bad throttle response at low rpms

Frank my 80 750 twin just had his carbs rebuilt and seems to run okay for a bit, but after a minute or two of being warm, he starts to really get difficult with the throttle. It becomes increasingly difficult to rev out of idle without either nursing it up or stalling. It has also been backfiring at low rpm's.Is it the float level? I checked the levels using the tube method and they both match up. I also hand checked the noise and pressure on the exhaust which seemed even. Could it be a simple adjustment I overlooked? Or am I tearing into the carbs again? little help here friends would be much appreciated.

Without actually hearing it, it sounds like the carbs are set to lean on the bottem end, richen them up and see what happens

May sound stupid, i dont care.. how do you do that?:S

toof wrote:

May sound stupid, i dont care.. how do you do that?:S

By throwing money at it! :silly:

Seriously, you can start with the mixture screws. Depending on what type you have, you will either open them up or close them down a bit. I'm not familiar with the 750 twin, so I don't know what kind of carbs you have. Some carbs had fuel mixture screws, which you would turn out to enrichen the pilot circuit. Others have air screws which you would turn in to enrichen it.

Next, you can look at changing the pilot jets to a bigger size.

I'd also look for a vacuum leak.....

I know one of the rubber diaphragms looks a lil chewed in one spot. Seemed like it was usable. Is there a way to replace JUST the diaphragm or do I need to disassemble the entire needle "thing"? Also my bike runs bs38's and I have yet to locate anything the seems like an adjuster screw otherr than the balance adjuster

If the diaphram looks damaged, that is probably the most likely culprit. Without a perfect seal at the diaphram, the bike will not run properly. Here is a source in the U.K. that sells replacement diagphrams. Even with the international shipping cost, they're probably still the cheapest source.

www.nrp-carbs.com/

The mixture screws on the BS38 are on the side as I recall. You really should get a shop manual or Clymer manual if you are going to work on the carburetors or anything else on the bike especially if you aren't an experienced hand.

I have a service manual from the dealer but it is xerox'd so the pics suck!
I cant seem to find any specs on my carbs for whatever reason. I have actually been having good luck with the manual so far. The bike is running better now than after I got it back from the only vintage mechanic in town! :S So if anyone can help me locate this magic mixtre adjusting screw, I will be forever greatful! thanks again! toof

After rereading your first post, you stated the carbs were just rebuilt, if you paid somebody take it back, they messed up. After it warms up then the problem starts. When engines warm up they expand slightly, causing an air leak,,,something is not tight enough.
Here is a good site for microfich so you can see an exploded view of the carbs www.cheapcycleparts.com

The idle mixture screw may be capped to prevent messing with. You may need pop off the cap to adjust. Better get a service manual.

BS38 1980 KZ750B4 and 1980 KZ750G1 models
Main jet, 125
Needle Jet Badge Y3,
Needle Jet 4HL14
Pilot Jet 40
starter jet 45
Service Fuel and Design Fuel Level the same,
pilot screw -blank
main air jet 1.0

There is no spec for the pilot mixture screw as it was set at the factory and not supposed to be mucked with. Try it at 1 3/4 turns out from lightly seated to begin with. Turning the screws out richens the mixture a bit. If you don't see improvement, go to a #42.5 BS30/96 style set of pilot jets. These carbs can also leak in several places... around mixture screws, at carb holders, where the throttle rods pivot, etc. so all potential leak areas need to be checked for air leaks but your symptoms are more consistent with lean idle mixture.