Topic: Why are there holes in my carbs vacuum port plugs? (750K4)

[curtisound]

I'm putting together a basket case 1974 CB750K4. One of the few things the PO did before giving up on it was to rebuild the carbs with a cheap kit. The new vacuum port plugs installed have holes in the center of them (they look like jets). However, reference pics of original condition Keihin carbs show the screw in plugs with no holes in them which to me makes sense.

I've looked at pics online of rebuild kits for these carbs and the ones I've seen all have holes in the vacuum port plugs? How is this a good thing? I ask because after syncing the carbs and doing a first test ride the bike bogs horribly off the like like it's supper lean which I'm guessing is because of a vacuum leak. Should I just fill these holes or do they serve a purpose?

-Curtis

[grcamna2]

I'm putting together a basket case 1974 CB750K4. One of the few things the PO did before giving up on it was to rebuild the carbs with a cheap kit. The new vacuum port plugs installed have holes in the center of them (they look like jets). However, reference pics of original condition Keihin carbs show the screw in plugs with no holes in them which to me makes sense.

I've looked at pics online of rebuild kits for these carbs and the ones I've seen all have holes in the vacuum port plugs? How is this a good thing? I ask because after syncing the carbs and doing a first test ride the bike bogs horribly off the like like it's supper lean which I'm guessing is because of a vacuum leak. Should I just fill these holes or do they serve a purpose?

-Curtis

Hi Curtis,
That doesn't seem like it should have holes in the synch plugs,no way. Any pics ?

[robvangulik]

Maybe they ARE jets? Both plugs and (main) jets have m5 threads, maybe he didn't have any m5 screws in stock and thought those small holes wouldn't make a difference?
The way some PO's think is unfathomable

[HondaMan]

They are probably jets! Are they brass?
Put some JB Weld in them and let it set up overnight. That will make it hard to idle smoothly, for sure. It will also make dark sparkplugs when riding in town. I once got a bike for tuneup that sounded weird, especially when starting from a stop (there is even a video of me riding it away from the 9 News reporters here who were talking to me about it, in which you can hear the sound) - it was missing 2 of those screws altogether. It fouled those 2 sparkplugs in 9 miles of riding it to the license bureau and back.

[curtisound]

Thanks for the replies. The "jet" doesn't have any size markings on it so I thought I was ignorant of some special purpose of this vacuum plug with a hole in it. Nothing should surprise me about this basket case though.

[Pielz]

Definitely looks like a cheap chinese jet! Lol since it doesn't have any markings

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[david 750f]

Does the bike run? Wondering if the vacuum port screws are being used as main jets...

[ekpent]

 Ha ha -what some people do is amazing ! I just took a front tire off a GS1000 and one side of the fender had no bolts at all holding it to the fork    Not a very good brace.
   Your bike will run much better soon when those are gone or plugged. Better look in those carbs to see what else he did and check the bike over well like valves,timing and all that other good stuff.

[ckahleer]

Maybe he was having a too rich problem and this was an attempt to lean it out.

[Alan F.]

Does the bike run? Wondering if the vacuum port screws are being used as main jets...

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Ha ha -what some people do is amazing ! I just took a front tire off a GS1000 and one side of the fender had no bolts at all holding it to the fork    Not a very good brace.
   Your bike will run much better soon when those are gone or plugged. Better look in those carbs to see what else he did and check the bike over well like valves,timing and all that other good stuff.

Yes, be very sure to check out your front and rear brakes, as well as your electrical system especially your fuses.

[ekpent]

Does the bike run? Wondering if the vacuum port screws are being used as main jets...

Like! Button

Ha ha -what some people do is amazing ! I just took a front tire off a GS1000 and one side of the fender had no bolts at all holding it to the fork    Not a very good brace.
   Your bike will run much better soon when those are gone or plugged. Better look in those carbs to see what else he did and check the bike over well like valves,timing and all that other good stuff.

Yes, be very sure to check out your front and rear brakes, as well as your electrical system especially your fuses.

Doesn't everyone carry around a .22 shell in their pocket in case a fues blows out ?? 

[curtisound]

Does the bike run? Wondering if the vacuum port screws are being used as main jets...

haha, yeah it ran just supper lean. My first test ride felt like I was starting off in 3rd gear. Engine bogged horribly. I've since pulled the carbs again and checked the main jet situation. Three are keihin 125 and one is an off brand 110  Apparently my 750K4 carbs stock size is a 105 (657B stamps on carb bodies)  though the bike only came with pods in it's buckets of parts so I guess ill start with some 115's

Ha ha -what some people do is amazing ! I just took a front tire off a GS1000 and one side of the fender had no bolts at all holding it to the fork    Not a very good brace.
   Your bike will run much better soon when those are gone or plugged. Better look in those carbs to see what else he did and check the bike over well like valves,timing and all that other good stuff.

