Carburettors slides not closing exactly together?

[Its been a long time]

I have just reassembled the carburettors from a 77 XS650D after cleaning and fitting new wear parts throughout. I didn't fit new diaphragms to the slides. Look as hard as I could, I can find nothing wrong with either of them. Before stripping, I noticed if I lift the slides together with my fingers, one returned slightly slower than the other. And after reassembly, it still does. I can hear the carburettor diaphragms breathing as I lift and release the slides. I can hear a definite slight clunk as the slides fully return. But they're slightly different rate of closing. I have no experience with this type of CV carburettor. Is my experience actually typical or can I expect issues when I get the bike running? Thanks.

 

[gggGary]

Not a big deal, slight difference in slide fall is normal. I'm pretty careful about checking slide and it's bore for corrosion/varish/dirt during the overhaul. checking for free motion. Sometimes overlooked; sealing of the choke (enricher) plungers affects slide performance. When the choke is open, a passage opens to the air chamber below the slide. Try and see, with choke applied the slides drop like a rock. The air part of the choke air/fuel emulsion is taken from below the diaphragm. It may serve two purposes provides air, and reduces acceleration when the choke is on, a good thing.

The enricher circuit, opened for clarity.

blue is air, note how it's supplied from the chamber below the slide.
Red is fuel pulled up from a dip tube in the float chamber
Pink is the mix supplied to the carb bore after the throttle butterfly. Opening throttle will lean out the mix why opening the throttle just bit will help a motor "catch" when starting.

 

[5twins]

To test whether or not the diaphragms are good (sealing with no holes or leaks), raise the slide with your finger then block off the oval shaped port at the top of the intake bell. If the diaphragm is good, the slide will drop very slowly. To help seal the diaphragm into the carb body, I apply red rubber grease to both the diaphragm and the groove in the top of the carb body .....

 

[Its been a long time]

Thanks for the comments. In the morning, I'll look at the suggestions to check it out. My guess is there's nothing (much) awry.

All the best.

 

[ckahleer]

If you swap slides, does the problem follow?

 

[Its been a long time]

OK, as a result of the above comments, I've found that the slide that's a little slower closing doesn't respond to the finger over the intake bell port test. So, there's a leak either at the diaphragm flange or a tiny leak in the diaphragm that I can't see. I need to sort that issue out then I think the carburettors are good to go.

Thanks for the comments, it's really useful.

 

[5twins]

Many times, pin holes develop along the edge where the diaphragm is attached to the slide. Hold the slide/diaphragm assembly up to a good light and gently pull/stretch the diaphragm away from the slide. Look along the edge that goes into the plastic ring for pin holes.

 

[Its been a long time]

Many times, pin holes develop along the edge where the diaphragm is attached to the slide. Hold the slide/diaphragm assembly up to a good light and gently pull/stretch the diaphragm away from the slide. Look along the edge that goes into the plastic ring for pin holes.

Thanks, yes, already tried and failed with that. I just cannot see anything even with a Maglight halogen torch with new batteries behind the diaphragm.

In the morning, I'm going to carefully tape over the joint between carburettor body, diaphragm and cover. 100% no joint leak. So that will tell me for certain if it's a leak at the joint or a very small perforation in the diaphragm. I suspect you are right and there's a tiny pinhole. My check in the morning should definitively tell me that's the case of not.

Thanks again for the input, it's appreciated. Though I have >5 decades experience of looking after my own bikes, this XS650D of unknown provenance is a voyage of discovery. Hopefully I will have the bike running and ready to go back on the road later thus year, hopefully in time for summer.

 

[gggGary]

Did you check the chokes?

 

[5twins]

Yes, if the choke isn't sealing, it will make the slide drop just like a hole in the diaphragm. Press down on the choke rod while doing the slide drop check and see if it helps. It's also possible you just didn't get the rib around the outside of the diaphragm into the groove in the carb body properly. Maybe it slipped out in spots while you were installing the tops. That's why I like to use the red rubber grease. It makes the lip stick into the groove a little bit and I also think it makes it seal better.

 

[Its been a long time]

Did you check the chokes?

Yes. One diaphragm is definitely 100% OK with the chokes checked.

Point taken that if my check in the morning to eliminate a diaphragm joint leak results in no change, then it's conclusion is that there's a pin hole I haven't found or I need to double check that choke function.

Thanks for the idea of pressing the choke rod while checking, I didn't think of that. That will also help me determine whether it's a diaphragm pin hole or not. Appreciated.

 

[5twins]

Yes, the end of the choke plunger is rubber and presses against a hole in the bottom of the choke rod's bore to seal it. If that rubber is damaged, it won't seal. Pressing down on the plunger will sometimes help it seal.

 

[Its been a long time]

Yes, the end of the choke plunger is rubber and presses against a hole in the bottom of the choke rod's bore to seal it. If that rubber is damaged, it won't seal. Pressing down on the plunger will sometimes help it seal.

Understood. The choke plungers are new. If the plunger isn't seating properly I should see that quite easily in the morning when I do the last couple of tests. Presently, I'm betting that there's a tiny perforation on the diaphragm that I've missed. But I would be happy to find it's a slight carb top joint leak as that's a lot less money to fix. Carburettor diaphragms are getting darned expensive for these old bikes!

 

[Jim]

Understood. The choke plungers are new.

Just to pile on here...It's not unheard of for the wrong choke plunger being sold as the correct part. You still have the old plungers to compare to?

 

[Its been a long time]

Just to pile on here...It's not unheard of for the wrong choke plunger being sold as the correct part. You still have the old plungers to compare to?

Fair point. Yes, I'm pretty sure. I gathered all the bits I removed in two pots. Then I compared each new bit with the removed bit as I fitted it. All the jets, needle etc were similar in the new and the used parts. Except the new parts are sized as per 1977 USA spec XS650D, the old parts were not correct per original.

 

[gggGary]

Quick choke leak check, put a few drops of oil in this hole.

A few minutes with a heat gun and some rust buster let me get the slide out of this gummed up mess with no damage.

 

[Its been a long time]

OK. Everything seems to be behaving how it should. Just one slide is falling a tad slower than the other. Using tape over the diaphragm port works better than my finger to block it when testing. Taping up the diaphragm joint to check for a carb top leak makes no difference.

I think at this point, I'll park the carburettors until it's time to fire the engine up. That's a few weeks away. Then I'll see where I go with the bike depending on how it runs.

 

[Wingedwheel]

Mine were both falling a little faster than I liked so I smeared them with red grease per 5T. Both drop really nice and slow now. I think a lot of it was not sealing well at the grooves.