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Discussion Starter · #1 ·
Hi

I am in the process of rebuilding a 900 carb engine. I have mikuni flatslides and have found that I must run them a bit richer than otherwise ideal to get rid of the popping in the airbox when cold.

I thought the backfiring could be caused by the wasted spark igniting the mixture when the intake valve is open if the mixture fails to ignite at the combustion stroke.

Converting the ignition to single spark would be an interesting experiment and would also reduce the power consumption of the ignition by half.

I am running the ignitech tcip4 so the conversion would be easy if I could fit the timing gears and pickup from an IE engine.

Does anybody know if the gears will "drop in" the carb engine? The place for the pickup hole is there in the alternator cover casting but would have to be drilled open (no big deal).

This mod would also allow more advance to be used at part throttle if I wanted to fit a TPS to the flatslides.
 

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yes, the i.e. timing gear will fit onto the carb SS timing shaft with no problem,
these parts are identical and should have the same part # IMO.

backfiring into the airbox is a matter of a too lean mixture IMO.

:)
 

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Discussion Starter · #3 ·
The timing gears are not the same, the carb version has helical gear and the ie engines have straight cut gears. The ie camshaft drive gear has 2 narrower teeth to give the signal to the pickup, which is why they would need to be changed.

I would need to confirm that the shaft diameters and the gear widths are the same to make them fit.

I realize the popping is caused by leanness, but the carb ignition is very sensitive to it, the popping starts already at afr 15:1. If the carbs are tuned to stay lower than that when cold, they run quite rich when warmed up. I thought maybe the single fire ignition would tolerate a bit leaner mixture. Saving about 30 watts of power consumption wouldn't hurt either to keep the battery fully charged.
 

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If it was me I'd have a quick check for a carb air leak, espacially with a new carb set up. What flat slides are you using?
 

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Discussion Starter · #5 ·
Tdmr flatslides. There's no air leaks, checked that. The stock carbs did also backfire to the airbox, but the cv design muffles the backfires so much that they're not really noticeable, but there was black soot in the stock carbs too.

I'm not really trying to debug the backfiring with this mod. I can do away with the popping by tuning the carbs rich and have now jetting that does not backfire but is about 11.5-12 afr when warm.

I'm interested in doing this because I want to make the bike run as smoothly as possible and also want to find out if this is a useful mod to achieve it. The cost for doing this would be minimal, the gears can be had for about 25 usd and used pickups are not expensive either.
 

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Make sure your pick-up coil cables are as far away from the coils as possible as the coils can interfere with the signal. I also wrapped the lines and igniter boxes in foil tape and earthed it to screen the. No more spitt-back through the carbs.
 

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Do the i.e. bikes only spark on the compression stroke then? I ask because I have a 916 which is obviously i.e. and uses the same type of pickup but I thought that both plugs sparked together every rev?...
 

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Discussion Starter · #8 ·
Dukedesmo: at least they have the pickup on the belt drive gear which rotates half as fast as the crankshaft.

Guess I'll have to find out if the ignitech has a wasted spark when set to the ducati 48-2 pickup type.
 

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Dukedesmo: at least they have the pickup on the belt drive gear which rotates half as fast as the crankshaft.

Guess I'll have to find out if the ignitech has a wasted spark when set to the ducati 48-2 pickup type.
Good point, but the reason I suspect the 916 has a wasted spark is that when setting up my digital dash, the rev counter has different settings for different cylinder configurations and it needed to be on the first (or last?) setting to give the correct RPM.

So unless the dash doesn't work on a single-cylinder bike then I assume that it is firing every time and that I'm actually on the single-cylinder setting? :confused:

900 i.e may be different to a 916? but it uses the same timing sensor (at least an ST2 does) also the Ignitech may allow different setup?

Interesting stuff though, especially as I am also in the process of building a 900 (944) with split carbs, Ignitech and other mods...
 

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none of the injected bikes run wasted spark. they all fire once per cycle.

i'm pretty sure the carb alt cover has the pick up casting marks for the digiplex or p7/p8 ecu positions, not the 16m. the p7/p8 uses a single trigger on the timing gear for cycle position reference, which is a welded boss about 3/4 distance out.

the 16m 48/2 gear uses the timing gear teeth, so the pick up is positioned further away from the timing shaft axis. so you'd need to check the casting bosses to see whether they're relevant or not.

the tachos read a certain number of pulses per revolution (maybe 4, i forget now) as sent out by the ecu. so the tacho drive needs to be set up correctly as required.

i'm sure all 16 spline crank (1992 sometime) onwards timing gears are the same in terms of crank size. but the timing shaft diameter varies at the lh end bearing. not sure if there's a gear variation also. maybe get a pre 98 748/916 set, should be more accurate than a post 98 set for example. or check the parts lists for bearing sizes and match as required.
 

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Discussion Starter · #11 ·
When I look at pics of the alt covers, there might be a little difference in the pickup boss position between the ie and carb engines as Brad said. If this is the case, it wouldn't be as straight forward mod to do.
 
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