Topic: No top end after carb rebuild.... what gives?

[sparty]

I can’t seem to get my bike to idle correctly after my carb cleaning, plus I no longer have any top end…what gives?  I disassembled the carbs, gave them a full cleaning, reassembled them and now I have big problems.  I crack open the throttle and I can only get about 4500 to 5000 rpms before it bogs down on me.  I checked the plugs and it doesn’t seem to be running rich or lean.  Can there be a problem with my accelerator pump?  I rebuilt the fuel line as it was before, so I know that I am getting enough juice to the carbs.  I blew out the jets and they seem clean.  Can my slides be sticking and causing the problem.  I love carbureted bikes, but they sure are a pain in the arse.  Any advise is appreciated.  Thanks in advance.

[sparty]

I should have asked, has anyone had similar issues?  If so, how did you cure them?

Sparty

[Bob Wessner]

Sounds like some sort of fuel starvation. How about the float heights?

[flatblack]

I just went through some of this with my 400F...

You probably set the air screws to the recommended default. That's where I set mine after a rebuild, too, but the bike wouldn't rev above 5K under load. I found my engine was very sensitive to the mixture setting and had to be leaner to rev and pull. Setting them to 1-1/4 did the trick. Matters where you are (elevation) and how warm it is.

Do a plug chop and make whatever changes the plugs tell you.

I also found my ignition wasn't up to par. Had a non-resistor plug cap on one plug and the other caps were all over the map when Ohm'd out. Replaced them with new NGK parts and no more worries.

Checked your timing lately? All the jugs firing?

HTH...

fb

[sparty]

Can you guys help me make a list of things to check to solve my problem?  I started the list, any additional input would really help.  I called my Honda shop today, and they can’t get me in until the 27th of the month… way too long to be without my ride.

The list:

1.   check float heights
2.   check plugs and wires
3.   check fuel line again
4.   timing
5.   check for manifold leaks
6.   

Thanks,
Sparty

[DammitDan]

Check to make sure the timing advance mechanism is OK, too (need a timing lamp for this).

[sparty]

Can you guys help me make a list of things to check to solve my problem?  I started the list, any additional input would really help.  I called my Honda shop today, and they can’t get me in until the 27th of the month… way too long to be without my ride.

The list:

1.   check float heights
2.   check plugs and wires
3.   check fuel line again
4.   timing
5.   check for manifold leaks
6.   

Thanks,
Sparty

Okay, I just finished checking all of the above.  And this is what I did and found:

1. The float hights checked out okay.

2. The plug wires checked out okay, however the plug wire on the number four cyl. is a bit loose on the plug.  I plan to order new plug wires tonight.  I have aftermarket Dyna coils which will accept universal plug wires.  These are the wires I plan to buy: http://www.jpcycles.com/productdetail.aspx?PID=381-116&cs=Metric&GID=5893D604-6ADD-4FDD-AC00-39FAEB16CF58&store=&page=&search=

3. I copied the old black fuel line with a new piece of blue polyurethane and I saw that I was getting air bubbles in the line, so I created a new fuel line with a shorter and more direct route.

4. Timing... haven't checked it yet.

5. I checked for manifold leaks and found none.

6. I drained the gas tank and pulled out the petcock, it was not blocked or stuck, I replance the o-rings and gasket and reassembled the bad boy.

7. When I drained the gas tank I had just a little over a gallonand a half of gas in it, I was wondering if my problem was caused by too little gas in the tank....hmmmmm.  After I rebuilt the carbs I put a minimal amount of gas in the bike just in case I had to take off the tank to fix any problems, I never considered that too little gas might cause "fuel starvation".

I haven't tried to start the bike yet since completing the above routines.  Just as I was about to put gas in the bike, we got a wicked thunderstorm.  I'll give it a try tomorrow.

[GroovieGhoulie]

How much would carb sync affect top end?

[DammitDan]

Shouldn't affect top end so much as bottom end I would think...  When the butterflies are open all the way (full throttle), flow would be at maximum.  Of course, a sync certainly can't hurt anything.  As well as checking valve clearances, etc.  I'm sure the sync would help the idle issues considerably.  Did you do a bench sync when you put the carbs back together after the rebuild?

This also could also be a jet issue (blocked/incorrect sizes), but I'm leaning more towards timing and advance.

Are all of your header pipes getting hot when it runs?

