BS38 carbs pod filters effect on mixture

[peanut]

I have been having some trouble tuning my carbs on my 79 2F US model and just cannot seem to get them running correctly.

Yesterday it occurred to me that my bike has almost certainly never actually run with the pod filters that came with it.
The PO probably bought them ready for the rebuild just before he sold the bike to me in bits.

Just how much of a difference is there to the air flow going from air filters to pods ?

I have now sealed the carb stubs to engine joints with gasket cement and the carbs to the stubs with HiTemp Silicon there is no air leak on the inlets now yet the idle is still high and it is now backfiring and popping out of one of the exhausts which it didn't do when it had an inlet leak ?

I have wound the pilot screws out to 2x turns which I am told is about right for this model but they were originally set to 3/4 turn out.This is the only thing I've changed

The butterflys are synced and the throttle stops are wound right out

I cannot run the bike for long due to the noise and angry neighbours so I'd appreciate any suggestions .

this is what it sounded like when it was first started last week

 

[5twins]

Yes, some pods are just plain no good. What ones do you have? Usually, the bad ones have their most detrimental effects in the upper midrange. Idle and low speed operation usually isn't effected that much. But you never know. Using the proper type pods will make the carb tuning process go much easier.

Your mix screw settings are a combination of what was stock (for a reference point if you've done mods) and pilot jet size. Changing the pilot jet size will many times require a slightly different mix screw setting than stock. If your carbs are the original '79 set, two turns out is probably a bit lean. Stock spec was 2.25 turns out but I've found this set usually likes between 2.5 and 2.75 turns on a mostly stock or mildly modded machine. 3/4 turns out would have been ridiculously lean. If you're up a size or two on the pilots then somewhere around 2 turns out may be possible. But popping out the exhaust is a lean indicator so I would open the screws more to see if it helps.

If you're running straight pipes and pods and haven't re-jetted, you need to. And there's no sense trying to adjust the carbs until you do. Mix screws are for fine tuning only and have a limited effect. They won't fix having the wrong jets in there.

 

[peanut]

thanks for the quick reply.
I have no way of knowing what jets the carb has but judging by the rest of the bike they are likely to be original I should have thought.

If you watch the vid you'll see the engine started immediately and ran well although I was unable to reduce the idle below 1500rpm. One warm the idle increased to nearly 3000rpm and although I removed the throttle cable and backed the throttle idle stop right out it would not decrease.

I managed to get the idle down to a about 250rpm by pressing hard on the LH throttle lever. I thought that probably meant the RH butterfly was set too high and preventing the LH throttle stop from going down further.

After removing the carbs and syncing the butterflys I refit the carbs sealing the inlets . Since then the engine now pops and spits and occasionly backfires out of one of the exhausts and the idle is still too high.

 

[5twins]

Yes, those are the bad pods. You won't get the best out of your carbs using them. You may get it to run OK but it will never be perfect or as good as it could be. The straight foam type pods work best. In this country, we use UNIs but I don't think they're available in England. But you have a similar filter, the Ramair. I would recommend you get some of them.

Your exhaust is very free flowing, practically an open megaphone. I wouldn't run the bike much until you check the jets and re-jet. If you have stock sizes, you're too lean. You will probably need to go up one or two sizes on the pilots and probably two to three on the mains. With the larger jets, you will need to lean the needle settings one step or the midrange will be too rich.

You say you've sealed the carb connections up but what about the exhaust? Make sure the headpipes especially are sealing well into the head. Muffler to headpipe leaks are not as bad but can still cause problems.

 

[xschris]

I would use a manometer to sync your carbs it's much more accurate.

 

[peanut]

I would use a manometer to sync your carbs it's much more accurate.

The butterflys are synced xschris, its not that which is the problem at the moment .

The problem at the moment is one of idle mixture and possible air leaks causing popping and backfiring on idle and de-acceleration.
Oh and also the bloomin weather .. gale force winds and sideways rain is not good for working outdoors on your bike lol

 

[peanut]

Yes, those are the bad pods. You won't get the best out of your carbs using them. You may get it to run OK but it will never be perfect or as good as it could be. The straight foam type pods work best. In this country, we use UNIs but I don't think they're available in England. But you have a similar filter, the Ramair. I would recommend you get some of them.

