Topic: Exhaust 2 and 4 much cooler than 1 and 3?

[ChopSticks]

Any idea why 2 exhausts would not be as hot as the other two?

I uncovered my cb750F engine after a couple years of neglected, was running through the periodic tune up, popped in a new battery, added gas and she turned over like a champ (prior to this I was only able to have her turn over she's never been ridden yet)

After the gas ran out and I started packing up I noticed that the exhaust 4 was dead cold, 1/3 were still hot, and 2 was warm.

So the next day I figured to check the plugs (all showed spark) and do compression testing while I was at it. Plugs looked normal and there was residue on 4 so it did fire at sometime. Fired her up again, this time all 4 exhausts were hot (assuming the first time I may have just let her cool down too much?) but I decided to grab a towel and grip each one. 1/3 I could hold on for a few seconds until it was getting way too hot. But 2/4 I could hold the exhaust forever through the towel as it was just warm/hot

Any idea what would cause different temperatures in different exhausts? or is this normal

[Jimray23]

Probably not getting enough fuel to those cylinders

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[flybox1]

First, verify bowl fuel levels using the clear tube test.

[ChopSticks]

Ahhhhhh completely forgot about this step, the manual has the carbs at the very bottom of the tune up. Just curious any idea if the drain hole for the earlier 750 carbs are 3mm? the picture just looked a difference

I did remove the carb bowl for 4 and it did have gas at the bottom, the overflow was leaking (assuming its because the carbs been sitting so long dried) but after I got it running the leaks stopped

[ChopSticks]

I think Flybox is always on target with his advice. But in this instance, if your bike has sat for a couple of years, and you did nothing physically to them, it's unlikely the float heights have changed. What is far more likely is the idle circuit is clogged. Probably best to give them a decent cleaning and see what comes of that. If you take them off and apart, certainly then a Clear Tube check is very needed.

I actually never got a chance to test the float height via tube the last time I put the carbs back together so that'll definitely be the next items I tackle. If the floats are correctly positioned; what is the idle circuit? I just did a quick google search and it looks like it's the idle screw? Just don't want to confuse it with something electronic.

Complete Disclosure: the bike doesn't turn over unless the Choke is completely closed, the last time I brought this up someone mentioned possible leak. So I bought all new intake rubbers and airbox rubbers. I tested for any leaks yesterday by having the bike idle and blowing compressed air around it and there was no change in RPM. I haven't gotten to carb sync yet since I was going in numerical order of tune up items, but guess I can tackle that this week also

[ChopSticks]

Well, that's a different situation than the "bike sitting for awhile...".

A Clear Tube test would be highly appropriate now given some of your other symptoms (requiring full choke). Vacuum leaks won't show themselves as well with compressed air, you need something "combustible" like an aerosol.

The idle circuit is the pilot jet, emulsion tubes (if relevant for your Carb type) etc... Nothing electronic at all with this 

Will do, do the carbs have to be off the bike / leveled to do a clear tube test? or should they be done at the angle when on the center stand? if it's the former, would propping up the front tire or frame with a lift to get the carbs leveled work?

Also any recommendations for the combustible compressed air that won't eat away at the rubber boots?

[ChopSticks]

Now that I think about it, if my carbs aren't leveled when they are installed on the bike wouldn't that effect the float levels?

[ChopSticks]

Alright after getting the carbs leveled (put some wood under the front tire)

http://i71.photobucket.com/albums/i139/bssv/carbgas003.jpg

(not my image)

but the float heights for 1&2 were about 1-2MM under this

3&4 float heights were about 2-4MM.

since the exhaust for 2&4 are cooler think it's safe to say it's not the float heights?

The bike fired right up again with these heights I was able to set the timing with a strobe light and after about pint or so of gas the bike shut off. I tried to get the bike running again but I think the battery is just too weak to start it up (11.7) last time I checked the multi meter so I pulled the battery and have it on a tender for tomorrow.

Once the bike is running again I'll test the jet pilots, since 2 & 4 are still not as hot as 1 & 3. (floats will get adjusted later when I pull the carbs again)

this time around I was able to keep the bike idling without any sporadic revs and was able to rev the bike up over 2.5k RPMs with it coming back down to idle (note: choke was on 95% the whole time, one problem at time)

Any other things that would cause 2 & 4 to be cooler? other than pilot jet and float heights?


EDIT:

I also switched out to brand new plugs. (the 2 & 4 issue was occurring with the old plugs also, all 3 of them were gapped to spec 1 was a little wider. All of the new plugs are gapped to spec)

[ChopSticks]

is it possible to change the gapping on the plugs to make 1 & 3 cooler?

