Well at least you have found the problem 
I’ve got big problems with fuelling for my bike carb conversion and I’d really appreciate some help.
Some background info first…
The carbs are Mikuni’s off a Yamaha FZR600. Ten quid eBay specials but they’re clean enough inside and out and as far as i can tell everything that
should be there is there. I’ve re-spaced them and attached them to a home made inlet manifold for my 1600 Ford crossflow. I’ve got the ubiquitous
Mitsubishi electric pump (reputedly also off a FZR600) feeding fuel to the carbs.
The problem…
When I turn the ignition on (not attempting to start the engine yet), the fuel pump does its thing and fills the carbs with fuel but then merrily
keeps on pumping away while they overflow. BAD THING!
Now, there are two fuel connectors on the Mikunis. From research on the web I found out the upper one was the fuel inlet and the lower one was the
overflow (counter intuitive, I know). However, I or my information source could be wrong about this. By the way, I’ve only got one fuel line from
the tank - there is no return line so I can’t pipe the overflow into that.
Help please…
Can anyone shed some light on what to do next please? Specific queries are:
(1) Why would the pump keep on running when the carbs are full? I though this type of impulse pump was supposed to stall or something when the float
bowls are full (but how would it know that? Back pressure?)
(2) Which fuel connector is which on a bank of 4 Mikunis? Is the upper the fuel inlet and the lower one the overflow or the other way around?
(3) Would I be better off using the original Ford mechanical fuel pump because it only delivers fuel when the engine’s running.
There's a float and float valve from memory - these have a habit of sticking if they arent absolutely perfectly clean which means the carbs overflow. Worth giving them a really good clean and soak in petrol
Ooh thanks. That's worth a look!
Is it the matching pump from the bike?
Have a look at where the pipes go. Id be surprised if the feed is at the top as the float valve will be in the bottom so would seem strange to feed
the top and have the fuel come down rather than in at the bottom and straight in.. Take the bowl off the bottom and you should be able to see in the
casting of the carb where the pipe goes. I reckon theyr fed from the bottom. It wont do anyharm to connect the fuel to the bottom pipe and see what
happens.
On the keihlins the top pipes are the vent pipes and i though mikunis were the same.
Also if your pump is from a Fuel injected bike then the pressure will be too high and may overcome the float valves.
Dont use the ford pump. The pump from the bike is perfect. Bike carbs are quite critical to fuel pressure so the pump from the bike is the easiest
way to regulate it.
quote:
Originally posted by Steve G
There's a float and float valve from memory - these have a habit of sticking if they arent absolutely perfectly clean which means the carbs overflow. Worth giving them a really good clean and soak in petrol
Thanks guys. I THINk the pump is from the same bike but I bought it separately from the carbs. I'm 99.9% sure it's not from a fuel
injected bike so the pressure should be OK.
Sounds like a good strip down and clean of the carbs would be a good start.
Danozeman, when you say "vent pipe" what do you mean? It vents to atmosphere? Presumably that pipe should be routed downwards (and away
from any potentially hot parts!)
quote:
Originally posted by skinned knuckles
quote:
Originally posted by Steve G
There's a float and float valve from memory - these have a habit of sticking if they arent absolutely perfectly clean which means the carbs overflow. Worth giving them a really good clean and soak in petrol
Defo what he says.
the pump is designed to stop when the pressure increases in the fuel lines. only way this isn't happening is if float valves not clean.
you say it has a home made manifold are the carbs sat at the same level that they would be on the bike so the fuel bow is level
Take a look at my Xflow/fzr600 bike carb conversion thread
http://www.locostbuilders.co.uk/viewthread.php?tid=115901
Do your carbs look like mine? If so, the fuel feed should be obvious (it's the pipe connected to the brown plastic T piece between the middle two
carbs.
There are two overflows which are located between carbs 1 & 2 and 3 & 4. These are located just below the diaphram plastic caps. I've run
pipes from these down to the lowest point of the engine.
I currently feed the carbs using the xflow's mechanical fuel pump and i've had no problems.
Cheers
Gary
[Edited on 7/2/10 by GaryM]
quote:
Originally posted by doddy
you say it has a home made manifold are the carbs sat at the same level that they would be on the bike so the fuel bow is level
yep that sounds about right
quote:
Originally posted by GaryM
Take a look at my Xflow/fzr600 bike carb conversion thread
http://www.locostbuilders.co.uk/viewthread.php?tid=115901
Do your carbs look like mine? If so, the fuel feed should be obvious (it's the pipe connected to the brown plastic T piece between the middle two carbs.
There are two overflows which are located between carbs 1 & 2 and 3 & 4. These are located just below the diaphram plastic caps. I've run pipes from these down to the lowest point of the engine.
I currently feed the carbs using the xflow's mechanical fuel pump and i've had no problems.
Cheers
Gary
[Edited on 7/2/10 by GaryM]
You only want one inlet pipe under the carbs the top one is a breather pipe if you are feeding from the top it will force fuel in and keep the pump running
The angle of the carbs won't make any difference unless they are upside down.
FZX6 carbs are pretty much horizontal pn the bike anyway.
Assuming the fuel connections are correct, the problem is most likely to be the float valve.
