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Mikuni PHH 40 jetting and idle speed


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My 69 510 is running a stroked and bored L16 with a ported and polished SSS head, a mild street cam, and a set of side draft Mikuni 40's on a short Mikuni manifold. Displacement is now about 1800cc.  The car has a header, and a nice exhaust with a bung for an O2 sensor. I have installed a "Dizzy-Matchbox" electronic ignition with the matching high energy coil, and the timing is set at 10 deg BTDC. I have been working through this car for the last month getting it back on the road, it had sat for almost a decade in a garage when a friend located it.


Just before it was parked some less than wise person tried monkeying with the jetting to get the car running on E85 type fuel and that is where it stayed until I towed it home.


Here are my cars cam specs:

Cam (have specs, manufacturer unknown) similar to a SSS cam or a Comp Cams 280S-10, .450” lift, 274 duration, IO-31 BTDC, IC-63 ATDC, EO-67 BTDC, EC-27 ATDC (222 duration, IO-5 BTDC, IC-36 ATDC, EO-30 BTDC, EC-1 ATDC, @ 0.050” lift) Valve lash is 0.008”, Exhaust 0.010” cold. Good to 7000 RPM


My "problem" is that I can't get the car to idle lower than 1,100 RPM. It's not that it doesn't idle, it idles fantastic, always returns to 1,100 RPM +100 RPM never lower. Today I went back through and made sure all of the mechanical adjustments for the carbs were sound.  synced the carbs, ran the idle speed adjustment screws all the way off the throttle plates, then brought them back to just touching, then re-synced the carbs. I reset all of the idle mixture screws, made sure the linkage arms are the same length, made sure the throttle linkage is pulling the linkage arms against the throttle shaft arm holding it against the idle speed screws, etc etc. No luck, car still idles solidly at 1,100 rpm.


My Innovate LM-2 digital wideband O2 exhaust reader says the car is running lean at idle. it registers a 15.7 to 16.0 instead of the desired 14.0 to 14.6. I'm beginning to think I might need to go up a pilot jet size (currently 62.5's and mixtures at 2.5 turns out) in order to get the idle richer and drop the idle speed. I sprayed the intake down with carb spray today and there is no obvious air leak.



1) Has anyone else out there in Ratsun land worked through a similar situation? Did you successfully resolve it and now have a car or truck that idles solidly in the 8-900 RPM range?


2) I have read on other Z-car oriented forums that the Mikuni PHH carbs should be jetted so that the pilot circuit (idle to low midrange) functions properly when the mixture screws are set at 1.5 turns out. It seems that the general opinion is that you jet for that screw setting, not use the screws to adjust the mixtures to work with what you have. I believe my pilot jets are on the small side, so am considering going up a size to correct the air fuel ratio and am curious if it will drop the idle some also?


3) I know some cams require a higher idle, but I would think that a mild street cam would idle down nicely?


4) The car doesn't ping or knock, so the timing set at 10 BTDC should be close to on the money, the vacuum advance and the mechanical advance are both fully functional. Anyone know what max advance should be so I can try setting the car's timing at higher RPM and letting the idle timing settle from there?

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Hi Metzman510, 

I can attest to your frustration and wondered why the idle on my set up was high too.


’ran the idle speedadjustment screws all the way off the throttle plates, then brought them back to just touching’


‘Just touching‘ is a mistake most people often think is the correct staring point, maybe for most other carb set ups. However with Mikunis, Webers, Solex or Dorlettos, the method for a lower idle speed is to turn the idle adjustment screws another half turn after contacting the throttle lever. 
Then synchronize each so they all run the same. If you can’t, then jetting is next.


The right order is... get the timing correct, then idle, then sync and then jetting. 
Have you taken a compression test yet?

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Compression tests?????? What that is going to kc?


you should ran about 12/15 degrees on the timing.


 I never had problems with idle on my Mikuni son idle so I been lucky.just kc all the jets are right I think my main jet is 130. Be honest I ran it on a stock  L18 and a cammed L 16 and was no difference as you 450 cam is minor .


however  I was never good with the idle mixture settings.

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The SSS cam is almost identical to the L20B cam with 0.413" lift and 248 duration* (it's not known if this measurement is exact or taken at 0.050" lift like most cam manufacturers do it.)