I didn't pay much attention to the carbs because they were vapor blasted and rebuilt according to the PO and since they looked good I just set them aside while I took on the rest of the bike. The rest of the bike had many issues and I have replaced a sickening amount of parts including the engine case. This is my first classic Honda (Ive owned several early BMW airheads) and when picking up this basket case I didn't look under the output shaft chain cover. When I got the bike home and got around to pulling the engine to paint the frame I found a piece of flat steel JB welded to the case where I'm assuming at some point the chain broke and punched a hole in the case 

Anyway, after rebuilding the engine bottom to top, and literally having every piece of this bike apart I don't know what else could go wrong (though I'm sure there's something). 

[ekpent]

 Nice that you are putting in the money,time and effort to save another one. You will be an expert by the time you are done. An option is to buy a cheap stock airbox for it as there are still a lot of them around if it fits the look and style your going for.

[HondaMan]

Does the bike run? Wondering if the vacuum port screws are being used as main jets...

haha, yeah it ran just supper lean. My first test ride felt like I was starting off in 3rd gear. Engine bogged horribly. I've since pulled the carbs again and checked the main jet situation. Three are keihin 125 and one is an off brand 110  Apparently my 750K4 carbs stock size is a 105 (657B stamps on carb bodies)  though the bike only came with pods in it's buckets of parts so I guess ill start with some 115's

Ha ha -what some people do is amazing ! I just took a front tire off a GS1000 and one side of the fender had no bolts at all holding it to the fork    Not a very good brace.
   Your bike will run much better soon when those are gone or plugged. Better look in those carbs to see what else he did and check the bike over well like valves,timing and all that other good stuff.

I didn't pay much attention to the carbs because they were vapor blasted and rebuilt according to the PO and since they looked good I just set them aside while I took on the rest of the bike. The rest of the bike had many issues and I have replaced a sickening amount of parts including the engine case. This is my first classic Honda (Ive owned several early BMW airheads) and when picking up this basket case I didn't look under the output shaft chain cover. When I got the bike home and got around to pulling the engine to paint the frame I found a piece of flat steel JB welded to the case where I'm assuming at some point the chain broke and punched a hole in the case 

Anyway, after rebuilding the engine bottom to top, and literally having every piece of this bike apart I don't know what else could go wrong (though I'm sure there's something).

Those 657B carbs should have #105 Keihin jets or #110 Keyster (even #115 Keyster). The ideal Keyster would be a #112.5 . Idle air screws at 7/8 to 1 turn out.

[curtisound]

 

Those 657B carbs should have #105 Keihin jets or #110 Keyster (even #115 Keyster). The ideal Keyster would be a #112.5 . Idle air screws at 7/8 to 1 turn out.

Thanks HondaMan. Yeah, Keihin #105 jets are what I saw listed as stock on the Honda Choppers site. However, I only have pods in my bucket of parts so I'm going to have to try and make those work for now (I'm keeping an eye out for a decent airbox). I was thinking of starting off with Keihin #115? does that sound right, at least as a starting point?

[grcamna2]

Definitely looks like a cheap chinese jet! Lol since it doesn't have any markings

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Could be they're #115 main jets and you can use them Inside the carbs.,rather than out there,Lol !
That's a 'First' for me. 
I have a Honda jet cleaner tool which has metered size rods all corresponding to each jet size:from #35 pilot jet,all the way up to #110;you could possibly use a known gauge wire size to see what size those 4) outer jets really are when compared to a #110 main jet.

[curtisound]

Thanks for the tip! I used a cheap set of carb cleaning pics I have that don't have size markings like yours. Though, using the different size picks as feeler gauges I was able to determine the jet "plugs" were smaller than the #125 mains installed, and larger than the single #110 installed. So I swapped out the installed jets for the matching cheap ones and installed some 5mm screws with fiber washers (thanks local true value) into the vacuum ports. Now it's running pretty good for just a quick carb sync on a cold engine!

[grcamna2]

Thanks for the tip! I used a cheap set of carb cleaning pics I have that don't have size markings like yours. Though, using the different size picks as feeler gauges I was able to determine the jet "plugs" were smaller than the #125 mains installed, and larger than the single #110 installed. So I swapped out the installed jets for the matching cheap ones and installed some 5mm screws with fiber washers (thanks local true value) into the vacuum ports. Now it's running pretty good for just a quick carb sync on a cold engine!

Lol  Nice !  They just needed to move down inside the carbs, Lol

[Maraakate]

I know you have already found something to plug those holes, but if this was an emergency and couldn't find something that fit locally I believe you can scuff the jet hole lightly with some sandpaper/scotchbrite and then use solder and flux to plug the holes on that jet.  That may have been enough to at least plug the hole so the bike is rideable until you can find something better.

[BoneDigger]

I ordered a rebuild kit that came with the same plugs with holes.  When I put them in, you could indeed feel back pressure from these plugs when the engine was running.  I'm sure it was causing a lean reaction, so I replaced them with the plugs from my old set.  Guess there are some Chinese rebuild kits out there with these, so watch out.

[grcamna2]

Those brass parts Are meant to be carburetor jets.. Lol 

[ekpent]

Those brass parts Are meant to be carburetor jets.. Lol 

Do we have a two-fer ?? 

[grcamna2]

Those brass parts Are meant to be carburetor jets.. Lol 

Do we have a two-fer ?? 

LOL