Author Topic: CB750 Rough Running, No Power, Won't Idle (Read 9177 times)

Weatherman · « **on:** May 21, 2006, 07:40:35 PM »

Hello everyone, I have become a Honda owner again and have just signed up.

I have purchased a '74 CB750K with 5600k miles on it. It is in wonderful shape for its age and the seller's ad stated it started and ran fine. The seller is in Virginia and had it transported by trailer to Texas, but not to my doorstep. I had to go pick it up. When I fired it up it wouldn't idle at all, ran rough with cylinders cutting out, and it was smoking a bit. Having had Honda's back in the 70's I knew they were cold blooded and I probably flooded it trying to start it. The fellow who unloaded it from the truck said it had started and ran ok for him.

I loaded it on a trailer and brought it home. Today I took it out for a ride hoping that some miles might straighten it out. However after about 3-4 miles it still wouldn't idle and it went from cruising along with cylinders missing out with some mild popping out the exhaust to having no power even with the throttle wide open.

Should I start by looking at the ignition system first or the carburetors? I have never worked on carb's like these and wouldn't know the first thing about setting and synchronizing them. Can any cleaning be done without removing them from the bike? If I can't fix this myself can anyone recommend a good SOHC mechanic in the Dallas\Ft Worth (I live in Arlington) area?

Thanks,
Dennis

ElCheapo · « **Reply #1 on:** May 21, 2006, 08:42:54 PM »

Ignition is easiest but usually wont cause it not to idle.

Lucky you the carbs are pretty easy to clean. Unles something is broken there should not be any parts you need. Normally I pull them down most of the way and let them soak in carb dip as long as needed. Then go about cleaning the rest, blow out the goods and then reassemble.

HondaMan · « **Reply #2 on:** May 21, 2006, 09:01:24 PM »

Welcome, Weatherman;

Your symptoms sound like one or more fouled spark plugs and old gasoline. Start with those 2 things before you go further and start throwing $$ at it.

Get back to us if that doesn't fix it up.

78_SaltLick · « **Reply #3 on:** May 21, 2006, 11:11:58 PM »

crazy question but did you check the gas? sounds like how my bike runs when its time for me to switch it over to reserve......

Jonesy · « **Reply #4 on:** May 22, 2006, 04:42:59 AM »

Try giving it a basic tune-up before you tear into the carbs. Also, flush out the gas tank and fill it with fresh fuel.

Dave Wyatt · « **Reply #5 on:** May 22, 2006, 07:07:24 AM »

Was it on an open trailer? If so, did it see any rain on the trip? If it did, I'd remove the points cover and see if it's wet under there. I've been down that road before with my 750. SOmetimes it would dry out without doing anything and sometimes I had to remove the cover and start drying.

dusterdude · « **Reply #6 on:** May 22, 2006, 09:26:58 AM »

damn virginians,ya cant trust none of us.

Weatherman · « **Reply #7 on:** May 26, 2006, 03:25:09 PM »

Hello everyone,

Thanks for the feedback. I did pull the points cover and there was no sign of moisture and the points looked ok. The gap on on set of points was a little wide.....a .016 feeler guage slides through with very little resistance. I have drained the gas, pulled and cleaned the plugs (gas fouled big time), checked the air filter (ok), and last but not least removed the float bowls from the carbs. The bowls were clean however the bottom side of the carb bodies had a light tan powdery residue that wiped right off. The floats were free and functional. I put the gas in my truck and it seems to like it ok.

Now for the good part. A coworker (who I discovered was a Honda mechanic in the 70's) suggested that I remove the jets and clean them out. He said the orrifice in the slow jet was extremely small and I would need a piece of fine wire or a bristle from a brush to push through them. I took the jets out and the orrifice in the slow jets look to be about .035 or better in diameter. A paper clip slides right through easily. Can anyone tell me what the actual ID is supposed to be? They are #40. The main jets look a bit large too, they are #105.

The bike has aftermarket slip-on-and-clamp mufflers that are extremely loud. And if indeed they have been drilled out I suspect that whoever installed those mufflers did it to compensate for the reduced back pressure. I took them over to the local Honda shop and they ordered me a new set. Now I have a week to wait to see if this fixes it.

I discovered that one of my float drain plugs has the head broken off and a new one is not available from Honda. Does anyone know where I might find one or something that will work in its place? It wasn't leaking but I'm not sure I want to trust it for the long term.

Thanks,
Dennis

VitaminCB550 · « **Reply #8 on:** May 26, 2006, 04:42:47 PM »

How are the chrome on those mufflers

? Want to get rid of them?

TwoTired · « **Reply #9 on:** May 26, 2006, 05:31:35 PM »

The jet numbers stand for the orifice size in mm. I.E. a #100 jet is 1.00mm in diameter. A #40 is 0.40mm. 0.4 millimeter = 0.0157 inch

The slow jet is a tube with a restriction or orifice deep inside it. Paper clip in my desk here measures 0.039 inch or about one millimeter in wire diameter.

