Topic: 1978 CB400t - no top end

[Patrick]

This is my son's starter bike and, while it's about time he moved up, I'd like to squeeze the rest of the season out of it. The problem? The bike seems to have lost its top end. Top speed right now is 65-70 mph. I can't figure out why. Low end torque is good, it just bogs at high speed and won't go any faster. I've rebuilt the clutch in case it was slipping and I've cleaned the carbs. Bike starts very easily. About 125 psi cold compression on both sides. Air filter is clean. It's running a bit rich - the plugs are kind of a chocolate brown - but I did that on purpose by going to 120s on the secondary main jets (from 110s) trying to make the bike run a little cooler. We're in Central Texas. A little cooler running is a good thing in summer. Main jets are stock 75s. I've never synched the carbs (beyond bench synching) because they put the synchronizer screw in a seemingly inaccessible spot, but it idles smoothly.

Did I kill the top end by going to the larger secondary jets? I never rode this bike much after I rebuilt it four years ago, so I don't know if it ever had a good top end, but 70 seems weak to me. Any suggestions on what could be causing it to lag?

Patrick

[KB02]

My Wife's bike (Same model) has no problem carrying the two of us at 70 down the highway, so it sounds like something is up. The first thing that jumps into my head is: Are you running an inline fuel filter and if so, is it clean? I can't remember if the bike has an in-tank filter (one around the fuel petcock), but if it does, check that out, too. - BASICALLY, it sounds to me like a fuel starvation issue at the higher speeds.  That's where I would start, anyway.

[Patrick]

I'll pull the petcock tonight and check for blockage. No inline filter. Just a thought: if the valves are not set properly (I believe they are, but I could easily check) could that limit the top end? The compression is decent, though, so I don't put much stock in the possibility. I'm just thinking of other things to check.

Patrick

[kslrr]

Failing ignition coils, improper timing and/or advance can also affect the top end.

[Patrick]

If the petcock is fine I have a couple other sets of coils. I'll try that. The bike has a CDI, though, so the timing cannot be adjusted.

Patrick

[Patrick]

I checked the petcock and it is clear. I also switched out the coil and it doesn't seem to make a difference.

If the accelerator pump diaphram in one of my carbs is damaged or compromised, could that cost me the top end? Like one side not firing along with the other? My son also tells me that the right cylinder gets hotter than the left. How do those diaphrams come out?

Patrick

[kslrr]

Difference in temperature indicates difference in cylinder fuel mixture.  When was the last time that the carbs were rebuilt and every fuel/air passage cleaned?  If it is just the top end, this points to main jets, needles and emulsion tubes.  I did not think that the 400 had an accelerator pump.  I thought the carbs were just like my 350, except maybe larger.

[mlinder]

Too rich of a mixture will for sure cost you top end.

Again, what are the stock sizes, and what are you running now?
What exhausts, and what filter?

Remember, your main jets are there for when you are moving. Once you are moving, the fins cool your bike down.
Overheating happens when in 5mph traffic or stopped.

[Patrick]

I am not too familiar with these carbs, so let me share what I know. The accelerator pump I am refering to is a diaphram under a circular cover on the left side of each carb. It is a diaphram on a peg that is not connected to any outside mechanism and moves with vacuum from inside the carbs. Maybe it's not an accelerator pump. If not, I don't know what it is. I just pulled off the carbs for cleaning this weekend. I did not soak them, but sprayed generous amounts of B-12 through all orifaces and then blew them out with compressed air. I got a good stream of cleaner through everything and do not think there is any blockage.

The jetting on these carbs is weird. Each has a main jet, a secondary main jet and a pilot jet. Stock is 75 main, 110 secondary main and I forget what the pilot is right now. The mains and pilots are stock. I replaced the secondary mains with 120s because I was told that would help the bike run a little cooler by running a little richer. I soaked and cleaned all of the jets and the emulsion tubes in B-12. All appeared clear and the little holes on the sides of the emulsion tubes also are clear. Needles and emulsion tubes are stock.

Exhaust is stock two-into-two. Air cleaner is foam, also stock. The only variable is I recently rebuilt the top end after the previous top end seized. I replaced the cylinder with a good cylinder off one of two parts 400s I own (a '78 and a '79) and replaced the rings. It ran well with no apparent restrictions on the top speed before the rebuild, and compression numbers afterward were adequate (125 psi cold on each side). Used the same head. Readjusted the valves, which I am not spectacularly proficient at. Maybe the valves need to be regapped. Could that be the problem?

Bike has a CDI and no mechanism for timing adjustment, just cam timing.

I'm thinking the problem is the carbs, but I'm not sure what would be wrong with them. I completely rebuilt this bike four years ago and it ran very well until my son ran it low on oil and the top end seized (seized while idling, not at speed). I obviously am overlooking something, but I don't know what. I am convinced the problem is with the air/fuel mixture, but everything seems to be set up correctly.

Also, my K5 750, which I also rebuilt from the ground up, has been running great for three years.

Patrick

[kslrr]

The diaphragms are air-cut-out valves.  They prevent popping when the throttle is let off by richening the mixture.

[Patrick]

Could they have any effect on the top speed? I'm running out of places to look.

[kslrr]

Only if they are leaking.  Then what happens is that there is then a passage from the intake to the idle circuit (I just found this on another thread) which screws with the idle mixture.

[HITMAN]

With the Kei-Hin's on the 78 Hawk I had after sitting in the garage for a year, the slides (diaphram) got a little laquered.  I had similar issues at WOT in high gear, they wouldnt rise high enough to let the needle open up.  Holding the cap upside-down and letting the slide fall, it should just slowly glide to the bottom.  Just watch out for that spring when you pull the two screws out.

[Patrick]

I can be really stupid sometimes. Know what the problem was? of course you don't, because I didn't give you enough information. Prior to rebuilding the top end of this engine I had always run this bike with only a single throttle cable, the pull cable. When I redid the top end this last time I decided, since I had a push cable on the parts bike, to install both cables just like the stock set up was meant to be.

The push cable may not have been the right one, because it was not allowing the carb to open fully.

I'm back to one cable and the top end is back.

Geez....

Patrick

[kslrr]

That's ok, It happens to the best of us .