Engine Has Gone MAD!

[Fergie]

I'm at my wits end with my engine!

I have a 1982 XS650 with BS34 carbs. I rebuilt the engine early last year (Jan 2009), but could never get it to run correctly, so it sat in my garage for a whole year. After I rebuilt the engine I couldn't get it started, so I rechecked the timing, and found that I was off two teeth on the cam gear, so after I reset the timing, the engine started up and idled, but it popped and backfired. When I say it popped, I mean it backfired through the carb. I took the carbs apart, cleaned them, and did a basic rebuild on them. This helped out some, but it still backfired and popped.

I checked the timing with a timing light, and it's right on. The jets are standard, and the engine ran good before the rebuild. I thought that it might be an air leak at the carb boots, so I bought new ones from Mike's XS, and installed them today.

After getting the engine started this afternoon, I had to use the choke for a long time before it would idle with out it. I adjusted the idle mixture screws using the dead cylinder method, but I haven't had a chance to check the synchronization of the carbs. Now the engine idles roughly, and sometimes it idles at 2000 rpm, 1000 rpm, 3000 rpm. Sometimes it will idle at 3000 rpm, so I'll rev the engine and it will go down to 1000 rpm idle, then I'll rev again and it will go up to 2000 rpm and hang there for a while, comes down to 1000 rpm and then dies.

Also, sometimes the throttle is very responsive, I'll roll on the throttle and it quickly revs up, and other times I'll roll the throttle quickly and the engine will sputter and die.

I'm lost here. I'm mostly a fuel injector guy, so carbs are dark and scary. Can any one give me guidance on where to start diagnosing this problem?

 

[xjwmx]

Since the timing is right, and the jets are standard, I predict this is badly adjusted valves.

 

[Fergie]

you mean the overhead?

I've checked and set the overhead several times.

 

[retiredgentleman]

When the engine won't respond to the throttle it usually means the pilot jet is partially plugged. The "dead cylinder" adjustment of the mixture screws was a good thing to do, but it would not be effective if the pilot jets are plugged up.

You can do a sych of the carbs on the bench, just by sliding a piece of paper under each butterfly, until they both have the same "drag". It will get them very close.

Have you actually removed the pilot jets and looked at them to confirm they are clear.

My recommendations:
Remove the carbs. Confirm pilot jets are totally clear.
Check that the butterflys will go completely closed (this is important in order that you can complete a proper mixture screw adjustment using the "dead cylinder method")
Sych the butterflys by sliding paper under each.
Re-install the carbs
Re-do the "dead cylinder" mixture screw adjustments
When you are setting the mixture screw, make sure you lower the engine rpm (on 1 cylinder) down to 300 or 400 rpm with the throttle position screw. The lower the rpm the easier it is to adjust the mixture screw.

Always make sure both spark plugs have a path to ground, to prevent damage to your ignition coil.

 

[tx650]

I could be wrong here to suggest this but is there a possibility that when U were crankin over the engine with the cam 2 teeth off that U are dealing with a bent valve/s?
I would do a compression test to verify this.

 

[Fergie]

Retired, thanks for the recommendations. I probably won't be able to get to this until later this week end. I haven't removed the pilot jets, but I did run a small piece of guitar wire through them to clean them out. I was also thinking of just buying all new jets and needles for the carbs.

tx650, I haven't checked the compression, but I don't have problems with starting, or much overhead noise. I should check the compression any ways. When I rebuilt the engine I honed the cylinder, replaced the rings, and the cylinder diameter didn't indicate that I needed to go up to the next ring size, but I could have been wrong on that.

Thanks for the suggestions guys.

 

[BigGeorge]

Heres another long shot. Make sure your throttle cable isnt sticking. Sometimes we overlook the simplest things...........BG

 

[Xumi]

Check for vacuum leaks by spraying some starter fluid or carb cleaner onto the carbs and intakes.. if the idle changes, then you have vacuum leak(s), which will cause lean running.

Also check float levels - if there isn't enough fuel in the float bowls, it will cause starvation at idle.

 

[Fergie]

Today I took the cars off and apart. I used almost a full can of carb cleaner on them, spraying each individual part, and even putting the straw into holes and passage ways to make sure those flow freely. I also adjusted the floats, but they weren't that far off. The right float was at 0.95", and the left at 1.01". The spec is 1.08", so I bent the tangs until they were both with in tolerance. I put everything back together, and reconnected the fuel line. The other piece of information that I didn't mention before was that I took the bike air cleaner boxes off, and replaced them with K&N pod filters.

I gave it full choke, cranked the engine for about 2 seconds, and it fired right up and idled at about 2000 rpm. I let it sit there for about 2 minutes and warm up, but while I did that, I forgot that I didn't put the filters back on, so when I started to put them on, the idle went up to about 3000 rpm. I took it back off, and the idle came back to 2000 rpm. I tried that a few more times to confirm that it's repeatable, and it was, so I left them off.

Once the engine warmed up a little more I started to take the choke off. I moved it just a little, and the engine died. I started it back up, adjusted the throttle stop a little, and backed the throttle off again until it would idle around 1000 rpm, but it was really rough at idle. I had to keep giving it gas every once in a while just so it wouldn't die. The engine also backfired a lot more than it did before, but I didn't hear or see any blow back through the carbs.

