82 xs400J(DOHC engine) backfiring and not starting?

[DanMach]

82 XS400J. Had it for about 3 or 4k miles now(its at 29k). Its been running great the whole time I've owned it. No popping on decel, stuttering, irratic idle, anything like that. Just runs great. Last time I rode it I shut it down and started it back up about 6 times within 10 minutes(Drive thru late at night, hard to hear me over the exhaust).

Went to go start it today so I could warm it up and change the oil(per manual) and it wouldn't start. Well, it did start for about 3 seconds then I stupidly touched the throttle and it died. So I tried to start it a few more times and it ended with a very loud exhaust bang. I figured it might of been too cold out(Its had issues starting in the cold before). So I tried it again around 4:30 today(75 out) and same deal. Just a loud bang.

It was semi low on gas so I filled it up with fresh gas from the station and same deal.

Any suggestions?? It was running fine 5 days ago and this is somewhat random. I'm thinking right now the best course of action would be this:

1. Check the fuel line from petcock to ensure proper gas flow.
2. Check air filter.
3. Pull plugs and check them. (Should I try to start this some before hand? To see if they're wet?)
4. Check spark.
5. Check valve gap. (Tight exhaust could cause backfire I believe.)
6. Check the carbs and make sure they're getting gas.

I'm trying to hold off on the carbs because an exhaust backfire should be caused by being too rich, right? I feel like its going to be the valves because its gotten harder to start as its gotten colder. At first I assumed it was the weather, but maybe the valves were just slowly going out of tune and causing it. It ran fine once it started though.

Anyways, I would really appreciate any suggestions or ideas. I'll be sure to post a follow up tomorrow night after I check everything out.

Thanks guys(and gals)!

 

[bcware]

The carbs may just need to be cleaned. How cold is "cold" where you are? If it's still hitting the 70's I don't think it's getting cold enough to have any dramatic effect.

I'd hold off on the other theories if it's been running fine all along and just suddenly started dying. It won't hurt to check valve rocker clearances, however. It just seems more likely, to me, that some crud is now blocking fuel flow or mucking up the passages in the carbs.

Do you have an in-line fuel filter? If not make sure you get one. Check the plugs and replace them if they are old-ish; they are so cheap that you might as well. Other than that I'd clean the carbs without changing anything to see what happens. One step at a time!

How does the inside of the tank look? Any rust?

 

[DanMach]

Between 45 and 75 or so. The last time I tried it was about 75, so I think cold weather can be ignored. I will check the carbs before valves then.

I have ran it on reserve a lot in the last few months, so maybe some crap has gotten into them. There is no in-line fuel filter. I will be sure to get one though, I thought about it before but just never did it.

Thanks for the input

 

[drewpy]

get an inline filter.

and only run on reserve in an emergancy!...

Unless you must be dying to find an excuse to strip those carbs down.

 

[DanMach]

Update:

So I pulled the carbs and went through them and cleaned them. Also wrote down all the info(jets and such) in case I ever needed it later. One of them had some jets clogged up (the left one, if it matters. Near the fuel inlet.).

Still haven't attempted to start it as I have the gas tank off of it right now so I can do the valves... but I wanted to just post a quick update about it.

Thanks everyone for the advice.

 

[drewpy]

clean out the tank too, you need some "shrapnel" to shake around in there and some fuel to rinse out.

 

[DanMach]

Thanks Drewpy!

So I was hoping someone could confirm the proper tool I need to adjust the valves? I've been looking at this: http://www.motionpro.com/motorcycle/partno/08-0020

As it seems to match the description in the service manual and it was mentioned a few other blogs I saw. However I can't find anyone who specifically says it works for the XS400J's DOHC engine.

Thanks ahead of time! Itching to get back onto it and want to wrap this all up soon.

 

[bcware]

This isn't about the shim tool, but if you're going to buy a set of bent feeler gauges make sure they get small enough. I picked up a set from an auto parts store without checking what I needed first; it turns out they simply do not have blades small enough to check the clearances on the valves.

 

[DanMach]

Thanks Bcware! That is 100% the type of silly thing I would do lol. I'll be sure to check the manual and make sure the feelers are of similar scale. I'm thinking about writing up a little "how to" about it because I can't really find one for the DOHC engine. Given its not a common thing to be needed, but can't hurt.

Cheers

 

[freymaster4449]

I have been having these same issues. its about 50 where I am and I also ride it in the 30's. I will also try the new oil filter and carb cleansing. i hope this works..

 

[DanMach]

Wanted to toss in a 'conclusion' to this. Sorry if its puts it to the top everybody...

Anyways. It was the valves.