Mysterious (to me) power drop

[RichardL]

Glen,

That's a very interesting article. Oddly, though, he does not mention fuel height in the bowl so he also doesn't mention where to measure it.

Richard L.

[worntorn]

Right, he just gives the float setting methods and measurement, which if followed , should give the correct fuel height in the bowl.
Methinks it's good to check fuel level with a  tube as you have done.

As far as tuning and fixing idle problems, the Bushman page is the Bible for a lot of Amal Retentive types.

Glen

[muskrat]

G'day richard.
All the tuning info is assuming the carb is horizontal. Once it's tipped over a few degrees the float hits the roof and won't close the needle. Notrun owners have the same problem.
 The fuel height is to be measured at the main jet which is in the middle.
Does your carb have the screw in pilot or the later fixed type? I prefer the screw in for tunability.
I had never tried adjusting the fuel height by moving the seat for fear of distorting the seat. Much easier to bend the tab on the new floats.
Cheers

[RichardL]

Muskrat,

Glad you asked about the pilot jet. Mine is the pressed-in type. I bought a #78 drill bit for keeping it clean, then discovered that the drill wasn't long enough to reach the jet when held in the pin vise I bought. So, I used the drill in the pin vise for drilling a hole in a toothpick that would hold the bit. A drop of super glue and it works a treat, as they say.

[Clive54bsa]

Richard, any music shop will have a .016 guitar string, that's what I use on my B50MX which constantly clogs, due to the crappy petrol we have in California.

[RichardL]

Richard, any music shop will have a .016 guitar string, that's what I use on my B50MX which constantly clogs, due to the crappy petrol we have in California.

Thanks Clive. I knew about the guitar string method, but figured the hardened steel of the drill would be a bit more positive and easier to steer. Maybe those aren't problems witht the string. While available more close by, I don't think the guitar string would be cheaper (except for getting 20 for the price of one).

Richard L.

[RichardL]

Well, I'm back with the story of the work so far, which I hope will be the solution, but it won't stop raining long enough for me to test it. Before I get into it, let me first give respect to those with actual complicated and expensive issues (for example, see "coffee grinder").

The two things I chose to do to address fuel starvation were to adjust the float and implement Service Bulletin #16-69 as posted by JulianS. I am holding off on raising the needle a notch.

First, and unfortunately, I went at cutting the slot per the service bulletin using the Dremel cutter shown in one of the pictures. It is 3/8" in diameter with an 1/8" shaft. This means that it cannot cut deep enough to fully open a passage through the side wall of the needle seat. (I wonder if the author of the bulletin ever tried it?) There was a small opening that I poked at with an awl but it didn't do much good. Next to the Dremel tool in the picture is a needle file that I wanted to use by hand, but the angle to access the slot was too tight to move the file.  Brainiac here says, "Hey, I'll put the file in a drill motor and it will work by rotation." Big mistake! I ran the file through the hole right into the needle seat. Many hours (not continuous) went into trying to dress the seat again and testing each new attempt. I'll spare you all the failed techniques and details I tried. After completely giving up and ordering another bowl and seat on eBay (also in the picture), I decided I might as well try again. I bought a brand new, high-quality 7/32" drill bit and twisted it by hand in the seat. Then I put brass polish on a Q-Tip, mounted it in the drill motor and ran it down the seat body. Surprisingly, that process worked. I was happy.

Now, on to setting the float level, again sparing all the details, I could not get it close with the needle seat where it was. The stay-up float wanted to rise too high to close the valve and it hit the carb body. Adjustment of the metal fork just didn't seem to get it. I ended up pushing the needle seat down a bit and then adjusting the fork. The result is shown in the picture below with the fuel in the clear tube. 

If I haven't adequately described my struggles with this, understand that I must have removed and replaced the float bowl twenty times in all this. That doesn't even count the level tests while not attached to the carb body.

Thanks for reading. sorry for boring.

Richard L.

[duTch]

 Wow- extensive stuff; I'm exhausted

 Do you not have mini cut-off discs in your Dremel set ?

[RichardL]

Yep, Dutch, I have them but I think their diameter would have cut into the vertical tubes (for the pilot path, I believe). You can see I knicked those tubes a bit doing the hand work.

Richard L.

[Greybeard]

I wonder if a hole rather than a slot would be sufficient. Would one be able to get a long bit into position? I realise that the hole would be angled.

[duTch]

 

You can see I knicked those tubes a bit doing the hand work.

 Oh Yup see it now, was looking on the phone in a bit of 'downtime'; I dunno if you have the same as me, but the little buggers don't have a chance to wear down to a useful diameter, because they seem to break real easy 

[Greybeard]

I wonder if you could get a dentist to drill that slot for you 

[muskrat]

G'day Richard.
That fuel height looks to be about as good as you'll get it.
Cheers

[Triton Thrasher]

I wonder if a hole rather than a slot would be sufficient. Would one be able to get a long bit into position? I realise that the hole would be angled.

Yes, I did that.

[RichardL]

I am returning to this topic, even though I introduced my recent nip-up in the Rally Weekend topic.

The top is all off, as obvious from the pictures. Before getting into it, I think I should mention that the previous "Mysterious Power Drop" did not result in engine stoppage, but the latest nip-up did.

I think the pictures and by-eye-and-feel-exam are saying that I am running too lean on the right side and only that side nipped-up

One picture shows the front of the right piston with me identifying what I think is the new contact streak, the other marks (still smoothed out) being from a previous event. Another picture shows me identifying the corresponding (but narrower) contact streak on the back side. There is also a picture of the fronts of both pistons for comparison. The right cylinder shows just a little bit of streaking, which I think is consistent with the minor marks on the piston. Then, the right exhaust valve is white-ish and the right plug is somewhat whiter.

Considering how much oversize the cylinder-wall clearances were, it makes some sense to me that just one side would nip-up since trouble would find the one just slightly closer to spec.

So, my plan is to give the lightest possible hone on the right side only, removing negligible diameter. Then, use my on-hand, half-a-set of 60-over rings. These rings came with the pistons some time ago, but I through them over for some genuine Hepolite. I think it may be OK to be less choosy right now.

Assuming I don't get too much pushback on my "too lean" diagnosis and the rework of the bore and piston, I am left with the question of what to do about "too lean". Raise the need on the concentric up from middle position? Change main jets up from (I think) 220? Add a bias gasket? All of the above? None of the above? Or. some mix of these?

By the way, after fixing the float level and cutting-in the early-supply slot in the float needle tunnel, I am pretty sure fuel shortage is not a/the problem, but feel free to throw it in if it still needs to be considered.

Your thought, please, gents.

Richard L.