Author Topic: 74 CB750 guzzling gas and 2 personalities (Read 3025 times)

piefairy · « **on:** June 24, 2018, 03:08:32 PM »

My bike build is mostly done, its back and running but its still not right. I went for an extended ride today out to my mothers house and back, about 75 miles mostly highway. The bike picks and chooses when it wants to go above 6k rpm, I really have to nurse it to get close to 8k, more disturbing is that I went through half a tank going to her house... about 36 miles. I filled it back up on the way back, it took in 3 gallons, meaning I'm only getting 12MPG?!?! I'm trying to figure out where to even start on this. The carbs were just sync'd and it runs and idles a hell of a lot better than it did before, but seems to be on the lean side. It will stall if I let it sit by itself. I know these old birds can have their own personality, but yesterday I could get up to 9k with no issue, then today its a struggle. First and second gear seem happier than the rest, also the bike gets a bit more power when its a bit cooler. The start of the day was around 90, it was starting to rain on the way back, temp down in the 70's but still not right. AND THE GAS!!!

I put it on newspaper when I stopped, came back 3 hours later and there were only 4 drops on it, 1 oil, 3 gas, but I did leave the petcock on. Any thoughts?

evinrude7 · « **Reply #1 on:** June 24, 2018, 03:19:01 PM »

When you crank the bike over and let it idle what do you smell?

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piefairy · « **Reply #2 on:** June 24, 2018, 03:25:25 PM »

Carbon and exhaust. It doesn't have the sweet smell like its running rich. Idle is also a bit jumpy leading to it stalling. Happy at 800 RPM, but will drop and flutter around 450, stalls out a little below that if I don't nurse the throttle. But sometimes it will perk back up and jump up to 1k.

evinrude7 · « **Reply #3 on:** June 24, 2018, 04:41:31 PM »

Pods or stock intake?

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piefairy · « **Reply #4 on:** June 24, 2018, 05:10:11 PM »

I'm running pods. I know that the carbs were synced with them off, but I am not comfortable running with no filter, and I'm not sure about going the velocity stack route, heard mixed reviews.

Coredozer19 · « **Reply #5 on:** June 24, 2018, 06:20:13 PM »

My bike is having this same exact issue. Stock airbox. 73 cb350f

Coredozer19 · « **Reply #6 on:** June 24, 2018, 08:07:14 PM »

What’s the condition of your gas tank lid. Someone told me the vent may be clogged. So maybe start there. Something simple. Ride it till it starts acting up at 6k (just like my bike) and you hit the throttle and it gargles and misses like a son of a. Stop real quick and pop your gas tank lid. And leave it open. Let me know if that works because you are litterally describing the EXACT same problem I’m having on my 350.

jgger · « **Reply #7 on:** June 24, 2018, 09:43:31 PM »

Quote from: Coredozer19 on June 24, 2018, 08:07:14 PM

What’s the condition of your gas tank lid. Someone told me the vent may be clogged. So maybe start there. Something simple. Ride it till it starts acting up at 6k (just like my bike) and you hit the throttle and it gargles and misses like a son of a. Stop real quick and pop your gas tank lid. And leave it open. Let me know if that works because you are litterally describing the EXACT same problem I’m having on my 350.

Coredozer19 if you are referring to the suggestion that I made in your post, you may have misunderstood. I thought I told you to REPLACE THE CAP and then flog the bike. I don't want to be blamed for a bike fire because of spilled gas, or someone getting a nasty nut rash from spilled fuel!

But yea you got the basic concept.

calj737 · « **Reply #8 on:** June 25, 2018, 03:09:47 AM »

What jetting changes did you make with your carbs when rebuilt? Did you set the proper fuel height with the Clear Tube method? That gas is going somewhere, unless you’re mileage is miscalculated. You should pull the dipstick and inspect the oil, and oil level.

Pods can cause lean conditions, but they won’t account for 3-5 MPG performance. Sure, maybe a 5 MPG change, but not a 50% change.

piefairy · « **Reply #9 on:** June 25, 2018, 04:20:04 PM »

The milage was tracked by my phone GPS, so its correct. I do not know about the jetting or how the carbs were synced, I dont have the tooling for that so I had a motorcycle mechanic set them up. It was running so rich I couldn't get over 50 mph without it stalling. Also, I cant really say it "Boggs down and gurgles" it just stops building rpms and will start just hold the speed. There is occasional popping in the exhaust, I haven't noticed any cylinders missing so I have attributed it to excess fuel or an incomplete burn. Also, I'm not sure how the carbs and fuel work together, but even when the bike is happy, 1/3 to 1/2 twist on the throttle and it's up at 8k. I've heard people tend to need to regripp to get to the high end? I heard the term throttle advance before but no clue what that is...

piefairy · « **Reply #10 on:** June 25, 2018, 04:51:21 PM »

Forgot to mention, tank is solid, it has some spotty rust, but I have held off taking care of it because I am hoping to replace it shortly. I am wondering if I should seafoam the engine. I did replace the plugs and the old ones had so much carbon on them you could use it for eyeliner.

