Topic: 1978 cb750f carb linkage adjustment

[majorzozo]

Hi All,

Sorry to bother but I cannot seem to find the procedure for the throttle linkage adjustment in the Clymers manual for the 78F.  Specifically the adjustment for the accelerator pump (Clymers only shows 750A) and the set screw and the jamnut on the throttle shaft.  Please see pictures.

In a quest for perfection I am trying to get the throttle to immediatly come down from revs as it just seems to lag a little.  I noticed there was no gap between the accelerator pump rod and tab and it was actually depressing the rod about 1/8 inch with the throttle closed. Before taking the carbs off I checked the accelerator pump and noticed it was not squirting fuel with the throttle but when I depressed a little more with a screw driver then it squirted fine.  I adjusted the gap between the rod and tab for just a little clearance but it still doesnt work with the throttle, only the screwdriver. To me, it seems like it is not being actuated enough.  Not enough throw.  So I was thinking about messing with the set screw and jamnut to see if I could get more throw.

Its a new diaphram, all the check valves work, interior of the carbs are spotless including the pilot jet, and the mickey mouse ears are open.  Timing advance works fine and no intake leaks.  Stock jets and airbox witha Delkivic 4 into 1.  Bike runs good but I just feel the throttle should come down a little quicker.

If anybody can share their expertise or maybe a manual it would be appreciated.  Thanks in advance.   

http://forums.sohc4.net/index.php/topic,191852.0.html   

 

[Floshenbarnical]

I've got those carbs. If memory serves, the two factors controlling the accelerator pump operation are 1) the length of the accelerator pump shaft 2) the spring attached to the bit at the top. It seems to me that if your accelerator pump shaft is depressed even in a throttle-off situation then either the spring is too weak or the shaft is too long.

There are lots of aftermarket accelerator pump kits available online. Quite often the shaft is not the right length. I had to buy 3 last year until I got one that was correctly sized.

Is the shaft the correct length? How is the operation of the spring mechanism that moves the shaft. Does it move freely and well?

[majorzozo]

Hi Floshen and thanks for the assistance.

The spring at the top is very week in other words when I throttle up fast it lags.  I tightened it up by bending in the spring ends and that helped but it still lags because it is weaker than the pump internal spring so when the top spring tries to retract it fights the extension of the internal pump spring.  I also adjusted the gap at the top just above the spring to 10.1 MM and adjusted the rod to just touch the tab as called out in the Clymers manual. That helped as well but the maunal references only the 750A.  From the best I can tell I can get about 3MM of full travel for the pump rod.  Not sure this is enough. 

Everything works smooth and free but still no squirt.  After removing the pump and carbs about 1,000 times I pinched the ears on the pump and damaged the diaphram now so I will be ordering another pump.

How do you know if the rod is too long?  Mine measures about 83MM but who knows if thats right?

Again thanks for assisting

[kerryb]

Accelerator pump has little to do with the hanging (slow to return to) idle so we have to discuss two topics.
1.  Accelerator pump travel of 2mm is plenty if the system is working.  I use a wire from a wire brush, clamped in a handle to open the nozzle ports in the throats.  There is a small gap called for in the manual. (.5mm?)   2 mm of pump rod travel should result in a Nice stream from The nozzles on the bench.

2. A hanging idle (slow to return to idle) can be attributed to timing, an intake air leak, or weak springs on your advancer not allowing The timing to return to normal.  You can begin checking with a timing light, and a flammable spray around the intake rubbers to listen for changes in idle.

[majorzozo]

Thanks Kerryb,

I have a new pump on order but out of town for a couple weeks so I will have to wait to try.  Timing is dead nuts and the advance springs had a loop cut off.  Fully advanced at about 2800.  New intake boots with no evidence of leaks but after I install again, I will check more vigilantly and hopefully find a culprit.

Thanks again... 

[newday777]

Everything works smooth and free but still no squirt. 

The no squirt gets the attention of plugged idle circuits and accelerator circuits in the PD carbs. The circuits of each are intricate and have ways they need to be cleaned properly so the pump can push the gas through to squirt. No squirt says they weren't properly opened and cleaned fully. Hardened old fuel is the culprit.
How much of the circuits did you remove and how did you clean them?

