TOPIC:

Hello,

I have a 1976 KZ400 - kick start only. After riding my bike ~20 minutes or so, the bike starts sputtering and bogging, getting worse and worse. I have to ride it at lower and lower RPM's until eventually, ~5 minutes later, the bike just dies and will not run for several hours.

The carbs are clean, gas is good, fuel petcock clean. I just rebuilt the bike. The float height on my Mikuni VM30 carbs is set at 22mm, the points gap is set, the timing is set, the spark plug gaps are set, the valve clearances are set, my 12V Emgo coil has a resistance of ~5ohms across +/- & ~12.5kohms across the plugs when cold. After this issue the resistance across the plug caps did not change (I forgot to measure the resistance across the +/-). It runs and idles great until this begins happening.

Below are pictures of the spark plugs after this happened. To me - they look extremely lean. HOWEVER, the bike will not start with the choke (indicating a rich mixture) and there is no popping or backfiring on deceleration (indicating it's not too lean.)

Do you guys have any ideas? I supposed i could try richening the main jet, but I'm really at a loss as to what is causing this. Maybe go to colder running plugs?

NOTE: This same issue happened back in the summer when it was ~90 degrees. Yesterday when this happened it was 45 degrees. Since this summer, I changed the float height from 25mm to 22mm (to spec) - which I believe richened it.

Thank you,

When it sputters, try popping the gas cap to see if it's not venting properly, or close the petcock and pull a float bowl drain and see how much fuel comes out.

Could be voltage drain too.

What kind of voltage do you have at the coils?
When it stalls you may want to pop off the fuel bowl drain screws to see if there is any fuel inside the carbs.
Is this bike modified and if so how? If you have pods and/or a loud exhaust, what was done to the jetting?

The plugs look okay to me. They appear to be a pale tan, which is basically what you want.

When the bike dies and cannot be restarted, have you ever checked for spark right then? I'm wondering if there is an issue with a specific part (e.g. coil, reg/rec) overheating and needing to cool down. I know you said you replaced a lot of parts, but unfortunately new stuff can still malfunction.

Are there filter screens built into the float valve seats? That's what got me - almost identical symptoms. In my case a little bit of lint from a shop towel clogged up one of the screens after I had methodically cleaned everything.

Kidkawie wrote: When it sputters, try popping the gas cap to see if it's not venting properly, or close the petcock and pull a float bowl drain and see how much fuel comes out.

Could be voltage drain too.

I did not do it this time, however this past summer when it happened I did try this - because my first thought was ran out of gas, so I opened to make sure I had plenty left, Then I thought of a vapor lock so I tried starting it for several minutes with the cap open.

Also at this time (past summer, same symptom/issue) I drained the float bowls which had a lot of gas in them.

Nessism wrote: What kind of voltage do you have at the coils?
When it stalls you may want to pop off the fuel bowl drain screws to see if there is any fuel inside the carbs.
Is this bike modified and if so how? If you have pods and/or a loud exhaust, what was done to the jetting?

I did not think to check the voltage at the coils. I will do that next time.

The only engine modification is VM30 carburetors with POD filters. It has a two in one exhaust with a shorty muffler (with baffle). I believe the main is 165 and the needle and pilot jets are as it comes, needle clip is halfway, I've played with all positions of air screw. Currently I believe it's 2 turns out - since I thought it was rich (won't start with choke) I tried leaning the screw as much as possible.

Nerdy wrote: The plugs look okay to me. They appear to be a pale tan, which is basically what you want.

When the bike dies and cannot be restarted, have you ever checked for spark right then? I'm wondering if there is an issue with a specific part (e.g. coil, reg/rec) overheating and needing to cool down. I know you said you replaced a lot of parts, but unfortunately new stuff can still malfunction.

I did not. I will inspect the regulator and rectifier per the manual once I get some free time.

ayeckley wrote: Are there filter screens built into the float valve seats? That's what got me - almost identical symptoms. In my case a little bit of lint from a shop towel clogged up one of the screens after I had methodically cleaned everything.

