1978 Kz650 Help please

I have a 1978 KZ 650 in my shop. I usually work on newer bikes but told the guy I would straiten this one out for him. It would not start.
I removed the plugs and carbs. Replaced plugs then cleaned the carbs very well paying attention to the jets. Installed the carbs. reinstalled the pods. set the air mixture screws to 1 1/2 turns out. primed the bowls. checked the bowl level, it was good ( within 3 mm of the bowl attachment seal). Synchronized the carbs got them real close. checked the spark advance and it was good. the bike starts and runs great.... but when I give it hard throttle from idle it bogs the engine. I cant figure this thing out! any suggestions?

Bike guy wrote: I have a 1978 KZ 650 in my shop. I usually work on newer bikes but told the guy I would straiten this one out for him. It would not start.
I removed the plugs and carbs. Replaced plugs then cleaned the carbs very well paying attention to the jets. Installed the carbs. reinstalled the pods. set the air mixture screws to 1 1/2 turns out. primed the bowls. checked the bowl level, it was good ( within 3 mm of the bowl attachment seal). Synchronized the carbs got them real close. checked the spark advance and it was good. the bike starts and runs great.... but when I give it hard throttle from idle it bogs the engine. I cant figure this thing out! any suggestions?

I have a 77 KZ650. Im no expert but if youve got good spark then your issues are related to carbueration/air. Either the main jet is the wrong size, or youve got a vaccum leak. Like I said, Im no expert, just trying to help with what knowledge I do have.

Once you install pods all bets are off regarding the jetting. VM carbs such as that bike has are easier to sort out than CV type carbs but still. Some serious jetting work will be needed and maybe moving the needle as well.

Nessism wrote: Once you install pods all bets are off regarding the jetting. VM carbs such as that bike has are easier to sort out than CV type carbs but still. Some serious jetting work will be needed and maybe moving the needle as well.

Usually its either or in re the main jet or needle adjustment. The theory is, keep the pilot, or slow jet the same, but in crease the mains. Lets say his stock main is #100, he should go up to #115 or higher. Ive had a LOT of headaches with pods and finally fingered it out.

You may find this helpful. Ppl swear by it and it helped me out a lot too:

Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

2 jet sizes for both single K&N and drilled airbox

2 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

Check plug color often, sync carbs after each jet change, make sure the floats are set correctly, and seriously consider purchasing a Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

Make sure your carbs are in perfect working order before making jet changes.

Example from my '79 XS1100 F:
Stock main jets: 137.5

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Jardine)

1 size for single K&N

1 size for drilled air box

4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 65K mile on it, of which I have put 33K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.

Happy Jetting

Before the smooth bores I used the 24mm carbs for 3 years with a open air box and then pods ..... used 17.5 pilots one size bigger went to the 5th clip up from the 4th and 112.5 mains....... ran exc with no problems at all off idle. Used them for 3 years after the rebuild with new holders with the same results. New carb holders on the 29s seem to have cured the off idle stumble they always had. And it starts much easier and quicker.

KZB2 650 wrote: Before the smooth bores I used the 24mm carbs for 3 years with a open air box and then pods ..... used 17.5 pilots one size bigger went to the 5th clip up from the 4th and 112.5 mains....... ran exc with no problems at all off idle. Used them for 3 years after the rebuild with new holders with the same results. New carb holders on the 29s seem to have cured the off idle stumble they always had. And it starts much easier and quicker.

Some say idle sizing up can alleviate problems. Whats your opinion on this? Do needles always have to be changed as well?

When I got my header I removed the guts to the filter (and a few years later pods) and went with the jetting stated so I never tried it with stock jets. Baldy runs his 810 with 15s and the clip in the 4th but with a shim under neath. Seems like it would be lean but have been told it runs great. Have read a lot that run my jetting on the 650 site. Might have to just try em and see and if they don't help it could then be the holders ??

Higher compression the stronger the intake vacuum.
Steve

KZB2 650 wrote: When I got my header I removed the guts to the filter (and a few years later pods) and went with the jetting stated so I never tried it with stock jets. Baldy runs his 810 with 15s and the clip in the 4th but with a shim under neath. Seems like it would be lean but have been told it runs great. Have read a lot that run my jetting on the 650 site. Might have to just try em and see and if they don't help it could then be the holders ??

650 site has been shut for some time.