I adjusted the throttle screw again to get a steady idle at 1500 rpm, and then sprayed some carb cleaner on the throttle seals and rubber intakes, and I didn't see any change in the revs or sound, so I don't think I have any vacuum leaks on the carbs.

The only thing I didn't do tonight is use the dead cylinder method to reset the idle mixture screws. That will be my next assignment tomorrow, but I didn't want to piss my neighbors off any more tonight.

 

[Xumi]

That's some good progress...

Put those pods back on - any tuning you do with them off is going to throw things off.

Sounds like you need to turn your air/fuel mix screws out about 3/4 turn - (Then use dead cylinder method to fine tune from there).

 

[Fergie]

I screwed the idle mixture screws all the way in, and then turned them out 3/4 turns. This is how the engine sounds. When I first start it up, it goes to 4000 rpm, and as you can see I give it a few throttle twists and eventually it comes down to 1200 rpm, sits there for a while, and then climbs back up to 4000 rpm.

 

[xsjohn]

If it starves on the pilots the engine will be lean and as the idol raises it begins to raise the slides and feeds off of the beginning of the needle slope...blending that with the lean pilot circuit......(no cutaway on these BS34's)...........pilot circuit sounds really weak (lean)..........when I ran stock pilots the mix screws were at 3 turns and above...........

Did you change the float bowl gasket..........some replacements there is a punch out hole that if left closed will cause lots of carb problems..............

xsjohn

 

[Fergie]

Yes I did change the float bowl gasket, but I punched out all the holes before assembling. I should also mention that this bike is a 1982 XS650, but when I rebuilt the engine I damaged one of the carbs, so I purchased a set of carbs from a guy who ran them on a 1979 XS650 that he was parting out. I have not done anything with the pilot circuit, so I don't know what condition the pilot jets are in...I'll try turning the idle mixture screws out to 2 3/4 and see what that gives me.

 

[xsjohn]

Do you know what jets ...pilot jets....air jets and main jets............are in the carb........

xsjohn

 

[Fergie]

I don't know for sure, but the guy I purchased them from said he had never touched the carb before, which I'm assuming means they are stock, so I'm guessing 42.5 pilots. The carbs came from a bike that ran in Arizona. I'm in Indiana at approximately 500 ft. altitude. Do you think it would be worth it to just buy a new set of jets all the way around (main, pilot, air)?

Also, since I'm running pod filters, I plugged up the hose from the crank case breather that fed back into the old air boxes. Should I let that be open to the atmosphere?

 

[Sundie]

Uhm. Pilot for 79's is supposed to be 27.5, innit?

main 135
needle 502-3 (z-2 type needle, 3rd position)
pilot 27.5
air jet 140
screws to 2.25

is what my manual has for 79 carbs... 42.5 pilot was for UK 78's.

I would really check all your jet sizes.

Leave the pods on. These carbs need vacuum in the plenum. Hell, i'd put the full boxes back on to get a stock tune, then go from there.

81 otherwise - so you're running TCI, right? Hm.

 

[xsjohn]

Ok.............What I see from the picture are BS34's I think...(verify if you would having a chrome steel tops)........this would tell me that it may be the later BS34 with the black plastic floats....you did mention replacing the carbs I think.....

if so they have the black plastic floats they should be set at 22MM.....(.866")..........you mentioned setting them a 1.08" which is 27MM which would be right for the 80 brass float BS34 ""..........and not the black plastic float 81-84 BS34................

this is a very improtant float setting difference and would cause your problem if the floats were 5mm too lean for the black floats

the brass float 80 carbs also had a brass drain plug in the bottom of the float bowl......the 81 on had no such plug..........

xsjohn

 

[Fergie]

Ahhhh, you are correct. These do have the black plastic floats. That makes sense now. It's too late for me to work on it now, and I promised the girl I'd take her out on the town tomorrow, so Wednesday it is...

 

[Xumi]

Great catch, John on the float levels.

Fergie, when you're in there, write down your jet #'s- if you need to rejet, that will be very usefull information (where you are now), so you know where you need to go next.

Another point of note - when I mentioned 3/4 turns out, I meant from current setting. I would suggest starting at 3 turns out from soft seat. - The further out the screws are set to, the more rich the pilot circuit. The further in, the more lean. After about 4 turns, the pilot screw isn't going to have much affect, but before that, it can make a big difference.

 

[Fergie]

OK, It's fixed...sort of. I reset the floats to 0.86", and cleaned them up again. Then I tried taking the pilot jets out, and they were both really tough to get out. I finally got them out, and they were not in good condition. I tried soaking them in carb cleaner, but that didn't help, so I took the jets out of my old carbs, which were much cleaner, and put them in my new carbs. Once I got every thing put back together, the bike started right up, and idle smoothly. I set the idle mixture screws to 2 1/2 turns, and then used the dead cylinder method to tune them. The engine starts perfect, revs up nicely, and comes back down to idle every time.

The only thing that concerns me is that the bike back fires (more of just a small pop) as it's coming back down to idle, and it hesitates just a little bit when you first get on the throttle. It got slightly better as I tuned the idle mixture screw, but it still does it. Does this mean I'm still too lean?

My pilot jets are 42.5, and my main jets are 135. Do you guys think I should go up to 47.5/140, for my jets? Once I get the bike out for a ride I'll let you guys know how it performs on the road. Might not be until Wednesday though.