Topic: CB750 K7 Idle Trouble

[mkoski]

Hello again!

Tuned the bike up and she is running very well now (above 4000 rpm/1/4 throttle) with just a bit of delay at the throttle. The PO had the low-quality Emgo pods installed so I know that most of you wont bother to help me out with this (which is very fair, going to buy K&N pods asap). I would toss the stock airbox on but the PO lost it so I have no choice in the matter! Anyways!

My bike has a rough/dying idle below 1800/2000 rpm, so I idle at 2000 rpm. I think I need to re-jet to a 125 main and go up around #10 on my slow jet. Does that sound about right? One thing I have noticed is that my bike makes frequent air popping/release sounds at idle and I have no idea what they are! I listened around, did some searching and couldn't find anything. Is this a problem anyone is familiar with? Also to note, my compression is around 120 psi. I am assuming that is the main reason my idle wont go lower..? (A top-end rebuild is in the works for this winter to lap the valves and get some new rings on the pistons.)

As always, thanks for your help guys!

[Stev-o]

Did you do the full 3000 mile tune up?  Check/change the spark plug caps??

The K7 was lean stock, [EPA thing] thinking you may want to consider buying an airbox rather than another set of pods OR plan on spending considerable time getting the jetting set just right.

[Spanner 1]

Stevo-o +1.... it's not worth it to try to tune the bike to pods IMO unless you have to have a 'cafe' look and don't need to ride the bike more than around town. I would look for a stock airbox and start there. Also allowing a motor to 'idle' at 2,000 rpms is very bad for it ( without any load, i.e. running down the road ).....do I need another IMO here ?... plenty of help here to fix this thing if you want.

[mkoski]

I will take a look around for an airbox then. For this season, though, I think I am going to take a stab at getting it to run reasonably with the pods. Here are my main problems, maybe one of you guys can point me in a certain direction to get started on fixing these:
1) Rough/high idle + low rpm lag. Bike wont pull or run well under 4k rpm
2) Lots of backfiring/popping and air release sounds on idle/deceleration
3) Gearing problem? My bike does about 6500 rpm at 60 mph, that seems to be very low gearing, does it not?

This weekend I pulled the carbs and cleaned them up beautifully, didnt help my bike run any better at all. I think (gut feeling) that I need to step up a size or two on both the main and pilot jets. Whats the opinion on that?

[flybox1]

Use your idle set screw to decrease your idle....

+1 to a 3000mi maintenance

+1 to getting a stock airbox (K&N makes filters for these)  We cant suggest jetting until you decide what intake to run.

+1 to correcting your jetting, but you havent said anything about the exhaust you have.  it does matter. We cant suggest jetting until you decide what intake to run.

Why would you pull the engine if youre compression is good across all cylinders? Ready for more $ than you planned for?

Backfiring and popping indicate your air/fuel mixture is off.

Gearing - count the teeth in your rear and front sprockets and report back.

[mkoski]

I have adjusted the idle set screw to the 2k rpm, as thats the point where it gets rough and unstable.

The 3k mile maintenance is complete, bar me setting/checking the points and valve tappet clearance as I dont have a thickness gauge yet... (working on it)

Stock airbox (while the simple solution to my problems) will probably be a last resort, as I dont mind spending the time trying to get it to run okay with pods.

As for my exhaust, its a 4-2 drag pipe style with no muffler or anything. I also live in Calgary, Alberta so the altitude is a factor to my A/F ratio.

I was only getting ~120 psi at the cylinders and want to do the top end (new rings, lap valve seats, ect.) to get a little more compression. I was planning on painting the engine as well, so that works out okay I think. (This winter is a full restoration for the bike.)

And yes, I will count the sprocket teeth but I am pretty sure they are stock, as my new chain was the exact same length as the stock one I pulled off, 88 links.

[flybox1]

I have adjusted the idle set screw to the 2k rpm, as thats the point where it gets rough and unstable.
Why so high?....idle is at 1000-1400

The 3k mile maintenance is complete, bar me setting/checking the points and valve tappet clearance as I dont have a thickness gauge yet... (working on it)
Soooo, are you implying you did a carb cleaning and vacuum sync BEFORE you have done points, timing and tappets?  This is incorrect order

Stock airbox (while the simple solution to my problems) will probably be a last resort, as I dont mind spending the time trying to get it to run okay with pods.  Gotta get the maintenance stuff done FIRST, and this is best done with a stock airbox.  once it runs right, make the change to pods and rejet.  right now you cant tell if the bike runs poorly because of the jetting or because its out of tune  

As for my exhaust, its a 4-2 drag pipe style with no muffler or anything. I also live in Calgary, Alberta so the altitude is a factor to my A/F ratio. this would make your bike run lean.  +10 on your mains.

I was only getting ~120 psi at the cylinders and want to do the top end (new rings, lap valve seats, ect.) to get a little more compression. I was planning on painting the engine as well, so that works out okay I think. (This winter is a full restoration for the bike.)  what makes you think 120 or higher would be better?  if compression is within 10% across all cylinders, its better to leave it alone unless there are mechanical issues needing attention.

And yes, I will count the sprocket teeth but I am pretty sure they are stock, as my new chain was the exact same length as the stock one I pulled off, 88 links.  # of links does not matter.  sprocket teeth (gearing) does.   chain should be replaced along with new sprockets

[mkoski]

Yes, idle is supposed to be ~1000 rpm but my bike dies if I let it idle below ~2000 rpm.

Yes, I did that in the incorrect order then, so I will go back and revisit all of those items before I move forward. I'm new to this whole thing (which I'm sure is apparent) so I am just doing my best to learn as much as I can. I'll look into everything you brought up and revisit these issues once everything necessary is dealt with.

[flybox1]

An idle of 1000rpm is nearly impossible, unless, all other components of your bike are perfectly in tune.
My bike idles best at 1200rpm.
if your bike chokes out below 1500rpm, you need to revisit the cleanliness of your carbs(idle circuit), along with valves, and timing....

look into getting a copy of Hondaman's book, and an electronic copy of the shop manual from the FAQ section here...
both are invaluable, and include the important maintenance items....

[martin99]

+1 to all the advice so far.

One thing I would add - have you pulled the slow jets? You can't clean them properly unless you do. Doing this had the greatest single effect on smoothing (and allowing me to lower) the idle on my bike.

[Stev-o]

+1 to all the advice so far.

+1.  Pods + open exhaust = a motor than can be a challenge to tune.  These motors like back pressure.