87 VR carbs, rubber plugs in jet block

[Gearhead]

Hi Folks,

 

Second in a series of carb questions. The Mikuni carbs each have two rubber plugs in the bottom of the jet block, below fuel level. I've never encountered this before. They are both hard and fit very loosely in the holes (all carbs). What are these for? Are they supposed to seal in the holes and block fuel flow into the jet block via those paths? Will they soften and swell at all when submerged in the bottom of the bowl for a while? If they leak, what would the symptoms be?

 

Mine were out on the bench and dry for 3 years, so I don't know what they were like before I first drained the fuel. I put the carbs back together this way. The bottom of the bowl prevents them from falling out, but they are loose and by no means seal the holes they are in. Maybe sitting dry allowed them to shrink?

 

Full Story:

https://www.venturerider.org/forum/showthread.php?141445-87-VR-carb-problem&p=1062867#post1062867

 

Thanks,

Jeremy

Edited June 16, 2019 by Gearhead

[Bob K.]

Yes, they seal. I reconditioned mine with a wintergreen oil & alcohol soak. I soaked the rubber in a jar for 24 to 36 hours. It re-plasticizes them. In a few cases, they over-expanded and were too large to fit back in, but I found that they tended to shrink a little bit if I let them sit out for a day or two. You can do an internet search for "wintergreen rubber recondition" for the process.

Edited June 17, 2019 by Bob K.

[Gearhead]

Thanks, I am going to try this.

 

But...does anyone understand the consequences of them being loose?

 

I've been searching the web and finding very little about that. The prevailing thought is something that I don't think makes sense. The pilot passage gets fuel from the main jet thru a hole that connects the passages [true]. Without the pilot jet rubber plug sealing, the pilot circuit will pull fuel directly from the bowl [true] and thus the pilot circuit will be rich [i'm not so sure]. I don't think this makes sense because the pilot jet is about a third the size of the main and will be controlling the fuel flow either way.

 

What I think is, since these passages are connected, and especially in the case of the Venture carbs which have two rubber plugs (one in the pilot jet passage and one in the main emulsion tube passage), loose fitting rubber plugs would cause the top-end main-jet metering to be rich because the main circuit can pull fuel past the rubber plugs, bypassing the main jet. Right? Wrong? Weird, because my bike is running well in the mid range to top-end.

 

The problem I am having is that my pilot circuit is lean...needs excessive turns out on the pilot screw (3 to 5 turns) and still doesn't take throttle quite right off idle, as when releasing the clutch. I'm positive everything is clean and installed correctly. I found one reference online to just such a symptom of leaking or missing rubber plugs. Something non-intuitive about the leaking plugs upsetting the balance of pressures in the pilot system which causes the pilot circuit to draw too much from the air-bleed and not enough fuel. I don't understand all the relative pressures enough to get why, but it's consistent with my symptom.

 

Do any of you know?

 

Thanks! Jeremy

Edited July 29, 2019 by Gearhead

[Patch]

Have you heard tell of my couz Bernoulli? He wrote extensively on the topic of flows?

 

The shape of the jet, the flow to the get, thru the jet are very much relevant. When you alter flow before the jet you alter velocity! Starting to make sense?

Patch

[Gearhead]

I have in fact met your cuz! Well, not personally...

 

I've run across reference on this forum, something to the following effect:

"The VMax guys have found that bad or missing rubber jet block plugs causes problems with idling and low speed"

This is encouraging to me that this might be my problem. Which is good, because I don't know what else it might be...

[Patch]

Take a closer look at the jet block and gasket while you're in there, notice the ceiling points impressions.

The flow directions and porting need to be operating as designed which requires a proper crush on that gasket.

[Gearhead]

Update on rugger plugs:

 

I soaked them per post#2 and it worked - the plugs are normal size and rubbery again, fitting snugly. That being said, I also noticed when I disassembled that they were already snug in the holes, so being immersed in fuel swells them as well. But the wintergreen oil soak did work. However, this did not fix my problem with the pilot circuit.

 

Full carb story:

https://www.venturerider.org/forum/showthread.php?141445-87-VR-carb-problem

[Patch]

Are you saying that you went thru the carbs as we mentioned, completely and the bike still won't hold idle without the choke or transition smoothly past a 1/4 throttle?

[Patch]

What on the list below has changed?

Do you know how to check the timing position?

 

SYMPTOMS - Lean idle and just off-idle

1) Very dependent on choke for good throttle response until it's really good and hot. I'm in Tucson, so I mean HOT!

2) Pilot screws are set at ridiculous settings for best idle - two carbs at 5 turns, and two at 3 turns. Before the rebuild, I had tuned them at some point in the distant past and they were all at or under 2 turns.

3) Even when it's hot and taking throttle, standing start with the clutch still doesn't feel quite right, a little hesitant. Before it's hot, if I try to take off quick with say 1/4 to 1/2 throttle, it drops cylinders / misfires for a second or so.

4) Runs smooth and goes like a bat out of hell once in the mid-range and upper RPMs. Man, this is a nice riding bike.

5) It wasn't acting like this before I took the carbs off.

 

 

[Gearhead]

Hey Patch, I'll respond to this over on the main thread.