BS38 carburation problem

[hrtnalbertan]

Hi guys,

I've been creeping on these forums for a while and find the site to be an excellent resource. That being said, I could use a bit of help! I recently picked up a near mint 78 SE with fairly low mileage. The bike is fitted with cheapo k&n tapered pods and running really lean. I've capped the intake ports, adjusted the cam chain tension, valves, and checked the points. I've made sure that the floats are set up properly, cleaned the carbs and been fooling with jets and needle adjustment with no luck. I noticed that one slide was dropping faster that the other under vacuum so I ordered replacement diaphragms. I just replaced the suspect diaphragm and it still drops quickly under vacuum. Is it possible that I have a vacuum leak coming from the enrichner on this carb? I'm running out of ideas and would appreciate some new points of view!

Thanks

 

[gggGary]

I'm not sure what you mean "drops quickly under vacuum"? Why do you think it's lean? Pretty strong bias here against the pleated pods. The foam UNI type filters usually help how the engine runs. Have you synced the carbs? Have you checked for air leaks at the intake flange and throttle shafts?

 

[hrtnalbertan]

I'm lifting the slides and blocking the large curved port on the top of the intake of each carb. One of the slides takes about 10 seconds to drop while the other side drops after around 3 seconds, even after a new diaphragm was fitted. I'm thinking that there is a leak on the enrichner circuit causing the slide to drop faster. I haven't synced the carbs yet but the spacing looks good on the throttle plates and it does idle fine. I'm at a higher altitude but I'm getting flat spots before the main jet engages on every size of jet I've tried (up to 4 sizes larger). I'm pretty sure that I'm not even at the stage where I should be chasing jetting sizes.

 

[xjwmx]

^The three second one is the good one, not the bad one. The ten second one is hanging somehow.

 

[hrtnalbertan]

Thanks for the responses, guys.

Just discovered that the inboard enrichner plate was loose on the carb with the fast dropping slide. I guess the PO had a hard a time getting those bastard screws tight. xjwwm, I'm now getting a 5 second drop on the slide fitted with new rubber. The other slide looked ok to me last I had it out, anything ideas on what I should be looking for that might be causing the hang up? I'll be checking the other slide and putting on new rubber as well.

Thanks for the input gggGary, I'll be sure to leak check and sync the carbs once it warms up enough to tune (should be good in about 5 months!)

 

[xjwmx]

I just re-read what you were doing and it isn't something I've done, but I do think ten seconds is too long because there should be a hole in the piston that lets air in.

If you can't figure it out, just compare the two. Put tape over passages under the diaphragm and see if the differences is in the passages. Also, do they fall at the same speed if your thumb isn't covering the slot?

 

[CDNTX650]

As always... Check intake manifolds for cracks.. And most importantly... Throttle shaft seals. I will never rebuild a set of bs series carbs without doing them. Every set i have had or bought has hae bad seals. If you spray electrical contact cleaner at the shafts and the bike dies or almost dies.. Time to replace.cheapo pods make you run rich..not lean.

 

[gggGary]

Did you fit only one slide with a JBM diaphragm? If so I don't think that's going to work. I would be willing to bet if not done in pairs it's going to create balance "issues".
Throttle shaft seals, yeah again. You will never regret replacing the points with a Pamco. I did my 79 this summer, night and day. Took care of an intermittent high speed miss.

 

[retiredgentleman]

I just did a drop test on my carbs, by covering the oval port.

My left carb does not start to drop until 12 seconds and it takes a total of about 23 seconds until the slide reaches the bottom.

My right carb starts to slowly drop after 5 seconds, and still takes about 23 seconds to reach the bottom.

The difference is just tolerences of the slider/carb body fit, and air leakage within the carb/diaphram area.

I'd say my left carb has an almost perfect seal in the diaphram area, but my right carb has a small amount of leakage.

That small amount of leakage on my right carb has no effect on the real world operation of the carb.

With the oval hole not covered, the slide should drop down smartly................less than 1 second.

 

[GreasyC]

I'm not sure who came up with that drop test, there's nothing in the factory manual suggesting 5 seconds or 10 seconds or 20 seconds is something to look for. It's rushing air that makes the slide behave the way it needs to. It's not designed to do anything in particular when the engine is off so if you want to check that the slides are functioning right start the engine with the air filters removed and look at the slides as you rev the engine. If you have a broken diaphragm on one that slide won't lift like it should when you rev it up.

There's a hole drilled through the slide just in front of the needle. The diaphragm is there to make sure the majority of the air going into or out of the slide has to go through this little hole. I imagine the hole size is important for controlling how quickly the slide can change position. Make sure it's clear of gunk and that you haven't inadvertently put a washer or anything over it when you installed the needle.

 

[weekendrider]

And if you decide to drill them out to lift your slides quicker you will get it too big only once and then your done. room-baa-room-baa-room. Or maybe there is a flutter tolerence?

There is nothing in the manual about the test but it is a good way to quickly check as oppossed to pulling them out and looking for pin holes. One off the benefits of countless hours at a bench these little time savers.
I'd say with the numbers Retiregentleman gave it would be a safe bet to say his are good. If they drop top to bottom in 3 seconds its tops off to look.

 

[retiredgentleman]

I'm not clear on how hrtnalbertan is measuring the time. His last time was 5 seconds he said. If that is 5 seconds, before the slide starts to move down, that would be just fine.

However, if he means that the slide takes only 5 seconds to drop from the top to fully down at the bottom, that would be rather poor.

Yes GreasyC, This isn't a factory spec. , just anecdotal information that may be helpful to others on the site.

 

[hrtnalbertan]

The test is just a quick way to check for breaches in the diaphragm without removing the tops. I got this from the carb bible linked to on this site. The fast dropping slide was caused by a loose enrichner plate on, not by a bad rubber. I just replaced both diaphragms anyway and now they both drop at similar rates. I suppose that it can't hurt to have fresh rubber since they were original.

Now I think I'll replace the throttle shaft seals as some guys suggested. Is there a way to do this without screwing up the sync?

 

[gggGary]

"Is there a way to do this without screwing up the sync?"
No but it's no big deal anyways. The carbs have to be split by removing the connecting angle iron from at least one carb. The synch can be preset visually with very high precision.