1986 VFR750 Carb help

Thanks for the info; just confirms my thoughts - very helpful. I will report back after I make some changes. When you reference the washers ("the same washers as the pilot screws have"), I am a little confused, can you clarify where those washers are that you are referencing?

 

When you remove the pilot screws for carb cleaning, each one has a spring, washer and o-ring. If you do not know what I am referring to, then a proper carb clean has not been done. Now you need to verify you have these items and they are in the correct order.

 

I believe I am getting caught up on the terminology. I believe you are talking about the fuel mixture screws right? The only reference I have heard with term pilot is for the low speed jets but that may just be my inexperience. I understand that there is a spring on the mixture screws if that is what you are referring to. If so, the next question would be where I get some of those washers; the OEM parts counter? Did you just use one washer to raise the needle or more than one?

sorry if I sound a little dense, just trying to get some clarification.

 

Yes, Fuel mixture screws / pilot screws... same thing. Fuel screw > spring > washer > oring.
You accounted for all of them when you had the carbs out and apart?

Washers not available separately, I just harvested them out of carbs over the years. You can source suitably thin washers from other sources though.

 

I've put Bridgestone Battlax tires on two different older Hondas ('84 CB700SC Nighthawk, and my current ride, '84 VF750F Interceptor) - awesome on both, especially the Interceptor. For the money they are worth every penny (IMHO); bought one set on motorcycle-superstore.com , the other set in Canada (Vancouver) from ITC (itctires.com). I think my price was just under $300 CAD, delivered to the garage where I get service done here in Vancouver. Got a skookum deal the first time, when the dollar was better and there was a 40% discount on MC, and happy with the price I paid here for the tires I got. I swear by them, great tire once they're broken in, point and shoot !!


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I am getting ready to order some jets and wanted to clarify something... I understand that each manufacturer has its own #'s for the jet sizes and there are many factors so there is really not any specific conversion formula; e.g., a 130 Dynojet is not the same as a 130 Keihin or 130 Mikuni so the numbers that you reference above - are those Keihin #'s? I was considering the Stage 2 Dynojet package as a baseline and looking at the jet sizes that come in that package (if my math is correct) I am approximating that a DJ124-DJ128 would be somewhere in the range of a 132-138 main for Keihin jets. Does this sound generally in the ballpark or way off? If you are running 125 mains on a 700cc bike with the specified mods what would be an accurate correction factor for the extra 50cc on the 750?

I understand that each bike will have its own jetting setup so I am not looking for the precise answer, just a general starting point (maybe some people will post up a few configurations so I can try to find an average) and play with the feel from there.

I am clearly way off with the 118's so I am not too afraid to make a larger step up but at the same time I would like to see what you guys think about the upper range of main jets that people have had luck with given an airbox and slipon exhaust mod...

Appreciate some more feedback. I would like to order jets tomorrow if I can get a general concenses...

 

The jets I used are genuine Honda Keihin. You can take the Honda part #s for those jets and change to what size you need. Take a look, you'll see what I mean, for both mains and pilots. Honda doesn't use a 40 in that configuration, but the 42 was perfect.

I also have a 750 that I jetted almost identically with essentially the same mods (but airbox snorkel in place). Runs perfect. I think I went 122 on that one for slightly better mileage (700 is a track bike, 750 is my rain/ camping bike). I recently went back to stock mufflers on the 750 and she still runs perfectly.

The jets you suggest are way too rich in my opinion.

 

Thanks, that is very helpful. Sounds like I should start where you are and see how she runs...

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OK, forgive the iteration here but I put a few feelers out to try and get a baseline and here is where I run into some confusion. As a pre-cursor, I should say that whenever I am looking for answers I will generally try to get as many inquiries out there as possible and look for common answers to help me filter out relevant information. With all the "voo-doo" of carburetor jetting, this issue is certainly challenging. I appreciate the feedback thus far and hope to get more to confirm I am on the right track.

The issue at hand: The proper jetting range to start with before working on a fine tune.

