Triple Mikuni Help

[madkaw]

Thanks for correcting. This carb tuning is tough enough to figure out--LOL.

It's pretty cool Dave could take the time to personally get your car running right. It's also good to know you can turn down the venturies since the larger ones seem hard to come by. So did you do the cam timing change all at the same time or did that after changing the venturie.

[ryant67]

So under what circumstances would you benefit from switching venturi size, and what does it directly affect?  The mikuni manual didn't really mention much about their function.  

Just asking as I pulled and "refreshed" my 44's today, and switched from 34 venturis to 37's while I was at it.  Haven't had a chance to play around with things too much yet, hopefully tomorrow.  

[cnwayland]

So did you do the cam timing change all at the same time or did that after changing the venturie.

@madkaw - based on how I had described my current driving feedback, Rebello - as with what I've always heard from l-series advice - started at the valve lash adjust, followed by moving the cam chain sprocket from the 2nd to 3rd timing mark before ever touching the carburetion.  I still haven't found a good explanation as to exactly how much each timing mark retards/advances timing but moving from 2 to 3 definitely brought where I 'felt' the cam 'come on' so-to-speak lower in the RPM range by 500-1000.  When I was building up the motor I put the new chain on the number 2 mark because to me #1 felt extra loose, and #3 way too tight on the chain.  Again, I'm still looking for a better explanation on the cam sprocket settings for future reference.  Anyway, most of us know to start with valve adjust and cam timing/distributor timing check before adjusting our carburetors.

 

The venturi discussion happened after I was asking about running the 40s with NO venturi.  Apparently an older Mikuni PHH model has a slight shelf in each chamber which would allow you run them without venturi, but mine (R Type - later model) do not - it's a smooth chamber.  So the largest the 32mm venturi can safely be lathed is up to 35mm.

 

In other words, PHH 40s are slightly under-carbureted for an L28 with a high compression ratio like mine.

 

So under what circumstances would you benefit from switching venturi size, and what does it directly affect?  The mikuni manual didn't really mention much about their function.  

 

Ryant67, I would say that also based on Leon's post, you're changing the amount of vacuum or draw achievable by the motor ~ maybe this is obvious but wasn't to me.  The larger amounts of air you're able to let it will allow the motor to run more power at high end because you aren't restricting airflow as much.  But alternately, like with a pure race motor, low RPM idling will be more difficult because of the lower vacuum draw and less friendly on the day to day street condition.

Edited May 3, 2015 by cnwayland

[madkaw]

Looking for way to preset my Mikuni's before installing I came up with this. My wife got excited when she saw me dragging out the Oreck, but I let her down as I carried thru the house and into the garage.

I thought the ole Oreck would supply a very stable SUCK on the carb that I could set all the carbs too. At the same time I could eliminate any possibilities of a bent throttle plate if each carb barrel pulled the same amount of air. I used my Unisync to measure and set all the carbs at the same level according to the unisync. I never liked using the feeler gauge on the idle scews because it was hard to see and feel.

 

I also didn't like the way my Mikuni intake lined up with the head ports. Looking at the gaskets you can see that the ports aren't centered . Without eliminating the step for anti-reversion control, I opened the holes enough to make them centered with the head ports. Be careful to check your gasket centers on the head . I have 2 felpro's and they line up differently. 

[Cbb]

Never would I have thought of using a vacuum to check for a bent throttle shaft/knicked throttle plate(s). Good thinking! Now I can steal this trick for any future Sidedrafts that I come in contact with

Out of genuine curiosity, how did you open up the gasket ports?

[madkaw]

1" drum sander in a cordless. 80 grit drum sander will do everything fast. Used it to notch my cylinder bores too. Very manageable and hardly a chance of misfires, if you know what I mean.

[madkaw]

1" drum sander in a cordless. 80 grit drum sander will do everything fast. Used it to notch my cylinder bores too. Very manageable and hardly a chance of misfires, if you know what I mean.

 

As far as Oreck procedure. My buddy stared at those throttle plates and poked and prodded them and I was sure that some of them were off by a little. With the Oreck they showed all the same. I couldn't come up with another idea to get a good measure on the opening. Feeler gauges don't work because of the size and shape of the area. Visual perceptions are just that.  

