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Old 10-12-2013, 09:48 PM   #1
CaligoneTx
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Quack attacking (infamous duc intake backfire)

The newest project is 99 748... oem filters, termingnoni exhaust unwrapped and inner tubes wrapped in exhaust wrap, I have ferraci chip enroute.

Since I picked it up, it was serviced by a Honda/Duc dealer on 45.... owner was French, loved the bike with a passion.

The vertical cylinder forward head cover was loose(several threads) causing a massive oil leak. O rings at fuel quick disconnects replaced due to leaking. The fuel regulator wasn't connected to vacuum; the tank vents to ground had engine vacuum(fkn duc mechanics), needed a chain, adjustments etc.

I performed the online tps adjustment to 150mv, then set throttles even using a vacuum gauge set.

My intake was backfiring horribly before I repaired the oil leak. After I adjusted the throttle balance I now only get backfire and studder at light held throttle ie cruise or decel/accel transition. Worse when cold, mild at op temp. The by pass screws are all the way in, the CO adjustment is a little rich from mid position in the ecm. Runs much better then when it had before.

I did notice, mind you I'm using vacuum gauges, the forward cylinder has a harder pull and vacuum drops of more, the pulses get wider until we hear the intake pop just off Idle. Vertical cylinder doesn't do this. Runs best as we adjusted it with throttle stop controlling idle, bypass screws in all the way and CO as mentioned above. At 3k on the vacuum measurements are exact on.

No decel popping etc.

No, I do not have a CO meter to compete the adjustments.

Since I'm a stickler for things via career and projects, yrs it's very tolerable but I know it can be better. It's my around town bike, I refuse to agree w all the online people saying "it does that cuz it's a duc"...

Suggestions from the gurus w experience?
I appreciate the time, thank you.
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Old 10-13-2013, 08:04 AM   #2
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More than likely it will always have some back firing on decel. My 99 sps does it and its been tuned by a certified Duc mechanic. Just saying
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Old 10-13-2013, 02:53 PM   #3
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Backfiring on decel out the exhaust isn't all that uncommon for a Desmoquattro, but intake backfiring isn't particularly normal. Who did the last valve adjustment, how long ago and how thorough were they when they inspected for flaking rockers?

Also, though the pressure regulator on the 748-998 tanks have a nipple for vacuum on them, nothing is attached - at least on the 996s that I've had. I'm sure it was sourced from an existing automotive application that provided the desired pressure and the pump module was designed to accommodate it. Try disconnecting that first.

An old 996 that I had back around '94 snorted through the intake occasionally when I got it. Valve adjustment took care of it on that bike. None of mine were flaking, but they were sloppy loose on a couple.

Taking the oil cooler out of the way and removing the front cylinder exhaust side cover would probably be the easiest to access if you just want to slip a feeler gauge in at TDC and get an idea. If they're off each other or one or both are way off, it's a good indication that the other 6 are out as well.

The Ferracci chips are a mixed bag. Some really like them. I had one in my first 996 when I bought it. I wasn't ever happy with part throttle mid-rpm response with it, and ended up going with a Power Commander. I still have a Ferracci chip programmed for a 996 with their slip-ons if you know anyone who wants to buy one.
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Old 10-13-2013, 04:13 PM   #4
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im coming up on the valve adjustment interval.

seeing all the valve covers loose from the shop this guy was taking to pretty much guarantees they didn't know what they are doing; and stories ive heard from Honda owners clarifies that concern.

I do need to figure out where the vacuum lines go under the tank; I was going based off internet pictures etc; I need to look through the repair manual (wasn't a vacuum hose diagram) to see exactly where the vents and vacuum supply went from the reservoir canister(assumingly to that regular; there isn't anything else vacuum controlled on the engine.)
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Old 10-13-2013, 05:06 PM   #5
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None of the lines on the old superbikes go to vacuum. The two under the tank are a vent and a drain - they both vent to atmosphere under the bike unless it's a California emission model in which case there's a carbon canister for evaporative emissions. The only places on those bikes to pick up any vacuum are the tapped/plugged holes on the intake runners and they're only used when you're synching the TBs and plugged the rest of the time. There's no MAP sensor, only a Baro sensor that's vented to atmosphere so no line there either.

The MM 1.6 injection on the old DQ motors is functional but prehistoric compared to a lot of other ECMs. Open loop speed density with no MAP. It only cares about temp, RPM, BARO and throttle angle. Not a lot going on, but the simplicity can be a good thing as well sometimes.

Main thing on the drain and vent hoses coming from the tank is make sure they're not in a place where the output from either is aiming straight at an exhaust pipe. It's not all that likely, but if raw fuel DID come out the hose, you certainly don't want it on your exhaust.
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Old 10-13-2013, 05:36 PM   #6
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no hose on the regulator, should be open, also valve specs are the most important of ANY engine to my knowledge, also the tbi sync is very important, and make sure and use the factory plugs
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Old 10-18-2013, 09:10 AM   #7
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Thank u guys.

I do have a California bike. I got the service manual and started reading. I had already inadvertently put both vents to drain when I saw vacuum running to one if them. The other shop didn't have one side of the canister connected. I'm going to go back into the system, as Chad mentioned above, and remove the vacuum supply from the nipple vent of the regulator, simplify the system also.

I also read the bypass adjustment factory is usually about a quarter turn off close... mine were at three full turns out at least. No wonder it ran so bad.

I know the valves were at least checked, the one cover was almost not screwed on when I got it... my finger could slide between it. But I'm doubting it was adjusted. As I review service intervals, there's no way that timing belt was replaced. So I got some tuning to do. I'll b visiting Motorcycles Unlimited in near future for some parts I'm sure.

Im a tinker-er much like you all that have responded, so looks like I got a good project to focus on. Thank you all. The advice I've used and applied already has made the bike 3x better since I've gotten it.

(Pics are old, already has open clutch, levers, modified exhaust and more... can't keep anything untouched for long)
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Old 10-18-2013, 09:15 AM   #8
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Good looking bike. I like the black wheels
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Old 10-18-2013, 03:17 PM   #9
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Seeing as you also own a triumph D675, like yours truely, a lot of people use blockoff plates to reduce popping on decel. Do Ducs have the same option?
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Old 10-18-2013, 03:24 PM   #10
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Doesn't have air injection in the exhaust.

I run a little different set up then most(or any I know of) on my brit.

Its a danmoto that allows plenty of breathing. Only pops once for each cylinder or transition to decal; not continuously nor on shifts.

(running a BMC, reprogrammed ECM also)
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