Fitting 32mm carbs to a Euro 86 VF500F2

In the time honoured practice of documenting changes you make to your ride, I submit my story on adding 32mm carbs to a 1986 Euro VF500F2. I bought a set of brand new 32mm carbs on Ebay from an Italian seller.
These were model VD56E. The slow jets were all numbered #38 and the main jets were #90. The embedded pilot(?) jet was a #45. Needles not checked yet. Float heights look OK.
I bought 4 new rubber isolators (16211MJ8000) from CMSNL.
When fitted, the carbs worked straight away. However the idle was low (1000RPM) and the bike was getting hotter than normal. I fixed the idle by using the (black knob) idle screw. I balanced the carbs at about 3000RPM using a Carbtune II. They were pretty close and it was hard to get them closer.

At high RPM the bike is fine. Maybe it is more powerful, I can't really tell. I top ended it at 177kph (110mph) but I am pretty sure I could get the same with the 30mm carbs. It does seem a bit snappier but hard to say. A few dyno runs would be nice but not going to happen.

I do have one issue. If the bike is hot and idling and I give it a fistful, it bogs and often stalls. The pilot screws were set at 2 turns out as they came, but to address it running hot, I had turned them out to 3 turns. I systematically varied the pilot screws to see if I could get rid of the bog. The only setting that seemed to reduce it was 4 turns out. Going lean (1.5 turns) made the bike run very rough. I couldn't get rid of it completely. As the pilot screws reportedly do nothing after about 3.5/4 turns, it looks like I will either have to live with it or maybe try a bigger size pilot/slow jet. (The thing is I never actually did this test with the 30mm carbs so the bike might always have behaved like this. It's never been that quick to rev).

You can still buy the pilot/slow jets on Ebay. As far as I know the part number involves the sequence 424-21 but I have not confirmed this.

I am a bit concerned that it now runs quite a bit hotter as evidenced by the steady state position on the temp gauge. The 32mm main jets are all #90 and in my 30mm carbs (VD41A) the front cylinder main jets are #95 and the rears #92. I might swap these in the future, but for now will leave as is to see how it pans out.

I did check for air leaks around the inlet rubbers using propane, but found none.

One thing I will say is I have great admiration for the mechanical engineers that worked on carbs. The way the 4 fit together and work together, with all the linkages and springs is remarkable. All that skill (CAD would have been very basic back in the early 80s) now rendered obsolete by technology.

Things I wish I had done before I started are to clean the K&N air filter and fit new plugs just to eliminate these as factors. Probably should have checked the valve clearances too. However the bike gets babied so I reckon they are all OK.

It's all been great fun and a good learning experience. Not that expensive either.

 

very interesting post. it sounds to me that you are running lean at low RPM and may want to up your slow jets. I have limited experience with carbs but I think that it's lean since it's running hot but still has good power at the top. Perhaps the accelerator pumps arent working properly causing it to bog. Too much fuel from the accel pumps during a blip of the throttle would be the cause, right? do you plan to do any performance evaluation besides the butt dyno?

 

Yes, I agree about running lean at low RPM. I have bought 4 off #40 slow/pilot jets on Ebay to have a go with. Will fit next week.
The VD type carbs on the VF don't have accelerator pumps. Just good old atmospheric pressure.
From what I have read, too much fuel or too little can cause an off idle bog but I'm pretty sure in my case it's too little.
I did check the needles in my new carbs and they are coded 2VG. I have no idea how to interpret these codes and I can't find anything either.
I do know they are very fine at the pointy end, much finer than the 3JV needles in my 30mm carbs. This means they should flow more fuel at higher throttle openings.
I am assuming the needle jets are the same in the 30mm and 32mm carb body castings but I have no evidence either way. I did notice the diaphragm springs are longer in the 32mm carbs and the diaphragms are larger diameter too.

 

I also took out a few spark plugs. They really just confirm that the engine is running lean. These are iridium plugs btw.
Their condition is probably not helping with the bogging.

https://goo.gl/photos/Qy287mqn5DR2PCd89

No performance testing until I have them sorted and I can switch between 30mm and 32mm with impunity

 

Be sure to check the emulsion holes in the #40 pilots. I had wanted to try some #40s in a bike I was building but the #40s were way different than the #38 or #42. I went with #42. Anytime you get more than 3 turns out on the pilot screws it's time to go up. Also be careful, that far out there is little spring pressure on the screws and they can vibrate out while riding.

 

OK. will give them a close examination when they arrive. When you say way different what do you mean exactly though? The holes were smaller than you expected compared to the #38?

 

Smaller and fewer, IIRC.

 

The bog is gone.

I bought #40 and #42 pilot jets off Ebay.

The original #38 jets were a lot more restrictive compared to the #40 and #42. I tested them by forcing water through them using about 1m of head. The difference between the #40 and #42 was very small. The bog is gone with #40 jets. There is a tiny hint of it but it's pretty much gone. I haven't optimised the pilot screws yet. When I fitted the carbs with the #40 pilot jets, it would not run with the "choke" full on without some throttle action. Kept stalling. Went for a ride and after choke removed would still not idle even when fully hot. Tweaked the idle screw and it was then fine. Seems a bit quicker to rev but probably wishful thinking.

Next thing to do is ride again, get it hot then set the pilot screws.

Then I should probably leave alone to check the mpg. However I could go the whole hog and fit the #42 pilot jets and the #92R/#95F mains while I have the carb removal technique just so. Just to see it it makes a noticeable difference.