Internal carb leak? (formerly "cracking the throttle kills it")

[Jimustanguitar]

I put about 50 miles on the Vision today and killed it twice at stop lights. If I give the throttle a twist it revs without hesitation, but if I barely crack the throttle it instantly dies. I turned my idle from 1400 to 1600 which helped a lot, but wondered what else to try tweaking. I have my sync set perfectly, found the sweet spot for the front mixture screw, but couldn't zero in on the rear.

Any suggestions on which adjustment may be the culprit? I'm going to try and set the rear with a vacuum gauge and resync, should I investigate something else while I'm in that mindset?

[fret not]

Jim, it sounds to me like your motor is relying on the accelerator pump a bit too much to keep running, and some other tuning for lower RPM is needed.

[Rikugun]

I'd start with fret nut's suggestion. Turning the idle up isn't a solution you want to live with long term!  :) I'd put some more miles on as you have it if it's rideable and then recheck your sync and idle mixture adjustments. Other than being able to force a stall, how does it run? Sounds like you're close to having it right?

It's really important to turn the idle way down while making those adjustments. Shoot for 1k or less. If you can't get it to run that slow (on both cylinders) the slow speed circuits cleanliness may still be in question.

[YellowJacket!]

I know its a bit OT but using my digital tach, I can get YJ to keep a steady idle around 850.  :D

David

[Rick G]

Consider an air leak in the rear carb plumbing.

[Tiger]

... but couldn't zero in on the rear.

Start here... ;)

[Jimustanguitar]

Well, I left the petcock on prime and ended up with a puddle on the floor. I traced it back to the rear carb, so I'm pretty sure that this has something to do with my inability to find the idle screw sweet-spot. Interestingly though, the fuel wasn't coming out of the overflow, it had filled up the intake manifold and was dripping off of the throttle shaft.

So... I have a leak that is dripping down the carb throat instead of letting the float bowl fill up to the overflow height... Where do you think my leak is coming from? Is there a vision "feature" that I haven't discovered yet :)

[Tiger]

Looks like you have dirt lodged in the float needle/seat...generally the cause of overflowing gas!! As for the were its coming from, I would check that the overflow isn't plugged up by blowing back through it first. All this is pointing to dirt in the rear carb my friend...

                 8) ....... TIGER ....... 8)
ps I would also check that you do not have gas in the engine oil!!! Spun bearing springs to mind!!!!

[Rikugun]

Interestingly though, the fuel wasn't coming out of the overflow, it had filled up the intake manifold and was dripping off of the throttle shaft.   This sounds like maybe the throttle shaft seals are not doing their job.

So... I have a leak that is dripping down the carb throat instead of letting the float bowl fill up to the overflow height... Where do you think my leak is coming from? Is there a vision "feature" that I haven't discovered yet :)  Are you assuming the bowl isn't filling? What if the overflow tube or the rubber lines that are routed under the back of the engine have a blockage? It's worth checking....

[Jimustanguitar]

So, if I remove the carb top, and put gas in the float bowl about half way up to the overflow tube, I get a drip out of the carb throat. So I don't actually have a float valve issue, something else is afoot.

After rocking it back and forth and watching it happen from every possible angle, I can clearly see fuel slowly flowing out of the very small holes above the butterfly... The manual calls them bypass holes.

The fine detail in the manual is a bit grainy, so it's difficult to tell what I'm looking at.

If I screw the idle mixture screw all the way in, it still happens. Yes it has the little o-ring under the washer. Needle, spring, washer, o-ring.

Any thoughts?

[Jimustanguitar]

This sounds like maybe the throttle shaft seals are not doing their job.

Rikigun, is there a fuel passage that touches the throttle shaft seals and could cause a leak like this? Or were you just referring to the fact that once it fills up the intake that's where it comes out first?

[Jimustanguitar]

There are 2 openings below the "water line". I've circled them in the attached drawing. I believe that number 1 goes to the accelerator pump, and number 2 goes to the chamber beneath the float bowl where the main jet is.

My suspicion is that the pilot/bypass circuit plumbing has a leak where it goes through the main jet chamber.

Does this sound plausible? Might I have a bad casting?

[Cdnlouie]

Jim it sounds like you are on the right track.  If filling the bowl under the overflow results in fuel coming out (I am assuming you mean the bypass holes below the throttle plate), then you most likely have a casting leak due to corrosion.  The pilot air mixes with the pilot fuel jet (in the bottom of the bowl) within the internal casting and feeds the bypass holes below the throttle plate, which normally should not leak because the mixture exit should normally be above the fuel level in the bowl, so it only works when the engine is drawing vacuum (I don't have x-ray vision, but I think this is what normally happens here).  I think the copper pipe that goes across the bottom of the bowl may deliver this mixture and so if it is leaking (due to corrosion no doubt), it could flood the pilot circuit with fuel.

