Tuning Mikuni's w/ wideband - need help

[heavy85]

Originally posted in the Misc forum with no replies so I'll try here:

 

It's been a big pain so far but it's finally working and I've got to say it's pretty slick. I've got the Innovative Motorsports (or something like that) LC1 (or was it LM1 ...) with the gauge in the dash. My A/F ratio's are ~14.0 at idle, mid 13's cruising down the interstate, and about 10.0 at WOT. Now I know why it stinks so bad at WOT! This is with my L28 w/ tripple 44 Mikuni's and I'm looking for some advise on where to go from here. Any takers? One other thing is it bucks once if you punch it from part throttle and it stumbled coming out of one corner pretty pad at the autocross yesterday but other than that it runs pretty good. Well the idle is pretty random as it always jumps all over the place. Last thing is yes fuel pressure is set at ~3 PSI, I have a heat shield and insulators, yes it is still deadheaded (but I'm working on that), and the carbs have all been fully rebuilt and set per the manual. Jets are 200 air, 150 fuel, 57.5 idle, 40 accel pump.

 

Thanks for any help.

Cameron

 

PS - it may be running a little retarded because when I set the timing (38 degrees total) there where three marks showing on the pulley so I picked the conservative one ... another thing to figure out.

[JMortensen]

When you're at idle and cruising on the freeway you're running on the pilot jet. The pilot will also continue to affect your air/fuel ratios well up into the rpm range. You're running on the main air and fuel at WOT with a little influence from the pilot. The air and fuel are apparently too rich. You can go bigger air or smaller fuel or both. The fuel doesn't seem that big, so I'd probably start with a bigger air jet.

 

The transition is the hard thing to get right with Mikunis IME. I spent a lot of time on this, and my conclusion was that running a larger pump nozzle helped a little bit, but didn't help anymore at all and just made larger and larger plumes of black smoke when it got beyond 50. What really helped was a larger pilot. I think I started out with a 52.5 pilot and I ended up with a 65 or a 67.5. MUCH bigger. I kept going up and up and up and it kept getting better and better and better. I don't know what effect that's going to have on the mileage, but I don't much care anymore either...

 

Also, what manifold are you running? There was a guy on classiczcars.com who said that he had a stumble and fixed it by going to a shorter manifold. That was news to me. I'm running a Cannon, which is the longest. I think this guy said he was running a TWM. If you search over there you might find it. I'm not totally convinced of the short runner argument, because my friends with 510s have the stumble and they're running Mikunis and TWMs, so ???

 

My understanding of the cause of the stumble is that when you snap the throttle open the velocity in the runners goes way down, and at the same time the acc pump squirts a big stream of gas in there. So there is no velocity to mix the gas, and you end up stumbling until the car kind of "catches up", then it takes off like a bat outta hell. So using a bigger pilot makes the car richer in the transition period without squirting the fuel in by fire hose, like the pump nozzle. That's why it works better. That's my theory anyway.

 

Also, fix your timing. If you're using the 240 pulley the 0 mark is the second from the top, and it is wider and deeper than the rest of them (barely). Get an adjustable timing light and use that to dial it in. If your timing is way off you're throwing lots of power right down the drain.

[clarkspeed]

I'm an FI guy and no carb expert but I do have some advise for a NA L6. You will want to data log your LM-1. Things happen so fast, sometimes they are hard to see on the gauge. RPM and throttle input even better. Jet for a steady WOT ratio a little above 12.5 to 1. Say 12.8 to 13.0. My LM-1 read a little high so I ended up about .5 above. Then tune for throttle transition per some of Jon's notes. You will end up with a comprimise. Going a little lean then a little rich is about right without going too lean and never rich or never getting lean and going too rich. That's what causes the stumble. Should clean up within 1000 RPM. Then if you really want to get it close, take it to a dyno, with LOTS of spares.

[Guest ZFury]

Sounds to me like you may want/need bigger jets. And you may have yur' pilot screw set a little rich. Though you will want it set as rich as possible. But,... wut do I know...

[heavy85]

Also, fix your timing. If you're using the 240 pulley the 0 mark is the second from the top, and it is wider and deeper than the rest of them (barely). Get an adjustable timing light and use that to dial it in. If your timing is way off you're throwing lots of power right down the drain.

 

Thanks for the carb advise - I'll soon be ordering some jets. On the timing I have what I think is a 240 pulley that has five grooves. The bottom groove is the wider and deeper than the rest and is painted white. The top two grooves are also painted white but are slightly smaller. On the engine side it's from a 280ZX so it has the 0-30 scale instead of just the pointer. Wouldn't the bottom groove be 0 deg so if I set the timing light to 36 degrees total and lined up the bottom groove with the 0 mark it should be set right? Why are you saying use the second from top?

 

Thanks

Cameron

[JMortensen]

I was going from memory so I may have switched top and bottom there. My Euro damper has a 5 degree ATDC mark, then a 0, then I think 4 or 5 marks.

 

Regardless, if you can identify the thicker one, that's TDC.

[JMortensen]

Forgot to mention yesterday, I think you need the 240 pointer for the 240 pulley and the 280ZX pointer for the 280ZX pulley. I don't think they have the marks in the same place.

[Guest ZFury]

One tip I forgot to mention is to try and up the fuel pressure and tighten the pilot screw a little.

[JMortensen]

I've heard of people lowering the float level and upping the fuel pressure to get rid of that problem where the fuel washes over the top of the jets and makes the engine run really rich on sharp right handers. There is a better way to get rid of that problem which involves JB welding the vents and running a vent tube out the top of the carb. Here's where I learned about it: http://www.classiczcars.com/forums/showthread.php?t=18948

 

Never heard of just upping the pressure and tightening the pilot. Is that supposed to get rid of the right hand stumble or just normal acceleration stumble or what?

[Guest ZFury]

I though side mounted carbs were suppose to be better in the corners when compared to top mounted carbs. I thought he was having this problem in the corners do to the drop in rpms, not the force in the corners. He mentioned his idle ratio's, so my advise was to solve that.

[Sam280Z]

I know this is an old post, but Forrest pointed it out to me...

The best thing I did to cure the transition stumble was to use a smaller air corrector.

 

You can find out where the transition to the mains should be by removing the jet blocks and driving in high gear to find the highest RPM that the engine runs.

 

To find out when the mains come on, with the jet blocks in, get it at a RPM and stab the throttle down. It will surge due to the accelerator pump and then either keep going if the mains are on or die if the mains are not coming on at that RPM. Repeat at different RPMs to find out when the mains actuate. Then you can change the air to get the transition point right. Smaller air jet lowers the RPM when the mains come on.

 

You can then tune the overall mixture with the main jet.

 

No books I have ever read have this info like this. Check out the LM1 forums. there is a user called type26owner who has done a lot of experimenting. The most useful info I have been able to find for side-drafts has been at that site.

 

FWIW,

Sam