rougher idle after rebuild

[pahako]

Just rebuilt the top end on my 81. It ran great and was completely reliable the 4 years I rode it before the rebuild. (one kick every time)Only rebuilt because the front cam chain guide was falling apart.
I had the carbs tuned quite nicely, so I just left them the way they were when I put everything back together
The bike is definitely got more power, than it did before, but when I come to a stop it seems to idle rougher and I have to give it more throttle when I disengage the clutch. Also sometimes I have to kick it more than once to get it started. Thought the pilot jets may have gotten something in them, but they are clean. My first instinct is it is a carb adjustment, but then started thinking it might be electrical. I have been running the same Wal Mart battery for at least four seasons now. It reads 14 volts at idle, but was just 11.75 with engine off, after sitting for one day.

 

[CDNTX650]

That's toast. 12.5 or 12.6 should be a good shape bat at rest.

 

[fredintoon]

Hi pahako,
I'm with CDNTX650, that low voltage number means that your battery is WAY past it's "use by" date.

 

[TwoManyXS1Bs]

Could the overtaxed charging system influence the TCI triggering?

 

[gggGary]

wait for it..

yeah check the float valve o-rings on the BS34s if you are still using the vacuum petcocks check for a ruptured diaphragm. Check for an air leak at front of carbs. Recheck your carb synch. |Do a compression test. Have you set the valves again after riding it? Done a retorque of the studs?

 

[5twins]

I agree with what's been said so far but it is also possible the carbs do need some slight re-tuning because the condition of the motor has changed.

 

[pahako]

Did the engine retorque and all that. No air leaks at the front of carbs. Put a fresh battery in and that seems to have fixed the problem. As far as tweaking carbs, should I set both mix screws the same and start from there? Before the rebuild I had to have one mix screw1/4 turn out more than the other to get the same color on both plugs. I don't think my exhaust valves were seating very well before, from all the carbon on them. I will also recheck carb synch.

Thanks guys for all the help.

 

[Brassneck]

I'd do the dead cylinder method with your carbs. Start from factory settings and go from there.

Make sure to ground your plugs when you do it, as you don't want to damage your ignition.

 

[pahako]

So it looks like the battery didn't fix the problem after all. After a couple of more trips to and from work it is doing the same thing again. The symtoms are this; It starts and runs fine, I have noticeably more power than before the rebuild, runs great, I have about a 7 mile stretch on the way to work at about 65 mph, and as the bike warms up and I come to my first stop light, I notice it seems to be idling slower instead of faster as it should after it is warm. There seems to be a bit of hesitation when I give it throttle to get going again from a stop. I have retorqued, rechecked camchain, reset valves and resynced the carbs. They are also both set at 3 and 1/4 turns out. I tried, but haven't had much success with the dead cylinder method for tuning carbs. When I turn the mix screws either direction even up to a half turn, I can't seem to distinguish any change in idle running on one cylinder. I'm not saying its not the carbs, but I am leaning toward more of an electrical issue than a fuel issue, but I am not sure. The battery reads 14.5 at idle, and 13 volts when off. The brushes have plenty of life, I did notice today that when I rev it, the headlight doesn't get noticeably brighter like it used to. Maybe it just did that because the old battery was to weak. I don't know. I have never messed with the timing because I didn't think I had to on an 81. Any thoughts?

 

[TwoManyXS1Bs]

...I have about a 7 mile stretch on the way to work at about 65 mph, and as the bike warms up and I come to my first stop light, I notice it seems to be idling slower instead of faster as it should after it is warm. There seems to be a bit of hesitation when I give it throttle to get going again from a stop...

Create this same scenario, then kill the engine. Pull the plugs and see if they're getting dark/sooty.

Another cheap test is to temporarily disconnect the regulator, ride about, note any changes...

 

[gggGary]

led volt meters are much less than $10 get one watch the voltage while you ride. I'd about bet a short or other elect prob is what's going on.

 

[retiredgentleman]

"They are also both set at 3 and 1/4 turns out. I tried, but haven't had much success with the dead cylinder method for tuning carbs. When I turn the mix screws either direction even up to a half turn, I can't seem to distinguish any change in idle running on one cylinder. I'm not saying its not the carbs, but I am leaning toward more of an electrical issue than a fuel issue, but I am not sure. The battery reads 14.5 at idle, and 13 volts when off."

When the mixture screw is 3 and 1/4 turns open, its fully open and moving the needle valve slightly either way, does not change the flow of the mixture very much. You could try changing the pilot jets to one size larger. Then when doing the dead cylinder adjustment of the mixture screws, the mixture screws would likely be closed in to around 1 and 1/2 turns. With them around 1 and 1/2 turns, they affect the idle speed more, and allow you to detect the best setting.

Something is strange, if you see 14.5 volts at idle. These bikes normally have around 13 to 13. 5 volts at idle (1200 rpm). Perhaps your voltmeter is not accurate? Try another meter. If your meter is accurate, what voltage do you measure when at 3000 to 4000 rpm? Your regulator may not be working??

