One Stubborn Carb.... Help!

[loopsandlogic]

Here's the scenario... I have a 84' Vf500.

At first the bike wouldn't come off choke. It would literally die if the choke was to come slightly off.

I took all 4 carbs off, blew the jets and jet passages, reassembled all 4, fired up on the 1 start! SMOOOOOOTH!

Then when feeling all for headers, the left rear cylinder just won't fire.
And being the carb on the outside, I've taken the bowl off 6 times already!!

The spark plug is completely dry. When I take the jets out, the main and slow, I spray carb cleaner through there passages, and at the same time, I look through the intake making sure the passages are clean.

So here are my questions:

1. Is there a idle jet?
2. Is the slow jet the one that comes out or the one that's stationary?
3. Fluid is suppose to come out of the second hole on the bottom of the carb bore when spraying in the slow jet?

4. What is the micro hole that's at the end of the carbs bore closes to the manifold boot?

And the choke does work properly on the left rear carb, cause when the carbs where only working on choke, that cylinder was firing!!

Help me!

Mike

[Longerfellow]

The slow jet and idle jet are one and the same and are removeable if my memory is correct. The passages between the throttle valve and the head should flow fuel at idle. You should be able to squirt through there with contact cleaner or something similar. The staionary jet (no slot for screw driver) is the enrichener (or choke). They go for more fuel as opposed to less air. I'm assuming compression is within spec in that cylinder? Does that cylinder kick in when you rev the motor?

[loopsandlogic]

So the slow jet is also the idle jet. That makes sense!

When carb cleaner is sprayed through the slow jet, a nice wide stream is released into the intake at the hole right by the throttle valve.
It's clearly visible.

How would I know if the cylinder kicks in when I rev it?
Will this test to see if my main jet is working properly?

I only assumed the compression was good, cause when I put my hands over all 4 carbs, the suction on all of them are STRONG!

And because the front 2 heads have 169lbs at cold

So what is the hole that's right by the end of the carb body closes to the intake manifold???

Thanks for the quick reply

Mike

[Longerfellow]

If you mean the larger hole in the carb throat, that would be where the fuel is let in while using the enrichener circuit. If you rev the motor slightly and the cylinder in question comes alive, that would point to a slow speed restriction, but from what you say, it should idle. Where did you det your pilot screws when you re-assembled it. There's someone here with a wealth of experience on these and I hope he chimes in. It really helps to see it. You can see by the way the carb body is casted (sp), where the fuel should be going.

[loopsandlogic]

Yeah the larger hole that closes to the cylinder head.

So, since the fuel should flow, I better remove my plug and switch it out with another one.

No change should indicate a problem with the fuel still.

I don't know what you mean about the pilot screws?

Are you asking if I set them as per the OEM shop manual, because if you are, then the answer would be yes.

I hope he chimes in too

This bike is killing me. I still need to take my very first VF ride

Mike

[Longerfellow]

The "pilot" screw are the low speed mixture screws in the bottom of the carb in front of the float bowl. They're for adjusting the air/fuel mix at idle to about 1/8th throttle. Did you see if the cylinder in question comes alive over about 2500-3000 rpm? I don't think a plug is the issue if it fires with choke, unless it's now fouled. Again, it'a always tough not being able to see and hear it.

[loopsandlogic]

Just got back from cleaning the garage and working on the VF.

I took a plug from the front cylinders and switched them to the left rear cylinder. No change!

All plugs nice and brown, except for the trouble maker. He's between brown and white.

I then decided to rev the bike just like you had mentioned, and after letting the bike come down to idle, the rear cylinder was hot, but not as hot as the other three.

Like when you touch a running cylinder, you pull away quickly, but with the left rear cylinder, it takes a moment longer until I yell ouch!
But it definitely felt like it came alive while revving!!

I don't know what I was thinking but I decided to put my Vacuum gauge on the cylinder, and what I found was surprising!!

The cylinder is pulling 42hg or whatever you call it, while the opposing front cylinder was pulling a normal 15hg.

What does this mean???

Could the stronger vacuum mean I have the throttle valve open too far???

I've rebuilt many classic CB 4's but I've always hated CV type carbs

And all pilot screws are 2 turns out.

thanks for the help

[squirrelman]

sounds like an air or fuel clog in some small passage.

to do a carb clean sucessfully sometimes a big (125 psi) air compressor is necessary, much more effective than a spray can!


try turning the mixture screw on the bad cylinder OUT while engine is running ........ (up to 6 turns ).....

....if that doesn't work, remove mixture screw entirely and shoot some carb cleaner into hole with engine running........

[Longerfellow]

Originally Posted by loopsandlogic

Just got back from cleaning the garage and working on the VF.

I took a plug from the front cylinders and switched them to the left rear cylinder. No change!

