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DB4 carburettors
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Thu Mar 21, 2019 10:13 am    Post subject: DB4 carburettors Reply with quote

I pulled the carbs out of the DB4 today to give them a clean as it’s been sitting for quite a while with stale fuel in it, probably well over 12 months.

These carbs were a set of NOS carbs that I bought from Italy a few years ago when I was going through a very long saga of running issues on this bike. When I got them I just fitted them straight to the bike without checking any settings, but I did synchronise them and it ran pretty good. In hind sight I probably should have checked setting etc. as I found some differences today.

I’m guessing they may have been set up differently in the factory? Maybe because the rear cylinder runs a little hotter? Not sure.... anyway the differences were....

#1 carb - pilot screw 3 3/4 turns out and no washer on the needle.
#2 carb - pilot screw 1 1/4 turns out and a .5mm washer on the needle.
All other settings and jets were the same.

I found it strange, as to my thinking #1 carb would be way richer in the idle circuit and #2 carb would be richer in the mid range circuit.

I’ve set them both the same at 2 1/2 turns out on the pilot screw and no washer on th needle. Back to work tomorrow so it won’t get a run until next week.
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'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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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Thu Mar 21, 2019 10:23 am    Post subject: Reply with quote

Tested out the new ultrasonic cleaner for the first time, ver6 happy with it Very Happy




Straight out of the cleaner, nice and clean...






No left over bits is always a good result.... except for the .5mm washer from #2 needle


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'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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Bud977



Joined: 03 Mar 2013
Posts: 525
Location: Sydney

PostPosted: Thu Mar 21, 2019 7:09 pm    Post subject: Re: DB4 carburettors Reply with quote

brian wrote:


#1 carb - pilot screw 3 3/4 turns out and no washer on the needle.
#2 carb - pilot screw 1 1/4 turns out and a .5mm washer on the needle.
All other settings and jets were the same.

I found it strange, as to my thinking #1 carb would be way richer in the idle circuit and #2 carb would be richer in the mid range circuit.



The pilot screw is an air screw so out is leaner. Raising the needle makes it richer. So carb #2 is richer both in the pilot and the needle.
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Thu Mar 21, 2019 9:08 pm    Post subject: Re: DB4 carburettors Reply with quote

Bud977 wrote:
brian wrote:


#1 carb - pilot screw 3 3/4 turns out and no washer on the needle.
#2 carb - pilot screw 1 1/4 turns out and a .5mm washer on the needle.
All other settings and jets were the same.

I found it strange, as to my thinking #1 carb would be way richer in the idle circuit and #2 carb would be richer in the mid range circuit.



The pilot screw is an air screw so out is leaner. Raising the needle makes it richer. So carb #2 is richer both in the pilot and the needle.


I'm pretty sure these screws are in the fuel circuit Lech, so out would be richer
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'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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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Thu Mar 28, 2019 11:45 pm    Post subject: Reply with quote

So... put the carbs back in, started it and it ran like shit. Fiddled with it for a while and came to the conclusion that I’d screwed something up in assembly of the carbs.

Pulled the dirty carbs out of the donor DB4 and put them in.... ran a little better but still ran like shite. After a lot more fiddling I ended up swapping the original twin can Bimota exhaust from the donor bike to this one... Ran MUCH better, but still not quite right.

Got the ‘carbtune’ out to hook it up and balance the carbs. Realised that the vacuum line from the front intake manifold to the Paioli vacuum fuel pump was left unplugged as I’m using a remote fuel tank to run it. Plugged the vacuum line and it ran almost perfectly. Balanced the carbs and seems to run fine.

Starting to think that the single carbon can on the exhaust has been a major part of my running problems right from the start, something that I never considered a couple years ago when trying to sort the running problems. The OEM mufflers are way too quiet and too heavy but if it runs nice I can live with that.

I’ll eventually clean the carbs that are on it now, but want to ride it first to confirm how it runs. Then I’ll check the other carbs that I just cleaned at the top of this thread.

.
_________________
'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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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Thu Mar 28, 2019 11:57 pm    Post subject: Reply with quote




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'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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Gammaboy



Joined: 25 Feb 2016
Posts: 188
Location: Brisbane, Australia

PostPosted: Fri Mar 29, 2019 12:52 am    Post subject: Reply with quote

brian wrote:
The OEM mufflers are way too quiet and too heavy but if it runs nice I can live with that.

