Topic: Air Box back on - 3/4 to WOT problems

[ken74-550K0]

I just got rid of my pods on my 1977 750F and replaced them with a stock air box with a new K&N filter.  The bike now performs better off the line and in mid range, but in 4th and 5th gear accelerating under load (up hill) the bike sounds like it is breaking up and does not want to climb past 6K.  When the bike is cool I don't have this problem.  Also with the pods on the bike I did not have this problem.

Here are the details on this bike:

Dyna S ignition (timing adjusted) last August (08)
Plugs NGK D7 (non resistor) replaced last August (08)
Accell coils and 8MM suppression wires
Valves adjusted and carbs balanced August (08)
Kerker 4 into 1 exhaust
Carbs are stamped PD 41B
I have put about 800 miles on the bike since it's last tune up
Bike has 22K miles on it

I checked the plugs and they are a nice tan color.  The bike starts easily and idles smoothly at around 1200 RPM.

The only other thing that I notice with this bike is, once it's warm at low RPMs under load it pings.  I tried running premium fuel, it pings a little less, but it still pings.

I bought this bike about a year and a half ago and I have no other knowledge of its history.

I have never been in the carbs on this bike, but I have verified that the accelerator pump works.

On one hand, I think the problem is likely the main jets may have been re sized to accommodate the pods and now with the airbox on the bike the mix is too rich at 3/4 to WOT.  On the other hand, it may be an ignition related problem?

Given everything that has been done to this bike; the question is where is the best place to start diagnosing this problem?

[Gordon]

On one hand, I think the problem is likely the main jets may have been re sized to accommodate the pods and now with the airbox on the bike the mix is too rich at 3/4 to WOT.  On the other hand, it may be an ignition related problem?

If the bike was running well with the pods then it was almost definitely rejetted for them.  If that's the case you will need to go back to stock jetting if you want it to run correctly with the stock air filtration.  Although the after market exhaust may require a change from stock, too. 

[Inigo Montoya]

You are most likely running rich now. The stock airbox even with a k&n will still restrict some. Also, I have found that with the later bikes, a very light dusting on the plugs is actually ideal but your results may vary as you have the accel coils but I would still tune the mains down a size. If you drop the bowl, the main jet should have the size stamped into the end. I think stock on the 77 was a 110 but I do not remember for sure. It was no bigger that 115 though. Another thing to try would be to drop the needle down or raise the clip as some would say. This will restrict some of the fuel intake a little.
I also have the dyna S and I get a slight ping too. I happens at idle and it seems that the dyna is somewhat advanced in its timing at idle.

[ken74-550K0]

Well I finally got around to pulling the cabs off the bike and checking the size of the main jets.  The earlier replies to this post were indeed correct - the carbs were fitted with 135 main jets.  The stock jets for this bike should be 105.

I clearly need to drop the jet size down to something closer to stock, so I just ordered 3 sets of jets, 110s, 115s, and 120s.  I did not order a set of 105s because I figured the Kerker exhaust on this bike should require slightly larger jets.

Does anyone have any suggestions on which set I should try first?  My instincts tell me to go with the 115s only because it's roughly the mid point between 105 stock configuration and the 135s that are in there now.

Thanks

[1timduke]

I'd start from the largest and work down, then go with the best result, if it stumbles at other positions, you may need to adjust the needle position (or resize the needle to match the jet, depending on what's in there.).   There's a chart around here somewhere that shows the correlation between all the jets and throttle positions that may help.   Might also help to mark throttle positions with some tape and a marker and then do some plug chops.   Prolly take a good day to get it all tuned, but it should ride really smooth when you're done.

-Tim