Mikuni Bog...Got Data, Need Some Analysis

[fp280z]

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

[JMortensen]

I screwed around with this for a long time. I went the wrong way and installed bigger pump nozzles and adjusted the pump jet linkage to no avail. What I've found to help is larger pilots. It will make the car rich during the stumble, but will also clear it up a bit.

 

 

Others have suggested smaller chokes and shorter runners as solutions.

 

Even what the cause of the stumble is is a bit of a mystery. My own feeling is that when you open the throttle suddenly the velocity in the intake goes down and the car has to kind of catch up for a second before it takes off.

[fp280z]

Ack! That doesn't sound good. I realize that a little stumble is quite common for triples on a Datsun but mine really seems extreme and got a whole lot worse with the free flowing exhaust.

 

Also, on these dyno runs there's no opening the throttle, it's already WOT when the run starts. I guess varying the load may affect the intake tract velocity the same way though?

 

-Rick

[JMortensen]

Try this if you can: Get the car rolling at idle and slowly roll on the throttle. In my car, this would completely eliminate the stumble. That's partially why I figure it is an intake velocity issue. I don't know how load would affect intake tract velocity, so I can't speak to that. My suspicion is that if you're WOT and pulling strong, and then the engine bogs down, then you might have something else going on.

 

I figure that you snap open the throttles, simultaneously the accel pumps shoot a big ol pee stream of fuel down the runners, and that is what causes the bog. I think the reason the larger pilots work is because they will give more fuel without it coming in that big stream. The fuel is atomized better, so no stumble. You already have small pump nozzles, so you're looking good in that respect. Most people run the accel pump linkage on the bottom of the carbs in the middle hole and say don't ever adjust it. I adjusted mine to the max output hole and it made the stumble worse, so I put it back in the middle hole. I think the smaller chokes may be a good idea for you too, because if my hypothesis is right that would keep the velocity up.

 

One other thing I wanted to mess with but haven't yet is the jet blocks. Everyone says use the OA blocks. But if I recall the Mikuni manual and various other sources also say that the 8 blocks are for "street engines". I want to try the 8 blocks, but like I said, haven't got around to it. If you take a look at Webers, they have a lot more parameters to adjust, and the "emulsion tube" is analogous to the Mikuni jet block. They have a whole bunch of them to choose from, where we only have 3 jet blocks and 99% of people are using the OA and won't even consider using anything else.

[Tony D]

Yes, the larger pilots will run the engine at partial throttle to 3000 rpms under normal conditions. The pilots on my car are 60.5 or possibly 63's if I recall.

 

Another thing you 'might' be able to try is to use a slightly smaller 'booster venturi' in there---normally they are 12's, and if you change to 10's, you will get the main jet coming into the circuit earlier, allowing for smaller pilot jetting. These are not the main choke, these are the ones that are on the 'stalks'---and using them to trim at what RPM the main circuit comes in will sometimes help---though it will make the upper rpm ranges more enrichened as the velocity through the carb increases as well. They give an increased vacuum signal on the wells that draws up more mixture into the barrel.

 

This bog is almost identical to the 'if you think carbs are easy' scenario I give for the reasons I ditched my mikuinis and went EFI. A few keystrokes and dyno pulls and you can experiment with stuff that will take you hours/days/weeks/seasons to sort out with jets tubes, etc...

 

From my understanding of the Jet Block setups, the "OA" was the last one on the market, the other versions were 'earlier' and had less optimized fueling solutions. Looking at OEM applications almost all the Toyota applications retrofitted 'OA' blocked carbs into the field when they were available, rather than keeping the earlier blocks out there. The drivability of the later carbs is greatly increased at partial-throttle over the earlier emulsion tubes. I suspect at the cost of upper-end horsepower, but the partial throttle (as we are seeing here) is far more important on a road-course/autox car than terminal power. Maybe if you were running Bonneville there may be an advantage for the earlier block configurations, but I doubt the tradeoff would be worth the effort.

 

Good Luck, man.

[JMortensen]

From my understanding of the Jet Block setups, the "OA" was the last one on the market, the other versions were 'earlier' and had less optimized fueling solutions. Looking at OEM applications almost all the Toyota applications retrofitted 'OA' blocked carbs into the field when they were available, rather than keeping the earlier blocks out there. The drivability of the later carbs is greatly increased at partial-throttle over the earlier emulsion tubes. I suspect at the cost of upper-end horsepower, but the partial throttle (as we are seeing here) is far more important on a road-course/autox car than terminal power. Maybe if you were running Bonneville there may be an advantage for the earlier block configurations, but I doubt the tradeoff would be worth the effort.

That's different than what I was told (don't ask who you can trace that one back to, it's been so long I don't remember... Rebello, Malvern, Nissan Comp, somebody who heard from some guy who had a friend who worked for an expert, ???) Regardless of the source, I was told that the OA blocks maximized top end power and the 8 blocks reduced it for the sake of drivability. I did ask Todd @ Wolf Creek Racing a while back and he did have the #8 blocks, so it would be easy enough to try them out.

 

It would be interesting to compare to a Weber emulsion tube and see what the differences are, I imagine it is the number and size of the holes in the tube.

[Tony D]

Hmmmm, interesting. The first gen Toyota R-G Series cars had 8 blocks, but later in the emissions carburettors they have OA blocks. Weren't the last sets sold out of Northridge all OA blocked carbs? That would support 'max power' for the aftermarket scenario...but that makes a strange dichotomy that they would use those same blocks in emissions laden carburettors on the 40PHH's into the 80's on the 2t and 18R-G motors.

[JMortensen]

I think the answer is to get Rick to try them out...

[Tony D]

Yeeeeah.... Rick, buy a set and let us know if it works out for ya!

 

LOL

[fp280z]

LOL! Believe me, I thought about it. Maybe after Nats when I can get a half day or so on the dyno. I know from my motor specs it may look like I'm made of money but I'm at the end of my budget at this point.

 

It would definitely be interesting and hopefully educational for everyone else to see mysterious #8 side by side with Mr. OA in back to back dyno pulls.

 

BTW, "preith" on the Classic Z car forum suggested that he had a bogging issue in the same RPM range and it turned out to be cam timing:

 

http://www.classiczcars.com/forums/showthread.php?t=31841

 

I didn't set my cam timing but my engine builder did and I know that he advanced (or retarded?) it to get a little more low-end torque for autocross. I need to follow up with him.

 

-Rick