Could always chop the cans off and have a pair of lighter, louder cans slotted on? If you did it with slip joints, you could swap the cans out as you see fit.
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Fri Mar 29, 2019 1:15 am    Post subject: Reply with quote

I don’t really want to chop up the nice original DB4 exhaust. I’ll be happier if it runs nice for a change. Once I know it’s running right, I could try the single carbon can again and experiment. The single carbon can definitely sounds great
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'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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polyesterpig



Joined: 30 Jan 2011
Posts: 76
Location: California USA

PostPosted: Mon Apr 01, 2019 6:50 am    Post subject: Reply with quote

The stock DB4 mikuni carburetors we’re always troublesome. Even the test articles complained about them. The easy fix in the day was to buy the DB4 Performance kit which contained a moto corsa exhaust, Keihin carburetors, air filters and a different gas tank so the Keihin carburetors could fit. If I remember correctly, the throttle cable location on the Keihins interfered with the stock tank.

I too had an electrical issue on my DB4ie. Went to get it dyno tuned and it was cutting out. I noticed it on the highway coming home before. It would stutter off and on. Ended up being a short in the wire harness. The rear cylinder injector was shorting out somewhere. We tested each wire with a light and figured that one out. Ended up buying a new harness from Bob Steinbuglar. Fixed the problem. After looking at the old harness, I could see where the harness was rubbing the rear shock spring from the factory. Probably chaffed a few wires inside the harness. I made sure my new harness was away from the rear spring.
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2002 Cannondale E440
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2000 Bimota DB4ie BiPosto
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DB1 860



Joined: 23 Mar 2014
Posts: 177
Location: Sydney

PostPosted: Mon Apr 01, 2019 11:38 pm    Post subject: Reply with quote

Brian, I know its abit of a hike... there's a Ducati mechanic down here who has a soft spot for Bimotas... he tuned my DB1 which is miles from standard and its the best its ever run... he has a dyno as well
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Mon Apr 08, 2019 6:43 am    Post subject: Reply with quote

DB1 860 wrote:
Brian, I know its abit of a hike... there's a Ducati mechanic down here who has a soft spot for Bimotas... he tuned my DB1 which is miles from standard and its the best its ever run... he has a dyno as well


Thanks Gerald. It’s running nicely now. Hoping to fit a set of flatslides at some point so may need the help of a dyno at that point.
_________________
'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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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Mon Apr 08, 2019 6:52 am    Post subject: Reply with quote

This was a bit of a worry, but doesn’t seem to have done any obvious damage.

I opened up the airbox to check the air filter when I had the carbs off. I found that the weaved stainless mesh had broken up and the foam air filter was GONE. I’m not all that worried about the faom air filter as it would just be burnt up and spat out.... the worrying thing is that there’s a section of the stainless mesh missing all together.

I have no idea how long it’s been gone for. I’m surprised that I haven’t got a burnt or bent valve as a result. In two minds about wether I should pull the heads to inspect or not.


_________________
'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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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Sun May 05, 2019 12:32 am    Post subject: Reply with quote

I changed the engine oil and filter on this DB4 yesterday and cleaned the long strainer in the bottom of the crank case. On the magnetic plug I found what I’m assuming is the remains of the stainless steel woven mesh that is missing from the air filter support in the air box.

My question is.... how did this get into the sump from the intake of the engine Question It can’t have gone past the rings.
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'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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DB1 860



Joined: 23 Mar 2014
Posts: 177
Location: Sydney

PostPosted: Sun May 05, 2019 4:25 am    Post subject: Reply with quote

Would the magnet have grabbed the mesh if it is truly stainless?
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brian



Joined: 22 Aug 2011
Posts: 3769
Location: Australia

PostPosted: Sun May 05, 2019 9:57 am    Post subject: Reply with quote

DB1 860 wrote:
Would the magnet have grabbed the mesh if it is truly stainless?


Lower grades of stainless are attracted to magnets. Good point though, I’ll check if the mesh does stick to a magnet. It definitely looked like the remains of the mesh wires to me though
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'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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