Clutch Slipping, or Oil to blame? (79 KZ650SR)

So, I ordered some Sea Foam to try and get rid of some gunk in my carbs, since my indy shop is apparently not taking any big jobs in at this time, and I don't have another shop close by to do the work. I figure, it won't hurt to put the prescribed amount of Sea Foam in and run the bike for a while, right?

I'm also going to replace the overflow tubes while I'm at it - does someone know off-hand what size fuel tubing I need to procure?

Last question: what causes a motorcycle engine to idle high (2,300 RPMs) when fully warmed up, versus 1,100 when cold (just off choke)? When I first start the bike, it'll idle low and then ramp up to about 3,000 RPMs before I slowly back off the choke, where it settles into the aforementioned 1,100 RPM idle.

FWIW, the intake boots and carb holders are new, I'll have to check out the air filter though (I thought it was replaced in 2017, but am not sure). Not running pods - it's got the stock air box.

If I can't find someone in the next month to work on it, I'll probably print out the relevant parts of the service manual and get a new tarp to throw over the darned thing while I take it apart and work on the carbs... which could be weeks of evening-time spent tinkering. If anyone lives in or near the Washington, D.C. metro area and is willing to help me, PM me... I'm more than willing to learn and will compensate you for your time.

-John

Didn’t find the size but here is the hose you need
www.johnnysvintagemotorcycle.com/product...owl-carburetor-hose/

If it only idles high when the choke is on or part way on, that is normal. Your best bet is to turn the choke off as soon as possible to prevent the engine from idling at the high rpm. On my KZ650 this only takes a minute or so.

Also, do not use more than the prescribed amount of Seafoam. Some folks dump the whole can in and then wonder why their bike doesn't run properly. :blink: Ed

650ed wrote: If it only idles high when the choke is on or part way on, that is normal. Your best bet is to turn the choke off as soon as possible to prevent the engine from idling at the high rpm. On my KZ650 this only takes a minute or so.

Also, do not use more than the prescribed amount of Seafoam. Some folks dump the whole can in and then wonder why their bike doesn't run properly. :blink: Ed

Thanks... it idles high and then lowers as I close the choke over the next several seconds. After 15-30 seconds (depending on ambient air temp), it'll settle into a nice idle with the choke off. After riding for 5-10 miles, the idle is closer to 2,300 RPMs.

At this point, I'm wondering if the carb mixture screws (1 per carb, right?) are set too rich or too lean, and I've been compensating by changing the main mixture (idle?) screw (thumbscrew by the #3 carb IIRC). Does that sound plausible?

Also thanks for the seafoam warning ... that's the same thing I read. The bottle says you can use more, but I plan to add 1oz for each gallon in the tank and see what happens, then keep running it through each tank for a while to break up any gunk without eating the O-rings and other seals.

-John

In my experience, climbing idle with temperature is usually lean. If it was rich, it would be more rich as it heats up and the idle might actually go down.

drakhen99 wrote: .................it idles high and then lowers as I close the choke over the next several seconds. After 15-30 seconds (depending on ambient air temp), it'll settle into a nice idle with the choke off. After riding for 5-10 miles, the idle is closer to 2,300 RPMs.
-John

Check to see if the carb holders are leaking as that can cause the idle speed to increase after the engine warms up. To do so, set the idle speed as low as possible and then spray carb cleaner around the carb holders especially where they mate to the cylinder head. If the idle speed changes the carb holders are leaking. Ed

loudhvx wrote: In my experience, climbing idle with temperature is usually lean. If it was rich, it would be more rich as it heats up and the idle might actually go down.

Hmm, that makes a lot of sense. Let me see if I'm understanding it:

Cold start-up (usually after sitting for several days), needs the petcock on Prime for 10 seconds before it'll fire up, and once it's running on full choke, it'll ramp up the RPMs to 3k or more RPMs, which indicates lean running, even with full choke?

Then, after getting it off choke, it idles OK (1,100 RPMs or so) until it's been ridden for several miles, at which point it's leaned out further?

So I can just back each carb mixture screws off a quarter turn or so and then ride it to see what happens, right?

I've got some carb cleaner, so I'll check the carb holders and intake boots for leaking, but it sounds like I have a good place to start.

Seafoam should be here by Wednesday, so I'll wait until then to muck around with it.

FWIW, I also have a carburated Vulcan 800 (no tach) that idles a little high but doesn't "ramp up" the RPMs like this bike does, and once off choke, settles into a nice idle and after riding there is no change in its idle RPM, so that's what I'm using as a frame of reference. Ever since my first car, these are the first two carbureted vehicles I've owned, and this is the only one I've had which has more than one carburetor.

-John

drakhen99 wrote:

loudhvx wrote: In my experience, climbing idle with temperature is usually lean. If it was rich, it would be more rich as it heats up and the idle might actually go down.

Hmm, that makes a lot of sense. Let me see if I'm understanding it:

Cold start-up (usually after sitting for several days), needs the petcock on Prime for 10 seconds before it'll fire up, and once it's running on full choke, it'll ramp up the RPMs to 3k or more RPMs, which indicates lean running, even with full choke?

NO! That is perfectly normal. With full choke the RPM will rise dramatically very soon after the engine begins to warm up. That is not lean running at all. In fact, that is perfectly normal.

