84 VF750F restoration - carb questions

ARRRRRRRG. I took everything apart again. Soaked the carbs in carb cleaner, blew out everything, verified the jets don't appear to have been messed with, the brass seats for the slide needles are in place, the aluminum washer is under each new float valve seat, the chokes are free, evrything is spic and span, the float level is 7mm, new plugs, etc. Air filter clean and airbox tight.

She fired right up with a bit of choke, almost idled, I was able to rev it up a few times. Still lots of black smoke out the pipes. After about 1 minute it started to falter and died. Started it up again, and needed full throttle just to keep it barely running - wouldn't rev up at all. Pulled the plugs - all 4 were black and sooty. Put in a fresh set, same thing after 30 seconds or so. It ate about a quart of fuel in this short time. What am I missing???? I have the shop manual and am no stranger to carbs. This has me snookered.

 

AND you're sure the float bowl vent tubes are open and clear ??

TRY resetting floats to 8mm just for a test.

 

You said you have a spare set? Try those or send the whole shit mess this way. I have a gen 1 I am about to do carbs on and can test on a known running bike.You pay postage.

 

Yup, the vent tubes are clear. I will reset the floats to 8mm for the heck of it. I am getting to be an expert in taking these carbs on and off. I am a bit confused that the front and read carb float levels are the same, given the fact that they are on very differant angles? I am also considering trying the jets out of my 83 carbs even though they are differant.

I may even rebuild the spare set and try them - a lot of work.

Thanks for the offer Toe Cutter - my ego is bruised so I am gonna keep trying till I get this figured out.

Steve

 

What do your pilot screws look like. Those are the ones in the side of the carbs that you adjust for idle mixture. I have seen some that the pointed ends are corroded off. So your adjustment would be really out of whack. Also they have a very small rubber washer backed by a metal washer to seal them. If someone changed those and put on a thicker size rubber or metal. It would be like backing the pilots out several turns more than what's needed. This would be a very rich idle condition then. Or possibly corrosion not letting them seat all the way? You got a good problem there I like your tenacity. Never say die.

 

The mixture screws look good. They have the pointy end and no corrosion. The o-rings are the old ones - I couldn't find new ones and they were in OK shape. I was careful to not overtighten them when I screwed them in and then backed them out to 2 1/2 turns. The only thing that cape to me in my sleep last night is the question of the new float valves and the little spring thing that holds them on the float. I wonder if it's possible that the little spring clip is backwards and interfereing with the valves working properly. I will set up a bench test with the carbs held at the same angle as they are mounted on the bike to see.

 

I don't think mixtue screw adjustment could make it run that rich no matter how they're set.

bench test...really good idea. If there's a problem with the float valves it'll save you removal and installation. If you watch you can see exactly which carbs are 'puking' fuel.

It's kinda rare but have you checked the floats themselves? If they're plastic (which I think they are) shake 'em to see if there's fuel inside. You can also place them in fluid to see if they 'sink or swim'.

Another thought. It doesn't take much to hold a float needle open. I once watched a coworker go through a similar ordeal only to find out the fuel in the tank had so much particulate matter in it that it would continually contaminate the tip of the float needles. It wasn't really obvious either.

 

What did you use for carb cleaner?

 

The floats were ok - not leaking. The fuel is pristine, and I am using a plastic push lawnmower tank for testing, and there is a new filter on the bike.
For carb cleaner i am using the Kleen-Flo carb cleaner. It is the only stuff that I can get here (it is made in Canada) looks like similar ingrediants to the Gunk product and the Tyme product when i compare the MSDS sheets. Soaked them each for 4+ hours, then rinsed in hot water, then 100psi air through all the passages taking care to ensure that I could follow where it was coming out, and comparing each carb one passage at a time. The only passages that are hard to verify are the three tiny bypass holes in the throat by the throttle plate. I have to assume that these couldn't be the problem because they must be fuel passages, and my problem is too much fuel.

Steve

 

Have you checked your fuel pump? Does it shut off once the carbs are full? Just wondering if the pump is out of it's pressure range and over riding the floats or running continuously?? One way to check is to make/get an external test fuel can. I made one years age out of a coffee can and 3/16 copper tubing. Still use it today. Let it gravity feed the carbs and see if they are still rich.

Dang beat on the starting line. I guess that answers the test can question.