I'll check it out - It has Mikuni VM30's on it. When I was cleaning the carbs I tried taking the float assembly off/apart but actually couldn't figure it out. I flipped through the carb manual and nothing was mentioned about this - but I will give it another look.

How does the kick start feel after it dies (ie do you still have compression)? It's obviously heat related and maybe there is something wrong with the valve tolerances?

I wouldn't think it was carbs if it runs fine for 20 min. You could try starting with ether after it dies (assuming itd be running lean). But it could be running way rich and this guy had the same issue

Otherwise id say heat soaked electronics.

Saablord wrote: How does the kick start feel after it dies (ie do you still have compression)? It's obviously heat related and maybe there is something wrong with the valve tolerances?

I wouldn't think it was carbs if it runs fine for 20 min. You could try starting with ether after it dies (assuming itd be running lean). But it could be running way rich and this guy had the same issue

Otherwise id say heat soaked electronics.

So this got me thinking... back when I purchased this bike the previous owner supposedly went through the whole motor and "rebuilt it." I rode it without issue up to 1-1.5hrs at a time.

I planned on doing a 2.5hr trip on the bike, and with me being particular about things and having some free time I decided to go through the whole bike. One thing I do remember changing is that when I went to check the valve clearances they were all on the "loose side" - from what I remember around 0.007-0.009".

The manual specs 0.004 to 0.006" so I set them to 0.004". After this adjustment, and maybe other adjustments (?), the bike ran into this problem after the 2.5hr road trip.

Currently the valves are set to 0.005".

As far as compression - I'm a bit embarrassed to say. It tested cold at 90psi both cylinders, and wet at 115psi both cylinders (but I think I may have put too much oil in.)

After this previous episode of it cutting off on me, I checked it when it was warm (after pushing the bike for 30 minutes) and to my disappointment it was still 90psi. Honestly I hope my harbor freight compression tester is just junk and those are incorrect readings.

I also want to add that when adjusting the timing the strobe light was more sporadic than I was expecting it to be (indicating miss-firing, inconsistent spark?) I also could not get the advance timing marks to align properly when the "static" timing was set properly. However because of the strobe light these readings were constantly jumping back and forth.

Even with this, it runs great across all RPM until ~20 minutes in (although idle could be better.)

So maybe the inconsistent spark (ie strobe light) is indicating a bad electrical part, and with time (heat) it gets to the point where it doesn't work?

Not the greatest compression, but it doesn't change so we can rule that out i think.

Have you verified that the cams are in time after those valve adjustments (not just the ignition timing)?

Is your timing advance mechanism functioning properly? (im new to bikes, i don't know if this would affect things)

If you have the same compression tester I got at harbor freight then it might be why you are reading lower compression. Loudhvx pointed this out to me. I bought a different tester, but I haven't used it to compare it to the HF one.

kzrider.com/kunena/2-engine/606826-low-c...n-s-on-kz1100#770728

Saablord wrote: Not the greatest compression, but it doesn't change so we can rule that out i think.

Have you verified that the cams are in time after those valve adjustments (not just the ignition timing)?

Is your timing advance mechanism functioning properly? (im new to bikes, i don't know if this would affect things)

From my understanding, the cam timing (ref cam chain and cam gear), does not change with changes to the valve clearances.

When the advance and static timing do not align, the manual says that the timing advance mechanism is worn. I don't have any way of definitively checking it, but it operates smoothly by hand.

M_a_t_t wrote: If you have the same compression tester I got at harbor freight then it might be why you are reading lower compression. Loudhvx pointed this out to me. I bought a different tester, but I haven't used it to compare it to the HF one.

kzrider.com/kunena/2-engine/606826-low-c...n-s-on-kz1100#770728

I am guilty of the Harbor Freight compression checker as well. I hope it is 60psi off!