Topic: Loss of Top End Power in 5th **SOLVED** FLOAT LEVEL IN 4TH CARB

[YoungBlood]

Hi everyone,

I need to see a family member, emergency, and I'd like to take the bike cross country. But I have something bugging me with the bike in 5th, 70+mph.
I recently did a cam-chain adjustment, valve adjustments (.002/.003), gapping point breakers (.014), and static timing. Needless to say, the bike runs very very well around the city. The bike ran strong, over 90, lots of power in 5th when I got the bike a couple months ago. However, the problem developed that I still have after adjustments. She feels to not have any more kick around 70mph, I can get it up to 80 (it will take a bit too long it seems), but that's a straight shot going through all the gears. If I have to shift to 4th, and then return to 5th, I will not get up to 80. Once I get off a highway, approximately having been running the bike for 7miles, the bike runs choppy, seems out of sync, a blurburing noise. This noise sounds like the bike is flooded on gas, it runs louder on exhaust, and weaker going through all gears. However, after about a few minutes off the highway, the bike seems to get back into normal (good) running condition.
I have no idea what the hell it is. I do have a head gasket leak, but not bad. There is not a noticeable amount of oil lost from checking the oil level. I have a carb sync that I am going to get out of the way tomorrow, hoping that might have something to do for it. Anyways, it all feels like a shot in the dark, and I need this bike to run well enough for long distance highway.
Edit: Oil was changed less than 200 miles ago, and new NGK D8EA spark plugs.
Thanks! Please let me know what info you all need, or videos?

[minimo]

What do your spark plugs looks like?
Around 3/4 to W.O.T. usually points to Main Jets


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[YoungBlood]

These are the old ones from about 200 miles.

[Jerry Rxman Griffin aka MuthaF'er]

Or a dirty air filter

[Alan F.]

Fuel cap vent? It sure would be simple and you wouldn't be the first.

What's your plug gap?

What fuel do you run?

Chicago heading where? One of us is nearby, on the way or at your destination. Check the Travellers Rest section of the forum.

Keep replying, the solution will be found, good luck.

[bryanj]

Be nice toknow what bike

[pjlogue]

I had a similar problem with my 750.  I had put an inline filter for the fuel.  It would run fine around town but on the open road it would bog down after a few miles when the bike warmed up.  I found two things happening.  The fuel tubing routing and the filter were below the float bowls and this caused fuel vapor pockets to  form in the lines which starved the engine.  Not sure what bike you have but fuel tubing routing seems to be critical to many folks with SOHC fours.  Tubing should (ideally) come from the petcock spigot and be all down hill with no uphill bends.  By up hill I mean the tubing should not be bent "n" like that.  A "u" bend is OK.  You don't want any pockets where vapor can get trapped.  I found inline filters only caused me problems.  Some folks on this board have no problems with inline filters but I always have.  Another thing to check is the very fine mesh filter in the carb float valve.  The 750 (and I assume some other models) have a mesh filter just behind (upstream) the float valve seat.  This should be clean.  Never mix up float valve parts!  Keep the same float valve needle with its matching seat and keep it with the carb it came off of.

Another thought is your ignition condensers (capacitors) Replace them if they are old.

-P.

[YoungBlood]

Alan:
When you say fuel cap vent, I am a bit at a lost. Looking at Honda Manual pg 82, I assume there's some small passage way within the top fuel cap? I'll take a look right away.
Spark plug gap is .028, I checked when I installed them. They were manufactured at .028. I believe that meets the gap range for this bike. Will double check this morning, and give accurate gappage for each cylinder.
Fuel usage varies, new bike so I'm not sure exactly what to run. Normally I've been putting mid-grade in it, 89 octane. Sometimes, high-grade... I believe it says high-grade in the Honda manual, no?
I live in Chicago, heading out West to Prescott if things go accordingly.
PJLogue:
I'll attach a pic, no inline filter for the fuel. From my memory there are not any up hill bends. There is a T fuel line connectors running into the carb fuel T-joint. I have not messed with the carbs, I only removed them to replace the carb boots because I found a vacuum leak. I do know the previous owner cleaned them (supposedly).
As to the capacitor, is there a way to cpnfigure how old they are?
I assume I'd have to pull the carbs to figure out if the mesh-filter is not clogged?
Bryan J:
1975 Honda Cb750F, I just update my profile thanks for bringing the missing profile info to my attention.

