1975 cb750k running rough

[SnowCrash]

Hi guys, I've been doing a lot of reading from this site, but I could really use some personal situation knowledge from the guys that live and breathe these engines!

The bike is a cb750 k5, 1975. It has the stock airbox and new stock air filter (bought from cb750supply), new carb rebuild kit (also from cb750supply) with a 110 main jet, 40 idle jet. The exhausts are MAC 4-into-2's (I think).

I have just done a valve clearance check, oil and oil filter change.

Next I will be doing a timing check (once I get a timing light) and also adjust the cam chain tensioner- I need to learn up on these first.

I have the carbs bench synced and have the gauges hooked up (from cyclex), they seem to be reading very close to each other even after blipping, but it just sounds like the engine is stumbling. Also, when I blip the throttle, the RPM's stick at 2,000 on the way down- then slowly drop down to idle.


Any help much appreciated!

[shinyribs]

Where are your idle mixture screws at? The manual calls for 1 1/8 out from lightly seated. Usually anywhere from 1-1.5 turns will get you going good enough to fine tune later.

Slow return to idle could be a vacuum leak. Most common sources of vacuum leaks are the carb boots, so I'd check there first. The normal test is to spray around the boots with some starting fluid while the bike idles and listen for an increase in rpm to indicate a leak.

The vacuum ports on your carbs can be a vacuum leak as well if the aluminum crush washers are missing or just no longer sealing.

Sounds like you got a good start on getting it squared away already, but a slow return to idle is most always a vacuum leak. Good luck!

[SnowCrash]

My idle screws are at 1/2 turn out, I will try tomorrow with one full turn out. The reason I only did half a turn was to compensate for the slightly more open exhaust.

I was hoping it didn't have to do with the carb boots, those are pricy to replace! About $100 for rubber!
But yes, I will get some starting fluid spray to check for where it's coming from.

My buddy is saying it might be the timing, does timing go bad from just the bike sitting?

[TOOLS1]

No timing does not go bad from sitting. You can recondition the carb boots by boiling them in water with wintergreen oil. Makes them soft aging. Then replace the band clamps, as they stretch over the years.
TOOLS

[SnowCrash]

wintergreen oil is a bit hard for me to find, I heard transmission fluid works just as well? All I see are very small quantities of wintergreen being sold for quite a high price at places like walmart.

[TOOLS1]

I get it at the local pharmacy. I know it is a little pricy ($6.00 for 2 oz) but you only need an ounce.
TOOLS

[SnowCrash]

So you mix an ounce of water and an ounce of wintergreen oil and boil it? I'm guessing you do it one by one boot because I don't think 2 ounces of fluid would cover all 4 boots

I just looked at clamps....any recommendation for cheap alternative replacement?

[shinyribs]

It's an ounce of wintergreen oil to a whole pot of water.

I found a quart of wintergreen on ebay for around $25 shipped to me, I think it was. I used waaay more than 1 ounce to my pot of water. Thats been a few months back and the corner of the shop where they are being stored still smells amazing. Still supple,too. Stuff works good.

[Re-run]

What brand of air filter did you get? Was it the honda filter or some cheap emgo or similar? I had an emgo filter once. Great quality but too restrictive. It caused a lot of tuning issues while I had it. Was going to change it out for the honda filter but for $10 more I got a k&n instead.

As for exhaust, seldom do carbs need to be tuned cause of an exhaust, unless it is some super free flowing thing such as drag pipes are a big 4-1. Your 4-2 most likely have a very negligible effect.

Also, check your spark plugs and see what color they are. Those are your tattletails when it comes to running rich or lean.

[SnowCrash]

I got it from cb750supply

Stock Air Filter - OEM Ref # 17211-405-000

Fits: Honda CB750K (1969-78)
CB750F (1975-78) Supersport
CB750A (1976-78) Automatic

IN STOCK
Part #: 15-0060

[SnowCrash]

Hi guys, just an update.

I've just got done doing the cam chain tension adjustment, timing adjustment, and tappet gap adjustment- along with carb sync.

My two problems are:
1) high idle (it's at 1200rpm, and I can't lower it because unscrewing the idle screw further has no effect)

2) slow return to idle (but will return right away when I force it closed with the throttle)

Here is a video showing the problems:

http://www.youtube.com/watch?v=2FWwYT0mCls&feature=youtu.be

There is NO vacuum leak, I sprayed carb cleaner all around the vacuum hoses connecting the carbs to the engine- no change in rpm. The carbs are all synced very close (not perfect, but close).

Any help appreciated!

p.s. Now that I remember, there is a bit of popping and a backfire every now and then when I was test riding it around the block.

[gkmaia]

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[Re-run]

High idle like this could be either the cable is not allowing carbs to fully close and needs adjusting. Or another option could be the mix screws are turned out too far and the mix is too lean. Those are common high idle causes. your plugs will tell you if you are rich or lean.

[TOOLS1]

The reason your idle is high, but goes down when you push on the linkage is because when your carbs were synced the slides were backed off too far. Turn up your idle, and then re-sync the carbs to bring the idle back down. You should have them at 7-9 lbs of vacuum.
TOOLS

[Re-run]

So, you think maybe the idle was off when last synced? I do mine at 1000rpm and it has always worked. I did not use the bike tach though since those are not overly accurate. I used a dwell meter with tach. Manual lists it as 1000 rpm too. Maybe he had the idle too high last time.

I know 1 time I had it happen and it was the pull cable not adjusted right. I could force the throttle closed a bit, but not all the way. Adjusted better and it took care of it.

[TOOLS1]

I have had this happen to me a couple of times now. If you look at the FSM it has a picture showing how many threads are showing on the sync adjusting screws past the locking nuts. When you sync the carbs, you are adjusting the height of the slides, and that is the same thing that the idle adjustment screw does to adjust the idle. Only it adjusts all 4 at the same time, while the sync screws just do one carb at a time. If you have the slides set too high with the sync screws, they will not be able to go low enough to idle correctly.
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[Re-run]

I guess I have never had to adjust my slides that much to have an issue like that. Crazy

[Lucky 1]

You do not need a timing light really. Check the advancer and make sure it is not rusted.
Just set the points static and use a dwell meter to make sure the points are still serviceable.
You can get a dwell/tach from sears for about $30.

Make sure all 4 pilot air screws (mixture screws) are set to the same number of turns.
Forget that lean best idle business.

[Re-run]

Dwell meter is by far the best way to set points gap. I would still use a light for making sure full advance works right. Main reason is that over time, the springs weaken and advance could happen before it is supposed too. Or if a person monkeyed with the springs and clipped them, basically just making sure advance happens when it is supposed to. I got a cheapo light from harbor freight for about 12 and it works perfect. Good enough for the few times I use it anyways.

When it comes to the idle mix, I guess this depends on what level of operation a person wants. If a person just wants a bike that runs decent, the appropriate # of turns out and idle it for a bit then check plugs and adjust as needed. This certainly works close enough and is kinda what I do. I probably put a bit more time into it but not a whole lot. Now if someone wants as much as they can squeeze out, well then they will need to tune the mix.