Topic: More 78 cb750 carb issues

[maxheadflow]

I've been messing with my 78 cb750 and have pretty much gone through the timing, valves, looked in the carbs and while everything looks reasonable, it has one heck of a flat spot if I open the throttle slowly right above idle. It's not just one cylinder that struggles, it's all of them  I can jack the throttle and the accelerator pump will get motor past the flat spot and take off.  Above 3K it's fine.  Compression checked and it's between 130-140 all cylinders. It has some new NGK iridium plugs.

The bike has the original air cleaner and paper element with what looks like new rubbers both on the intake AC and head sides with new clamps.  The intake manifold spigots have been sealed with epoxy. As fa as I can tell the are not any manifold leaks. Chop the throttle and the motor drops right back to idle and idles smoothly. As far as jeting goes, I've not looked at the needles but the pilots appear to be stock as is a 110. The idle screws are all turned out 3 turns.  The exhaust is some sort of aftermarket 4 into 2.  Not sure if the headpipes are stock but they might be as they appear to be double wall.

Bike seems to have a hondaman ignition module and 1 coil that looks to be aftermarket. Other oil is a typical ND (IIRC).

AFAICT some sort of knew what they were doing tho there are a number of buggered screws. I suspect that they were fighting the evil flat spot and gave up due to all the things changed.

I know that these bikes had a bit of a flat spot when new (serviced a few of them) but this one is almost undriveable.  I've searched through the carb threads here and on another site and I really did not finds anything that looked like a possible fix except to possibly toss a washer or 2 under the needle and possibly go up on the pilot.

One thing that seems to be ignored is the slide cutaway.  I suspect it could stand to be smaller.  I've modified slides in the past by taking 0.020" off the bottom to make the cutaway smaller.  I'm also thinking about filling some of the upper holes in the emulsion tubes with solder to help pull fuel earlier.   

Anyone else got an easy fix for this beast?

Pic shows the holes I'd fill.  Other tubes are just for reference.

[HondaMan]

These carbs have an accelerator pump on the #2 float bowl. The diaphragms got hard from the days of MTBE gasolines, so yours needs to be replaced. This can be done right on the bike, too, so you're in luck! Then set the idle air screws back where they belong (1.5 to 1.75 turns) and run midgrade gas or regular, instead of premium. That will restore the throttle response, provided the ignition timing is where it should be, too.

PartsNmore sells the diaphragms, and even the Keyster version of it will work fine. Make sure the emulsifier holes in those mainjet holders are all clean and clear, and when you get it running well enough, get a can of BG44K at the auto parts store and run about 1/5 of the can in a tank of gas until it is all gone to help the engine and carbs seal themselves back up.

[kerryb]

I've done 3 sets of those carbs (Pd42b), never had a problem that wasn't solved by cleaning them again!  You might want to start by being really sure all the accelerator tubes are squirting fuel and the the accerator pump is being activated early enough on the throttle shaft movement.  There is a clearance spec in the carb rebuild section.  I once had to modify the accelerator pump diaphram rod because the new one I recieved was too long.  I've never experienced that "flat spot" with all stock jet sizes and stock airbox into an aftermarket 4 into 1 exhaust. 

A piece of very fine music wire clamped into an xacto-knife handle will allow cleaning the accel nozzles without removing them from the carb....it does take patience and a very steady hand!

Good luck, don't give up....just clean and adjust to stock specs.
Also, I've read on here many people say to only use genuine keihin brass parts.

Edit:  I was warned that someone else had posted while I was pecking away,  they didn't warn me it was "the" guru of honda repair, so take what I said with a grain of salt...or carb dirt.

[Mr. Mike]

Iffin it were me, I’d do the following first:

Set those iridium plugs aside and install the spec’d NGK plugs.
You’re certain all jets are clear? Side holes and center bores?
You’re certain all accelerator pump nozzles squirt a good stream into each carb? The pump rod is just a hair under the tang?
You did a spray test around those “epoxy sealed rubbers”?
And your bike has this flat spot even well after warmed up right?
I wouldn’t mess with modifying the slides, and I also wouldn’t fill emulsion tube holes.

I’d lean towards #3 on the list.

[brianc]

I had a similar sounding issue on my 78 750. Turned out the pressed in jet (?? 35 size ) was seriously blocked. Made a little tool, got them out, cleaned and all good again.

[maxheadflow]

Thanks guys,

I'll work my way down the list.   

One thing to note is that the rubbers are not epoxy sealed.  It's the spigot that screws into the heads that has been resealed.  Rubbers and clamps look new.

I've got couple sets of 61 to 80 drill bits. The 80 is about 0.34mm.   I guessing if the jet size is 35, it's likely .35mm.  IIRC only early Mikunis were flow rated instead of hole sized.

Not giving up.  I still have my old Action Four dwell meter and honda vacuum gauge set.  Had to break down and buy a new timing light tho.

[HondaMan]

Set those iridium plugs aside and install the spec’d NGK plugs.

+1 to that!

[maxheadflow]

So D8EA, D8ES what about X24ESU?  (ND)

[HondaMan]

So D8EA, D8ES what about X24ESU?  (ND)

The "X" plugs have been my favorite since they showed up in 1972. I've used them almost exclusively, except for experiments with other plugs, and ALWAYS on the hiway.

[maxheadflow]

I liked the hot Us.  Funny thing was that If the Hondas that we sold came in with ND plugs (non hot U), we'd replace them with NGKs.  Not sure what the issue was but they would foul before the first service.

[maxheadflow]

While I haven't gotten around to changing the plugs, I did spend some time cleaning the carbs. The fuel pump diaphragm was good, liquid in the bowel pumps out the passage just fine. I pulled the air cleaner to make sure that all the pump nozzles sprayed.  I pulled the emulsion tubes and blew out the passages. I ran a piece of wire brush wire through the pilots and blew out all the pilot needle passages making sure I didn't lose any any of the washers or O rings.  LIke I said the carbs look clean.

Started the bike up with the adjustments at 1 1/2 turns and warmed the bike up.. It really does not like to idle and at that pilot screw adjustment and not take throttle. Brake clean on the manifold rubbers made no difference in idle so I backed out the pilot screws first to 3 turns then found the best idle at about 4 turns. 

While 4 turns helped with the flat spot slightly it's still there.  While I have not changed plugs,I don't think they are the issue.  I do still suspect that the motor simply has a real lean spot as the throttle is opened slowly.  With the AC off, I can crack the throttle and just enough to get the bike to run rough and hold it there. If I cover each inlet slightly with my hand I can make that cylinder run smoother.  I expect that the first thing to do is replace the pilot jets with something bigger. Easiest way is to drill out the pilots with a number 77 or 78 wire drill and a pin vise.  It's best to do this with the carbs off the bike so I'll pull them.

I got side tracked fixing the oil leaks so it may be a few days.  I also have a set of 120 and 130 main jets to play with. May try them.

The idea is to fix the oil leaks.  Get the bike running properly then eventually tear it down and start cleaning/painting/PCing.  Since this is not a stock restoration, I plan on upgrading the forks with RT valves and springs, timken style steering bearings (can feel the dents), try a set of ikon shocks, new alloy rims with SS spokes, and convert the front brakes to dual disks. I would also like a little more pep, 4 into 1, KnN, possibly a cam. Not sure about a bore kit.

Any help / recommendations on the mods would be appreciated.