Author Topic: cb750 carb problem (Read 17552 times)

efyou · « **on:** March 10, 2009, 12:00:32 PM »

I recently bought a 1975 Amen Savior special construction with (at least) a cb750 engine. The bike is in SOCAL and was sitting since 1994-1995 until July 08, when the previous owner bought it. The previous owner got it running and upgraded to Dynatek electonic ignition. The bike has stock appearing (from outside) Keihin carbs. It currently has empty dual air cleaner boxes (no filter element/foam)

When I start the bike it's on the first kick (not notoriously cold blooded as other cb750s) The bike runs fine on surface streets, but when I take it on the freeway and open throttle beyond 1/2 there's no more power...just sort of flattens out. Feels like it's fuel starved. There's no smoke at any speed, and the only time there's any backfiring is a little bit coasting downhill (surface street) with the bike in gear and throttle closed.

Is the problem as simple as running lean because of air intake mods? Is there an easy way to tell like adding foam elements for the filter containers (gonna do it anyway), or going to a bigger main jet?

upperlake04 · « **Reply #1 on:** March 10, 2009, 12:35:43 PM »

Sorry, just can't get past your username.

Johnny5 · « **Reply #2 on:** March 10, 2009, 12:43:20 PM »

Did you go through all the regular maintenance? Valves and cam chain and such?

Carbs synced?

If that's all done, it sounds like it's running lean. Check your spark plugs. That'll let you know if you need to rejet.

mcpuffett · « **Reply #3 on:** March 10, 2009, 02:29:48 PM »

sounds like its running lean, you could try putting your choke on and see if it picks up at all, this may point towards running lean, cheers Mick.

gerhed · « **Reply #4 on:** March 10, 2009, 02:37:54 PM »

sounds lean

TwoTired · « **Reply #5 on:** March 10, 2009, 02:45:58 PM »

Sounds like...
... you are trying to insult/berate everyone who might potentially help you with your user name.

Good luck with your problem!

efyou · « **Reply #6 on:** March 10, 2009, 06:28:34 PM »

Thanks so far for the replies. Please don't take my username as some personal affront. (now i unnerstand if yer a little miffed that you din't think of it 1st!) It's merely my username!

efyou · « **Reply #7 on:** March 10, 2009, 06:35:31 PM »

More info: All regular maintenance valves, plugs, oil done. Exhaust is Mac drag pipes.

Plugs look normal. I'm pretty sure they would only show how engine was running when shut down, and I haven't tried shutting down when its runnin on the freeway (no kill switch. girder front end.... you get the picture).

Gordon · « **Reply #8 on:** March 10, 2009, 06:43:06 PM »

F you, too.

Johnny5 · « **Reply #9 on:** March 10, 2009, 06:44:06 PM »

I would check where the needle is set and what size main it has and then go from there. Maybe it was never jetted properly to begin with.

mystic_1 · « **Reply #10 on:** March 10, 2009, 06:51:22 PM »

How does it do above 1/2 throttle in lower gears? Maybe it's geared too tall? Sounds like a chopper, give us more specs.

mystic_1

efyou · « **Reply #11 on:** March 10, 2009, 07:09:46 PM »

Was told engine that the bike is stock from the original build (except electronic ignition) in 1975. Amen Savior frame, girder front end. told engine is at least 836. Except for this problem it runs beautifully. Starts on the 1st kick every time and will idle without choke almost immediately. Runs very smootly. I haven't had carbs off to check jet size, but when I pull breathers, everything still looks clean and shiny on the inlet side of the carbs.

stock rear cb750 hub and sprocket laced to a 15" hoop

scondon · « **Reply #12 on:** March 10, 2009, 07:24:32 PM »

You should be able to check jet size by pulling a float bowl and looking at them. No need to wait until carbs are pulled. Simple and quick procedure.

