Cant get the old boy firing

[Sampo]

Hi all,

New to this site.

ill brake it down to you all.

Bought a Cb750 f Dohc 1982 modell september last year. The bike run perfect.
I took a part the whole bike and chopepd and sanded the frame, did all the "cosmeticts" changed bars to clip on, new paint to the rims, new speedometer, battery, ingnition, headlight, taillight, signallights and a bunch of more nice parts and details.

I but the engine back to the frame after i cleaned it and gave it new colour.

The bike is 95% ready and i tryed to fire it upp for 3 weeks ago fpr the first time.
It started after 10 second but run only on cylinder 1 and 2.

1. I checked the wiring from battery to coils and from coils to parkplugs.
I get a good decent charging. and the spark are good on all for plugs.

2. I have cleaned the carbs 7 times. Checked and rechecked all jets. floatbowls.

After have been pulling of the carb several times and putting them back the biek wont start at all.
spark 1 and 2 are wet. 3 and 4 are bone dry.

Could it be the fuel petcock on top of the carb 3 and 4? A dissembled it and put i back to gether but i am not sure if i put the spring back the right way. If it should sit on top or on the bottom??


I shall do a preassure test this week for all of the 4 cylinders and check also the timing
But it gets air and spark, fuel only to 1 and 2 cylinder. Can it be the timing or preassure in this case?

Could a carb swap solve this problem? I can order a set of Mikuni Gsxr 36mm Flatslide carbs that are in good condition for 120$ from a friends friend of mine.


I have the Keihin CV carbs with out the Stock airboc that the carbs seem to love alot.
I have rejetted the many times and now i have 35 slow, 115, 75

Tried also with the stock airbox but no luck.

[seestheday]

Welcome to the forum

If it's firing on 1 & 2, but not 3 & 4 I'd say it couldn't be the petcock.  The 1982 carbs just have the one fuel pipe on it right (I have a 1981)?  I recall some of the SOHC bikes had a petcock that split into two hoses so fuel entered the carbs from 2 locations.

My vote would be that it is a compression issue.  If you have zero compression in 3&4 then fuel can't be sucked in.  The compression test will tell you.

Carb swap will not fix this, unless your old carbs are in bad need of cleaning, but since you've cleaned them 7 times this shouldn't be the case.

Just to be sure, can you elaborate on how you cleaned them?  A simple soak/ultrasonic often isn't enough for these carbs.

Probably unrelated to your starting issue, but those are some big jets, so I'm assuming you are running fairly open pipes and pods or velocity stacks.  These carbs are notorious for being difficult with anything other than the stock airbox.  I would focus on getting the bike running well with a fairly stock set-up, then start messing with pods etc.

Another aside - if you're going to run the GSXR carbs, how are you planning on making them line up well?  I'm interested in this mod myself, but can't seem to find a good manifold solution.

Edit: I just did a quick search and I believe that the 36's are going to be way too big.  The appropriate Mikuni's are the 29's.  That said, don't take my word for it.

[Sampo]

Hi,

Forgot that. It a 4-1 mac with a canister (mechanical baffle)
I took of the carb for cleaning and have disembled them and used compressed air, soaked the jets and carb parts. I havent pulled the apart totally. Perhaps I should follow the MacGregors guide and go thrue them totally.
I shall do the comp. test tomorrow for each cylinder. There is a threat on cb1100f.net how people have done the manyfolds of radiator hose with copper or aluminium spacers. I was thinking of trying out those.

[TOOLS1]

From what you describe, it is a carburetor problem. Find someone with a sonic cleaner, and have them clean the carbs for you. Those CV carbs are notorious for being hard to clean. They have so many small passages, and complicated workings that it does not take much to mess them up.
TOOLS

[Sampo]

Wich of the carbs would be a nice fit for my bike?

http://www.dynoman.net/carb/mikuniRS2.html

Keihin RS seems to cost $$$$ way out of budjet

[Sampo]

Or other carb options?

Could these work?

86-88 GSXR1100, 34mm Mikuni.
86-87 GSXR750, 34mm mikuni

Have googled around and cant just figure out with carbs would be the best alternative.

[seestheday]

Honestly, I'm not the right guy to ask about this.  I think you'd have better luck asking on the cb750c or 1100f site.

For performance technical advice on the DOHC's the 1100f site can't be beat, you just have to have a thick skin in case someone rips your head off.

This forum is much friendlier .

Found this thread on the c site: http://cb750c.com/modules.php?name=Forums&file=viewtopic&t=19020

Looks like the 88 GSXR 750 carbs work with some tuning.  Might require drilling air bleeds to get rid of rich idling problems, so.... not for the faint of heart.