79 kz650 loss of power

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10 May 2015 09:41 #671373 by bikerdave79
79 kz650 loss of power was created by bikerdave79
Just got a new to me 79 kz650. Bike starts, idles and revs up nice. Problem I'm having is under hard acceleration, or higher rpm it breaks up. Feels like cylinders are dropping out. I also get a backfire on the left pipe (cyl 2-3) on decel. I've cleaned and adjusted the points to spec, set the timing to spec. It helped some but it still bogs terribly on hills or under heavy throttle. If I baby it under light throttle it seems to be OK. The spark advance seems to be lubed and moving OK. Only thing I've seen is the points are a bit pitted. I cleaned them without much difference. Not sure how old they are or the condition of the condensers so i ordered a new set should be here this week. My question is am I on to the right thing here or an I completely going in the wrong direction? Thinking it could also be a bad accelerator pump diaphragm because if I snap on the throttle quickly it responds for like half a second then bogs. Wondering if maybe the diphram is leaking fuel. Any ideas?

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10 May 2015 10:12 #671375 by Nessism
Replied by Nessism on topic 79 kz650 loss of power
If the bike stock? All the various maintenance tasks like valve adjustment up to date?

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10 May 2015 10:18 #671377 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power
Bone stock as far as I know. Stock airbox, pipes. It only has 20k miles. I'm the second owner. I had the vc off to replace a gasket. Clearances all seem to be in spec. I doubt they'd be off much with so little miles. It feels like I'm losing cyl 2-3 at heavy throttle or up hills. The coil has been replaced recently by the po. The 2-3 points are deeply pitted so I'm thinking that's where the problem lies. It revs all cylinders fine and idles fine in neutral. Only when your riding up hills or trying to romp on it does it act up. With a passenger on it feels super sluggish

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10 May 2015 10:21 #671378 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power
I also had the bowls off the carbs to check for dirt. Clean as a whistle inside

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10 May 2015 10:42 #671384 by Nessism
Replied by Nessism on topic Re:79 kz650 loss of power
20k miles is a lot of miles for a bike that never had a valve adjustment. If you checked them and found all are in spec then it's almost certain that someone adjusted them previously. At any rate, those points should be replaced, and you may as well do the condensers at the same time. Breaking up under load could be a dirty carb issue, or maybe a clogged up air filter. Not sure. After you replace the points you may want to do some plug chops if the situation continues. You need to determine if the engine is running lean/rich or just misfiring. Oh, and check the plugs if you haven't already done so.

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10 May 2015 10:43 - 10 May 2015 10:47 #671385 by Tyler
Replied by Tyler on topic Re:79 kz650 loss of power
Problems like you describe can be caused by a restriction in the fuel line. How does the tank look inside? Clean the pickup screen, add an external fuel filter and a new fuel line that may help.

How do your plugs look?

Checking the carbs is good, did you pull a main jet? Check the float levels?

I agree with need better sort out the points first. Our covert to electronic, if your gonna keep the bike for a while you may want to think about that, but you should get it running properly before changing anything.

If I knew what I was doing all the time life wouldn't be any fun.

'80 KZ650 E 700cc, dyna ignition and coils, frame up restoration, daily driver
'81 KZ1300 A3 full restoration, custom big bore pistons, 1400cc 6 cylinder super bike
"77 KZ650 B1 - Barn Find, work in progeress
"74 Yamaha DT 400 Enduro
Last edit: 10 May 2015 10:47 by Tyler.

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10 May 2015 11:02 #671387 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power
Jets are clean. Primary's clean. It was my brother in laws bike. He kept it maintained well. He started having this issue lately and just sold it to me and bought a new bike. Gas tank is very clean, screens clean. I have a new inline filter, newer coils, plugs are new, oil is new. Plugs looked pretty much the same across the board. Yea I'm leaning towards the points too. He said they may have been changed but can't remember when. Denso points and condenser kit is cheap so I ordered the whole thing. If that solves the problem I'll run them till they die and upgrade to electronic. I didn't wanna dump the money until I knew that was the cause. He put a bunch of $$ into this bike last season. And this season it started giving him troubles and he got a settlement check and bought a brand new vulcan instead of messing with it. So now its my problem lol

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10 May 2015 11:09 #671388 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power


Should the accel pump adjustment be all the way out like this?

Sorry I'm new to manual carbs mostly experienced with CV type.

He had his "buddy" try to fix the bike and idk what all he adjusted. I checked the synch with a manometer and set the idle mixtures with a color tune. But the rest of it I don't know much about

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10 May 2015 15:38 #671417 by 650ed
Replied by 650ed on topic Re:79 kz650 loss of power
Change the points as you plan because pitted points will affect performance. Let me know if you want the procedure, as I already have it written up for the KZ650. The valve clearance being correct is no surprise to me. I have only changed 1 shim on my '77 KZ650 since it was new (more than 57,000 miles) and that was at the 500 mile mark just to set that one clearance dead center between the spec tolerances.

If after changing the points there is still a problem it may be the 2/3 coil, especially if the replaced one is not the correct one for the bike.