[ChopSticks]

Ugh... alright so I fixed 2 and 4, but now 1 is running cooler than 2,3,&4

Pulled the plugs, left side is 1 right is 4

1 also has a lot more carb vacuum than 4 almost twice as much. All floats heights have been set with clear tube, and both main and pilot have been cleaned. Throttle stop screen backed out one full turn also

[Gene]

Why are those running so rich? Check your spark and your plug caps, make sure you're getting fire and it's consistent.

[ChopSticks]

Why are those running so rich? Check your spark and your plug caps, make sure you're getting fire and it's consistent.

Which one is running rich? I got exhaust 1 to heat up again the carb needle was set to high but now 1&2 run hot and 3&4 run warm. All have the same vacuum now

[flybox1]

Did you get the fuel levels in the bowls corrected? (on the center stand is fine.  dont overthink it)
Vacuum sync done...good
Have you adjusted the IMS to get the richness out at idle?

[Gene]

Why are those running so rich? Check your spark and your plug caps, make sure you're getting fire and it's consistent.

Which one is running rich? I got exhaust 1 to heat up again the carb needle was set to high but now 1&2 run hot and 3&4 run warm. All have the same vacuum now

Both of those look rich/fouled to me. Better minds than mine may speak otherwise - reference this chart - plugs should look like those in the 3rd row.

I take it back - 4 looks prolly ok.

[ChopSticks]

Did you get the fuel levels in the bowls corrected? (on the center stand is fine.  dont overthink it)
Vacuum sync done...good
Have you adjusted the IMS to get the richness out at idle?

Yes I did the fuel levels via clear tube while on the center stand. I put a couple of blocks of wood under the front tire to get the carbs leveled.

What is IMS?

[flybox1]

Idle Mixture Screws
These allow you to fine tune the Air/Fuel mixture at idle.

[ChopSticks]

Why are those running so rich? Check your spark and your plug caps, make sure you're getting fire and it's consistent.

Which one is running rich? I got exhaust 1 to heat up again the carb needle was set to high but now 1&2 run hot and 3&4 run warm. All have the same vacuum now

Both of those look rich/fouled to me. Better minds than mine may speak otherwise - reference this chart - plugs should look like those in the 3rd row.

I take it back - 4 looks prolly ok.

Thanks, based on that chart Plug #1 looked like 5th picture (not wet) which would agree with #1 being too cool. After I adjusted the needle on #1 and readjusted the Idle Adjustment Screw on the rack I got it back to idle again. But this time 3&4 are running cooler. Any idea how 1&2 relate to 3&4?

To summarize:

Originally:
1&3 ran hot
2&4 ran cool

Fixed float levels and needles
2,3,&4 hot
1 cool

Fixed carb 1 needle
1&2 hot
3&4 cool

I just don't see any relation in regards to the adjustments made

[ChopSticks]

Idle Mixture Screws
These allow you to fine tune the Air/Fuel mixture at idle.

This is the stop throttle screw right? If so, I've backed it out one full turn from fully seated in each Carb. I was able to get the same vacuum when carb syncing without touching the stop throttle screws and just adjusting needle heights at 1500RPM

[flybox1]

Each carb has its own IMS.  small, brass, for your carbs, it will be between the slide and the airbox.  see below.

Use the IMS to adjust your idle mixture to get proper plug color.
Use new plugs.  Plugs stained with darker deposits will not read properly unless media blasted before re-use.

[ChopSticks]

Each carb has its own IMS.  small, brass, for your carbs, it will be between the slide and the airbox.  see below.

Use the IMS to adjust your idle mixture to get proper plug color.
Use new plugs.  Plugs stained with darker deposits will not read properly unless media blasted before re-use.

Literally was just going to post a similar picture with a red circle around it! 

I thought this screw was just to adjust the during the carb sync? Currently it's backed out one full turn, when I get new plugs what's the proper way to test? Assuming backing out will make it more rich? and seating (clockwise) will make it leaner?

[flybox1]

there is a different screw to adjust for the vacuum sync 
so if youve been adjusting the one i pictured, your carbs really have not been sync'd after all   

Put all of the pictured screws at 1 turn out for now, and leave them there.  Time for a proper vacuum sync. 

[flybox1]

adjust these for vacuum sync

[ChopSticks]

there is a different screw to adjust for the vacuum sync 
so if youve been adjusting the one i pictured, your carbs really have not been sync'd after all   

Put all of the pictured screws at 1 turn out for now, and leave them there.  Time for a proper vacuum sync. 

The one in the picture has only been 1 turn out. I've been syncing the carbs based on the top needle screw 17mm/10mm lock? The only reason I mention the IMS is because this video mentions to fiddle with it during carb sync

but since I was able to sync the carbs up without fiddling I just left it as is. Should I be syncing the carbs with 2 cooler exhausts?

