My 1971 521

[mainer311]

If Mikunis were the same as Webers (are they? I don’t know) I could give you a crash course. The way a DCOE works is almost identical to a DGV, it’s just pointed sideways instead of up. With a PHH, I don’t know. Idle jets are usually 50-55. That’ll get you all the way through the progression circuit. 
 

I noticed that it stumbles right at throttle tip-in. Typically that means it’s already running lean, and cracking the butterflies makes it lean to the point of stumbling. You could go one size higher on the idle jet and/or a little bigger on the accelerator pump.

 

It’s also possible that your idle screws are too far turned in, and to compensate you turned the idle stop screws in too far. As they say in Maine: hard tellin’ not knowin’

Edited March 8, 2021 by mainer311

[datzenmike]

Meter the + side while the charge light is still on. If +14 then charging and the light doesn't matter. If not charging until the light goes out the alternator is probably struggling. Take back till you get a good one.

 

30 PSI is great oil pressure, but your video seems to be idling high. Driving pressure is 55-65 PSI.

 

My R-1 carbs are louder than my exhaust. More addicting than crack. Even when not 'giving it', it sounds like you are.

 

The front side draft, has a dedicated vacuum advance port??? Always run a ported vacuum advance if it is available. It improves part throttle response and fuel economy. It may remove some of the 'over carbed' sound that Hainz was talking about.

[d.p]

1 minute ago, mainer311 said:

If Mikunis were the same as Webers (are they? I don’t know) I could give you a crash course. The way a DCOE works is almost identical to a DGV, it’s just pointed sideways instead of up. With a PHH, I don’t know. Idle jets are usually 50-55. That’ll get you all the way through the progression circuit. 
 

I noticed that it stumbles right at throttle tip-in. Typically that means it’s already running lean, and cracking the butterflies makes it lean to the point of stumbling. You could go one size higher on the idle jet and/or a little bigger on the accelerator pump.

yeah I just don’t know enough about them it trying learn.   Mains are 180 and 160 at the bottom. Pilots are 60 and the accelerator pump is a 45.    

[d.p]

4 minutes ago, datzenmike said:

Meter the + side while the charge light is still on. If +14 then charging and the light doesn't matter. If not charging until the light goes out the alternator is probably struggling. Take back till you get a good one.

 

30 PSI is great oil pressure, but your video seems to be idling high. Driving pressure is 55-65 PSI.

 

My R-1 carbs are louder than my exhaust. More addicting than crack. Even when not 'giving it', it sounds like you are.

 

The front side draft, has a dedicated vacuum advance port??? Always run a ported vacuum advance if it is available. It improves part throttle response and fuel economy. It may remove some of the 'over carbed' sound that Hainz was talking about.

Got it from rock auto so hard to take back but I guess I’ll try.  Yeah they are loud and at my age not sure it’s necessarily what I’m looking for.  And yes it has a vacuum so I’ll hook it up and see what if anything it changes.  

[datzenmike]

Just tell them the red light comes on when I start it. But check that it's not charging while the light is on.

 

I put a 280zx air filter on.... no difference. But it might, if I could have kept and used the entire enclosure. Made my own because there wasn't enough room. At around 3,800? the bark from them increases. Even at part throttle there is a change in the loudness. Covering the under side of the hood with sound deadner padding might help.

 

Used an aluminum tray for a heat shield.

[d.p]

I got a heat shield under them already and k&n filters just not installed.  Like the look of the open horns vs the filters.  
 

How do I confirm the alt is charging again?  Cold today but supposed to be warm tomorrow so will spend some time on all

of this. 

Edited March 8, 2021 by d.p

[d.p]

2 hours ago, mainer311 said:

If Mikunis were the same as Webers (are they? I don’t know) I could give you a crash course. The way a DCOE works is almost identical to a DGV, it’s just pointed sideways instead of up. With a PHH, I don’t know. Idle jets are usually 50-55. That’ll get you all the way through the progression circuit. 
 

