Mikuni Bog

[fp280z]

I posted this on HybridZ.org last night but I figured I'd cross-post here since I could really use some help from the carb experts...and quick! -Rick

I don't know if this is too much data (I'm an engineer, can there ever be too much?) but I was hoping someone could assist me in honing in on the root cause of my issue and I always enjoy the threads that have a lot of data to go along with the HybridZ troubleshooting!

Ok here's the deal, the SCCA Solo National Championships are less than two weeks away, I spent all last week thrashing to install my new Stahl header and fabbing up an exhaust for it, dyno'd on Saturday and got some decent numbers up top but I seem to have introduced (or exacerbated) a bad bog between around 2250rpm to about 3250rpm. This is a terrible place for a flat spot on an autocross car.

Obviously, eliminating the bog is my goal but even reducing before next week it would help tremendously. Attached is the dyno chart from Saturday and a chart from April that is pre-exhaust work, the only major change between the runs. As you can see, the exhaust work really woke up the motor above 3250rpm!

These dyno runs were performed starting with a load at WOT at 1800rpm then then removing the load and allowing the car to accelerate the drums, at least that's how I understand the basic concept. I only mention this because this method takes most of the mechanical action of the carburetors out of the loop (in other words, this is probably not an acc pump issue, progression drillings, etc.)

Also plotted on the dyno charts is the AFR for each run. I tuned the jets so that the target of ~12.9 hit just over 5350rpm and the upper AFR "step" in the midrange was a little leaner than that but not dangerously so. Based on my understanding of the carbs (I have read the Mikuni manual and several Weber books) that seemed like a logical tuning plan. If I'm way off base here, please educate me. Maybe the "step" is a clue?

At the point of the bog the AFR goes way rich (<10:1) and the torque drops. The same dip in the AFR is on the pre-exhaust pull but it's not as drastic, and you can see a similar dip in torque. So, it looks like this issue may have been present before just not as bad.

One troubleshooting step I tried at the autocross today was to drop the main jet down one step and the air down two. Seemed to help a little and the in-car wideband didn't look too lean but I have not been back to the dyno. I also leaned out the idle circuits which helped the idle but didn't to much for the bog.

Here's the other pieces of the puzzle:

13.5:1 GT2 motor

.540/292 cam

Stahl Header 1-5/8" primaries

2-1/2" to 3" Y-pipe to Magnaflow muffler

Mikuni PHH44 carbs

39mm venturis

170 mains (165 mains)

230 airs (220 airs)

55 pilots

35 pumps

Fuel pressure 3psi

Electromotive XDi, crank triggered, timing is set to 15 at idle and 36 total by 3000rpm. Spark plugs are a half-season old, autocross only. Valves adjusted beginning of the summer.

That's about it. I was hoping one of the carb gurus may have seen something like this in the past. Or it's something simple like my jetting being way off. I would welcome any technical feedback, I enjoy learning about the carburetors but books don't really tell you everything. At this point, a "dude you didn't have to write a huge post, it's obviously ..." would be great too. If it's just a matter of dropping jet sizes, I can do that and if I need to head back to the dyno, it's a possibility. Even if so, I'd love to know the theory around "why".

Thanks for reading.

-Rick

[Nigel1943]

You say you fabricated the exhaust system yourself? Z's are reknowned to have a flat ("bog") spot around 3000-4000rpm. This can be eliminated to an extent by having the longest secondary pipes you can get in, preferably back to the gearbox and then run into the Y piece. I'd be interested to know where your Y piece is. Not sure about running 2.5" into 3.00" either, depends whereabouts on the system it is.

[fp280z]

Lots of pics of the header, Y-pipe, and exhaust here:

http://www.typeischeap.com/stahl.html

-Rick

[Curtis240Z]

You AFR during the blog is way too rich basically flooding the engine. Try and use a different emulsion tube because your gas/air jets look to be about the right ratio.

[preith]

I had an almost identical issue on my road racer, it would fall on it's face in that rpm range. I spent quite a while playing with the jets on the Mikuni's but it ended up being CAM TIMING. It was advanced a bit too much, spent the winter degreeing it and now th stumble is virtually non-existent. I traded some emails with Dr. Sideways (Alan) afterwards and I beleive he said he had the same problem as well. It was kinda fun driving it that way, once past that rpm range it felt like a turbo spooling up, she would just take off.

My exhaust is a Nissan Motorsports 1 5/8 primary 3 to 2 header with custom 2.25" duals straight out the back capped with Supertrapps.

EDIT: I should add, the stumble wasn't present until I opened the exhaust up a bit by adding more discs to the 'trapps.

Edited September 9, 2008 by preith

[fp280z]

Thanks Preith! Good info. I know that my builder either advanced or retarded the cam to help with low-end torque, I'll follow up with him on exactly which.

So many variables!

-Rick

P.S. And, yes, it does feel like a turbo spooling up. Great fun but not the best for the autox course.

Edited September 9, 2008 by fp280z

[jmortensen]

You would want the cam advanced for low end. Retarding the cam gives high end power. Opposite of spark timing.

[preith]

Thanks Preith! Good info. I know that my builder either advanced or retarded the cam to help with low-end torque, I'll follow up with him on exactly which.

So many variables!

-Rick

P.S. And, yes, it does feel like a turbo spooling up. Great fun but not the best for the autox course.

No problem, sounds like that may be the problem and if so, glad I can help! Just curious, what brand cam & lift/duration do you have? Mine is a Schneider .560/300. On a side note, I've been very happy with it, no premature lobe wear problems, etc.

[fp280z]

The cam is a Schneider .540 lift and 292 duration. No problems here either, I used the "above 3000rpm for 20 minutes" break-in procedure.

-Rick

[Andydando]

fp280Z

Try going to a 40 or 45 pilot jet,then readjust your idle mixture screws for proper idle.I had this problem also when i was racing FP. I also would not count out adjusting the cam.With the size of your cam i would consider what type of track you race most ,then make the proper adjustment,ie... advance the cam for short tight tracks,retard the cam for long high speed (auto x speed) tracks.

Andy Craig FP Champ 85,87,88 BSP 98

p.s. first time i posted on this site.

p.s.s. you need stiffer spring and better shocks to go faster(saw your pics on spokes)

Edited September 12, 2008 by Andydando
added info

[jmark]

Hope you got it solved Rick. Good luck!

[carusoracer]

fp280Z

Try going to a 40 or 45 pilot jet,then readjust your idle mixture screws for proper idle.I had this problem also when i was racing FP. I also would not count out adjusting the cam.With the size of your cam i would consider what type of track you race most ,then make the proper adjustment,ie... advance the cam for short tight tracks,retard the cam for long high speed (auto x speed) tracks.

Andy Craig FP Champ 85,87,88 BSP 98

p.s. first time i posted on this site.

p.s.s. you need stiffer spring and better shocks to go faster(saw your pics on spokes)

Hey Andy!

Long time! I hope your doing well. I'll send ya a PM.

Hopefully your troubles were cured with the above posts. Good info to have. Andy knows Z Cars, I had a great many conversations and parts I bought from him.

Cheers, Mark

Former BSP 240Z now heading to FP?