[YoungBlood]

Alan:
#1 spark gap: .025-.026
#2 spark gap: .025-.026
#3 spark gap: .026
#4 spark gap: .025-.026
Question: is that tiny hole at the top of the cap the vent? If so, I cannot see through it when removed. Should there be some light able to shine through?
The order of pictures follows order of cylinder number: 1-4
PJ: Also a pic of fuel lines that previous owner installed. The wire tie you see is my own doing, still need to cut that off. I have more clamps, just no leaks so it hasn’t been an issue.
Finally a pic of the head gasket leak, just a side problem that I’m curious on how long I might have, and whether this could have any bearing on problems above 70mph? I have not done a combustion pressure check, but I have someone I could borrow a gauge from.

[Alan F.]

I don't know much about the F bikes, I was just trying to cover some simple general stuff that might help.

[Don R]

 I'd have another look at things you worked on since it seems to have changed after working on it. Re-check the valve lash and the point gaps. Plug caps can be an issue if the resistor has become too high and what's the plug number.

[minimo]

That last plug looked mighty rich and sooty.
#4? Are the plugs posted in sequence, 1-4?


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[YoungBlood]

Okay, thanks Alan, do you know if that port should be clear for air passage?
As to the pictures, yes that last picture is #4.
As to rechecking the adjustments I made, I could do that, but this problem existed before the adjustments. If anything, now it runs better around town, but once on the highway the same problem persists.

[69cb750]

I need to see a family member, emergency,

Fly or Amtrack

[Alan F.]

Yes that vent port on the fuel cap should flow air. If you wipe it off and put it to your lips like a trumpet, you should be able to blow air through it.

[Alan F.]

Here's a stupid guess, but looking at your pictures in reply#13..... Does anything (cables or linkage) push against the fuel lines when the throttle is wide open?

That would account for good around town manners and bad high speed highway.

Another thing might be to change the clip position on the slide needles. I'd say one groove higher to lean it a bit, but I'd get a second opinion before cracking into that.

[YoungBlood]

Thanks Alan, I’m running the bike right now. Weird, cylinder four seems to be cold, I’m slightly tapping the exhaust pipes 1-3 (very hot), number 4 not very hot

[JLeather]

Any chance this started after your tuneup?  If your valves are just a shade too tight it can cause an issue like this.  When the bike is thoroughly hot they aren't quite sealing any more.  I usually run mine at .003/.004 instead of .002/.003.  An inexpensive check would be to run your valves a few thou looser and see if it stops.

[YoungBlood]

Thanks, I’ll double check when back home. All cylinders are firing now that it’s sat for a minute. But very rich gas smell, exhaust smells like unburnt fuel.

[Alan F.]

Other things to check:

Verify your plug cap resistances. 5K ohms (5000 ohms) with a meter. As these caps age their resistances can vary.

Before you screw the plug caps back onto your plug wires, snip 1/4" off the wire ends so the plug caps have fresh meat to screw into. (Plug caps attach with coarse threads like a wood screw straight into the ends of the  plug wires)

Also if this is a new-to-you bike there are tons of things you want to verify. One good idea is to unplug and clean each electrical connector, inspect for corrosion, use an aerosol electrical contact cleaner, then add copious amounts of dielectric grease to keep things clean.

[HondaMan]

This sounds like either:
Fuel is falling low in the float bowls (slow fuel feed from gas tank to carbs).
Bad Daiichi condensors, if points are in use. New Daiichi condensors will be the same (or worse). If you have those, and have some old TEC or Hitachi points plate lying around, install the old condensors from the old plate instead. They don't "go bad".

