79 engine rebuild

Hey, what projects are you planning or preparing for? CBX, other motos, workshop, WHATEVAH!

Re: 79 engine rebuild

Postby desertrefugee » Mon May 07, 2018 5:51 pm

Agreed about the synch discrepancy between banks. I just finished mine and had the same symptom. The two banks were off similar to what you're seeing.

Oh, and I think you win the prize. Which one, you say? The prize for getting the most tools on the table under the patient. I am not worthy...

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Re: 79 engine rebuild

Postby Syscrush » Mon May 07, 2018 9:06 pm

wyly wrote:until it arrives I'll be searching for the adjustment screws on the carbs, I've found only one on carb #1 (maybe a 2nd on #6) so if you have a picture of where the others are that would be of help.

The adjustment screws aren't really on the carbs, they're on the throttle linkages between the carbs. I don't know exactly what it looks like on a CBX, but on the flat 4 and I4 engines I've synchronized, it was not difficult to find a screw that would adjust the relationships between the throttle plates. There should be one screw between 1&2 and another between 2&3, likewise between 5&6 and between 4&5. Between 3&4 will be a screw that balances the two banks against each other.

And forgive me for saying this, but you did balance the 6 gauges on your tool by connecting each in turn to a known constant vacuum and adjusting those dials, right? I notice that they're each in different positions and I assume that that's because you dialed them all in correctly, but I figure it can't hurt to be explicit about the basics. Maybe because I only understand the basics. :lol:
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Re: 79 engine rebuild

Postby desertrefugee » Mon May 07, 2018 11:33 pm

I don't think it's humanly possible to synch a CBX with the tank in place. I ran my external IV bottle with the gas tank on the shelf during the operation.
'97 Valkyrie Bumblebee Tourer, '79 CBX-Z Troublemaker, '78 GL1000 (not your father's Goldwing)

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Re: 79 engine rebuild

Postby mlynch001 » Wed May 09, 2018 11:58 pm

daves79x wrote:Wyly, also from experience, seating and lapping valves is not rocket science as your man might have implied. If each of your valves would have been simply hand-lapped, a bent valve that was not seating would have been very obvious. And a simple kerosine leak test on the finished head would have shown which valves still were not seating. Now, replacing and reaming guides is a whole other matter. Bottom line is someone didn't do a very good job of a fairly straightforward procedure.

Now if you decide that you trust this guy you found, you'll have to go the whole way with his recommendations. You are correct, it will be expensive.

Again, I'll help you with any parts I can.


Very sad situation. Another person with tools that probably should not be working on something as complex as a CBX. As Dave stated, valve seating is not rocket science. I always vacuum test and kerosene test just to make double sure that I have a good seal. Seems that because there are more than two valves per cylinder, the Previous "mechanic" might have been overwhelmed, since his last rebuild was probably a Briggs and Stratton. This story repeats itself year in and year out, only the names and the serial numbers change. SMH!
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Re: 79 engine rebuild

Postby mlynch001 » Thu May 10, 2018 12:00 am

desertrefugee wrote:I don't think it's humanly possible to synch a CBX with the tank in place. I ran my external IV bottle with the gas tank on the shelf during the operation.

It is impossible. The tank must go.
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Re: 79 engine rebuild

Postby mlynch001 » Thu May 10, 2018 12:28 am

wyly wrote:engine fires up easily, and idles well revs easily but the rpm gets hung up at 2,000-2500rpm and doesn't come down
I've check the throttle cables and choke several times all seem to be working well
checked the advancer and that snaps back when twisted
checked for vacuum leaks at the carb boots front and back, sprayed ether at them and there was no response in rpm

hooked up vacuum to check sync, 1st photo is at 1000rpm
2nd photo at 3,000rpm, number one is a bit high but is it significant enough to cause the problem
7586
7587

Dave has been trying to help me with this but if anyone has any ideas from left field let me know, it's quite possible I did this in the assembly but I don't know what I may have done.


Your # 3 carb is closed too much, which, in turn, holds # 1 and #2 closed as well this is why the indicated vacuum on those cylinders is higher. I am Assuming gauges are attached 1 to 6 right to left in the picture. Open throttle plates, Low vacuum . . . . Closed throttle plates, High vacuum. Basically 4-5-6 are trying to drag the RPM up and 1-2-3 are acting as a load to try to keep the engine slowed down. You appear to have decent balance on 1-2-3 as well as 4-5-6, so get the two sides or banks equal will very likely be a huge help. #4 is base and the idle adjust carb, and the balance between #3 and #4 is critical. These things have almost no vacuum at idle, so the slightest balance issue becomes magnified. You will not be able to get a proper idle mixture adjustment on all the carburetors, since the throttle plate position in relation to the idle ports is off one way or another, until the left-right balance is perfect. #3 screw is accessed from the front side of the carburetors and is to the left and slightly below the #3 carburetor link. #2 and #5 access from the rear with the long tool and #1 and #6 require the short tool. The slight variation in Vacuum at 3000 RPM is irrelevant as these are CV carbs and each cylinder can have the slide sitting at a slightly different position. I made a short tool from some bits and pieces that I had in the shop, I am a cheapskate.
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Re: 79 engine rebuild

Postby daves79x » Thu May 10, 2018 6:57 am

Thanks Mike, for the excellent explanation. Agree on all counts, except, in my experience, those reading are close enough for decent idling and just need fine-tuned from there for best smoothness and engine quietness. Thing we don't know is if the gauges were synced to a common source first. Perhaps a needle-width on these gauges is more significant than on mine.

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Re: 79 engine rebuild

Postby mlynch001 » Thu May 10, 2018 11:37 am

daves79x wrote:Thanks Mike, for the excellent explanation. Agree on all counts, except, in my experience, those reading are close enough for decent idling and just need fine-tuned from there for best smoothness and engine quietness. Thing we don't know is if the gauges were synced to a common source first. Perhaps a needle-width on these gauges is more significant than on mine.

Dave


Dave, I agree that the first thing that needs checked and balanced are the sync gauges themselves. This is the most important and most overlooked part of any carb sync job. In my opinion, any gauges used MUST be spot on equal, or the effort is wasted to sync the throttle plates. I would say that he needs to get the left/right balance a little closer and call it good. People spend WAY too much time getting the gauges perfectly aligned, it is simply NOT that critical. That being said, left to right balance needs to be as close as possible to equal. It all boils down to that Idle speed screw and the #3 sync screw on this example. As I recall, HONDA gives a 1.6 in Hg or 40mm variation as the maximum allowable variation from high to low. Bottom Line is that the measurements are only as accurate as the instruments used to obtain them. One final thought, I used the word "perfect" in my original post, that was probably NOT a good choice. The better way to say it would have been as close as possible to the minimum variation. You are EXACTLY correct, they do not need to be perfect, the margin of error on the gauges does not allow for "perfection".
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