Spanner 1 · « **Reply #13 on:** March 10, 2009, 07:39:04 PM »

The words 'fyou' ,' Amen ' and' Savior' never belong in the same post...your username is inappropriate, go away and come back 'G' rated !

billybobobrain · « **Reply #14 on:** March 10, 2009, 08:44:26 PM »

And we all forgot the UTFSE funktion!

billybobobrain · « **Reply #15 on:** March 10, 2009, 08:47:12 PM »

http://forums.sohc4.net/index.php?action=search2

177 pages of carb goodness, Have fun, happy reading.

mrbreeze · « **Reply #16 on:** March 10, 2009, 10:42:36 PM »

Hey efU......back at ya!!!!!!!!! Don't worry....we aren't all as squemish as the few that have posted!!! I agree with Johnny.....needle setting. You say it goes flat after half throttle......mine did it pretty bad at quarter to half.Putting the needles to 2nd from top did the trick.You have to figure in what altitude you are at,what you are using for air filters,etc. I played hell getting anyone here to guide me through it at first so I experimented on my own.Best thing to do is take the carbs down....ID what mains you have and where the needles are set. Clean them real good and when you are all done....synch is the icing on the cake.

efyou · « **Reply #17 on:** March 11, 2009, 06:52:35 PM »

Thanks to all of you who seriously tried to help. You've given me more than enough to get started, and confirmed what I already suspected. I especially appreciate the tips from those who obviously knew what they were talking about and cared to share.

to the rest of you:

The references to "AMEN" and "Savior" have nothing to do with religion. Go here and open your mind: http://www.hondachopper.com/amen.html

To the rest of you who have a problem with my name, that is your problem. You really need to think about getting a sense of humor. Any kind of a sense of humor. Even a crude sophomoric sense of humor. like that guy

efyou!

Chops are Tops!

Gordon · « **Reply #18 on:** March 11, 2009, 07:04:55 PM »

You're sadly mistaken if you think naming yourself "efyou" has anything even remotely to do with having a sense of humor. It's sophomoric, that's for sure, but not even close to being amusing.

If you want to have a productive and helpful time on these forums, you need to change your username and your attitude.

upperlake04 · « **Reply #19 on:** March 11, 2009, 07:19:17 PM »

Amen

mystic_1 · « **Reply #20 on:** March 11, 2009, 07:19:30 PM »

Quote from: Gordon on March 11, 2009, 07:04:55 PM

If you want to have a productive and helpful time on these forums, you need to change your username and your attitude.

I dunno, I think that his attitude (going off of what he's actually said) seems just fine to me. You just dislike his username, which is your prerogative.

mystic_1

Spanner 1 · « **Reply #21 on:** March 11, 2009, 08:19:14 PM »

Explain why I need to have an inappropriate sense of humor , if you please.

Well, i guess I'm just flat-out jealous that out of 4000 members worldwide, I didn't think of the most fabulous username..damn....it's a lovely thing to send around the globe, representing your country, oh!, wait we don't know where you are?, figures.....

cherry bomb · « **Reply #22 on:** March 11, 2009, 08:37:55 PM »

To piggy back off this problem Ive got one of my own...ive been doing a lot of reading on this forum and think ive nailed the issue down from some earlier posts but I'd like to make sure so here it goes...I have a 77 CB750F chopped and bobbed...It appears to be the stock engine setup except the air filters are K&N knock offs and it has some lowboy pipes. the bike had been sitting for some time and only fired on the 1&4 cyl after the carbs were cleaned it fires on all four the issue at hand now is that when the gas is twisted more than 1/4 throttle or so it holds the RPM and the only way to get it to come back down is by un-screwing the Idle adjuster screw (the big brass one)...once it comes down I have to screw it back in to keep it idling...its only a turn or two on the screw...from what I have read it leads me to believe its a lean condition...it also misses pretty good when its cold and pops a bit when throttle is twisted but only higher RPM...I havent had a chance to test it under load due to weather but if I cant get the Idle to come down i dont want to ride it around looking and sounding like a clown...just want to make sure im looking in the right spot. the electrical has not been upgraded but I have it all ready to go just want to devote my attention to the carb for now. as far as compression goes...easy to start and no smoke. please let me know...ill post a pic and youll see why I want to ride it.

mrbreeze · « **Reply #23 on:** March 11, 2009, 08:50:37 PM »

That sounds like an ignition problem that should be sorted before taking on the carbs.1&4 on one coil....2&3 on the other.Check your points and timing.

cherry bomb · « **Reply #24 on:** March 11, 2009, 09:39:27 PM »

yeah at first they werent firing...when the carbs were taken off originally the 2 was almost entirely blocked and 3 was totally blocked...so thats why they werent working...when the passage ways were cleaned they fired off...ill double check the timing on it but its firing on all four right now...

News:

Author Topic: cb750 carb problem (Read 17552 times)

efyou

upperlake04

efyou

efyou

efyou

billybobobrain

billybobobrain

efyou

upperlake04

cherry bomb

cherry bomb