Dual mikuni's 28's running rich

[Bob Wood]

Steve,
Thanks I did see the Floyd info a while back I will build one makes sense...
I have not braced my carbs yet sort of played with it and tried a stiffer intake hose but still shaking and venting a small amount of fuel at random times during the run. I was hoping as I got the engine tuned between the two carbs it would smooth out. I also was thinking as those carbs shake the float bowl needle is bouncing all over causing more fuel in the bowl than needed possible causing a rich condition I am seeing and also the fuel venting issue. Am I right?  I will get back to the bracing I did see his pic ,looked better than my idea of running braces from the carbs to the bottom drilled holes on each corner of the case. Did you suggest the cut case does not have the shaking carb issues as bad.

Bob

[Steve]

Steve,
Thanks I did see the Floyd info a while back I will build one makes sense...
I have not braced my carbs yet sort of played with it and tried a stiffer intake hose but still shaking and venting a small amount of fuel at random times during the run. I was hoping as I got the engine tuned between the two carbs it would smooth out. I also was thinking as those carbs shake the float bowl needle is bouncing all over causing more fuel in the bowl than needed possible causing a rich condition I am seeing and also the fuel venting issue. Am I right?  I will get back to the bracing I did see his pic ,looked better than my idea of running braces from the carbs to the bottom drilled holes on each corner of the case. Did you suggest the cut case does not have the shaking carb issues as bad.

Bob

Bob:
If the carbs are vibrating around the engine will never run right - brace the carbs at the air filter side using the Homan example on full case engines - the cut case engine by design dosen't have the aft case flange to anchor on - the brace for a cut case can look like the rigging Joe Spencer has but don't try doing something anchored from the engine mount - it'll vibrate worse...  1/2 Vw's shake - a good balance job makes them better - soft engine mount bushings are essential - the Balance Master disk a plus - a dynamic engine/prop balance job the final touch...
Steve

[Bob S.]

OK...So I finally got out to the hanger to inspect my LE Better-Half Mikuni's...
I was totally flabberghasted to see what I saw when pulling the jets... I don't know how or why but I have a 180 jet on one side and a 120 jet on the other! THAT sounds TOTALLY screwed up, I know, but the bugger is running just fine! NOW I am at a quandary to know what I really should have in it! I THOUGHT I was running 140 or 145's...... I guess I need to do some more testing.... One side LEAN and the other RICH?? but both sides running just under 1400 EGT's....only have one CHT and it is on the #120 side and is running 350 or so...
BTW, Both needles are set to the mid-ring...

[Bob Wood]

Bob,
Many thanks for this. Are you running Carb supports to eliminate the vibration.  I put them on last night per Steve K recommendation and ran it up and I noticed a couple of things. The EGT temps seemed to come up faster and were more stable on my LED Belite gauges.  I am currently at 950 and 1150 EGT with 120 and a 115 jets. Needle clips in center slot.  The other good thing is all the weird air spaces and bubbles in my fuel lines to the carbs after the split have gone away. I never liked to see the air in the fuel lines!!! The vibrations must shake the hell out of those carbs and disturb float bowl operation etc.....

 Do you remember how many jet sizes it took you to get up those last 200 degrees? My One CHT probe barely hits 250 during these run ups. I am turning a TN 54-22 prop and getting 3100 Static. Does that static go up a little when flying? I am running 2.3 QTS and only see about 125 degrees oil temp on the ground during my test run ups. I have hot flown yet. My Mosler 37 has a compression ratio of 8:1 after I lowered it from 9.2:1. It runs and starts beautifully with the 009 dist set up. I have a couple lower jets coming this week . Going to run up a 105 & 110 and see where I get to on EGT.

Thanks for your help, nice to have a similar power plant to discuss. this is my first Ultralight and VW project. Been flying a Tripacer for 28 years, having a ball with this, really excited to get it in the air!!

Bob W.

[Bob S.]

I am NOT running any carb supports but am using a pretty stiff & short diesel heater hose for the coupling between the carb and head. I may have a MAX 1/4" between the intake and the spigot of the carb. and my intake tubes on the heads are pretty short too...MAX 2" from the head. I have black fuel lines between the pump thru the "T" to the carbs so I don't see any air there.... ONLY in the blue clear tube from the tank to the pump and that goes away pretty quick.

What jets DO you have...120 & 115 now in the carbs and what others have you tried? I have a stash of smaller jets but they were all too small!!
Carbs COME with 200's (IIRC)...and they weren't too far off really... Like I said, I THOUGHT I was at 145's before going to Reklaw....Don't suppose someone changed my jets as well as re-timed my distributor do you???

I don't really bother too much with CHT unless you are getting too hot...with all that oil, you shouldn't be too hot. I barely run 2 qts plus a 1/2 for the filter.

I liked the TN 54-20 and my BEST was a 54-22 I had cut down to 54-20!!! It had the smaller hub too... I ordered a NEW 54-20 from TN and they sent a WIDE, THICK HUB stick that was terrible! I also liked a thin 54-20 from Culver...but the prop I have from Performance is the best of the best!!!!

If your engine is newly built, all the numbers will change every time you run it until you get 25-30 hours minimum! And then they all get better from there... 

I was terribly disappointed on my thrust numbers at first, but you will be surprised how even they come up. If you have a good runway area, I suggest getting it out on there and make some runs up and down. Get a "FEEL" for the T/O speed/rpm... There are those who don't approve of 'Crow Hops' but I taught myself to fly doing them! Just remember where the end of the runway is. If you can't get it back down smoothly before the end, Guess what? You better fly it!

[Bob Wood]

Bob,

I tried extra stiff hoses also and still had vibration....My supports go from the air filter side to the corner of the case (metric threaded hole there on all 4 corners. I went just from the back hole on each side of the engine to the carb back with  3/8 aluminum tube...

My VM28's came with 200 jets and I went to 170- 145- 130-  120 now hovering at 100- 110.  What happens if you go too low? do you not get the full rpms or do temps go above 1400?? I did not even see anything above 900 degrees until I but the 130- 120 in.  (My #4 was a little richer than the #2)

Still get some spitting out of the carbs so I ran both vents to an aluminum tube discussed in the forum to take the venturi effect out of the equation.  I still wonder if my floats are set too high....Did you adjust your float levels?

So does your static on the ground match your flight WOT RPM? What size is your performance prop? What does it pull for static?  I ended up with the TN 54-22 per Scott and Leonard at Airventure last year. 3100 static was the magic number according to  them.  

My motor has about 50 hours on it, I put another 5 on it taxi and running up with these carb issues, feels good, and familiar. My airport has over 5000' so I was going to do some hops on calm nights to feel it out. going to be strange taking off below 60......

Thanks for the feedback

Bob

[Bob S.]

 
What happens if you go too low?

Generally your temps & RPM's go DOWN as you go leaner as well as going richer.... going Lean is quicker than rich, IIRC. Think of the bell curve. TOO Lean and it quits running, Hard to start too.

I failed to do as I am telling you: Keep a chart of CHT/EGT/RPM for each test jet... If you do that you can SEE things happening albeit very small. AND if you are doing it all static, put your THRUST values in there too... My values were all over the place.. I just had to settle on one and use it. HOPEFULLY you won't get the 1400 EGT...not right away...

You say 'spitting'....is that out the venturi? I had fuel coming out the overflow tubes and yes, I had to adjust the float levels quite a bit as the floats do pound up and down a bit! I set it so the tabs are pretty level with the bottom of the carb body.

 So does your static on the ground match your flight WOT RPM?

My "static" is lower than WOT in flight but only by about 200 RPM or so. Prop unloads in the air somewhat.

What size is your performance prop? What does it pull for static?

I BELIEVE it is a 58-22 in Frank's way of measurement... Each prop mfg. has their own way to measure. I never really measured static on the PP.

I ended up with the TN 54-22 per Scott and Leonard at Airventure last year. 3100 static was the magic number according to them. My motor has about 50 hours on it, I put another 5 on it taxi and running up with these carb issues, feels good, and familiar. My airport has over 5000' so I was going to do some hops on calm nights to feel it out. going to be strange taking off below 60...... Thanks for the feedback Bob

Well, you will definitely get more RPM with a smaller pitch prop (54-20) your top speed will be less. But we arent really looking for SPEED...
anything over 3100 RPM is a bonus! I start my T/O run open all the way at 3250..climbing at same then level off at 3400-3450 before throttling back to 3300, where I cruise at 55-60 IAS It WILL take-off at lower RPM, I just like to feel the surge and clear the trees at the end of my field. I get to 100' AGL by half our 4000' runway and up to 200' by the end if everything is working well.

[Bob Wood]

Thanks Bob will keep in mind and have been charting..by spitting I mean overflow tubes. I had one spitting worse than the other so I set it to the one that spit less, since I did not have a float  measurement to go to. My tabs are not level they are down a little when you look from the side not parallel to base plate. so they are leaner than yours I think if I am looking at this right. ( little tab down closer to needle means faster shut off which is leaner right??

Maybe I can go a little more lean on the float level... I will work on temps first

Bob

[Bob Wood]

Bob. S. Question for you. All my testing and running so far has been with 91 mogas. (Eth free)Are you burning 100ll or mogas. I was thinking my low EGT may have something to do with that. I burn it in my tripacer and get 1400 no problem so no issue I keep thinking maybe vw different

We discussed float tabs & float level does being too high add to fuel spitting out overflow tubes?  If you are too low I assume engine would loose power at High power settings. When I look at my float plate from the side my tabs are just the slightest down (toward the float needle). Seemed leaner than yours (being level)

After I put my carb braces on I kept going down from 120 to 110 to  105. When I hit 105 the engine loses power at about 3/4 throttle. So too lean I assume. Temps still not over 1050 So I went back to a 110 and 120 and see temps of 1000 -1050max in both sides..

