Topic: wierd idle problems, really bad mileage too... need help!

[dakeddie]

I just finished putting my rebuilt 836cc in my bike and I was really hoping it would make this horrible mileage problem i was having disappear.  When I first got the bike, I was getting >70mi per tank in the city.  It gradually got worse and now I only get ~40mi per tank.  Btw, my bike has never run on reserve, so by "tank" I mean 4.5 - 1.3 = 3.2 gal.  The other problem I was having before and still after the rebuild is that when I pull up to a stop, the bike often stalls, or sometimes will idle very high, like 2000 - 3000rpm for around 30sec.

My first instinct was that the float levels were set too high and that an over-rich mixture was occurring which would either stall out the motor, or cause fuel pooling that made the high idle situation.  So I checked the float levels with a modified drain plug and clear tube.  The results were that the fuel level was about 1/8" below the lip of the carb.  That seemed pretty high to me, so I adjusted the level to be 2mm lower.  Btw, before I did the adjustment, the float was set to 26mm as shown in the manual.  I took the bike out for a ride, and it's behaviour isn't really different.

I guess it's not the float level then... is it the mixture?  I'm currently running an open 4-1 with k&n pod filters and 125mains.. I never moved the needle positions.  Carbs are synced, and it runs pretty good... a little roughish at idle, but it was always like that.  The cam picks up at 4000rpm, runs awesome there... I can easily float the front wheel.

If a mileage as bad as this is due to mixture, wouldn't the bike be running horribly?  I haven't checked the plugs yet, I'll do that tomorrow if it's not raining.  And what about this weird idle problem, anyone ever had this?

[liaudio]

The idle problem might be that air is sneaking in somewhere.  possibly around the rubber intake manifolds.  When it's running, try spraying WD-40 around the rubber where it joins the head and carbs. 

[Gordon]

And what do the plugs look like?

[736cc]

I never heard of 70 mpg on a CB750; 40 is average.

[Jonesy]

He was saying 70 miles per tank- not per gallon.

[OldSchool_IsCool]

Could your choke linkage be broke?  Are you sure the choke valves are actually opening when you push the cable in/flip the leaver?

[scondon]

Hey 'keddie, glad that cam is workin' well

     13mpg is pretty darn bad, I would be shooting for something over 30. Your original float setting sounded right to me, but leave that alone for the moment. Could you fill us in on what coils, ignition, plugs/plug wires you are using and their condition(new,used,ancient?). Also, where your timing is set and how you set it?

      You can check for leaks as liaudio suggests, if there are vaccuum leaks then the idle will rise when boots are sprayed. If your carbs have not been cleaned/dipped thoroughly then they should be in order to rule out blocked air/fuel passages.

       The thing about tuning carbs is that you want to make sure that EVERYTHING else is working properly or you will be chasing your tail and, more than likely, getting further from where you want to be. That said, I was running '75 carbs on my bike(836,4-1 open,pods) with 125 main/40 pilot/ needle set in 4th clip(one clip below richest setting) and was getting 30+ mpg. The needles and needle jets were worn and I could never get the carbs running perfect so I pulled them from the bike while I consider rebuilding them.

[dakeddie]

Yeah, I'm pleased with the cam.

I put in a Dyna2000 with new 3 ohm coils and new 7mm leads.  The plugs I bought late last year and I cleaned them up a bit before putting them back in.  I cleaned out the carbs last year, put in new jets and float valves and seats at the same time.  I didn't dunk the carbs though, only sprayed them with carb cleaner.

I timed it statically as described in the Dyna guide to 30deg, but after looking at the guide again, I think I should change it to 40deg.

I will definitely check for intake leaks and the condition of my plugs, but only after I get home from work tonight.  I'm pretty sure the choke is getting out of the way, but I'll double check tonight.

[scondon]

Yeah, I'm pleased with the cam.

I put in a Dyna2000 with new 3 ohm coils and new 7mm leads.  The plugs I bought late last year and I cleaned them up a bit before putting them back in.  I cleaned out the carbs last year, put in new jets and float valves and seats at the same time.  I didn't dunk the carbs though, only sprayed them with carb cleaner.

I timed it statically as described in the Dyna guide to 30deg, but after looking at the guide again, I think I should change it to 40deg.

I will definitely check for intake leaks and the condition of my plugs, but only after I get home from work tonight.  I'm pretty sure the choke is getting out of the way, but I'll double check tonight.

  Sweet set-up     The 7mm wires your using are "suppresion" type wires recommended for Dyna 2000?  I haven't got mine up and running yet, but Mrieck emailed me the degree settings (which I can't find now ), I think it was 33. You would be a bit retarded? at 30, and advanced? at 40. I would set for 35 and see what happens.   Mike, what was that setting

   The air jets and fuel passages within the carb bodies can get quite gunked up over time. You should at least remove the main/ idle jets, spray carb cleaner through the air jets(located in the carb air intakes, remove a pod filter and you'll see them) and make sure that cleaner comes squirting out of empty jet holes. I mention this method 'cause it can be done with the carbs still on the bike and is a quick and dirty check which has helped me in the past.

[dakeddie]

I got home from work late and didn't have a chance to do too much before it got dark.  I checked the plugs and they are sooty black or shiny black depending on which plug you look at.  So it's rich, but I wasn't really running the bike, so it's rich at idle... maybe not rich at open throttle.  I've read that since our carbs don't have accel pumps the idle mixture is set rich to give it some jump off the line.  Is that true and would it make the plugs read black?  Or is it just a little rich so that the plugs still look fairly good?

I checked for intake leaks using WD40 and there was no change in rpm, so that's good.  Might an overly rich mixture cause my idle problems?

Tomorrow is an important hockey game and it's a 5pm game, so further work will have to wait.  But I plan on inspecting the needles for wear and then maybe rejet to 120mains.

Go Canucks!!!   

[scondon]

    I just re-read the Dyna 2000 instructions and they state that the older 2-valve engines run better with 40deg total timing. Maybe Mike told me 37 deg. Anyway, if your timing is set at 30deg(advance timing) then your fuel is probably not igniting soon enough to achieve full burn. You might still want to drop your mains a size, but I would get the timing set first.

    I hope to be firing mine up in the next three weeks and can have some actual experience to share then

[dakeddie]

I missed that paragraph in the Dyna guide when I read it, thaks for pointing it out...  I'll adjust it to 37deg and see what happens.