1978 CB400T- Rebuilt top end and Mikuni VM30 tuning

GarageBuiltMoto

GarageBuiltMoto

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Well hello again folks. After a long hiatus of work, kids and life, I finally got around to rebuilding the top end of my 1978 CB400T.

I am a little concerned because my initial compression readings are lower than they were prior to the rebuild (145psi before, roughly 125psi now), but I am using a cheap, crappy gauge and I've read that it can take a while for the rings to seat? Is that true or should I be concerned? I am also kick start only, so I wonder how much that does or doesn't impact the reading.

My other problem is that the OEM carburetors were completely shot and not repairable, so I bought Mikuni VM30s. The bike has literally never run right and I could never get it to idle prior to rebuilding it, so I have no baseline to even ride it or compare against. After the rebuild it literally started on the first kick, but its running super rich and still wont idle. Prior to the rebuild, the plugs would come out wet and oily after running it a bit. This time they came out dry and fouled so maybe that's a good sign that the rings are better??? During the leak down test I did before rebuilding it, I could hear the air rushing past the rings and into the crank case through the dipstick hole. I wasn't able to do another leak down test yet because the wife and kids were asleep and my compressor is LOUD.

Last night I pulled the carbs apart to see how I had them set up and remembered I had purchased the "Performance Tuning Kit - Carburetors - Scott Turner - VM30 - CB350 - CL350" from Dime City Cycles thinking it might get me closer to where I needed to be:

www.dimecitycycles.com

Performance Tuning Kit - Carburetors - Scott Turner - VM30 - CB350 - CL350

Color/Finish: Raw Metal Material: Steel Fits: Mikuni VM30 Carburetors Note: Kit includes parts to upgrade two carburetors.
www.dimecitycycles.com www.dimecitycycles.com

Here's the Jet Setup I found in the carbs last night:

150 main jet
3.0 slide cutaway
159 P-0 needle jet
2.0 air jet
50 pilot jet
6F4 needle, clip in middle position


Since it was definitely running rich with fouled black plugs, I decided to replace everything with the leanest jet setup combination I have, which is:

145 main jet
3.0 slide cutaway
159 P-0 needle jet
2.0 air jet
20 pilot jet
6F5 needle, clip in top position


Since the top end was shot before, I'm ignoring all tuning work done previously as I don't think it it helpful. My main question is- Has anyone ever gotten VM30s with pod filters to work on a CB400T successfully? If so, can you help me with jet setup starting point at all?

TIA. Pictures of the build for attention:

1000023458.jpg

1000023481.jpg
 
Awesome looking bike. I’m not far from you, near Utica ny. All I can say is make small changes to the carbs and document the changes.
 
I know you want carb help, and sorry I’m not that guy, but [drool] the bike looks fantastic! Beautiful work.
 
So after reading the ignition diagnosis thread for the CB400T, I decided to check all of that to make sure I didn't miss anything. Overall I think the numbers seem OK.

Coil primary 0.7
Coil wires 7.8k
Brand new spark plug caps 4.8k and 5.3k

Green and pink 141.4
Blue and white 83.7
Brown and light blue 215

Alternator yellows
0.8 1.0, 1.0
All 3 open to ground

I also did the leakdown test and at 100psi I only had about 3% loss

Looks like my problem is truly the carbs which makes sense because it will start up first kick and run with some throttle but I cant get it to idle and the plugs look super rich if I ride it around the block. Lots of hesitation at half throttle.

I saw a set of VB21B carbs on eBay so I bought them as insurance in case I cant get these VM30s to work. Seems to be a lot more success stories with pods on the original carbs and documented jet settings. I may swap these in just for my sanity. Ive been messing with these Mikunis for far too long.

Stay tuned.
 
I have some modest experience with those carbs and jetting in general, can help. What I don’t have is any experience with that model bike (nicely done btw!) so can’t help there.

This is a bit more work but please, only one change at a time. So, put the needle clip back in the middle position and let’s start there.

I like the PJ at #20. Fwiw, the orifice size is 0.2mm. How does it run once at operating temp? Will it stay running? Popping? (If so, where, out the exhaust or back through the intake?)

Where is the idle mixture screw set? Recommend start at 1.5 turns out. Note that screw with the air jet, primarily affects low speed operation and as the screw controls air, turning in/clockwise, riches the idle mixture.

So, agree that carbs are the issue. Please get some run time. Feel free to PM me also.
 
In terms of MJ, the hex Mikuni jets are calibrated to a flow standard. Unless you can measure (pin gauge or optical comparator come to mind) the orifice size, can only assume the MJ is close enough…for now.

The Keihin VB jets are to a size/diameter standard.
 
Welp... I officially gave up on the Mikuni VM30s. After years of intermittent playing around with jetting to no avail, I decided to rebuild the VB21B carburetors I got off eBay.

First try, first kick, no adjustment it ran AWESOME. Idled great and rev'd up well. I followed some other forum guidance I found on the jetting with pods setup and achieved a ridable result. I'll start a separate thread to talk about the CB400T pod filter jetting (95, 125, 1 needle shim washer), but so far its running the best it has run since I built the bike. I am very happy to have confirmed the carbs were my problem this whole time. That was my assumption but I kept second guessing myself.
 
Last edited:
GarageBuiltMoto said:
Welp... I officially gave up on the Mikuni VM30s. After years of intermittent playing around with jetting to no avail, I decided to rebuild the VB21B carburetors I got off eBay.

First try, first kick, no adjustment it ran AWESOME. Idled great and rev'd up well. I followed some other forum guidance I found on the setting with pods setup and achieved a ridable result. I'll start a separate thread to talk about the CB400T pod filter jetting (95, 125, 1 needle shim washer), but so far its running the best it has run since I built the bike. I am very happy to have confirmed the carbs were my problem this whole time. That was my assumption but I kept second guessing myself.
Click to expand...
Aside from the adapting it takes to use pod filters, Honda carbs are always the best choice for all around rideability and performance. Always have been because they knew what they were doing.
 
Good news that you got it to a good ridable condition. I'm curious what your gas mileage will be with that jetting. when you figure it out report back.
 
This was definitely a "smiles per gallon" build hahaha but I'll try to get some numbers once I get my final setup dialed in and get some longer distance rides in.

I think im scarred from the number of times I had to push it home when the other carbs fouled the plugs so I haven't taken it on a long ride yet.
 
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