Topic: Yay! I burnt my self! 78 CB750F

[Porscheguy912]

Alright, So i'm an idiot. I know big surprise.

After rebuilding carburetors for 13 years i thought i was hot #$%*.
The secret is i have never rebuilt or tuned these damn F3 carbs before. Plenty of webers and solex, hell, even a bosh jet-tronic mechanical fuel injector pump. But i am at my wits end with these dang 78 F3 carbs.

My 74 550 was so easy. I just soaked, cleaned, rebuilt, bench synced, set mixture screws and tossed em on the bike and it worked fine. But not these F3 carbs. Now i know why they cause such a ruckus here on this forum. I always thought "what could be so hard about them", when i would see posts about em before i got one.

After a nice clean rebuild (over night soak for each carb, compressed air in all the orifices and new everything but slow jets *duh*) It still wouldn't ildle with out the choke on.

I researched the problem here, i used a guitar string to poke into the slow jets. I slipped a clear PVC hose over the jet and blew into it and it seemed clear. i got some resistance with The guitar string but nothing that i felt would raise an alarm.

Put it all back together and cranked her up. **Note that i am running 133 main jets with new pods and 4-1 wrapped mac exhaust with pilots @ 2 turns out**
Once she warmed up she began to smooth out. I dropped the choke and she sounded like she was going to die but she didnt! It coughed and sputtered but kept the rpm a little below 1,000Rpm.

"Yes!!" i thought, maybe i can actually take this bike around the block....Mind you that I tightened down the mac muffler clamp as much as i could with the crappy tools i have here at my apartment. I have most of my "real" tools at my folks house where they cant or wont get "jacked" but some alley bum in Chicago.

So i jump on her and roll off of the two stand and i'm off! It was really sputtering and spitting though. I got into the street and opened her up and wow is she fast! I zipped around quite a bit. all the while maintaining a good handle on the RPMs with down shifting or the throttle.

I get on to my home stretch a block from my house and i decide to open her up and see what happens.

Well, i'll tell you hat happened alright... I blew off the Mac muffler tip! It must have worked loose and i look behind me and the damn thing is bouncing around the street like a jack rabbit! So i turn around (the bike has gotten significantly louder by the way and people are beginning to stare and look out their windows) to get my tip, and for some reason i just reached down and grabbed it. Yes, it was hot. Very hot.

I now have a very fine 3rd degree burn across the finger tips of my left hand and palm.
Strange thing is it doesn't really hurt that bad now. but the skin is really tight and i have no finger prints.
Good thing my night job is working the door at a bar where is can hold a nice cold brew while i work. Dual purpose, for the cold on the hand and the brew in the belly.

Thing is, i though it was running to lean without the choke. and once that baffle blew off it actually ran alot better. Not perfect by any means but better. Could i be too rich?
I noticed that i was getting some flames from the end of the mac baffle when i would rev it up.
I haven't pulled the plugs but i will do that tomorrow if i can manage to get my shoes tied with one hand.

[fuzzybutt]

i had a real problem getting the muffler to stay on my mac header too. i put a couple of spot welds with the mig on and it didnt move again till i wanted it to.

[Gungosa]

I don't know about your 'too lean/too rich' question, but i do know that engines will generally run better without a muffler because you have created a "free flow" exhaust. Too bad it's loud enough to be illegal and to do permanent damage to your ear drums.

[MacM2010]

Man, that sucks.  Though flames coming out the muffler would be a cool effect, especially if you're going to be chasing alley bums away from your garage.

Anyway, you have my sympathies. And my permission to put ice in your beer, without me comparing you to my grandma. 

[WFO]

Did you say ouch or various selected nasty words?

[eurban]

Sorry to hear about your hand   You ask if you could be rich. . .Did you take a look at your plugs?  133 mains could very likely be a bit large for a stock motor with pods and 4-1.  What means did you use to determine the size of your mains?  133s are somewhere between 14 (105 size main in 78F) and 9 (115 in 77k) steps in size larger than the  stock pieces which is a huge jump in jetting.  Also sticking a steel guitar string though a brass pilot jet has some potential for damaging the tiny opening. How did you arrive at 2 turns out on the IMS screws? Did you shim your main jet needles? If you decide to replace your pressed in pilots, new #35s can be had for little $ from Sirrus Inc.

[Porscheguy912]

Did you say ouch or various selected nasty words?

Believe it or not, i think all is aid was "ah #$%*" under my breath. LOL

Sorry to hear about your hand   You ask if you could be rich. . .Did you take a look at your plugs?  133 mains could very likely be a bit large for a stock motor with pods and 4-1.  What means did you use to determine the size of your mains?  133s are somewhere between 14 (105 size main in 78F) and 9 (115 in 77k) steps in size larger than the  stock pieces which is a huge jump in jetting.  Also sticking a steel guitar string though a brass pilot jet has some potential for damaging the tiny opening. How did you arrive at 2 turns out on the IMS screws? Did you shim your main jet needles? If you decide to replace your pressed in pilots, new #35s can be had for little $ from Sirrus Inc.

Well for starters i have a set of 120 main jets on order from sirus. It has been a week and a half and i was getting rather impatient. So i used some charts online to find out what drill size i needed to drill out my old jets a bit bigger. Martin at sirus seemed to think that 120 mains was going to be a little conservative but i ordered them anyway thinking i could always experiment with drilling to find the best size.

I drilled my old jets out to a #55 drill bit, which i found to be in the middle of 1.30mm and 1.35mm.

