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ViragoTechForum.com Power Adder KJS Mikuni HS40 can you help ? (SOLVED)

#1:  KJS Mikuni HS40 can you help ? (SOLVED) Author: Jim Davis Post Posted: Fri May 14, 2010 3:16 pm

This thread (Need single carb help) is a life-saver for me. I have a '96 xv750 with the KJS Mikuni HS40 kit installed. After three manifold installations it's ridable, but I must still have a small vacuum leak--everything I'm reading here confirms it. I also have calculated that I'm only getting 26 miles/gal., instead of the 46/gal. that the stock setup supposedly delivers. Any thoughts about where all that gas is going? I don't smell it.

Last edited by Jim Davis on Tue Sep 07, 2010 5:15 am; edited 1 time in total

#2:   Author: 1986 XV700 Post Posted: Fri May 14, 2010 6:03 pm

Hey Jim, the mods like for us to start a new post when we start asking questions about another bike, it helps to avoid confusion about which bike we're talking about. When you make the new post let us know which pilot jet you've got in the carb, seems like he used to put a 20 in them but was considering going to a 17.5 or vice versa also the pilot air jet size. Let us know where your problem is in the throttle range, sounds like its just above idle since you mentioned a similarity to the original post. The cause of your bad mileage could be due to riding at wide open throttle to adjust the needle, or in my case it seems that I ride the bike harder now with the KJS setup.

#3:  Re: KJS Mikuni HS40 can you help ? Author: Jake Post Posted: Sat May 15, 2010 7:11 pm

Welcome to VT Jim.

I split your post off from the thread you posted it and edited your post by adding a link to the OP.

Please read the links here Members Announcements and be sure to add your signature.

#4:  Re: KJS Mikuni HS40 can you help ? Author: eaglebeak Post Posted: Sat May 15, 2010 11:07 pm

I rode my '86 XV-700 brand new off the showroom floor and never got more than 42mpg on its best day. Perhaps they did a lot of improving during the next 10 years if the 750 was supposed to get 46mpg?

After installing the KJS manifold and Mikuni HS-40, I typically average 38 to 40mpg riding solo and depending on terrain and/or headwind; but the bit lower gas mileage is primarily due to a full mechanical carb with an accelerator pump where the OEM duals were strictly vacuum operated with no accelerator pump(s). The great increase gained in hard/quick acceleration from virtually any speed comes at a price of a bit lower gas mileage and the loss of a few MPH off the top end.

However, your 26mpg is most definitely an indication of a problem somewhere, but may not necessarily be a carb or manifold problem. Checking for a possible vacuum leak is very easy by using a can of starting spray. Do the test outside in a well ventilated area and only do it on a warm (not hot) engine that's warmed up only enough to idle without any choke. Shoot little squirts of starting spray around the manifold connections to the engine heads and around the carb to manifold boot (taking care not to spray toward the carb's air intake throat). Any change you hear in idle speed or engine sound will indicate you've just sprayed a vacuum leak.

If the leak test shows no leaks, then it's entirely possible the carb is outfitted with too large a main jet or the metering pin isn't adjusted at the correct height. If the engine seems to run hard and good at full throttle with no bogging, misfiring, or evidence of dark exhaust, the main jet is probably OK. If there's a lot of hesitation or "dead spots" when going from 1/4 throttle to mid-throttle and/or sluggish response in the mid to 3/4 throttle range, then you may need to adjust the metering pin to the next lower position.

If the carb seems to perform well through all throttle and acceleration ranges, then you may have an ignition problem or dirty/restricted air cleaner element. Pull your spark plugs and "read" them to see if they are black and sooty looking to indicate a rich condition, then let us know what you find.

#5:  Re: KJS Mikuni HS40 can you help ? Author: monty Post Posted: Mon May 17, 2010 11:57 am

I have been trying to get all of the bugs worked out of my KJS conversion and here is what I have done so far. Mine is a 93 750 so it should be similar.

Pilot Jet 22.5 - It's big I need to install a 20
Pilot Air Jet .9
Needle Clipped 4th Notch with a shim
Main 160

I commute about 60 miles a day and have been riding it on and off since March. Mine will still cough on a cold morning like 34F and at the end of the day it's about 60-65F. I will average about 36 mpg. Now that it's getting warmer like 46-48F in the morning and at end of the day it's about 75F. I'll be at 40-42 mpg. I think that I installed my manifold 4 or 5 times before I finally got it right.

#6:  Re: KJS Mikuni HS40 can you help ? Author: monty Post Posted: Tue May 18, 2010 12:48 pm

Here is what I did to seal my manifold. First clean the intake flanges on the head and manifold to remove any old silicone. I plugged the intake ports with shop towels. Then I took sand paper and cleaned the flanges. Then cleaned them with brake cleaner to wash off the residue. After that was dry then take the gaskets put a thin layer of black rtv silicone on 1 side and a thin layer on the intake port flange on the engine. Carefully put the siliconed surfaces together. Take your old carb boots and bolt them back on to hold the gaskets in place. Now go take a break for 10 to 20 minutes. When you come back take off the carb boots and the gaskets will stay there where you want them to. Then once again put a layer of on the gaskets and on the KJS intake manifold flanges. Then take another break for about 20 minutes. Now you can bolt on the intake manifold. The best way that I found to not mess up the silicone was first put in the bottom bolt on th rear cylinder then rotate the manifold down to where I could put in the top bolt of the front cylinder. Follow the instructions that Ken sent for tightening the manifold into place and you should be good.

I hope this helps.

#7:   Author: Jim Davis Post Posted: Tue Sep 07, 2010 5:14 am

I just discovered, several months late, that this thread had been created. I really appreciate the replies and the excellent info I received. Thanks to everyone. I'm gonna mark this thread as solved.

Thanks again.

Jim D.

#8:   Author: 48vintage Post Posted: Tue Sep 07, 2010 5:20 am

Jim . . I edited your "SOLVED" to the way the powers have asked it to be marked.
ViragoTechForum.com Power Adder KJS Mikuni HS40 can you help ? (SOLVED)


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