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First ride is in the books! 74 KZ400, mikuni carbs, pod filters...It was sputtering and popping like crazy though.
I think it's getting too much fuel, didn't have a chance to check the plugs though, and the mufflers are rusted through, so that could be part of it. It was also revving unsteady at idle and at speed. What I'm most concered about though is the lack of power. Can a product like AMSoil, make a big difference for the clutch. I'm not even sure what kind of oil I put in it a few months ago when I first changed it. It revs up nice but doesn't seem to be catching anything. I also thought the mufflers might be bad enough that it's missing backpressure.
I should've said too, this is the first time these wheels have turned under it's own power since 1978. I seemed to get better power the longer I drove it, maybe the clutches have gunk on em.
I am doubtful oil has anything to do with the sputtering problems, but since you asked, I would use a conventional (non-synthetic) motorcycle oil or even a cheap SM listed oil for older vehicles (more zinc).
As far as the popping, have you changed/checked the points and coil, particularly the voltage to the coil? What Mikuni's did you put on it? I have VM32's from Dime City on my 1976 KZ400 and they were not jetted to suit out of the box.
Thanks regal. I found an unkown crack in the left muffler, photo attached. Most of the exhaust is coming out of it. It's about 3-4 inches back from where the weld is.
I haven't checked the coil yet. I put in VM30s. I have consistent rev up to 5000 rpm, but above the rev noise is the noise of the muffler popping and carrying on. It's kind of weird, it could almost sound like two bikes running next to each other, one sounds shitty and one sounds good. I'm thinking the muffler hole is causing me alot of the power problem/backfiring/bike not running consistent?
I got the hole fixed in the muffler(Sea Foam metal and pop rivets) and it still has no power and runs bad. Too rich I think. Time to start rejetting. If I'm getting too little or too much fuel can the motor rev well and still not move the bike as fast as it should be going. I need to sync the carbs too...first I'm thinking?
Revving an engine while it's sitting, or under load, are two different things. You need to get everything back to base before you start making a bunch of changes that will get you farther away.
Get the exhaust buttoned up. Yes you repaired the exhaust, but are the baffles in? Are they baffled at all? Do you have the stock air box? Put it back on. Check your intake manifolds and make sure if they are good. Make sure the carbs are set in them properly. Once you find a constant idle, then you can start making mods. The popping (some of it could be the lack of back pressure) is usually intake, and this is usually from a lean situation, not a rich. Get everything back to base first, or you'll spend weeks chasing crazy theories... I usually find my theories are about 180 off, and get reigned back in by calmer minds. Don't even look at rejetting till you find the actual issues.
Thanks for the insight.
It idles well and revs well while it's sitting. It has problems with load. I've tried loading a vid but they all seem to be too big, wanted to give the pros a listen.
The stock air box is gone so that's not an option. It's stock mufflers, a bit rusty, so I'm thinking all is baffled still. It's new intake boots, and sealed with high temp gasket sealer. I'm thinking the next step is to sync the mikuni vm30s?
Ok, stock box is gone, we'll have to work around it. Are the pipes blocked in any way? You say they are rusty, they haven't collapsed on the inside? See if you can get the carbs. synched up the best you can. Check float levels, and of course look for any gunk or garbage in the bowls...
They aren't blocked, good exhaust coming out. I don't think they're collapsed, I've looked in with a light and it all looks solid as far back as I can see and that's past the baffling. I was going to get some new ones from Dime City later than sooner. http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-chopper-custom-motorcycle-exhaust-parts-16-inch-reverse-cone-megaphone-muffler-80-84033.html
Being brand new carbs I didn't think I'd have to check for any gunk or anything, I added fuel filters to both lines before gas from the tank got to the new carbs, the tank inside though is super clean.
I'm also thinking the clutch is full of gunk, it's been sitting since 1978 with the same old oil and gas in it. I'm thinking about running a 5 minute oil clean solution in it, and replacing it with AMSoil synthetic motorcycle oil, it's supposed to help the clutch tighten up I guess.
Didn't know you had new carbs. Might still want to see if they are at least bench synched. It might be over kill, but when I get a bike, I change the oil three times in a few hundred miles. Change the oil, run for a 100 miles, check oil filter and change oil, run for a few hundred and check filter and plug again. Is your idle hanging at all? High idle? Have you sprayed any carb cleaner around the intake manifolds to see if idle changes? Are you sure the carbs are seated all the way into the intakes? Have you checked valve clearances? It's been a few decades since I've had a KZ400, but mine was really a good bike. Also your clutch springs could be sticking a bit. Are you getting any smell from the clutch? Where are the smart guys who usually jump in here???
Careful with the carb cleaner. Most of them can destroy rubber which can mess with some of the parts in the carb.
If it were me I would be syncing the carbs and checking that the pod filters are not blocking any of the intake. Check the picture attached.
Another great thing to remember when working on old bikes is if you are unsure of the problem to change one thing at a time. That way if it gets worse you know what caused it.
Also go for a run and let it rev out as far as it will until the problem occurs. Once it does shut off the bike and pull the plugs out. Check the colour. If it is white it is lean if it is black it is rich.
What everyone is saying is good advice. While it was good to repair/replace the mufflers, I doubted that would be the fix. Also, since these are VM carbs, ditching the airbox probably won't cause big tuning issues like on CV carbs.
"Bench Sync" the carbs first. Do this by simply making sure the slides rise at the same time when the throttle is pulled. Make changes by adjusting the cable slack adjusters on top of the carbs. Make sure the slides are also set at the same start point. Do this by adjusting the idle speed screws to make both slides have the same gap at rest.
What main jets are in the VM30's you got? They are cheap and very easy to change...so do a "plug chop" and grab a magnifying glass and inspect the plug for mixture condition.
So Carbs Synced. Had Enough Play In The Cable So Both Adjustments On Top Of Carbs Are All The Way Out. Both Flapping Down At The Same Time. Carb Holes Arent Blocked By Air Intake. How About Wd To Check Intakes? Also Both Idle Screws Are All The Way In And Will Start If I Hold Electric Start For 10 Seconds. Its Gotta Be Too Lean Im Thinking.
So Carbs Synced. Had Enough Play In The Cable So Both Adjustments On Top Of Carbs Are All The Way Out. Both Flapping Down At The Same Time. Carb Holes Arent Blocked By Air Intake. How About Wd To Check Intakes? Also Both Idle Screws Are All The Way In And Will Start If I Hold Electric Start For 10 Seconds. Its Gotta Be Too Lean Im Thinking.
There is an idle speed screw (on the side of the carb, parallel to the ground) and idle mixture screw (on the side of the carb but pointing down at the ground at an angle). Which adjustment have you made?
Did you clean the tank completely? If it was idling well, and now it isnt, you really should check the carbs for rust particles. And check float heights, and check the gas tank cap seal/vent, and.....
My point is, if the bike has been sitting since the 70's, it is going to be difficult to just slap new carbs on and go. Did you check the voltage to the coils, change points, set points gap, set timing, etc?
You got it...one thing at a time. It is hard, I know! You just want the thing running and want to ride!
Seriously though, how is the interior of the tank? Check the vent in the gas cap too and make sure it is not clogged or stopped up at all. If air can't pass through it, the tank won't vent and the bike will run great for 5-10 minutes then cut off. I chased that problem down for quite a while, thinking it was running rich. Backed the jets down and it ran worse, but it was acting as if it was rich. Things are not always what they seem...
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