One Exhaust Pipe Cooler Than Others at Idle?

Hey guys, I don't know if this is a problem or not, but it bugs me. On my 86 VFR700 the number one exhaust pipe doesn't warm up like the others when idling.

It runs fine, has lots of power and doesn't seem to be an issue, but it bothers me.

I checked the spark and it's nice and blue at idle so I figure it must be carb related.

I tore the carbs apart and cleaned them when I first got the bike and everything looked good.

I believe all the pilot screws are supposed to be 2.5 turns out, but you can smell the raw fuel from the exhaust, so I set them 2 turns out and you can still smell it's rich.

I can close the #1 pilot screw or open all the way and it doesn't seem to effect the idle speed at all.

Any ideas?

 

You might have to clean that problem carb yet again, wire cleaner in all orifices, carb cleaner spray and comnpressed air in each orifice.
Check your float level also, might be too high on that problem carb.

have you checked your carbs with a carb synch gage?

 

As Dukie suggests, just sitting for a week or 2 is sometimes enough to clog slow jets with our corn-infused fuel, so get in there again and put a .012" wire through the idle jet hole, then inspect the throttle plates to see if the #1 is closed more than the others.

Since the mixture screw has no effect on #1, you need to remove the screw, spring, washer, and O-ring, then blast carb cleaner and air through that hole, best done while the idle jet is removed.

It's usually good to try switching sparkplugs from a cylinder that is firing to one that isn't as a test.

 

yep, a quick sparkplug swap or a new plug would be a quicker check than pulling the crabs for another cleaning. If the pipe is just a shade cooler it does usually point to dirty crabs.

 

I did sync them.

I have removed the pilot screw and blown compressed air into the hole while blocking the 3 little holes in the venturi and it didn't change anything.

Does that pilot circuit go from the venturi, through the pilot screw opening and then through the idle jet hole or is there another opening somewhere?

Can anyone show me what the idle jet looks like? It's possible my wire was a little too large and didn't go through all the way.

 

pilot jet cleaning.....

Note that there are 2 axial holes that need to be clear.

 

I dropped the #1 float bowl the other day and removed the idle jet. It is clear. I also ran a fine wire through the other, fixed brass tube. I'm not sure what it's called. I can feel the wire go through the oriface and then go 1/4" further and botton out on something solid. I blasted some 100psi air through both of these tubes. (I did the same things back when I had the carbs off and cleaned them). After putting it back together there was no change. The pilot valve adjustment still has no effect.

I think I'm going to pull the float bowl once more and also the pilot valve and then try blasting air through the circuit.

I could use some help in identifying the jets in the pic.

 

If I blast air with the idle jet installed, the air isn't going to flow very quickly through that part of the circuit, correct? That might be why the air blast had no effect last time I tried it. The jets were all installed.

 

You could have a sticky float also. That pipe of the sticky float will be cooler until the bike warms up.

 

The needle and seat look new. Plus the engine temp doesn't have any effect.

 

I'm working on it right now. I can blow air into the removed pilot valve screw (+ spring, washer, o-ring) and get air out of the pilot valve jet as indicated in red in my attached photo. Air also comes out the 3 holes in the carb throat by the butterfly valve. So as far as I can tell the pilot valve circuit is not obstructed.

Is there more to the circuit than what I'm describing? Maybe a pilot valve AIR jet?

These are Keihin VD carbs, right? I can't seem to find any good diagrams on them.

 

I know I set all the float levels a while back by turning the carbs and letting the floats just rest on the valve. With them on the bike, I'm forced into raising the float until I see the needle stop moving and it looks a little too low. The fuel level might not be covering the pilot jet hole. I'm going to raise the fuel level a little and see if that's my problem.

Are the floats suppposed to be 7mm or 9mm from the carb base?

 

7 or 9 ?? Interesting question !!

If your plugs look darkish, try 9. Whiteish, try 7.

It's an enduring mystery why Honda speced two different setting for the same carbs on the same bike. Emissions might be one answer, or not ??

I posed this question to a former employee of American Honda and former chief Honda instructor at MMI and former champion roadracer on a Honda and got no real answer.

 

Well raising the float didn't help.

Is there a pilot air valve? Maybe it's blocked.

I'll pull my plugs and check them once the engine is cooled down.

 

I checked the carb sync since it may have changed since the last time I did it and sure enough it was not sync'ed to the main carb #2. I balanced #1 and #2 together and the idle seems a bit smoother.

I'll report back if that fixes things.

 

I'm pretty sure the red pilot jet is correctly named. Since it mainly works at idle, I suppose you could also call it the idle jet.

Is the blue jet for the fuel enrichment (choke) circuit?

 

If I remember right the blue jet is a starter jet that engages when you pull the choke in and the red is a slow/pilot jet.
You may need to measure the static head of the gas in your float bowls with 4 clear lines hooked up to get your true fuel level in the bowl if after a thorough cleaning the problems persist.

 

I personally hate cleaning my crabs. What a messy job. lol

 

Yep, crab cleaning can get quite messy. :wink: