Keihin Carb Question

[biscotti]

After a few hours of work, I finally got my 80 Sportster running – almost 17 years since it was last put away.
Poured some oil into the cylinders to give the pistons and cylinder walls some lubrication, changed filter, changed oil, put in new plugs, cleaned the inside of the tank and fuel filter – and darned if it didn’t start. Didn’t run well unless the carb was on at least half choke, but it ran.

Took the carb apart and soaked it in solvent overnight, than blew it out with compressed air. Got a carb rebuild kit, and made it like new again.
Along with new gaskets I put in a new 72 low jet ( stock is 68 – but I had changed that to a 72 years ago ); a new 170 main jet ( stock is 160, and that was what was in it ). A fuel valve and accelerator diaphragm finished the job.
Put the carb on, along with a new fuel line – and again it started right up.
This time it would idle nicely at about 900 rpm.

Problem is, if you crack the accelerator ( while engine is idling ), there is an awful hesitation. If you crack the accelerator from about 1200 rpm, there is no hesitation.

I have not ridden the bike yet because I still have to do the tires and brake fluids, so I do not know how it will react on the road.
Did I put in too much main?
Any other suggestions on where to look to get rid of the hesitation?
Thanks for any help.

[newultraclassic]

Originally Posted by biscotti

After a few hours of work, I finally got my 80 Sportster running – almost 17 years since it was last put away.
Poured some oil into the cylinders to give the pistons and cylinder walls some lubrication, changed filter, changed oil, put in new plugs, cleaned the inside of the tank and fuel filter – and darned if it didn’t start. Didn’t run well unless the carb was on at least half choke, but it ran.

Took the carb apart and soaked it in solvent overnight, than blew it out with compressed air. Got a carb rebuild kit, and made it like new again.
Along with new gaskets I put in a new 72 low jet ( stock is 68 – but I had changed that to a 72 years ago ); a new 170 main jet ( stock is 160, and that was what was in it ). A fuel valve and accelerator diaphragm finished the job.
Put the carb on, along with a new fuel line – and again it started right up.
This time it would idle nicely at about 900 rpm.

Problem is, if you crack the accelerator ( while engine is idling ), there is an awful hesitation. If you crack the accelerator from about 1200 rpm, there is no hesitation.

I have not ridden the bike yet because I still have to do the tires and brake fluids, so I do not know how it will react on the road.
Did I put in too much main?
Any other suggestions on where to look to get rid of the hesitation?
Thanks for any help.

If your carburetor is one of those without an accellerator pump, that is one characteristic. You could swap it for a later CV40 with an adapter and be much better off.

If it has an accelerator pump, then it could be that the idle jet is too lean.

Good luck.

[biscotti]

This carb does have an accelerator pump, and I did put in a new diaphragm.
When you say idle jet, if you mean the idle mixture screw – this carb has that screw sealed so you cannot make adjustments to it.

I was doing a bit of reading, and understand that it might be a leak in the carburetor to intake manifold area. I did install a new gasket here, after cleaning both surfaces well – but who knows.
Anybody use RTV sealant here to insure a good seal?

[FL84FXWG]

Get the bike warmed up then spray some wd40 around the intake.

If there's an intake leak, the idle speed will change.

Intake leak will also causing popping and backfiring.

[newultraclassic]

Originally Posted by biscotti

This carb does have an accelerator pump, and I did put in a new diaphragm.
When you say idle jet, if you mean the idle mixture screw – this carb has that screw sealed so you cannot make adjustments to it.

I was doing a bit of reading, and understand that it might be a leak in the carburetor to intake manifold area. I did install a new gasket here, after cleaning both surfaces well – but who knows.
Anybody use RTV sealant here to insure a good seal?

Wel, you said that it will idle at 900 rpm. Ordinarily, that would rule out an air leak.

As to the idle jet, right, they come sealed. If you should choose to unseal it, an adjustment to about 2 turns out will cure your stumble off idle.

[biscotti]

I’m going to try the easy test first, with the WD40.
If that doesn't work, I’m going to drill out the seal and re-adjust the idle mixture needle.
I hope it isn’t that difficult to remove the seal though – I don’t want to turn this carb into a pile of junk.

