Topic: cylinder 3 not drawing fuel in to cylinder?

[75 super sport]

Just got the bike fired up, but cylinder 3 is not firing. 1,2&4 are good but number 3 not so good. It doesn't seem to be drawing fuel into the cylinder. Brand new 836 wiseco kit and webcam 63a. Does this sound like cam timing is off? I sure hope not

[BobbyR]

It is either not drawing fuel or the carb is not working well. Shoot some carb cleaner or ether into that carb. If she fires, it is the carb. If not, you have some work to do.

[75 super sport]

As far as what? There is no suction or drawing of fuel into the cylinder.

[eigenvector]

If that were the case then you would have either no compression of that cylinder or the valves would be shot (same thing really).

You said #3 isn't firing, which means to me it isn't getting spark.  If you mean it isn't getting fuel, that's another issue.  If it's not drawing in the mixture - that's another issue as well.

[75 super sport]

It is not drawing the mixture in. When I put my hand over the intake side of the carburetor thereis no suction. When we reassembled the head we did a leak test on the valves and all was well. It is getting spark and we just rebuilt the carbs last night. There is just do suction or drawing of fuel into that cylinder.

[mrbreeze]

Do a compression test and go back and make sure the valves aren't adjusted too tight starting with that cylinder.

[75 super sport]

Ok, will do. If there is not good compression in that cylinder what would be my next step?

[mrbreeze]

A leakdown test will show you where it's not sealing. Make sure the valves aren't too tight or if one is stuck open.

[75 super sport]

Ok, thanks for the help. Any other pointers would surely be appreciated. Thanks

[calj737]

Check for a vacuum leak around the intake side of that cylinder. With a significant leak, you'll have reduced suction.

[75 super sport]

Ok. I have new boors, so I don't think that would be it but I will sure give it a try! Thank you

[75 super sport]

Any other ideas? Anyone ever seen this before?

[72 yellow]

Interesting.  I have a 72 CB750.  All stock except for a Kerker w/baffle.  Does the same thing on 1, 3, and 4 when cold.  Taking the screw out where you insert the adaptor for carb balancing and adding a few drops of gas will cause it to fire up.  Sort of like tickling the Amals on my BSA.  Once started and shut off it will fire right up. 

[75 super sport]

That doesn'tsound llike the same problem I am having. I am getting fuel into the carbs, its just not drawing it into the cylinder

[Bodi]

Cam timing would affect all four the same.
Sounds like a stuck exhaust valve? Did you check piston clearance?
There's not many other choices assuming you have a spark plug in the hole - hard to miss that. Or forgot a piston or its rings... ditto.

[75 super sport]

Yes, I checked piston clearance and it was to spec, rings were installed to spec as well.

[toolguy]

step one is a compression test. .and  check the exhaust valve clearance as pointed out previously. .
 guessing isn't going to help at this point.
If the intake was too tight, ie not closing , you would have blowing compression back thru the
 carb which you didn't mention. .  too tight exhaust would blow into the muffler which you'll
 never hear or see. .  and prevent suction on the intake side. . . 

Head Gasket

ARe you thinking bad cam timing as in just that one cylinder?  In today's age of CNC cam grinding
I'd doubt one lobe is off on a new cam. . . You can check cam timing  just
like on  a car with a dial indicator. .

[75 super sport]

About can timing, I just brought that up because it was the first time I have ever timed a cam and was unsure about it. But I don't think that's the case now. There wassome fuel coming up of the exhaust like it was just passing through the valves, and no suction on intake side of carburetor. It sure would be nice of exhaust valve was just a little too tight causing it to not close all the way. I highly doubt it is the head gasket either. I would love to not have to tear this motor down again.

[Don R]

The first thing to do is check the valve lash, the most likely and easiest thing to fix. Don't panic yet,   there's an order to go through, that is #1.  Exhaust then intake, you might want to go through the order again, I double check them on my race car sometimes.

[75 super sport]

So set exhaust first, and then intake?

[Bodi]

Neither is necessarily first. You just adjust a tappet with its valve fully closed. There is a "fast way" minimizing crankshaft turning, or you can start with, say, #1: turn the crank until the intake valve is fully closed (like at BDC on power stroke), adjust that tappet. Crank around to have exhaust valve fully closed (ie TDC on compression), adjust that one. Move to next cylinder.
If a tappet gap is abnormally super huge, that valve is probably stuck open.
Remove sparkplugs so you're not fighting compression.

[75 super sport]

Here is a little video of what cylinder #3 is doing. Fuel out of exhaust pipe. I'm starting to think this is a spark issue. None of the valve clearances were exceptional, they were just as we set them when we reassembled the head. Last night I just assumed we had spark cause we had just checked a couple days ago, but #3 carb was pissing fuel everywhere so maybe it fouled the plug? Anyway here is the video.

[BobbyR]

Usually spark issues will drop two cylinders due to the waste spark system. The block is part of the circuit. Some years back there was a guy going crazy over a single dropped cylinder. He finally discovered that a faulty spark plug was able to complete the circuit but not spark an ignite the fuel. Maybe swap the plugs between than and another cylinder and see if the problem moves with the plug.

It is a long shot, but who knows, and it is simple.

[mrbreeze]

Starting to sound like you flooded #3 and fouled the plug. Take another good spark plug....plug #3 wire onto it and ground it to the case. If it fires a nice healthy spark...... install it. If the valves still have the proper clearance you should be OK in the compression dept. If there is no spark....check for spark on #2.

[Don R]

 I said exhaust first just because you were suspicious of the exhaust. Did you test compression on 3?

[75 super sport]

I was not able to check compression, the tester we had did not have the right adapter. A little update tho, at first cylinder three will fire and run good, then it immediately fouls the plug and starts not firing. I got a jet kit from 6sigma. They gavee 125 mains for outer two cylinders and 127.5 mains for the inner two cylinders. #2 and #3 are both running wayyy to rich. #2 seems to keep firing but #3 stops. O think what we have is just a fuel issue that I need to sort out. I will try to pick up another set of 125 mains and see if that helps the issue.

[calj737]

You need to make sure the fuel level is correct, regardless of the jet sizes. Use the "clear tube" method and correctly adjust that. Then, once the bike is running, tune the carbs with jetting changes.

I don't know your supplier, but be dead certain to only use Keihin brand brass in these carbs. Aftermarket jets and needles (especially) are prone to create tuning issues as they differ from stock.