IT RUNS!!

[jbrightwell]

Thanks to all on this forum who helped me to get my 81 750K operational.  Your info has been invaluable to someone new to these bikes.  I can get mine to idle about 1500 revs, a little high.  But when I ride a few miles and stop at a stop sign, the revs may go to 2500+ at idle.  I keep backing down the adjuster knob and it will settle back down, but ride a bit and goes back up.  The carb boots show no visual signs of deterioration, but do you think vacuum leak ?  Should I try the starting fluid spray on the boots or start elsewhere?  Once again thanks all - and by the way when you hit that throttle these things are wicked fast aren't they?  No pods, all stock except old Jardine 4-2 exhaust.  JON

[seestheday]

Sounds like a vacuum leak. My insulators looked fine, but when I finally tried to pull them off the head they disintegrated.  

Easy to check. Do the spray test and see if the revs increase. I'm betting they do.

[jbrightwell]

The rubber boots going from carb to head are very supple and look like they have been replaced in the past due to the condition.  BUT - the boots from the airbox to carb #2 has a big ol' split in it.  Now - since these DOHC don't run well with free flowing pods - would this split have the same effect?

[seestheday]

Uneven air intake will definitely affect how it runs. The boots between my air box and carbs are very supple. Are your other ones hard and deteriorating as well?

Sent from my BlackBerry device

From: jbrightwell [via Honda CB750'S]

Sent: Sunday, April 7, 2013 5:27 PM

To: seestheday

Subject: Re: IT RUNS!!

The rubber boots going from carb to head are very supple and look like they have been replaced in the past due to the condition.  BUT - the boots from the airbox to carb #2 has a big ol' split in it.  Now - since these DOHC don't run well with free flowing pods - would this split have the same effect?

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/IT-RUNS-tp4038986p4039056.html

To unsubscribe from IT RUNS!!, click here.
NAML

[jbrightwell]

Only the boot from air box to carb 2 has any splits, although the rest of the boots from air box to carbs are obviously older than boots from carbs to engine.  I plugged the split with silicone but it started raining before it set up so we shall see.

[LukeM]

That would cause problems.  When they mention the "spray test", don't use starting fluid or WD-40.  Your best bet is some carburetor cleaner.  For a short term test, if you can do it without destroying the boot, wrap some vinyl electrical tape (other choice: duct tape) around the boot and see if it runs better.  If so, then your hunt for new airbox-to-carb boots is on.

Glad you got it going.  Nice feeling, isn't it?  If I weren't helping my wife's buying and selling business, and swapping the front shocks on my truck, my Shadow would have been out there.  After a cold and snowy winter, it was 70 in central Ohio.  Warmer weather is coming.

Luke M

[jbrightwell]

Thanks Luke:  I am still puzzled as to why I get this bike to idle in the driveway at 1000 rpm.  But I ride for 10 minutes and when I pull up to stop sign the engine is at 2500+ rpm and doesn't seem to want to come down.  I have to wait for better weather to do more testing but there was a thread about this same problem a month or so ago, danged if I can find it now.

[seestheday]

I had vacuum leaks that wouldn't present themselves until the bike had warmed up sufficiently. 

Sent from my BlackBerry device

From: jbrightwell [via Honda CB750'S]

Sent: Tuesday, April 9, 2013 9:49 AM

To: seestheday

Subject: Re: IT RUNS!!

Thanks Luke:  I am still puzzled as to why I get this bike to idle in the driveway at 1000 rpm.  But I ride for 10 minutes and when I pull up to stop sign the engine is at 2500+ rpm and doesn't seem to want to come down.  I have to wait for better weather to do more testing but there was a thread about this same problem a month or so ago, danged if I can find it now.

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/IT-RUNS-tp4038986p4039160.html

To unsubscribe from IT RUNS!!, click here.
NAML

[LukeM]

One more thing to check are the throttle cables.  When you twist the throttle, does it snap back pretty quickly?  They should.  If not, you may have a binding cable or a bad return spring on the carb bank. If it's not a mechanical problem, then I'd go chasing the vacuum leak.

After you find the source of the problem (vacuum leak wise), getting your carburetors sync'd up will probably help as well.  Search for "carb sync" at the home page of our happy little forum, and you'll find LOTS of discussion about it.  The tools will cost you about $75-100 US. Or, if you wish, you can take it to a repair shop, and have them sync them for you.  Everything, and I mean EVERYTHING else needs to be working well before doing the carb sync (valves adjusted, no vacuum leaks, compression good, etc). If not, you'll just need to do it again after you fix the other stuff.

I remember getting my carbs in sync, and I had to turn the throttle stop down quite a ways to get it to spec.  It ran SO much better afterwards.

Luke M

[icerigger]

Also check your choke idle lever. If this doesn't contact the throttle linkage to raise the idle while the choke is on and you need to increase the idle with the knob to idle. When the bike warms up your idle will run about 3K so you have to turn it down. If the coke idle lever is properly adjusted, your idle is the same.
 

[jbrightwell]

Good snap back on the throttle yes.  Im sure carb sync is in order after all else is ready.  Sure hope I can get those revs down.

[jbrightwell]

Icerigger I will try and check this very thing this weekend.  This sounds very plausible.

[RodneyCB750K1979]

Some thoughts off the top of my head:

Vacuum leaks in carb boots. Check each end of each boot. Are the boot clamps tight? My boots looked good until I removed them and found severe cracking and deterioration.

Make sure the sync tool connection port plugs are tight.

Check the carb pistons for cleanliness and freedom of movement.

Lube the throttle cables.

Make sure the throttle plates in each carb return to home position. I found during my carb reassembly the throttle plates would not close completely if there was the slightest pressure on the shafts.  The carbs not being tight on the two mounting rails could bind the throttle shafts. To fix, you need to loosen all the throttle plate screws, tighten the carbs on the rails, and then retighten the plate screws with the plates in their closed position.

Check the choke cable and arms and springs.

Confirm during high idle whether it is being caused by one cylinder or all four.  Place hand over the exhaust if you have 4 into 4 system or use a carb synchronizer gauge. If it is being caused by one cylinder, the problem could be easier to locate.

My bet is you have a vacuum leak or a throttle plate or carb piston sticking.

[jbrightwell]

IceRigger - Can you give me an idea what I am looking for?  Tank and seat are off and I have been checking things out as far as cables and such but not sure where or what the choke idle lever is.  Thanks again.

[jbrightwell]

Rodney I have some new airbox to carb boots on the way so that should cancel out that problem.  When those are on I will try the carb cleaner test and see if that changes anything.  I would like to buy a carb sinc guage - any suggestions on that?  

[Jebbysan]

A member here mentioned the high speed choke deal....good advice....look and see if
the choke lever move during riding.....easy peasy...then you can rule it out. Impossible to
see if you run the airbox.

But....I cannot express how many bike I have diagnosed due to dry card boots.
They look goo...but simply are not...for those that are bucks down I have a fix.
Take your boots off and wipe them down with Silicone grease. (Dielectric Grease)
Rub them liberally....do it a couple of times...it will bring some pliability back to the rubber.
Not to mention the grease helps them seal....

But in reality this is a stop gap fix....Randakk's offers new boots....Amen.

Jebby

[jbrightwell]

After I get my airbox to carb boots installed, I will test with carb cleaner on the carb to engine boots.  And then probably shell out the $$$$ for new ones.  Its just money...