Dual mikuni's 28's running rich

[Bob S.]

28 Mikuni's worked very well on my 92x69 halfVW... and working excellent on my 92x78 engine right now!
I am not real sure what size my jets are, but I am thinking they are smaller than what you are using!
I'll go look and report back.

My EGT's are 1350-1400 with CHT's around 350 at 3450-3500 RPM Max.

[Bob Wood]

Bob. You have exactly what to am shooting for. My 28 came with 200 jets.  It is possible I missed the correct temps and am on the lean side of the curve as Steve brings up. It is just so odd that my plugs were so sooty and rich. I am really anxious to see what jets you are running. The other thing I am interested in is what slot your needle clip is in. 
Thanks much!
Bob

[rockiedog2]

my 45 with 28s and 200s was way rich...went all the way down to the current 150-160 to get it right. judging from that i don't think youre LOP yet especially with the sooty plugs and it being a 37. 
Bob where are your needles?

[Bob Wood]

Joe
Both my needle clips are in the middle notch #3

[rockiedog2]

ok
i was wondering where Bob S's are.

[Bob Wood]

He was going to check his jet sizes and needle position  I am very interested to hear!!
Bob

[Sam Buchanan]

my 45 with 28s and 200s was way rich...went all the way down to the current 150-160 to get it right. judging from that i don't think youre LOP yet especially with the sooty plugs and it being a 37.  
Bob where are your needles?

I mentioned this previously in this thread but will bring it up again.

Be careful about drawing conclusions based on sooty plugs. The plugs in my Lycoming O-320 were always a tasty grey while burning 100LL avgas, but when I switched to car gas they went sooty. Jetting didn't change, performance didn't change, but the color of the plugs certainly changed.

However, you can hit the sooty plugs with a shot of brake cleaner and the soot is immediately removed and the true color of the plugs is revealed. My Casler 45 has always had sooty plugs and exhaust stacks and this is a known result of using car gas. But a shot of brake cleaner reveals healthy tan plug insulators.

I know this goes against old-school motorcycle jetting theory (been there, done that), but don't pin your entire jetting efforts on plug color. Mapping EGT will be much more productive. But even then don't blindly accept absolute EGT temps. You are looking for mapping based on "relative" temps because the numbers can vary from one engine to another due to different gauges. 1400F on one engine might be the same fuel mixture as 1250F on a different engine due to variations in gauges, probe location and turbulence in the intake tract.

[rockiedog2]

right. i agree. mine run sooty too always have on cargas and the stacks too. the Lycs and Continentals too...all of em.
maybe the plugs are a confirmation maybe not can't tell from here...the plug in the lean cylinder was lighter color than the richer(or lower egt)one.
but both his egts are so low that looks like a valid indication to me; regardless of plug color. and the jet/carb sizes seem to indicate it may be rich yet. guess work w/o being there.
looks like he's gonna find it whatever it is...

[Steve]

Here's a pretty clear article on tuning the Mikuni: http://www.iwt.com.au/mikunicarb.htm  How they go at it Down-Under...

[Bob Wood]

Good info in that article Steve thanks,  
Joe,getting back to your earlier post. I marked my throttle throw on my quadrant and found the top 1/4 of slide opening is really doing nothing. My rpm maxes rout at about 3/4 throttle. This is with the needle clip moved from the middle position to number 2 slot. I ran it up this way with the same mains in from previous run up and got about the same EGT temps 1000-1050 in both sides. It seemed to me I got more rpms in the last 1/4 throttle with the needle clip in the middle on my previous run. Does that make sense?
Should I go back to the middle clip position and continue to decrease jet size and try to raise temps?

Still very interested in what clip position and jet sizes Bob S  is running in his 28's

Thanks,
Bob

[rockiedog2]

SK has more knowledge on this particular thing(carb too big which is what it sounds like you have)than i do but the way i understand it is the smaller motor can't suck enough air thru the carb to use all the venturi(air) thats available so you're getting the fuel metered by the jet size but not the air. yeah it will run "pretty good" but not max performance which is what youre after.  i would probably continue to go to smaller jets still and see what happens and move the needles from mid to down back and forth to see what happens but i wouldnt move them up. even tho the manual says the needles control the mid range and the main jet the upper range in my experience each seems to affect the other...it was trial and error for me when i got into that. i would also be thinking about smaller carbs
i messed around with mine for a couple months. keep at it.
just cause youre not getting 1300 egt doesnt mean you can fly it...many(most?) are too rich. (the general thinkng is it' s the static rpm that determines go no go). that involves your personal judgement of what you got...prop, gross weight, max static, runway length available, obstacles, wind temp, pilot proficiency...all those critical factors that only you can evaluate.
have fun. be careful.

[rockiedog2]

>>>ust cause youre not getting 1300 egt doesnt mean you can fly it

should have said doesn't mean you *can't* fly it

[Steve]

Should I go back to the middle clip position and continue to decrease jet size and try to raise temps?


Thanks,
Bob

Bob:

Put the clips in the middle slot and continue putting smaller main jets in if the EGT temp rises...
Steve

[Bob Wood]

Ok thanks Steve I will continue.. I have a 110 and 115 jet coming this week to try,.  Is it a good idea to use a carb sync gauge of some kind to even out these 2 carbs so they are working together? I used a carb suction gage when I first put on the carbs to set idle evenly on both and double checked that the slides were equal. Is that enough?

[Steve]

Ok thanks Steve I will continue.. I have a 110 and 115 jet coming this week to try,.  Is it a good idea to use a carb sync gauge of some kind to even out these 2 carbs so they are working together? I used a carb suction gage when I first put on the carbs to set idle evenly on both and double checked that the slides were equal. Is that enough?

At the beginning just setting the slides so they open at the same time is adequate... Tim Floyd built a dual manometer on one board to do the WOT & 2000 rpm adjustments after he had a few hours on it - his engine revs smoothly and pulls strong... I think he ended up just balancing at idle and WOT... Tim has the TM28 carb which is designed to perform better in the mid slide ranges...

Have you stabilized your carbs with the Homan brace or is your engine a cut case?... The carbs must not be vibrating...