Mikuni 28 spitting fuel from vents

[Bob Wood]

I am running Mikuni V28's on my XL 37 hp Mosler. All running well, I have about 5 hours on the plane since I finished it. I had spitting due to vibrating carbs and braced both carbs & snugged mounts,  and it almost went away. I did most of my testing on the ground before I started flying the plane. Now in the air it seeems to be worse. The right seems to be worse than the left.

I am wondering If I need to sync my carbs better to smooth things out? I carefully set the slides to match and it seems to be running well and not shaking any more than before.

Would my rubber mounts be giving out already, maybe need to be tightened? I searched out the soft ones & I have witness marks on the bottom, and the tops are cotter key safety'd so   I know that the bolts are not loose.  Maybe the rubber is wearing.....

I did check the float levels when I get he carbs and set them to what the list recommended ( cannot remember right now)

It seemed t be the tightness of the motor mounts & carb bracing that fixed it the first time, now that I think about it. I have about 24 hours run time in these mounts to date. Not tightened the mounts since I originally snugged them up probably 10-15 hours ago.

Any other ideas?

Thanks
Bob

[Steve]

I am running Mikuni V28's on my XL 37 hp Mosler. All running well, I have about 5 hours on the plane since I finished it. I had spitting due to vibrating carbs and braced both carbs & snugged mounts,  and it almost went away. I did most of my testing on the ground before I started flying the plane. Now in the air it seeems to be worse. The right seems to be worse than the left.

I am wondering If I need to sync my carbs better to smooth things out? I carefully set the slides to match and it seems to be running well and not shaking any more than before.

Would my rubber mounts be giving out already, maybe need to be tightened? I searched out the soft ones & I have witness marks on the bottom, and the tops are cotter key safety'd so   I know that the bolts are not loose.  Maybe the rubber is wearing.....

I did check the float levels when I get he carbs and set them to what the list recommended ( cannot remember right now)

It seemed t be the tightness of the motor mounts & carb bracing that fixed it the first time, now that I think about it. I have about 24 hours run time in these mounts to date. Not tightened the mounts since I originally snugged them up probably 10-15 hours ago.

Any other ideas?

Thanks
Bob

The Mikuni carb in normal conditions will spit some fuel under partial throttle settings... Equalizing the motor mount bolt tension helps... Have you checked your prop balance, tracking and bolt torque this season?...

[Dan_]

  I did most of my testing on the ground before I started flying the plane.  Now in the air it seeems to be worse.  The right seems to be worse than the left. 

Any other ideas?
 Thanks Bob

It could be that the slipstream is affecting the pressure inside the float bowl...  Especially since you say it is worse now that you are flying.

Some connect the bowl vents together with one tube and drill the tubing much like a static port.  Equal size and number of holes on the upwind and downwind sides of the tubing.

Could be worth a try...

The purpose of the vent is to keep the pressure equal inside and out of the float bowl, but also serves as overflow if the float sticks open.

[Bob S.]

You could try setting your float level just a tad shy of the recommended height so you will not be overfilling.. I think that is what I am doing... Are you using the gravity feed or a fuel pump??

[Bob Wood]

I know my prop torque is good check it every other flight. Tracking I need to check. Will also check mount tightness to try to even out all 4 bolts.....

The vent tube drilling is worth a try.  Both my carbs are set to factory spec for float. Is a last step I will takes carbs off and recheck. Maybe the right carb that spits more is set higher than the left??? Any idea how much lower these can be set without going lean....

Thanks for these tips, will report back.
Bob

[Bob Wood]

Bob S., to answer your question, I am set up with gravity feed, not running a pump. so a tad shy of recommended setting like 1/32" or so??

Bob

[Steve]

I know my prop torque is good check it every other flight. Tracking I need to check. Will also check mount tightness to try to even out all 4 bolts.....

The vent tube drilling is worth a try.  Both my carbs are set to factory spec for float. Is a last step I will takes carbs off and recheck. Maybe the right carb that spits more is set higher than the left??? Any idea how much lower these can be set without going lean....

Thanks for these tips, will report back.
Bob

Check the prop balance and trueness of the blade profiles too... Wood absorbs and looses moisture unevenly...

If you have multiple vent tubes there are ways to arrange them so they see the same static source... The Mikuni VM has many venting schemes... If you have a vent on each side of the slide barrel tie them together and make equal length - some use a single loop of vent tube on both ports with weep holes drilled in the bottom of the tube loop... If there is a third vent coming out of the bottom of the fuel bowl - plug it...

[Bob Wood]

Thanks Steve,
I believe I have 2 vents, will try this..

Is there a prop balancer  like used in RC a guy can build to check the prop balance?

Bob

[Steve]

Thanks Steve,
I believe I have 2 vents, will try this..

Is there a prop balancer  like used in RC a guy can build to check the prop balance?

Bob

Try this VID http://eaavideo.org/video.aspx?v=1218646859001 - nice simple method... The best final balance is a dynamic balance on the prop/engine combination... FBO's have the electronic gizmo that does the job and on the average charge $150 for the work...

[Bob S.]

Bob S., to answer your question, I am set up with gravity feed, not running a pump. so a tad shy of recommended setting like 1/32" or so??

Bob

Bob Wood, yeah....I doubt you will starve your engine with that.. good starting place...

[Bob Wood]

Bob,
I looked back in the Mikuni manual and saw the recommended setting was 15-17mm. I think I went 15.  if I go 17 or a hair more maybe I will see a difference.

The other interesting thing is this spitting carb is on #4, which always has been running richer. In fact I have a smaller main jet in that one to get the EGT up to match #2. I have a 130 jet in #4 and a 150 jet in #2.

I am not sure if this is related or not but maybe...

I am going to re torque the mounts ,and do the joined vent tube with the holes first to see any difference, then pull the carbs.

Will report back,

Thanks,
Bob

[Bob Wood]

Ok, interesting results on the venting /float level.  I checked my floats and the rich side#4 was at 15 and  the other side was at 16.  So I made both sides 18mm.  I ran it up and at 3/4 throttle it was coughing and cutting out. Recommended settings for the VM 28 was 15-17mm

So I went back to 17mm on both sides and all ran well. I taxied around a bit and saw no spitting. Before i did have some even while taxing.....

I also re-torqued the mounts and tried to balance the 4 bolts as best I could.

I did connect the vents with the single tube drilled with holes both sides, and for the float chamber vent which I had capped ( upon advise from this forum), i uncapped it and put on a single hose 3" long with the end cut at an angle so as not to venture in slipstream.

I have not flown due to winds for a couple days, but will this week and report back.

Bob

[Steve]

I did connect the vents with the single tube drilled with holes both sides, and for the float chamber vent which I had capped ( upon advise from this forum), i uncapped it and put on a single hose 3" long with the end cut at an angle so as not to venture in slipstream.

I have not flown due to winds for a couple days, but will this week and report back.

Bob

(All 3 tubes need to see the same static port)... This carb was made to hook all the vents  into a common plenum that also has the total induction air going thru it... It's a major tinkering (tuning) area for the Snowmo guys...

[hawkaleuge]

I had spitting 28's and started playing around with mounting hose links to get the least amount of vibration between 2500 and 3200 rpm.  My carbs were spitting because of the vibration and quit when I found the right hose length to damp the harmonic that the above rpm range.
Ed L.

[Bob Wood]

Interesting  thought Ed.  Right now I am using blue silicone fuel proof hose from the Marine industry. Seems very stiff, but maybe not stiff enough. A have them set up so my carbs are about 1/16" away from the 1 1/4" head intake tube. I was thinking the shorter the better.... I have not played with that.

Will take a look thanks!
bob