Jetting the pilot circuit, need some help!

[wilsonishere]

So I have a very modified 2001 Vmax and I am trying to get my carbs right. First when I say modified I mean it has the web cams 80-321's in it, a set of extremely ported and milled heads, vboost is eliminated, a set of Dale Walker 4 into 2 pipes with 6in cans, and individual k&n air filters. Now to the problem I currently have. The bike has a slight occasional back fire at idle and a surge at 70 to 85mph when cruising on the highway. If i give it throttle it will get better but then it goes right back to it once i get into cuising speed. Also if i have been on a wide open run when I let off the throttle the bike will feel lean if I rev it on the way down almost like a non-responsive issue. When I started the build it was way worse, as in lean everywhere. So what i have done to this point, went through main jets until the best top end was reached currently 167.5 mikuni, set the float levels and got them good, moved the needles out until I lost some response and the idle would dip on throttle release then put them back in one notch seams good now. So on this idle circuit design i know there is a pilot fuel jet and two pilot air jet and i am trying to figure out if changing out just the fuel pilot jet would be best course of action or if messing with the air control jets would be. The current air jets are a set for the stage 7 dynajet kit and my pilot screws are at 4 and 3/4 turns out any more out does nothing and the screws start to feel loose. I was told that if the screws were over 3 out that I was in wide open or unable to control territory is this true any advice. Please help.

 

[Vmaxnl]

Do you use the airbox?


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[wilsonishere]

No there is no air box.

 

[blaxmax]

What did you do with the 4 overflow hoses that go from the carbs to the airbox?
A few guys have had the same surging problem at around 55-65 rpm. I ran mine together along the frame open end facing towards the front of the bike. The same routing as if there were an airbox and in the same general direction of where they were with the airbox on.
Also I went up one pilot jet size at a time until I matched the main jet now 157.5 to now 42.5

 

[wilsonishere]

I have them all ran down to the bottom of the engine currently. So they are supposed to be under pressure then? Everything eles I have ever had was ran down to the bottom the way I have it. Guess I can give that a shot though.

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[blaxmax]

I have them all ran down to the bottom of the engine currently. So they are supposed to be under pressure then? Everything eles I have ever had was ran down to the bottom the way I have it. Guess I can give that a shot though.

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I did the same thing running them to the bottom as they are overflow hoses but later found out from a friend that they need to be up top to stop that surge. Close to where they were mounted to the airbox should do it.

 

[tugla]

+1 I just tied mine up to where the coils go so they are up high...
Start there and test before doing more set up changes.
What needles are you running?

 

[blaxmax]

So I have a very modified 2001 Vmax and I am trying to get my carbs right. First when I say modified I mean it has the web cams 80-321's in it, a set of extremely ported and milled heads, vboost is eliminated, a set of Dale Walker 4 into 2 pipes with 6in cans, and individual k&n air filters. Now to the problem I currently have. The bike has a slight occasional back fire at idle and a surge at 70 to 85mph when cruising on the highway. If i give it throttle it will get better but then it goes right back to it once i get into cuising speed. Also if i have been on a wide open run when I let off the throttle the bike will feel lean if I rev it on the way down almost like a non-responsive issue. When I started the build it was way worse, as in lean everywhere. So what i have done to this point, went through main jets until the best top end was reached currently 167.5 mikuni, set the float levels and got them good, moved the needles out until I lost some response and the idle would dip on throttle release then put them back in one notch seams good now. So on this idle circuit design i know there is a pilot fuel jet and two pilot air jet and i am trying to figure out if changing out just the fuel pilot jet would be best course of action or if messing with the air control jets would be. The current air jets are a set for the stage 7 dynajet kit and my pilot screws are at 4 and 3/4 turns out any more out does nothing and the screws start to feel loose. I was told that if the screws were over 3 out that I was in wide open or unable to control territory is this true any advice. Please help.

I guessing stage 7 needles with the vboost out and individual k&n's. If you have run your a/f mixture screws out too far and still have a lean pop you need to go up to the next size pilot air jet.
Read through this-------as you are about right here now----

If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.

Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.

http://www.vmaxforum.net/showthread.php?t=3809

 

[DMAN999]

I did the same thing running them to the bottom as they are overflow hoses but later found out from a friend that they need to be up top to stop that surge. Close to where they were mounted to the airbox should do it.

Excellent point.

 

[MaxMidnight]

As I see it, you are trying to best guess what the problem is without any hard data.
I guess it depends on how much patience and what price you put on your time which will determine how you address this.

The advice regarding the float chamber vent hoses is good but as you have altered cam, inlet and exhaust ports and exhaust I doubt that there will be an off the shelf fix.

You don't say if you have done a plug chop at the revs where the problems occur. That would give you some indication as to what the fueling is dong.

However, as that takes time and time is money then (IMHO) it would be far better and probably cheaper in the long run to get a dyno base line reading.

You are then able to make future jet changes on factual data and a good dyno man should be able to offer advice on the most appropriate jets to try.

A second run would then allow you to fine tune.

 

[wilsonishere]

I moved the fuel vent hoses and that helped but did not solve the issue thanks for the advice. I love this old bike so my time spent working on it is not an issue, lol This bike was on an dyno about 6 years ago, i have done work since then, i will get it back on one soon.

 

[blaxmax]

I moved the fuel vent hoses and that helped but did not solve the issue thanks for the advice. I love this old bike so my time spent working on it is not an issue, lol This bike was on an dyno about 6 years ago, i have done work since then, i will get it back on one soon.

You will get surges from being too lean also. Outside of going to the internals of the carbs try opening your a/f mixture screws.

 

[wilsonishere]

For everyone the replied thanks for the info. I ended up putting 42 idle circuit jets in from the 37.5 stock and I was able to get the idle circuit screws adjusted down to around 2 turns out from the 4 and 3/4 turns I had to have them at before. The stock jets were just not enough with the modified heads cams intake manifolds air fillers and exhaust set up I am running. The increase of fuel over the whole range from that move aloud me to change the 167.5 mains to 165 and the bike runs great!! No more surge at all and snappy throttle all through the power band! She breaks the wheel loose easy in 2nd and 3rd gear now and pulls hard all the way to the rev limiter! If any one know a easier way than using rpvc block off caps in place of the v boost eliminator tubes for synchronizing the carbs i would love to know. These thing don't last more than two synchronizings before the fuel and heat screw them up.

 

[one2dmax]

Put the vboost butterfly back in and use that to sync. Just unplug it in the open position or run a manual cable to open/close it.