I noticed that it stumbles right at throttle tip-in. Typically that means it’s already running lean, and cracking the butterflies makes it lean to the point of stumbling. You could go one size higher on the idle jet and/or a little bigger on the accelerator pump.

 

It’s also possible that your idle screws are too far turned in, and to compensate you turned the idle stop screws in too far. As they say in Maine: hard tellin’ not knowin’

going to check the idle screws tomorrow and will let you know.  

[thisismatt]

Sure you don't have the alternator plug wires reversed?

[datzenmike]

3 hours ago, mainer311 said:

 

I noticed that it stumbles right at throttle tip-in. Typically that means it’s already running lean, and cracking the butterflies makes it lean to the point of stumbling. You could go one size higher on the idle jet and/or a little bigger on the accelerator pump.

 

 

 

This and possibly more accelerator pump output. The whole point of the accelerator pump is to transition from idle to primary and richen up that lean area.

[mainer311]

1 hour ago, datzenmike said:

The whole point of the accelerator pump is to transition from idle to primary and richen up that lean area.

 

Kinda. That's more what the transition circuit is for. The accelerator pump is more for when you lay into it.

[datzenmike]

Not sure what you mean. Idle is too slow, there is hardly any air traveling through the carburetor. If you step on the gas the throttle opens and sucks air but not enough flow to form a venturi vacuum to suck any gas in so the engine goes extremely lean and bogs. The pump squirts raw gas in to compensate.

 

The 'transition circuit' is more for a slower opening of the throttle. As in just driving away from a stop. In the video that was not a slow opening.

 

 

[d.p]

I really need to drive it more to get a feel for what it’s doing. Around the block and parked in my driveway (getting hot no airflow) are obviously less than ideal.  
 

Did I post what jetting I have? If so fuck it here it is again. 

mains - 180

bottom of the mains (air correctors?) - 160

pilots - 60

pump nozzle - 45

 

 

Mainer maybe this pic will help ?

Edited March 9, 2021 by d.p

[mainer311]

26 minutes ago, datzenmike said:

Not sure what you mean. Idle is too slow, there is hardly any air traveling through the carburetor. If you step on the gas the throttle opens and sucks air but not enough flow to form a venturi vacuum to suck any gas in so the engine goes extremely lean and bogs. The pump squirts raw gas in to compensate.

 

The 'transition circuit' is more for a slower opening of the throttle. As in just driving away from a stop. In the video that was not a slow opening.

 

That's basically what I said. Laying into it = butterflies flying open. Sudden drop in vacuum needs a shot of fuel. But, the accelerator pump just covers dips, and isn't for the transition. 

[mainer311]

Drew, I believe the "pilot" jet on a Mikuni is the same as an "idle" jet on a Weber. 60 sounds big, but I think you can use it. Your pilot adjustment screws will just need to be turned in slightly more. 

 

Things should be done in this order:

-verify correct ignition timing

-get the engine hot and double check valve lash (this affects flow through the carbs)

-The throttle stop screws should be set to get a workable idle. Check with the sync tool that you're getting the same flow at idle with both carbs.

-You'll need to set the idle mixture screws individually across all 4 tubes. The method to doing this is the same as the Weber DGV, but instead of 1, now you have 4. 

-Reset idle speed to desired RPM. (The speed of the idle may change after the pilot screws are adjusted.)

This will get you the idle sync and mixture dialed in.

 

-Carb sync should also be checked at higher RPM to verify that the linkage is pulling the butterflies open evenly. This is remedied by tweaking the linkage or dog bones.

 

Once all this is done, you can verify high speed mixture with: spark plug color, a Gunson Colortune, or O2 sensor setup. This is all adjusted with the main jets and air correctors. 

[demo243]

Sorry late to the party here— 

 

What engine and what size carbs?

 

Here is what I ran on my old L20b with a hot cam and 44s. Came recommended to run this way from Todd at wolf creek and didn’t require any tuning. I basically set the idle per the mikuni manual and drove it that way for a couple years till I threw the crank pulley and went KA after that.

