Topic: CB750K4 what did I get myself into?

[BrockCB750]

I know on car throttle bodies there are sometimes coatings and you need to be careful about what chemicals you use to clean them to avoid stripping that coating.

Is that the case with the CB750k carbs? As far as cleaning products/chemicals anything I should stay away from?

[calj737]

That might not be corrosion in the boots.
Someone might have used white lithium grease to get the carbs on last. Put some brake cleaner on a rag and see it it comes off.

+1

[denward17]

I don't think these carbs has any kind of coating on the inside.

I used a mixture (in the ultrasonic tank) of Aircraft type Simple Green and Dawn dishwashing detergent.  Worked pretty well to clean.

[PeWe]

WD-40 spray is good too for carbs.

The carbs need to be cleaned in an ultrasonic cleaner.

I feel the difference with my CB750 K2 that recently got another set of K6 carbs I bought in pieces. Bodies ultrasonic cleaned according to the seller 3 years ago.

The old set, also K6 carbs worked fine in the 80's. I cleaned them with carb cleaner, naphta and vinegar 2018 when building the bike.

Not an ultrasonic clean though.
I could not fully solve a light hesitation at take offs.

The "newer" set works fine, no hesitations at all  and work generally better.

Carbs clearly showed that bigger main jets was needed at higher speeds and much appreciated by the engine. They seems to breath better thru all holes

My old set need a bath in my 30L ultrasonic cleaner.

[BrockCB750]

Thanks for the advice on the carbs. Sounds like I can't do much to harm them. I will be getting an ultrasonic cleaner but for now going to take them apart and order the rebuild kits.

I also pulled a bunch more stuff off the bike this morning before it got too hot out. Brake lever, foot rests, spark plug wires and other electronics, tool tray, etc.

I also tried to break loose some of the screws on the carbs. Most were not a problem but two are giving me trouble. After two rounds of PB blaster and heat they aren't budging yet.

Also, the tach cable from engine to gauge is probably going to give me some trouble. Having a hard time describing this but the screw head is really shallow where the driver mates to it. It looks like its all but certainly going to strip. A couple rounds of heat and PB blaster will probably do a couple more. Going to attack both with the impact driver if I can't get them to break loose with moderate preassure. I am using JIS screw bits/drivers. Just worried about even attempting that one on the engine.

The carb screw that aren't budging:


The engine screw that scares me:

[newday777]

Those screws are no problem if you are using a hand impact driver and the correct bit.

[PeWe]

Avoid cheap rebuild kits jets.
Needles usually way wrong taper, stock most likely OK.

I use all stock jets except for main jet, bigger Keihin main jet when K&N filter and Yamiya no number 4-4 breath much better and need more fuel.

My K6 carbs have nothing from a rebuild kit.
The old rubber gaskets, float valves.

New o-rings for fuel T connections, bought together with other o-rings for engine in 25 or 50 pack.

[BrockCB750]

I was looking at the rebuild kits on 4 into 1. I am not looking to get the cheapest parts so if there is a better source please let me know. The amount of effort I am going to have to put into this build that would be silly to go cheap. I am not going to buy anything until I actually take apart the carbs fully but given the condition of the rest of the bike I am thinking they will need a full rebuild.

Jets is a discussion for later I guess. I am sticking with the airbox with at least new rubbers for the airbox and likely engine side as well given how hard they are. Going to bite the bullet and get the Honda parts. I am willing to pay for something like the Cognito breadbox but I am just not confident it resolves the problems with flat spots and tuning the jets.

Speaking of the headers. I pulled off terrible condition originals. I was going to toss them but wanted to make sure that they aren't something someone would want? They have rust holes... Very rough.

[RAFster122s]

Delete 4 into 1 from your address book if you want to avoid cheap or poor fitting or poor quality parts. Get complete points a d condenser plate from South Sound Honda of Olympia, Washington, you will spend less than buying points and condensers separately and you will have both points and condensers that fit and work.
South Sound discounts part prices on OEM partss and charges lowest shipping rates I have seen...

[newday777]

Speaking of the headers. I pulled off terrible condition originals. I was going to toss them but wanted to make sure that they aren't something someone would want? They have rust holes... Very rough.

Well someone might want them.
I made up some patches from flat stock 040 steel and having a friend tig them on a set of Swiss cheese mufflers on my other friend's original K4 that he bought new and doesn't want new repops at this point as they won't match the patina of his faded paint tank. I'll spray the patches with silver high temp manifold paint. It'll get him by for now better than the fiberglass reinforced bondo he used before and was going to do again......
Most are on the backside or on the bottom so they won't be seen much.
I used an old driveshaft that is 3" diameter and my vise to form the radius with a plastic deadblow hammer and a ballpeen hammer to finalize the fit. A 4" grinder did the cutting along with tin snips.

[BrockCB750]

RAFster122s that is exactly the kind of suggestion I was hoping for. I figured the rebuild kits were good to go since those show up everywhere.

It looks like they are the cheapest as well for OEM parts. Seriously, thank you for saving me from possibly getting something that doesn't fit correctly.

New rubber/insulators, rebuild kits, floats, dust cover caps... that added up FAST. haha

[BrockCB750]

Forgot to mention, the impact driver did the job on both the carb screws and the other one for the tach cable. That screw was damaged before I got to it and dangerously close to stripping out. I am adding a new one to the parts list. So thankfully broken fastener count is still at 1.

I am pretty scared I am going to break off another screw/bolt so I am avoiding using excessive force wherever I can. I probably don't need to worry about it as much as I am.

Oh also, carbs pissed old brown liquid (fuel?) all over me. So that was fun.

