1975 BS38 Carb Advice

[TimG]

So here is what we have. 1975 XS with BS38's from 1975 as well. I have owned the bike for 2 years.
The bike idles and runs well. It pulls clean through the gears and RPM range.
The bike is stock with the exception of pod filters and aftermarket pipes. They are the EMGO Commando from Mikes. Floats are set at 24.5mm from the top of gasket to the top of float. I have checked cam chain tension, valve lash, timing chain tension and timing "in that order". I just finished cleaning the carbs and pulling the main jet, pilot jet and needle jet. They are shown in the attached picture. The needle is set to 1 and the pilot is a 45. The main does not have any markings on it. I bench sync'd the carbs before adding a pair of carb boots with vacuum ports on them and reinstalling. The carbs were then synced using vacuum gauges. The air fuel is all the way in (full lean).

My air fuel mix screw does not want to backed off at all. the fastest idle I can achieve is definitely all the way in. I have also always thought the bike ran a little rich. Plugs definitely show as such. I guess I need to change the main jet, because right now I have no idea what size it is. I was thinking about starting with the 127.5. and leaving the needle were it's at. Running it and seeing if there were flat spots and then raising the needle to 2+ until things improved.

Any advice would be greatly appreciated. Thanks.

 

[5twins]

Yes, you'll need to determine the main jet size. If you can't, just replace it, they're not that expensive. It should be marked on top of the jet. You may need to clean it to read the number. The little wire wheel attachment in a Dremel works well. Besides the number, you should also see an "R" and the Mikuni "square in square" logo if it's a genuine Mikuni jet .....

Check the number on your needle. The one in your pic looks like it may be a bit short, and that would make it rich. Many of the rebuild kits come with a generic Y-22 needle and it's too short for any of the '74 and later carbs. If your needle is stock, it should be a 4N8 and measure about 45mm from the top clip slot to the tip. Stock clip setting for it is the 4th slot from the top .....

Your float setting is off too. The spec for your carb is 24 ± 1mm, and that should be measured from the gasket sealing surface on the carb body, not off the gasket.

 

[TimG]

Thanks 5T. I am going to order a pair of 4N8's just in case and I'll pull the current ones and measure them. Any thoughts on which main to start with? I ordered 125-132.5.

 

[5twins]

Here's a stock carb jetting spec chart. The early carbs were set up richer so changes needed for your "pods & pipes" mods shouldn't be that big. I'd start stock but wouldn't expect having to increase the mains more than a size if bigger was needed.

 

[TimG]

I pulled the needle and it is a 45mm from the top clip slot. I changed the mains to 127.5 and it had no effect on the mix screws. They still only slow the idle as they are backed out. All the way in still creates the fastest idle. I did not change the needle clip setting and it is still in #1 (didn't want to change 2 things at once).
Bike idles well and first gear runs strong second gear has flat spot almost immediately. Thinking of lowering the needle to #2 and seeing what happens and then continuing to drop if I can.

 

[TimG]

Lowered straight to #4 and the flat spots were gone. Had a couple minor stumbles the first 2 times I jumped on it really really hard. Rode it around for a little bit and then pushed it hard a few more times but did not notice any problems. Seems like an improvement in throttle response overall.

 

[5twins]

That's probably because you're now closer to the original settings. The original needle setting was the 4th slot (from the top). Having it all the way up in the #1 slot was a very lean setting, the leanest there was. That may have been responsible for the flat spots. Your needle seems to be the right length but the taper looks different than the original in my pic. Yours might be a generic one from some aftermarket rebuild kit, a Y-31.

If your float levels are off, the mix screws can have very little or sometimes no effect. I would carefully re-set them. Remember, don't measure off the gasket, measure off the carb body surface under it. I use a 6" steel rule graduated in MM on one side. It's thin enough to stick down in between the gasket and the lip around the carb body, and reach that gasket sealing surface below it. Also, on the BS38s, there is no jet tower sticking up between the two float bulbs. This allows you to place the ruler on the far side and sight across the tops of both bulbs. Often the floats are ''tweaked" or twisted on their mount. This makes one side sit higher than the other. For this reason, you must measure them twice, once from each side, to make sure they measure the same both ways .....

If they're off and don't match side to side, bend and/or twist them back so they do.

 

[TimG]

Thank you very Much 5T. I will lower the floats. They were actually racked and I twisted them to get them back. Are the Mike's needles ok. Or should I be trying to find them somewhere else?

 

[5twins]

I don't think you can find original needles anywhere, the repops from Mike's are the only option besides used originals. Are there any markings on your current needles? You would find a size code along with the Mikuni "square in square" emblem on originals.