Is my rod too long? Slabby Accelerator pump question

[Robinjo]

Allright, after  formulating my  "maybe problem" into a dirty question I hope some flatslide experts are  persuaded to check out  my topic

With the throttle closed the rod that is being actuated from the top of the carbs and pushes on the plunger in the float bowl is already sticking out 6 mm, total travel of the plunger is a little over 7 mm.

I ultrasonically cleaned my VM29SS carbs off my Slabby track bike because it started to developed a stumble coming out of a corner in the middle of the rpm range which got worse and worse. Everything under 4k rpm was allways crappy btw but on a track bike not really important but the big stumble was...

[Robinjo]

I had the carbs apart completely so the chance I f*cked something up myself is allways an option. 

[nlovien]

mark the throttle in 1/4 steps and look for the throttle position when the accel pump squirts - if it doesn't = something wrong

check you have the piston that the plunger rod push's ( the bit inside the float bowl socket) the right way around

that looks like Spa ?

[Oilyspanner]

Sounds like you have a rich stumble, with it getting worse with increasing temperature.

[Jaydee]

I have worked about 6 sets of vm29ss carbs this year alone! Yes perfectly normal to have 6-7mm of accelerator rod sticking out. As said, check that the 2 pistons in the floats move freely. Also push back the 4 brass accelerator nozzles and check their (2x1mm) o-rings. Every set I worked on had perished o-rings which would stop the nozzles spraying to their full capacity since the accelerator system is pressurised.

[Robinjo]

3 hours ago, Jaydee said:

I have worked about 6 sets of vm29ss carbs this year alone! Yes perfectly normal to have 6-7mm of accelerator rod sticking out. As said, check that the 2 pistons in the floats move freely. Also push back the 4 brass accelerator nozzles and check their (2x1mm) o-rings. Every set I worked on had perished o-rings which would stop the nozzles spraying to their full capacity since the accelerator system is pressurised.

Oood to know the stickout is correct, now the problem is that the plunger in the floatbowl has a travel of about 7 or 8 mm from the top of the floatbowl so it's already near the end of it's travel. Maybe the plunger is not the correct one, will get a pic up tonight! Nozzle looked replaces not to long ago, other rubbers too

[Robinjo]

7 hours ago, nlovien said:

mark the throttle in 1/4 steps and look for the throttle position when the accel pump squirts - if it doesn't = something wrong

check you have the piston that the plunger rod push's ( the bit inside the float bowl socket) the right way around

that looks like Spa ?

See my previous reply about the plunger, and yes the pic was at Spa, love that place! 

[Jaydee]

13 minutes ago, Robinjo said:

the plunger in the floatbowl has a travel of about 7 or 8 mm from the top of the floatbowl so it's already near the end of it's travel

Don't assume that the plunger pins stay in this position. As soon as you lift the throttle , the pins move, along with the pistons in the float. The pistons should lift with the aid of their springs (and maybe a little fuel pressure from accelerator circuit, not 100% sure on that). Flatsides don't like to be wacked open suddenly at they will take a huge gulp of air. The whole idea of the accelerator circuit is to spray fuel in to this extra air when the throttle is snapped open. You can see the 2 plastic cams that the plunger pin rest against between carbs. They slap the pluggers when hard accelerating. When overhauling the carbs, make sure the plungers are cleaned properly and new o-rings on accelerator nozzles. Use nozzle slots to reposition facing in to engine.

[Robinjo]

3 hours ago, Jaydee said:

Don't assume that the plunger pins stay in this position. As soon as you lift the throttle , the pins move, along with the pistons in the float. The pistons should lift with the aid of their springs (and maybe a little fuel pressure from accelerator circuit, not 100% sure on that). Flatsides don't like to be wacked open suddenly at they will take a huge gulp of air. The whole idea of the accelerator circuit is to spray fuel in to this extra air when the throttle is snapped open. You can see the 2 plastic cams that the plunger pin rest against between carbs. They slap the pluggers when hard accelerating. When overhauling the carbs, make sure the plungers are cleaned properly and new o-rings on accelerator nozzles. Use nozzle slots to reposition facing in to engine.

I hope these pics clarify the situation a bit.

The plunger hole is 23 mm deep, the plunger is 14 mm long, when it bottoms out there is 7.5 mm left, the rod (at zero throttle) sticks out 6 mm, after 1.5 mm there is no more room tot travel for the plunger. 

Does it only need 1.5 mm travel? 

Is my plunger too long?

The carbs seem to be rebuild in the last couple of years so  wrong parts could be possible.

[Robinjo]

Quote

Use nozzle slots to reposition facing in to engine.

Thanks for the tip, they are now adjusted correctly!

