Hello, poor running and setting float level

Hi,

Firstly, this is my first post on this site, so hello! I bought an 83 VF750F a couple of weeks ago (European model - I'm in the UK). It's only got 22K miles on it, and is in very good condition, however it was lacking power and generally running poorly when I got it. Thanks to this site I got a copy of the workshop manual and lots of useful information, but still haven't got to grips with the problems.

When I got the bike it smelt rich, would sometimes tick over at 3000rpm or 1000rpm, but not inbetween. It suffered from a lack of power, particularly at the top end. The plugs were very sooty.

So far I found the air pipe that links the 2 banks of carbs was missing, and made sure the choke was operating correctly, but that had no real effect. I've had the carbs off, cleaned the jets (no drilling!) and attempted to set the float levels to 7mm as per the manual. When I did this though, fuel started running out of the venturi with the carbs set up on the bench (with the top of the airbox horizontal). I increased the level to 8mm which solved that, but on the bike the clear tube suggests that the level is still too high (just a gut feeling though). The pilot screw plugs had been removed, so I reset the screws to 2.5 turns, though foolishly I didn't count the original settings.

Now the bike starts fine from cold (with lots of choke), but dies when you touch the throttle unless you're verrrry gentle. Once warmed up it refuses to start at all! At first when I checked the spark it was occasionally missing, so I replaced the leads with some old car ones (resistance is in spec) and checked the coils which were OK. I got an old oscilloscope on both spark units and they appear to be working correctly. The spark appears OK now (as in it's there at all) but nothing like as powerful as a car one would.

So, firstly, has anyone got any idea what's wrong? I'm far more au fait with fuel injection on cars and carbs are still somewhat mysterious.

Secondly, can someone confirm where to take the float measurement? I've had the carbs at an angle so the float tang just rests on the valve. Then I've been measuring the distance perpendicular to the gasket face, from the gasket face to the point furthest away on the bottom of the float. If that makes any sense.

The fuel pump is functioning correctly, and I've jumped it by shorting the black and white wires on the fuel pump relay connector. The float valves look in good condition and the washers on the valve seats are present and (presumably) correct. The filters are all clean. The bike hasn't run much in the last few years (100 miles in the last year) so the fuel may be quite old. I'd like to get the bike running well enough so I can ride it to a petrol station, brim it and then give it a thrashing which I'm sure it needs!

Cheers,

Rich

(Sorry went on a bit longer than I intended there...)

 

have you done a compression test? should be between 157-213 psi
and do a check on amplifier/rectifier they act up sometimes the screws on side of carburetor should be 3 turns out
from lightly seated position they are for idle mixture I believe.
sorry i can't help on floats.
if bike starts fine cold but crappy and rich hot you getting to much gas hope i helped a little.

 

Start with fresh fuel first.

Sooty plugs just won't do. Is it rich at idle or high speed or both??

If you have standard jets and still darken the plugs, you may need to lower fuel level in bowls.

Using a heat gun to cheak header pipe temps is useful for tuning.

Did you blow out carb air and fuel passages with compressed air ??

Does it run better whrn cold or hot ?

Doing a quick run with the air filter removed might be a good test to try.

Since mixture screw settings are determined to comply with US EPA emissions regs............. in practice the engine starts, idles and runs better with richer settings up to max 3 1/2 turns. Use the idle drop proceedure from the service manual.

And keep checking the PLUGS until they're looking all proppa like.

The desert groundsquirrel is a greyish-to-sandy brown color like the sparkers should be when it's running right.

(disclaimer: i don't know EU fuels, only US.)

 

Hi and thanks for the responses...

To go through the points raised above...

Don't have the equipment to do a compression test, but will look into it if needs be. The alternator connector had melted but I fixed that some time ago. The regulator/rectifier is fine.

I took the carbs off again and this time gave them a proper clean. Carb cleaner, compressed air etc. Verified that the chokes and sliders etc are all in good condition and move freely. I couldn't verify one passage in each carb - the very narrow one that starts just under the diaphragm and runs down to (presumably) the starter jet.

Anyway, I reassembled it and the bike runs a lot better now. Well enough to go for a quick blast up the motorway and fill up with fresh fuel. However it's still running rich and chewed through 2 3rds of a tank in 55 miles (albeit 55 relatively fast miles :wink. The bike pulls well at low revs, feels a bit stuttery from about 4k rpm to 7.5k, after that it flies! Now when I pull the plugs the front two are really sooty still, but the rear two while still too black, aren't too bad. Could this be to do with float level given that the two banks are at different angles? The floats are set to 9mm which is already off spec.

So what do I do next? My inclination is to lower the floats another mm, as the bike seems to be running rich right through the rev range. Would I be right in assuming that the pilot screws are the last thing to adjust? Failing all of that I guess I'll just have to assume that someone has messed with the jets.

Finally, are the air link pipes supposed to be joined together or not? I can't find an answer anywhere! It looks like they should be, but I also read that the top of the float chamber should be at atmospheric pressure so they should be left open....

Cheers,

Rich