Help me tune my Mikuni!
06-06-07, 08:06 AM
#26
Brap, Brap, Ole!
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The vacuum advance is fine left undone. I have never run vacuum advance on a Mikuni set up ever.
What trochoid is trying to point out in the FPR is that you are regulating pressure then you have a filter inline afterwards which may drop pressure again. You want it to drop right before it enters the carb.
Fuel pressure was around 4.5 PSI if I remember correctly. But the gauge that was on their was a cheap one so I can't confirm if that was the actual pressure.
If the only issue is surging when trying to stay at a steady cruise speed. Then the issue is jetting. As I have stated in the past. It takes experimentation to dial in an aftermarket carb for your specific driving habits.
If you are having an issue when you go WOT and leave it there and it falls on it's face between shifts. That is a fuel supply issue not a jetting issue. The most common things there are. 1. Kinked supply hose from the pump.
2. Low voltage supply at the pump or poor grounding there.
The carb was in perfect working order. Float level set correctly. Same set up as you have now. Even same type fuel pump.
The engine made excellent power and no driveability issues with the jetting that is in there now. This was at near sea level though.
What trochoid is trying to point out in the FPR is that you are regulating pressure then you have a filter inline afterwards which may drop pressure again. You want it to drop right before it enters the carb.
Fuel pressure was around 4.5 PSI if I remember correctly. But the gauge that was on their was a cheap one so I can't confirm if that was the actual pressure.
If the only issue is surging when trying to stay at a steady cruise speed. Then the issue is jetting. As I have stated in the past. It takes experimentation to dial in an aftermarket carb for your specific driving habits.
If you are having an issue when you go WOT and leave it there and it falls on it's face between shifts. That is a fuel supply issue not a jetting issue. The most common things there are. 1. Kinked supply hose from the pump.
2. Low voltage supply at the pump or poor grounding there.
The carb was in perfect working order. Float level set correctly. Same set up as you have now. Even same type fuel pump.
The engine made excellent power and no driveability issues with the jetting that is in there now. This was at near sea level though.
So then, i will change the location of the fuel filter, that is no problem, but since i will be ordering some new jets; can you guys make a list of all the jets i should order? I have ordered parts from Todd at Wolf Creek before and he is great, but it takes a few days to get them. So if i had a complete list i think i could get this thing dialed in just right.
Also, should i order up the smalle FPR from summit too?
Again, thanks a million guys.
06-06-07, 04:11 PM
#27
Brap, Brap, Ole!
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Put the new RB muffler on today, and all i can say is wow, smooth sound and really sounds like a rotary again, i love it!
then i
Changed the location of the filter today and that seemed to stabilize the readings on my gauge a bit.
so,
I dialed in the fuel pressure to 3.5psi and took her for a spin, shifting through gears the problems seem not as bad (not falling on its face as much).
But the problems with around town cruising are still there, but not as bad. Maybe its not that bad at all??? Is there going to be a bit of a compromise in driveability? I mean its not like bucking or anything it just seems to sputter a little bit when cruising at a moderate speed (like 25 3rd gear 3k rpm or 55 5th gear 3k rpm). Am i just trying to be supper picky???
Maybe different idle jets? One size down? Should i try that?
then i
Changed the location of the filter today and that seemed to stabilize the readings on my gauge a bit.
so,
I dialed in the fuel pressure to 3.5psi and took her for a spin, shifting through gears the problems seem not as bad (not falling on its face as much).
But the problems with around town cruising are still there, but not as bad. Maybe its not that bad at all??? Is there going to be a bit of a compromise in driveability? I mean its not like bucking or anything it just seems to sputter a little bit when cruising at a moderate speed (like 25 3rd gear 3k rpm or 55 5th gear 3k rpm). Am i just trying to be supper picky???
Maybe different idle jets? One size down? Should i try that?
06-06-07, 04:47 PM
#28
That is how a Mikuni can be. The transistion stage from light to medium throttle is touchy. That's why it's a matter of getting adjusted to it. And after while if you don't like the way it feels then you start experimenting with the jetting for your style of driving.
06-06-07, 10:46 PM
#31
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wow doc, I thought you would have tried the vac hooked up and not, just in case and be able to note the differance if any. I always hooked them up just in case there might be untill I started using forced air. how do you feel about the dead head fuel set up vs the return set up?
06-17-07, 10:18 AM
#32
Brap, Brap, Ole!
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Well, i thought i would give you all an update on the mikuni.
Turned out that getting the fuel pressure just right with a little trial and error and relocating the second filter to before the regulator made all of the difference!
Runs awesome now.
But...
My idle does not seem very smooth at times, is this normal? I have the idle set at around 1200-1300 rpm's and the rpm's on the gauge seem to stay in one place yet it just sounds like its going up and down (a little) when you listen to the exhaust note...
??? Normal or not?
But again, thanks everybody!
Turned out that getting the fuel pressure just right with a little trial and error and relocating the second filter to before the regulator made all of the difference!
Runs awesome now.
But...
My idle does not seem very smooth at times, is this normal? I have the idle set at around 1200-1300 rpm's and the rpm's on the gauge seem to stay in one place yet it just sounds like its going up and down (a little) when you listen to the exhaust note...
??? Normal or not?
But again, thanks everybody!
