Author Topic: 1977 cb750k throttle characteristics (Read 2804 times)

cuz · « **on:** May 04, 2016, 09:34:07 PM »

Hey guys. I'm still waiting on paint for my build to be complete but have been making some test rides up and down the block with an auxiliary tank hanging on the bars... Real safe I know. The bike is running really nice with the exception of a wot crack from low rpm or idle where it will fall on its face. I have read a few posts that describe this as a characteristic of these carbs even with the accelerator pump working correctly. I can roll on the throttle pretty aggressively and burn through the gears and hit wot from above 4000 rpm or so with no hesitation. Its just a really hard hit from idle in neutral or low rpm in gear will make it stumble. The bike is stock with the exception of a cyclex 4-1 open exhaust, carb sync done, slow jets removed and cleaned, pilot jets set to about 1.75 turns out. Thoughts from those that ride a 77?

flybox1 · « **Reply #1 on:** May 04, 2016, 10:31:49 PM »

Idle plug chop color?
Slightly lean here is ok, so when the accel pump kicks in, the mixture is right for that second you get the spray

cuz · « **Reply #2 on:** May 05, 2016, 06:22:56 AM »

I'll get some fresh plugs and do a good idle plug chop to confirm. Thanks Flybox. Should I be able to hit wide open throttle from low rpm and have no stumble? Like I said earlier, I can Rev it a bit then crack wot with no issues.

flybox1 · « **Reply #3 on:** May 05, 2016, 06:54:52 AM »

This is what you're looking for. I get no stumbling, or popping thru the exhaust.
Remember the accel pump only works within the first 1/3 (+/-) throttle travel.
If you're at any throttle position above this, and you get your stumble going to WOT, its your jetting, not the accel pump circuit.
Remind us of your current jetting/intake/exhaust.

cuz · « **Reply #4 on:** May 05, 2016, 07:31:41 AM »

I'll do an idle plug chop tonight with some fresh plugs after a good warm up. The setup is stock jets, pilot screw 1.75 turns out, stock airbox with new emgo filter, cycle x 4-1 exhaust with open silencer.

flybox1 · « **Reply #5 on:** May 05, 2016, 07:39:49 AM »

Quote from: cuz on May 05, 2016, 07:31:41 AM

I'll do an idle plug chop tonight with some fresh plugs after a good warm up. The setup is stock jets, pilot screw 1.75 turns out, stock airbox with new emgo filter, cycle x 4-1 exhaust with open silencer.

#35pilots, #110mains?
Keihins or an aftermarket brand?

cuz · « **Reply #6 on:** May 07, 2016, 12:37:07 PM »

Here is the result of my plug chop at idle. Engine warmed up, then fresh plugs in. I idled it for 3 minutes then shut it off. The plugs look exactly like they looked new, maybe slightly tanned up on the insulator.

cuz · « **Reply #7 on:** May 07, 2016, 01:15:43 PM »

I did a second idle plug Chop just now. Before this chop I turned the mixture screws out two full turns and rode it down the block a couple times. While riding I noticed that my exhaust rumble disappeared a bit more and it accelerated a bit smoother. I brought it back to the garage and let it idle again for three full minutes. Plugs are now slightly more tan but still look like they just came out of the package. I'm thinking that some 41 idle jets may be in order. What do you think flybox? Seems to run strong otherwise.

flybox1 · « **Reply #8 on:** May 07, 2016, 04:56:20 PM »

too lean at idle. My above picture was from a 3-4 min idle.
go up to Keihin #40 pilot jets, and 1 turn out on the mixture screws.
2 turns out is getting to the limit of what this circuit can provide. too hard to tune when youre at the end-range.

cuz · « **Reply #9 on:** May 09, 2016, 07:13:01 AM »

As an experiment I dialed the mixture screws back to 2.5 turns out, rode up and down the block a couple times and did another 3-4 minute idle plug chop. The results are in the picture. Much better plug reading. Thoughts flybox? I know that 2.5 turns is beyond much of the usable range of the pilot circuit but I'm wondering if my pilot jets are a bit different than stock. I replaced my stock ones with the screws than came with my rebuild kit as the originals looked pretty crude on the tapered ends. Should I be happy with the results of this last plug chop and put some miles on the bike before ordering new idle jets?

flybox1 · « **Reply #10 on:** May 09, 2016, 07:28:32 AM »

That plug looks good, but its an invalid test.
You cant ride a plug and then do an idle plug chop with the same plug. Its contaminated with the deposits of the other throttle positions you used during your ride. Who knows where that throttle position 'color' came from.
Bike at operating temps. New plugs. Idle test.
Why DR8ES-l plugs? Do you have different coils or plug caps? (D8EA's are standard)

cuz · « **Reply #11 on:** May 09, 2016, 07:50:55 AM »

Damn, good point on the contamination. A guys going to go through a few sets of plugs to tune these things. The plugs I am using are recommended in the service manual in the K7 supplement. Although I live in Canada I don't believe my bike is a Canadian model, however the shop I buy my plugs from only had the resister type plugs in stock. I don't think the resister would make any difference would it?

flybox1 · « **Reply #12 on:** May 09, 2016, 08:05:40 AM »

Yes, R plugs do make a difference.
Do you have the original, or NEW plug caps? What is their ohm rating?
What coils are you using?

