Please forgive my ignorance

Just wondering if I'm on the right track here. My 83 vf750f I just traded for has carb issues. I've cleaned them (one at a time, important later) twice, repaired 2 diaphragms, still won't rev right and is very sluggish. Checked all the normal stuff like cams, valve adj, firing on all cylinders. It has full cobra exhaust and a uni filter. Is a cali emission bike so the jets were 122, I replaced them with a set of 135 that a buddy had, no change. After all the back story, the question. If the number 1 and number 3 carb springs and needles were switched with the number 2 and 4, would it cause this? As stated earlier I did them one at a time but the previous owner had been in there and likely switched them. I didn't know there was a difference till after I went through them. Any opinion and input would be greatly appreciated.

 

Where are you located?

 

How do the plugs look? How is the compression? Have you done a good sync? Have you checked that the needles are OEM spec?

If the 1-3 needles, etc. are different than 2-4 and were accidently reversed there isn't enough of a difference to cause the slugishness that you describe, IMO.

 

North Georgia

 

Haven't checked compression, plugs are a little sooty, but it a little rich at idle, not enough to stall. Sync shouldn't affect revving or pulling, just idle and starting issues. The needles and springs are the only idea I can come up with. According to the manual, the 1 and 3 carbs use thinner needles and shorter springs.

 

Cam timing and valve adjustment is right, changed from 122 to 135 on the jets thinking it would go way rich, didn't change. Will check out the link tho, thanks.

 

what part of north ga? I'm in NE GA, exit 166 on 85.

my 500 has cobra and K&N, on it when I bought it and it was as sluggish as a 50cc scooter from about 4-9k rpm even after all the normal carb work 2 or 3 times.

I do NOT condone doing what I'm about to mention as most people here will also tell you not to..

Drill the slides. I drilled mine the same size that the Dyno Jet kit requires (did not install jet kit) and it made a big difference. Now, that said if it is sluggish during the first 1/4 throttle then drilling will most likely not help anything. My problem was the restrictive air filter not in place reduced the amount of vacume available to the slides. I still need to buy another set of slides and spends at least a day on a dyno till I get the hole the correct size to match the aftermarket parts. It's still a little bit flat in the mid and around 8-9k when the velocity gets up the slides will go full open and it's all most like having a V-Tec.

An easy way to test if this is a possible problem is to restrict the airbox inlet, I used a piece of duct tape, block off about 1/2 the inlet and ride it to see if it seems better, if it does you can either go the easy route (OEM filter) or work on the slides.
BUT before that I would replace, not repair the diaphragms. You may still have a tiny leak that will cause problems.

BTW have you checked for leaks at the carb boots? It can cause similar symptoms.

 

Checked the slides already. there non molested. I read the post, it basically went over the same thing I went through when I put aftermarket exhaust on a gs750 I used to have. Up 2 sizes on the mains and all was well. This thing acts totally different. Have you ever drove a car with the catalytic converter stopped up? Acts like that. Idles good, lo rpm revs fair but a little lazy. Will not pull, won't rev passed 8000 in neutral even if you hold it wide open for a couple seconds.

 

I sprayed the boots with carb cleaner, no change in idle. I took one of the rubber air horns off the box and taped up the hole, didn't change. 1/4 throttle is a little sluggish, gets way bad after that. Could be a leak in the diaphragms i guess, but with the airbox off if you rev it all the pistons are moving, kinda pulsating tho. When you say drill the slides, do you mean where the needle seats in the bottom of the carb above the main jet? Or the actual slide piston itself?

 

Checked twice.

 

Was thinking about trying a gravity feed to make sure the pump isn't taking a dump. I took the fuel line off the carbs when I first started working on it and wound it over. The pump worked, but couldn't promise it still is. getting a full stream of fuel from the tank when you remove the line.

 

Drilling the slides, there is a hole in the bottom of the slide that has nothing in it. The hole is there so that as air passes it speeds up and the pressure drops, creates a vacume (through the hole) to the diaphram and raises the slides.

Mine pulsate too on the 500 on my Sabre they are pretty smooth but still pulsate, just a lot less than the 500. I have to assume this is due to the porting from the diaphram chamber to the area between the butterfly and intake.

 

K I would assume then that this would change the rate that it goes back to idle then? it's kinda slo in that department too.

 

Not really, once the butterflies close the intake charge is choked off regardless of how slowly the slides close.