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hi, im finally coming to the end of a build that has been going for quite some time now. the whole bike has been stripped and rebuild from the ground up and im finally at the stage of starting it up for the first time
the whole engine and exhaust system is standard, but it has a set of 32mm mikuni's on it because we wanted to run pods (i belive the carbs were originally on an FZR 600).
any way at the moment the jetting is as follows...
MAIN - 107.5
STARTER JET (CHOKE) - 52.5
PILOT - 35
now i gave it a go yesterday, and it didnt even try to start, i pulled one of the plugs and im getting a spark it is abit weak so im goin to try another set of coils tomorrow.
when i turned it over with the plug out there was a faint smell of petrol from the cylinder but it didnt seem like theres nearly enough petrol getting into the cylinder :/ my first thought was to buy the next 2 or 3 sizes up on the pilot and starter jets.
has anyone else on here had a similar situation ? if so any info would be greatly appreciated.
FZR600 carbs on a DOHC Honda is an interesting project.
Before you spend too much time and cash on jets, double check ignition timing etc and make sure the right coils are wired up and not wired to the wrong coils. And make sure you have a decent spark and new plugs and a well charged battery.
Those carbs are similar to the ones on our FZR400 and are a PIA to keep the small jets clean. With modern gas, even a can of Seafoam is not enough if it sits for a few weeks. Usual symptoms are failure to start and if it does start it misses at higher revs. So it may be stale gas or carbs need to be cleaned (again).
Yours should start with the "choke" on even if the jets are a little too small.
A simple test is to squirt Starter fluid into the air box and with a fire extinguisher close by, hit the starter button. If it catches and won't continue to run, it's a fuel issue. It it still won't even fire once, it's likely to be an ignition issue and if it fires and can be coaxed up to run at higher throttle openings the slow jets and passageways are blocked or the jets are too small.
Do not try that trick inside or without a fire extinguisher though.
A thought just hit me. FZR's use a fuel pump and very small float needle/jet. For gravity feed you need larger float jets or a pump. On VM34 carbs sleds with pump use a #1.5 and on bikes we use #3.0-3.5 in the same carbs.
ah brilliant, im glad i got a reply before i went ahead and bought the jets ok well tomorrow ill be swapping the coils to see if that improves the weak spark and checking the ignition timing (will have to do this static obviously, 1 quick question, is there a way to tell that im at TDC on the compression stroke as the DOHC doesnt have valve inspection covers and would like to avoid removing the cam cover at all costs ?)
i dont think it can be blocked jets or bad fuel as the carbs were put through me ultrasonic, the jets are brand new and the fuel is fresh.
do you think it would be overkill installing a fuel pump? i dont think it would be too much work its just a pump and a relay isnt it ?
also ive just realised surly the pump is just used to get fuel to the carbs and once the float valves cut off the feed the pump would shut off and would make no difference compared to gravity feed ? i bought a low preasure fuel pump last night (hastily before i realized this but im sure it will be of use), so the first thing im to look at is the float/needle jets?
cheers,
josh.
Check sparks first. You can get a cheap spark plug "tester" ie variable gap to see how far a spark will jump, from your local auto parts store. Do not crank it too long with too large a gap.
Pump is there to raise fuel on an FZR to the carbs because the pressure is higher than on a gravity feed set up, the float needle valves are typically much smaller than on gravity feed.
But let's step back for a moment. FZR carbs are a steep downdraft - almost vertical and a DOHC is almost horizontal. How is that set up? Mounted horizontally FZR carbs, the jets will be too high relative the fuel level. Some of the jets may even be above the fuel.
And the floats cannot work properly if mounted horizontally.
that is very true. s*$t.
although i hav been lookin at this post...
http://older-carsandbikes.blogspot.co.uk/2011/01/gsx-r750-caruretor-swap-for-cb750c.html
and they are very similar to the carbs i am using, and definitly mounted in the same position originally :/
and i have seen several builds using carbs originally mounted almost vertically without the issuers you speak of, im not douting you its a very good point
Josh, those carbs in the blog are GSXR and were originally mounted at more or less the same angle as they are on that DOHC.
I don't want you to take my word. I want you to KNOW by checking it. Grab the carbs and drop the float bowl of one (fuel off of course) and see how much fuel was in the bowl and look at the float and try to work out the angle it's at when it cuts off flow. (turn on the fuel to test that and then off again).
And/Or fix a length of tube to the drain on one carb and see what the fuel level is with the fuel turned on. It will rise to the same level in the tube attached to the drain as it is inside the bowl.
Based with that info drop a float bowl and see where the fuel level was in relation to the jets and then compare that to the FZR600 carb diagrams in a Fizzer manual. Post some pics if you are not sure what the results mean.
we've decided to take the easyer route and get a new set of carbs. ive been trawling ebay and am finding it hard to find a set for a decent price.
i have found a set at a local bike breakers that came off of a 1991 GSXR600, they are 32mm, would they be too small ?
the standard set that were on the bike are 34mm, and from what ive read most people use either 34 or 36mm mikuni's
cheers,
josh.
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