Stu
quote:
Originally posted by jacko
You only want one inlet pipe under the carbs the top one is a breather pipe if you are feeding from the top it will force fuel in and keep the pump running
Many thanks for all the help everyone. Out to the garage later to try to fix this, or at least to get a better understanding of what's going on.
If ALL the carbs are leaking, then its probably not the float valve. Because for them all to leak all the valves would have to stick open.
You could post a picture, as we all like those.
The mitsubishi pumps are dead simple. (take the back off and you can see for yourself). Anyway as it starts pumping higher pressures it slows down,
if you have no leaks then eventually it will stop. I ticking fuel pump on a stationary engine means a fuel leak. Interestingly they work even if you
wire them backwards (someone off here told me that) when wired the wrong way around the flow rate drops, but not by very much. Weird.
Matt
quote:
Originally posted by matt_gsxr
If ALL the carbs are leaking, then its probably not the float valve. Because for them all to leak all the valves would have to stick open.
You could post a picture, as we all like those.
The mitsubishi pumps are dead simple. (take the back off and you can see for yourself). Anyway as it starts pumping higher pressures it slows down, if you have no leaks then eventually it will stop. I ticking fuel pump on a stationary engine means a fuel leak. Interestingly they work even if you wire them backwards (someone off here told me that) when wired the wrong way around the flow rate drops, but not by very much. Weird.
Matt
clamp the fuel hose going into the carbs and see if the pump stops. if it doesnt then its the pump. if it stops then its the carbs.
simples.
i have got the same pump and it doesnt take much pressure for it to stop. good little pumps but the worst thing about them is they are noisy.
quote:
Originally posted by ashg
clamp the fuel hose going into the carbs and see if the pump stops. if it doesnt then its the pump. if it stops then its the carbs.
simples.
i have got the same pump and it doesnt take much pressure for it to stop. good little pumps but the worst thing about them is they are noisy.
What I didn't admit to earlier was that initially I had the inlet and outlet pipes of the pump the wrong way round. Nice bubbling noise from the rear of the car but no fuel in the carbs. Duh!
quote:
Originally posted by Russell
What I didn't admit to earlier was that initially I had the inlet and outlet pipes of the pump the wrong way round. Nice bubbling noise from the rear of the car but no fuel in the carbs. Duh!
Well at least you have found the problem
quote:
Originally posted by jacko
Still peeing petrol out of the vent/oveflow pipes. No idea where to go from here.
It can only be the float valves or associated parts.
Oh and about the same amount of petrol comes out of each vent pipe (one pipe for carbs 1&2; one pipe for 3&4) so it's some kind of
problem common to both pairs of carbs.
I can only think that one or more float valves isn't closing properly when under pressure from the pump????
The pump you have is of a bike that has carbs and not injection ?
How i checked my carbs was when the carbs are not fitted blow down the fuel pipe and turn the carbs upside down it should cut the flow of air if not
you nead to look at the floats / cut of valves
quote:
Originally posted by jacko
The pump you have is of a bike that has carbs and not injection ?
How i checked my carbs was when the carbs are not fitted blow down the fuel pipe and turn the carbs upside down it should cut the flow of air if not you nead to look at the floats / cut of valves
I had the same problem a couple of years ago when I put a set of Fazer 600 carbs on my car. I had spaced the carbs correctly to fit on my home made
manifold. Then to keep it all looking neat and tidy, I removed the pipes and t-pieces at the top of the carbs (1 t-piece between 1&2, and another
between 3&4) I then connected nice new clear pipes between 1&2, and 3&4 with a single T-piece between 2&3. (hope that all makes sense,
as I can't find the photo at the moment)
Anyway, it all looked brilliant - until I ran the engine and ended up with fuel spilling out of the carb inlet bell mouths!
I hadn't realised that the carbs only vent to one side, even though the tubular casting appears to be open at both sides. By putting unvented
pipes between 1&2 (also 3&4) I had connected the carb vents to its neighbour, but not allowed them to vent to the open.
In summary: left hand carb (No1) vent faces carb No2. Carb No2 vent faces carb No1. The same rules applies to the other pair of carbs ie carb 3
vents towards carb 4, and carb 4 vents towards carb 3.
simples eh!!
quote:
Originally posted by FazerBob
I had the same problem a couple of years ago when I put a set of Fazer 600 carbs on my car. I had spaced the carbs correctly to fit on my home made manifold. Then to keep it all looking neat and tidy, I removed the pipes and t-pieces at the top of the carbs (1 t-piece between 1&2, and another between 3&4) I then connected nice new clear pipes between 1&2, and 3&4 with a single T-piece between 2&3. (hope that all makes sense, as I can't find the photo at the moment)
Anyway, it all looked brilliant - until I ran the engine and ended up with fuel spilling out of the carb inlet bell mouths!
I hadn't realised that the carbs only vent to one side, even though the tubular casting appears to be open at both sides. By putting unvented pipes between 1&2 (also 3&4) I had connected the carb vents to its neighbour, but not allowed them to vent to the open.
In summary: left hand carb (No1) vent faces carb No2. Carb No2 vent faces carb No1. The same rules applies to the other pair of carbs ie carb 3 vents towards carb 4, and carb 4 vents towards carb 3.
simples eh!!