Stumbling and hard start at Idle and High RPM

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ccatinella
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Stumbling and hard start at Idle and High RPM

Postby ccatinella » Sat Apr 25, 2015 2:57 pm

First Timer Here. I recently became owner of a great little '79 Honda CB650 project. It's an absolute blast to ride, when I can keep her running. I have been having some trouble getting the bike to run smooth at both idle and high RPM (above about 5k).

The bike came with the pod style air cleaners and I don't think the previous owner was all that savvy with these things, so I doubt the jets were switched out. When I give it throttle from idle there is this tremendous sucking sound of air going in, but the bike just stumbles and stalls. If I keep revving, and I mean constantly revving, and if I keep the RPM up, I can keep it alive, but I think that's only because the accelerator pump is dumping fuel into the carb to keep it going. I just don't think it's getting enough fuel to stay running. Once warm it gets a little better at idle, but at high RPM and speeds of about 40-50mph she doesn't have much left in her. I just don't think it's getting enough fuel.

So, forgive me if I am bringing up old post knowledge, but I couldn't find a definitive and specific answer to my problem. Can someone recommend if this is the most likely culprit, and if so, what size jets would you recommend?

I know the '79 is notorious for carb issues, and I know parts are hard to find, so any help pointing me to the right parts shop would be great too.

Thanks.
-C

jcasey007
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Re: Stumbling and hard start at Idle and High RPM

Postby jcasey007 » Sun Apr 26, 2015 4:22 pm

I would start by looking at an air leak between the head and the carbs, there are rubber boots that degrade with time and will cause the symptoms you have.

ccatinella
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Location: Massachusetts

Re: Stumbling and hard start at Idle and High RPM

Postby ccatinella » Sun Apr 26, 2015 8:48 pm

Thanks. I ordered a set of boots yesterday actually as I noticed the cracks, and they were stiff and a bitch to get back on the bike. I'm hoping the new boots help in both cases.

I'll post the results after they arrive.

Pbiggs
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Re: Stumbling and hard start at Idle and High RPM

Postby Pbiggs » Sun Apr 26, 2015 11:51 pm

Sounds like boots to me also, Here's a good place to get mains and slows.

http://www.jetsrus.com/a_jet_kit_street ... _1980.html

To tell you what jets to run would be trouble, take a look inside your carbs check sizes and go up a size or two, while your at it give them a nice cleaning.

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Folsoml
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Re: Stumbling and hard start at Idle and High RPM

Postby Folsoml » Mon Apr 27, 2015 4:16 am

Definitely start with a carb cleaning. If a bike sits for any length of time with the ethanol gas we must use, the carbs will certainly gunk up. Dirty carbs can cause just what you are describing.
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Volker_P
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Re: Stumbling and hard start at Idle and High RPM

Postby Volker_P » Tue Apr 28, 2015 2:22 am

Welcome here! :D

As you already suspected, PD carbs with pods and stock jetting (90-92 mains) will be too lean. For jetting suggestions, please have a look at the 79 cb650 carb jetting question thread. Sizes may still differ for your particular configuration, but it should be a reasonable point to start from.

For a quick verification of the current main jets, you may pull the choke a little bit above 5krpm to check if things go a bit better then.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

ccatinella
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Joined: Sat Apr 25, 2015 2:18 pm
Location: Massachusetts

Re: Stumbling and hard start at Idle and High RPM

Postby ccatinella » Tue Apr 28, 2015 1:28 pm

Thanks Volker. I'm almost certain it's the jets now. I pulled the choke at 4500rpm, right when the bike started to stumble. It started running better up to about 5500, then it stumbled again. I also get better power in the 2k-4k range with the choke on.

I'm ordering a set of slow jets and a mains jet kit from Sirius and I'm going to do a carb rebuild and re-jetting. I'll reply with results when it's done.

ccatinella
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Re: Stumbling and hard start at Idle and High RPM

Postby ccatinella » Sat May 02, 2015 11:24 pm

I received the new jets and during the process of rebuilding the carbs I found that someone stripped a bunch of the screws for the bowls. Everything was barely holding on. I also found stripped screws that mount the carbs to the aluminum plate that goes towards the head. I'm going to have to get some helicoils and attempt a repair.

In the meantime I was able to get everything together, albeit not as tight as I want, and I took the bike for a quick spin. The throttle response is much better with the new jets. I Love Seeing Results! But I have another weird issue.

I can only ride the bike for about 1 minute at cruising speed and then it's starving for fuel. It either stalls or I can let it idle for about two minutes. Then I can ride it for another minute or two again and the same thing happens. It's almost like the fuel can't drain from the fuel tank fast enough.

The fuel filter is new, but do I need a faster flowing filter? Do I need to upgrade my petcock because of the jets? The new jets use more fuel, but I'm skeptical that it's now using so much that the tank can't supply fuel fast enough. Any ideas?

cb650
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Re: Stumbling and hard start at Idle and High RPM

Postby cb650 » Sun May 03, 2015 5:10 am

Maybe the cap is plugged and air cant get in.
Did you clean the tank out?!?!?!?!

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tario
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Re: Stumbling and hard start at Idle and High RPM

Postby tario » Sun May 03, 2015 12:52 pm

Are the fuel mixture screws are set to about 2 turns out? Manual says to start at 1-5/8 out and adjust out until the idle flattens out. I've found a few people that say 2 turns out is about right.
1979 cb650
1980 cb650

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Volker_P
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Re: Stumbling and hard start at Idle and High RPM

Postby Volker_P » Tue May 05, 2015 2:02 am

Pull the fuel line from the carbs and hold it into a clean jar. Then open the petcock to get an idea if you get enough clean fuel that way.
There is a filter screen at the petcock in the tank that may be partly plugged or your inline filter may be too restrictive.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

ccatinella
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Joined: Sat Apr 25, 2015 2:18 pm
Location: Massachusetts

Re: Stumbling and hard start at Idle and High RPM

Postby ccatinella » Sun May 24, 2015 8:01 pm

I finally finished repairs on my carbs. Upgraded the jets, added helicoils to some of the stripped screws, replaced a couple of fuel lines, put it all back together, and SHE RUNS LIKE A CHAMP! I can't believe how much of a difference it is. She still leaks a ton of oil from the head, down the side of the engine, and all over my leg, but that's a project for a different day.

One question regarding the carbs. The guy before me replaced the air filter system with pods. I assume that the old filter system used to take some of the weight of the carbs because the only thing holding them on the engine now is the rubber connectors from the carbs to the head. So they're kind of just cantilevered out there hanging out in the open space inside the frame. Has anyone every had any issue with this setup? Seems like they would experience a lot of movement.


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