Carb issue! 750 TWIN

Ok, I got some great answers from folks in the electrical posts. I have isolated the problem with my bike to the carbs and need some help.

The bike will run up to redline in idle with the choke all the way out. In fact, it will run great on the road (except for a poor idle) with the choke all the way out. It will go all the way up to redline with no problems!! The problem occurs when I push the choke all the way in and try to run the bike. With no choke, in gear, it won't go above 2500-300 RPM and it will stall after a few hundred feet.

So, it has to be the carbs and the choke. Any ideas?

Thanks

Cincinnati, OH

The choke on your bike is an enrichment circuit and adds gas to the "normal" air/fuel mixture for any given range of operation. Since you need the choke on to run normally, it would seem that you have either an air leak where the air/fuel mixture is oriented to the lean OR you have modified the stock airbox or stock jetting or both (or some distant owner from the past) and the carburetors need to be rejetted. Provide some info on your current airbox and jetting and I can help you clear up the latter possibility quickly.

George,



FYI... It is running the stock airbox. Would a loss of vacuum BEFORE the carbs make a difference?

Also, I have no history on the past jetting of this bike. I know the kid who owned it before me told me that a guy worked on the carbs for him.

Will checking the seals before the carbs make a difference with the running of this bike?

Thanks!

We nned to know which 750 twin as the 76-80 use BS38 carbs and the 82-83 use BS34.

Stock jets one the BS38's are 125.

Assuming it's a 76-80 750 twin, remove that carbs and set the float level at 25mm (1" will work) from the bottom of the float to the base gasket when holding it upside down.

The above method is not in the manual, but I have done it that way on 3 sets of carbs, and on each set the fuel level has been right in the middle of the specified range when checked with a hose attached to the bottom of the bowl.

Use starting fluid to check for air leaks at the manifolds (between carbs and head) and the hose that goes between the two manifolds while the engine is running.

I just checked your thread in the electrical section and it states that you have an 82 KZ750M1. I don't know if my method of setting the fuel level will work on the BS34 carbs that your bike had stock or not.

Ron Jones (another member here) has had extensive experience with the carbs on his 82 750 twin, so maybe he will chime in.

Post edited by: steell, at: 2007/11/10 12:09

I prefer to check both the BS38 and BS34 wet as that is the most accurate and reliable method but if some other method works for you, that is fine with me.

I would also avoid using starting fluid for checking leaks... I like WD40 as it doesn't go up in a ball of fire if there is a spark or backfire. The WD40 will either kill the engine if there is a leak or cause white smoke to come out the exhaust. It eventually evaporates and won't leave a perma-puddle... only spray it on the carb holders.

Getting back to the original issue, if you have the OEM airbox and OEM jets and the carbs cause the bike to die, you either have an obstruction internally or an air leak... sounds like an extreme air leak condition. Besides the carb holders, check all vacuum caps/hoses. Last, make sure the slides don't have holes in them.

I will double check for air leaks!

Really wierd. I took it out last night. It started up fine and idled at 1000RPM. I was able to ride it around the street for about 10 mins and it was doing fine. All of this with the choke pushed all the way in( no choke).

Then, after about 5 mins, it started giving me problems again. Coming out of first, around 2000RPM it would just stop climbing, start to bog, then I would have to pull in the clutch, rev to 4000RPM and release the clutch to keep from stalling. Then, I will get to a stop sign and the darn thing will die on me. Can't figure it out!

I will check for air leaks and will update if I see any changed in performance.

Also, would altering the screw on the front of the carbs possible help this problem? Something I might try next!

Thanks

Are all valve clearances set to specs?

Take the petcock off the tank and check the filter screens to make sure they are not plugged/restricted.