 

[5twins]

The BS34s won't run right with only 1.5 turns open on the mix screws. They need that 3 to 3.5 turns because they're so E.P.A. strangled. Lots of the later carbs on E.P.A. mandated bikes need these larger openings. My dealer told me he's had to open the screw to 4 turns or so on Viragos with an exhaust change to stop the popping.

Being so E.P.A. strangled, the BS34s aren't going to respond like an earlier set when doing the dead cylinder method. It may take a half to 3/4 turn either way before you see any effect. Not really much good for "fine tuning". I don't bother with the dead cylinder method on them. I do little throttle blips and observe how the response is. I look for popping or slow-to-fall R.P.M.s which tells me the screws are still set too lean. Starting at about 2 or 2.5 turns out, I open them a little at a time (same amount on each one) until that goes away. As I mentioned, that's usually in the 3 to 3.5 turns out range.

 

[retiredgentleman]

The BS34s won't run right with only 1.5 turns open on the mix screws. They need that 3 to 3.5 turns because they're so E.P.A. strangled. Lots of the later carbs on E.P.A. mandated bikes need these larger openings. My dealer told me he's had to open the screw to 4 turns or so on Viragos with an exhaust change to stop the popping.

Being so E.P.A. strangled, the BS34s aren't going to respond like an earlier set when doing the dead cylinder method. It may take a half to 3/4 turn either way before you see any effect. Not really much good for "fine tuning". I don't bother with the dead cylinder method on them. I do little throttle blips and observe how the response is. I look for popping or slow-to-fall R.P.M.s which tells me the screws are still set too lean. Starting at about 2 or 2.5 turns out, I open them a little at a time (same amount on each one) until that goes away. As I mentioned, that's usually in the 3 to 3.5 turns out range.

What I'm saying 5twins, is that going up 1 size on the pilot jet and with the mixture screw set back in around 1 and 1/2 turns, provides the same amount of mixture flow as the stock pilot jet size and mixture screws at 3 and 1/2 turns. With the mixture screws at 1 and 1/2 turns they could actually fine tune the mixture flow, while at 3 and 1/2 turns they have almost no effect on mixture flow.

That dealer tuning a Virago, should have gone to a larger pilot jet. At 3 and 1/2 or 4 turns, a needle valve has no control function anymore. A larger pilot jet would put the mixture screw back into a range that it could actually be used to fine tune the mixture flow.

Surely BS34 owners use larger pilot jets, and then find the mixture screw needs to be in around only 1 or 2 turns open.

 

[5twins]

It would be nice if it worked that way but it doesn't. The BS34s still need larger mix screw settings even with larger pilots. Their E.P.A. type mix screws are very restrictive.

 

[pahako]

I have been running one size up on the pilot jets, 142.5 mains, pod filters, and the mix screws have been around 3 turns out for a few years now. The carbs were a little out of sync before I resynced them a couple of days ago. Both sides seem to be running the same when I put my hand behind the exhaust. Before I resynced them I noticed I was getting more soot from the right exhaust when I would put my hand behind it, and the right plug is quite a bit darker than the left. I cleaned both plugs after I resynced the carbs, and put the mix screw both back at 3 turns out. I did have the idle set up to high last time I checked the voltage, and am getting around 13 volts now at idle. The needle broke off on my tack a while back, but I believe I am close to 1100-1200 rpm's at idle. I am going to get an led volt meter to check the voltage while riding.
So what does black sooty plugs indicate, and what am I looking for if I ride with the regulator disconnected?

 

[5twins]

If carb related, a darker or black sooty plug would indicate a rich condition. If electrical related, it would be a weak spark. There are a few tests you can do to try and decide which it is.

Your single coil fires both plugs at the same time and with the same strength or power. It's very uncommon for this type of coil to have one weak side. They usually work good or not at all, fire both plugs or neither. But, you can get a weaker spark on one side if that side has a bad plug, plug wire, or spark plug cap. The first thing I'd do is switch the plugs from side to side. If the former right side dark plug stays dark on the left, I'd say the plug is bad. Next would be to switch the plug wires from side to side. Since they both fire at the same time, you can do this. Connect the current right plug wire to the left plug and the left one to the right plug. If the dark plug moves to the left cylinder then that plug wire or plug cap is bad, or their connections are faulty. Re-doing the connections may remedy that.

If nothing changes with these tests (dark plug doesn't change sides) then that would seem to point to the carbs. If the jetting and mix screw settings are matched, and the carbs are synced, about the only thing left is float levels. Your '81 float bowls have a nipple on the bottom below the drain screw. You can attach a clear hose to that, bend it up along the side of the carb, open the drain screw, and the hose should fill with fuel to the level that's in the bowl. It should be around the seam between the bowl and carb body. Compare the levels between the two carbs.

 

[pahako]

Looking like it was a fuel issue after all. My float heights were a little off on the right side (the side with the black plug). After getting home today, the plugs were both similar looking with neither one being overly dark. Idled much better at stop, and took off smoothly. Good thing too. I was in stop and go traffic due to an accident coming home. I must have accidentally bent one when I had the carbs apart cleaning the pilot jets. Thanks again for the help.