lots of issues

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XS6fiddy

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I am new to xs 650's so be kind,
I have an 83 US Custom that does run but bogs down anything over 60, its fine up untill then. The bike has a new set of Mikuni's on and is being run through a set of hand made pipes with a set of cheap baffled short mufflers.
The petrol tank has the two vacume taps still intact with just the right hand side split with a Y peice which feeds both carbs. I was told to run it on prime to allow enough fuel through, but am wondering if this is the issue with the running and blacked out plugs. Also due to there not being an off location on the tap I am unable to turn the fuel off, which has been dripping a bit when it is stood.
The needles have been set to 3 slots from the top, but to be fair I hold my hands up, I don't know what I am doing but would like to learn how to set it up so that it runs better.
The bike is also very thirsty, I did 75 miles the other saturday and it drank a full tank of petrol. Surely that's not right.
I live in Colchester,UK is there any one close ish to me who may be able to show me how to set it up properly to run well.
I am not a mechanic, but I am willing to learn.
Any help would be appreciated.
Many thanks in advance Chris..
 
Running that rich I suspect you have float valve problems. Be very careful, while sitting, fuel can leak from carbs into cylinder and down to crankcase diluting engine oil. disaster lurks................
 
XS6fiddy said:
I am new to xs 650's so be kind,
I have an 83 US Custom that does run but bogs down anything over 60, its fine up untill then. The bike has a new set of Mikuni's on and is being run through a set of hand made pipes with a set of cheap baffled short mufflers.
The petrol tank has the two vacume taps still intact with just the right hand side split with a Y peice which feeds both carbs. I was told to run it on prime to allow enough fuel through, but am wondering if this is the issue with the running and blacked out plugs. Also due to there not being an off location on the tap I am unable to turn the fuel off, which has been dripping a bit when it is stood.
The needles have been set to 3 slots from the top, but to be fair I hold my hands up, I don't know what I am doing but would like to learn how to set it up so that it runs better.
The bike is also very thirsty, I did 75 miles the other saturday and it drank a full tank of petrol. Surely that's not right.
I live in Colchester,UK is there any one close ish to me who may be able to show me how to set it up properly to run well.
I am not a mechanic, but I am willing to learn.
Any help would be appreciated.
Many thanks in advance Chris..
Click to expand...
Hi Chris and welcome to the forum.
Lots of willing help available here; even for the 'new to mechanics' members.
Always be clear in asking questions; and support that with actual pictures of your bike.
It sounds like you have had the carbs apart as you know where the needle position is?
My bike is also a US import and showing similar running issues to your bike.
The moto here is go through all the ignition first; fuel issues second. But your observations about the fuel system are valid.
No 'Off' Position may flood the carbs (even cylinders) if left standing overnight.
Bogging down could be float position which is very sensitive on these carbs / bikes.
Carb slide diaphragm may have a tiny hole causing a similar issue; even if your carbs are new if opened up the diaphragms are delicate.
@5twins is our go to carb guy here.
Ads.

You can read my build thread about my recent investigation it might help: https://www.xs650.com/threads/welsh-flat-track-revival.60449/page-10
 
Identify what carbs. VM's or still CV's.

US Custom is an original Euro model with quite a few differences from the US models.

Post some pics of the bike.

Running vacuum petcocks on prime means they are never shut off. Relying on the float valve to stop petrol from running through the carbs when not running the engine.
 
I have US Customs 2 of them 1980 Both had BS 34 s stock
https://www.xs650shop.de/en/carb-mikuni-bs-34/

New set can be anything.
50 U.S. MPG =
4.70 Liters per 100 km

I have taken notes recently and
are running
0.472
0.465
0.427
0.451
0.428

Don think it is to lean
The dripping I would suspect the float valve.
I am not good at carbs but picture of how the spark plugs look I believe will help others have an Opinion
 
thanks gggGary, The carbs are VM34's. I need to strip them down to see whats in them.
Do you know if it is possible to use the original petrol tank taps which are for the Vacumn carbs. I have tried leaving the taps on all settings on/res/prime. but can't stop it from dripping over night. Do I need to get a different Petcock with an off position?
I have set the carbs to :air mixture 1 1/2 turns out, and idle out to 4 complete turns out, to see if that helps but havent had a chance to take it out for a test ride. Also would the bike get enough fuel to run corretly using one petcock only or do I have to plumb the taps up differently. Sorry about all of the questions but its the only way I am going to get it sorted and ripping down the road properly.. Chris.
 
Chris, a lot of you questions can be answered by looking at the Tech Section of this site: https://www.xs650.com/pages/tech/
I have no experience of VM34 carbs as I run standard BS34 on my import 1980 US model.
Heres a carb setting guide from the tech section to use as a guide.
XS650 CARB SETTINGS.png


Regarding the vacumn petcocks, they normally only work when demand is requested by a running engine creating a vacumn.
Which negates the requirement for a 'off' position. It seems gravity is drawing fuel down through your petcocks and onto the carbs.
Is the leak at the petcock or further down the fuel line?
I changed my petcocks to gravity feed when I built the bike up. They have Reserve / On / and Off positions; which work.
Fuel comes from one side of the tank down the fuel line, through an in-line filter, and to a splitter between the carb bodies.
A simple system that works.

When I first got the bike I once left the tap on Reserve overnight, resulting in flooded cylinders and a pool of oily petrol below the bike.
A full oil change followed for fear of fuel in the sump oil; a carb rebuild & refurbishment, and things got better.
Ads.
 
Yes one petcock is fine I did flow tests. Petcocks can be rebuilt. I use only Yamaha 4 hole valve, rubber disk, the aftermarket tend to be too thin and of inferior rubber. The vacuum valve, o-ring and seat can be issues also. There's a good overhaul thread in here
Note on VM 34 the idle mix screw is turn OUT to lean, in to richen mix.
It tough to sync vm's I tend to use a mirror then hand over muff outlet. There's also a thread on suggested brass in VM34s.
 

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