[nlovien]

I had a similar issue when rebuilding a set of mikuni 38's - flipped the plunger , the spring now sits on the shouldered top cap - the rod sits inside plunger = the plunger now swallow's up the 6/7 mm of rod as its hollow from the other-side - no idea if this works for you but it looks similar 

[Robinjo]

The top and the bottom of mine look like this, having a counterbore in the plunger would resolve the lack of space...

Are the plungers supposed to have a counterbore? That would explain a lot. 

[Robinjo]

On 19-9-2017 at 8:41 AM, nlovien said:

mark the throttle in 1/4 steps and look for the throttle position when the accel pump squirts - if it doesn't = something wrong

check you have the piston that the plunger rod push's ( the bit inside the float bowl socket) the right way around

that looks like Spa ?

Do yours look like this Nlovien?

[Macduff]

4 hours ago, Robinjo said:

The top and the bottom of mine look like this, having a counterbore in the plunger would resolve the lack of space...

Are the plungers supposed to have a counterbore? That would explain a lot. 

My slabbby carb plungers look exactly like your pic .

[Cheeky4648]

My 29s have a raised area on one side for the spring location but have a 5 to 6 mm counterbore for the rod to sit in. On another note @Robinjo on the right hand carb as Sat on bike there are some numbers and letters (model code) I think. What does yours say? 

[nlovien]

I have a counter bore on the top side for the spigot thing to sit in - as above, it swallows the extra length - as I 1st rebuilt the carbs with the plunger the wrong way around I got the same symptoms as you - hence why the focus on this -  is your 2 plunger picture one of the top and one of the bottom ? or two diffrent ones 

[Robinjo]

On 21-9-2017 at 3:04 AM, Cheeky4648 said:

My 29s have a raised area on one side for the spring location but have a 5 to 6 mm counterbore for the rod to sit in. On another note @Robinjo on the right hand carb as Sat on bike there are some numbers and letters (model code) I think. What does yours say? 

My plungers are solid.

And my carbs have the codes:

27A

10

Like that, so one above the other, on the right side. 

[Robinjo]

On 21-9-2017 at 9:02 AM, nlovien said:

I have a counter bore on the top side for the spigot thing to sit in - as above, it swallows the extra length - as I 1st rebuilt the carbs with the plunger the wrong way around I got the same symptoms as you - hence why the focus on this -  is your 2 plunger picture one of the top and one of the bottom ? or two diffrent ones 

The pic where I'm holding the two plungers is showing one top and one bottom side so there is no counter bore.

Since then I've fired up the lathe and made a 6 mm counterbore in the plungers, when installed there is a little bit of clearence between the rod and the actuator.

And when you put fuel in the carbs and twist the throttle they squirt! 

[Macduff]

My VM29s are from a Jap import , I wonder if the solid plungers are a feature of Jap market bikes?

 

[Robinjo]

Bike is running much better now! 

While I had the fuel tank off I adjusted the valves (they were all exactly even, exhausts precisely in spec but intakes all exactly the same but 0.05 tighter than spec, adjusted them to 0.10-015 too. Balanced the carbs, checking with the tops on too, and adjusted the idle mixture per carb.

The bike now has a steady idle running on all four all the time so that's a change  still not quite right under 4000 rpm where it's most likely too lean? but above that it's soooo nice! Just wants to rev straight to redline and has great throttle response.

 

Thanks to all who helped!!! 

[yyt]

I was planning to continue this subject on my VM29's post, but then in my searches I found this post..

On 9/24/2017 at 7:00 PM, Robinjo said:

Since then I've fired up the lathe and made a 6 mm counterbore in the plungers, when installed there is a little bit of clearence between the rod and the actuator.

If I understand correctly, you added 6mm to the 1.5mm you got at first. so now the plunger can have a 7.5mm travel down, and you're happy with the improvement.. Did i get it right?

I also measured the plunger potential movement and was puzzled why out of 8mm of plunger depth only about 3mm are actually used opening to full throttle.

My bike is not ready yet, so I cannot ride,make a change and feel the difference.

Just finished assembling the carbs.

I'm planning to simply change the rods to shorter ones (instead of drilling the plunger).

Hope to share my experience ASP.

Appreciate if you share your impression .

[Robinjo]

On 6/4/2018 at 5:56 PM, yyt said:

 

Yes I bored out the plungers to get full travel and it worked for me. 

[yyt]

I have made a set-up to test the output of the accelerator nozzles.

With the original size of push-rod / plunger (2.5-3mm movement of plunger) I measured about 0.4cc for 10  throttle openings,  which equals to 0.04cc for each squirt.

I just shortened the push-rods by 3mm (5.5-6mm movement of plunger-out of 8mm) from 74mm to 71mm.

I measured about 1.1cc for 10  throttle openings'  which equals to 1.1cc for each squirt.

Almost 3 times more fuel. I think its enough for me at this stage.

I'll inform how it feels after rides..