Do you think I need to re-jet?

Hey guys,

I'm trying to sort the old girl out before some quality riding weather teases me.

I'm planning on sending the carbs out to Billy C to be rebuilt and I'm trying to figure out before hand if I require a re-jetting. The carbs have not been touched in my 5 years of ownership.

It's an 86 VFR750F that originally came with a oem paper air filter and a dual exhaust. These have been swapped out for a K&N and a 4-2-1 system with a straight through exhaust.

As you can see in this picture, it's a pretty free flowing exhaust: http://i346.photobucket.com/albums/p435/Amos830_photos/SL733906_zps777aaf17.jpg

Some of the symptoms that make me question if they need re-jetting is an exhaust popping on deceleration and quick throttle releases while coasting and letting the engine brake. She still pulls hard; granted I don't know anything else.

I just need to let Billy C know what route to take and figured maybe someone here has a similar equipped bike and has some experience with the pro's and con's of re-jetting.

Cheers

 

I've always bumped my bikes up a jet size or 2 when I put on a free flowing exhaust and air filter and dial in the air/fuel mixture screw. Someone with your bike will chime in but jetting is always a give and take. You can jet for better top end, better mid range, etc, but usually you'll end up with a spot somewhere that isn't perfect. When doing it without a dyno, it's more of an art than a science. The problem with V4's is, taking the carbs off alot to experiment with jet sizes is a pain. Hopefully someone with your setup has a good base starting point.

Keep us posted. You should be happy with Bill's work. He's done 100's of V4 carbs and usually has parts on hand if any are needed.

 

Also consider your altitude above sea level. With out a proper setup it hard, not impossible, to get the jetting right. A wide band o2 sensor and afr gauge work wonders to figure out if your too rich/lean. The jets just set the range, so watch your adjust screw. If it is under half a turn from bottom or less than a turn from full open then changing your jets will bring it back to a reasonable setting.

 

Geeze, Matt, you don't mind touting, do you ? What a loaded post it is, highly propagandistic, the both of you.


freeflowing exhausts won't produce extra power unless they're custom jetted by trial and error on a dyno. And i fix carbs professionally for alot less and quicker!

 

Huh?? no comprende ? If the carbs don't idle ok at somewhere between 1 1/2 and 4 turns with stock idle jets there are probably partially-clogged idle jets or passages. bike won't run long if screw is out past 5 turns cuz it will fall out and be gone.

 

Is this comment in reference to me? If it is, I have absolutely no idea what you are talking about.

I'm just looking for an idea on a baseline jet size for my current setup so that I don't have to mess with these carbs anymore than I have to.


I noticed before you edited your post that you also rebuild carbs. Is the propaganda line because I mentioned Billy C? I have no affiliation with him. I bought some metal fuel tubes from him in the past and found him to be prompt and professional so I just figured I'd return to give him some more of my business.

By the sounds of your post I assume there is some history there... I apologize if I stirred something up.

 

no worries, matt, let us know how long it took, willya ?

 

I sure can.

I haven't even got my carbs off yet so it probably won't happen for at least a couple weeks.

And I'm sure the closer it get's to summer the busier he'll be. :shrug: I'm not too worried about it.

 

Matt,

If it is any help I can offer you my own opinion on the subject of jetting with the understanding that I don't rebuild carbs professionally nor do I deal with anyone who does but I am a technician with decades of experience as are many others on here.

Your bike was probably running lean when it was completely stock as this is generally the case with Honda and the need to meet certain emission standards. With the mods that your bike currently has, it is probably even more lean so it is safe to say that you could probably benefit from a jet change to richen things up across the board. The bike would start easier and take the throttle more readily while running much cooler at the same time. As has already been posted, the actual process of trial and error jetting takes time and experience as well as tools such as a dyno to do really well. Many have benefited from jet kits that can be purchased for just this purpose but they only get you in the vicinity of where you would like to be and final "fine tuning" requires the tools already mentioned. Personally, I would have the fellw doing your rebuild put just such a kit in at the same time and when you get the bike back you can determine if it is acceptable in terms or rideability. If you aren't happy, you haven't wasted any money getting where you are at that point and you can take the next step to get it fine tuned. I suspect that you will be happy with the results of the kit, I was.

I should also mention that a little backfire on the over-run (gearing down) may not mean that you mixture is way off. Remember that in this condition the engine is spinning fast, the throttle plates are closed, the vacuum is enormous and it's running on the idle circuit so minimal fuel is getting in there. I personally like a little snap crackle pop and even my fuel injected 6th gen with PCV does it.

The credit goes to you for spending the time and money to make that puppy right!

 

Just stumbled on this this beaut of a gold mine.

Figured I'd link it just in case anyone else in the future asks a similar question.

http://vfrworld.com/forums/showthread.php/7754-Jet-Kits-Dynojet-vs-K-amp-N

 

Hi vfrMatt86 I have a 4 into 1 KerKer and pod filters on my '82 cb750k and I went up one size jet on the primary and 2 on the main as well as I added 2 shim's per needle and she runs so much better. So I say yes to changing the jets but you're going to have to take your time and try different combinations to get it right good luck and post your results after to help anyone else who plan's on doing the same mod

 

Well said!!

 

I did drill the slide valve and didn't like the results and it cost me a $100 for another set so I would advise not doing that mod

 

Basically everyone has told you to test, test, and test some more. Jet sizes only select the range your are adjusting in. Go to big and you not only waste fuel but can wash your cylinders. Go to small and it may not run or runs so hot and finicky that its not ridable. Not two engines or sets of carbs are the same, so play with it.

 

Assiniboia hey? I'm pretty sure that's really near where I bought the bike from originally. Did you used to own a yellow 86? :loyal:

I 100% agree with your summary. Unfortunately I'm not in a position to be testing over and over, though I agree that is the proper way of doing things. I'm just looking for a best guess suggestion based on previous experience. And if what you're saying about no two engines/carbs are the same.. then I guess all those suggestions are pointless.

It was running fine for me with the stock setup (although maybe a tad hot but definitely not finicky) so I may just leave it. The most I'd do is 1-2 sizes up on the mains and shim the needles.

I have a hard time seeing it but do you think 1-2 sizes larger could cause a cylinder wash situation? I'm not worried about wasted fuel. lol