CB750 rat bike?

Nice! 8)

Oh, and the charging system seemed to work fine! IR thermometer indicates it is only running on the center two cylinders. Also #3 and 4 carb float valves leaked. And after leak checking with alcohol before mounting! Just really looking forward to pulling them off again, but then it's a long winter ahead.

pidjones said:

Oh, and the charging system seemed to work fine!

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Thats great, I haven't been able to check mine. :

trek97 said:

Thats great, I haven't been able to check mine. :

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I had read so many complaints about the charging system (and mine looked awful inside the alternator cover) that after running the bowls dry I turned the fuel back on, got the DVM, fired it back up, and checked the voltage. Right after cranking, it went up to 14.7 @ ~3500 RPM and stayed above 13 at idle (headlight fuse pulled for now). I like this system so much more than the GoldWing permanent magnet with shunt regulator system.

Found a problem with the accelerator pump (o-ring too fat at bowl/carb junction) and remounted the carbs. New front cam tensioner bolt came in, so I went through adjusting and (lightly) locking it down. Ready to re-mount tank but broke the inline fuel filter I had added so will have to wait for next week to fire it up again.

This morning I did another (I hate brakes) rebuild on the 750's front calipers. They were still leaking and the bleed screw on the right leaked. After draining the system (again) and removing the right caliper, I put the bleed screw in the lathe and polished the corrosion off the tip (don't know why I didn't see it before). Disassembled the caliper and once again thoroughly cleaned the seal groove. This time, I used one of those cheap 12V Harbor Freight rotary tools (I love this thing, will buy another when it burns out) with the flimsy brass brush tool in it to clean the seal groove. Wow, that works great! Reassembled with a new seal (had reused the seal previously) and then repeated on the left caliper. Hope this finally takes care of it, because I HATE BRAKES! Have a 39cm line on order to replace the 50cm that I'd put on it that was just too long and may have been kinking. I'll wait until that comes in (supposed to be in this week) before refilling the system.

Meanwhile, the 750 goes in the corner to permit layout of the Hunley parts I hope to clear coat tomorrow after church. Forecast is for humidity <60% and temps >60F for a 5 - 6 hour period tomorrow afternoon. I'll get the shed prepped this evening so that I can make best use of the weather window without hurrying. This will be rattle-can 2K gloss over lacquer. I'll lightly wet sand the parts this afternoon, wash with Dawn, and rinse thoroughly.

Sounds pretty good, really. I had the same issues as you with the triple tree on my 350. There was nothing I could do to get the threads back into a working condition. Nice save, and nice work on the mountain vac test.


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Beginning to pull on my sparse hair. Fired it up again after fixing the float valve issue. Starts easy, even idles after a good warmup. BUT, exhaust for cylinders 1 & 4 are not heating up. I have great hot blue spark on them. Much better now that I've run an extra ground lead to the spark units (was reading 150 ohms from mounting stud to (-) terminal before and had weak yellow spark). Compression is not great on any cylinder, but is actually 10 psi better on the outside pair at 130, 120, 120, 130 when last checked. Have not balanced or messed with pilot jets yet (all set at 3 turns because the aircuts are disabled). Even with throttle open for ~5k RPM, the #1 stays ~60C, #4 ~90C while 2 & 3 go over 300C (read on outside of bend at head exit).

Ideas? Carbs have been off twice (fully cleaned 1st time, just looking for problems and fixing float issues 2nd), so I'm getting better at it. I heat the "isolators" with a heat gun until warm to the touch to make it easier on them and me. Spray of brake cleaner around carbs and isolators yields no change in idle. Full airbox and filter. New plugs. 5K ohm plug caps, new plug wires, coils ~11K ohms secondary, 2.5 ohms primary. Pulled #1 cap while idling and no change.

pidjones said:

Beginning to pull on my sparse hair. Fired it up again after fixing the float valve issue. Starts easy, even idles after a good warmup. BUT, exhaust for cylinders 1 & 4 are not heating up. I have great hot blue spark on them. Much better now that I've run an extra ground lead to the spark units (was reading 150 ohms from mounting stud to (-) terminal before and had weak yellow spark). Compression is not great on any cylinder, but is actually 10 psi better on the outside pair at 130, 120, 120, 130 when last checked. Have not balanced or messed with pilot jets yet (all set at 3 turns because the aircuts are disabled). Even with throttle open for ~5k RPM, the #1 stays ~60C, #4 ~90C while 2 & 3 go over 300C (read on outside of bend at head exit).

Ideas? Carbs have been off twice (fully cleaned 1st time, just looking for problems and fixing float issues 2nd), so I'm getting better at it. I heat the "isolators" with a heat gun until warm to the touch to make it easier on them and me. Spray of brake cleaner around carbs and isolators yields no change in idle. Full airbox and filter. New plugs. 5K ohm plug caps, new plug wires, coils ~11K ohms secondary, 2.5 ohms primary. Pulled #1 cap while idling and no change.

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Have you synced the carbs?

