Engine won't idle

[panofish]

I've read dozens of threads, but I still feel stumped.
Here are list of symptoms for my new build:

1. Engine starts with medium choke and slight throttle
2. Must hold slight throttle and feather choke to keep engine running
3. Engine will rev with throttle, but is rough at times (because it is choked)
4. If I release throttle then the engine dies immediately
5. If I turn choke completely off then the engine gets weaker and weaker until it dies
6. Adjusting the idle screw has no effect

My best guess is a vacuum leak between the carb and intake?

 

[panofish]

OK... I disconnected the fuel line from the tank and opened the petcock to drain the fuel. The fuel drained out slowly in about the thinnest stream of fuel possible without dripping. How fast should it have drained? I also removed the tank cap while draining and that made no difference.

I would have thought it would drain as if through a thin straw. I removed the petcock... the filter above the petcock looked clean. So the thin flow of fuel was caused by the filter screen, a smaller orifice inside the petcock valve, or something else inside the petcock?

Tomorrow, I will try running the engine with the fuel remaining in the the fuel line (unattached to the fuel tank) to see if there is any difference.

The petcock screen looks like a very low quality plastic mesh, but isn't clogged
The screen unscrews easily from the petcock.
I disassembled the petcock and found no clog.
The internal holes are smaller than the fuel line connector, but I would still expect a faster flow.
Tomorrow, I will reattach the petcock (without the mesh screen) and add some fuel to the tank and check the fuel flow.

 

[bluegoatwoods]

Let's see.....to start with, you've got a nice camera. Future photo post of yours might be valuable.

Your mesh fuel filter looks just fine. But I'd also add a better inline filter between the tank and the carburetor.

I don't really know, but I'd bet that the flow of fuel you described from your valve is enough to run these engine. They're known fuel misers, after all.

the symptoms you described in the first post certainly sound like an air leak. And those are common enough. There are vendors with seals of a higher quality than those that come with these kits. Can't remember who they are for certain, but I think sickbikeparts is one of them. I sealed mine with a whole bunch of silicone sealer smeared all over it. Seems to work pretty well.

good luck.

 

[MotorBicycleRacing]

I've read dozens of threads, but I still feel stumped.
Here are list of symptoms for my new build:

1. Engine starts with medium choke and slight throttle
2. Must hold slight throttle and feather choke to keep engine running
3. Engine will rev with throttle, but is rough at times (because it is choked)
4. If I release throttle then the engine dies immediately
5. If I turn choke completely off then the engine gets weaker and weaker until it dies
6. Adjusting the idle screw has no effect

My best guess is a vacuum leak between the carb and intake?

How did you seal between the carb and intake?

You have a large air leak at either the carb neck or at the intake gasket or ....

I include an O ring with all the kits I sell to seal at the carb to intake tube

When your motor runs applying full choke should kill the running engine quickly

Do not touch the clip -- adjustment yet as that would not cause your symptoms

Your fuel supply is fine......Fix your air leak

 

[panofish]

Fuel flow was still a trickle when I removed the petcock screen, but I don't think that is the problem.

I added an o-ring between the carb and the intake and that made little difference.

I was able to take a short ride today, here are the symptoms:

1. I had to keep the throttle at 5%.
2. After warming up, the choke was fully off (down)
3. If I released the throttle then the engine dies
4. It I try to increase the throttle then the engine bogs down and loses power.

 

[MotorBicycleRacing]

If you full choke a warmed up running engine does that kill it?

That will tell you if you have an air leak

Make a better gasket for the intake tube to the head

Unscrew the float bowl and check that the jet is screwed tightly into
the bottom of the carb throat
Sounds like your jet is coming unscrewed

Did you fool around with anything else on the carb? Like the clip on the needle?

Where did yo ubuy your motor from?

Is that a Honda 500 single in the pic?

 

[angrysamoan]

Hey Panofish, I can almost see a gap where carb throat connects to intake tube. If you take off carb filter, with rubber mallett slightly tap flat 1x3 piece of pine wood so carb scoots completely over intake. Do that to all my installs {I personally have four bikes, and have now built an additional three for others}, also idle can be adjusted just right at throttle cable adjust on handlebars and another adjustment on top of carb. None of my bikes require choke ever, just a few pumps on carburator and start up. Good Luck

 

[panofish]

Where did yo ubuy your motor from?
Is that a Honda 500 single in the pic?

I bought it off ebay from a reseller. The kit was new.

The other bike is my Triumph Thruxton (no problems with fuel flow there).

 

[panofish]

PROBLEM SOLVED!

I finally opened the carb and guess what I found?
The brass jet was totally unscrewed and just flopping around in the bowl.

I screwed it in place and now she runs and idles like it should.

Wow, this thing puts out a little more vibration than I'd like... maybe she'll run smoother as she breaks in.

I shot video of the entire build process (including the problems) and I will post it on my blog when I have it edited. I am going on a 2 week vacation next week, so it will be a while before I can finish the video. I think it will be the BEST video you can find on this subject.

 

[MotorBicycleRacing]

PROBLEM SOLVED!

I finally opened the carb and guess what I found?
The brass jet was totally unscrewed and just flopping around in the bowl.

I screwed it in place and now she runs and idles like it should.

Wow, this thing puts out a little more vibration than I'd like... maybe she'll run smoother as she breaks in.

Unscrew the float bowl and check that the jet is screwed tightly into
the bottom of the carb throat
Sounds like your jet is coming unscrewed

I had a customer that had 40 miles on his bike with the jet unscrewed.....

Your engine is a PK and they just don't compare with my Grubees
I used to sell that junk....