I'm guessing that if you can push a paper clip all the way through your slow jets, someone has drilled them. I would not expect the Idle Air Bleed Screws to be able to compensate for such a large enrichment of the slow system. And, fouling spark plug with prolonged idle will be the norm.

Your main jet size is probably about right for your bike, if it, too, hasn't been drilled. But, that one is tuned or sized for WOT.

If you have the stock paper air filter, it should be a light cream color. The darker it is, the more trapped dirt it has, and the more restrictive it becomes. This also causes rich mixtures as it behaves as partial choke. It's your call, but it is difficult to tell its flow characteristics by looking at it. From experience I can tell you that a new one *can* make a profound difference. I used to keep a new one on hand to swap in for a short test run after the tuneup. If I could tell a power difference between the two, I threw the old one away. If not, I put the old one back in for the next tuneup cycle. You can only backblow these things a limited amount of time, as the dust and grime gets ever more tenacious at holding on.

Cheers,

endobendo · « **Reply #10 on:** May 26, 2006, 05:42:54 PM »

Hey weatherman,

From what you describe, it could simply be fouled plugs. I think that was mentioned already. The guy said it ran good before he loaded it, but he probably didn't warm it up. Also, was the gas shut off when you picked it up? All that bouncing around could have helped flood things.

So maybe you want to try some new plugs or dry the ones you got before you do much else?

Weatherman · « **Reply #11 on:** May 27, 2006, 07:44:35 AM »

The gas was on when I arrived to pick it up, however I don't know if it had been on during the whole trip. Very likely could have been though. I will wait for the new jets to arrive before I try to run it again. I just hope the "driller" didn't make any other alterations to the carbs.

VitaminCB550,

If you are referring to those replacement mufflers the chrome is very good. However I haven't got down and looked at their underside. And providing it doesn't cost me a fortune to get it running right I do want to put OEM's back on it. If you're interested in buying them I will keep you in mind. Might be a month or so.

thanks,
Dennis

VitaminCB550 · « **Reply #12 on:** May 27, 2006, 07:56:32 AM »

Quote from: Weatherman on May 27, 2006, 07:44:35 AM

VitaminCB550,

If you are referring to those replacement mufflers the chrome is very good. However I haven't got down and looked at their underside. And providing it doesn't cost me a fortune to get it running right I do want to put OEM's back on it. If you're interested in buying them I will keep you in mind. Might be a month or so.

thanks,
Dennis

Please... DIBS!

Bodain · « **Reply #13 on:** May 28, 2006, 06:12:08 AM »

I'm just down the road from you in Fort Worth. CB 550 and 750 in my garage. My money is on the carbs.
If you need parts to swap and troubleshoot. I might be able to help. 817-798-8201

Weatherman · « **Reply #14 on:** May 29, 2006, 03:04:16 PM »

Thanks Bodain, I'll give you a call if new jets don't fix it.

I thought while I am waiting on parts I would pull the carbs off the engine to check the float settings and give them a more thorough cleaning and visual check. How far down can I take them without affecting the synchronization?

Thanks,
Dennis

TwoTired · « **Reply #15 on:** May 29, 2006, 05:14:45 PM »

Carb sync is related to the slide positions of each carb relative to the others.
Separating carbs, or checking slide needle position will disturb the current sync.

How do you know they are synchronized now?

Weatherman · « **Reply #16 on:** May 30, 2006, 07:59:20 AM »

Acutally I don't. But I would rather not alter any settings (that I don't have the means to fix

) before I try running it with new jets.

Dennis

Weatherman · « **Reply #17 on:** June 23, 2006, 10:38:11 PM »

Hi All,

An update. I cleaned the carbs but didn't remove the slides or separate them, set the float levels, and installed new main and slow jets. I also installed a new set of plugs, air filter, and a tank of fresh gas. I hit the switch and it started right up, but here is what it does now.

Once warmed up it doesn't want to idle down. Even with the idle adjuster screw backed all the way off. I can turn the throttle backwards and it will slow down some but when I release it it runs back up. I have tried to adjust the throttle cables (to the best of my understanding of the Clymer manual) and the throttle has the correct amount of free play. The slides are closed at this point and twisting the throttle backwards doesn't move them. It does move the spring loaded linkage that prevents undo force on the slides. Turning the air screws in all the way has no effect. If I turn them way out it will start to blubber a bit but the speed doesn't drop. I have checked the tops of the carburetors and the connections to the engine and they are tight.

When driving down the street and easing out of the throttle at low (2500 rpm or so) engine speed and slowing to stop or turn it "pops" out the exhaust. It doesn't do it when accelerating from a stop, under light load, or at idle. Throttle response is real good.

Does this all sound like carburetor issues? Tips and suggestions/solutions are welcome!

Thanks,
Dennis

TwoTired · « **Reply #18 on:** June 24, 2006, 12:36:26 AM »

I'd guess that the intake couplers are leaking air. Or, the carbs are out of sync.

Spray some fluid around the rubber connections; brake cleaner, or somesuch. If the idle speed changes, it's leaking.

Cheers,

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Author Topic: CB750 Rough Running, No Power, Won't Idle (Read 9177 times)

ElCheapo

VitaminCB550

endobendo

VitaminCB550