Topic: Wrapped exhaust = idle jumped to 2k-2.5k (‘74 CB750)

[Yoshimitsuu]

Hey all,

I took my exhaust off of the bike this weekend to wrap them but I encountered an issue. Before the wrap, my bike used to idle at 1200 and was pleasant on the ears. Since wrapping the exhaust, the bike idles at around 2k-2.5k. I’ve never wrapped an exhaust but this doesn’t seem like something that should happen simply by wrapping the pipes, right? When I was wrapping, I replaced the copper crush gaskets as well with new ones. Pipes are tightened and clamped. What can be causing this?

TL;DR: Wrapped pipes and idle jumped from 1200 to 2000. What bonehead thing did I do?

[Ichiban 4]

Yoshi..Do you know if the pipes or gaskets were leaking before you wrapped them?  I'm thinking that they were before..so when you tightened everything-up..you had more back pressure..which could affect your idle.

Anyway..doesn't sound like a real biggie to me.  I'd just re-adjust your idle down a little more now.

Anyone else have some ideas for Yoshi?
Mattane..Ichi

[HondaMan]

If the exhaust is 4-1 type, wrapping does increase the back pressure slightly unless the muffler is a straight-thru type. Usually this increases the idle burn efficiency (sort of like how a catalytic convertor in a car does), so if it's not related to a prior leak (like Ichi said above), it's just idling a little happier than before. These carbs are NOT known for their efficient idle characteristics, which is where most of their perennial plug-fouling troubles originate. The carbs actually start "mixing" right about where yours is now idling, near 2000 RPM.

[Yoshimitsuu]

THanks for the responses guys, I went downstairs to check on the bike and noticed that my idle adjustment screw didn’t have much play and pretty much any more turning will take it right off. Turning it in only raised the idle when I need to lower it. It’s starting to seem like a simple wrap of an exhaust will be a more involved project haha. The question remains: How will I be able to lower the idle of the screw is already almost at its lowest?

I’ve read through a few other threads and some have said that timing could affect idle (which I’m checking later on today) among other things. Since I changed my exhaust, I feel that I should include some tuning info about my bike;

- 115 Main / 40 Slow Jetting
- 4 into 1 exhaust with Supertrapp muffler
- Set floats to spec as per manual over the summer

I’ve done all of the 3k tuneup steps and now this has me scratching my head. Thoughts? I was also thinking of trying a cooler plug but I’m not sure that will lighten up my mixture to the extent that I need it to.

[madmtnmotors]

I seriously doubt the header wrap is the source of the problem. What model are we talking about?

As Ichiban said, the original exhaust gaskets may have been leaking, but it sound more like you may have a vacuum leak between the carbs and the head or your throttle cable is in a bind or mal-adjusted. Have you checked the throttle cable adjustment?

[Yoshimitsuu]

I seriously doubt the header wrap is the source of the problem. What model are we talking about?

As Ichiban said, the original exhaust gaskets may have been leaking, but it sound more like you may have a vacuum leak between the carbs and the head or your throttle cable is in a bind or mal-adjusted. Have you checked the throttle cable adjustment?

The model is CB750K4. I went downstairs and checked for leaks. I saw that spraying the carb boots and checking if there are any changes to the idle might be able to tell me if this is the issue. Upon spraying around them, there was no significant decrease in idle so I don’t think that’s it. The throttle cable I will have to check. It is really possible that it can go out of adjustment so easily though? I feel like the exhaust has nothing to do with throttle and before I wrapped the headers it would idle at 1200 all day. Thanks again for the suggestions!

EDIT: Went downstairs for a look and was greeted by this. It looks like from the top of the carb the slides are closed but this throttle cable is definitley not doing anything like that.

[madmtnmotors]

EDIT: Went downstairs for a look and was greeted by this. It looks like from the top of the carb the slides are closed but this throttle cable is definitley not doing anything like that.

Looks like a good place to start!   

Get the end of that cable properly mounted and I'll bet your idle adjustment will be easier to make. Sometimes things will come loose at the same time that something totally unrelated was worked on thus making diagnosis a little more tricky. I think you've found the culprit for sure. 

[ekpent]

 Ha ha - don't you just love a good coincident. As madman said that should get it back where it was once you fix and reset your idle again with the screw.  Maybe you were turning your handlebars really sharp doing the pipes and made that pop out of its holder.

[Yoshimitsuu]

Thanks guys, it helps to have people guide my brain  . My friends brother always tells me that usually it’s something stupid but when you get frustrated you don’t want to think like that haha.

I actually think it might have come loose when I removed the old exhaust flanges. The bolts were shredded from a P.O. so a drill out and heat was necessary. I didn’t want to get that heat near the tank so I removed it. Maybe when I pulled the tank off it took the cable with it. Of course something like this would happen...

[ekpent]

  That sounds like a very plausible explanation.

[Yoshimitsuu]

UPDATE: So I secured the throttle cable that was loose and the problem persists (glad I fixed that though). Something interesting that I discovered though is that if I turn my throttle the other way (it has a bit of play but not too much and it’s not tight enough to increase my RPMs), it will lower my idle. I’m assuming this is a throttle issue I’ve created. I tried the adjuster at the bars on the cable but this didn’t do anything really. I found the picture below and adjusted the eccentric link pin to manual specs (2-3mm) and I’ve found that now if I overturn the throttle the other way, it’ll idle at 1200 but once it snaps back it goes right  back to 2000. Also adjusted my timing, it is on the mark (no pun intended).

Maybe my slides are sticking open and I need to bench sync. Any thoughts?

[TwoTired]

Are you certain the individual slide adjusters aren't all skewed enough to prevent the slide from reaching the "floor" of the carb throat?

This is often done during a carb sync when "chasing" vacuum levels across the bank and tweaking them all.

Later carbs removed the adjuster from one carb to prevent this.   For the older ones, you designate a master carb that you know has full travel capability and then adjust the other three to it during vac sync.   That way they are all certain to have full travel capability and the idle knob can control the lower reach of the sildes.

Cheers,

[Powderman]

I doubt you can see any measurable difference between the use or not of header wrap on a motorcycle. There is absolutely no measurable performance gain from header wrap. It's only positive is subjective and that's in appearance at the cost of lots of negatives.
It had no impact on the idle.

[PeWe]

TwoTired, you're right.
I noticed that when cleaning my old K6 carbs a week ago. Possible to set the nuts too deep down and the throttle will not reach the floor. The upper lock nut must almost be flat with rod upper end, not many threads visible above the nut Honda painted yellow.
Probably no problem for me when I always did a bench sync, no vacuum clocks +30 years ago.

[PeWe]

I stumbled over this description and remembered this thread.
http://www.hondachopper.com/garage/carb_info/bench_sync/bench_sync.html

[Yoshimitsuu]

I stumbled over this description and remembered this thread.
http://www.hondachopper.com/garage/carb_info/bench_sync/bench_sync.html

This was super helpful! Pulled the carbs today and went through the carbs. Also took the opportunity to replace the old float needles (hopefully my carbs won’t drip anymore). Anyway, fired the bike up and was able to dial the idle down to even better than before! Thanks so much guys