You mention it has the stock air box, etc - that's a good thing for sure. Is the air filter inside the box in good shape? Ed

1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
The following user(s) said Thank You: Tyrell Corp

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10 May 2015 18:00 #671435 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power
Air filter is brand new just changed it. I put 130 miles on the bike this weekend and she rode awesome. I figured out that I have to leave the choke partially on to get throttle response (kick the center cylinders on) but when I decel it backfires bad. So that tells me its getting plenty of gas. And a vacuum leak somewhere probably 2 or 3 or both. I'm gonna spray it down tomorrow and see what I find and I will let you know. Points showing up in a day or so, gonna change them anyway. I have the clymer book for it the procedure seems pretty straight forward.

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10 May 2015 18:34 - 10 May 2015 18:35 #671440 by 650ed
Replied by 650ed on topic Re:79 kz650 loss of power
I don't know what Clymer says, and those manuals often have bad info, but take a look at the following instructions. They have worked for me for 38 years on my KZ650.

The manual has several pages of instructions, pictures, etc. on how to set the timing using the static method plus timing light, plus dwell meter. I can give you a fairly easy way that will get you in a very close ballpark without a timing light or dwell meter, but you will need a 0.35 mm feeler gauge and a multi-meter or other device to test continuity. To be absolutely precise, the timing light and dwell meter will be needed.

Before attempting to replace or adjust the points the following is important to understand; timing is comprised of two separate components, and BOTH of these components MUST be set properly if the engine is to run well:

--- The GAP – this is the distance the points spread apart when fully open. This is set by adjusting the points. The GAP is the element that determines the DWELL. In essence, the DWELL is the number of degrees of points cam rotation that the points are closed and this controls the amount of time the coils receive a charge before firing the spark plugs.

--- The TIMING of the initial opening of the points (the point at which the continuity across the points breaks) controls the precise instant that the coils receive the signal to fire the spark plugs. This is set by turning the backplate AFTER the gap is set.

Remove the points cover on the right side of engine. Under it you will see 2 sets of points. The set on the left fires cylinders 1 & 4; the set on the right fires 2 & 3. When replacing points observe carefully how the little bits and pieces are arranged where the wires attach. Some of those pieces are actually insulators and if you leave any of them out or put them back in the wrong place the points will be grounded and won't work. Take a very close look at the contact surfaces of the points. If they are pitted you really should replace them. You can sand down pitted points, but they will quickly pit again. Replace one set of points at a time so you can look at the other set in case you get the little bits confused.

When adjusting the points, use a 17 mm wrench to turn the nut NEAR the end of the crankshaft clockwise while looking in the hole above that nut. (Do NOT use a wrench on the smaller bolt on the very end of the crankshaft to turn the engine.) Inside that hole you will see a vertical pointer cast into the casing. As you turn the 17 mm nut you will see a 1 & 4 and F and T roll by and then you'll see a 2 & 3 and F and T roll by. Each F and T has a line next to it.

Here's the method I use for static timing. I turn OFF the ignition. I disconnect the green wire near one coil and the black wire near the other coil. (This is not in the book, but it makes checking continuity much easier for me.) After installing the new points or cleaning up the old ones, turn the 17 mm nut while watching the points. When points set 1&4 are at their widest gap adjust them (by loosening the 2 screws that hold the points to the backplate) so the gap equals 0.35 mm. Turn the 17 mm nut clockwise through a full revolution again and double check this gap. Then repeat this for points set 3&4. Now set your meter to test continuity and clip one wire to the leaf spring on points set 1&4 and clip the other wire to ground. Turn the 17 mm nut clockwise until the 1&4 "F" mark aligns with the pointer mentioned above. You want the continuity across point set 1&4 to just break when the F mark aligns with the pointer. The idea is that when the continuity just fails is when the points will fire their respective coil and cylinders. In order to adjust the point at which continuity fails you loosen the 3 screws that hold the backplate to the engine and slightly turn the backplate until the meter shows a break in continuity. Once you have the 1&4 set timed properly you can check the 2&3 set to make sure they break when the 2&3 F mark aligns with the pointer (they should or something is not right). Don't forget to plug in the green and black coil wires when you are done, and put a little grease on the rubbing block felt. Assuming you are using new points of the correct type this should enable you to get the timing very close. Trying this with old points may give poor results, especially if the points are pitted and/or the rubbing blocks are worn.

After you have set the gap (which in effect sets the dwell) and the timing using the method above you can use a dwell meter and timing light to fine tune dwell and timing. If you have followed the above procedure carefully, very little if any fine tuning will be needed.
Sorry this is so long. It's not as difficult as it sounds. Ed

1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
Last edit: 10 May 2015 18:35 by 650ed.

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10 May 2015 18:54 #671444 by bikerdave79
Replied by bikerdave79 on topic Re:79 kz650 loss of power
I have a 77kz1000 that looks almost identical. The process seems pretty similar too. I don't think I'll have a problem. I'm almost 100% sure that the major issue is a carb boot seal leaking. Dunno of they've ever been changed. Partial choke seems to clear up the problem. So I'm gonna say its got a vacuum leak. I'll know more tomorrow

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