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Old 07-23-2019, 09:07 PM
Tobiczyk Tobiczyk is offline
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ignition combinations

Hey all.

Been working on an SG build for what seems like forever now (going on 3 years i think) and finally getting ready to get it put back together. With that I have a questions about the ignition that I couldn't find on my own.

So I have a ton of spare parts. vacuum advances from all years. 2H7 and 4RO TCIs. etc...

I am replacing the original engine with another 3H5 unit that currently has the stock mechanical and vacuum advance units on it. What I am thinking about is the difference in running the 2H7 and 4RO TCIs.

From my understanding, all models except 81 have the mechanical advance (with some variation) while 81 added it to the TCI but still retained the vacuum advance. I also have read a few times on here the vacuum advance really doesn't effect performance, just efficiency. I am replacing the carbs with an aftermarket that does not have a accessible vacuum port so I would prefer to just cap the vacuum advance as well and save the hassle.

SO with all that said, I plan on keeping the 3H5 mechanical advance that is already on the motor. I plan on plugging the vacuum advance. Would there be any benefit to running the 4RO to pick up some of the lost vacuum advance or would it advance it too much too early and not help at all where i need it? Do you have a better combination of stock parts.

(side note. I also have a set of 2H7 cams I may swap in as well if that is of any consideration)
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  #2  
Old 07-24-2019, 09:12 AM
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skids skids is offline
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If you use the 4r0 from the 81 you also need the ignition advance and you need to bipass the ballast resistor and use 3ohm coils. I have run xs11s with the vacuum nipple on number 2 carb plugged but the bike had original components. I did not do that for much time.
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Old 07-24-2019, 09:29 AM
Tobiczyk Tobiczyk is offline
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I already am using 3ohm dyna coils and the resistor is removed.
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Old 07-24-2019, 11:16 AM
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jetmechmarty jetmechmarty is offline
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All XS-XJ1100 ignitions are vacuum advance dependent. The XJ has a separate box doing the job. Vacuum advance is a fuel saver.
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Old 07-24-2019, 12:43 PM
Tobiczyk Tobiczyk is offline
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The reason I am asking this is because I came across a post from TC (cant remember where) that mentioned a story of a guy who had an XJ and his TCI died. He had a 4RO laying around so used that. This obviously is without the cent/vac adv. He said everything worked great without issues. TC said that using modern carbs would inherently be more efficient and running the 4RO solo would be good, just may have to advance the initial timing to compensate.

So I was thinking that maybe using the 3H5 centrifugal and the 4RO TCI but plug off the vacuum might be a good combination but also could be too much.

I do not know the exact amount of timing each creates and where so i cant say. I was hoping someone with more specific information could help answer if this would be a viable solution or not.
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Old 07-24-2019, 08:05 PM
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bikerphil bikerphil is offline
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I would stay with the 2H7 TCI and use the mechanical advance and no vacuum advance. 2H7 tci dime a dozen, 4R0 harder to find if it goes bad.
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