83 VF750F idle problems

Hi, excuse my english.
This is my first post in this forum, (I read it a lot, of course).....and, of course, I need help.... eaceful:

The situation is:
A couple of days ago, the bike starts ok but, running at idle speed (yes, she run at idle speed ok), and leaving the throttle untouched, she began to accelerate.
If you use the throttle and reach 4.000rpm, for example, she stabilizes there, don't return to idle speed. I checked the throttle cables, ok.
So....the problem must be on carbs.....

I clean and rebuild the carbs, new jets, new pilot screws, etc..... The problem changes, now the bike don't accelerate herself, but no idle......

Jobs done:

- Changed all jets except starter, float bowls, pilot screws. Checked the passages with compressed air prior to install jets, all ok.
- Float bowls set to 7mm.
- Removed the old pilot screws, with washers and o-rings and installed new pilot screws. Set them to 2,5 turns from closed.
- If you remove pilot screws, turn on the bike, and blow carb cleaner through the passage, the carb absorbs it, so I understand that all is ok.
- Checked spark plugs, the spark is strong and good.
- Checked fuel pump, ok
- Checked with 1,2, 2'5 and 3 turns in pilot screws. No idle.
- Diaphragms are ok, no broken....
- Testing with another ignition coils...no idle.
- In decelerations the bike don't stop at 1.200-1.300rpm. go down till die with a detonation and few smoke in the carb 4.
- If you maintain around 1.500rpm with throttle, you could see the vacuum valve vibrating in front cylinders, but no vibration in rear ones. ¿Is this ok?

I am out of ideas....any help is greatly appreciated.....

 

All four slides should move when you work the throttle or slightly vibrate at idle. If the 1/3 doesn't then I'd suspect you may not have the diaphragms in correctly for 1/3. There is a small loop/hole on the edge of the diaphragm that fits over a vacuum port hole on the edge of the carb where the cap goes. Sometimes when putting cap on the diaphragm slips out of place and covers the hole. This keeps the slide from working. Or that passage is plugged up. Seems odd it would be plugged because it seem to work before, right?

I'm assuming you put the right spring length in the right carbs. Shorter in 1/3 plus thinner needles. also you did bench sync'd the carbs. If not your sync could be way out also. I've seen that. Your main idle set and base carb is #4 that you set the rest of the carbs to.

When you set the 7mm float height, it was when the float tab just touched the center pin with out compressing it right?
Shouldn't cause your current problem. Pilot screws will be between 2 - 3 turns depending on your bike.

That's all I can think of now of what to check from your description. Good luck and get back.

 

You are right. The problem was this. I set the height with the carbs in a incorrect position. After reading your post I tried your suggestion and all worked ok. The bike idle fine now, well, all fine she could without sync and balancing carbs....this is the next job.....:lemo:

Thanks for your help!!!!

 

Cool, glade you got it fixed.

 

Geez, I know who to go to for advice now! Nice work!

 

The idle speed adjusting screw is your friend; learn to use it.

 

One other point, when you "bench-synch" the carbs, you want to blow compressed air into the t-fitting and watch the slides go up and down in synchronicity. All four should move together. eace:

 

Thanks to all for your suggestions!!!! You are a great team!!!! :stung: