Russell
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| posted on 7/2/10 at 11:16 AM |
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Help! - Bike Carb Installation Fuel 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.
I'm a bilingual illiterate. I can't read in two languages.
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Steve G
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| posted on 7/2/10 at 11:21 AM |
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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
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Russell
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| posted on 7/2/10 at 11:25 AM |
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Ooh thanks. That's worth a look!
I'm a bilingual illiterate. I can't read in two languages.
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coozer
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| posted on 7/2/10 at 11:31 AM |
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Is it the matching pump from the bike?
1972 V8 Jago
1980 Z750
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Danozeman
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| posted on 7/2/10 at 11:33 AM |
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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.
Dan
Built the purple peril!! Let the modifications begin!!
http://www.eastangliankitcars.co.uk
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skinned knuckles
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| posted on 7/2/10 at 11:40 AM |
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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.
A man isn't complete until he's married, then he's finished
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Russell
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| posted on 7/2/10 at 11:46 AM |
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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!)
I'm a bilingual illiterate. I can't read in two languages.
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Russell
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| posted on 7/2/10 at 11:48 AM |
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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.
So I could plug the outlet from the pump and see what it does? It should stall or whatever.
I'm a bilingual illiterate. I can't read in two languages.
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doddy
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| posted on 7/2/10 at 11:51 AM |
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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
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GaryM
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| posted on 7/2/10 at 11:54 AM |
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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]
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Russell
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| posted on 7/2/10 at 11:55 AM |
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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
Not sure what angle they were on when on the bike but I made the manifold pipes so that the float bowl gaskets are horizontal (the carbs themselves
are angled upwards maybe 15 degrees). Hope that makes sense.
I'm a bilingual illiterate. I can't read in two languages.
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doddy
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| posted on 7/2/10 at 11:56 AM |
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yep that sounds about right
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Russell
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| posted on 7/2/10 at 12:00 PM |
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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]
Very similar carbs but not 100% identical. However, from reading your post and looking at the pics it looks like I have got the fuel connections the
right way round. Good post by the way, wish I'd seen it sooner (how did I miss searching for that?!!!!).
Sticky float valve is looking like the favourite root cause of my probs.
I'm a bilingual illiterate. I can't read in two languages.
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jacko
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| posted on 7/2/10 at 12:15 PM |
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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
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whitestu
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| posted on 7/2/10 at 12:16 PM |
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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
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Steve G
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| posted on 7/2/10 at 12:22 PM |
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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
Yes - missed this point in the original post. the fuel goes in at the bottom and through the float valves at the bottom of the carbs. The air is
displaced via the breather at the top which sounds like where you've got your fuel going in at the moment. Chances are the fuel has cleaned up
the float valves by now so give it a go with the fuel connection switched.
If the valves are sticky then you'll simply get fuel overflowing out of the top
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Russell
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| posted on 7/2/10 at 12:50 PM |
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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.
I'm a bilingual illiterate. I can't read in two languages.
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matt_gsxr
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| posted on 7/2/10 at 01:33 PM |
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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
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Russell
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| posted on 7/2/10 at 01:40 PM |
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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
It's difficult to tell if all the carbs are leaking because they are joined in pairs then tee-d (teed?) together to a single outlet. Good
thinking though - I could take the tee off and see if the overflow/leak is from one pair or both. If both then it's less likely to be a sticky
valve.
Piccy of the pump I'm using below (eBay image of the one I bought). I haven't got pics of the installation yet.
Image deleted by owner
I'm a bilingual illiterate. I can't read in two languages.
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ashg
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| posted on 7/2/10 at 01:45 PM |
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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.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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skinned knuckles
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| posted on 7/2/10 at 03:30 PM |
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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.
clever, would never have thought of that. this is why i like forums
A man isn't complete until he's married, then he's finished
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Russell
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| posted on 7/2/10 at 03:36 PM |
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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!
I'm a bilingual illiterate. I can't read in two languages.
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skinned knuckles
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| posted on 7/2/10 at 03:54 PM |
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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!
you are not the first and won't be the last
A man isn't complete until he's married, then he's finished
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jacko
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| posted on 7/2/10 at 04:35 PM |
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Well at least you have found the problem
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Russell
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| posted on 7/2/10 at 05:33 PM |
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quote:
Originally posted by jacko
Well at least you have found the problem
I wish!!! I've had the carbs off the car and the float bowls off the carbs. I now understand how the valves work and I've checked each
one statically (i.e. not under pressure) to confirm they all work OK. All cleaned up with carb cleaner just in case even though they looked fine.
Also, I clamped the outlet hose from the pump and, sure enough, the pump stops.
Put it all back together and hey presto - exactly the same   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.
I'm a bilingual illiterate. I can't read in two languages.
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