I know the stock setting for the pilot screws is somewhere in the middle of 1 1/2 and 2 so figured that would be a safe starting point. I have not pulled the plugs yet either, (work all day) but i will let you know what they have to say. Thought the slow jets were pressed in and not replaceable? I will check that out.
And did not shim my needles either. should I have?
Oh and the carb boots are original but in my opinion in working order. I will crank her up and spray them with starting fluid to be sure.

[alltherightpills]

I now have a very fine 3rd degree burn across the finger tips of my left hand and palm.
Strange thing is it doesn't really hurt that bad now. but the skin is really tight and i have no finger prints.

If I'm remembering what I learned for my first aid merit badge, a third degree burn means that all of your skin is charred off and it is black and nasty.  A second degree burn will give you a nice big blister and make your skin tight like that.

http://en.wikipedia.org/wiki/Burn

Can't really help you with your lean/rich issues.  Also, where did you find those drill bit conversion charts online?

[crazypj]

stock MAC silencer is actually pretty restrictive so you may be lean at idle then rich everywhere else.
A bigger pilot jet would probably be a better bet than going huge on main jet. Are the F3 carbs the three jet ones?
PJ

[mark]

....I now have a very fine 2nd degree burn across the finger tips of my left hand and palm....

I hate when that happens.

[eurban]

Your slow jets are indeed pressed in but they need to be removed to be properly cleaned.  I would suggest adding a set of pressed in slow jets to your order from Sirus; should run you less than $20 and might save you alot of hair pulling. They don't seem to carry anything other than the stock #35 pilots but I really doubt that you will need to go to a larger than stock pilot. On your carbs, the "easy" carb tuning adjustments you can make are: main jet size, main jet needle position (78s don't have multiple positions for the needle clip so your option is to use small washers to shim the needle thereby richening the mix) and turning the IMS's in and out. Fix any functional problems with the carbs (and the rest of the bike for that matter) before you attempt carb tuning or you are wasting you time! For instance, pilot jets and their associated circuits need to be clean, the accelerator pump needs to function, the fuel level in the bowls needs to be correct (you can use clear tubing on the overflow nubs to "see" the level), the carbs should at least be bench synchronized, etc etc. While you are waiting for parts / letting the fingers heal up, you might want to read up on carb tuning and working on the 77/78 carbs.  Searches under "pressed in idle jets" (or "pressed in pilots") , "accelerator pump", "carb tuning", "plug chops" will give you some good info to read.

[Porscheguy912]

Thanks for the good info!
I certainly dont mean to turn this into another slow jet thread.

But... I just got to my other garage and re-cleaned the slows. I pulled them out carefully soaked them in carb dip and blew them out with compressed air @ about 120 psi. They didnt appear to be dirty though, which makes makes me even more mad. I was hoping to find them jammed with crud. Oh well.
 I also blew out and poked around in the two tubes that feed air to the main jet emulsion tubes and the feed passage to the slow jet. everything appears to be crystal clean.

My 120 mains arrived form sirus today aswell. I took out my mfg'ed 133 mains and put the 120's in. I also opened up the IMS or pilot jets to 3 turns from seat. I was unable to look at the plugs before i left because i realized that my plug wrench was in the burbs aswell.

I will be back to the bike tomorrow (with my tools) and i will toss these buggers back on the bike and hold my breath. I also think i will leave the stock Mac baffle out. I will prob get a monza or cherry bomb type tip instead. The way that damn thing blew off of there makes me think that it will be much happier with a free'er exhaust. Besides i was really disapointed with the way it sounded any way.

 

[eurban]

Why three turns out on your pilots?  Thats pretty much the end of their adjustment range. I would go back to 2 turns as a baseline.

[crazypj]

Unless you use a good magnifying glass (or preferably a x5 jewelers loupe, cheap from Harbor Freight for set) and a strong light, you cant tell if jets have corrosion inside the 'hole' Air will blow through ok. Carb cleaner wont shift oxidation. I usually 'polish' the inside of jets with a strand or two (or three  ) of copper wire as its much softer than the brass jet.
 As mentioned, if you need more han 2.5~3 turns ou on mixture screw he pilot jet is too small.
PJ

[Porscheguy912]

Why three turns out on your pilots?  Thats pretty much the end of their adjustment range. I would go back to 2 turns as a baseline.

10-4 !

[Porscheguy912]

So i got the bike to idle with out the choke on.
I was 4 turns out on the Pilot screws. It idled really low and wanted to die.
I also shimmed my needles with a 1mm washer to make the mix richer.
*Note i'm using the needle seats, Pilot screws, main needles from a 1977 kit but they dont have clip positions on them.

The bike still dies the moment that you twist the throttle.
My accelerator pump squirts but it seems that i have to give it a few rapid twists to get a solid stream.
(insert "thats what she said joke" here ) I opened up the diaphragm plate to try to clean up the holes in the rubber a bit and i noticed that there is a small rip in the the rubber under one of the holes. I dont think that it is compromising the pressure but i still have the old one and its in decent shape so i'll try it. Its just frustrating that new parts are junk.

I'm convinced that the reason that i need to open up the Pilot jet screws so far is that the passageways must be dirty. I'm ordering new slow jets tomorrow. I disassembled all the carbs today.
My Dad is a jeweler and i talked him into loaning me his ultrasonic tank for a while. They will be "Hot-Buzz-Dipped" tomorrow.

I'm about to throw these on ebay, score a quick $150, add a little more cash and pick up a set of smooth-bores. This is driving me nuts!