06-17-07, 11:27 AM
#33
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Sorry I haven't read every one of the posts but make sure your ignition timing is advanced with this setup. I have a weber dcoe (very similar) and I found that I needed bigger fuel jets than air correctors, otherwise I would lean out at high rpms. Right now I am running 190 fuel and 165 airs on the main circuit with 40 venturis. I know its running a bit rich, but it's close to being right on. Also you may want to experiment with venturi size to move your power band around a bit, if you'd like. With smaller venturis you have more low end power but not so much up top and with bigger or no venturis you will have power up top but not as much down low. Note that no venturis basically makes it a true 44 carb but will yield a rougher idle and you will stumble a bit between shifts, but the top end pull is simply amazing! Not very streetable IMO but damn was it fun to feel the power from 5k up. I was up to nearly 100 in 3rd before I knew it! I recently bought a wideband and will be installing it soon to get really dialed in and will let you know what jets I settle in with. Also I am running the low pressure holley regulator at 3 psi. I think you need to switch yours. Low pressure and high flow is what these carbs are about.
As far as the idle smoothness, again make sure your timing is advanced, that will bump up the rpms a bit and then you have to adjust the idle and mixture screws accordingly (I'm assuming you have that adjustability with the Mikuni). Here is a link to lean best idle tuning procedure on the weber, it should help:http://www.redlineweber.com/html/Tec..._typical_i.htm
Nice engine bay btw! I really like the blue hoses.
As far as the idle smoothness, again make sure your timing is advanced, that will bump up the rpms a bit and then you have to adjust the idle and mixture screws accordingly (I'm assuming you have that adjustability with the Mikuni). Here is a link to lean best idle tuning procedure on the weber, it should help:http://www.redlineweber.com/html/Tec..._typical_i.htm
Nice engine bay btw! I really like the blue hoses.
Last edited by djjjr42; 06-17-07 at 11:36 AM.
06-17-07, 04:13 PM
#34
I didn't look to see what porting you have, but I've been able to get my 12A 1/2bp, with the Mikuni PHH 44 to idle as low as 800 rpm. And they say it can't be done, lol. As far as the difference between dead heading the fpr or using a return style, that depends on the size of your fuel pump more than anything else. The Holley 110 gph pump works better and lasts longer with a return style fpr, the 72 gph and smaller is fine dead headed.
On the Mikunis, 0-20% throttle is run off of the pilot system, which is where most of the steady state cruising is, except perhaps highway speeds with a head wind or going up grades.
I have 2 different Mikunis, one was set up for a 12A, the other came off of an S4 13B. They were bought used off eBay so I have no idea if either engine was ported. The pilot jets were 52.5 and 57.5 respectively. If you're surging when cruising on the idle circuit, your jets may be too big. This is where buying/borrowing a wideband pays off as sometimes the rich/lean conditions will exhibit the same symptoms.
On the Mikunis, 0-20% throttle is run off of the pilot system, which is where most of the steady state cruising is, except perhaps highway speeds with a head wind or going up grades.
I have 2 different Mikunis, one was set up for a 12A, the other came off of an S4 13B. They were bought used off eBay so I have no idea if either engine was ported. The pilot jets were 52.5 and 57.5 respectively. If you're surging when cruising on the idle circuit, your jets may be too big. This is where buying/borrowing a wideband pays off as sometimes the rich/lean conditions will exhibit the same symptoms.
08-03-10, 01:38 AM
#35
The vacuum advance is fine left undone. I have never run vacuum advance on a Mikuni set up ever.
What trochoid is trying to point out in the FPR is that you are regulating pressure then you have a filter inline afterwards which may drop pressure again. You want it to drop right before it enters the carb.
Fuel pressure was around 4.5 PSI if I remember correctly. But the gauge that was on their was a cheap one so I can't confirm if that was the actual pressure.
If the only issue is surging when trying to stay at a steady cruise speed. Then the issue is jetting. As I have stated in the past. It takes experimentation to dial in an aftermarket carb for your specific driving habits.
If you are having an issue when you go WOT and leave it there and it falls on it's face between shifts. That is a fuel supply issue not a jetting issue. The most common things there are. 1. Kinked supply hose from the pump.
2. Low voltage supply at the pump or poor grounding there.
The carb was in perfect working order. Float level set correctly. Same set up as you have now. Even same type fuel pump.
The engine made excellent power and no driveability issues with the jetting that is in there now. This was at near sea level though.
What trochoid is trying to point out in the FPR is that you are regulating pressure then you have a filter inline afterwards which may drop pressure again. You want it to drop right before it enters the carb.
Fuel pressure was around 4.5 PSI if I remember correctly. But the gauge that was on their was a cheap one so I can't confirm if that was the actual pressure.
If the only issue is surging when trying to stay at a steady cruise speed. Then the issue is jetting. As I have stated in the past. It takes experimentation to dial in an aftermarket carb for your specific driving habits.
If you are having an issue when you go WOT and leave it there and it falls on it's face between shifts. That is a fuel supply issue not a jetting issue. The most common things there are. 1. Kinked supply hose from the pump.
2. Low voltage supply at the pump or poor grounding there.
The carb was in perfect working order. Float level set correctly. Same set up as you have now. Even same type fuel pump.
The engine made excellent power and no driveability issues with the jetting that is in there now. This was at near sea level though.
I hate to bump and old thread but this one is a goodie, I just installed a mikuni 44phh carb on my s5 streetport and I'm having a "similar" issue, it was already jetted for a streetported 13b but which year and size port? I dunno. I followed pineapple racing large 6pt streetport.
anyways it holds power fine between shifts.
I have a wideband
leans out and falls on its face when you give it heavy load or near wot
the problem isnt I can't really drive it since it only has about 10 miles on the break in so im not driving above 4krpm.
cruising speed and low/medium throttle driving is excellent.
any thoughts?
holley electric external pump
holley 1-4psi regulator
dizzy ignition, timing dead on
RB header/street dual exhaust
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