[Gene]

Each carb has its own IMS.  small, brass, for your carbs, it will be between the slide and the airbox.  see below.

Use the IMS to adjust your idle mixture to get proper plug color.
Use new plugs.  Plugs stained with darker deposits will not read properly unless media blasted before re-use.

Literally was just going to post a similar picture with a red circle around it! 

I thought this screw was just to adjust the during the carb sync? Currently it's backed out one full turn, when I get new plugs what's the proper way to test? Assuming backing out will make it more rich? and seating (clockwise) will make it leaner?

It depends on the location of the screw - on your carbs, if it is between the air intake and the body Clockwise restricts the air and makes it richer. If it's between the body and the head, clockwise restricts the mixture and makes it leaner.

Tell me if I'm wrong Fly - hell, half the time I'm still wingin' it.

[flybox1]

Youre right Gene.

His IMS are on the airbox side, so they meter air into the idle circuit.

Chop...if your vacuum sync was done properly, using the lock nut and screw on top of the carbs, then great.  leave it.


The IMS now need to be adjusted for each carb to get the proper air/fuel mixture. 
If the plug for carb 1 is Darker than you want, turn the IMS for carb 1 OUT about 1/4 turn, and retest/repeat until the plug is med brown.
(If the plug is too white, turn the IMS IN .25 turn, retest/repeat until the plug is med brown)

Repeat this for each carb/plug....but you gotta be certain the vacuum sync is done properly...first.
revisit it if you are uncertain.

[ChopSticks]

Youre right Gene.

His IMS are on the airbox side, so they meter air into the idle circuit.

Chop...if your vacuum sync was done properly, using the lock nut and screw on top of the carbs, then great.  leave it.


The IMS now need to be adjusted for each carb to get the proper air/fuel mixture. 
If the plug for carb 1 is Darker than you want, turn the IMS for carb 1 OUT about 1/4 turn, and retest/repeat until the plug is med brown.
(If the plug is too white, turn the IMS IN .25 turn, retest/repeat until the plug is med brown)

Repeat this for each carb/plug....but you gotta be certain the vacuum sync is done properly...first.
revisit it if you are uncertain.

Alright just to confirm

Buy new plugs
Install into Carb #1
Let bike idle
1/4 out
turn off
inspect?

How long should the bike idle for between each plug check?

Also any idea why exhaust 3&4 temp would change if the IMS was never touched between the adjustments?

[flybox1]

changes in vacuum will have an effect on temps.
idle plug chop:
Set all IMS to 1 turn out
Put new plugs in ALL Cylinders.
Put a fan on your engine.
Start the bike and let it idle 4-5 minutes with no blipping.  Just idle.  turn off the engine.
Pull out the plugs and post a picture of all of them, side by side.  1-2-3-4.

[ChopSticks]

changes in vacuum will have an effect on temps.
idle plug chop:
Set all IMS to 1 turn out
Put new plugs in ALL Cylinders.
Put a fan on your engine.
Start the bike and let it idle 4-5 minutes with no blipping.  Just idle.  turn off the engine.
Pull out the plugs and post a picture of all of them, side by side.  1-2-3-4.

Since all the vacuums should be the same now via carb sync what else would cause temp differences?

Also should I look to purchase the plugs in bulk? I know I'll eventually have to do a plug chop when I get it on the road, do I have to use new plugs then or could I reuse old ones?

[flybox1]

A/F mixture issues
Unseen vacuum leaks
Weak spark (check your plug caps with an ohm meter)

I try to start my idle plug chop on the lean side, and work up to brown from there.
Once they are rich/black, they're only good for riding unless you can blast them.

[ChopSticks]

Haven't been able to do a idle plug chop since other things have been popping up but I did solve the cooler 3 & 4 exhausts. Their respective idle screws were completely blocked (probably from using auxiliary gas tank while tuning), couldn't even get it out with air. I've soaked them in cleaner but had some spares laying around.

Now 3 & 4  warm up perfectly, but 1 @ 2 are suddenly cold...  the left exhaust also backfires on idle. I checked the pilot jets when I did 3 & 4 and they were clear... any ideas?

[ChopSticks]

In case anyone stumbles upon this same issue here was my solution:

While trouble shooting this and other things I noticed that the pilot jets were clogged (couldn't see light through the main hole at the bottom) switched them out for different ones I had lying around that's when the other exhausts were cold instead of the original.

Then after other issues, I finally decided to pull the whole rack, disassemble the carbs entirely, and do an ultrasonic clean. I'm assuming the clog was me not being the greatest about making sure the auxiliary gas was debris free, also there was a bunch of crust and crud on the bottom of the carb bodies. I ordered new OEM pilots and mains. Reassembled everything, mounted them, and now all 4 exhaust appear to be running at similar temperatures.