Have you fully read through this thread on the PD carb cleaning? Lots of good information on the idle and accelerator pump circuits and how to get them clean fully, getting the brass plugs out and all.

http://forums.sohc4.net/index.php/topic,48858.0.html

[majorzozo]

Thanks Newday,

Yes I read through the thread and cleaned and validated all curcuits clear.  It does squirt but not by itself as I needed to use a flat tip to push down more to make it work.  Another pump is on the way so hopefully that works.

Thanks again...

[enwri]

I noticed no squirt after messing with mine, I had used a slightly too thick an o-ring on the bowl to pump seal. Was squeezing in and partially blocking the passage when the bowl was tightened.

[Floshenbarnical]

I noticed no squirt after messing with mine, I had used a slightly too thick an o-ring on the bowl to pump seal. Was squeezing in and partially blocking the passage when the bowl was tightened.

Where did you buy the correct replacement?

[majorzozo]

South sound Honda.  An assembly for a pricey $51  The last on was from 4 into 1.  I am trusting this is better quality and the rod is the correct length

Thanks all . 

[majorzozo]

Hi all,

South sound Honda sent an email that the pump I ordered was shipped but after reading more carefully a week later it stated not in stock.  Still awaiting a new pump, O ring and gator.

Another question for the experts.  After vacuum synching do the carb screws get installed with any thread sealant or O rings or do you just reinstall the screw with nothing?  Thanks in advance..

[denward17]

If I recall, my '78 K did not have any o rings or washers, been running without them.

[majorzozo]

Thanks Denward, greatly appreciated...

[HondaMan]

Some of the "F" carb's vacuum port screws came with the soft aluminum sealing washer, but toward the end of production this was often missing. So long as it seals, not a problem.

There are 2 different accelerator pump diaphragms for the PD42 carbs: the PD42a uses the longer rod of the two. I have often only had the long-rod one on hand and just cut off (and polished the end) the difference and it worked fine. The shorter-rod version is for the tighter-emission carbs (PD42b versions), and the linkage to engage it is slightly different. Not having both carbs on hand at the moment I can't supply a picture of this, though.

[majorzozo]

Finally got the new accelerator pump and there was still no heavy squirt.  What did make it squirt was tightening up the linkage spring.  As stated before when twisting the throttle, the spring force was too weak to push the rod down which activates the pump.  Tightening up the spring now forces the rod to fully actuate the pump driving it all the way to the stop.  Works good now.

Not sure how I was able to cure the hanging idle but after having the carbs on and off about ten times this is what I did.  Adjusted the valves, tightened the cam chain, adjusted the timing, carb sync, new spark plugs, and replaced the stock 35 pilot jets with 38.   Very careful whie tightening up the boots.  Idles fine now.  I wish I knew what the cure was but I don't.

Thanks for all the help and suggestions....

[Floshenbarnical]

Finally got the new accelerator pump and there was still no heavy squirt.  What did make it squirt was tightening up the linkage spring.  As stated before when twisting the throttle, the spring force was too weak to push the rod down which activates the pump.  Tightening up the spring now forces the rod to fully actuate the pump driving it all the way to the stop.  Works good now.

Not sure how I was able to cure the hanging idle but after having the carbs on and off about ten times this is what I did.  Adjusted the valves, tightened the cam chain, adjusted the timing, carb sync, new spark plugs, and replaced the stock 35 pilot jets with 38.   Very careful whie tightening up the boots.  Idles fine now.  I wish I knew what the cure was but I don't.

Thanks for all the help and suggestions....

These are all things I did to solve my hanging idle & im not sure which one solved it. Could’ve been a combination. Valves, cam chain, good boots, correct new spark plugs, correct jetting.

Take the win!

[majorzozo]

Went to start my bike today after about 3 weeks and wouldnt start.  My accelerator pump spring is just too weak to actuate the pump rod.  Picked up a spring from Ace hardware and cut about 10 coils off and now works.  The spring on the right is the stock spring.  Hopefully this lasts.  Also the throttle return spring is very stiff.  Not that it is too hard but I prefer a little less tension.  I basically unwound the torsion spring and added another pull type spring.  Feels better..