Just bought "78 K 750 Runs ONLY with Choke full on

[Pabeye]

Hi all...NEW member here....today I brought home, bought from the original owner a 1978 750K. Cosmetically the bike is almost like new. BUT right now it is running like CRAP.

The only way it starts and runs is with the choke full on. Even when warmed up, if I take off the choke it dies. One additional note, IF i keep the engine turning about 4K RPM, it will run. Mind you with the choke on it runs pretty well. PS: Bike is completly STOCK

Any suggestions as to cause and fix will be greatly appreciated.........best to all

 

[dirtdigger]

idle circuit in the carbs are plugged.

 

[Pabeye]

idle circuit in the carbs are plugged.

Damn, DirtD, that was a quick response! Thanks............Guess I will have to pull the carbs and investigate that!

Any hints on doing said problem would be appreciated..........perhaps a total 4 carb rebuild would be in order?

Best parts source for said rebuild would be appreciated!

 

[dirtdigger]

could be as simple as the pilot jets plug or could be the passages in the carb body themselves. go to http://www.cb750supply.com/products/6/fuel-system look about half way down, need 4 kits. These guys support this sight and do a monthly giveaway. You could try just taking the bowls off and see how they are first but if its been sitting then they will probably be pretty bad inside and the bowl gasket will junk. If they dont look too bad it may just be the jets but you will need compressed air and some good carb cleaner. If you have never had a carb apart then do just one at a time so you dont mix parts up and have something to compare to if you get lost. Get a good Honda manual so you have it for future repairs and for tuneup information. These bikes require periodic maintenance and adjustments they are not ride and forget bikes you gotta take care of them or they will not run good.

 

[amc49]

Modern fuel containing ethanol only makes that much worse.................

STOCK, the most beat up word on the planet, I see non-stock exhaust (can affect motor running) as well as non-stock seat. And who on earth slicks up bike tires with tire black or coating, the quickest way to throw somebody off a bike there is. Dangerous.

 

[Pabeye]

Modern fuel containing ethanol only makes that much worse.................

STOCK, the most beat up word on the planet, I see non-stock exhaust (can affect motor running) as well as non-stock seat. And who on earth slicks up bike tires with tire black or coating, the quickest way to throw somebody off a bike there is. Dangerous.

I hear ya.....guess I should have said "almost stock"............It does have a VERY 70's king/ queen seat ( which MUST GO ASAP) and high bars with I guess a 6" or so rise ( These REALLY must go ASAP) and the exhaust is a two into one exiting with a baloney cut to the side.

****Has anyone ever used the carb cleaner that domes in a gallon can? I have a friend who uses it and he lent me a can of it. He says if you take the plastic off the carb and then soak it in this stuff over night and ten blow out the orifices with air it might do the trick. Opinions?

 

[dirtdigger]

I use the gallon of carb clean all the time but you have to take each carb off the rack which means dealing with all the linkage assemblies etc. If you have never done it before dont know if I recommend it. Any rubber bits will be ruined. I would start with just some spray carb clean and compressed air if it looks good inside the bowls. The jets are the thing that plugs the easiest. I have fixed a lot of bikes with no idle and many times just the plugged jet is the issue, doesn't mean a passage is plugged but if cleaning the jet fixes it no reason to go deeper. If its bad inside then you will have to go with complete disassembly.

 

[madmtnmotors]

I use the gallon of carb clean all the time but you have to take each carb off the rack which means dealing with all the linkage assemblies etc. If you have never done it before dont know if I recommend it. Any rubber bits will be ruined. I would start with just some spray carb clean and compressed air if it looks good inside the bowls. The jets are the thing that plugs the easiest. I have fixed a lot of bikes with no idle and many times just the plugged jet is the issue, doesn't mean a passage is plugged but if cleaning the jet fixes it no reason to go deeper. If its bad inside then you will have to go with complete disassembly.

+1

The later carbs are a bit different than the earlier carbs. Complete disassembly of the rack can leave you with more problems than you started with if you are not familiar with them. You can remove the bowls, remove the jets and clean/adjust everything thoroughly. About all you should need are bowl gasket (molded o-rings) and new float valve needles. Original Honda parts are highly recommended here, but they are pricey. If you must use aftermarket, make absolutely sure these rubber bits are made of Viton rubber. The float valve needles in these later carbs are rubber tipped, so theoretically there is no wear on the seat. Good thing too, since the float needle seat is not designed to be replaced, nor have I ever seen a source for them, other than extracting them from another set of carbs (if that's even possible).

