Topic: ...embarrased to ask...Jetting on my '78 750k

[surveywaters]

I have a '78 750k with about 800mi on the top end(1st over), valves and Pamco Ultimate ignition(coils and cables included). It was getting hard to crank so I was going through the normal: clean the carbs, set valve lash, check timing. I pulled the plugs and found heavy carbon build up on all four as well as carbon on the piston tops. I am running a Mac 4into2 with a K&N filter in the factory air box and an NGK D8EA. The jetting is a 120main, 35slow, 2.5 turns out. Was running good with no flat spots.
Does the jetting sound right?
I am wondering if it's a weak spark from this electronic ignition How would I check this? The spark looks good

To make it more frustrating, I have a new set of Mac performance baffles to replace the tinny sounding baffles that came with the crappy sounding pipes.

[b52bombardier1]

What do you have for compression in PSI vigorously cranking with the starter and the throttle held wide open?

What needle clip height are you using on the carbs?

Rick

[calj737]

2.5 turns sounds wrong too 

[flybox1]

Reduce to 1 turn out and complete an idle plug chop w new plugs.  Plugs should be light tan

[BobbyR]

I am running the exact same 120 main jetting with stock airbox. I am using stock exhaust. There are no needle clips on the 78 pumper carbs. I have had no carbon issues. Try adjusting the mixture screws again. I did mine to smooth running rather than a given turns out. You may also want to use the D7EA plug which I have read was the original plug.

[surveywaters]

Thanks for the replies, seeing that others are running a similar setup is reassuring. I will dial the mixture screw back, that is way past the initial setting. I got a new set of plugs I'll install and do an idle chop. I'll also check the compression, but it is a fresh build.

[surveywaters]

compression 1-4: 132/129/129/137

One observation to note. When comparing the new plugs to the old, <1000mi on them, the center electrode is shorter on the old. It is NOT melted, it looks even and flat with no grooves. There is also a silverish discoloration on the insulator under the boot, not surrounding the electrode. What would cause this?

[flybox1]

Your compression is great.
Please post a picture of your idle plug chop results for us to see. 
Put all plugs next to each other and snap a closeup pic of their tips/electrodes

[surveywaters]

mind telling me how to do an idle plug chop? I let the bike idle while syncing the carbs and setting the timing (timing was a couple degrees off), then pulled the plugs and they still looked brand new.

[flybox1]

Warm up the engine to operating temps.
Put a brand new set of plugs in, and a fan on the engine case to keep it cool.
Start the engine and let it idle for 4-5 min.  No blipping of the throttle.  Just let it idle.
Cut the engine and pull the plugs. Line them up 1-2-3-4 and take a closeup picture of their tips and post it here.
similar to this (rich idle circuit)

[surveywaters]

alright, pretty much exactly what I did while tuning and they still looked brand new. I try again.

thanks

[flybox1]

Make 1/4 turn adjustments OUT of the idle mixture screws, until plugs show light tan like this.

[surveywaters]

Okay, I hope it doesn't mess with it too bad but I have a like 6mi on these plugs.  I cleaned them and then did the idle chop. There is a nice gold on the ground electrodes but the center electrodes look a little lean to me.
I opened them up another 1/4 turn (loosened), and took it for a ride. She back fires on hard deceleration, this is what previously brought me to the 3 turns out. I will admit that I have a good bit of experience tuning carbs, but four carbs is an unknown for me. Are the PMS typically set the same or do they vary from carb to carb on a bike? (not from bike to bike)

[flybox1]

If those are 1 to 4 (L to R), then make #1 richer by 1/2 turn, and 2-3-4 richer by 1/4 turn, and retest.

[surveywaters]

that's the order, will do. thankyou.

[BobbyR]

Okay, I hope it doesn't mess with it too bad but I have a like 6mi on these plugs.  I cleaned them and then did the idle chop. There is a nice gold on the ground electrodes but the center electrodes look a little lean to me.
I opened them up another 1/4 turn (loosened), and took it for a ride. She back fires on hard deceleration, this is what previously brought me to the 3 turns out. I will admit that I have a good bit of experience tuning carbs, but four carbs is an unknown for me. Are the PMS typically set the same or do they vary from carb to carb on a bike? (not from bike to bike)

You should not get a backfire with a K8. You have an accel pump to richen the mixture. This is probably unrelated to your current concern, it should be looked into.
 