I have read numerous posts with negative feedback regarding DynoJet but my main objective here is the find the proper jet size range for optimal performance from the bike. When you look at the Dynojet kit, it has one for the '86 VFR750 which is good. I take this to mean someone has put some time in on the 2nd gen and crunched the numbers to find an appropriate jet range. They (Dynojet) recommends a DJ124/128 main jet with a different needle and slide vacuum mod so whether or not this is optimal for every bike is a different matter, I am assuming this is a range that works from a general perspective. Now, the issue I am seeking to clarify is that when I try to convert the DJ124/128 jets to a Keihin equivalent I end up somewhere around the mid 130's (132-138) but most of the feedback I have been able to get suggest that this is way to rich. Ok, no problem other than how can I confirm this without physically making some changes and map it myself?

So, I decided to take a look at other jet kits for our bike and went to carbjetkits for another reference to see where they are at. unfortunately, I could not find a kit for my specific year like with Dynojet but I found one the is reasonable close (84vfr750) and after using their jet calculator their site recommended a 135 main with a 40 pilot and shims for the needle. This is where I would like to get some more feedback (not to offend any previous feedback - it is all appreciated) because I now have two separate companies offering jet kits that have mains running in the mid 130's.

Let me take a step back here and explain that I have been a motorcycle enthusiasts for years and have had many bikes so I feel like I have a pretty good handle on how a bike should perform but also realize that I am dealing with a different era so I want to make a careful assessment before I rip the cabs apart and start playing battleship with jets. One example I have is that the previous owner of this bike thought it ran great; and it does as long as you don't lean into the throttle so I believe it is possible that riders with this year VFR's that are running main jets in the mid 120's likely feel the same way - and it is also possible that we could still have untapped potential in the bike considering I have found multiple jet-kits that place the proper mains in the mid 130's. The problem I have from a common sense perspective is that the discrepancy is quite large (a good 5-7 full jet sizes) so I would love to open up some more dialog and get people talking about it. I have scoured the .net and have not found many people talking about what they have tried and what worked and what didn't.

So, to add - below is a chart I found that gives an estimate for jet size equivalencies across vendors so take a look at that and consider the two jetkits I have found and their recommendations and let me know what you think?

Jetkits I have found:

DynoJet - stage 2
DJ124 to DJ128 main jets

carbjetkits:
135 (Keihin) main jets

 

Please don't drill your slides. Since this is a new bike for you, why not ride it for a thousand miles or so to really get a feel for what it does and what you need. Chances are you didn't clean the carbs properly unless you verified that all the jets and cross-drilled bleed holes were open to the correct diameter.
http://vfrworld.com/forums/showthre...s-(and-avoid-buying-jet-kits-with-drill-bits)

 

I appreciate the info and link. Personally, I have no idea if it is better or not. My current plan is to clear up the lean condition on the top end that I have now and I am on the confident side that the carb has been cleaned correctly. I will simply start by introducing larger mains until I find a result to confirm or deny if I am on the right track. What I am really trying to achieve it a starting point to eliminate unnecessary trial/error. My reference above is that we have two jetkit vendors that are both recommending mains in the mid 130's and every other post I can find has people running much lower mains so I wanted to provoke some discussion about the main jet range that would be best to start testing with. Drilling the slides is a good topic and I will certainly not go there unless I can backup some + experience on the action.

Initially, I need to fatten up my top end jetting and need to decide if I want to tear the bike down a ton of times starting with lower #'s or see if I can get lucky and start in the right zone to begin with...

I am tempted to start with a 135 main and 42 pilot and go from there but hoping there is someone out there that has tried all of the theorized combinations and can save me (and others that read this post) some time.

 

Since I don't have any dyno charts to post, all I can go on is 20+ years building, riding and racing VFs and VFRs and finding out what works for me. I tune them so they start properly without excessive warm up period and the "choke" can be dialed out pretty soon, pull clean to redline, give me correct spark plug readings, and don't over heat when pushed. I have essentially stock bikes, lightly modified bikes and track bikes.

I'm sure there are some gains to be made by spending a day and my paycheck on a dyno, but I feel I don't need to.

Remember, "Stage 2" usually implies a modified airbox (not just removing the "snorkel"). We are probably talking about "Stage 1.5" with most of our stuff.