I tried a set of non-Viton float needles and bowl gaskets once. Within one week of being exposed to 10% ethanol fuel they turned to goo and started leaking profusely,

 

[Pabeye]

WOW, you guys are AMAZIN! Thanks.....I'm about to take off the tank and the remove the tank and DIG IN!

 

[madmtnmotors]

You might not want to remove the slides unless absolutely necessary. Removal of the emulsion tube from the underside should provide enough access for a thorough cleaning.

 

[amc49]

FYI, in any carb the passages that gunk up the easiest are always those at or below the fuel level line. Ethanol adds to that by drying up so much faster than straight old school fuel ever did. Also because it draws water like a magnet and residual sugar in it can stick parts together too. Carbs being vented allow that to happen; modern EFI has no vent other than for tank and even lots of those shut off now.

Your typical cheap Chinese repro parts are commonly the ones that melt with ethanol. If bike not a stunner, I use old fuel bowel o-rings as a groove stuffer and commonly cut a paper gasket to seal bowel placing it over the old o-ring. Parts for pennies and work perfectly. I bypass $20+ aircuts in later carbs using simple hardware store o-rings at $.79 each. Buna type and they swell up but what I want them to do in that case.

A little more work but retired and farting around anyway. On a 550 with almost same exact type carb I tore every single piece down including all linkage to then get carbs back up and working perfectly for less than $10 in parts plus the jets. Modded, I threw a header and pods on it. Runs great.

 

[dirtdigger]

The parts from cb750supoly seem to be holding up well from the couple I have used. I dont use ethanol fuel unless absolutely necessary so all my use has been with regular fuel. But wont know how bad they are till the bowls come off then go from there.

 

[Pabeye]

While I'm at it........lets talk jetting........

I plan on replacing the stock air box with pods.........may change the exhaust too........input on jetting appreciated.

 

[dirtdigger]

Subject has been beat to death with a stick but I'll go at it again lol No one can tell you what you need for jetting...too many things affect what you need....engine size, engine condition, tune up condition, intake setup, exhaust setup, fuel type and quality, altitude, temperature, how you ride, etc It all determines what you need, no two bikes are the same and each needs what it needs there are no set numbers. You need to ride it, see how it runs, look at the plugs then adjust and try again. Make sure engine is good and tune up is fresh before you start though.

 

[Pabeye]

Thanks DD........I know it is a tough question........not even sure what's in there now.....I guess I'll find out when I pull the carbs apart

 

[madmtnmotors]

If you switch to pods now (which suck by the way IMHO) and if the bike still doesn't run right, how will you know what the problem is? I recommend that you get the bike running right first before proceeding to screw it up making modifications.

 

[Pabeye]

If you switch to pods now (which suck by the way IMHO) and if the bike still doesn't run right, how will you know what the problem is? I recommend that you get the bike running right first before proceeding to screw it up making modifications.

That's good advice, which I will follow.......as for the pods "sucking".........well I guess they do, the IS WHERE THE AIR COMES IN RIGHT!......, I would have to disagree......Us 'em on an XS 650 and a Kawi KZ 650 never had a problem. ANd they were not the fancy schmancy K&N pods either, just the cheapo ones.

 

[amc49]

Pods are wonkier on stock exhaust than on a header, the header increases throughput quite a bit. If there's one thing I learned after years in Dad's hot rod garage, you pretty much always mod the exhaust before you mod the intake as exhaust will limit any intake improvements. I've never seen that proven wrong in that direction but I have the other way. Just as an idea I always bump up one size or maybe two with pods and header and look for a possible needle clip change but not always needed. The 550 I did one jet size bigger and nothing else using UNI podfilters custom cut to length, 3" instead of 4''. Bike runs to 9000 rpm all day long even though getting tired.

I'll echo one of the last sentiments to say get stock engine running right before you pod, could make a big fiasco there otherwise.

 

[Pabeye]

I'll echo one of the last sentiments to say get stock engine running right before you pod, could make a big fiasco there otherwise.

Point well taken

 

[Pabeye]

Just ordered this kit ( four of em actually ) from CB750.com......hope it is the right thing.

Honda CB750 Carb Packing Kit (Float Valve) OEM Ref. #16010-405-004
Fits: CB750K (1977-78)
CB750F (1977-78)
Part #: CB-70109

Will worry about jets after I take the carbs apart....