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1992 CB 750, tackling the carbs

AlwaysAdar

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Picked up a 1992 CB750 about two weeks ago. 1-owner bike that had been traded to a tradesman in exchange for work had kept it in a shed for a couple of months. Carbs needing work were discussed at the time of purchase.

I’m familiar with doing some basic vehicle maintenance. Came into this figuring I’d tackle some repair/task on every other weekend.
Something to keep me busy, learning new things, and an excuse to build up my tools.
I have digital copies of the common service, model-specific, and Cyclepedia (got it for $2.50) manuals.
Regardless of my efforts, I will have a mechanic look over before I do any riding (which will make the Miss a bit happier).

The bike starts with some choke but dies when throttled unless warmed up.
Ran some sea foam through it. Kind of helped, but still dies.
The bike recently stopped running, but it seemed to be a battery problem. The battery is from 2018; I put a tender on it for now but plan on switching it out when it’s closer to rideability.

I’ve never worked on carbs.
A problem I encountered was that there seemed to be so much other work that would be optimal to do while cleaning the carbs since the tank would be off. Seemed like a good time to change the throttle cable, rubber manifolds, fuel lines, and spark plugs, along with rebuilding the petcock and synching the carbs since I’m already taking off the gas tank.

Also, got stumped about what carb rebuild kit to buy. I’m kind of assuming the kits that seem affordable though busy with all included parts, are probably of a lower quality.
I definitely need/want to tackle the carbs. I think I just need to get them off and cleaned first; then I’ll have a better idea of what I need. I can always take the gas tank off again.

Suppose the carbs sit off the bike for a week or two while I figure things out. Not a big deal.
I've so far done an oil change. I'm cleaning the battery terminals to add pigtails and replacing the air filter.

I was going to get an ultrasonic cleaner, some carb cleaner,
Any advice on figuring out what to tackle, in which order, and what to buy?
 
I wouldn't bother running the bike anymore until the carbs are rebuilt (I use all ballz racing). Rubber boots are a good idea but only if they are truly cracked and need it. I would also flush the brake system. Definitely going to want to change that battery, I think. Also wouldn't hurt to look at the chain (clean and lube) and the battery ground at the frame.
 
To remove the carbs, I identified that just a basic tool set would be needed.

I’m going to pickup an ultrasonic cleaner, there are a couple of other things I could use it for.

I’ll look into the ballz racing.
General advice seemed to be on keeping the original jets.

Any recommendation for switching out the tubing or getting a synchronizer?
 
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What tubing do you mean?
The carb sync gauges are largely the same but they are far from precision tools. Much easier to use a 4-gauge than a single or double.
 
the great thing about the nighthawk is a can have that fuel tank removed within 5 minutes.
the factory jetting is really lean. i put a stage 2 jet kit in mine and even with the stock exhaust and air filter it runs so much better. it now has 38 pilots, 118 mains and a shim under the needle.
 
The carb sync gauges are largely the same but they are far from precision tools. Much easier to use a 4-gauge than a single or doubl
Going to probably pick up a 4-gauge sync.

Looking at a vevor ultrasonic cleaner along with the ballz carb kit.
Read about using silicone grease, oil of winter green, or armor all for the rubber manifolds.

Meant the ?rubber? fuel tubing used with the petcock and carbs. Sorry, not sure of the material.
 
rubber today is so much better than it was back then. i still have the original radiator hoses on my 2000 ford..
That’s what I was thinking. Some parts may have been better built but material specific parts (like tubing) had greater advances.

Are there any specific brands or materials I should look out for?
 
no i just worked at the ford dealer since the eighties and i remember we were putting belts and hoses every 30k. now they are worry free. i think any good fuel hose from a parts store will be great. economy cycle has good fuel hose. make sure you use a good vacuum hose to your petcock so you don't end up like me on the side of the road with a crack in the hose so no vacuum no fuel.
 
That’s what I was thinking. Some parts may have been better built but material specific parts (like tubing) had greater advances.

Are there any specific brands or materials I should look out for?
When they rebuilt my carbs (before I bought the bike) the shop used the Randaak kit. From what I understand the rubber parts in that kit are made of alcohol resistant materials more suited to today's fuels. I do know for sure that the instruction booklet that came with the kit is pretty comprehensive and easy to follow.
 
Unfortunately, Randaak doesn't seem to make a carb rebuild kit for my bike (92 CB750). Had looked into them, the brand seemed reputable!
 
How did you go about finding the sizing for the fuel & vacuum lines along with the joints? I'm looking at changing out the rubber and other worn-down pieces as I'll be reassembling after cleaning.

Getting mixed results with search filters from sites like ebay, amazon, and revzilla.
Some research shows the fuel line to be 3/16 or 5.5mm. Some people went with 1/4 and clamped it with varying success.
 
How did you go about finding the sizing for the fuel & vacuum lines along with the joints? I'm looking at changing out the rubber and other worn-down pieces as I'll be reassembling after cleaning.

Getting mixed results with search filters from sites like ebay, amazon, and revzilla.
Some research shows the fuel line to be 3/16 or 5.5mm. Some people went with 1/4 and clamped it with varying success.
Going up a 16th of an inch will have almost zero impact.
 
How did you go about finding the sizing for the fuel & vacuum lines along with the joints? I'm looking at changing out the rubber and other worn-down pieces as I'll be reassembling after cleaning.

Getting mixed results with search filters from sites like ebay, amazon, and revzilla.
Some research shows the fuel line to be 3/16 or 5.5mm. Some people went with 1/4 and clamped it with varying success.
I bought the rubber seals for the carbs from IMPEX. The o-rings for pipes and T-joint were in the kits. As I said the carbs were easy to clean and it was just the float bowl gaskets, o-rings, and float valves that needed replacing. The vacuum line is readily available at at any rubber store ( it’s the same as my ‘69 Beetle vacuum advance line actually! ) I didn’t need to change the fuel line, just chop about 20mm off to get a nice flexible end, but I do have some eebbay 8mm line which would fit if I needed it.
 

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