Tag: Repair

Carbs 1, 3, and 4 have synchronizing screws. They let you synchronize the throttle plates. That is a “bench synchronization”. When I did so, carb 3 would not stay adjusted correctly. It kept slipping out of alignment.

Continue reading “CB750F: Day 37: Carb 3 Linkage Rod Out”

I couldn’t find a throttle plate rod for the carburetor on ebay. I couldn’t even find the part on the microfiche. A kind soul sent me the link for that part though, used, on ebay. The search term is “carb linkage assembly”. Another kind soul explained that some of the carburetors on the ’79 CB750F were recalled and maybe half the bikes got the work done. He went on to explain that the carburetors were very different between ’79 and ’80-’83, and even more, that the carburetors were very different between the supersport and the classic. Great to know; great to know. Those should be here soon. The other option presented was to weld some steel in there and tap out a new thread. That may still happen, we’ll see.
The next project is to rebuild the front suspension.
Addendum: 2015-02-09
Here is the Motamec Tools Thread Insert Nut & Stud Rivet Gun Riveting Rivut Blind Riveter that was suggested on Facebook. Very cool.

Left unable to bench sync the butterfly valves, I finally pulled the bolt out only to find that it was in clearly bad shape. When the nut reaches a certain point, it just won’t go any further. On my carbs, the nut is threaded, and so too is the mounting point on the rod (that the butterfly valves attach to). Running the nut up and down the bolt (viced in) cleaned it up, but when it goes through the rod it gets “screwed up” again. Looking closely you can see the rough edge after going through the mount point on the rod.









Kind of a funny way to end the semester, but knowing that it will get repaired “right” makes me feel pretty happy. On the way out noticed a nice placard on the door, too.

Today I received my set of Vessel JIS screwdrivers and at the shop the screws on the carb shrouds didn’t slip once they are pretty great.

In preparation to start the bike I re-attached the exhaust system and torqued it all to spec (98in/lb header bolts, header-pipe attachment 16ft/lb, rear shock 18ft/lb, hanger 30ft/lb. Some of the bolts didn’t specify a spec, so Pat told me about this guide to estimate it and I used that. The exhaust headers may get replaced someday. A u-bolt holds the headers to the pipe so I turned that inward to be safe.




Got the fuel line attached and tightened up; found a nice hand-tool to compress that thing, human finger (at least mine) can’t do it.


The old flasher relay had to go, it didn’t work anymore. Finally got around to starting it and it had a super high idle.


We checked for a tight throttle cable, air leaks with carb spray, and finally just pushed on the throttle body assembly to see if it was stuck and nope, so we had pull them out again. If you look at the picture you can see the butterfly valves are not aligned. Measuring them with a gauge they were totally off. The bad thing was they just would not stay adjusted no matter what, so class ended and I will tackle it more on Thursday; but before that I reinstalled the chain protector.

Tonight I completed installing the carburetors but did not have time to test them. One of the mistakes that I made was that I was babying the carburetors and the air-box side intake shrouds. When you stare at and literally play with the few parts (carb rack, shrouds, and retaining clips) you see that there are only so many ways for the thing to go back together (cylinder head <-> shrouds <-> carburetors <-> air-box) but it doesn’t work like a puzzle. You kind of have to play around and force things and squash down the shrouds on the air-box side (not the cylinder head side those are hard and do not flex, they support the carbs almost entirely so you can see why). One curiosity was that when pulled out the air-box shrouds to clean them months ago I found that they were glued in with silicone or something. It was baffling because this is not normal. Only upon re-installation last night, were I found that the carbs would slide in and then out of a correct seating on both the cylinder and air-box side did it become apparent that specifically on the air-box side, the shrouds will only seat correctly to the air-box at the point of “final installation” when everything is correctly lined up, and that includes the location of the air-box itself. Until that point was reached, the air-box side shrouds kept popping in and out of seating, and even worse, they appeared to not even be capable of seating. For the #1 and #2 shrouds, I put them in the hot water ultrasonic batch to loosen them up, and that was one of the final keys because basically I had everything liked up but for #1 and it just wouldn’t cooperate. Finally warming it up and strong arming it and then gently guiding the shroud into seating with a flathead screwdriver was the key.
When things finally looked right I tightened the cylinder side shroud on the cylinder head side, then the air-box side (only one retaining clip on that side), each time making sure it looked right, then the cylinder head carb side, and thought it was done only to find the #1 and #2 were out further than I had wanted, so I loosened those and the key was to loosen the airbox location bolt, and just strongarm the assembly, and then tightened those up. Upon doing that, the #1 and #2 airbox shroud seat popped out, so I reseated #2 and then #1 but boy it didn’t want to cooperate. The cylinder side shrouds to appear to pop up a bit under full tightening, but as Pat told me, there is a metal lip in there that is important to seal, so it is OK since it is locked down at that point. It is really kind of “funny” because the airbox side shrouds will sort of go in and out of seating, with very little effort, and then at the end will seat perfectly when everything is lined up perfectly, that is really a noticeable and un-intuitive point. I had wondered why the service manual was so non-descript about re-installation, but now I see that it is, there is only one way for it to go back in, and it is kind of a process of playing to make it fit. That said I would love to have seen how these were installed in the factory!
In preparation, I re-installed the airfilter, hooked up the spark plugs, and connected the exhaust headers and exhaust pipe, but I didn’t torque down the header bolts, the u joint between the headers and pipe (also need to turn it inward), and the exhaust hanger bolt yet. I didn’t get a chance to use the new exhaust pins because one was drilled out on the cylinder head and wouldn’t fit and the other I ended up reusing the old one because the new wasn’t needed. Nonetheless I learned about a new to me tool, to hand-turn stuff like that in. The bike is looking closer to being on the road again; I think I’m only going to invest in mechanical stuff this year and maybe cosmetics next year (I hope lol but maybe can’t resist).