[newultraclassic]

Originally Posted by biscotti

I’m going to try the easy test first, with the WD40.
If that doesn't work, I’m going to drill out the seal and re-adjust the idle mixture needle.
I hope it isn’t that difficult to remove the seal though – I don’t want to turn this carb into a pile of junk.

The seal is an aluminm plug that is approximately 5/16" diameter and 3/16" thick. It is a press fit in the tube where the idle jet is situated.

Take a 7/64" drill and put a hole thrugh the plug but only just enough to get through it, not enough to touch the idle jet.

Tighten a small sheet metal screw into the hole and just pull the plug with a pair of pliers on the screw.

When you get the engine warm, turn the screw in until the engine falters and then out to about 2 to 2-1/2 turns. You should be good to go.....unless there is an air leak.

Sometimes when those early Keihins get a lot of miles on them the bushings wear loose around the throttle shafts and they will suck air. Then it's impossible to get them adjusted. It's not worth the trouble to rebuild them. At that point, you should be ready for a newer CV 40, also a Keihin but a much better carb.

Good luck.

[SteveB]

Yes, a small intake leak would cause these symptoms by allowing you to close the throttle plate enough to block the low speed bleeds at idle thereby causing the hesitation. The edge of the plate should be just at the edge of the first hole so as to pull very slightly on the low speed circuit at idle. I went with a bigger plate from NAPA and shaved it to fit. Didn't need an accellerator pump but had to also build an adjustable idle mixture screw.

[biscotti]

Sprayed WD40 around the intake after the engine warmed up – no change, so I ended up removing the mixture screw plug.
Turns out that the factory setting for the idle mixture screw was ¼ turn out !
I changed the setting to the recommended starting point of 2 turns out, and what a difference that made. No more stumbling.

Does anyone know if anyone still makes the idle mixture screw with the knurled knob on top instead of the screw driver slot?
Neither J&P or V-Twin carry them any longer.
Because the bottom of the stock tank is so close to the top of the mixture screw tower, it would make it so much easier to adjust this thing if I could find one.

Thanks all for the suggestions and help.

[newultraclassic]

Originally Posted by biscotti

Sprayed WD40 around the intake after the engine warmed up – no change, so I ended up removing the mixture screw plug.
Turns out that the factory setting for the idle mixture screw was ¼ turn out !
I changed the setting to the recommended starting point of 2 turns out, and what a difference that made. No more stumbling.

Does anyone know if anyone still makes the idle mixture screw with the knurled knob on top instead of the screw driver slot?
Neither J&P or V-Twin carry them any longer.
Because the bottom of the stock tank is so close to the top of the mixture screw tower, it would make it so much easier to adjust this thing if I could find one.

Thanks all for the suggestions and help.

I use a small straight screwdriver with a piece of rubber tubing on it. Hopefully you won't have to adjust it that much. I haven't seen one of the knurled knobs for a long time. If you change to a CV40, the idle jet is on the bottom and is much easier to adjust.

Yep, when you get the idle jet correct, they will be a lot easier to run.

[L.Linkert]

Newultraclassic gave the best tip, replace that dreadfull carb with a cv40.
Put the old one on a shelf in case the antique road show comes to town.

[newultraclassic]

Originally Posted by L.Linkert

Newultraclassic gave the best tip, replace that dreadfull carb with a cv40.
Put the old one on a shelf in case the antique road show comes to town.

Not a bad idea! The main probklem with the older side-draft "butterfly" type Keihins is the fact that even in good condition, they let too much air in. The basically suck a lot of draft and, on a bad day, you won't get your bike to start.

Couple that with the fact that most of them are now about 25 years old with loose shaft bushings and consequent leaks, and you have a bad deal.

The nice thing about the CV40 is the excellent starting an idle that it provides for older bikes.They are very tight when the slide closes down. This keeps the mixture correct when starting. My Shovelhead with CV40 idles like a clock.

I honestly believe that the CV carbs are the best that were ever made. They are used on lots of foreign bikes as well to good advantage.

Yep, as Mr Linjkert says, you've got a museum piece there!