Seems to me that if you can't lower the idle below 1,100 then air is getting past the throttle plates (or in from somewhere) regardless of the mixture. If you correct the mixture, closer to perfect, I can only assume that the idle will go up, not down. The opening and closing of the throttle plate is what controls an engine's speed.


Disconnect the vacuum advance and rev above 3K. The initial advance (100) and mechanical should add to around 33-340 on an L series.

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There are 3 ignition advances... initial, usually 12 degrees, vacuum which is load dependent and mechanical which is RPM dependent. If checking total advance revved to 3k, you do not want the part throttle vacuum advance added to it. Just pull the hose off, it's ported and is NOT a vacuum leak at the intake. When revved above 2,500 your mechanical advance should be 'all in' and with the initial 12 degrees will show your total advance. I think 32-34 on an L series.

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Rechecked my source and came up with this Low Speed Circuit Tuning Guide. It definitely helped my set up of dual DCOE Webers on an L20B, w/modified W58 cylinder head, stock cam, larger diameter exhaust, and electronic ignition.

I did encounter a slight problem while setting the carbs, a small random ticking noise. Once I associated it to the distributor, it would go away when the idle was rev'd off idle. To eliminate this ticking, I disconnected the vacuum hose to the distributor diaphragm and plugged the tube and intake fitting.

My apologies for asking about the compression on the cylinders but I was mainly interested in what your readings were so I could compare them with mine. People do get rather jumpy on this site if a question is randomly asked and doesn't pertain to the original problem.


It is most important to verify all linkage and levers are installed without binding and the linkage opens to full throttle and is allowed to close to the Idle Speed Screw. This is the number one and two reasons for tuning errors, improper linkage installations and over tightened linkage nut, causing binding in the linkage assembly.


The Individual Runner carbs, DCOE, have individual Idle jets and mixture screws for each barrel. They also have an additional air bleed screws and lock nuts. This is not used for idle adjustment or idle quality. The settings for these screws should be closed.


Standard DCOE Settings:

Speed screw ¼ to ½ turn in after contact with lever maximum.


Mixture Screw 1 turn out from lightly seated.



“Lean Best Idle” Procedures


After confirming the linkage allows the throttle lever to seat against the Idle Speed Screw. Back off the Idle Speed Screw, then turn the screw in until it contacts the throttle lever and turn it in ½ turn. Turn in the Mixture Screw in until it “LIGHTLY” seats, then back it out 1 full turn. Loosen the 8mm wrench size nuts on the “air bleed” screws, turn in the air screws until it seats then tighten the nut.


a. Start the engine, it will run slow and like a tractor. As long as it will stay running, the idle speed is not important at this point.


b. First, turn in the mixture screw until the engine runs worse, then back out the screw ¼ turn at a time. The engine should start to smooth out. Continue to back the screw out ¼ turn at a time until the screw does nothing or runs worse. Then turn it back in to the point where it ran best. You want to tune the engine by sound. Adjust each mixture screw to the best, fastest and smoothest running point. Do this procedure with each mixture screw.


c. Now you may adjust the Idle Speed Screw. It should be sensitive and only require ¼ turn in or out to achieve the idle speed you like.


d. These carbs are commonly used in pairs, this makes the synchronization important, be sure to bring the high flowing carb down to the low flow carb. Then bring them both up to “proper” Idle speed. The Idle Speed Screws are not opened more than ½ turn in.


e. After synchronizing multiple carbs, reconfirm steps b. c. & d.


“Simple Rules for Calibration”

If your mixture screw is out more than one turn like 11/2 turns then your idle jet is too lean, go up one half size on the Idle jet.


If you mixture screw is not out one full turn, something like only 1/2 turn out from seated then your Idle jet is too rich, go down one half size on the idle jet.


This is all based on the important fact that your speed screws are not open more than ½ turn if they are then that is also an indication that you have a lean Idle circuit. You are cheating by opening the throttle plates and exposing additional progression holes in the transition.