I wrapped up literally at the end of class and didn’t want to fire it up, I’ll do that next week (only 4 classes left).

The front-most frame bolt was nagging me so I figured now is as good a time as any to get it in. It was kind of funny because when that bolt was in there the front-right forward frame bolts would not go in. Pat said to get the jack and use the tapered metal rods to move the frame into position after loosening the bolts on the engine, too. After playing around with the jack and then more importantly the tapered rods, everything just lined up all of a sudden it was really super. After getting all those bolts torqued down I went back to the carburetors.



Last week I had left them in half-installed state with the carbs seated but not mounted on the air box side. Staring at the shrouds I realized that it was wrong, I should have connected them to the carburetors first before installing them for two reasons. The first was that there is physically no way to position them on the carb, so it has to be wrong. The second reason is that it is also nearly impossible to pull the shrouds out of the airbox at that point, too. This time I mounted them and then kind of squashed them to get the rack down into the assembly, both time actually because I installed the 1 and 4 shrouds 180 degrees off!






That did work, but I forgot to loosen the airbox so the shrouds wouldn’t seat nicely into the airbox. So, I loosened it and they still did not want to seat properly. I tried just about every push pull and twist I could and they just didn’t want to seat. That wrapped up the night, next time I will see maybe I can move the airbox more and just play with it until everything fits.

Reassembly continues along with lessons learned.

• Shifter rubber install: As you can see it doesn’t look like it would fit, that is, until you boil the rubber in hot water!
  
• Drive sprocket cover install: Without incident.
  
  
• New kickstand rubber: Pried the old one out. New one slipped right in with a little grease.
  
• Began front frame bolt install: The holes didn’t line up so I disconnected the front right frame, put a jack under it and raised it, which made the bottom hole line up but made the front right frame hole not line up. This is a work in progress; also need to check the chain slack after I get these bolts back in.
  
  
  
1. Began carburetor installation: This was interesting; the service manual is strangely not-detailed here. For me it wasn’t a race and I made mistakes but was OK about it and took my time backing out mistakes until it made sense. What I did seemed to work well so far, but we will see because I didn’t finish.
2. What I did was to first hookup the throttle and choke cables because there is no way to do it easily when the carbs are in the bike; I used a plastic sharpie to hold the throttle open so I could more easily reach the “pulling” throttle cable mount. Before hooking up the cables, I shoved the air intake shrowds inside the air box leaving their mount points to the carb intake exposed. You can see that they are designed for this once you try it; you will also notice there is probably no other way to get them installed has you left them off when you installed the carb bodies.
3. I only slid the carb bodies 1/2 way in so that I could hook up the cables and sat on a stool nearby so they could rest on my knee. The choke worked just fine but the throttle was way too tight, Pat explained that it can be loosened up at the throttle itself; I had no clue what that thing was before he told me! Vio helped me out and made the adjustment for me; it helps here to have a buddy around to help. To give you some perspective on having the carb rack 1/2 the way in, once you have it hooked up, you can move it around because the cables are at the right length; this helps for installation.
4. The next challenge is sliding the entire carb rack through the empty space because it is difficult to mount the carbs in the cylinder head side exhaust rubber and also the engine air intake tubes are in the way. For the rubber, I very lightly lubed it with some grease, on the air intake side also. For the air tubes, I disconnected the T-connector on the top so I could move the tubes around. I knew that they sit between the 1-2 and the 3-4 carburetors, so before and during sliding the carb rack in, I lined up those intake tubes so they would be in the right place at the right time. If you don’t do this, you end up pulling them apart and or moving them into the wrong position, and even better their metal clips fly off leaving you wondering whether they fell into your cylinders or heavens knows where (I found it lol). With the carburetors in place, you can stand over the bike and guide/force them into the exhaust rubbers and using some force they just seat themselves. That wrapped up my evening, I will be back at it soon.