Coredozer19 · « **Reply #11 on:** June 25, 2018, 06:25:55 PM »

My plugs are extremely carbon soaked aswell. What’s the state of your valve seals? are you sure it’s not oil.

754 · « **Reply #12 on:** June 25, 2018, 09:22:32 PM »

I had a set of carbs would only get around 20 mpg. People in traffic were complaining of the smell. Had them apart a bunch of times, I suspect bad brass.
Changed carbs, got better real quick. 45 to 50 mpg now.

calj737 · « **Reply #13 on:** June 26, 2018, 03:31:58 AM »

Well until you perform some plug chops with fresh plugs, and verify the fuel level, you’re guessing. Since a “mechanic built your carbs” you have no earthy idea what was done, what parts were used, and where they were setup. Way too many unknowns to diagnose performance issues.

Good luck with all that-

piefairy · « **Reply #14 on:** June 26, 2018, 02:31:31 PM »

He synced the carbs, he didn't built them. All the parts are stock or NOS as far as I can tell. If I am going to have to play with the fuel and jet settings, I guess I'll have to do just that. I have a copy of Hondaman's book coming, but I will need to get parts together to get ready to set them up. Am I going to need the 4 gauge vacuum setup I have seen people using? I will order a set of jets so I can adjust them as well as new plugs. What else do I need to properly set up the carbs?

Robbo · « **Reply #15 on:** June 26, 2018, 02:56:13 PM »

Before ordering jets, I would open up the carbs first and see what needles and jets you have to begin with.

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piefairy · « **Reply #16 on:** June 26, 2018, 03:32:00 PM »

Quote from: Robbo on June 26, 2018, 02:56:13 PM

Before ordering jets, I would open up the carbs first and see what needles and jets you have to begin with.

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Will do. I already had a set of clear fuel line ordered, I am ordering new plugs, and Daiichi points

calj737 · « **Reply #17 on:** June 26, 2018, 04:04:57 PM »

You don’t need to re-sync the carbs simply because you change jets. And ditch the Daichis and buy TEK points.

piefairy · « **Reply #18 on:** June 26, 2018, 04:30:10 PM »

Quote from: calj737 on June 26, 2018, 04:04:57 PM

You don’t need to re-sync the carbs simply because you change jets. And ditch the Daichis and buy TEK points.

I saw the reviews on the Daiichi points after I ordered them. I'll either return them or keep them as emergency backups. I am looking to put Hondaman's transistorized system on here as well to hopefully prevent more issues in the future.

piefairy · « **Reply #19 on:** June 26, 2018, 04:42:17 PM »

I am not sure where to get TEK points, but I can get NOS points, 55 for a pair. Should I do that or look for the TEK?

calj737 · « **Reply #20 on:** June 26, 2018, 05:37:25 PM »

TEK are NOS.

piefairy · « **Reply #21 on:** June 26, 2018, 06:46:55 PM »

Ok, I have seen them posted as TEC and TEK, same thing?

piefairy · « **Reply #22 on:** June 28, 2018, 02:27:30 PM »

Well, the new starter solenoid came in today, got it on and decided to take off the pods I was running. Its an entirely different bike with them off. It redlined with no issue and kept wanting to pull. The throttle response is MUCH better, I even broke the back end loose in 3rd gear. So it seems the pods were the power issue, but would they account for the fuel consumption? Also, this is a stupid question, but in reguards to the petcock, which direction is "On" rather than reserve? Long fin up for on?

BobR · « **Reply #23 on:** June 28, 2018, 02:48:43 PM »

Quote from: piefairy on June 28, 2018, 02:27:30 PM

Well, the new starter solenoid came in today, got it on and decided to take off the pods I was running. Its an entirely different bike with them off. It redlined with no issue and kept wanting to pull. The throttle response is MUCH better, I even broke the back end loose in 3rd gear. So it seems the pods were the power issue, but would they account for the fuel consumption? Also, this is a stupid question, but in reguards to the petcock, which direction is "On" rather than reserve? Long fin up for on?

Too much oil on the pods and/or too big main jets? Perhaps opening up the throttle more to "hold the speed" as you put it ruined your MPG? ...."On" is with the big part down.

piefairy · « **Reply #24 on:** June 28, 2018, 03:28:10 PM »

Thanks. Sounds like I was also riding with the reserve on. I don't know if that would effect gas either. I plan to do a ride saturday morning and see what I get for mileage. I am picking up a set of Steel Dragon velocity stacks tomorrow from DCC.

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Author Topic: 74 CB750 guzzling gas and 2 personalities (Read 3025 times)