Mikuni Carb Vent Question

[DaveBrit]

I have some questions on the correct venting of the Mikuni carbs on an '85 FJ1100 and I want to make sure I reassemble them correctly (the bike had no venting lines when I received it).  I have done the usual searches on the forum and I think what I am suggesting below is correct but wanted to confirm.  I have included an image (red border) I 'borrowed' from Randy's site which shows for sale a new set of carbs with vent lines attached as a reference



1. Vents labeled "A" I believe are the choke vents (one on each carb), should there be a vent line attached to each and, if so, do these vent lines need to run down the back of the engine and finish near the rear suspension swing arm.  In addition, can I run them into one common tube to achieve this?  Alternatively, should these lines be connected to something else?

2. Just behind the fuel feed line labeled "B", there is an additional T piece vent made of off-white plastic between carbs one and two  and another between carbs three and four.  Is this an additional pair of vent lines that need to be run down to the swing arm?

3. Is the clip labeled "C" for the vacuum line that runs back to the IC Igniter?  I believe this vacuum pipe should be secured to the small vent on air intake manifold #2.  Further, am I correct in that the small vents on air intake manifolds #3 and #4 are blanked off and the one on #1 has nothing on it and is open to atmosphere?

Thanks - DaveBrit

[FJmonkey]

1. Vents labeled "A" I believe are the choke vents (one on each carb), should there be a vent line attached to each and, if so, do these vent lines need to run down the back of the engine and finish near the rear suspension swing arm.  In addition, can I run them into one common tube to achieve this?  Alternatively, should these lines be connected to something else?

There should be a line on each, you could combine them but having them separate aids in identifying which one is leaking when you get a stuck float or crap in the float needles. Run them down to drain to ground just in front of the swing arm

2. Just behind the fuel feed line labeled "B", there is an additional T piece vent made of off-white plastic between carbs one and two  and another between carbs three and four.  Is this an additional pair of vent lines that need to be run down to the swing arm?

That sounds like you have gravity feed carbs improperly converted to fuel pump. The top right pic shows GF, they have two 10 MM fuel inlets, the reference pic from Randy is a FP set up with one 6 MM fuel inlet.

3. Is the clip labeled "C" for the vacuum line that runs back to the IC Igniter?  I believe this vacuum pipe should be secured to the small vent on air intake manifold #2.  Further, am I correct in that the small vents on air intake manifolds #3 and #4 are blanked off and the one on #1 has nothing on it and is open to atmosphere?

One should connect to the "IC igniter", another to the fuel petcock, the other two blocked off. If you have an after market petcock that does not have a vacuum shut-off then block that one off as well.

[56 CHEVY]

2. Just behind the fuel feed line labeled "B", there is an additional T piece vent made of off-white plastic between carbs one and two  and another between carbs three and four.  Is this an additional pair of vent lines that need to be run down to the swing arm?

I am pretty sure the white T pieces are bowl vents. I am not sure if they also go down the swing arm or not but I do not see any reason they couldn't.

[DaveBrit]

Appreciate the responses Chevy and FJMonkey, everything is clear on the answers for questions 1 and 3 other than confirmation of what goes into the clip labeled "C".

In regards question 2, I realise Randy's pic is for the later version with fuel pump, however I included it as it does clearly show the two white T pieces betweens carbs 1 & 2 and carbs 3 & 4 and, based on Randy's picture, I suspect they are drains but wanted to confirm this (the two T pieces are obscured in my carb's pictures by the two incoming fuel lines).  My setup is the dual gravity feed as FJMonkey observed.  I believe my fuel petcock is the original as it has the vacuum shut off pipe to carb #1.

[racerrad8]

A) Choke/Enrichment circuit atmosphere/draw tubes. They must be open to atmosphere. If combining them you must have an ID of the new hose equal to the combine size of the four individual hoses.

B) Those are the float bowl vent tees. You can also combine the two hoses but again the single size must be the same as the two combined.

C) That clip is to hold the vacuum line to the petcock as the petcock is "behind" the carbs. The CDI vacuum line had a different clip located on the #2 carb that is not there in your picture(s).

All hoses went down behind the engine on the right side and there is a clip on case bolts #35 & 36.

Intake nipples 1 & 2 are the vacuum supply for the petcock and CDI, 3 & 4 are capped.

Randy - RPM

[FJ_Hooligan]

The petcock line has a clip on the bottom side of the tank.

Do not combine the choke air tubes.  Or if you do, do as Randy suggests and use larger hose.  I tried this once on someone's FJ in an attempt to tidy things up around the carbs and it didn't work well.

[racerrad8]

The petcock line has a clip on the bottom side of the tank.

.

The vacuum line goes from the intake through the clip and hooks to the petcock.

I just looked at the bottom of a tank and there is no clips there for the vacuum line. (for the early model FJ's)

Randy - RPM

[FJ_Hooligan]

Must be a subtle difference between models. 
The tank on my '85 has a built in tang on the bottom of the tank that the petcock vacuum line clips into.  It comes straight out of the petcock, under the tang then bends 90 degrees down to the #1 vacuum port.  When I take the tank off all I have to do is reach in there and pull the hose off the vacuum port and yank the tank (of course this is after wrestling the fuel line off and unplugging the fuel gauge connector).