I don't think hole 1 which feeds the accelerator pump can go anywhere except the nozzles, and hole 2 is the main fuel feed to the internal passages (main, choke, pilot) and you really can't see what is going on in there.  I do think you are correct in saying that there is a casting failure here because it should not leak out the bypass holes when at the correct fuel level, too high a fuel level and it can leak out anywhere.  Throttle shaft seals (as mentioned above) only seal vacuum (not fuel) and only seal one-way which is from the outside of the shaft so that air is controlled through the venturi.  They are not an issue for fuel leakage.

I would definitley try another casting.  If you check the front one at the same fuel level and it seals properly then you can be pretty sure you have a casting leak in the internal passages.  This would explain why you cannot set the mixture screw because it is flooded with fuel rather than just mixture, therefore no adjustment is possible.  You are going to love your vision after you correct this issue.

[Rikugun]

Rikigun, is there a fuel passage that touches the throttle shaft seals and could cause a leak like this? Or were you just referring to the fact that once it fills up the intake that's where it comes out first?

The latter.  At the time I had no idea how or why gas was running down the venturi, just that it  leaking from the throttle shaft seal was an indication the seals are no good on top of whatever other problems you were experiencing... :(  :) I'm unaware of any passages that interconnect the shaft/seal area but that doesn't mean there isn't one.  :)  ;)

That's very interesting about the "bypass" holes.  Are these the 4 holes that are uncovered as the throttle plate opens up just beyond idle? I've seen the term "transition circuit" holes used to describe these in other cycle carbs. They take over as the idle circuit outlet begins to be ineffective the more the plate is opened. I'll have to look at my spare set later to get a better handle on this and the holes you referenced in the bowl. In the meanwhile maybe there is someone well versed in Vision carbs that might chime in with some valuable insight.

edit - while I was posting the very guy I was thinking about showed up  ;D

[Jimustanguitar]

Yep, we're all talking about the same 4 little holes. Bypass holes, off idle bleed holes, transition circuit, etc. The little tiny little ones that are on the same side of the bore as the mixture needle... I agree with Louie, it appears that this circuit is fed by vacuum from above the water line. I think it goes to the hole in the casting (looks like a figure 8 ) that one of the tubes from the upper fits into. The one with the little non-removable brass orifice in the bottom of it. And I agree that the little tube across the bottom of the float bowl is probably where it's rusted out...


Does anybody have a spare '82 rear lower carb body sitting around? I don't need the whole carb, just the casting. I'll post a want ad in the swap shop... Maybe that straight accelerator rod that Lucky sent me will end up getting used after all :)

[Lucky]

Before going thru all that, check your float level. If its too high & your drain is blocked (you should be able to blow thru it easily) it will do exactly the problem your describing. Simple.


Sent from my iPhone using Tapatalk

[Rikugun]

Before going thru all that, check your float level. If its too high & your drain is blocked (you should be able to blow thru it easily) it will do exactly the problem your describing.

I think the test Jim described earlier indicates the leak occurs even with the fuel level well below the overflow stand pipe top.  I agree the horizontal brass tube referenced earlier may be the culprit and not the carb body itself. If water sat in the carbs for some time, dissimilar metal corrosion ate a hole through the bottom of the tube where it is in contact with the aluminum bowl. Gravity would then have fuel pushed through the pilot circuit outlets in the venturi. It would not need to rely on atmospheric pressure pushing fuel up the cast tower above the fuel level and then back down the other to where it connects to the horizontal run of tube as is normally the case. Either way (failed casting or tube) it seems unlikely a fix could be easily made. This of course is just an unproven theory.  ;)

I suppose if you were dexterous enough you could block the myriad of holes with your fingers and with some water in the bowl introduce air where the mixture screw would be. If bubbles emanate from the area around that tube, you've found the leak.  :P

[Jimustanguitar]

The picture that I posted earlier shows the fuel level that I was getting drips at, you can see that it's a good centimeter below the top of the overflow tube.

it seems unlikely a fix could be easily made. This of course is just an unproven theory.  ;)

I completely agree that it's either the tube itself, or the aluminum hole that it terminates into... If I'm intending to replace it anyway, I wonder if a few drops of POR15 would flow around the tube and plug the hole without blocking the passage. I don't really have anything to lose, I may try it after I have a replacement and am inspecting them side by side...

Replace it is still plan A.

[George R. Young]

A little pic showing transition hole function (on a CV carb)

http://web.ncf.ca/ag136/cvCarb.jpg

When the butterfly is closed, they act to bleed air into the idle mixture, and when it's more open, they 'transition' into fuel delivery jets.

[Jimustanguitar]

Now they're called progression holes too... Maybe there's a different name for each of the 4 little ones on our Vision carbs :)

That's a neat diagram. My friend just bought a Maxim with slide carbs and he's been trying to wrap his head around the principles that they operate on. This will help, thanks!