ok, carb wizards....

ugh!

I guess I'm missing something. I cannot get the carbs on my 750 to lean out. :x When I twist the throttle under load, the motor sputters a bit. If I keep the revs high, it's hardly noticeable. At high RPM, the bikes has all kinds of power. Plugs turn sooty-black around the perimeter after 20-30 miles of riding.

History:
Carbs have been cleaned. Thoroughly.
PO installed the dynojet kit, because the bike has Mac 4-1 exhaust.
The slide holes have been drilled out, per DJ kit instructions.

I went back to factory jets, factory needles.
The slides "clunk" very nicely. No wobble, no hesitation.
I adjusted the idle jets to the best of my abilities. The bike idles like a sewing machine, 1100 RPM. I have to use about 1/2 of choke to start and warm up. Leaning out the idle screws doesn't affect the sputtering all that much.
Spark is great, plug gap is to spec.

Could the size of the slide holes make a difference? What is the relation of the slide hole diameter to how slow/fast the slides lift up?

Also, I have a feeling that this particular Mac 4-1 is a tad more restrictive than what the PO thought.

Any ideas on what to try next?

 

I think I partially answered my own question.

Here's what I did:

I used a length of metal wire to make four c-shaped "clips" with the ends extended and pointing down. The two ends very loosely fit in the lift holes from the top of the slide, and the clip itself is held down by the plastic screw that keeps the needle and spring in place. This way the lift holes are partially blocked (maybe 25-30%).

Throttle response off the line is much more noticeable. There's still an occasional hesitation, but at least I'm heading in the right direction (I think).

Tomorrow the weather is supposed to be very nice (in the 60's). Guess who's going riding? :mrgreen:

 

Two other things that could cause a rich mixture are the fuel level in the float bowls and open choke plungers. Float level can be checked with carbs on the bike but is easier to set with carbs off. Choke plungers are checked by pushing the choke lever all the way in and then pushing down on the plungers to see if they close more.
Actually stock jets on a 4 to 1 header should make for a lean mixture which would be white plugs. The holes in the diaphraghm pistons can be filled with JBWeld.

 

First of all you need to find out what speeds and throttle openings are too rich and which are too lean or just right. Any time you make changes to the exhaust or intake it will change the mixture but it may be too rich at some throttle openings and RPM's and too lean at others. If you replace the exhaust and the filter at the same time with less restrictive units they will tend to cancel out at some engine speeds but not at others. I put on a 4-1 exhaust and pod filters and it was too lean at slow speeds and full throttle but too rich in the mid range so I had to go up a size on the pilot and main jets but lower the tapered needles a notch. The jets have to be determined by trial and error so I would stay away from jet kits because they are not likely to have the exact size jet you need unless you are just extremely lucky and they don't usually include pilot jets anyway. On the other hand the main jet only works at full throttle so you won't notice any difference one way or the other unless you drag race a lot. If it's too rich either the jets are too big, the float level is too high or the needle valve is leaking or the tapered needle is set too high because someone may have put washers under it or something.
You can find out if it's too rich at slow speeds just off idle with a Colortune but for faster speeds in the mid range and at full throttle you need either an exhaust gas analyzer or you will have to read the plugs a lot which isn't fun. If it pops at slow speeds it's too lean but if it fouls the plugs it's too rich so adjust the pilot jet a size at a time up or down accordingly. If the main jet is too small it won't develop full power at full throttle and may miss, burn the plugs or act up some other way but the only way to tell for sure is on the freeway at high speeds or on a dynamometer. In most cases you can go up one or 2 sizes and it won't hurt anything unless you drive at full throttle a lot then it will use a lot of gas and foul the plugs. If it's too rich at cruising speeds with half throttle you will have to adjust the tapered needle. That's where drilling out the holes in the vacuum slide comes in. Changing the hole size will make the throttle respond faster or slower but it might also cause the slide to ride at a different lever and if the tapered needle ends up at a different level because of it your mixture will be off. In any case you have to adjust the needle's level to get the correct mixture and it's not possible to adjust it very far so I would try to get the holes as close to the stock size as possible. Maybe you could plug the holes with something like JB Weld then drill them out to the correct size again? That process works for adjusting the jets, if I have to go smaller I solder the holes up then drill them out the the smaller size that I need with a tiny drill the correct diameter. However you do it you are in for a long period of trial and error changing jets and adjusting the needles then riding it to check the results unless you can use a dyno that has an exhaust gas analyzer to give you a readout of just how much too rich or too lean it is at different RPM's so you will know what to adjust.

 

cool! I'll epoxy the holes up and re-drill them tomorrow. Any idea what the stock diameter might be?

 

On the Hitachi carbs, the air holes in the base of the diaphram piston are actually two different sizes, one hole is 1.70mm (.0670") in diameter, the other is slightly larger, 1.75mm (.0690") in diameter. At first I thought that the hole size difference may be just have been due to production variances, but I measure 8 of them, from different sets of carbs, and they all come out like that, so I suppose it's intentional, for what reason(s) I don't know..........

 

thanks Chacal! That's great info.

1.75mm and #51 bits ought to be good then. Time to put that bazillion-bit set from Harbor Freight to good use! :-D

 

Sweet! The hesitation is gone. Thank for the help, guys. Once the bike cools down, I'll take a look at the plugs to make sure I'm not on the lean side.

I ended up using a #50 and a #51, cause I couldn't find a 1.75mm bit.