Mikuni 44PHH Tuning Questions

[okayfine]

To adjust the rate in which the advance applies, or what rpm it applies at you have to change the weights or springs to get them to spread out later / sooner.

We're on the same page, then. It sounded like you just tweeked the springs, which I would equate with adding pre-load. This can have an effect, but not the same as shimming the slots, which reduces the amount of total advance.

[marx1]

Sometimes What I'm thinking doesn't come out right - My son just started school; so i've been fighting with homework

I appreciate everyone's input - It's good to be able to bounce ideas off of other people that know the engine

[okayfine]

In your OP you were citing a rich condition, but further down you got rid of a backfiring by richening the mixture. Now you said you have a mis, likely due to fouling.

Have you checked the plugs at this stage to verify condition? Also, would it be possible to do a cut test to inspect the plugs when the mid-range poor running occurs?

I know the Lynx manifold is supposed to be the way to go with a single dual-barrel sidedraft, but maybe the convoluted path is causing a cylinder or two to be of a different mixture at speed. The cut test might be a way to check things out.

[marx1]

In your OP you were citing a rich condition, but further down you got rid of a backfiring by richening the mixture. Now you said you have a mis, likely due to fouling.

Have you checked the plugs at this stage to verify condition? Also, would it be possible to do a cut test to inspect the plugs when the mid-range poor running occurs?

I know the Lynx manifold is supposed to be the way to go with a single dual-barrel sidedraft, but maybe the convoluted path is causing a cylinder or two to be of a different mixture at speed. The cut test might be a way to check things out.

LOL- I must of conveyed it wrong - lets try it again

First At idle - I was too lean, backfiring though the carb, using 57.5 pilots - I couldn't get it rich enough (i didn't want to take it past 2 turns out - as the SM said 1 1/8) so I went to 62.5; and got it to stop with 1.5 turns out. I then found out my O2 sensor was bad - so I replaced it, then found the rich condition when under 20% throttle. I went back to the 57.5's and turned the idle mixture out until it stopped backfiring while warm (180deg). At this point the O2 showed leaner by 3 bars. Once it was hot (about 200 when in traffic) it started misfiring because of rich. I figure this was because I had it a little too rich, then when it got hotter it went just over the edge.

When driving over 20% throttle it shows full rich. Because of misinterpretation, I had the air correctors at 210; I put in 190's and it leaned up 1-2 bars; and overall feel of the engine got better.

I then advanced the timing a hair (I didn't have the timing light - I just moved it enough to smooth out the idle some) and the engine responded much better, and the flat spot moved from 1000-3000 rpm to 1000-2300 or so.

I then took your consideration and re-read Jason's page; and pulled the dizzy apart- to find the vacuum advance bad; and all of the internals nasty - weights where hard to move, the vacuum advance plate was stuck, and the shaft was extremely stiff. I pulled it all apart, cleaned and re-greased the vacuum plate; and re-lubed the bushings. how it spins, not freely, but easily, with no sideways slop (some vertical but that's just how it is) and I added about 1mm of epoxy to the inside of the slots (+/- .2-3 mm close enough to be ok), and filed it smooth so the weights would not get hung up.

Hope that helps

[okayfine]

Once it was hot (about 200 when in traffic) it started misfiring because of rich. I figure this was because I had it a little too rich, then when it got hotter it went just over the edge.

This would be a good point to do a cut test. It'll show if you're missing 'cause it's lean, rich, or unrelated to fuel.

I then advanced the timing a hair (I didn't have the timing light - I just moved it enough to smooth out the idle some) and the engine responded much better, and the flat spot moved from 1000-3000 rpm to 1000-2300 or so.

Gots to get a timing light. Without it it'll be really difficult to tell if the mechanical advance is working properly.

I pulled it all apart, cleaned and re-greased the vacuum plate; and re-lubed the bushings. how it spins, not freely, but easily,

Have you driven it since? I really think that could be your (main) issue.

[marx1]

Have you driven it since? I really think that could be your (main) issue.

I have not driven it since; I'm waiting on a new vacuum advance. once It is in, I'll drive it agian; I do think this was a lot of the issues as well. It will be here Tuesday I'll re-time it, and see how it does; I think it will still be rich; but we'll see. A little rich is OK IMO, a lot isn't.

[okayfine]

I have not driven it since; I'm waiting on a new vacuum advance.

No vac advance won't be a big issue for a trial run around the neighborhood.

[marx1]

True, but the whole canister was seized in place; so I had to break the can, and cut it out of the dizzy

[marx1]

UPDATE: Got the dizzy re-curved and re-installed; set the timing to 15 BTDC, and did some other needed stuff (Gear redution starter, diff oil change, clutch adjustment, top off tranny etc) and I installed 135 Main jets. So the resulting setup is 135 Mains, 190 air, 57.5 pilot. T

Then I drove down to oakland (about 75 miles round trip) and it's MUCH better, the flat spot is hardly noticeable, It's still rich when cursing (under 20% throttle - so the pilots) and under heavy acceleration / WOT , it leans out a hare too much (just under stioch). Under moderate (30-50%) it leans out to the upper edge of stioch. (Yes I know I need to do the plug read - but I had to go fix stuff so I took the 510)

So once I get my 55 pilot jets, I'll put those in, and I'm going to put the 140 mains in, and go up to 200 air to help richen it up across the band; and stop it from leaning out so much at high rpm. And a bonus- It idles faily good at 850rpm

Thanks for everyone's help - I now have a firm grasp on what to do

[okayfine]

Good work, glad there's progress!