Your exhaust is very free flowing, practically an open megaphone. I wouldn't run the bike much until you check the jets and re-jet. If you have stock sizes, you're too lean. You will probably need to go up one or two sizes on the pilots and probably two to three on the mains. With the larger jets, you will need to lean the needle settings one step or the midrange will be too rich.

You say you've sealed the carb connections up but what about the exhaust? Make sure the headpipes especially are sealing well into the head. Muffler to headpipe leaks are not as bad but can still cause problems.

Thanks for the advice.

I'll order up some exhaust compound to seal those and check the headers are sealed ok.

I'm not going to start messing about with the jets just yet as it has run fairly well when first started up so I don't think the jets can be too far off but once I have got the carbs running well enough to do a test ride I'll have a better idea of how it runs right through the rev range .

The rain has just not stopped here for days so I have had no chance to do anything.

First thing I'll do is remove the tank and rig up a temporary gas feed then I'll warm her up then check the inlet carefully for leaks then
adjust the pilot screws and throttle stops. See if I can eliminate the popping and backfiring at least.

 

[5twins]

You may not be able to totally cure the popping without larger pilots. As I said, your mix screws have a limited effect. The fuel/air mix from the pilot is delivered into the main bore through 4 tiny holes. 3 are clustered together and come directly from the pilot. The 4th is off to the side and comes from the pilot, through the mix screw, then into the main bore. This gives you control of roughly 25% of the flow from the pilot jet. You're not changing the strength of the fuel/air mix though, just how much comes through.

To change the overall strength of the fuel/air mix produced by the pilot circuit, you need to change the jet size. The air jet that feeds it is fixed on the BS38s so that is a constant.

I suggest you drop a float bowl with the carbs still in place and check the main and pilot jet sizes. Write them down.

 

[ippytattoo]

After watching your video it is apparent you definitely have something "off" there. It sounds like your exhaust is very free flowing as 5twins says. Listen to 5twins when it comes to jetting, he really knows his stuff and with pods and free flowing exhaust a change in jetting is in order. A manometer is really needed to get the sync just right and is very inexpensive to make, check out the home made tools thread. Also you may want to check your timing if you are running the stock ATU, if it is too far advanced it can cause a constantly rising idle (advanceed timing will cause a higher idle which can cause the advance weights to open, causing a higher idle, causing the advance weights to open more, etc. etc. etc.). If you have not done so yet it is a good idea to go ahead and pull the carbs and clean clean clean to make sure you do not have any partially plugged pilot circuits as that will cause a lean condition.

 

[peanut]

After watching your video it is apparent you definitely have something "off" there. It sounds like your exhaust is very free flowing as 5twins says. Listen to 5twins when it comes to jetting, he really knows his stuff and with pods and free flowing exhaust a change in jetting is in order. A manometer is really needed to get the sync just right and is very inexpensive to make, check out the home made tools thread. Also you may want to check your timing if you are running the stock ATU, if it is too far advanced it can cause a constantly rising idle (advanceed timing will cause a higher idle which can cause the advance weights to open, causing a higher idle, causing the advance weights to open more, etc. etc. etc.). If you have not done so yet it is a good idea to go ahead and pull the carbs and clean clean clean to make sure you do not have any partially plugged pilot circuits as that will cause a lean condition.

thanks ippy but the timing is absolutely spot on and it has a fully electronic Boyer so no mechanical advance.

The carbs are well in sync and definitely doesn't require a manometer to adjust for a satisfactorily idle ,a good ear and eye is all you need . I used to set up double and triple SU carbs with nothing more than a vacuum gauge and a piece of rubber tubing and my ear.



You need to get the basics right first before changing jets. The more things you change by fiddling the more difficult it is to troubleshoot a problem. I need to make sure that there are no air leaks on the inlet and exhaust is first . Then I will adjust the idle mixture screw and throttle stops .
Check the needle valves are working properly etc If that still gives me a lean mixture then and only then will I look at changing the pilot jets.

