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Discussion Starter · #1 ·
Hey guys,

Been working on a 78 cb400t for awhile no, resto mod. Getting a fresh top end tuned in with some VM30s, UNI pods, Stainless 2 into 1 exhaust.

I am getting close with the VM30's, current setup:
slow jet: 35
Needle: 64F 2nd from top
Slide: 3.0
Main: 140
Air: 1.75 out

Slightly lean up top, rich at idle. Have 145s on order.

However, while I was doing plug chops I noticed some wet plugs coming out. This happened after I adjusted valves. They were bad. I adjusted during motor assembly per FSM but this is the first time after really riding the motor. So I am assuming this boosted the chamber pressure to the point that is is starting to really test the rings. Here is my plan to trouble shoot and I would like some input from the forum on anything to look out for.

compression test.

oil in chamber compression test.

Leak Down test.

I will report back this afternoon with results.


Here are some pics. Trying to do a scrambler type build. I was humbled in my my first stainless exhaust attempt, love it but can't wait to do another to make it better. See ya in a few hours.

IMG_0875[1].JPG IMG_0737[1].jpg
 

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Oil or fuel fouled? Once the plugs are fouled they are garbage, yeah you may be able to sort of use them but any diagnosis is pointless using them a second time.
How new is the engine? Has it been broken in?
A couple of unrelated items: you really do want to have a front fork brace. Either the fender or use the fender mounting bracket after drilling out the fender rivets. That keeps both fork sliders working in unison.
The exhaust appears to become a single pipe at the front of the engine, if so then the primary pipes are too short which will sacrifice low and mid range power.
 

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Discussion Starter · #3 · (Edited)
Did compression on cold engine

Left: 187 right .185 ish

So that was good. Added oil and the starter couldn't turn it over. Couldn't kick it due to bad kickstart gear or what ever it catches in there.

Could be unburt fuel. Plugs show uneven carbon ring on the porcelain. Could that be from bad spark? Anyway, idle was a tad rich. It is missing and gurgling from 1/8 to 1/2 throttle. Top end ripped. So I need to do a full throttle chop and a half and I should have a direction to go next.

Couldn't do the leak down test. Small compressor I have took a crap, time to get a real one.

Pipe is what it is at this point. Will keep the down tube length in mind for future builds.


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I would verify that the ignition system is 100% since compression is great. No need for leak down with those readings. There's a sticky in the Electrical section on the ignition
 

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Discussion Starter · #5 ·
Tuned in my air screws per the manual and changed to 2nd clip position from the top on the needles and she running pretty damn good. Here is a shot of the plugs after a quick spin through the gears


Might be a bit on the lean side. Had really good power. Surprised me a bit.

Do any of the aftermarket ignition mods make any more power than a properly working stock system?


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I can almost guarantee that your ignition system isn't working 100%. The timing advance will be late/slow if it even hits full advance. This is due to the resistances in the stator and pickups as well as the CDI unit. I experimented with 3 different CDI's and stators in various combinations and got close but not right. One combo wouldn't get full advance even at 7K rpm.
I've been running the GM coil conversion for 5 years now which gives better and cleaner spark, especially with wire core plugs wires, not graphite.
I've been running the Ignitech CDI for 3 years now which allows you to program the advance curve via laptop.
Proper spark and timing are important to creating horsepower.
Member Datah tried creating a CDI fix which utilized a GM control module. Those modules control the coil signal to fire it but have no timing advance since GM still relied on the original mechanical distributor for that. It worked as far as getting the bike to run and it would run down the road but never developed any real power.
BTW, your plugs look great. These engines always produce plugs that are lightly tanned to barely any color on them.
 
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