Topic: Why would a stock '72 cb750 have #110 main jets?

[Hondawggie]

Sheesh yet more proof that people loved putting on a 4-into-1 back in the day and running the bike lean.

I have #40 pilot jets all the way across so I'm stock in that regard.
I also have the stock airbox and filter, no mods there.

So my Honda Factory Shop Manual says this about standard main jets:

1969-72:  #120 main
1973:  #105 main

I'm at sea level.  If ANYTHING my main jets should at a minimum be a #120, the stock main jet.  In reality at sea level here I would have expected the bike to have been delivered with a set of #130 mains from the dealer (perhaps).  Bike has under 5000 miles just out of break-in.

Then I might expect an even larger main jet if the Kerker installer (prior owner) had checked his plugs to see if he was lean.

I am inclined to buy a set of four #120's and four #130's.

Any idea why my under-5k-miles  '72 cb750 came with 110 main jets?

[Stev-o]

Someone changed 'em

[Hondawggie]

Someone changed 'em

I tried to reason that out and I came up with a 1970's stoned-out local Honda mechanic somewhere in Acapulco-Gold-rich Santa Cruz in 1973 saying "uh, oh wow man lets see, less backpressure means bigger jet, no wait (toke-toke 'ahhhh') less back pressure so less jet, I'll lower the jet size down, then I'll mount the kerker and throw out the stock pipes the owner said he didnt want 'em back   (toke-toke "screw posterity")'

[ekpent]

I don't think Pot was involved, at least on that, actually they are probably stock. Here is the jet timeline.
#120 used up to engine #2200000
#110 from 2200001-2304500
#105 2304501 and later
Put that in your pipe and smoke it   

[Terry in Australia]

Yep, I'm with Eric, and unless your bike was running like crap with them (and they are the actual cause of it running like crap) I'd leave them alone. Cheers, Terry.

[Hondawggie]

205043x  is my engine number.  My frame number is cb750-20426xx   The 22000xx engine started with the 1973 bike -- this is a 1972 k2 cb750.


Somebody did a shigga-moh-doo-hah on my snappy-doo on this one.   Them gott-dang 110's are outta here man.  I am 100% going to put the stock 120s in.


The bike has not run yet in my possession I bought it 12 days back it came with a Kerker so I *actually* may need 130s --

Has anyone out there found that these early 750s need a 130 main (sea level is where I live such as in Australia, and California) jet when running a 4-into-1 like a Kerker?

[Greggo]

Maybe someone swapped a whole set of carbs on without bothering to check/swap the jets?

[Hondawggie]

Maybe someone swapped a whole set of carbs on without bothering to check/swap the jets?

Thanks Greggo, would be plausible explanation.

 If this bike had higher mileage it would be a reasonable guess.   However this '72 has less than 5k miles on it and has no signs of any tinkering except the Kerker.

For those of you who have bought and worked on a lot of old bikes -- you know how you get used to seeing loose wiring harnesses due to the Honda wire-loom frame tie-downs gone missing, and missing rubber drain hoses from the bottom of the carbs, missing chain guards, and hardware-store hose clamps on the carb intake manifolds?

We get used to seeing signs of other tinkerers who came before us when we buy an old vintage machine.  We see things like allen bolts instead of the stock phillips screws on the engine cases, clear tubing and inline filters on the gas lines, poorly-routed throttle cables and missing stock airboxes.

This bike is like from the factory.  When I pulled the carbs, the 4 rubber drain tubes on the bottom of the carbs were routed in a way that looks like factory due to its neatness.  All the original Honda clamps were on the carb airbox boots and the carb intake manifolds.  All the Honda wireloom frame tiedowns are there.  The handlebars have the wide plastic wiring tiewraps too. The gas hoses are black and everywhere there is zero sign of tinkering.

After having time to think about these 110 main jets, one other guess I have is -- maybe this bike was originally *not* purchased at sea level here in California -- maybe it was set up in Denver the 'mile high city' or something and the dealer was in the habit of going with a smaller main jet due to the thin air. 

I was hoping someone would recommend, that running with the stock airbox on this '72 cb750, and the Kerker 4-into-1, and being at really thick air at sea level -- should I go with 130 mains?

Or just stick with the stock 120's?

[hoodellyhoo]

My understanding is the The early K2's with hm300 exhaust had 120 mains and the later ones with hm341 exhaust had the 110 mains. I just rebuilt the carbs on a K2 with 13k miles this summer. There were ZERO signs of previous tampering with the carbs and they had 110 mains so I believe you have the stock jets.

[Terry in Australia]

Yep, I'm guessing that they're the originals also, and I definitely wouldn't run 130's with the stock airbox, which works like a choke anyway, or it'll run like crap. I'm running 130's in my hotrod 836cc engine fitted with Tintop's "Anti pod" filter assembly, and it runs slightly rich, so 130's in your stocker with stock airbox would be awful. Before you waste your money on #120 main jets that you probably won't need, get the bike running and see if indeed it needs bigger than 110 mains. Cheers, Terry.

[seanbarney41]

yep...airbox/intake mods have a much greater affect on jet requirements than exhaust mods...it's even debatable whether a four into one is less back pressure than the hm300's, especially if it is baffled...

[Hinomaru]

Any idea why my under-5k-miles  '72 cb750 came with 110 main jets?

The correct main jet for your 1972 K2 as originally delivered from Honda depends on your carb casting numbers:

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

My Honda published shop manual also shows a 120 main jet for a 1972 K2. However, I have several Honda published CB750 parts books from the period that shows either a 110 or 120 main jet as standard for the K2. So it’s back to the casting numbers for a positive ID. Which carb casting number do you have?

[andy750]

Just for your reference my K2 (when it was still 750cc) with stock carbs/airbox and HM300s ran with 120 main jets. I actually went from 110-112.5-115 -116-120 to get it right. Good luck!

