Low vacuum across all carbs

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lonestarmedic

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Ok, got a good vacuum source. Measured it at 22.5 mmhg on my MightyVac gauge. Pulled vacuum and the rods do come way up the scale. But uneven as heck. I used the Food Saver accessory port! Worked perfect.uploadfromtaptalk1438908720946.jpguploadfromtaptalk1438908743002.jpguploadfromtaptalk1438908773481.jpg
 

Tonywylie

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1986 Venture all stock. Has 80K miles. Cylinders showing 170 or better on compression. Using a Morgan Carbtune 2. All 4 carbs at the bottom of the glass. Just barely visible. Cleaned the carbtune and a few drops of light oil applied. Bike used to pull rods about halfway up. Running well. I inverted carbtune and checked. My bench sync is pretty good that way. Sprayed starter fluid around boots and intake with no change. Cannot figure why showing low. Starting to suspect gauge. Going to outfit a vacuum gauge with a restrictor valve and check. If I have compression and closed throttle plates I would think vacuum should be better. Any ideas or stop worrying?
I have a vmax 91 full power. And its doing the same . My carbtune is not reading untill i get to 2000 reves. Did you find out what the problem was ? Tony
 

MaxMidnight

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Is this a new problem i.e. have you used this device for balancing before?

If it is new to you does it need restrictors?

If it was due to an air leak then I wouldn't expect it to affect all cylinders only the one (s) with a leak.
Assuming the problem is across all four pots then the first thing I would check is that the V Boost butterflies are fully closed.
 

Tonywylie

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Is this a new problem i.e. have you used this device for balancing before?

If it is new to you does it need restrictors?

If it was due to an air leak then I wouldn't expect it to affect all cylinders only the one (s) with a leak.
Assuming the problem is across all four pots then the first thing I would check is that the V Boost butterflies are fully closed.
I have had and used the vacuum gauges for 20 years but not in the last 10 . If i take the restrictors out the bars jump up and down. But when in theres nothing till 2k revs apx. You said about v boost butterflies . Is there a check to do to see if there closing without taking the carbs out ?
 

Tonywylie

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Section 5-13 to 15 of the Service manual describes the adjustment procedure for V Boost so I would start there.
I have just had a look at the v boost and it looks as if its working right when the ignition is turned on and off. Think i will get some new gauges and try them .
 

MaxMidnight

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I have just had a look at the v boost and it looks as if its working right when the ignition is turned on and off. Think i will get some new gauges and try them .
Just because it is cycling OK it doesn't mean that the butterflies are in the correct position.
I'm not aware that they go out of adjustment but if the bike is new to you it is possible that they have been fiddled with.

Unless you have a burning desire for some new gauges then I would test them on another bike first.
If you get a similar problem then it is the gauges but if they perform correctly you will have saved some money. The issue therefore would be on the bike.
If you put your location on your profile there may be someone who could help.
 

Tonywylie

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Just because it is cycling OK it doesn't mean that the butterflies are in the correct position.
I'm not aware that they go out of adjustment but if the bike is new to you it is possible that they have been fiddled with.

Unless you have a burning desire for some new gauges then I would test them on another bike first.
If you get a similar problem then it is the gauges but if they perform correctly you will have saved some money. The issue therefore would be on the bike.
If you put your location on your profile there may be someone who could help.
My location is whitehaven cumbria england. And i have had the bike for 20 years . I have ordered a cheap set of bike tek gauges .so i will find out if its the gauges or some thing else. It could just be the dampers in the pipes closed up with age and not giving the correct reading. But i will find out when i get my new gauges. Will let you know the outcome. Tony
 

MaxMidnight

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Another Cumbrian Max...that makes at least two then. 👍

Given that you've had the bike for so long then I'd probably suspect the gauges as well.

Let us know how you get on.
 

Tonywylie

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Another Cumbrian Max...that makes at least two then. 👍

Given that you've had the bike for so long then I'd probably suspect the gauges as well.

Let us know how you get on.
Well put the new vac gauges on and got a reading they are not far out . So sett them and it still runns like shit. Think i will play with the mixture screws tomorrow if that dont work its carbs off again and restripped.
 