Then, after getting it off choke, it idles OK (1,100 RPMs or so) until it's been ridden for several miles, at which point it's leaned out further?

So I can just back each carb mixture screws off a quarter turn or so and then ride it to see what happens, right?

I've got some carb cleaner, so I'll check the carb holders and intake boots for leaking, but it sounds like I have a good place to start.

Seafoam should be here by Wednesday, so I'll wait until then to muck around with it.

FWIW, I also have a carburated Vulcan 800 (no tach) that idles a little high but doesn't "ramp up" the RPMs like this bike does, and once off choke, settles into a nice idle and after riding there is no change in its idle RPM, so that's what I'm using as a frame of reference. Ever since my first car, these are the first two carbureted vehicles I've owned, and this is the only one I've had which has more than one carburetor.

-John

Before fiddling around with mixture and quite possibly messing things up bigtime do yourself a serious favor and check, as I described above, for carb holder leaks. Carb holder leaks WILL make the rpm rise after the engine is warmed up. I experienced that on my KZ650 when the carb holders were quite old, so for sure I know what I'm talking about. BTW, I've owned my 1977 KZ650 since it was new (43 years; 62,000+ miles) so I have a fair amount of experience with the KZ650. Ed

650ed wrote:

drakhen99 wrote:

loudhvx wrote: In my experience, climbing idle with temperature is usually lean. If it was rich, it would be more rich as it heats up and the idle might actually go down.

Hmm, that makes a lot of sense. Let me see if I'm understanding it:

Cold start-up (usually after sitting for several days), needs the petcock on Prime for 10 seconds before it'll fire up, and once it's running on full choke, it'll ramp up the RPMs to 3k or more RPMs, which indicates lean running, even with full choke?

NO! That is perfectly normal. With full choke the RPM will rise dramatically very soon after the engine begins to warm up. That is not lean running at all. In fact, that is perfectly normal.

Then, after getting it off choke, it idles OK (1,100 RPMs or so) until it's been ridden for several miles, at which point it's leaned out further?

So I can just back each carb mixture screws off a quarter turn or so and then ride it to see what happens, right?

I've got some carb cleaner, so I'll check the carb holders and intake boots for leaking, but it sounds like I have a good place to start.

Seafoam should be here by Wednesday, so I'll wait until then to muck around with it.

FWIW, I also have a carburated Vulcan 800 (no tach) that idles a little high but doesn't "ramp up" the RPMs like this bike does, and once off choke, settles into a nice idle and after riding there is no change in its idle RPM, so that's what I'm using as a frame of reference. Ever since my first car, these are the first two carbureted vehicles I've owned, and this is the only one I've had which has more than one carburetor.

-John

Before fiddling around with mixture and quite possibly messing things up bigtime do yourself a serious favor and check, as I described above, for carb holder leaks. Carb holder leaks WILL make the rpm rise after the engine is warmed up. I experienced that on my KZ650 when the carb holders were quite old, so for sure I know what I'm talking about. BTW, I've owned my 1977 KZ650 since it was new (43 years; 62,000+ miles) so I have a fair amount of experience with the KZ650. Ed

OK, I'll check the carb holders first, and I certainly don't doubt your expertise! I am only skeptical that they are leaking since they were replaced in late 2018, approximately 500 miles ago. I had them replaced along with the intake boots. Both were leaking badly before being replaced.

-John

Like Ed said, the leanness I was referring to could also be due to a vac leak.

OK, quick update:

Today I poured about 3oz of Seafoam into the gas tank (I figure there was 2.5 to 3 gallons of fuel in there, so just a hair over the minimum). Fired the bike up, she ran well, I let her idle a few minutes and then adjusted the idle speed screw to 1,100 RPMs or thereabouts. NOTE: I only adjusted the idle screw, not the individual carb mixture screws.

Once the bike was warmed up, I sprayed carb cleaner on the intake boots, carb holders, and any fuel lines and junctions I could see, testing for vacuum leaks. None were found.

Went on a 35-40 mile ride, no leaks, parked at my mom's house to do a wellness check on her.

Waited about 45-60 minutes (had a cigar), and then got back on the bike. A few miles away from home I noticed what smelled like fuel, and you could see it smoking off the exhaust while at a stop light. Ugh.

Got home, and figured it was the #2 carb again... not so fast! It was the #4 carb this time, leaking fuel through the overflow tube. Whacked it pretty good with a screwdriver for a while until it stopped puking fuel. It was starting to leak and then stopping for a while, then finally stopped. Idled, revved it up a bit, rode it up and down the street, no more overflowing fuel, so I parked it.

I'll go out in a few days and see if she fires up and then if no leaks, go straight to the gas station to fill up.

Should I add more Seafoam at that time, or give it a tank or two to work through?

My current theory is that the idle screw was just set too high, and with working some gunk through the carbs, she's running better but the gunk is what's causing the overflow. The real test will be to see if she fires up in a few days. In the past, if I did this, I couldn't get her to start with the idle screw set properly. I'd have to set for a higher idle when cold than when warm. This time, when fully warmed up, the bike was idling below 1,500 RPMs, which to me is quite acceptable!

-John

I would not add more Seafoam until at least I was positive that all the fuel in the previous tankful was used up. Too much Seafoam will cause problems; they may be temporary but they will still be a royal pain in the neck. Ed