After spending 2 months waiting on parts for '86 VFR I finally finished getting it up to being roadworthy again. I learned a hard lesson though on the airbox modification. In 2010 I started to get this bike going again. It was leaking gas and needed a fuel pump which I replaced. At the time I decided to drill a few 1" hole in the top and front of the airbox while I had the carbs off for cleaning. I had to hose out the airbox of my CRF250X to get it running good at this altitude and also according to the jetting kit. A very small intake like this VFR. Sounded like a smart idea!! I got very busy for a few years and in March of this year I decided to get it going again. I made sure the needle/seats were replaced, 115 Mj's installed for high altitude, Uni foam filter, new intake boots, Samco hoses and a new waterpump. I decided to drill a few more holes in the airbox. The bike fired right up after getting a sniff of high octane gas. It settled into a smooth idle and seemed fine. The new bowl o-rings leaked for a short time and then sealed. No leaks anywhere now. During reliability testing I noticed a 1/4 throttle to needle jet stumble and some backfiring. I got the chance to ride it in first gear all through the range to the top end on a non-windy day in a canyon. I could hear it backfiring into the airbox and the front cylinders seem to be doing it. I had just synced the carbs after waiting 10 days for a Morgan Carbtune. It was in perfect sync with cyl #2. So what was going on? I spoke with several friends who were great mechanics and no one seemed to come up with an answer but I got a few hints. One friend in Idaho said he had heard his neighbor revving up his bike and heard backfire at certain rpm points too. I know some machines need a still airbox just like this. And many pull off the airbox and install single K&N air filters and have problems. I personally will not run them after seeing sand pulled through them on Banshees. At this point I did not want to pull the carbs and drop the needles. Anything but that! So, I pulled the tank and taped up the holes, reinstalled the tank and off I went. What happened? The backfiring went away and no stumble. It was victory for the humans! I went for another ride, longer this time and it solved the problem. I am thinking that the holes caused a problem with the vacuum in the airbox and the front carbs gulped more air than the rear. So now I have a very well mannered VFR again. It runs hard up to 11000 rpm redline now. I was equally impressed with the brakes and suspension. This still is a nimble bike for it's size also. The canyon I was riding in alone was a great reminder of just what the earlier VFR's were really about and why I did not buy a CBR at the time. Now it is time to ride!
Holes made you run too lean and you fixed that. No, i don't believe that front carbs sucked more air than rears unless the synch is way off.
I think you are right squirrelman. I just know the holes made for a lean situation. Probably all 4 carbs were lean. I also could have corrected it with raising the needles, but I did not want to pull the carbs again. After 350 miles this bike runs great after a few canyon test sessions. It was difficult to believe it could be a lean mix at high altitude. I turned the fuel screws out more to the stock setting. Dynojet says 2 turns out. But these are general settings. My shop manual says turn them in 1/2 turn for high altitude but that is for stock pipes. I may go even a little richer later. The Yoshimura 4-2-1 pipe MAY need it richer. My snowmobile friends say that high altitude lowers float bowl pressure and lowers compression. They are sometimes at 9k-10k' and that means a density altitude of much high that this. That is what the bike thinks it is running at.This bike does not lack anything though. When the engine is on the cam in my test canyon, it flat out rips now.