Screwloose

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Well put the new vac gauges on and got a reading they are not far out . So sett them and it still runns like shit. Think i will play with the mixture screws tomorrow if that dont work its carbs off again and restripped.
As an aside mate:
I have the same gauges and you need the restrictors in, or you will definitely have very bouncy readings. The gauges will show correctly at low revs, so at tick-over the gauges have no problems showing very stable readings.
When I was working on mine one of the main problems was split rubbers where I was drawing air after the carbs, and got worse when the engine got to temp as the rubbers soften.
I did not find much advantage fiddling with the mixture screws, you turn them out until the engine revs are up and smooth, and that's about as best you get. Mine were 2 1/2 to 3 turns out for correct settings.
When I have the carbs out I always set the carb flaps individually with a drill bit shank, it will not be perfect on the gauges at start-up as the butterflies and surroundings are not precision made, but gives you a reasonably smooth start up point that will be not far out on the gauges, and allow immediate tick-over.
 

Tonywylie

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As an aside mate:
I have the same gauges and you need the restrictors in, or you will definitely have very bouncy readings. The gauges will show correctly at low revs, so at tick-over the gauges have no problems showing very stable readings.
When I was working on mine one of the main problems was split rubbers where I was drawing air after the carbs, and got worse when the engine got to temp as the rubbers soften.
I did not find much advantage fiddling with the mixture screws, you turn them out until the engine revs are up and smooth, and that's about as best you get. Mine were 2 1/2 to 3 turns out for correct settings.
When I have the carbs out I always set the carb flaps individually with a drill bit shank, it will not be perfect on the gauges at start-up as the butterflies and surroundings are not precision made, but gives you a reasonably smooth start up point that will be not far out on the gauges, and allow immediate tick-over.
[/QUOT
 

Tonywylie

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The bike tek gauges have a little damper valve on them were as carbtune 2 gauges have a insert. Allso carbtune need higher vacuum to work. Not to good on a vmax. Just flushed the tank out and got rid of the nasty after market petrol filter. Waiting for the new filter to arrive then its time to play with the carbs again .
 

Screwloose

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The bike tek gauges have a little damper valve on them were as carbtune 2 gauges have a insert. Allso carbtune need higher vacuum to work. Not to good on a vmax. Just flushed the tank out and got rid of the nasty after market petrol filter. Waiting for the new filter to arrive then its time to play with the carbs again .
Hi.
Aha; it's Carbtune Pro I have, and they work great at VMax tick-over revs.
Anyway happy carb playing, hope it goes well.
 

desert_max

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Hi.
Aha; it's Carbtune Pro I have, and they work great at VMax tick-over revs.
Anyway happy carb playing, hope it goes well.
Yeah, I was getting worried. I've had the Carbtune Pro for a good many years. It has always given me accurate enough information to sync anything I've hooked it to - including a fair number of Gen1 Vmaxes.

Good luck getting this one sorted out, Tex.
 

Screwloose

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The bike tek gauges have a little damper valve on them were as carbtune 2 gauges have a insert. Allso carbtune need higher vacuum to work. Not to good on a vmax. Just flushed the tank out and got rid of the nasty after market petrol filter. Waiting for the new filter to arrive then its time to play with the carbs again .
Hi Mr T.
if you strip the carburettors then there is an opportunity to follow on with D_M as it appears he has done a few of them. I’ve stripped mine a few times and followed the different posts on here that are 95% great info, but i’m standing on the shoulders of giants, even as I make these comments.
There are a number of ups and downs but one of my regrets is not getting the carbs ultra sonic cleaned. I used about £30 of carb cleaner and feel I replaced the needle seats without need to.
Also I should have done a running commentary on this site so people could comment as I feel that by progressing on my own this sent me down the wrong route now and then. I.E. aftermarket needles and seats did not work out as good as I wanted and I will be replacing them, maybe not this year as I have Big Tiigger to deal with as a project during the summer.
 

Tonywylie

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1986 Venture all stock. Has 80K miles. Cylinders showing 170 or better on compression. Using a Morgan Carbtune 2. All 4 carbs at the bottom of the glass. Just barely visible. Cleaned the carbtune and a few drops of light oil applied. Bike used to pull rods about halfway up. Running well. I inverted carbtune and checked. My bench sync is pretty good that way. Sprayed starter fluid around boots and intake with no change. Cannot figure why showing low. Starting to suspect gauge. Going to outfit a vacuum gauge with a restrictor valve and check. If I have compression and closed throttle plates I would think vacuum should be better. Any ideas or stop worrying?
I put my carbtune gauges away and got a cheap sett of biketek ones and they work fine vacuum all set at 200mm.
 

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