 

Good idea - I don't think the fuel pump keeps running, but before I pull the carbs again I will try your suggestion and feed directly from my test tank to the carbs without going through the fuel pump just to make sure. Thanks
Steve

 

Ok try this. Set your floats really low. so the bowls only fill about half of what they should. See if that changes your condition of being rich.

"is the question of the new float valves and the little spring thing that holds them on the float."

Just wonding if they are the correct ones or may be a revised version or what ever. The richness is an idle circuit situation only normally. Almost sounds like you may be sucking fuel thru the main or secondary circuits. That should only be able to happen if there is to much fuel in the bowels.

 

Just for clarity, the little 'bypass' holes are tied into the fuel enrichener. When the plungers are open, extra fuel is supplied on those holes, downwind from the butterfly valve...to allow a fast idle during warm up. You probably couldn't verify them without manually holding open the enrichener plungers and then only through the enrichener circuit itself.

 

The work day is over, I'm going to go put a fire on in the shop and play. I'm going to leave the carbs on and connect the test tank, then when the carbs are full take off the fuel line and see if the fuel valves are closed. Should have done it sooner - we will see.

 

It's possible to observe the fuel level externally. Take a section of clear hose that fits on the bottem drain for the carb(s) and curve it up so the open end is any amount higher than the actual float level in the carb itself. Simply open the drain screw and allow the carbs to fill up. The level in the clear hose will be the same level as is in the carb.

 

Dizzy: I will pick up some clear tubing tomorrow and try that - thanks.

Here's what happened tonight: With the carbs on the bike I filled the bowls and then took off the fuel line. Blowing on the line I could just percieve some pressure loss. When I pulled the carbs I could see evidance of fuel on 3 of the rubbers. The wet rubbers matched the fouled plugs. I turned the carbs upside down on the bench and carefully adjusted the float levels to 8mm. I tested it by blowing on the fuel line to ensure that the valve closed at 8mm. Put it all back together. She ran for a bit, still not long enough to warm up, and the plugs were wet. This time there was some backfiring from the carbs and from the exhaust. I'm tired and there is a Canada vrs Russia game on. I'll think about it and try again tomorrow night.

 

The good news is Canada Beat Russia. The bad news is that something was bothering me while I was watching the game - I went out to the shop and pulled the oil dip stick and sure enough there is strong evidance of gas in the oil. Not sure what the next step is, but maybe i'll set up a jig to hold the carbs on the bench at the same angle as they are on the bike and see what happens when I feed them gas.

A question on checking the float level with a piece of clear tube on the drains: where should the level be?

Of course I will change the oil before I try starting it again.

Steve

 

There are no specs for that kind of test. It's useful more for ID'ing if and which carbs are leaking...an immediate feedback kinda thing.

Another thing I've done on occasion is remove float bowls, prop up the floats so you know they're closed and feed fuel. Of course they should all hold and you can visually tell if one doesn't. Helps differentiate between a level problem (you KNOW the floats have closed the valves) and a bad seat or needle.

Personally I've never had much luck with psi testing...without any 'pop off' specs.

 

I snuck out to the shop at lunchtime.
I propped the carbs up on the bench at exactly the same angle they are mounter on the bike. I connected clear tubing to each drain, and filled the carbs with gas. Lo and behold the levels were way out from each other by up to 1/2"! How can that be??? I had adjusted them so carefully! I guess I need to try again. For the heck of it I filled a can with gas and dropped all the floats in. They all floated equally, and didn't show any signs of leaking when I submerged them. I guess I will make a float adjusting jig instead of using a steel ruler. The floats are kind of flexible, so maybe one needs a guage that spans both floats to properly adjust them. These things make the old Amal's on my Triumph look awful simple.

My apologies to those that suggested float levels early on - I thought I had them accurate, honest. I sure hope this is the issue.

Steve

 

OK, so I made a guage out of plastic margerine container lid - exactly 7mm, spans both floats. When I measue them they seem right on - if anything they are tending towards 8mm. Carb assembly on the bench, carefully propped so the float tang is just touching the spring loaded pin in the valve, 7mm from bottom edge of float to bowl mating surface. Needle valves and seats are brand new OEM parts, c/w aluminum washer. When I blow into the fuel line and rais the floats they seal. While I was at it I checked the fuel pump relay per the manual. Fuel pump seems to be working properly - doesn't stay on. I am obviously missing something.