Topic: new owner of a '72 cb750, spark plugs black as coal

[Hondawggie]

I'm glad I did all the easy stuff first.

If you can call losing the #3 spark plug for about 25 minutes today 'easy.'  When I was removing the old plugs to install the new ones, old plug #3 slipped back to the rear of the head in a gap behind the #3 sparkplug socket that Honda thankfully designed as a emotionally stimulating bigass hole that is (luckily) big enough for the fat end of the spark plug to slip into.  As I pulled my plug socket out after unscrewing the #3 plug, the plug dropped and I went to grab it and pushed it into the bigass hole.  Nice emotionally stimulating Holy Head design there, thanks Soichiro.   I had to remove the horn so I could get a visual on the hole.  At one point I was nearly committed to pushing the little f!@#ker back as deep as possible and leave it there and install the new plug.  That's how deep I found the spark plug could go, nearly enough to clear the plug socket in the head.  What a laugh in Japan they must have had on that accidental design flaw.

I'm such a novice around the 'gotchas' on this bike.

So all the easy stuff is done, bike runs strong for a '72 bike with only 65 horsepower.  But it's got too much unburned fuel.

Another bit of feedback I'd like from you pro's -- if my coils were bad, what's the chance that
- both coils would be bad
- and given both bad, would they have the plugs being very sooty like they are?

I'm guessing the carbs are very dirty.  It won't surprise me if there is noteworthy junk in the bowls and blocked air passages.

The cool thing is,  this durned bike runs strong now, held back by the sooty plugs, I think I have a pot-o-gold of performance gain waiting when I pull and clean the carbs.

[ekpent]

OK l have read most. Your powerband on a good running 750 will start at 2500 and run to the red.Most of my riding is between the sweet spot of 3-7 thousand RPM'S. It should pull very strong anywhere in that range,over redline if you want to,these engines are known for good low end torque and power. If you have a flat spot in there get the carbs sorted if everything else checks out.These bikes do not have a narrow power band,thats why I like them.

[Hondawggie]

Only reason I'm hesitating now is the amount of time I have in this project.  I pretty much plan to sell it, it has 50k miles, it looks weathered, I've got about 2 1/2 days of my life into the work already.  A weathered '72 cb750 with an old 4-into-1 pipe (quite rusty header pipes, was going to sand and paint black) -- also needs a seat cover -- would bring $750 ($1 per cc) if I was LUCKY.  In this economy?  Who knows.  So I'm not sure if pulling/cleaning/possibly rejetting is a good investment of my time.  It runs pretty strong.

If this bike had 18k miles and was in pristine cosmetic shape and had the highly regarded hm341 stock exhaust still that was also in great shape -- it would be much less of an issue to commit more time to it.  As it is -- if I pull/clean/maybe rejet the carbs, the bike would still sell for.......$750.  If I'm LUCKY.  I have a couple other projects that look much better I might better spend time with.

I realize these motors go for a long time, but the bike is cosmetically beat.  No serious flaws, just faded original paint, seat needs re-cover, that yucko-looking 4-into-1, and the perception of a buyer "hey it's high mileage."

This bike reminds me of the movie "The Mask" where Stanley Ipkiss (Jim Carrey) is out in front of the nightclub with Cameron Diaz, he's waiting for the valet service to retrieve his car.   The valet drives up with a clunker, Ipkiss says "That's not mine."

[TwoTired]

I get it now.  You just want us to help you make money on the bike.

Good luck!

[Hondawggie]

Nah, just mad that I always underestimate time.  I got this to hand off to (hopefully) someone who does restos, which I lack the talent and experience to do.   I really appreciate the help from everyone.  I don't know if anyone makes $$ from buying nearly dead vintage bikes and making them roadworthy again, but I like to do it.  I have this '74 cb750 with a stuck motor that only has 18k and the hm341s still on her, bike was garage kept and is stuck from sitting over a decade, that '74 is calling me.  I'm just trying to get real with myself here.

[Logan80]

Why don't you take literally 5 minutes and pop one of the float bowls off and look at the jets? Don't even have to pull the rack of carbs to examine ALL of the jets if you wanted to.

[Hondawggie]

Why don't you take literally 5 minutes and pop one of the float bowls off and look at the jets? Don't even have to pull the rack of carbs to examine ALL of the jets if you wanted to.

Once I get started I wont stop thats why.  I wanna get started on my '74.  I cant just pull the bowls it will be like my niece asking me for a nickel.  She then moves up to a dime, 'You gave me a nickel, a dime is only a little more' and she's right.  Before longn she will empty my wallet and have a college fund!

Only have so much time.

[Logan80]

I only figured that since you've been asking/suggesting over and over that it's jetting improperly, you would spend the time to find out. Don't have to rebuild them to find out what they jet sizes are. But if you want to give up, that's fine too.

[Hondawggie]

Well theres no more mystery about it, fun's over.  Gotta be the carbs.  I know how to do carbs, old hat.  I am ready to do the hand-off.  Let's see if I can breath life back into my ramshackle '74!  This one will be a challenge.    I'll leave some wiggle room for the '72 but I had wanted to start the '74 few days back now.

[KingCustomCycles.com]

Next time, start with the carbs.