Quick stupid question

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cybrhick

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We have our motor on a stand right now. We have adjusted the cam, adjusted the valves, checked for compression, rebuilt the carbs, installed new condensers, installed new plug wires, installed new plugs, changed the oil, and I think we are finally ready to test fire the engine. Nothing is hooked up to the engine except the reg/rec unit and a TC Bros chopper basic wiring harness.

To test spark, I am assumeing I need a battery connected to the harness. Is this correct? Also, do I only need a battery connected to the ignition key wire, or do I need the battery going to the reg/rec as well just to start the motor?
 
OK, we tested spark. Getting spark. Hooked it all up feeding fuel through a hanging funnel. Carbs started leaking. Uhhhgg. Checked floats, and they were higher than they were supposed to be. (24mm) Seems they would close the needle early and stop it from flooding, right? Adjusted floats lower until they stopped leaking. Tried to start, and nothing. So frustrated.
 
75 stock motor. Points. Not sure the carb model. They have the L bracket connecting them underneath, and a bracket connecting the caps up top. They also have the linked throttle.
 
Basically all the carbs for these have those parts, but assuming they are stock for '75, then they would be BS38's.

Read up really good on www.amckayltd.com/carbguide.pdf

Those floats, for a '75, need to be set at 24mm +/- 1mm. You need to be very accurate when doing this measurement. 2mm off will make the carbs not work correctly.
What size jets are in there? Are they cleaned thoroughly? I mean every little port, hole, rubber o-rings replaced? Float valve ok? ect.
These carbs, and every carb, need to be cleaned and adjusted correctly. They will not work, at all, if you skip one step. There is a lot to them, but simple when you understand them.

Here's a carb ID guide.
carb_page.jpg
 
littlebill31 said:
Basically all the carbs for these have those parts, but assuming they are stock for '75, then they would be BS38's.

Read up really good on www.amckayltd.com/carbguide.pdf

Those floats, for a '75, need to be set at 24mm +/- 1mm. You need to be very accurate when doing this measurement. 2mm off will make the carbs not work correctly.
What size jets are in there? Are they cleaned thoroughly? I mean every little port, hole, rubber o-rings replaced? Float valve ok? ect.
These carbs, and every carb, need to be cleaned and adjusted correctly. They will not work, at all, if you skip one step. There is a lot to them, but simple when you understand them.

Here's a carb ID guide.
View attachment 13501
Click to expand...

Judging from the pics, I would say the middle set looks the closest. (The top set of BS38s) We has the floats set at 24mm, but the carbs would flood and overflow. We ended up raising them a bit to stop the overflow. Something is obviously not right. Just not sure what. My son spent 2 weeks tearing these down and cleaning them at night after school. They are pretty dang clean.
 
Did you replace the float needle, float valve? They, if damaged or the o-rinds are bad, will overflow the carbs.
48-1445.jpg
And you measured the floats from the gasket base to the top (actually bottom) of the floats when the float tang just touched the pin in the float valve needle?
48-5014.jpg
XS650_Carb_Float.jpg
The needle (pic #1) has a pin in the center, under the wire that hangs it from the float tang. This pin should move/spring freely. You want that float tang to just touch it (pic #2), but not pushing it down, when you take the measurement.
 
littlebill31 said:
Did you replace the float needle, float valve? They, if damaged or the o-rinds are bad, will overflow the carbs.
View attachment 13505
And you measured the floats from the gasket base to the top (actually bottom) of the floats when the float tang just touched the pin in the float valve needle?
View attachment 13502
View attachment 13503
The needle (pic #1) has a pin in the center, under the wire that hangs it from the float tang. This pin should move/spring freely. You want that float tang to just touch it (pic #2), but not pushing it down, when you take the measurement.
Click to expand...

Measured the floats with digital calipers. They were dead on.

I'm not sure if we replaced the float valve o-ring. I'll have to check that.
 
If the float valve o-ring is bad, then fuel will get around it.
If the float valve needle tip is bad, it won't seal into the float valve, cutting off the fuel supply.
If the float needle pin doesn't move, same thing.
I mention this because, to the naked eye, they may look fine, but it doesn't take much.
A slow moving/springing float valve needle pin will not work fast enough to cut the fuel off either or not shut it off completely.

And you have brass floats, correct?
20-6500.jpg
 
littlebill31 said:
If the float valve o-ring is bad, then fuel will get around it.
If the float valve needle tip is bad, it won't seal into the float valve, cutting off the fuel supply.
If the float needle pin doesn't move, same thing.
I mention this because, to the naked eye, they may look fine, but it doesn't take much.
A slow moving/springing float valve needle pin will not work fast enough to cut the fuel off either or not shut it off completely.

And you have brass floats, correct?
View attachment 13506
Click to expand...
Correct. The only thing that doesn't make sense here is that the flooding stopped when we adjusted the floats high. Seems if the o-ring was the issue, it would still flood.
 
So when the carb is upright, in it's correct, mounted position, you essentially raised the floats? If so, you probably forced the float needle shut, therefore not letting any gas at all in, but there was enough in the float bowl to float the floats.
There is a very fine line with these between flooding and working.
Have you tried the method of using a clear line, attached to the drain bolt in the bottom of the bowl to see where the level is?

I drain the bowls, attach the clear tube to the float bowl nipple, raise the tube vertical, pointing up (you don't want to drain all the gas on the ground) above the carbs and then turn on the gas. The line will fill with gas and tell you how high in the bowl it is. If it is above the bowl gasket, then it's incorrectly set.
 
Last edited:
If the carbs are upright, I actually lowered the floats, which would cause the valve to close with less fuel in the bowl. However, if the o-ring were an issue, wouldn't the carb still flood?

I'm not sure what you are speaking of when describe this clear line thing.
 
cybrhick said:
If the carbs are upright, I actually lowered the floats, which would cause the valve to close with less fuel in the bowl. However, if the o-ring were an issue, wouldn't the carb still flood?

I'm not sure what you are speaking of when describe this clear line thing.
Click to expand...

You are correct. And yes, that would stop the flooding, but, if too low, will starve the carb.
If the o-ring were bad, then the bowl would flood.

If they are still flooding at the correct height, measured from the gasket surface to the float, then I would suspect the flood valve needle tip is bad. It's not closing in the float valve hole tight enough to shut off the fuel supply. I've had perfect looking needle tips that still flooded. Put new ones in, fixed.
 
OK, so most likely those o-rings are good, because we were able to get the carbs to stop flooding by lowering the float. Now I guess it's just a game of finding the perfect level for those floats.
 
You have no o-rings on your float needle seats, that's a BS34 thing. Your seats simply screw in and seal against a washer. Check your floats from both sides. They may be tweaked or twisted. You think you're setting them at the correct level but if the other side is off, that will throw the overall setting off.
 
Don't feel bad, these carbs can be very confusing what with all the variations over the years. Although we've established they are a later linked set of 38s, they could still be a '76-'77 or '78-'79 set. The two sets are similar but very different in that they use totally different slides, float bowls, and jetting. You have to know exactly which carb set you have in order to adjust and tune it properly. I can't stress this point enough.
 

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