Who knows about 1980 honda CB750 Carbs

[Jason's CJ5]

I am having trouble getting this bike I picked up a few weeks ago. The bike was sitting for the past few years, so I took apart the carbs and cleaned the slides, jets and needles. I still only will fire on cylinder 1 and 3. They are on different coil packs so the coils are working. If I spray carb cleaner into the dead carbs the engine will take off so there is spark in the cylinders. If you have any good ideas please post them, I am at my wits end. I guess the next step is to buy 5 gallons of carb dip and soak the whole assembly.:shrug:

Thanks

[DenverDude2002]

I know from exp the CB 750 carbs are a ****** pain in the ***. Your best bet is to get a Clymers and Haynes manual. Id check to make sure fuel is getting to the carbs. Maybe there is something plugged on both of them as well, those carbs have lots of little holes in them and all need to be cleared. good luck. You may also wanna invest in a carb sync tool as well

[satan]

If it sat long enough to gel the fuel... you're likely chasing a blocked passage or some float issues...

with #1 & #3 running you're kinda hosed in that you can't swap a running carb into a "bad" cylinder ...

I'm not sure if you're working with the 2 or 3 jet carbs on that (though I'd suppose the 3-jet with it being and 80 and all) ...

I'm reading that you've cleaned the "slides, jets, & needles", but what of the rest of the carb? and does this include the needle/seat for the float?

I'm with DenverDude2002, in that fuel supply (sticky pins or just plain crappy floats / settings) are common - THE CB750s are known for not wanting to "push" fuel down their lines anyways, so open the bowl drains on the "suspect" carbs to see if you're at least wet! & let em drain for a bit to get fresh/clean past the needle and seat, they stick closed OFTEN!) I'm in the habit of putting a half-pint of Seafoam into the tank (with about a gallon of fuel) when I start chasing these kinds'a things on the bike... sometimes having the cleaner in the fuel will help to soak-away or chase your demons (at the very least it'll help to lube things that are moving now for the first time in a while, and it will make the fuel more aggressive at cleaning off anything you may have missed)

Also, the "carb cleaner sprays" can be handy to shoot through the passages of the carb... every hole leads to somewhere! Blow em all! (the little tubes that come with carb cleaner are handy, but don't "seal well" to some of the odd shapes - I find that about 3/8" length of 1/16" Silicone fuel line, for model airplanes and such - can be had at most hobby shops, can help to seal your carb spray straw to some of the surfaces, so you can really blow the holes) -- Eye protection is good here! high risk of splatter or spray-back

[Jason's CJ5]

Thanks for the info, When I cleaned the carbs the first time I took off the bowls, removed the float, and needle. Removed two jets from each carb and soked them for a few days. I also took the top of the carbs off that house the springs and the slide assemblies. I also checked to see if there was gas in each bowl after reassembly by turing the little drain screw, and there was.

I am going to make a solution of gas and seafoam and soak the whole assembly for a few days to try and dislodge any remaining crap blocking the internal packages.:shrug:

[satan]

Sounds like a good approach -- blowing through the passages with air or cleaner is highly recommended... they just seem to collect goo and contamination.

Have you looked at the throttles? Like are #2 and #4 opening and can oyou get em to open more (yeah- you'll likely get a little imbalance but, generally if you can get em to flow and start drawing - they'll be better after running some fuel (or fuwl w/ solvent) through.

Synchronizing em is pretty easy, but they'll all need to be running first!

another thought:

are you 100% sure it's afuel issue and not a bad half-a-coil (like did you swap plugs and plug wires coil for coil -- just to be sure? Also, have you verified your firing order and compression?

[DenverDude2002]

If it sat for over 5+ months remember unless the carbs we're drained that old gas has turned into a nasty heavy varnish. worse comes to worse check ebay for rebuilt carbs or take them to a dealership and have them done, its a job id never tackle again. The sync will be impossible without special tools just so you know ahead of time.

[Jason's CJ5]

Sounds like a good approach -- blowing through the passages with air or cleaner is highly recommended... they just seem to collect goo and contamination.

Have you looked at the throttles? Like are #2 and #4 opening and can oyou get em to open more (yeah- you'll likely get a little imbalance but, generally if you can get em to flow and start drawing - they'll be better after running some fuel (or fuwl w/ solvent) through. Not yet, will check out

Synchronizing em is pretty easy, but they'll all need to be running first!

another thought:

are you 100% sure it's afuel issue and not a bad half-a-coil (like did you swap plugs and plug wires coil for coil -- just to be sure? I put new plugs in and sprayed the carb cleaner through the dead carbs. Also, have you verified your firing order and compression? No, I just purchased a book of e-bay
:shrug:

[Jason's CJ5]

If it sat for over 5+ months remember unless the carbs we're drained that old gas has turned into a nasty heavy varnish. worse comes to worse check ebay for rebuilt carbs or take them to a dealership and have them done, its a job id never tackle again. The sync will be impossible without special tools just so you know ahead of time.

It sat for like 2 years, I am getting a book and attempt to tackle this myself. If worst comes to worst I will take it to the dealership.

[Guammy]

Does it have the mixture screws at the motor side of the carbs plugged off?
They would be aluminum plugs that are there for EPA. They NEED to be pulled and cleaned.

Its covering number 23 on this page

http://www3.telus.net/dougsimpson/CVcarb.html

[Jason's CJ5]

I found a set of rebuilt carbs on E-bay for $209.

I contacted the seller and he will rebuild my carbs with new o-rings, gaskets and sync them for $65 plus shipping. So then are going off to him at the end of the week.

[briani]

not sure if your on points on that bike i have a 73 and that was exactly what happened to me last week i went to fire it up and i wasnt getting fire to 2 cyl i checked the points and i had no gap gaped it cleaned it and ran like a raped ape those cb's are some smoking bikes ive got the 836 kit with the 4 into 1 header with the megaphone exhaust if you have any questions let me know ill help if i can