Would need all cylinders working, wouldn't I? But, no I haven't yet. Waiting to get them all on the team. I did bench sync - and checked it again when they were off the second time. Think it would do any good to do it now? I've done it many times on GL1000s. Need to get the fancy screwdriver for the 750 and get the pilot screws right too, I guess. I've seen some say sync with the gas in the float bowls and tank off. I fear that will change carb operation as level falls - plus I'm slow as mud. May finally have to make an external tank.

advCo said:

Have you synced the carbs?

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You'd think a bench sync would have you much closer to where you are now. Unsynced carbs will certainly show different temps on the headers. You definitely want to sync them by vacuum and make sure all of your air mix screws are set the same. It's a strange coincidence to have 1/4 cold and 2/3 hot though, considering that's how the coils are oriented. Have you swapped the coils to see if the problem moves?

Haven't tried swapping yet. Probably should swap coils and then spark units? Just popping a plug cap off and putting it on a spare plug, then cranking gives a nice blue spark now. Tomorrow, I can at least try it with a coil swap. Weather forecast shows three days of rain to start tomorrow evening. No way to test it in the basement garage. I'll be working on a fuel cell while it rains. Have several tripods that I can rig up to suspend one from.

Swapping coils gave no change.

Got a little more checked (or really, re-checked) today. Did the ignition test with screwdriver on the pickup coils - good hot spark on old plugs with the gap opened to ~ 3/16". Voltage test on spark units in spec. Buttoned that up and pulled the valve covers. checked the valve clearances again. Tightest is .003, loose one was .006 inch. Need a couple 295s to get any closer to the .004-.005 I'd like to have. Spent a lot of time getting RTV off of the valve cover from a PO putting it on the gasket. Now, I believe in tacking the gasket on the cover to make it more convenient, but not squishing RTV out everywhere. Anyway, cleaned that up and tacked the gasket to the cover with Indian Head.

Then, while digging out the RTV from the head I noticed something that I had not seen before. Right end intake cam holder is broken. This thing just keeps getting worse! Anyway, found a full set on eBay for $40 and ordered them.

Finished up my aux fuel cell. Presently using a 1/2 liter isopropyl bottle for the reservoir. Can go bigger if ever needed by just getting a bigger bottle to put the nipple in the bottom of. I like this one because I can just cap the bottle when I'm done with it and take it out to the shed, drain it in a tractor.

Re: CB750F rat bike FINALLY hitting on all 4!

A good test this afternoon. Backed her out of the garage and hooked up to the aux fuel cell. Fired as soon as I touched the start button, and came off of the choke a lot faster than before. Reved up fast, not lethargic as before. All pipes are getting warm (~50F outside today)! Didn't bother with balancing carbs yet. #3 float must be sticking as it was overflowing (I put clear drain lines on this time). But it is firing on all 4 finally!

Holiday duties will be keeping me busy soon, so when I bring it in (have to leave it outside to air out so the wife doesn't complain as much about the exhaust smell) it will be pushed aside for a while to make room for getting the Hunley prepped for a show mid-January. Guess it is time to order a chain for it, too. Still won't be able to ride it as the tires are too old to even ride up my driveway, but at least it can be taken through the gears on the center stand.

"L" cam holder is broken pretty badly. Could have been over-torqued, but I really suspect they just rammed in a too-long screw for the valve cover (only had a couple proper valve cover bolts when I bought it). Also, this is not the first holder to need replaced. As I looked at the others to see if they were damaged, I found a J in position for an F (far left end of intake cam). And yes, there is a J in the J position. Guess this one need to be Plastigage checked and also check oil hole alignment, too.

I was going to see what I could do to fix up the tail section on the seat as it seemed to have a lot of repair fiberglass under it. Turns out, almost the entire tail is packed with Bondo and fiberglass, which is all separating from the remaining ABS part which is the top part. Need to figure out what to do with it. I could just glue it back together with ABS cement, but it weights 3.4 pounds with all of this Bondo packed in it! Think I'll check forums and ebay for another. Cracked/broken doesn't matter as I can always glue with ABS cement and reinforce from below with fiberglass cloth, but all of the bottom/back of this one is missing.

A good bit of what I do at work is fixing the things "someone before me" found to be an acceptable repair. Ugh

Then keep digging deeper to repair the original problem properly.

Just like that tail...It would have been less work for them to fix it properly, than do all the shit work they did.

Now its up to you to completely un-fuck it.

Yup, we're in similar boats. The cam holders in my F aren't in great shape either, but yours take the cake. I hope that set you picked up from Ebay will fix the problem though.

When I bought my F up a PO had turned the bike into more of a K - different tank and different tail section. In fact I thought I'd bought a K until I looked at the plate on the frame and saw it was an F. Otherwise I'd've been more than happy to hook you up. If I come across anything through I'll let you know.

Have you guys seen my 750f engine? The PO turned also turned it into a K and left it in great shape.

F's into K's. Flavour of the month =)

Where are did all the F tanks go? I still need one to fit on a Suzuki........