Occasional bike fire through carb and excessive vibration

[cmills83]

I am finishing up my 1980 XS650 bike build and the bike starts up and seems to idle fine. However, sometimes there is a sort of backfire through the carb. Also, when riding it seems like there is a lot of vibration to the point my vision is blurred and the bike, although responsive, doesn't seem like it has all the power it should. The battery is new and the charging system is working fine.

I am now running these pod filters from mikes xs:

https://www.mikesxs.net/parts/yamaha-xs650-54mm-tapered-oval-pod-air-filter-2-1-8

and Delkevic 2 to 1 exhaust, other than that the bike is stock.

As far as tuning goes I installed a #45 pilot and #137.5 main jet and the carbs are synced. So I guess my question is, what are some things I should check? and how do I know I have the bike jetted correctly?

 

[figure8]

What do the spark plugs look like?

 

[fredintoon]

Hi cmills,
the "usual suspect" for excessive vibration is bad carb balance;
for backfiring it's a partially blocked slow running jet.
I'd suggest a carb triple clean and re-balance.

 

[cmills83]

Thanks. That's is what I have read regarding the vibration. I bought a digital manometer and from 0 - 4000 rpm the biggest difference between the carbs is 1/10th of a psi, what is an acceptable limit? Looks like they both stay around 13psi on this manometer.

I have cleaned the carbs a few times and the pilot/main jet only have about 3 miles on them. I have read that backfiring through the carb is typically a symptom of running lean. Does that sound right? I will also pull the plug and check those, although they only have about 3 miles on them too so might be hard to see anything yet.

 

[warrenlevihursh]

I'm assuming you have the BS34s?. What do you have on the hose barbs on the side of each carb? Are they blocked off with a cap? I found that running a 4mm hose from one carb to the other can help them sync and may help with the backfire through the carbs. Make sure the plug isn't cracked or something if you have those, it can suck air, cause a lean mixture and backfire.

 

[cmills83]

Yeah I have the stock BS34s, right now I am running a vacuum line from the intake manifold barbs to the petcocks and then the barbs on the inside of the carbs there is a line connecting both. Are those what you're referring to?

 

[warrenlevihursh]

Yeah I have the stock BS34s, right now I am running a vacuum line from the intake manifold barbs to the petcocks and then the barbs on the inside of the carbs there is a line connecting both. Are those what you're referring to?

Yes. I do not have the vacuum petcocks, just a single gravity one. My line connects the outer barbs on each intake boot. I don't have inner barbs. I just know I was missing a barb plug when I first got the bike running. It was breaking up and backfiring through the carb. Once I ran a hose connecting the two it made a huge differences. Sounds like you are good though!