Next step is to take carbs fully apart, strip the spraypaint overspray, clean up the rust, buy an ultrasonic cleaner and order parts.

Anyone know if the rubber dust caps on top of each carb is necessary? Looking around at pictures a lot of bikes aren't running with them on. Would save ~$50 if I can avoid getting replacements. Mine are completely deteriorated.

[PeWe]

I have tried 4 into 1 pilot jets.
I tested one size bigger, 42.5.  Crap from China?

The jets were bigger on the outside too.
2 of them broke inside carbs. The thin part with holes.
I had to remove carbs and drill the brass debrise out with a 2.0 mm drill.

I drilled all 4 just for sure to make them equal. Very small amount of alu removed too, like hone oxidation away.
Back with stock Keihin 40

I have tried aftermarket pilot jets in my Mikuni smoothbores and TMR that worked as good as stock. Lower price similar qiality.

[Stev-o]

Anyone know if the rubber dust caps on top of each carb is necessary?

I dont have them on either of my 750's. 

[PeWe]

There is an old thread were one guy found an alternative.
I ordered a bag from eBay China after his recommendation. 100pcs.

Look good and work fine.
Keep dust outside.

Search:
4mm-Inner-Dia-Double-Side-Rubber-Grommets

[BrockCB750]

Got the carbs completely disassembled. Outside of playing a game of hide and seek with four springs that shot out of there it went well.

Lots of brown fuel, grit and mush in there. Leave for a wedding Wednesday so probably won't make more progress until next week.

[BrockCB750]

Okay back at it. Finished cleaning up the carb bodies last night. Each still had remanants of wasp nests in them. After several runs through the ultrasonic:

[BenelliSEI]

Nice work….. what are you using for liquid in the ultrasonic tank?

[HondaMan]

I could not fully solve a light hesitation at take offs.

Here's something you might try with those hesitant carbs, Per: change the air screws. I've found that with some cams the holey-tipped ones worked better, and sometimes vice-versa. When the "7A" series carbs came out, Honda went back to the holey-tipped air screws that had disappeared during the K4 series bikes. The K4 required the idle be raised to 1150-1200 RPM because taking off from a stop could actually stall the engine: this led then to the slipper-plate clutch business that we saw appear then to let the engine slip slightly more. It would have been cheaper for them to leave the air screws alone!

[BrockCB750]

Worked on the float bowls last night. Covered in brown sludge. Got them clean enough that I think I can toss them into the ultrasonic tonight. Those were really gross.

[MRieck]

I have been using Simple Green Aircraft Cleaner for about 4 years now. Great product that is non corrosive.

https://www.cleanitsupply.com/p-460795/l-cs7pp/simple-green-extreme-precision-cleaner-1gal-clear-smp13406.aspx?gclid=Cj0KCQjwldKmBhCCARIsAP-0rfwlZcwIYrDaJv_ubN6JD28kxtHZ4oA3ZPLKvu5TXyT2QY_i-24UpJEaApwDEALw_wcB

[PeWe]

I have been using Simple Green Aircraft Cleaner for about 4 years now. Great product that is non corrosive.

https://www.cleanitsupply.com/p-460795/l-cs7pp/simple-green-extreme-precision-cleaner-1gal-clear-smp13406.aspx?gclid=Cj0KCQjwldKmBhCCARIsAP-0rfwlZcwIYrDaJv_ubN6JD28kxtHZ4oA3ZPLKvu5TXyT2QY_i-24UpJEaApwDEALw_wcB

Which mix with water?
1-3 or stronger/weaker?

[MRieck]

I have been using Simple Green Aircraft Cleaner for about 4 years now. Great product that is non corrosive.

https://www.cleanitsupply.com/p-460795/l-cs7pp/simple-green-extreme-precision-cleaner-1gal-clear-smp13406.aspx?gclid=Cj0KCQjwldKmBhCCARIsAP-0rfwlZcwIYrDaJv_ubN6JD28kxtHZ4oA3ZPLKvu5TXyT2QY_i-24UpJEaApwDEALw_wcB

Which mix with water?
1-3 or stronger/weaker?

Mix with water Per....ratios are printed on the bottle. You can make a very strong solution and it will not harm aluminum. I use it a lot to degrease heads in my US unit and it does a fantastic job.

[BenelliSEI]

I have been using Simple Green Aircraft Cleaner for about 4 years now. Great product that is non corrosive.

https://www.cleanitsupply.com/p-460795/l-cs7pp/simple-green-extreme-precision-cleaner-1gal-clear-smp13406.aspx?gclid=Cj0KCQjwldKmBhCCARIsAP-0rfwlZcwIYrDaJv_ubN6JD28kxtHZ4oA3ZPLKvu5TXyT2QY_i-24UpJEaApwDEALw_wcB

Which mix with water?
1-3 or stronger/weaker?

Mix with water Per....ratios are printed on the bottle. You can make a very strong solution and it will not harm aluminum. I use it a lot to degrease heads in my US unit and it does a fantastic job.

Thanks! I just use Simple Green Concentrate (with water) but need to watch out with aluminium. Too long and it starts to black. I’ll try that.

[RAFster122s]

The Simple Green purple Industrial cleaner available from Home Depot is aluminum safe in concentrations of up to 50% water to S.G.I. and it also does a nice job cleaning in an ultrasonic cleaner. This Aircraft version looks to be cheaper possibly... haven't priced the HD version lately.
Simple Green green formula got banned by DoD as it was found to cause corrosion of aluminum and was affecting composites leading to delamination issues. So, it got banned from base.  They had been using it a lot prior to that ...

David