As for the Main jets that is way off yet. one thing at a time is the rule for troubleshooting

 

[xschris]

The carbs are well in sync and definitely doesn't require a manometer to adjust for a satisfactorily idle ,a good ear and eye is all you need . I used to set up double and triple SU carbs with nothing more than a vacuum gauge and a piece of rubber tubing and my ear.

Then why ask us for help

 

[retiredgentleman]

In your first post you asked for suggestions. OK, my suggestion is you follow 5twins advice. If your jet sizes are unknown, 5twins recommended you remove the bowls and record the jet sizes. You won't make any progress unless you know what jet size you have as a baseline for tuning.

If the pilot size is wrong, for your pods and exhaust, then the mixture screw will be out of range and have no affect at all.

 

[5twins]

Well, it may have some effect, just not enough to cure the problem. When this happens, it's telling you that a jet change is required. You may need to open the mix screws 3 or 4 turns before the popping stops. That's another indication that a jet change is required. The carb makers design the mix screws to meter and work best at usually between 1 and 3 turns out. Get above or below that and a jet change is indicated. Also, the mix screws are spring-loaded. This keeps tension on them so they hold their setting. Open them too much and you lose that spring tension. Your mix screw setting can change or in an extreme case, the screw can fall right out.

 

[peanut]

In your first post you asked for suggestions. OK, my suggestion is you follow 5twins advice. If your jet sizes are unknown, 5twins recommended you remove the bowls and record the jet sizes. You won't make any progress unless you know what jet size you have as a baseline for tuning.

If the pilot size is wrong, for your pods and exhaust, then the mixture screw will be out of range and have no affect at all.

you should read the thread properly RG before making recommendations.

When the engine first started it started first time (even though it idled at 1500rpm at first } ,with pressure on the throttle stop the idle fell to a nice even 250rpm! with the idle mix screws set at 3/4 turns!

If you watched the video to the end you would have seen this for yourself and realise that the pilot jet cannot be too far off or it would not have done this .

It was only later that I adjusted the pilot mix screws out to 2.5 turns at the recommendation of someone else on this thread. Since then it now idles at 3000rpm and pops from the exhaust.

It is not unreasonable to assume that the jets in the carbs are correct and were functioning correctly when the bike last ran. However the only jet that is relevant here is the pilot jet as we are discussing idle.

It is only amateurs that start replacing components to correct a fault before carrying out a proper inspection and testing which I have not yet had a chance to do.

 

[peanut]

Then why ask us for help

I was hoping to get some advice from someone that knows what they are talking about

you don't use a manometer to correct idle or correct backfiring.

 

[ippytattoo]

I was going to reply by pointing out some of the faults in your logic regarding; the need to document/change jets according to intake and exhaust changes based on information provided by others that have more experience with these machines and the fact that if you can put "pressure on the throttle stop" and the idle changes (although you state that it goes to a nice idle of 250rpm, which these machines do not idle at 250rpm, these bikes idle around 900-1100rpms if in tune) indicating either the throttle cable is sticking or the butterflies are closing differently (out of sync); but figured I would just wish you luck.

 

[650Skull]

If that was on a cold engine then a new sump screen is called for.

 

[650Skull]

It is not unreasonable to assume that the jets in the carbs are correct and were functioning correctly when the bike last ran.

It is only amateurs that start replacing components to correct a fault before carrying out a proper inspection and testing which I have not yet had a chance to do.

No where does it say the jets have been checked for correct sizes for that year carb.
That is the starting point before changing for a free flowing exhaust and pods

Assuming is for amateurs

At the end of the vid when the idle was reduced it sounded like it was running around 900 rpm or close to it then when you blipped the throttle it went back to hanging up

 

[grizld1]

Peanut, no offense meant, but when 5twins and I wrote the XS-650 Garage Carb Guide, he did a very nice job in the chapter on tuning for modifications, and you won't get more information by asking him to write it all again just for you. And for cryin' out loud don't smear gumpucky on your carb spigots (they aren't stubs unless somebody decided to "build" them with a Sawzall).

 

[mrkil]

Hanging throttle at idle is also a sign of a lean mixture. Which is what pods will create.
I think there is lots of great advice here and you should be a little more open to solutions that may not make sense to you.