[Tony D]

You need tapered feeler gauges, not the valve setting 1/2" wide flat variety.

 

We have the same issue with guys taking radial clearances on Impeller Tips.

 

There are actually feeler gauges you can get that are long and about 1/4" wide, and they work great for setting that...alternately since it's a 0.003" starting point generally recommended you can buy a spare individual feeler gauge blade and cut it down with a good scissors to an appropriate width.

 

One important thing to remember is after setting all barrels equally using a vacuum flow bench setup (or feeler gauge) that they are moved equally afterwards when setting idle speed.

 

Especially with a physical flow bench setup, if you have variation cylinder-to-cylinder it's in the engine!

 

Valve adjustment, rings, stuck,valve, worn cam lobe, plugged exhaust port.... The flow reading you see is related to the flow generated by the cylinder. If you have a standard source and have set them to that standard...variation is coming from the engine cylinder and must be investigated further!

 

It's usually a valve adjustment or cam / rocker arm / lash pad issue. That's why the tune up instructions from Nissan always start with a valve clearance adjustment. Yes, it makes that much of a difference!

Edited May 17, 2015 by Tony D

[madkaw]

I agree to all that Tony,  the Oreck method(for me) almost seems dumb proof for getting everything set to the same opening in the beginning. Then once on the car, it's a measured turn on the screw for each carb. I only opened them enough to get a reading and set a standard for all of them.

[ryant67]

Ryant67, I would say that also based on Leon's post, you're changing the amount of vacuum or draw achievable by the motor ~ maybe this is obvious but wasn't to me.  The larger amounts of air you're able to let it will allow the motor to run more power at high end because you aren't restricting airflow as much.  But alternately, like with a pure race motor, low RPM idling will be more difficult because of the lower vacuum draw and less friendly on the day to day street condition.

 

 

Cheers mate!  That makes perfect sense - I somehow missed Leon's post on my read through the thread, d'oh!

[ryant67]

I've been trawling through Japanese websites about S30s lately, and surprise surprise, I came across people over there struggling with the old pilot to main jet stumble.  From what I gather, they blame the OA style jet blocks and replace them with different ones.  May I present to you the "Wako SPL Jet Block" from AS Watanabe!  

Pics stolen from this link:
http://yosyos227.jugem.jp/?eid=341

Looks interesting.   

http://www.as-watanabe.com/parts4.htm

5000 yen is about $40 USD each.  

[Cbb]

I've been trawling through Japanese websites about S30s lately, and surprise surprise, I came across people over there struggling with the old pilot to main jet stumble.  From what I gather, they blame the OA style jet blocks and replace them with different ones.  May I present to you the "Wako SPL Jet Block" from AS Watanabe!  

 

Pics stolen from this link:

http://yosyos227.jugem.jp/?eid=341

 

Looks interesting.   

 

http://www.as-watanabe.com/parts4.htm

 

5000 yen is about $40 USD each.  

 

Good find! They look similar to Weber Emulsion tubes.

 

I also learned recently that there is another type of Mikuni/Solex jet block, Type 8. They have interchangeable bleed pipes inside the jetblock to change the characteristics of fuel atomization(Quoted from my PHH service manual. Swapping bleed pipes and modifying the holes is similar to swapping Weber E-Tubes.)

Edited May 19, 2015 by Cbb

[madkaw]

Post 108

[ryant67]

Bored while waiting for other engine things to fall into place, so I figured I would give my Mikunis a good look over as they weren't perfectly balanced when I put them back on the engine after rebuilding them.

Enter "The Madkaw Method"!  

I grabbed some dense closed cell foam and made a few gaskets to fit over the end of my vacuum hose, and they seem to create a good tight seal - measurements with the gauge are repeatable with pretty much no discernable differences in readings.  This actually works pretty nicely, since you can adjust the throttle stop screw to get a higher or lower airflow reading, and see if it's balanced through other amounts of flow.  

The first two carbs were perfectly balanced on both sides, but the third was a full kg/h off on one side @ 5kg/h.  Checked the seals and all were good.  So figured it's probably a twisted shaft.  Pliers on both ends of the throttle shaft, a gentle twist, check, nope... twist again, check, and we're all good!  

