Dual mikuni's 28's running rich

[Bob Wood]

Thanks for this Joe, I am also going to move up my Probe to 2" and see if that matters.

Will report back with the 125's  results
Bob

[Bob Wood]

Jetting 37 HP update:

I put in 130's in both sides last night only because I could not get 125's at my local cycle shop. My EGT is on #4, and it is still very black and sooty...On #2 the CGT side it was very clean and looked good. Is it likely #4 is still too rich to get above 900? My LED gauge has  a 900 minimum reading...

So i still get no minimum 900 degree EGT reading on #4  and when I shut down I took a temp reading on the head where my CHT probe is under the head bolt. It read right at 230 under the LED gauge minimum 250 mark.

I checked my probe location on #4 for EGT and I am 2" below the bottom end of the pipe flange. Seem ok....

So i am going to try a 120 in the #4 EGT side. I also am going to install another egt probe on the #2 side. 

Any thoughts? when you are close to EGT of  1400 ..say 1200, how much of a jet reduction does it take to gain 100 degrees?

I kept my needle clip in the center slot on both carbs

Thanks for the help,
Bob

[Steve]

Bob:
Use a Multimeter with the Milli-volt scale and check the EGT probe voltages per attached table:
Steve

[Bob Wood]

Perfect !! thanks steve.
Bob

[rockiedog2]

maybe you told us this in the OP...have you checked the float level for the rich cylinder? also have you verified that the float needle is seating?
as a check on the egt gage try swap it over to the nice running cylinder and see what thats doing. if its in the ball park say 1100-1300 and you got near the same jets both cylinders i would start looking at stuff besides the jets too. you're getting a nice hot spark at the plug on the rich cylinder? i'm starting to think i would try swap carbs over and see what you get then. should be a reversal if its in the carb. no change if it's something else

[Bob Wood]

Thanks Joe, good stuff here. On the floats...should I just see if both are the same, or is there a height measurement I should set?  am going to take volt meter measurements on the probes and see the differences between the good and rich side also.

On the gas spitting situation  it seems like the carbs are vibrating on the hoses and shaking fuel out of the vents.. Maybe the floats are jumping around and opening the float needle....

Will be back you after my next run up.
Bob

[rockiedog2]

IIRC the top of the float should be set level with the top of the bowl. if the rich carb is set too high that could be the problem. mine drip and spit randomly and that could mask a high float overflowing the bowl. basic carb stuff...
the 37 doesn't hit as hard or shake as much as the 45...maybe when you get the carbs balanced it will settle down and stop shaking and spitting so much. if not a brace would be good. here's a pic of mine
IMO dual egts are important for fine tuning these things. if we don't get the carbs closely balanced then its like 2 diff power motors hooked to the same crank...they fight one another. course that costs power and we don't have much. and the routine daily temp swings will affect the egt so its good to know  what its doing. if you got it tuned to say 1350 for summer afternoon temps and go out and do a cool early morning flight the egt may go up as much as 100 degrees and nor we/re over the 1400 max.

[Bob Wood]

Many thanks Joe,  I will report back.
Bob

[Bob Wood]

I checked my float levels yesterday, sure enough the rich side was set richer than the lean side by about 2mm. So I evened them out and will run up and take probe voltage measurements to see what is up with EGT. I ahve my second probe on order.... Raining so no runup....

So i went back to the Mikuni handbook and looked at float height measurements for the VM28 and I see it says 15-17mm. That seems even lower than where I am now. So i am going to recheck after my run up. With the independent float set up you just measure the plate height when the tab just touched the needle. I  did not do this I just compared my lean to rich and made them the same...

Do you guys go with the Book float setting. After seeing this set up it is easy to see why the carbs spit fuel. My rich side was the only side spitting alot.  Just a couple drops on the other side.

Thanks for the help, I feel like I am making headway....
Bob

[Bob Wood]

I ran up again with a 120 main on #4 carb(Richer side) and it is still very sooty and black. The #2 side looks nice and clean with the 130 in it.

I took EGT readings with the volt meter and got 19.8 on the lean side #2  and my gauge was indicating about 925-950 on the rich side #4 so very close to each other.

So I was going to try 100 & 110 main jet  in the rich side, and maybe110- 115 in the lean side to try to get up to 1300 or so.

The other interesting thing is I checked my air screws at 1000 RPMS. I have had them both set at 1 1/4 turns out and went in and out 1/4 turn to see if I could gain any RPMs.. No big effect (smooth acceleration) I left them at 1 1/4 out.  Then I closed each one and opened back up to 1 1/4 while the engine was running at 1000 RPMs. On #2 the leaner side the engine idled way down seemed like it was going to quit...On the rich side there was very little effect when I close the air screw ( slight RPM reduction)  

Is this telling me the rich side is so rich already  the extra richness ( when closing the air screw) is not effecting anything. And on the lean side the extra richness is bogging things down?  Seems logical , am i correct on this?

So I continue decreasing jet size to increase the EGT.  Looking for 1300-1350 or so....correct?
Thanks for any suggestions.
Bob

[rockiedog2]

>>>On the rich side there was very little effect when I close the air screw ( slight RPM reduction)  

not sure which airscrew youre talking about but if there's no change when rotating that indicates its probably stopped up. a stopped up airscrew would probably cause it to be rich. have you removed it and checked for that? i'd blow air thru it put it back in and see if any change. you should have gotten at least some improvement on the rich side with the jet changes and float adjustment. since it sounds like you haven't seen any improvement atall i'd look elsewhere for a while...like the airscrew and possibly casting scale in the passages inside the carb altho that's not very likely i've heard that it's happened. are you running air filters? no obstruction there? try taking the filter off and running it. i think youre short on air somewhere. you haven't said anything about the choke.

have you got this manual?



Sam and SK yall got any more ideas?

[Steve]

Joe: I think you're right on track with observations re: the still rich carb - maybe switch carbs to eliminate cyl or ign anomalies... Steve

[Bob Wood]

Ok ,did another run up tonight and I went from a 120 to a 105 jet and went from 925 or so to a about 1050 or 1100. EGT. That was the rich cylinder. I am using a motorcycle air filter that seems like good flow when I blow through it.  I will do a run up with no filter and see what happens. I am waiting for a probe to test the lean side. I was going to swap carbs to see if it is carbs..I was going to try a 95 or so down from the 105 to see where the EGT goes. After I went to the 105 jet the carb did stop spitting fuel from the vents.......

Does it seem like a 95 jet will get me up to 1300 pr so?

Think I am on the right track?
Bob

[rockiedog2]

i wouldn't expect it to take more than a couple sizes diff to match up 2 otherwise same same cylinders. believe you said you had a 120 on the lean side. youre down to a 105 jet now which leads me to think the problem is air and that's where i'd be looking at this point. seems all youre doing now is matching the fuel to a restricted air source
that non responsive air screw is job one. if that doesn't fix it and the air filter removal doesnt do it then next is swap the carbs side to side. i wouldn't do anything else til i had done those 3 things one thing at a time. always one thing at a time and then test.

[rockiedog2]

i looked back and see you have a 130 in the lean side. that sounds about right for a 37...my 45 is a 150 and a 160 IIRC. naw, that rich side definitely looks like it has something besides the jetting going on.