[YoungBlood]

I'm not too sure about there being low fuel. Exhaust smells of gas, it seems #4 is not sparking once too much gas floods in?

I also did a carb synch thinking this might change the rich/lean condition. I will attach a video of where the vacuums were at before I was done messing with them. The problem still exists, after the first shot on a highway, the fourth cylinder gives when I let up on the gas.

HondaMan, as to the condensors, I am going to order new condensors, new coils, wires, caps, and rectifier. I want to cross my I's and T's with ignition (spark). Today I noticed that there is arcing going on in #1 wire, I reached down to touch the cable and got shocked! This is why I'm going to just replace everything spark related. The points looked good, but then again I'm not sure what bad looks like.

Also, I do not recommend anyone to buy the Barracuda vacuum gauge set, the copper adapter tubes are extremely short! This caused me to be unable to tighten the port in #2 carb properly. It still sealed, but I stripped some damn threads from trying to toggle the damn tube to rotate in and when rotating out. PISSED. There's still enough thread for me to put the screw in, but I'm afraid I cannot sync these carbs again and mess with that thread.

Lastly, I am having trouble with idle after the carb sync. The bike only wants to idle at 1400, if it dips to 1200 it'll die. Can someone point me to the correct steps to proceed with to get the bike to idle around 1000rpm when syncing? The air screws are all 1 turn out from fully seated. The tip of the throttle adjuster lock nuts (1-4 carbs) all have a thread exposed. However, I could not figure out where my "Throttle Stop" screws were, and I believe this has to do with idle adjustment? Or just lower vacuum pressure when synchronizing all four to the same value?

[YoungBlood]

Also Alan, I cut back the wire on 4, and reinstalled the cap. I don't have a volt-meter, but I assume the cap is fine if it'll run around town with all four cylinders. In addition, I checked the fuel cap vent, it is not blocked. And I went through all the adjustments one extra time: I changed intake to .003, and exhaust to .004; all tight .003s/.004s, and rechecked the timing. Timing is dead on the F mark for 1/4, 2/3 with my volt light toggling on when rotating crankcase.

[minimo]

As for your idle issues, maybe an air leak? While running idle, spray some carb choke around the carbs, boots, and hoses to see if you notice any changes


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[YoungBlood]

Thanks Minimo, I will double check that. I did that initially when I got the bike and replaced all 8 boots. Could an air leak occur in the air-box itself and cause problems further down the intake line?

[YoungBlood]

Here's the video of the vacuum sync. I think I may have goofed and not ran the bike at higher rpms?? when setting the adjuster screws, and then the idle?

[minimo]

It could affect the whole intake, but it appears, based on your #4 plug and your one less-than-hot exhaust pipe, the issue seems to be particular to a cylinder


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[minimo]

Here's the video of the vacuum sync. I think I may have goofed and not ran the bike at higher rpms?? when setting the adjuster screws, and then the idle?

Be sure to blip the throttle after every screw adjustment to get a better read


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[YoungBlood]

Thanks, I was doing that before hand. This the final cut video of when I was done messing with the screws. Does this look good enough? The manual says ~3 cm hg in between all needles, but mine is in mm markings.

[minimo]

Might also want to check compression across all four cylinders. Loss of power could be piston ring-related also


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[YoungBlood]

Thank you, that sounds like a good idea. I have a friend with a compression gauge I'll reach out to.

[YoungBlood]

This sounds like either:
Fuel is falling low in the float bowls (slow fuel feed from gas tank to carbs).
Bad Daiichi condensors, if points are in use. New Daiichi condensors will be the same (or worse). If you have those, and have some old TEC or Hitachi points plate lying around, install the old condensors from the old plate instead. They don't "go bad".

HondaMan, you were right. I'll never second guess your judgement next time. The float in #4 was set too high, thus a very lean condition starving the bike in 5th. I bent the tab back, richening the mixture, setting float lower. Bike is running great!!!!!! Very happy.

[Alan F.]

That's why someone so many years ago called him by that name and it stuck.