticed that compression on the rear cylinder was a little lower than on the front, but thought I'd ride it for a bit to see if it came back with a little "exercise" after sitting so long. Never did a leak down test. Turns out that the intake valve on the rear cylinder was very slightly bent. The bike would start and idle, but as revs increased, the vacuum in the intake track wasn't (because of the "leak"). So.... the signal to the CV carb was all wrong. Replaced the guide and valve and all is good. Bike runs great! Next time, a leak down test for sure.

Hope the above is of interest.
Chuck S.
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quikduk



Joined: 13 Aug 2016
Posts: 287
Location: Southern California, USA

PostPosted: Fri Nov 01, 2019 7:53 pm    Post subject: Reply with quote

How did that valve get "very slightly bent"? Was the adjustment too tight or did it get over-revved (sp?) and float until the piston nicked it a little?
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wallaman



Joined: 01 May 2012
Posts: 58
Location: USA

PostPosted: Fri Nov 01, 2019 11:39 pm    Post subject: Reply with quote

Would love to tell you I knew how the valve got bent, but now I know why the bike had sat for as long as it did with the fairing and seat/tank cover removed. (That's how I got it.) The valve adjustment seemed OK but guessing it might have depended on the valve's position radially in the guide when it was checked. I found a good size piece of the tip of the intake valve guide (~1cm x 1cm) siting in the intake tract! Given the coloration of the edges of the cracked surfaces, it must have run like that for some period of time. How the piece didn't get sucked into the cylinder, I don't know, but I got lucky there! Given the bend in the valve, thinking the valve adjustment might vary depending on how much the valve rotated in the guide. There were no obvious marks on the top of the piston that would indicate a valve-piston "collision" (doesn't mean it didn't happen) so I don't know if the guide broke first resulting in the bent valve, or if the valve just "kissed the piston" and bent and broke the guide (guessing the later). Other than replacing the guide and valve, everything else looked really good. I've been riding the bike for almost a month now with no issues at all and thoroughly enjoying it! The only downside is I'm old enough that I don't "fold up" as well as I used to, so longer rides leave me a BIT sore. This age thing is a PITA! LOL! You'd have thought I'd have remembered why I sold my DB4 a few years ago! Smile
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Ray916MN



Joined: 29 Apr 2015
Posts: 71
Location: Orono, MN

PostPosted: Sun Nov 03, 2019 7:47 pm    Post subject: Reply with quote

If you change out the brake and clutch masters for ones with remote reservoirs, you can fit clip ons with more height and adjustability to make the bar position more comfortable.
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wallaman



Joined: 01 May 2012
Posts: 58
Location: USA

PostPosted: Sat Nov 09, 2019 1:39 pm    Post subject: Reply with quote

Ray,
Appreciate the thought. I already have Brembo radial masters fitted so there is a bit of room for some "higher" clipons. That said, The lower back surgery I had some years back has taken about all the abuse it's going to, so upright is getting to be a requirement and clipons a thing from my past. Still enjoying a ride to get a coffee, but not sure how much longer the DB2 will stay in the garage. I still love riding her though! Brings back memories of my GP days! Very Happy

Thanks.
Chuck S.
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Fri Dec 06, 2019 2:30 am    Post subject: Reply with quote

Glad you got it sorted. A tricky one to diagnose for sure. Strange how the valve was bent though
_________________
'99 DB4 #104, '96 SB6 #1165, '94 DB2 J #652, '99 DB4 #088, '08 VTX1800, '93 ZXR750R M1, '95 ZXR750, '95 ZXR750 Race Bike, '94 CBR400rr NC29 Race Bike, '94 CB250, '49 BSA C10 250, '61 BSA A10 650, '89 ZXR750, '91 Ducati 851
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2bims



Joined: 03 Apr 2010
Posts: 7289

PostPosted: Fri Dec 06, 2019 9:37 am    Post subject: Reply with quote

Quite a simple one really....but not potentially good news....Ducati 2 and 4 valve engines are an "interference engine"...as in the pistons and valves occupy the same space...but not at the same time....If the belts aren't properly set with the crank marks...OR...they aren't replaced regular and one snaps....then the valves occupy the same space as the piston...at the same time....if youre lucky...and it sounds like you were....its "just" a case of a bent valve thats been hit by the piston...if youre not lucky.....its a knackered piston and barrel also
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wallaman



Joined: 01 May 2012
Posts: 58
Location: USA

PostPosted: Sun Dec 08, 2019 6:55 pm    Post subject: Reply with quote

2Bims,
All conventional 4-stroke engines are "interference engines" by your definition. On mine, no belts were broken, and belt timing was correct on each belt. NO marks on the top of the piston and no broken bits other than the tip of the guide and the very slightly bent valve. I am told by a very experienced Ducati mechanic that the original valve guides on the the early 90's era 2-valve engines were quite brittle and prone to breakage like the one in my engine. Maybe there was a collision that caused the valve guide to break, but I'm not certain that that was the order of things. As for "simple" to diagnose, maybe if you were here, you would have gone straight to it, but I suspect not. The good news is its fixed and the bike runs great! Glad it worked out as well as it did.

Chuck S.
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