Can't get any better than this...Vacuum values problems

Hello everyone. I noticed that my VFR 750 RC24 1986 was running lean, so I decided to check its values with a vacuum gauge, and what I found is that cylinders 3 and 4 are equal, while cylinders 1 and 2 have significantly different and low values. I tried spraying starter fluid to check for leaks and noticed that the RPMs only increased when I sprayed near cylinder 2. So I bought new intake manifolds and installed them, but to my surprise, the RPMs still increased when I sprayed near cylinder 2...

Setting aside cylinder 2 for a moment, I focused on cylinders 3, 4 and 1. By tightening the valve opening adjustment screw all the way (without forcing it over), I managed to get cylinders 3 and 4 to have equal values, while cylinder 1 only reached 5 (values taken with the new intake manifolds in the photo).

What do you recommend I do?

 

"Running lean" and out of synch are two different things.

Lean or rich is controlled the jets (and slide needles). A smooth idle and just off idle is controlled by the synchronization.

If your RPMs increase when spraying fluid around the boot, you have not installed the carbs correctly and/or have damaged the new insulator. No sense trying to ascertain lean or out of synch until you correct that known problem.

 

In order to get less errors as possible I asked ChatGPT to translate...bad idea I guess. I didn't mean "running lean", I meant that I heard some issues like not synchronized, I didn't hear the engine running equal, I don't know how to explain it better.

I installed the insulator like the shop manual says, with "CARB" faced to the carburetor and aligned the narrow groove of the insulator with the tab on the intake port of the carburetor. I doubt I damaged the new insulator because I popped the front intakes first and then the rear ones with a flat screwdriver to gently open the insulator in order to fit the carbs, I expect those in the back to break first.

What are the correct values that need to be shown in the vacuum gauge? Are the values in #3 and #4 correct? (I attached the hoses sequentially, so to far left is the #1 and far right is #4)

 

That is a problem. You can chase your tail all you want, but that is all you are doing until you address this.

Maybe you didn't damage the insulator, maybe you did. Something is wrong. That is called an intake leak, and in that location can cause running problems.

If you were going down the road faster than walking speed, carb synch is largely irrelevant. The butterflies are open too far too make any synch inconsistencies on a bike that starts and idles at all, not a factor.

 

Do you find useful to seal with black joint silicone all around the insulator where is getting the carb?

 

No. I find it useful to install them correctly, and without damaging them. You have not done at least one of those two things.

Sorry to be the bearer of bad news, but you need to investigate and find out what is wrong. Gooping silicone is not the answer when mounting carbs.

I can describe how I baked a pie, doesn't mean I did it right or it came out good. The pie either tastes good or not. The idle either doesn't change or it does when the boots are sprayed down.

 

Do the clamps for the carb boots have limiting collars?




If so, over time the clamps can change enough that even if tightened all the way, they will not clamp enough and can even still move or spin.

Remove them if they are present and you find nothing else wrong.

 

this is absolutely something that could do the trick! Gonna remove them right now and try, be right back

 

removed the limiting collars and still increasing RPMs if I spray starter. So I swapped the insulator and still on #2 carb the RPMs increases...
For now I managed to get them all at the same level on the gauge, at least now they're synchronized. Tomorrow wanna check the gaskets, maybe they'll need some silicone

 

What gaskets?

 

the gaskets of the bowls, maybe they don't seal good, I don't know what could be the problem now

 

If the float bowl gaskets don't seal, fuel leaks out, the bowls are not under any vacuum.

I assume that the port for vacuum synchronisation is sealed up nicely?

 

This ∆

So much.

There is ZERO gaskets in a fuel system that takes silicone.

 

Yes, I always spray on that first and nothing happens, unfortunately

The fact is that if I spray around the bowl area it increases too…
What other carburetor parts could be involved in an air leak besides the intake port?

 

Check to see if the screw in the intake used to install synchronization plumbing is installed, along with any other open-port in the intake. If it was an "emission" model, like we had in California, there was alot of extra plumbing and paths for a vacuum leak.

 

They were not any extra plumbing or stuff around there, just checked.

I installed the front ones in the carburetor, tightened them good before positioning them on the cylinder head, and then I pushed the back carbs on the insulator that I seated on cylinder head, but always the #2 is leaking air, whatever insulator I put in that position. This problem is driving me nuts

 

Are the pilot screw o-rings present & in good condition?

 

Yes, I changed them a few months ago. Do you think the pilot screw could be related?

I noticed that the rpms increase only if I spray starter on the front part of the insulator, the back facing carb #1 doesn’t even flinch.

 

Earlier you stated it was #2 you sprayed & rpm increased- ?

 

Yes, and it still. If you imagine splitting the intake mainfold in two parts, the part facing the front is the one who leaks. The part (of insulator of carb #2) facing the carburetor #1 doesn’t seem to be affected