Pump By-pass Valve:

The pump by-pass valve is designed to allow fuel into the pump circuit and when depressed, by-pass a percentage (hole on the side of the valve) of the fuel delivered to the accelerator pump nozzle/jets. We have included the zero by-pass valve in our jet kit. This will deliver all of the available fuel to the engine and not by-pass fuel back to the fuel bowl. This will increase the duration and volume of the pump shot with the original pump jets. To decrease the duration and increase the fuel volume we have included in our jet kit the larger 55 pump nozzle/jets. This is for maximum fuel delivery from the accelerator pump circuit. The accelerator pump by-pass valve is located in the bottom of the fuel bowl, one is required per carburetor.



DCOE Adjustment controls Tuning & Adjustment



1. Critical! Be sure for initial carburetor set up all air by-pass screws should be in closed position. These are not commonly used in standard carburetor adjustment.


2. Set the idle speed screw at ¼ to ½ turn in after contact with throttle lever. When doing multiple carburetors all linkage should be disconnected. All carbs should be bench adjusted to same setting.


3. Set the idle mixture screw to one turn out form lightly seated. When checking the seated position to make only light contact with seat, aggressive seating will damage needle and seat of carburetor,


4. Start engine as long as engine starts and runs do not set up idle speed first. First adjustment if possible should be to find smoothest idle with each mixture screw on all carburetors. Some prefer to do one barrel of each carburetor then come back and do the second barrel


5. After preliminary lean best setting of idle. Check carburetors for synchronization. Commonly done by checking lead or front barrel of each carburetor.


6. You will always want to bring high flow carburetor down to match the low flow carburetor. If this cannot be done you will need to recheck bench assembly for binding throttle in high flow carb. Once you have matched both carburetors you will need to set the idle to the desired idle speed setting. This will be done by adjusting both carbs up or down the same amount and re checking for synchronization.


7. Make one last check of lean best (smoothest running position) idle on all mixture screws one last time.


8. Best idle should end up with the mixture screws at or near one turn off seat.



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Thanks for the information!

Rat-a-tat-Dat, yes I have run the compression, all cylinders are within 10% and compression is a solid 150 psi per hole. Just to make sure all was well went under the valve cover yesterday and set the valve lash while eyeballing the internals trying to ID the cam. Everything looked great, cam just says "Japan" on it, and does not appear to be re-ground. I then used a feeler gauge to set the throttle screws to just touching, then added a 1/2 turn. I double checked the timing and it is set at 10 BTDC. I need to calculate where 32-34 BTDC is on the crank pulley so I can see where full advance runs out to (my timing light does not have an advance feature).


datzenmike, All 3 timing advance mechanisms are are working smoothly, no waver or hop on the timing light when I check it at idle through 3,000 RPM.


Rat-a-tat-Dat, thanks for the Weber tuning information, the process laid out is very useful and nice to reference.


Having done the valve adjustment and reset the carbs I test drove the car and it was running superb! The car still idles at 1,100 RPM, maybe as low as 1,000 RPM? But man did it run nice, until after running through the gears hard it sucked something into a jet and started to spit and burp and fuss. I will be pulling the carbs tonight to do a complete cleaning and inspection. I suspect there is some sediment in the accelerator pump circuit on one or both carbs because this is not the first time it has been running great then taken a crap after a good couple pulls under load across the tachometer. Before you politely suggest it, yes the fuel filter on the car is brand new and yes the fuel is also brand new (super no less) and yes I cleaned the particle trap below the fuel pressure regulator.


It's frustrating, but so much fun once you get it right.

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What I was asking is what is the total timing at, say 3K?


The high idle must be air getting in somewhere. If a leaking mount or intake gasket the idle mix would compensate for the extra air but the idle would go up. You should be able to turn the idle speed screw down till the engine starves for air. Have a look at the throttle plates, they should close 

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So, once you have everything back together and running again but have the same or similar problem, D/M suggested an air leak at some point. Might I suggest using ‘brake clean‘ spray-in-a-can. Yes, brake clean! Less harmful than Ether. Use this on all areas around the intake mani, carb throttle body levers ( stay away from carb intakes, of course ) and inlet vacuum lines. Spray conservatively while engine is idling. If the engine rev’s higher during a specific location of choice, then that’s where the air is getting in. 
No issues? Then something else.

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