CB360 Rat Rod got to run on the dyno

[tweakedlogic]

I came across what I think is a good deal on a pair of used Mikuni VM32s for $75. It is worth buying them and tuning for the 360? Or should I hold out for some VM30s? The 32s would basically need all the internals replaced since they have way too big jets and slide is 3.5. I think a 2.5 or even a 2.0 would be better, but would performance be better with the 30s? would I have as much money tied up in tuning these used 32s as I would with new 30s?

I've been digging around and found what jets others used in VM32s on 350s and 360s to get a baseline starting point. But I can't find anyone that used both and did a comparison, or subjective performance evaluations of either carb (on these bikes).

FYI: The engine is stock internally now, but future plans include the ported head, cam, and larger pistons.

 

[3DogNate]

I came across what I think is a good deal on a pair of used Mikuni VM32s for $75. It is worth buying them and tuning for the 360? Or should I hold out for some VM30s? The 32s would basically need all the internals replaced since they have way too big jets and slide is 3.5. I think a 2.5 or even a 2.0 would be better, but would performance be better with the 30s? would I have as much money tied up in tuning these used 32s as I would with new 30s?

I've been digging around and found what jets others used in VM32s on 350s and 360s to get a baseline starting point. But I can't find anyone that used both and did a comparison, or subjective performance evaluations of either carb (on these bikes).

FYI: The engine is stock internally now, but future plans include the ported head, cam, and larger pistons.

VM32s are too big. VM30s are kinda too big too but can be jetted ok enough to work pretty well evidently. I ended up choosing a pair of VM28 Flat Slides for my 360. I haven't fired it up with them yet, but that should be coming in the next week if I can get enough time in the garage to get the rewire complete.

 

[rockcitycafe]

if they're charging you for dyna time, I'd go get a wideband a/f gauge, weld it into your exhaust and tune as you ride, they're about $150 now, and once you've got your bike dialed in, just screw a plug into the weld in bung

 

[rockcitycafe]

i found that jets r' us has the primary main jet, but it's actually an air jet, the thread matched though, so I bought some and they worked fine, used pin gages to check that the orifice diameter was what I assumed it was

 

[crazypj]

VM32's for $75.00 is a pretty good price, but, as stated, they are too big for stock 360, even if your running at 9K+
I've run RD350 26mm carbs on 390cc motor and been pretty happy with results
If you can find them they have a much better fuel curve than the ones sold by 'everybody'
Slides are 2.0, emulsion tubes are specific for RD and will probably fit VM30?
Early Yamaha Banshee 350 carbs are pretty much the same, later ones have all sorts of weirdness going on (plus, they are generally overpriced)

 

[teazer]

According to my notes, all RD350 and 400 used 28mm crabs but DS7 used 26 and of course the RZ350 uses 26's to make around 60hp with decent exhausts.

Emulsion tube/needle jet on those carbs is usually a #175 bleed type and that's common to several sizes of VM carb. Cutaway are 2.5 on RD350 according to my notes, which are not infallible, and 2.0 on RD250E and R5. R5 and DS7 used the primary type #169 needle jet. Of course different countries sometimes used different sized jets, so treat my comments with caution.

I'd expect a set of RD350 or 400 carbs to be a good starting point for a 4 smoke twin.

VM30 use a 176 series needle jet and that's slightly longer and has a thicker top collar 2.5mm versus 1.5mm on a #175 series.

 

[crazypj]

Could be RD250, it was a long time ago, done way too many swaps over the years ;D
I know I've used YPVS 350, Suzuki GT250, AMAL monoblock, whatever I could find cheap
My brother threw out just about all of my notes 10+ yrs ago (I think he may have stashed some stuff but he had some issues after out mother died)
I'd probably have to move back to Britain fo several months to find out

 

[teazer]

............. done way too many swaps over the years ;D

We're still talking carbs right?

All my old notes are in binders spread across the world and is more or less unrecoverable. Example: I tried dozens of different clutch plate combinations in a CB77 race bike until I came up with the perfect clutch pack. I measured and calculated and tried piles of different springs and alternate plates and of course those notes are nowhere to be found now after so many moves.

For Carb specs I created a LOTUS 123 spreadsheet that I keep updating but there will always be gaps in it and it only includes bikes I have owned and pulled the carbs off or carbs I bought to see if I could use them in a swap..

 

[crazypj]

I've got Lotus 97 somewhere ;D
Should have transferred everything 5~6 computers ago

 

[teazer]

So much stuff (to input), so little time.