cuz · « **Reply #13 on:** May 09, 2016, 08:27:08 AM »

I'm not sure about the caps or coils as they came with the bike. I'll have to check that. My understanding about R type plugs was that they are necessary for vehicles with electronics etc, but using them on vehicles without electronics wouldn't make a difference. I am running a pamco electronic ignition if that makes a difference.

flybox1 · « **Reply #14 on:** May 09, 2016, 08:32:32 AM »

Sorry, I dont know enough about the Pamco setup to answer.
Might want to PM Hondaman and ask what plugs you should be running. Its important you have adequate spark.
He will need to know the type of coils and the cap resistance you have.

cuz · « **Reply #15 on:** May 09, 2016, 08:48:42 AM »

Thanks Flybox. Assuming stock caps, coils and ignition, what effect does running an R type plug have?

flybox1 · « **Reply #16 on:** May 09, 2016, 09:00:00 AM »

let me see if i can find an old Hondaman thread....

If you think your coils/caps setup is original, you'll benefit from checking your current plug cap resistance.
R plugs have a built in 5k ohm resistor. With original caps, your total resistance might be too much to supply adequate spark.

http://forums.sohc4.net/index.php/topic,67543.msg1776674.html#msg1776674

cuz · « **Reply #17 on:** May 09, 2016, 09:33:31 AM »

This is a good thread. I'll measure my cap resistance asap. I'm glad I started this thread. Thanks Flybox! I can't wait for my paint to be done.
http://forums.sohc4.net/index.php/topic,67096.msg739250.html#msg739250

flybox1 · « **Reply #18 on:** May 09, 2016, 10:14:16 AM »

Post some pictures of what youre working on

cuz · « **Reply #19 on:** May 09, 2016, 10:24:55 AM »

Cb750k7. By this weekend I'm hoping to have everything together as the paint should be done. The side covers in the picture are the color of the tank as well. Here is a link to my Facebook build page:
https://m.facebook.com/1977-Honda-cb750-Restoration-1535650020001724/

flybox1 · « **Reply #20 on:** May 09, 2016, 10:32:02 AM »

Nice!
Might I suggest a nice Tarozzi fork brace to finish off that front end

Without the stock fender/brace, it'll feel quite mushy up front

cuz · « **Reply #21 on:** May 09, 2016, 10:41:12 AM »

Thanks. I shortened the fender a bit so it is getting painted as well and will be going back on.

cuz · « **Reply #22 on:** May 09, 2016, 11:29:35 AM »

I measured my caps. They are 5k caps. I'm going to pm Honda man to get his opinion on plugs with my current configuration.

cuz · « **Reply #23 on:** May 09, 2016, 06:11:49 PM »

Flybox, I haven't heard back from Honda man yet but I have copied a quote from one of his posts:
From 1976 until the end of the SOHC4 series, the ohms value was 10,000 (10k) from the factory, and the coils came with thicker spark wires because of this higher resistance (to prevent pretty night effects!). This later design was intended to reduce AM radio interference for cars next to your bike, and for riders who were sporting fairings with built-in radios for touring. It also made the plug cap life shorter, as the "burnout limit" was (is) only 11,000 (11k) ohms. These coils can be retrofitted to the earlier plug cap resistances with improved spark being the result, as the internal parts of the coils did not change.

With my 5k caps and a resistor spark plug, that would be pretty close to factory specs. Would agree with this logic?

cuz · « **Reply #24 on:** May 09, 2016, 07:36:20 PM »

Reply from Honda Man. Looks like I got lucky and am running the right plugs for my bike.

The K7 and K8 (especially the K8) have lean-burn carbs, which makes them cold-blooded critters. If you add some more resistance overall, it helps against this: Honda used 7500 ohm caps on the early bikes and 10,000 ohm caps on these bikes to help with this, specifically. The higher resistance makes the spark last slightly longer. So, as you have good plug caps, I'd recommend using the resistor plugs with those. Also: I have had good results with the new superbike plugs that just HAPPEN to be the right heatrange for our engines, but have a longer tip reach: these are the DPR8ES-L (I haven't found any of the ND equivalent yet, but I am sure they will be even better, always are). These are also only $2.50 each at the local OReilley Auto, which makes them a bargain for our bikes that is hard to pass up! These worked real well in the K8 I just finished restoring last month (took 2 years!). They are about 2000 ohms inside, so it brings the resistance up to 7500 ohms, which is closer to the OEM value Honda used back when.

flybox1 · « **Reply #25 on:** May 10, 2016, 06:50:09 AM »

Good info, but nothing comes up on the NGK site about that plug.
it loops me back to the DPR8EA-9

cuz · « **Reply #26 on:** May 10, 2016, 09:06:04 AM »

The DR8ES-L is an automotive spark plug. Maybe it's not listed under the motorcycle stuff.

flybox1 · « **Reply #27 on:** May 10, 2016, 09:14:57 AM »

Hondaman mentioned the DPR8ES-L.
Same thing? type-o?

cuz · « **Reply #28 on:** May 10, 2016, 05:06:05 PM »

I'm not really sure. He referred to it as a superbike plug.

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Author Topic: 1977 cb750k throttle characteristics (Read 2804 times)