[surveywaters]

The acceleration pump should only work when twisting the throttle, it helps avoid the stumble when taking off. I am backfiring(popping) on hard DEcelleration which is an indication of a lean idle circuit.

[madmtnmotors]

The acceleration pump should only work when twisting the throttle, it helps avoid the stumble when taking off. I am backfiring(popping) on hard DEcelleration which is an indication of a lean idle circuit.

or an exhaust leak, especially where the pipe joins the head

or one or more exhaust valves too tight (not closing fully)

or there is one or more vacuum leaks at the manifold boots (between the carbs and head)

Three turns out is a lot. If the carb to head rubber boots are very hard I would suggest replacing them as a means to remove the possibility of vacuum leaks. If none of these correct the issue then I would suggest a more careful inspection and thorough cleaning of the carbs. People bash the later PD "keyhole" carbs but I have had good results with them. I am running the flat top 836cc (65mm) cruzinimage pistons, a CX-1 (Web Cam 41) cam, K&N filter in the stock airbox, and a MAC 4 into 1 header with a straight through baffle. I started with 125 mains and mine fell on its face at 5000rpm when the main fuel circuit would come in fully. I ended up reverting back to 110 mains with stock idle jets, idle mixture screws 1-1/2 turns out, carbs carefully synchronized, and am still showing a slightly rich condition. This is on my Street E/T dragbike that spends most of its time between 7500 and 9200rpm with a Dyna S ignition, Dyna 3ohm coils, and Denso X24ES-U plugs to light the mixture.

I did use one shim under the main jet needles (equivalent to about half a notch) when I installed the CX-1 cam to bring the main jet circuit in just a little sooner.

[surveywaters]

I appreciate your input. I was at 3 urns when I started this thread and was experiencing a slightly rich condition, no  backfire. As a listed above, I just cleaned the carbs, adjusted the valve lash(wasn't out), checked timing(was slightly advanced), synced the carbs(were good), and the PMS was set to 1turn for the idle chop which showed lean. I turned it up to 1.25 turns and rode a bit, this is when I heard the popping on decel. I feel I'm heading in the right direction.

[surveywaters]

I will check my pipes at the head and double check valve lash too. Thanks for pointing those out. My intake manifolds are good.

[flybox1]

I am backfiring(popping) on hard DEcelleration which is an indication of a lean idle circuit.

You are making the assumption its not un-burnt fuel (rich condition) igniting in the headers because 1/2 or 2/3, or whatever throttle position you decelerated from, was too rich when the slides shut.  Does the popping go away when you are idling in your driveway or at a full stop?

First  -  Fix your idle circuit AF mixture.  Continue idle plug chops till you get your plugs the proper color.

Second.  -  1/4 throttle position plug chops.  Is your pilot jet the right size?  Plug color will tell you.  35s or 38s should be about right but you need to be sure.

Third  -  Move to 1/2 throttle position plug chops.  This will show you your AF mixture for the needle.   Adjust the slide needles as necessary to achieve the same plug color as your idle plug chops.

Last  -  WOT (or at least 3/4 throttle) plug chops.  Get your mains dialed in.

Then....test if for popping on hard decel.

[BobbyR]

The acceleration pump should only work when twisting the throttle, it helps avoid the stumble when taking off. I am backfiring(popping) on hard DEcelleration which is an indication of a lean idle circuit.

My apologies I read it too quickly. When I was experiencing a deceleration popping on my K8 using the clear tube method I found one float bowl low. Luckily I was able to adjust it on the bike.

[surveywaters]

Thanks guys. I have a good list here. I'll do the clear tube float check also, I just set the float height on the bench. I'll get back in a few days when I finish all these great suggestions.

[madmtnmotors]

That's one advantage to the PD carbs in that you do not need a special drain bolt to use the clear tube method, just connect it to the bowl drain and open the drain screw.