And, it sounds like you still need to verify some things in your carbs. Fuel screws have the orings and passages clear for sure? Diaphram springs in the correct position? (they are different front to back).

Start conservative. Shim your stock needles, don't drill, go up on your pilot and take a stab at a main jet. The only thing you will have to dial in is maybe a couple sizes on the mains.

IMO, the only thing you'll notice going over 128 (10 sizes over stock) is your fuel economy and range going down even further.

 

I must regretfully disagree with Capt 80's on one point only: I think the small washers on the idle mixture screws are too thin to make any noticible difference and that washers of about .010" thickness, possibly even 2 of them, are whacha need to start your experiments.

 

so do we have any idea why the two jetkit packages both recommend 135 mains? I cant seem to get away from so much difference from a 125 to a 135 yet two different vendors have similar recommendations. Is there anyone out there that has tried the recommendation or does everyone stay on the low side for gas mileage? I am assuming that the mains only come into play when the throttle is over half to wide open so for normal driving would the mileage stay about the same?

 

Got some news, Squirrel...

 

What is their recommendation based on? I have stock cams in stock heads, stock airbox, and stock headers with a 4-1 slip on. I chose 122 on my 750 with the slip on because my years of doing this on 500s to 1000s said that would be about right. It was. At least my plugs, temp gage, ears and butt say so. There might be a sweeter spot, but I'm not worried about it. It runs fucking sweet. Not even my 1000R with a full Sudco and modified air box needed 135s.

 

The jet size is the diameter of the orifice in 0.01 mm units, so a 38 pilot is 0.38mm diameter (0.0149").

Calculate the area of the main jets (eg 118 vs 135) to determine how much larger "stage 2" would be versus stock. The volume of fuel flow is directly proportional to the area, so for example, if the area is 10% larger there will be 10% more flow.

Now consider the air intake and exhaust mods--have they increased the air flow by the same calculated percentage as the jet size increase? If the cams have not been changed then i would guess it will be very difficult to make much increase in air flow.

The correct main jet is the one that provides the same % increased fuel flow (volume) to match the increased % air flow (volume) since weight is proportional to volume and the stoich 14.7 air:fuel mixture ratio is based upon weight.



ps. @Squirrel, that's why i use a #14 guitar string to clean out the pilots and verify that they are fully open. #14 Go, #15 NoGo

pps So i calculated the areas and found it was a 31% increase from 118 to 135--that's yuge!

 

All great information. I went back to the Dynojet site and checked to see what they have listed for a stock bike and it roughly equates to about a 120 main and with a slip on or modified exhaust jumps to a 125 so that is in the ball park. Their stage 2 kit must take into account some more serious modifications than they list in the documentation but at least things are starting to add up correctly. There is quite a large jump in the jet sizes for the stage 2 kit. I will start off with a 125 main and see how she runs. I appreciate all the info; I am certainly not challenging anyone just seeking to understand the wide spectrum. It is always nice to get as close as you can on the first try to avoid having too many teardowns.

Thanks again. I will report back once I get the new jets in.

Also, to clarify did we say that I will want about 0.30 of lift on the needle spacers?

 

I think if everything is properly setup, you will be very happy with 42 pilot, 125 main and .020 under the needles. It will run sweet.

In my experience going much over .020 under the needles starts to affect how they float under the spring loaded cap. You can compress the spring all the way and the needle can "bind" causing uneven wear on the needle.

With 42 pilots you will probably only have to have your fuel screws at about 1-1/2 to 1-3/4 turns out from closed, but you can dial that in. Remember to swap your float bowls around so the limiting "tab" doesn't stop you from adjusting the screws where you want. Take a look, you'll see what I mean.

 

perfect, interestingly enough I have a friend that works at Honda and he tried to order some jets for me and was told that they are no longer allowed to get anything but stock jets (some kind of EPA crap again) so I called my local parts shop and they said that there are two different types and I will have to remove the old ones and take them down so they can match them. Does this sound right?

I'm going to pull everything apart again tonight so I can do that but thought it sounded strange...