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Discussion Starter #1
98' M900 with racked 41s, Keihin blue velocity stacks, very short/free-flowing exhaust. Generally speaking there is backfire when decelerating from 4k+, or at least a deep burping sound.

When at WOT for a few seconds it feels as it the bike is running out of fuel and the motor will die within a matter of a few more seconds if I continue to keep the throttle pinned. When it dies all electronics remain on. After a few seconds of the motor being off the motor fires right up again and operates normally.

If, however, I immediately let go of the throttle completely while it begins to feel like the bike wants to die and then wait a second or so and gently get back on the throttle just a bit it then the bike will ride just fine. It's as if we gave it time to replenish the the float bowls.

When I had 39s this issue did not exist. This leads me to believe it is probably not the fuel delivery pre-carb, and instead is related to fuel circuitry in the carbs themselves; the only thing that has changed has been the carbs (and the exhaust).

I have rebuilt most of the easier to access gaskets, seals, and float valves + accelerator pump. I should mention that I slightly damaged one valve seat, then used a dremel to smooth it again and tested the operation of the valve by holding the carb at the angle it sits on the intake and ensured the most gentle touch to the float would push the valve into the seat without any resistance. But let's say that valve was stuck inside the valve seat. There would be no fuel for one cylinder, but wouldn't it still run on the other cylinder? And if the valve was stuck outside of the seat, there would be a lot of fuel, but since the bike is already at WOT wouldn't it consume all that uninhibited flow of fuel anyway? If not, it'd be a rich condition for one cylinder, but the other cylinder would still have to run, no?

I'm buying a set of new set of seats anyway, but I am not yet convinced the seats are playing a dominant role in this.

Running 155 mains with "W" heads.

Cheers.
 

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Discussion Starter #3
That was my initial thought as well but when I had the 39s the bike ran perfectly, and it was only one week later when I switched to the 41s did this fuel issue arise. It seems to me the only variable that changed was the carburetors, not the fuel system.

I'll have another go around of the fuel lines but I did not find anything pinching during the last two inspections.

Would you happen to know the required inner diameter of the fuel line?



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Discussion Starter #5 (Edited)
The fuel line was slightly kinked. I installed a new and longer 5/16". There is still the most subtle of kink from the petcock, but it runs much better. Thanks for your help.

That having been said, it did nearly cut off twice again at WOT.

I still feel like the main jet is either too big and/or the valve is being stuck open (but at WOT does that even matter?)

I'll be ordering new valve seats from Sudco next week to rule out the one valve seat as the culprit.
 

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Discussion Starter #6
I am beginning to think that my gravity fed setup may not be appropriate for the 41s. The bike initially came with 39s and gravity fed fuel and did not have issues of any kind.

I ordered two new valve seats from Sudco. I don't think they're related to my issue, but it's useful to rule out issues by merit of fact.
 

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Agreed. I’d check the fuel flow at the carbs. Do you have a tank filter ? Could be clogged or partially, or just inadequate flow. Make sure your tank is venting, that vent line can get plugged too. How short and wide open is the exhaust ? That might be causing the noise when you close the throttle . Think about raising your float level, if the carbs are getting enough fuel in the first place. What mains did you run with the 39s ?
I’d start there, but now you have lower velocity at partial throttle so you might end up going smaller. W heads with short exhaust and 41’s, I’m not sure if that’s an ideal combo, but I’m sure you can get it to run well.
 

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Discussion Starter #9
Your bike doesn't have a fuel pump in the tank? If not, you might try a larger external fuel filter inline.
I do not think so. I double-checked my parts fiche for verification. It seems to have an externally mounted vacuum pump.

4 is petcock which leads to the fuel filter (unmarked).

16 is vacuum fuel pump.

Considering I have W heads - that is to say, smaller valves - I wonder if a larger fuel filter makes sense to be the solution. V heads have larger valves and even with 41s they seem to operate fine using an OEM capacity fuel filter which leads to me think the size of the fuel filter may play a small role if anything? Hm.
 

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Discussion Starter #10
Agreed. I’d check the fuel flow at the carbs. Do you have a tank filter ? Could be clogged or partially, or just inadequate flow. Make sure your tank is venting, that vent line can get plugged too. How short and wide open is the exhaust ? That might be causing the noise when you close the throttle . Think about raising your float level, if the carbs are getting enough fuel in the first place. What mains did you run with the 39s ?
I’d start there, but now you have lower velocity at partial throttle so you might end up going smaller. W heads with short exhaust and 41’s, I’m not sure if that’s an ideal combo, but I’m sure you can get it to run well.
Fuel filter: I have a brand new fuel filter I will replace right now.

Venting: Is the vent line also also know as a breather pipe? I tried looking for it on a parts fiche diagram. I couldn't find a "vent line" but I did find "breather pipe" / #22 in the parts fiche. Pic attached.

Exhaust: Pic attached. I also pictured the header because the cross-section is substantially wider than OEM and maybe it's important for your to see it.

