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I have recently experienced a delay in throttle response, which caused me to check everything associated with the carburation. One of these items was the fuel pump relay. I read all the comments on this item as recorded in the forum, but I still cannot understand how it works. The concensus seems to be that it activates the fuel pump when the starter button is depressed, but after that the float bowls fill under gravity. Much to my surprise I checked this and found 12V only appeared on the black/blue lead to the pump while I was depressing the starter button. I'm a mechanical engineer, and understand how the starter relay works - power is supplied to a coil to generate a magnetic field and move a solenoid, which in turn closes a heavy duty switch to supply the serious power to the starter motor. However, the fuel pump relay is solid state, and I assume contains transistors or diodes which activate other circuits when power is applied. But I don't understand why there is no direct connection to earth to complete the electrical circuit. The only earth I can find is the black lead which connects to the coil and IC igniter. This earths for a period, and is then interrupted at an appropriate moment by the igniter to create an overvoltage in the secondary side of the coil and hence a spark at the plug. So the fuel pump would seem to be intermittently earthed, yet there is no sign of the fuel pump operating. But when the bike is running, I note that there is 4V on the black/blue lead.
A car uses a fuel pump to raise fuel from a low level tank to a higher level carb. But if a motorcycle tank is above the carbs, why bother with a pump at all - especially if it doesn't operate once the engine is running. My next problem is how fuel moves through the pump under gravity feed, because both inlet and outlet ports have check valves on them to ensure that the pump pulls fuel through one port and then pushes out through another. I applied air pressure to the feed port,but there was no air flow because the check valve on the outlet was firmly closed. Applying a vacuum to the outlet port caused both valves to open, but it needed significant drop in pressure - do the carbs "suck" fuel in?
Another thought - why do race bikes carry a dead lump of metal around the track if the bike is only working under gravity feed? I have seen race wiring diagrams which include the pump and relay, although some racers seem to prefer the pump wired directly from the battery to keep it going continously.
I have run out of fuel while the tap was in the ON position, and when I switched to RES it took a good 25 seconds to fire up. This confirms the comment in the manufacturers handbook: "when the carburettor is completely empty it takes about 30seconds for the engine to start". So - why did the carbs not fill up under gravity?
Sorry this is lengthy, and advanced thanks to anyone taking the time to reply with hopefully constructive comments (yeah I know I'm a bozzo)

Some downdraft engines are greedy buggers on fuel and the demand at high rev's can't be met with gravity feed.

Your right about the carbs drawing on the pump once the bike is running as the head of fuel in the tank drops it effects this draw and the pump takes over again ( I would assume this can also happen under heavy acceleration too ) so the pump will run constant when in reserve, this leads onto why racers stil use them -they run their tanks almost empty by the end of a race they are also thrashing it from the off so having a constant supply is paramount. An upgrade for them is fitting a ZZR1100 fuel pump especially if running FCRs as these are greedier than CVKs.

You can just hear the pump when you 1st start - it switches off pretty quick, but while running if you put the tap back to off you will really hear it.

So although the pump/carbs switch to gravity feed the engine does have to be running initially. You can bypass the fuel pump and plumb directly from the tank but the internal shape of the tank would mean your most likely going to have to keep topping it up to full every gallon or so used.

Hope that helps, MM!

Many thanks for the comments Mori Man. I knew that the fuel pump was activated when the starter button was depressed - 12V on the black/blue wire to the pump confirmed that. I was still uncertain about the pump operating when the engine was running, since there were only 4V on the wire. However, I started the bike and although I couldn't hear the pump due to engine noise, I used a long screwdriver with the blade pressed against the body of the pump, and my ear against the handle. Every few seconds the pump could be heard clicking briefly. I then turned the petrol tap to the OFF position, and listened again. After about 20 seconds the pump started clicking incessantly, clearly indicating that it was starved of fuel. Turning the tap ON again produced the intermittent clicking again. I can only assume that there is some sort of "economy" ciruit in the fuel pump relay, which supplies 4V, to ensure that the pump does not run continuously, but will only kick in when there is a lack of fuel.
Since I started this enquiry, I have realised that I changed the old "foam" air filter for a K&N. Reading other comments about this change would suggest that the mixture is now too weak, hence the apparent problem with carburation. I have replaced the old filter to see if the performance returns. If it has then the needles will have to be adjusted - at present the clips are all in the top groove, which I deduce is the weakest position.

A very comprehensive post 'Ewetea' and your assumptions i have found to be correct. The zxr has a 'pumped system' and as you rightly pointed out how can a pump work then 'switch to gravity feed' ????fuel will not gravity feed through a pump with the check valves/one way valves? Some people who think they know havent a clue it seems. A zxr400 fuel pump will pump 1.7 litres/minute so, will move 17 litres of fuel in 10mins which is about 4 times more than you could use, even at the blistering pace of the i.o.m. So as for 'upgrading' to a zzr1100 pump...never heard of anything so ridiculas.
And while on the subject of 'gravity feed' there are two types of float needle valves...one for pumped systems and one for gravity fed systems...using a pump with gravity fed needle valves causes the float bowls to persistantly flood.
Im glad youve got to the root of your problem and havent been misled too far, by the way R1 needles work very well in the zxr400 and require very little modding to fit...maybe a cheaper option than the dynojet needles.

I'll find the part where it talks about the pump being used in reserve - might not have been the workshop manual

BTW, Rodger - I don't make this stuff up , I am able to READ ! Plus mmmmmmm experience. For someone who allegedly has a wealth of knowledge on these bikes you contribute nothing to this forum unless of course it involves receiving money

M Man
Where in that manual does it say its gravity fed?????????
Pretty sure you make this stuff up :wanker
or maybe you just reply when under the influence of something else?
Better get myself on ebay for a ZZR1100 fuel pump.

I've also been a bit misled by previous posts on this subject into believing that the carbs are gravity fed unless demand is high / tank levels are low, then the pump kicks in. So are we now saying for deffo that this is a fully pumped system and the fuel pump is required AT ALL TIMES to keep the float bowls fed?

Yes 'Tirpitz' thats correct...even the draft from the manual confirms that. Simple enough to put it to the test. When your tank is reasonably full, disconnect your relay or pump and see how long it runs for. It will run untill the carbs empty i can assure you of that.

Hands up , I got this completely and utterly wrong - apologies to everyone

MM!

: to lead in a wrong direction or into a mistaken action or belief often by deliberate deceit

And I've just sold all my fuel pumps after seeing this post and I've ended up replacing the fuel system with a somewhat cumbersome 6ft header tank.
See link for example:

<a href="http://www.tripadvisor.co.uk/LocationPh ... html"><img alt="Photos of Chanteclair, Cannes" src="http://media-cdn.tripadvisor.com/media/ ... ><br/>This photo of <a href="http://www.tripadvisor.co.uk/Hotel_Revi ... teclair</a> is courtesy of TripAdvisor


Am gonna look a right bugger now arnt I!!

Thanks Roger, nice to know in case in get a problem in future. Don't want to be chasing my tail.

No probs MM, everyone can make a mistake and it was made with the best intentions of helping. Only those people who never do anything never make mistakes.

I had forgotten how entertaining and informative this forum can be - wonderful!

Please restore Robbies access - I cant wait for the next instalment

Seems I was banned for upsetting somebody so for that please accept my apology.
Or the easy fix is dont talk utter pish then I wont have to upset you.