Triple Mikuni thread

[bpilati]

This is AWESOME! Would it be possible to make a PDF of the booklet and post it? Especially the tuning section.

May be asking too much, but thought I'd ask

We might need to consider copyright on this. Better check the book for copyright markings.

[Mike]

We have a release for Nissan/Datsun material.

[Travel'n Man]

I'm dropping in 185 airs today and I'll let you know what that does to my AFR. I would like to lean out my cruise just a little. I'll post the results this evening. If the smaller airs don't lean me out in cruse a little - I'll put the 190 airs back in and drop my pilot to a 62.5. I do like to make one change at a time so I can keep everything recorded to see exactly what happens per 1 item change to my AFR's and plugs................stay tuned.

[240260280z]

................stay tuned.

bad pun

On DCOE's, the main jet typically affects fuel delivery between cruise and max RPM. The Air corrector changes the non-linear behaviour of a/f to lean it out as max RPM is approached. Too large an air corrector will affect a/f lower in rpms and cause over-lean at high RPM's.

If the fuel level is too high on DCOE's, the fuel in the main jet will be pulled too early/easily and over enrich the cruise A/F as well as the cruise to WOT. Changing mains will have little effect on the cruise A/F apart from taming the excess fuel at higher RPM's.

Lowering fuel level and going with a larger main is one method for tuning the cruise-power transition.

Edited June 18, 2014 by Blue

[Travel'n Man]

185 kept me on the rich side - put back in the 190's. Out of town for a couple days on business. This weekend I'll change the pilot jets with the 190's. I'll record that session with all the edits I did to that series of jetting, pick the best recorded AFR's and spark plug condition.

I plan on doing this same systematic series with 3 or 4 setting using different Main Jets and get getting my favorite 3 setting (with 3 different Main Jets) and off to the dyno's to see the best one.............

[madkaw]

Chasing my tail lately I believe. My 'studder' is back(randomly) and seems to be directly tied to a rough idle I intermittently get. These carbs use to idle at 750rpm -rock solid, but now everyonce in a while the idle bops around. I no longer have a working tach, but my idle speed hasn't changed much, but just gets erratic and shows super lean . It seems when my idle gets crappy my transition issue re-appears. I guess it's time to start from square one.

Tomorrow I'm pulling the float covers off and start double checking all the settings and looking for anything that would cause a float issue. Yes I am focused on the floats because it is one of the few adjustments that can effect all parameters of operation. Not to say I don't have something else going on --and I will check. The mikuni manual eludes to checking for carbon build up of butterflies or pilot screws, but these carbs are clean.

[Travel'n Man]

Anybody keeping up with this thread make sure you have the right tools to keep your SU's or triples balanced - this is the best on by far.

[Travel'n Man]

Just in case you are following this and wondering where all this jetting is located and how do you get to it and how difficult it is - here is a quick snapshot of how easy it is to change a jet...............

[madkaw]

Had a long day of getting my hands smelling like fuel. I pulled my float covers off and sure enough my settings were off. Probably a case of not properly adjusting the float within the parameters of the adjustment screw. There is only about 5mm of adjustment externally but the float has to be set so you can get 2.5 either direction.

Anyway it wasn't off much and didn't help my issue so I moved on for now. Plugs looked good. I checked timing on my MS and timing light- that was good. Sprayed starter fluid everywhere looking for vacuum leaks an couldn't find any.

Checked vacuum at idle and it was 12. I can't ever recall measuring this before so I can't compare, but it seems low. I measured at my vacuum log .

Checked valve clearances and some are tight, but I knew that because I set them that way- might need to undo that. It's been that way for many months with no issue.

My issue seems to be getting worse- maybe it will just break so I can figure it out.

Sounds like a miss and my AFRs are bouncing around at idle

Compression check next

[LeonV]

Did you check your bleed pipes to see if the solder job is holding up?

[Travel'n Man]

Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.

3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.

4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.

.

[madkaw]

That's the first thing I jumped on.Everything is holding.

This issue is worse and effects all parameters. Basically it's running like poop! It acts like a slight miss, just enough to hear and notice in performance. AFR's are not stable which is easiest to see at idle and it causes the idle to sound like a galloping 3/4 cam.

For now I am going to readjust my valves to stock specs to eliminate that. Maybe an internal fuel leak that is allowing some fuel to bypass normal metered paths and causing the roughness. I am going to recheck the needle/seat assemblies.

I also eliminated the brake booster and PCV by plugging them at the vacuum log.

Did you check your bleed pipes to see if the solder job is holding up?