 

 

 

Edited March 9, 2021 by demo243

[demo243]

check this thread out too - on my phone so sorry didn’t really read back to far.

Edited March 9, 2021 by demo243

[datzenmike]

On the O₂ sensor mentioned earlier it only reads the O₂ level of all cylinders combined. No way to know which cylinder is running lean/rich or if 2 3 or all 4 are causing it. Reading the plugs would be the only way. Once dialed in the O₂ would be a check that everything is ok.  

[d.p]

9 hours ago, demo243 said:

Sorry late to the party here— 

 

What engine and what size carbs?

 

Here is what I ran on my old L20b with a hot cam and 44s. Came recommended to run this way from Todd at wolf creek and didn’t require any tuning. I basically set the idle per the mikuni manual and drove it that way for a couple years till I threw the crank pulley and went KA after that.

 

 

 

similar setup L20b, mild cam and dual 40s. 

[d.p]

Thanks @mainer311 I figure some work to do with the carbs. And as far as lash goes I set it to the cam specs but at the rocker arms versus the valves like this thing says.  @demo243 can you or anyone else what tell what level of cam I have? Mild hot big??
 

 

Edited March 9, 2021 by d.p

[Crashtd420]

10 hours ago, d.p said:

 

mains - 180

bottom of the mains (air correctors?) - 160

pilots - 60

pump nozzle - 45

 

 

Mainer maybe this pic will help ?

Fyi the main is at the bottom and the air corrector up top... should be similar to the Weber with an emulsion tube connecting them...

 

And in that picture it looks like that carburator has a dedicated port for vacuum advance..... 

[d.p]

1 hour ago, Crashtd420 said:

Fyi the main is at the bottom and the air corrector up top... should be similar to the Weber with an emulsion tube connecting them...

 

And in that picture it looks like that carburator has a dedicated port for vacuum advance..... 

 

Per the Mikuni service manual the top is the 'main air jet,' bottom is the 'air jet' and they are connected by a 'bleed pipe and jetblock.'   ANd it does have a vacuum advance port but I have been told to just plug it.  

 

http://www.mikunipower.com/PHH01.htm

 

 

[datzenmike]

2 hours ago, d.p said:

Thanks @mainer311 I figure some work to do with the carbs. And as far as lash goes I set it to the cam specs but at the rocker arms almost the valves like this thing says.  @demo243 can you or anyone else what level of cam I have? Mild hot big??
 

 

 

 

The '78 620, 710 and 610 L20B cam is

 

I opens 16 BTDC

E closes 14 ATDC

I closes 52 ABDC

E closes 54 BBDC

 

Lift 0.413"

 

The '73 620 L16 cam is...

 

I opens 12 BTDC

E closes 26 ATDC

I closes 48 ABDC

Exhaust closes 50 BBDC

 

Lift 0.413" though some I think are 0.385"? (this may be 521 and 510?)

[d.p]

Ok great Mike but I have racer brown cam which someone has referred to as a "very good street grind."

 

 

[Crashtd420]

3 minutes ago, d.p said:

 

Per the Mikuni service manual the top is the 'main air jet,' bottom is the 'air jet' and they are connected by a 'bleed pipe and jetblock.'   ANd it does have a vacuum advance port but I have been told to just plug it.  

 

http://www.mikunipower.com/PHH01.htm

 

 

Look at the picture again....

Top is the main air jet 

Bottom clearly says main jet not air jet... 

 

And who says dont connect the advance? 

The weber side drafts dont have a ported vacuum like that, usually people hook up to an incorrect vacuum source which is why its recommended to leave it disconnected.... I see no reason not to use it having the proper port..

[d.p]

Lol my bad, you are right.  I mistyped that...bottom is MAIN jet not AIR jet. 

 

I read it online and was told by the guy who rebuilt my crabs that its not needed because Mikunis 'have a very week vacuum signal.'    I trust the dude and he has come HIGLY recommended.