All plugs nice and brown, except for the trouble maker. He's between brown and white.

I then decided to rev the bike just like you had mentioned, and after letting the bike come down to idle, the rear cylinder was hot, but not as hot as the other three.

Like when you touch a running cylinder, you pull away quickly, but with the left rear cylinder, it takes a moment longer until I yell ouch!
But it definitely felt like it came alive while revving!!

I don't know what I was thinking but I decided to put my Vacuum gauge on the cylinder, and what I found was surprising!!

The cylinder is pulling 42hg or whatever you call it, while the opposing front cylinder was pulling a normal 15hg.

What does this mean???

Could the stronger vacuum mean I have the throttle valve open too far???

I've rebuilt many classic CB 4's but I've always hated CV type carbs

And all pilot screws are 2 turns out.

thanks for the help

You may be onto something now. The high vaccum would say that it's not open far enough as opposed to too far. Do you have the tools to do a carb synch?

[loopsandlogic]

I'll try the pilot screw (6 turns out).

I first spray the passages with carb cleaner, then I shoot them with 90psi of air.
Been doing that for years.

I pretty much have the tools to rebuild a Trans am to a 75' CB400F.

I just bought my other 2 vacuum gauges to make it easier on me. I now have 4.

So, what you are saying, is that my throttle valve is open too much or too little??
From what I've read into this, I think it's open too much....???

I'm getting close...if worse case scenario, I pop the left rear carb only instead of taking all 4 off. It'll be much easier as I'm looking at it.

Other than that, it sounds pretty good. I can hear the carbs are out of sync cause theirs some ticking noise up top at idle indicating uneven firing....or an early sign of shot cam lobes

Again, thanks for the help

Mike

[Longerfellow]

Well, the vac would be drawn from the cylinder and measured between the "butterfly valve" and the head right? If you open the throttle then vac would go down. If they're that far off, it could be the cause of your problem. I would verify their snch before taking anything apart again... especially v-fours... yuch-
If that cyl draws no air, it draws no fuel as well.
Mark-

[loopsandlogic]

That makes sense....but I thought the vacuum increases due to the increase in motor speed??
But if the flow is restricted, then there would be a much greater increase....man my head hurts

Tomorrow I will sync the carbs and post any differences.

Thanks again respective members,

LL

[loopsandlogic]

Problem fixed! After looking at the carbs sync screws, somehow this carb was really off. It nearly took 5-6 turns just for it to come down to 25hg.

Well, after much time and effort, the bike responds swiftly and sounds like a V-twin but with 4 cylinders!!!

But just one question, is there any easier way to adjust the screws without getting my hand burned with a craftsman short butted slotted screw driver??

My snake screw driver is even harder to get in there....

Thanks everyone for the assistance,

LL

[Longerfellow]

Good Deal!
What I use is a 90 degree handle like you describe that has a 1/4 drive on the end. That and a 7mm socket (I believe that's the right size) and away you go. Sometimes you have to open the throttle slightly to get the socket on then do your adjustment and see where it settles out. If you've gotten as far as you have, you'll get the hang of the carb synch quickly.

[loopsandlogic]

What I found at Sears is a 1\4" drive and at the handle is a spindle so I can actually turn the head without swinging the ratchet.

It's really nice! I gotta get me both, 1/4 and 3/8

You never really appreciate how simple Honda CB4's are until you have a VF

Now wait until I find me a VFR800....just another horse to add to the stables and another excuse to by some expensive tools

Thank you Longerfellow,

LL

[Longerfellow]

Glad to help. I love it when something works.

[DKC'sVFR]

Wish I could have been there. Sounds like you were struggling with some counterintuitive thinking! It's very easy to think you see what isn't really happening. When the throttle is closed and the motor is idling is when the vacuum in the manifold is at its highest. When the motor is redlining and the valve mechanism is floating because the cam is turning so fast that the valve springs haven't the capability to close the valves before the cam is pushing them open again is when the vacuum reading in the manifold is lowest or nil.

The counterintuitive interpretation of manifold vacuum gets lots of people confused when setting up carburators. It's that we humans have a tendency to consider that we glide around in a vacuum all the time, because to us the air is invisible. We think of the air as nothing. Its the scientific revolution and a bunch of curious Europeans who figured out the gas laws back in the 17th and 18th century that make our dilemmas even possible. The vacuum is actually inside the cylinder of the motorcycle piston, and its the sea of air that we live in (like fish in an aquarium) that is pushing air through the carburetor and picking up fuel along its way past the throttle plate into the cylinder as the piston descends, that creates the low pressure in the manifold that you measure with your Hg (mercury) Meter or barometer (AKA vacuum gauge). The fact that we make a kind of reverse barometer called a vacuum gauge only adds to the confusion. Its a fun game to think about when your falling to sleep.