overheating, air screws, and other questions

[wsup55]

new the the forum, this is my first post.  I apologize in advance for a long post.  I'm new to working on bikes so I don't know what info is important and what isn't.

Background:  I recently had my '77 SOHC in to the shop.  They cleaned out the carbs and replaced needles but didn't check floats or rejet.  When I got the bike back it was running great except for taking a long time to warm up and had some heavy backfiring while decelerating.  I followed some advice which suggested to adjust the air screws to help on with the backfiring.  The original position was 3 turns.  The backfiring stopped at just over 5 turns.  After adjusting, the engine seemed to run very smooth and warmed up much quicker but there was a noticeable performance loss and the bike seemed to be running hotter in general.  I've also read the oil weight can affect how hot a bike runs so it's worth mentioning that in the same time period I changed the oil using 10w-40.

Questions:

Is 5 turns too much?  Should I be looking into rejetting?

Any advice on opening the air screws to stop the backfiring and the resulting loss in performance?

Today i hit heavy traffic for the first time in a while.  After about 10 minutes of stop-and-go I saw a DRAMATIC loss in power and the engine seemed to be running quite rough.  Overheating?  Thoughts given the above info?


Thanks in advance.

[CB750Winger]

  I think the 77 carbs are different than my 75 and I think your air screws should be backed out from seat at about 1 1/2 turns. You may have an accelerator pump on #2 carb also. Did you sync the carbs after installing? You also need to check your points and timing on 1/4 and 2/3 cylinders to make sure this is right. Have you checked spark on all cylinders? You should not have to worry about the jets unless you have altered the air box or exhaust. Your floats should be set at 26 mm from base of carb to bottom of the float but do not compress the pin into the valve, just touching it.

[2tires]

too much air and you will burn your valves...floats are not the problem unless they bent them while working on them..the  loss of power is too much air...less gas gettin in...oil  weight is a tempory fix...air  filter clogged??  overheating is too tight valves...02 in and .03 out.. and believe it or not a bad charging system will cause overheating....14.2  volts at your battery terminals when the motor is running..

[wsup55]

i replaced the air filter a couple days ago and saw no change.  I'm trying to formulate a plan for next steps.  I haven't synced the carbs since I got it back from the carb rebuild.  So the likely problem areas are the syncing and checking timing?  What are the usual suspects for backfiring under decel?  It didn't backfire before i took it in for the carb rebuild.

[Re-run]

Lets start at the beginning.  5 turns is WAY too much for a 77. Unless you have earlier carbs on the bike.
Do you have the accel pump on #2? If so then you have the right carbs. the FUEL screw base position is 1.5 turns from lightly seated on the K, for the F it is 1.75. If you have popping on decel, you are running rich. The 77/78 are different from the earlier carbs. People get confused by that. See our carbs have fuel screws, NOT air screws. If you turn our screws out, the mix becomes RICHER not leaner as it did in the earlier carbs.
Turn them back in to 1.5 or so. Also, what air filter do you have? This makes a BIG difference sometimes. The honda paper is a good filter. The emgo filter( a lot of places sell this rebranded) is built very well but very restrictive. I have a k&n in the airbox and it works well. Pods run the engine very lean though and take a lot of work past simple tuning to work effectively.

Engine oil removes heat, just like air would. Thicker oil will hold more heat. A 10-40 is ok if it is not too hot, it is in the manual after all. I use rotella, it IS better than regular 10-40. It stands up to abuse better. It is a diesel oil.

Ultimately, you need to find out what carbs you have for sure. Also, we need to know the filter style. I am also going to tell you this. Most likely, if you have the correct carbs, your shop did NOT clean them right.

[wsup55]

Thanks for the detailed post.  It's very helpful.

Yes, accel pump is on #2.

The air filter that I have on it is ordered from JC Whitney.  Not sure if it's emgo rebranded but not k&n.
http://www.jcwhitney.com/AIR-FILTERS/GP_2002763_N_111+1977+200730429+600014687_10111.jcw

"Most likely, if you have the correct carbs, your shop did NOT clean them right."

I've been realizing this more and more.  It's my first bike so I wanted to try to get it set up correctly the first time before I started tooling on it.  I'm feeling more and more like I got worked over.  It's probably time for me to start learning how to do it right.

So what are my next steps after resetting the screws right?  Digging into the carbs?

[Re-run]

I hope the filter they send is not the one in the pic. That would be the wrong filter. I have ordered from whitney before and they did send the right stuff.

The best thing to do is get a shop manual. The honda one is best and CAN be found online for free but it is not easy to find. the other ones will help but they show mainly 69-76 pics with a little 77/78 thrown in.

The biggest problems with cleaning is that many never pull the idle jets. People think they cant be removed, heck I thought that until told better. You wrap a piece of leather/rubber/something tear resistant around them and then grab with a pliers and pull while twisting gently. They pop out. you will most likely see a small amount of gunk and crud. This is enough to make them not work right.

I will see if I can find a link to manual for you.

[wsup55]

The filter I received had CB750 stamped on it and fit the air box correctly.  I also changed the filter after I was experiencing issues.  Definitely still worth while looking at it as part of the whole though.

I have a Haynes manual and just recently bought a Clymers.  

Is the treatment for cleaning the idle jet the same as the main jets i.e. carb cleaner and compressed air?  Should my next steps be to clean out the idle jet, double check the rest of the carb, then sync?

Thanks again for the advice.

[Re-run]

If your jets are not too bad you can get away with that. Most end up using a strand of copper wire though to push through the holes, then air and cleaner. I know a lot cry not to do this but unless you roto router things, you will be ok. Just go easy.

So when you clean, pull all the fuel rails off and any tubes. Remove the jets and the idle screw taking care to retain the parts. Clean everything and then clean it again. Clean it to where you would eat or drink out of it pretty much. THEN you are clean. Anything less and you will end up doing it again. but once done right, it will be years before you do it again unless you leave fuel in there a long time. I did mine 4 years ago and this spring, I did it just cause and they were still clean.

otherwise, follow all the tune up steps in the manuals.

[Re-run]

this site should allow you to download the 78 manual, the 77 is about identical. very minor differences.
Much better than the clymer of haynes.

www.mediafire.com/?5ttzdg9yyuz

It will start your pdf reader right away. Save it then.

[dave Esch]

When I got my bike (just last week), one thing I lvoed about it was the open exhaust. Four to one header full dump. The carbs have been jetted and it sounds great runs great, but I get heavy backfiring as well. I just assumed this was due to the zero back pressure.

[Re-run]

If properly jetted, backfiring should not happen or be very low unless your headers are very short as in make it to the bottom of the engine and thats it.

If your exhaust is backfiring, then your mix is too rich. A common issue on the pre77 bikes.