The real test will be once they are back on the car, but if it works it will be pretty impressive.  MUCH easier to play around with these things when they are not mounted on the car - thanks Madkaw!

 

[madkaw]

Might add to the Oreck method that make sure you start with a clean bag in your vacuum. My reading the second time around were not stable. I thought I should check the bag and sure enough it was plugged full. New empty clean bag cleared that up.

Feeling pretty good about the carbs I moved on to changing up the vacuum log. I shortened things up so I could keep the vacuum hoses from running all over the engine bay(it seemed). this will look some cleaner when all together!!

[cnwayland]

Madkaw,

 

Those are tapped for your brake booster vacuum and what else? 

[madkaw]

Pcv and for my Map sensor

[madkaw]

My Mikunis are giving me a bit of a fit lately. Strange issue of the car dying or wanting to die at idle. The engine acts like it is running out of fuel. I can watch the AFR gauge slowly but surely go way lean as the engine starts to idle rougher and eventually it will die. It is kind of arbitrary when it happens, at other times it idles okay. In the past I have always been able to have a good idle with these carbs.

I installed a fuel pressure gauge so I can drive and watch, because I am still dealing with transition. I suspected it has been a fuel supply issue. I have cleaned my tank, replaced fuel pump, bypassed my regulator, replaced fuel lines, replaced fuel filters. The pilot adjustment(idle) screws are pretty new and I can not find any issues with them. The carbs have been off and cleaned several times. The float adjustment has been checked and adjusted per Mikuni manual. I examined the needle/seat assembly and they look new, which they are pretty much since I bought them. Checked sync of carbs.

I have watched the engine die and the pressure gauge shows 3.5 psi , so I think the supply is okay to that point.   

So would an issue with one cylinder kill the whole motor? If I had an issue with an idle jet with just one hole do this, or more likely a whole carb? I have the original needle/seat assemblies which I think I will swap back in to see if that makes a difference.

I DO know that I am  missing the accelerator check valve balls, which I got today(via Amazon, pack of 100-anyone need some-LOL) They got lost during one of the carb cleaning processes here lately.

 

Float adjustment. So I have always done the inverted lid and measuring distance between top of float and lid at 12.5mm. The manual also gives you a height for the fuel in the bowl-24mm. It's not very user friendly since the measurement is from the center line of the butterflies upward, so you have to do some math to a get a number to measure from the top of the carb. When I did the math and measures the fuel height was off by 4-5mm-WOW. Can that even be right?  I like this method better because that's actually what's in the bowl. I went with it and increased the fuel height and the car ran alright and seemed better in transition, but nothing to do

with idle, still dies and idles rough. Curious if anyone else has gone down this route with fuel height adjustment ? 

 

I'll take some pics and get back with everyone

[madkaw]

Still plugging away here with carb tuning. Swapping around the wideband to the other bank of exhaust ( advantage of duals), from the back to the front cylinders I noticed the front carbs were starving. The odd thing is that I have to run the mixture screws 1/2 turn richer to get a smooth idle. So I suspect air leak, but can't pin anything down. I have replaced all O-rings ( 50mm X 2.5mm is what I found) and not much difference. Sprayed down everything with starter fluid with no response.

now I can live with this because I can tune it in and the motor runs really well-- -EXCEPT for #6.  By looking at the plugs 6 is sputtering rich with small black spots diluting the tan underneath. I can use a hose and listen inside the carb inlet and hear the sputtering occurring. I suspect it's the accelerator pump leaking fuel by the packing, though I have changed it once already. With a mirror I can see nothing happening on my side of the butterfly. Trying to come up with an idea to seal off the accel chamber to verify this issue. I've looked down the accel pump hole and can't see any obstruction to keep packing from sealing.

Anyone ever try an Oring to seal the pump? I tried tightening the cap as much as I care too, since I know this isn't the way it was made to seal. 

[madkaw]

I am changing my suspicion of the sputtering to the pilot mixture . I can dial in the mixture screw until the sputter goes away, but then it's about to lean to run. Not sure what's going on so I guess I will yank the carb off and look inside the mixture hole.

Any ideas?