Topic: popping... carbs or pipes?

[Vintagelover]

Hello all!
Anecdotal info: I'm 29 and have finally stopped traveling! Good job + spending money + free time = Time for a hobby! The goal: build my own motorcycle by 35! Yes yes, I know. go ahead... laugh it out. Anyway, the SOHC4 I have is a '77 CB 750F that I got about 1.5 yrs ago and have been slowly improving using to learn how to work on engines. I also have an air cooled '78 BMW. I got the Honda running really well and it stayed that way until about a week ago when... I pulled the plugs and noticed that a couple of them were white. Time to tinker, right? Lean mixture, screw the air screw out, right? Edit: since I posted this, I learned that it's not the same on all carburetors. IN for some, OUT for others. Which pretty much makes the next paragraph useless.

The Problem: The plugs I noticed were 3 and 4, which looked... whiter than I expected, but weren't blistered, I figured they were lean and screwed the air-mixture screw (coming straight off the bottom of the carbs. screws which, up to then, had been to spec: 1.75 turns out) OUT. First 1/2 turn, then 1 turn. But what actually happened was that I started noticing WAY more popping than the previously existing seemingly-random pop here and there. Two types of pops: The carbs themselves backfire into the pod filters on acceleration and low throttle; the pipe (4 into 1) backfires on mechanical deceleration (downshifting). I checked plug 4 and it's still white. More popping + still lean? I've only gone on a few rides since I disassembled the carbs, cleaned them, set the floats, and put in new gaskets. Could something have clogged up again already?

Also, I took a look at the 4 into 1 pipe and noticed the actual baffle/muffler part is wrenched as tightly as it will go, but still moves when I apply 2 hands and push/pull. I assume there's some air flow here and am going to make a shim this evening, which will fix the thing in place, but as for sealing off air flow.... ? ... and as for the connection between backfiring and a sealed exhaust system... ?

Needless to say I have a lot to learn about carbs and exhaust, but that's why I come here
Feel free to let me know if I've rambled too much. I've read plenty of posts that are too sparse on details; I'm purposefully erring on the side of "as much info as possible."
As always, any wisdom is appreciated,
Andrew

[ABikeNamedTrouble]

Yes, you have a lot to learn about carbs  .  But don't worry, I bought a 650 that needed rebuilding with absolutely NO mechanical experience.  I didn't even know how to drive it once I got it running...  and now I have a classic motorcycle running in excellent condition  .

Anyway, rather than trying to tackle all the issues your post brings up, I'm going to refer you to some resources that were a REALLY big help to me:

Carburetor theory & tuning:
http://www.carbparts.com/keihin/needles_tuning/jetting_your_carb.htm
http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm
http://www.gadgetjq.com/keihin_carb.htm
http://justkdx.dirtrider.net/printcarbtuning.html
http://keihincarbs.com/tips/gate.html

Spark plugs:
http://hondachopper.yuku.com/topic/20355#.TpNN9JzrUwI
http://www.4secondsflat.com/Spark_plug_reading.html
http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html
http://faq.f650.com/FAQs/SparkPlugFAQ.htm


Enjoy the reading! 

[ABikeNamedTrouble]

Oh, one more thing:  I wouldn't play with ANY adjustments from the stock configuration until you make SURE every passage and pinhole in your carbs are clean and working properly.  If you have a hard time getting your idle jets clean, just replace them.  The cheap price tag is VERY worth the trouble it will save you.

siriusconinc.com and jetsrus.com are both excellent sources for jets if you need them.

[BobbyR]

Some basic things. The 77/78 750s have a different carb than earlier models in two ways. The adjustment srews are not air screws they are mixture screws. You also have an accel pump.

I had some popping that cropped up. I used a piece of clear tubing slipped over the carb drain and held next to the carb bodies and found one low float level. You can find pic of the procedure by doing a search. Eevn if that is not your problem, you may also have low float level anyway.

With these old bikes everything needs to be checked. 

[the technological J]

build a motorcycle? like from what to what? an exsisting engine then a custom frame? no laughs here other than what might take you so long... thats what half of us are on here doing... check out the project shop

[Vintagelover]

Adam, Thanks for the resources 

Bobby, so I did know about the Accelerator Pump. What I didn't realize is that Air screws and Mixture screws are different. I have a lot of reading to do, maybe that will become clearer for me.

Tech J, I'm thinking right now that if I can take a welding class at the local comm college, then I can weld a frame. Don't have the slightest idea what kind of metallurgy would be involved in making an engine. So right now the goal is exactly what you said: make a frame, find a good engine and start from there

[phil71]

welding is such a small part of making a frame. Take a CAD class!

[Vintagelover]

So I took out all 4 plugs and figured I'd borrow a camera and show you guys what I'm looking at. Pics are in ascending order of plug (plugs 1-4). As I was taking them out, I noticed that it was pretty cruddy around the cylinder threads. Any tips on how to clean that out without getting gunk into the cylinder?

WOW, I was pretty far off, considering those plugs to be 'white.' What I was looking at was the ground strap... well it SEEMED white when I last checked it (now I'm doubting my memory/untrained eyes/camera).