cb750 Carb mystery

keeleydavis

keeleydavis

Been Around the Block
I bought a 76 cb750 with pods already mounted and 115 size main jets. It always ran great in the morning and then once it warmed up at the end of my commute and also on my ride back home at the end of the day, it would run a bit crappy. If I held the throttle at a cruising speed and then gave it a little gas it would sputter. It runs great and smooth at high rpm. Its mainly in that lower rpm cruise and if I give it a little gas after cruising at any speed.

I put the factory air box (new seal and the filter is clean) back on and replace the 115 mainjets with 105s. Still feels the same. Any thoughts? Idles fine.
 
I'm having a pretty similar problem with my 350. Idles great and runs smooth all over except for the 4000 to 4500 rpm range at which point it sputters pretty badly. Does it in every gear. I'd also be interested if anybody had an suggestions.
 
I dont have much experience with that era of CB750's but 4-5000 rpm is at the top of the range for running on the pilot jet and transitioning onto the needle jet so maybe try the needle clip on the next lower ring or put a shim under the clip to raise the needle and allow more fuel to flow in that range. Sounds like the main jet is ok if it runs well at high rpm's.
 
Sounds like it's time for a set of FCR's ::) I can't help you with the jetting situation, I had a DOHC 750, but don't anymore, and never had to mess with the jets. But I'm sure someone with a SOHC will chime in sometime soon and probably give you way more advice than you ever planned.
 
If it runs well cold and then gets worse as you heat it up, its likely too rich. The idea is that, when the engine is cold, the fuel doesn't stay in the air as well (drops out) and the effective air/fuel is lower (leaner). As you heat the bike up, the air heats up and holds the fuel better. That being said, there are a lot of things that could be an issue. Its often said that you should check your ignition points and valve clearance before you make carb changes. After you're done that, start dicking with your carb.

I subscribe to the school of thought that suggests working from the main jet down. So, Main Jet, Needle Position, Pilot, Float. If are riding when its warm and you are at 4k rpm and you open the throttle fully, what happens? If its not right, change the main until it is. Then adjust your needle for mid-way throttle positions (50-75% throttle opening) and then finally adjust your pilot jets. If you need help, PM me!

Darryl
 
I took it for a longer ride today. It seems to run great at high rpm and crappy in 2-3k realm. Its def running rich cause I looked at the plugs and they were black sooty. Ill check the ignition points later. hmmm
 
keeleydavis... I didn't caught an age on the bike or carbs... on old Honda carbs and CB750's in specific... folks forget that they "buzz" and the wear is evident on the needle jets... check the shank (light scalloping... on one or both sides) and bore (oval distortion wear... may need a magnifier)... this wear can play havoc on tune... at specific RPM ranges... do all the other checks and verify... then consider this also... best of luck...
 
Thanks much. The bike is a 1976 cb750 and I'm pretty sure it's the original carbs. I just got back from carb syncing the bike but still it's lugging at low throttle, but mostly when it's hot. Would this have anything to do with ignition coils?

Also would a 4-1 muffler change the stock settings in the carb? Right now I am one full turn out on the air screws.
 
keeleydavis... How'd the sync look... even between all four? The coils could be weak (which might explain the spark plugs)... but I've run some pretty battered coils without issue... As to your question on the exhaust change.... yes... both intake (ie; open filters) and exhaust (increased flow) impact the carb settings... How free-flowing is the 4-1... I have run BUB on my 750's with great results... different units cause different needs.... Do you have a manual on the bike? It's a great help as a starting point... the air screw setting seems off (but it has been a while since I tuned one last)... I use the old fashion method for initial tune... warm the engine up to operating temp... set to 1K RPM... and tweet the airs up/down from a base setting (that's where the manual comes in)... typically 1 to 1-1/4 out from set (and did you check the air screw tips? If they are worn... bent, grooved, etc. you'll be chasing your tail)... to set middle of RPM lag/increase... There's quite a few other things to look into... but this may give you a little more of a start.. also... how many miles are on the "clock"? Age and mileage can reek havoc on old Nippon rubber bits... again I hope this can help...
 
captainvr said:
keeleydavis... I didn't caught an age on the bike or carbs... on old Honda carbs and CB750's in specific... folks forget that they "buzz" and the wear is evident on the needle jets... check the shank (light scalloping... on one or both sides) and bore (oval distortion wear... may need a magnifier)... this wear can play havoc on tune... at specific RPM ranges... do all the other checks and verify... then consider this also... best of luck...
Click to expand...

often over looked and very good advice

http://www.factorypro.com/tech/needle_jet_wear.html

~kop
 
captainvr said:
keeleydavis... How'd the sync look... even between all four? The coils could be weak (which might explain the spark plugs)... but I've run some pretty battered coils without issue... As to your question on the exhaust change.... yes... both intake (ie; open filters) and exhaust (increased flow) impact the carb settings... How free-flowing is the 4-1... I have run BUB on my 750's with great results... different units cause different needs.... Do you have a manual on the bike? It's a great help as a starting point... the air screw setting seems off (but it has been a while since I tuned one last)... I use the old fashion method for initial tune... warm the engine up to operating temp... set to 1K RPM... and tweet the airs up/down from a base setting (that's where the manual comes in)... typically 1 to 1-1/4 out from set (and did you check the air screw tips? If they are worn... bent, grooved, etc. you'll be chasing your tail)... to set middle of RPM lag/increase... There's quite a few other things to look into... but this may give you a little more of a start.. also... how many miles are on the "clock"? Age and mileage can reek havoc on old Nippon rubber bits... again I hope this can help...
Click to expand...