When PCs first came out, I used to calculate mean gas velocity and port shapes and stuff like that. I tried in on a programmable calculator and those were just nuts. We installed the first PCs in FORD back when CPM was flavor de jour and Bill gates was still in his parents garage. Then came Visicalc and Supercalc and Lotus and that was it.

Created a capacity planning program in Lotus about a decade before CRP was thought of by the software guys and used it to work out capacity shortages and shift patterns. It never occurred to me that companies would pay stupid amounts of money for that functionality. Ah, the benefits of hindsight.

Now back to that CB360. I'd speculate that the loss of HP was really more a function of points bounce.

For the purposes of a test, could you replace the alternator outer cover with a flat sheet of 1/4" Al with a hole in it to take the rotor through and bolt a DYNA S to that. Wire it so that the points could be unplugged and the DYNA plugged in and test it back to back. That would be interesting to see on teh dyno on the same day.

Eliminating ignition system issues has to be the best way to start setting up carbs.

 

[crazypj]

At present I'm less than impressed with Dyna-S, definitely looks like Brendon's problem was module overheating and dropping a cyl. at low speed.
Running on 1 for a few miles (@50mph) and it cools down enough to run crazy fast, but, wait at a red light and it's back on 1 cyl
He's back on points and bike is running on both cylinders but says it definitely doesn't feel as 'crisp'

 

[teazer]

I have heard of people having overheated issues with a DYNA but maybe we just got lucky. If they are cam mounted, they need airflow - as do any electronics. I like them crank mounted to keep them cool(er). We ventilated the cover and the points housing to get air through there before we changed to a crank mounted system on our CB160 race motors. I just completed a similar set ip on a CB72 race motor.

 

[crazypj]

I modded the points cover to increase airflow but didn't want to carve on the head in case I had tore-fit points (I did )

 

[tweakedlogic]

Good news and bad news. First the good news, I my a:f is much better. I made it up to the low 11s between 5k and 7.5k, and all around 13 above 8k.

Bad news, I made no more power (well, 0.7 HP bump), and the new points didn't help my spark at all. I still have the ripple @5-6K and drops off dramatically at 9K.

My changes for this run were a new stock primary main jet and the new points. I'm starting to think it may be my Dyna coils, but maybe it's the solid copper plug wires, maybe my points start to ark badly. I noticed today after changing the points that they arc to ground sometimes. Right to the points plate. but that was at idle, I didn't see it happen at higher RPMs. Doesn't mean it wasn't doing it, I just couldn't see it.

I am still super lean below 3k. I guess I'm going to have to drill the pilot jet because I can't find anything bigger than the 38 anywhere. If I have an intake leak somewhere, it is very elusive. The intake boots are maybe 2 years old, and the diaphragms look perfect.

The quest continues. I'm not sure where to go next. I could ignore my spark problem for a while and keep playing with the carbs cause that's pretty much free. I do have a ported head waiting to go on, but I wanted to wait for more top end goodies. But I know I need to figure out this spark.

On a side note, the guy that ran an R1 after me, identical power and torque curves every time, I want a fuelie.

BTW: he included last weeks run for comparison on this sheet.

 

[teazer]

There are always some ripples in a power curve, and yours are a little more rippled than normal. PJ is the 350/360 carb guru, but I'd suggest getting that bottom end richer - as long as it's not a false reading from air being pulled back into the tailpipe at lower speed. That's common on short pipes or ones with mechanical baffles.

It's interesting that it is so much richer even at very low revs now compared to earlier, but that's probably both pilot jet and primary main at work.

If the issue is not the measurement, I'd start by adjusting the pilot jets to full rich and I'd also look at the primary air jet. If it's got its own air circuit, I'd be thinking about fitting a smaller air jet to see if that cleans up the bottom end. It's way too rich in the middle from 4750 to7750 which suggests that the change in primary main jet went too far, so I'd go back to the originals and see if you can get the bottom cleaner just on the pilots.

The alternative is to ignore that very bottom end for now and just drop the needles a notch or two to lean out the part lift mid range.

The top end dips and peaks looks like a misfire, so check/replace the condensers and borrow a spare set of coils to test. It''s unusual for DYNA coils to fail like that, but anything is possible. Any idea what battery voltage is like at 10k revs? If it drops that would cause the situation we are looking at. A DYNA or pertronix might be enough to eliminate the misfire. It could be points bounce that's causing those swings and the new points look like they took some of that out, but only to about 9500.

 

[crazypj]

You need to sort out ignition before you go any further, do you have a timing light?
Use it to check for missfire
The transition from primary to secondary mains is even more noticeable now (about 4,750 to 7,750?)
I think Dynojet can't measure below 10:1 A/F ratios,you may be a lot richer than you think mid-range?