84/85 Carb Swap into an 86

I recently purchased a rebuilt set of carbs for my 86 500 off ebay. I had already purchased a set of some gunked up 85 carbs a while ago at a good price to get a hold of the k&n air filter so I knew what they looked like compared to the smaller, less shiney ones for my high school sweetheart. When the rebuilt ones arrived, I noticed they are the same size as the ones from the 85. I know I have seen post where people have put them on their 86's but a couple of details are missing.

1) What other parts am I going to need? Will they shoe-horn into the smaller insulators for the 86, or do I need to get new insulators and bands?

2) Are there any other mods I am going to have to do in regards to fuel flow? I know that the 86 lacks the fuel pump and therefore the lines are a bit different. I'm mildly competent so I should be able to re-route fuel lines and a fuel filter to make it work. Mainly, will I need to throw in a fuel pump and some funky jetting?

3) Am I mentally deficient in trying to get the seller's screw up to work, or should save the time and money and send them back?

I've seen some sporadic posts, but nothing definitive about the swap. It looks like Jamie has the most experience with that sort of thing. That 500 was my first bike back in 91 and I fully intend to ressurect her after 20 years of sitting in the garage. I love that bike.

 

I have read the larger 32mm bore carbs are a bolt and go. New boots (insulators) are required. Gravity flow will be fine. The petcock outlet on the '86 is actually larger. The '86 carbs have two inlet points (coming from a 'Y' downstream from the petcock) the '84/'85 carbs only have one inlet, so you will need to run a new fuel line.

One question I have is the design of the '86 intake air passages themselves. I am not sure if these are different based on the smaller bore.

Also the cam profile is different. The '86 cams having a milder profile. I would like to posit that to take full advantage of the larger bore carburetors you'll need to change the cams (perhaps the heads?) as well.

Look forward to hearing other thoughts on this.

 

I ran 85 carbs with an 86 engine in my race bike with no issues. No fuel pump needed, and I routinely ran it low on fuel to save weight. Never once experienced any symptoms like starving for fuel. I used a reducer to go from the 86 petcock fuel line size to the 85.

Although the 84 engine I also ran sounded way nastier (in a good way), my fastest time was with the 86 engine.

 

Boots Boots Boots

So grabbed one of the isolator boots off the 86. They are indeed a bit smaller than the 85 outside bore so they won't work. I went to the Honda shop and ordered up some 85 boots, be in next week. The bands to hold the boot appear to be the proper size and fit both sizes of boots. About a 15 years ago I bought a new set of boots and clamps cause the old ones were like poo. Both the bands and boots were superceded with newer, slightly different parts

Fuel line size is indeed a bit smaller, will get a line reducer and throw in a fuel filter for good measure.

I don't really want to hot rod the machine to make more power, faster, nor flashier. I just love riding that bike. So smooth, lots of power for me, and well mannered. If I want brute force and ignorance, I hop on my VTR.

BTW, I have pretty much become an expert on removing and replacing the carb assembly on the bike. I can easily do both in about 15 min now. Key to the exfil is jockeying up the rear and then the front multiple times. Eventually they pop out. On the way in, I seat the front first and then the back. To make it easier and to help the rubber, a small rub of silicone lube helps a lot. Grease and other oil products actually attack the rubber, causing it to harden and crack. I found while working at car dealers and washes that they use the silicone lube to rejuvinate door seals and CV boots. A small application once a year keeps them well conditioned. I've never cracked a CV boot or had a leaky door seal since. Like I said, my newer boots are 15 years old, and are still as soft as when new with no cracks at all. When I stored my 500, I sprayed the entire engine bay with the silicone to help preserve it. I do it every winter with my XJ650RJ as well, and it helps keep all the rubber bits soft like new.

 

Other way around - the 86's have hotter cams. That's why the 84-85 carbs on an 86 engine has always been the hot setup.

 

Not sure about this JD. From what I have read the '84/'85s had the hot cams.

 

100% positive bro - that's why the 86's (with the smaller carbs) were able to net the same hp numbers.

 

This is a bit of puzzle. If I could ask, have you compared the cams side by side?

The article above specifically notes that the cams are a milder profile on the '86.

Also, the HP numbers are not the same.

Here is an overlay, the '84 represented by the red line (53.16 BHP) the '86 the blue (51.09 BHP).

 

It's pretty easy: that article is wrong. The 86 engines made the same peak hp as the 84-85 models. The magazines from the 80's were known to be incorrect more often than not - my suggestion would be to take care not to use what you find in them for anything except fodder.

 

Had not seen that article before, thanks IC.

 

I may be missing something here.

I double checked another copy of the FSM (one that covers all years) and this clearly shows a milder cam for the '86 model year.

Honda Service Manual, Date of Issue June, 1986

 

Yeah, you are missing a couple of key points:

- The FSM you reference shows "lobe height". This is not lift. It's the difference between the base circle and the lobe tip. The 86's have a smaller base circle.
- The 86 cams also have more duration.

Once again - just reading and doing web searches often leads you astray. You need some practical experience to know what is really going on.

 

Adding to confusion my FSM pairs the 30mm carbs with the 225 degrees of duration for both the inlet and exhaust.
Alternative setup according to my (questionable?) notes was the 32mm carbs with 230 degrees of duration again for both the inlet and exhaust.

I agree with JD that magazines of the 80's were more guessing than measuring. The overlay of power curves above, though rpm do not accurately correlate, shows torques almost identical when power is much different

 

Here is a link to a previous thread that I had started when I was gathering info on this subject:

http://vfrworld.com/forums/1st-2nd-generation-1983-1989/32470-vf500f-cam-specs-84-85-vs-86-a.html

I may have misunderstood, but I thought the takeaway was that the '86 cam has more duration and less lift (post no. 37)

http://vfrworld.com/forums/1st-2nd-generation-1983-1989/32470-vf500f-cam-specs-84-85-vs-86-a-2.html#post283536

 

Something more relevant to the OP who is using 86 cams: you may experience a decrease in power in low end and mid range with the larger carb bore with a gain at the top. That is the typical trade-off of going to a larger bore carb. We used to do a lot of that on 2-stroke MX'ers when you desired more top end out of a bike at the expense of some lower RPM performance. The problem is that you don't want to ride the 500 up there anymore like they were road many years ago when parts were available. This might not be too noticeable, but just FYI.

 

Isolators

So I got the 85 isolators and they are indeed bigger on the carb side. Just fashioned a fuel hose with an inline filter and an adapter to fit the larger petcock for the 86. Time to strap 'er on and see if she'll fire up.....

 

My suggestion is to not run a fuel filter on a gravity feed system. Honda did not put a filter on any gravity fed motorcycle that I know of, if you put it in there I think you'll find that it just causes you trouble.

Good luck!