What exactly is a bub? Yes I do have a manual and it said to start with air screw 1 turn out and then sync carbs. I haven't really played with the airscrews. I got the sync pretty good between all four. When I first looked at it , the left two carbs were running a little lower than the right two. If I adjust the air screws now will it throw off my carb sync?
 
keeleydavis.... Let me start over... as I said... I'm 'Ole Skool... I use the KISS system of trouble-shooting (Keep It Simply Stupid... me... not you)... learned it in the military. First... a BUB is a moderately priced 4-1 exhaust system (header and muffler with good flow, back pressure and sound)... so... is yours a Kerker or some other after-market system (it helps to know). Second... you've gone back to a basically stock air box (the stock '76 unit is somewhat restrictive and the inlet rubber sometimes doesn't seal well... I usually retro-fit the earlier '72 style box... not as restricted and filter changes are simpler...). Third... you said that you went back to the stock 105 main... now more questions... did you clean the carbs (and if so how did they look? Current day fuels are nasty when left to sit just a short time). If so... did you verify all the air/fuel passages (either by air or fluid cleaning) are clear of debris/gum. A little side step... be absolutely sure that ALL passages are clean... including the jets (main orifices and bleeds... a carb cleaning wire tool kit is best). Prior to any of this... I check for intake leaks in the rubber parts (WD-40 works well... if the idle "bogs"... replace the rubber first and move on). OK... too much I know... but I'll lay it out as a step by step...
1) Valves are set... did you check/adjust the cam idler?
2) Ignition timing set... static or dynamic?
3) Bring bike to operating temp and check for intake air leaks... all... air box inlets to carbs, carb tops, and carbs to head.
4) Pull carbs and clean/inspect... critical step (a partially clogged pilot jet will give your symptoms too) Check the needle/seat for wear... per previous comment and check out the other gentleman's link.
5) Check the jetting you have... should be 105 main, 40 pilot, and needle mid-clip... unfortunately... this is a base-line... but gets you in the ball-park for tweaking.
6) Re-assemble and recheck for sealing.
7) Check sync
8) Test ride
Again... sorry for the lengthiness... It really is hard to point you in the right direction... sometimes trial and error is the only answer... best of luck... and I hope this has helped a little...
 
This is good stuff captain. Ill answer some of your questions and then try and test these methods out.

Im not sure what exhaust system it is since it came with the bike and is looking pretty old.
I did go back to 105 main

1. I have not checked the cam idler. Is there an obvious way to know if its off?
2. Im not sure what you mean by static or dynamic ignition. I havent touched the timing at all yet.
3. When you mentioned using WD 40, do you just spray it all over the rubber parts and listen to the engine?
4. Carbs have been cleaned by a mechanic, not sure what kind of job he did but I trust he did a good job.

So should I not mess with the air fuel screw at all in these steps?

Thanks much!
 
keeleydavis... What I should have asked about your exhaust is whether it is baffled any at all or not... that's a huge difference. Easiest way is to tell is just look at the end of the muffler... though not the definitive... if it's "gutted" (no baffling at all)... all this is moot. I'll assume that it's not. Not knowing the actual condition of the carbs... cleanliness or what the other jet sizes are... (pilot/slow and needle)... really limits getting to the root of the problem. So we'll jump to the list for now....
1) No obvious way to tell the idler is off... other than sound (but other issues can sound similar) and performance. Check your manual... it really isn't that hard to adjust... basically a self tensioner... but... must be adjusted "by the book".
2) Ignition timing is critical... especially if your still running points (they are a bit of a pain in the behind... mine typically only lasted 1500 miles before I noticed performance issues... which can be at concern here). Static ignition timing is merely setting you points and timing... none running. Dynamic timing is with the bike "warmed-up" and with a strobe light... again reference your manual. Side note... whether you have points... or say a"Dyna" electronic pickup... be sure the ignition flyweights are lubed... the environment for the "points" is problematic (high temps with ambient moisture and "road" moisture generating rust... if they are "sticking"... here we go again).
3) As to the leak checks... yes... just warm the bike up to op temp... set idle to roughly 1000 RPM... and spray WD-40 around each carb inlet/exit... one at a time (try and not get any in through the air box... sounds touchy but can be done... at least you have the "box" on)... if the RPM "bogs"/stumbles... try and replicate it... if it does... fit a new piece.
4) Not disparaging your mechanic... but without "seeing" what they look like and knowing the full compliment of jetting that they have (plus float level)... I can not be much help. Don't fiddle with the air right now... try and "nail-down" the other items... I'm sure this sounds like a ton of stuff... but just break it down into steps... and you'll do fine... regards.
 
first check all obvious(compression,leak down) then aau operation, valve clearances and timing set properly . is there stock needles and needle jets? if they are keyster parts it is probably the off idle richness problem since theirs are too rich for most later cb750k's. or it could be reversion if you are using a reverse cone muffler but your carburetor is probably the problem if the integrity of the engine if ok and all stock set up adjustments and parts are fine.
joe
www.joesvcycle.net
www.joesvcycle.wordpress.com
 
Yeah. I have adjusted points and valves. I haven't looked in the carbs but I too suspect its more carb related. It's always once the bike is warmed up and between 2500 and 3000 rpm that I get the stuttering.
 
again if the bike runs as you say it is too rich off the pilot........lower the needles until it cleans up or replace all the needles and needle jets with oem NO key ster kits.. points and the aau require maintenance but work fine on stock and lightly modified bikes with no problem.....
good luck, let us know how it goes.
joe
joesvcycle@wordpress.com
 
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