[five&dime]

Found this thread in a search.

Some of you may recall I put the 510 on a dyno a few months back. Jamie (batfastard) was there so he may be able to chime in. First a little background info. Rebello 2.3L L20 with twin Mikuni 44's. Carter low pressure electric fuel pump running at about 3-3 1/2 lbs.

The car put down 125 RWHP but was running rich across the band. At about 4k RPM, it ran so rich that the plugs started to foul and the car started to sputter and backfire.

Jamie stated I should try smaller jets first. I took the cover off this past weekend and there is a 200 stamped into the top of the jets. I assume that is how you know which ones? I had been told the #'s were on the side not the top.

So.... Should I start running smaller mains first? I was going to order 190, 180, 170, 160 & 150's to start. Or would you all suggest ordering the other replacement jets? I think the others were 52.5's (excuse my ignorance as I know nothing about the mikuni's) I have read the manual several times but still don't really understand them Working on that though. Batfastard also has the manual and is obviously much more technically savy than I am so maybe he can interject. He may also be able to post the dyno sheet. It was so dissapointing that I didn't even keep a copy.

Thanks,

Brandon.


Edit: I wasn't expecting a reply from Todd so quick. It only took him like 5 minutes

Here is what he said. I figured I would post this in case anyone else needs help with this. I will post the results as we work through this.

"You need to revisit your manual to touch up on jet function and placement in the carb. The jets you see when removing the jet cover are the air jets and these are not where you start the tuning. You have to remove that entire jet block they are screwed into, to get to the bottom where the fuel jet is. This is the one you have to work with on intitial tuning. the air jet is at the very end when you are trying to tune the very top end AFR. Check out what you have for fuel jets and we'll go from there. Air, fuel and pilot jets are $4 each. The fuel goes by 5's and the air by 10's. You probably have something like 155 or 160 fuel jets. I would suggest you get the next four smaller sizes, based on your findings, don't guess. Then, you step down one size at a time and get the AFR reading from the dyno operator. He'll be your best guide in going about the tuning and by how much."

[marx1]

I found this page http://www.rockinrich.com/socalroc/imag ... mikuni.pdf that shows 180 main (on the bottom of the emulsion tube) and 260 air (on the top of the emulsion tube) I have a friend that had a set that came off of a l20b, and it had 150 main, 210 air and 51.5 (I think) pilot jets.

I have 150 main and 210 air with 57.5 pilot jets on a single carb on a lynx manifold. I have a 140 main jets- but they are too lean - so I'm going with 145's. When you change the main jets you need to change the pilots as well - as you also change the band where the fuel is being applied - The leaner you go the higher the range. To test this, take your car out, with it warm, and go 10-15mph, then put it in 5th or 4th and go wot. DO not progress from closed to wot, just mash it. If you accelerate decently it's too rich, and if it backfires through the carb your too lean.

Keep testing this until you find a good middle point- where it doesn't backfire but doesn't really accelerate fast either. Then you need to play with your pilots - Take the main jets out and accelerate slowly until the carbs start popping. This rpm should be about 300 higher than what you want to idle at; it should also be pretty close to where the mains start flowing. Once you find this - you can adjust your idle mixture screw to where it idles smoothly but doesn't pop.

My friend that has the dual's (speedman here on the board) just got the colortune and I think that will make the tuning process better. I also found out that my fuel pressure is very low (1.5psi) so I need to swap it to a higher-pressure pump before I start tuning.

[marx1]

Ok so I've had some time to sit down and figure this whole deal out; and that I have!

So: The main problem - a major stumble from 1500 rpm to 3000. major richness on idle and cruse

What I tried before: Leaning out mains and pilots; playing with air correctors to get rid of stumble.

The fix: a balance tube and hooking up vacuum advance. yes. really! The LYNX manifold does not have any kind of balance between the sides of the carb - so the intake pulses where very short and abrupt. I drilled 2 holes and tapped them for 1/4NPT and put in a 3/8" tube between them - at the same time I put in a barb for vacuum advance and a PCV valve for the cank case vent (I was tied of oil in the engine bay)

After looking at my old Cannon dual manifold i see it has balance tubes between each side of each carb; this explains why it does good on low end as well as the high end.

So now without changing jets - I have the mixture at 1-3/4 turns; 60 pilots. Mains are 145 with 190 air correctors. the mixture screws do make a noticeable change on the O2 sensor when adjusting at 1/8 turn increments.

Now just with a quick test drive after the mods i'm still rich on idle - but that can be adjusted - and rich on cruise - this has been narrowed down to the pilot jets - I have every size from 55 to 67.5 so I'll try what I have to get that where I want it. The mains are a little too lean when accelerating hard so I'll try 147.5's and if all else fails I'll use the 150's. There is a little stumble when it transition from the pilots to mains on acceleration - but I think this is because the pilot jets are rich. once I get them settled down I'll see how it works.

But currently the car is getting 21.5mpg - I'm going to drive to work tomorrow and go down on my idle jets when I fill up. I'll see how it does then.

Thanks for everyone's help!

[five&dime]

Hahaha. This is great. I also found out that the best mains for my car are 147.50. I had no idea going from a 150 to a 145 would change the A/F by 2!!!!!

Seems like we are both getting our cars tuned at the speed of slow