Andy

[Hondawggie]

Any idea why my under-5k-miles  '72 cb750 came with 110 main jets?

The correct main jet for your 1972 K2 as originally delivered from Honda depends on your carb casting numbers:

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

My Honda published shop manual also shows a 120 main jet for a 1972 K2. However, I have several Honda published CB750 parts books from the period that shows either a 110 or 120 main jet as standard for the K2. So it’s back to the casting numbers for a positive ID. Which carb casting number do you have?

Hi, thank you, that is informative -- at the front of the carbs is a flat spot and although I see writing after wiping off the dust I can't make it out -- the stamping pressure they used there was p_ss poor man.

What am I looking for -- what do your docs tell you the range of values is for a 110 main jet on these?

[Hinomaru]

What am I looking for -- what do your docs tell you the range of values is for a 110 main jet on these?

Well, I’m not showing a specific engine or frame serial number range that separates the fitment of 110 and 120 main jets to the K2 carbs. If you can somehow make out your hard-to-read carb casting number and it’s stamped 657A then it was originally equipped with 110 main jets.

[Hondawggie]

What am I looking for -- what do your docs tell you the range of values is for a 110 main jet on these?

Well, I’m not showing a specific engine or frame serial number range that separates the fitment of 110 and 120 main jets to the K2 carbs. If you can somehow make out your hard-to-read carb casting number and it’s stamped 657A then it was originally equipped with 110 main jets.

I really appreciate your effort here -- I don't know why some of the '72 cb750s had the 110s and others had the 120 main jets -- but it's reasonable that if Keihin had different series of carb numbers, maybe Keihin had a design-change and the carbs I have simply flowed more gas (or less air or both) and hence the need for a smaller main jet.

I wish the manuals I have showed that change, they only mention that 105 mains were used for the '73 and the manuals I have are not even consistent about that.  Based on what you found I'm going with the idea that the 110s came stock because Keihin altered the carbs somehow hence the different stamping numbers, then to wrap this thread up, I'm going to wait until she's back together to find out if the Kerker 4-into-1 is affecting performance.  Thanks for everyone's help on this.

[dave500]

get it running first.

[Hondawggie]

Did you pull the spark plugs to take a look before complaining, they may have a nice color-that'd be the normal thing to do. It could be the  PO knows better than you, maybe maybe not. The book is but a guide and if it were me I'd drive it and make a decision rather than ripping and tearing half cocked.

rt

Not sure why you characterize my surprise at a deviation from the Honda Factory Shop Manual as "complaining" that's a cut low there son.   I'm reviving the bike and the manual says 120 mains.  I am keen to set her right from the get-go.

Unless you're trying to tell me "Hey if I bought a used bike that had a non-stock 4-into-1 and main jets in the carbs that were too small as per the Honda factory manual, I would never question it."

I think when the factory manual says 120 mains, man you EXPECT to see 120 mains -- kapiche?

[Rgconner]

From "Thoughts of Hondaman" in FAQ

4. Jetting. ALL of the inline fours came rich from the factory. At sea level, we started with a 10 size less than what it came with. Here in Colorado, it's not unusual to drop a 30 size main jet. The jet needles are almost always set too high (except the 500/550, too low). The cutaways on the slides were close for sea level, too low for high altitude by about .030". My 750K2 runs at 20 less than stock for general cross-country touring.

So there is one possible explanation.

[Hondawggie]

From "Thoughts of Hondaman" in FAQ

4. Jetting. ALL of the inline fours came rich from the factory. At sea level, we started with a 10 size less than what it came with. Here in Colorado, it's not unusual to drop a 30 size main jet. The jet needles are almost always set too high (except the 500/550, too low). The cutaways on the slides were close for sea level, too low for high altitude by about .030". My 750K2 runs at 20 less than stock for general cross-country touring.

So there is one possible explanation.

Man, this is a ton of great info, thanks Rg, that explains a lot.  I'm at sea level, maybe that's why they didn't drop the mains from a 120 down 30 pips to a 90 main.   Today I'm reassembling the carbs and am going to go with the 110s in there based on your and Hinomaru's VERY helpful findings  -- thanks dude and thanks to everyone for helping out on this.

[ekpent]

Just recently picked up two 1972 cb750's this week and have knocked the carbs off for cleaning. Will report engine serial numbers and jet size next time I go to the shop.  I know pics or it didn't happen 

[lucky]

Well what happened??

[KC_Northstar]

Why would a stock '72 cb750 have #110 main jets?
well for the most part that's what most of them came with from the factory.
The very late K1's were dropped down to 115's to stop the plug fouling problem
and most K2's because of the more restrictive HM341 exhaust ran 110's
My totally stock K2 had 110's  If its a totally stock K2 110's are probably your
best bet.  120's will more than likely foul your plugs.

[MCRider]

I didn't follow the whole thread. I would just caution Hondawiggie and others to not get too hung up on the model year. At that time in the Honda production, changes were made during the calendar year, and during the "K" runs, for various reasons. So the VIN for frames and the Engine number for engines, rules, usually.

And regardless of what the Honda Shop Manual says, the parts book (microfiche) rules as to part changes and modifications.

Honda didn't correlate model changes to calendar years until 1974 or so Due to losing a lawsuit. Its only a coincidence that with the CB750 the K numbers run close to model years. They did not with other models. All attempts to assign model years prior to that is a USA convention.

This from an old Honda parts man.

[Retro Rocket]

I think when the factory manual says 120 mains, man you EXPECT to see 120 mains -- kapiche?

You need to read all the posts dude, it isn't unusual to see 110's in a K2, i have a mate with one that has 110's. There are a few differences in reality that never made it into the manual, listen to the guys that know....