[flatblack]

2. The plug wires checked out okay, however the plug wire on the number four cyl. is a bit loose on the plug.

5. I checked for manifold leaks and found none.

7. When I drained the gas tank I had just a little over a gallonand a half of gas in it, I was wondering if my problem was caused by too little gas in the tank....hmmmmm.  After I rebuilt the carbs I put a minimal amount of gas in the bike just in case I had to take off the tank to fix any problems, I never considered that too little gas might cause "fuel starvation".

Just hold the wire with one hand and give the plug cap a few twists with the other, like you're screwing in a light bulb, and it should tighten up.

How did you check for leaks?

The only thing a full tank will do for you is build up some head pressure. If the tank vent is good and the fuel level is above the selected petcock opening (normal or reserve), it shouldn't matter. What matters is that good, clean fuel flows freely from the petcock. A good petcock on even a partial tank will flow more than the engine can burn, even at full throttle.

I'm gonna put in another pitch for you to monkey with the air screws. I wonder what would happen if you leaned it (turned the screws in) a half turn to lean it out a bit...

HTH...

fb

[Noel]

I was gonna ask about carb synch too. Were the carbs unracked for cleaning? If they were racked back together without any attention given to synch, the slides could certainly be far enough out to affect running at any RPM.

[sparty]

Okay, let me see if I can answer your questions.

I have not performed a bench synch – I don’t have the tools. 

The header pipes are hot.

I checked for manifold leaks using WD-40 as recommended on this site.  If the WD-40 gets sucked into the manifolds there is an air leak, but before I did that, I readjusted the boots and retightened the clamps.

Yes, the carbs were completely disassembled and yes, the slides do seem to be sitting a bit higher than before.  Will a synch correct that?

I wasn’t getting a good clean flow of gas to the carbs because I had a large air bubble in the line, which is now corrected.

My slides did show some wear and slight grooving, should they be replaced?  Where can I find new slides for 78 carbs?  I am running 78 carbs on the 72 CB750.

Also, when I bought the bike, it only had a pull throttle cable.  I’d like to put on a push cable, but should I buy the 72 or 78 cable?  My lever is a 72.

I can’t work on the bike this evening because I have grad class – I am currently working on my PhD which sucks up a lot of my spare time.  That is why I was thinking of taking the bike to my Honda shop for the carb synch.

Thanks for all of the input thus far.

Sparty

[DammitDan]

The tools required for a bench sync are fairly simple and can be fashioned at home.  Essentially what syncing does it make sure all the throttle butterfly valves are opening and closing at the same time and allow the same volume of mixture to pass, which allows an equal amount of mixture to enter all the cylinders.  So if the carbs are out of sync, one cylinder may be getting more/less gas than the others (thereby making the piston push stronger/weaker), making the engine "fight" itself.  This is especially evident at idle because your #2 throttle valve will be sealed closed but the other 3 may be at different vacuum levels.

If you disassembled the assemblies (split the carbs for cleaning) then it's likely that this sync was thrown off.  Just jiggling them on the brackets can throw the sync off.  You can learn everything you need to know about syncing here:  http://www.sohc4.us/forums/index.php?topic=5410.0

As for the bubble in the line, it shouldn't have affected gas flow as much as it did.  Perhaps you have a major blockage in the petcock or the in-tank screen.  I remove/install my gas line all the time and the flowing gas just pushes past the air bubbles.

Unfortunately I don't know much about slide carbs (I have CV piston carbs), but I fairly certain that grooving on the shaft is a bad thing.  For me it's a part failure, but then again I have to maintain a vacuum seal around the piston.  Slides may be different.

I'd probably go for the 72 cable, cause the connector is likely gonna be the same on the carbs and different on the bars.  But that's just guessing.

[Noel]

My money's on carb synch. If your bike ran okay before the cleaning and now doesn't, well, it doesn't make much sense to start asking questions about plug wires and such, seems to me.

You can't unrack the carbs and then rerack them without doing a synch -- and IMO, bench synch isn't good enough. Once your carbs are unracked you're on the hook for either a synch tool or a visit with the mechanic.

IMHO, as always.

[sparty]

Thanks guys, I guess that I'll have to wait on the Honda shop to take me in for a synch.  The weather next week is supposed to be awesome and I'll be putting around in my car

In the mean time, I'll replace my chain and install new plug wires, my old wires are old, early 80s, so it is about time anyway.  I found a Dyna plug wire set for $21.99.

I didn't realize that I could throw the carbs so out of synch even if I didn't meass with the screws.  As for the slides, I have no idea were to get replacements?  Any help in this department is most welcome.

Thanks again,
Sparty