How much can the exhaust affect fueling at high speeds? I know it can affect cylinder scavenging and valve closure but I figure our desmo heads have return springs for rockers anyway so maybe it doesn't apply much to our bikes. I have V heads with oversized valves, IE cams, polished and ported to match 41s ready to be installed when I have time. Maybe the exhaust will be less of a variable for fueling in that situation. I'm saving up for custom cans anyway, this setup is temporary.

Float levels: Doh! I totally forgot about the float level! I'm going to sync the carbs right now using the drill-bit method. I will adjust the float level while they're off.

Mains: I am 99% sure I ran 155 mains on the 39s which are same as the 41s. I need some time but I can verify that.

The issue seems to only arise at full speed at WOT not partial throttle, which leads me to believe there might be a fuel issue and also that issue is unique to variables involved only with the demands of higher flow rate such as a fuel filter or like you had mentioned float levels. I know the 41s have issues with lean bog at low RPMs presumably due to the lower vacuum not being enough to pull in adequate fuel thus the need for the accel pump, but even so that pump seems to need tuning. Next project >:)
 

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same jet in a 41 will run leaner than a 39. like 13.5 instead of 12.2:1. 5th graph here - BikeBoy.org - Ducati 900SS Carb model with Keihin FCR 39 and 41mm carbs

if it had enough fuel for the 39 it will have enough for the 41. the swap would make fuck all difference with the same heads. can't imagine the exhaust would do much either.

i think the 41 issues with lean bog are due to them having the same jetting as the 39. they need a richer needle with a smaller root diameter. call it bad tuning for simplicity.
 

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Discussion Starter #12
same jet in a 41 will run leaner than a 39. like 13.5 instead of 12.2:1. 5th graph here - BikeBoy.org - Ducati 900SS Carb model with Keihin FCR 39 and 41mm carbs

if it had enough fuel for the 39 it will have enough for the 41. the swap would make fuck all difference with the same heads. can't imagine the exhaust would do much either.

i think the 41 issues with lean bog are due to them having the same jetting as the 39. they need a richer needle with a smaller root diameter. call it bad tuning for simplicity.
Before I mess with float settings it might be sensible to first work with the main jet based on the article you shared. Apparently the 41s have 11 - 15% less vacuum than 39s. But what confuses me is that Sudco told me the 41s are shipped with 155 mains because "that works for most applications". Other people with stock 900 motors also used 155s with success (at least not with the bike dying), however to be fair I am running velocity stacks and some of those people mentioned they were using the airbox.

I might play around with the leak jet to address the bog, or even a dual-spray kit. You solved the bog with a needle that had a smaller root diameter?
 

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Yes 22 breathe line, make sure it’s clear and can’t kink. Staintune headpipe with essentially no mufflers, I could see that popping on shut down. It’s basically straight pipes.
 

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Discussion Starter #16
none of that shit is going to make it run out of fuel as quickly as it is. it must have some sort of fuel supply issue, possibly inside the carbs themselves.
Noted regarding the EMR needle.

I see. I was thinking about the main jets being an issue, but I just remembered that when this happens the solution is to back off the throttle or, if I ride until it loses power then to give it a few seconds before starting it back up. If indeed the jet was too small, would I need to give it as much time completely off the throttle (it's about 3-5 seconds) to be able to give it throttle again? The fact that I need to wait before I use the throttle again leads me to believe that maybe it isn't the jets but rather availability of fuel, ie. float levels.

Is my thinking correct?

I'm buying jets anyway because I want to tune the top end. I think I will purchase 158 + 160, and 150 + 153 and EMR needle.
 

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Discussion Starter #17
Yes 22 breathe line, make sure it’s clear and can’t kink. Staintune headpipe with essentially no mufflers, I could see that popping on shut down. It’s basically straight pipes.
It sure as heck sounds like it.

Breather pipe checked out fine. I'll be ordering some main jets but I think the float level is playing a role in this. Will update once I can go for a ride.
 

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Discussion Starter #19
Before opening up the carbs - that fuel filter has been replaced - right? You said you had one available. Don't skip this step.
Turns out the SS filter isn't plug an play for the Monster. I could make it work, but I'm worried the internal design may not be compatible with my geavity fed system since after all the fuel filter for the SS was used in conjunction with a fuel pump due to tank location being higher than the carbs.

I'll have to order a new one.

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Discussion Starter #20
Alright, I feel like dumb or dumber. Pics attached. Zoom in on second and third pics to really see the kink.

I wanted to do one last check for kinked fuel lines. Guess what I found? The fuel line from fuel filter to the petcock is kinked to hell.

Why didn't I catch this earlier? When I investigated for kinks the fuel tank was UP, thereby exposing that fuel line for inspection but also stretching that fuel line enough to remove the kink.

However, this was clearly a false positive because when the fuel tank sat down, that fuel line hid behind the frame, and I didn't notice that it had also now kinked in that position.

I'll address this tonight and post the results. We may have found the culprit. Oh, and the velocity stacks were slightly loose, 1/8" gaps. I didn't want to overtighten them because the previous owner really did a number on the bolts. Time to call Sudco.


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