Then I put the 140 and 130 back in just to see if the carb bracing changed anything and got no EGT. On either gauge. My LED gauges start at 900. I even put a 140 and 150 in to see if I was on the wrong side of the curve and again less than 900. EGT.

So...the only other thing I have not tried is moving the needle clip I notch richer.  I think I mentioned I tried one notch leaner and it lost power just like when I put in a 105 jet...if moving just one notch leaner killed it maybe I am just too lean at the needle.....

So I am going to try that today.  Go richer and try the jets again 120 - 180

Any chance this low temp issue is due to timing or valves. I set my points at .020. Plug gap ar .035, timing at 0. Valves .006Per Leonard.  I set my timing with a light gun at 0 and checked that I do get the advance at 2500 rpm or so

I have not rechecked my valve lash since my run testing (8 hours) everything else rechecked.

Thanks for any input.
Bob Wood

[Tom H]

Bob, you mentioned timing.  I wonder if the advance is not going as far as you think.  I set my timing, on both distributor and magneto, with a timing light.  It gets a little windy, but is doable.  At higher rpms, say over 2500 rpm, the timing is set to 28 BTDC.

[Dan_]

Bob,
Are you sure that EGT gage is Kosher..?  What brand is it, and is it new..?

There are J and K types of thermocouple probes and they must be a certain distance down the exhaust pipe from the valve seat, and reside in the center of the flow.  Are the probes the same type, the right type for the gage, and in the right place..?  Polarity correct..?


If there is any doubt on these questions, perhaps you could take the whole EGT package to another engine with a known good EGT setup and compare the readings you get..? This would eliminate a variable in the simultaneous equation.

[Steve]

Bob:
I think it's time for you to try a prop with less pitch - see if you can borrow a thin tip 54x20 from some one in your area or the woodbar dyno from Les Homan if you have the the 3 1/8" bolt pattern (Les may have the 4" pattern in the dyno too now - ask if you need the 4" setup)...
Steve

[Dan_]

So...the only other thing I have not tried is moving the needle clip I notch richer.  I think I mentioned I tried one notch leaner and it lost power just like when I put in a 105 jet...if moving just one notch leaner killed it maybe I am just too lean at the needle.....

Like Sam said the 2 strokes are very sensitive to air/fuel ratio, and die easily when it is not right.  So their carbs are fine tunable.  We are basically using a 2 stroke carb here, so it is very finely tunable over a very wide rpm range.  

So we must keep the big picture in mind when tuning it.  First and foremost it must not only be in the ball park... it has to be in the infield --before it is going to respond well to fine adjustment.   We must have the floats right first, we must have the shaking out first.  If the fuel is frothing... it ain't gonna run right.

Best way to get to the infield is start tuning at wide open throttle, and work your way down.  WOT is main jet only...  nothing else in the carb affects it, get the best (highest) rpm and plug color reading until you have the main jet nailed down.  Then worry about the needle setting and if you need another needle jet.  

Put a new plug in one cylinder, fire it up, run it wide open... kill it while at max rpm, read the plug.  What ever the plug tells you, make that main jet change on both carbs.  Rinse and repeat.

You may want to get hold of some avgas to make the plug reading easier.  Here is plug reading info.

Or you could try a google search on plug reading with unleaded

[Bob Wood]

Tom H,

I set my points to .020 at tdc, fired it up and set it with the light to Zero degrees,  and it was getting to the  28BTDC mark on my hub at around 2200 to 2500 RPM

Is the the correct way to do it. I realize that if you reset points you have to re time.
Thanks,
Bob

[Bob Wood]

As i said I was going to run up with the needle 1 notch richer. It ran rough and choppy as I got to 3/4 throttle. really bad.
I did that with the 120 and the 115 jet in that gave me the highest EGT temps to date when I was at the middle clip setting.  See chart below

I charted my egts on both cylinders with the needle clip in the middle.

My Belite LED EGT/CHT  gauges start at 900 degrees and the lights go up every 50 degrees. My probes are both 2" below the flanges. Through all the run ups my CHT is between 250 and 300.  I only have 1 side probed.

#2  200   less than 900
      170   less than 900
       150  less than 900
       130   950    ( first time the gauge showed an increase)
      120   1000   I thought I was getting there.....
      115    950  This was after I braced the carbs
       110   950
       105   bogged down and lost RPM at 3/4 throttle




#4    140  less than 900
        130  1000
        120  950-1000
        120 1050  after the carbs were braced picked up 50 degrees
        115 1150
        110  1150
        105 1050   bogged down at 1/2 to 3/4 throttle like #2 did.
       
my static RPM are always about the same 3090 to 3150  hard to tell above 3000 on the tine tac A2  but I checked it with a digital hand held. The only big reduction was with the 105 jet with the big cut out at 3/4 I am assuming lean cut out.

Not sure where to go from here......I thought my 3100 static was good on the prop? In the air it will be 3300 maybe......

Still wondering if my floats are set too rich.....

Maybe run 100ll and see what happens my engine is 8:1 compression  I am running 91  no ethanol )

Thanks for any pointers

Bob W