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Administrator
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I travel for a living, and like to fire up "the avenger" on the weekends. No one is there to start my bike during the week. Typically, I'll leave the petcock ON, and my bike has the vacuum valve that cuts off fuel flow to the carbs when it's not running.
So, on Saturdays I'll go out to the garage, hold the choke open, and crank for 2-3 minutes until the float bowls fill up.
Last week, I cranked it for about 30 seconds to fill up the carbs, then twisted the throttle twice, and it stuttered. Repeating this process cut down on cranking time from 3+ minutes down to about 1. Easier on the battery/starter/etc.
What is the preferred starting sequence for these machines? Is there something I'm missing here?
Comments are welcome.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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If your tank has no rust in it this may help allot.
when I am going from empty carb bowls and want to fill them, instead of putting the valve to on I put it on reserve. Some bikes reserve seems to flow better like on my kawasaki.
another way is to spray some carb cleaner or wd-40 int the ait filter intake port. this will give it something to burn and speed up the pulsations to the pump.
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Administrator
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I don't think it's an issue with fuel flow: more a matter of that fuel manifold valve taking its time filling the float bowls.
I was thinking of starting fluid in the intake, but since my seat is a bolt on, using that method involves a ratchet wrench and sockets.
This past weekend, I cranked it for about 10 seconds with the choke on, then twisted the throttle twice, then cranked again. It sputtered a bit, so I repeated the process. 3 times of that, and it started and ran. So, I'll try that again this weekend when I get back into town.
Finally got to ride with my older son last night. Him on his Ninja 250, me on the Avenger. Freeway onramps can be fun, and a good test of Newton's laws.  The old machine did well, but I don't have the courage to scrape the pegs...yet.
Thanks for the input. I'll give it some more thought.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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why not remove that vacuum fuel valve and remember to leave the fuel on off when you're not riding?
on my wife's vulcan, there is a prime position on the petcock, but I don't think these bikes have that.
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Administrator
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two reasons, for now:
1) The bike is almost totally stock, and I'd like to keep it that way until I can't find parts for it or have to make drastic changes. It's the 10th anniversary edition.
2) The petcock, sadly, is not the original. To get it to fit properly, the previous owner has the lever facing inside under the gas tank. Getting to it is not as convenient as a stock petcock would be. I could and probably will switch it back to a stock model, but $50-100 for that item is a bit pricey at this time.
3) I like the safety feature of having the gas get cut off when the engine stops.
So, for now, I'll stay with it as is. I got the start time down from 5 minutes to 90 seconds, so that's an improvement. I'm thankful there's a fuel pump on one of the carbs, and going full throttle squirts some fuel into the chamber enough to get it to stutter to life.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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It sounds like the petcock you have is made for the other side of the tank.
You can try looking into what years/models honda used your petcock.
Once you have a few choices you can keep an eye on Ebay for an origional.
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Administrator
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The petcock on there now has 2 outputs on it. One goes to the carbs, and the other is capped off. Presumably, this other output feeds another bank of carbs, or maybe the PO didn't want to pull the tank to drain it. I'll take a picture of it next time I'm in town.
Either way, I've been looking on EBay for a stock unit, and they run anywhere from $60-$120US. I'll keep looking, and might even check with the Honda dealer to see how much they gouge...errr...charge.  Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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yes I would like to see a pic of the one you have.
Honda did use dual output petcocks on some bikes.
what side is yours mounted on while your sitting on the bike?
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I can get my '81 cb750fz to start quicker, particularly in colder weather, by cranking for a few seconds with the kill switch on, to get a bit of fuel through first. Something else worth checking is if you have a good spark. I hadn't, and have just done the GM coil mod, and coil relay, and increased the plug gaps, after reading good reports on the forums. Starts much easier, and seems to run smoother.
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do you have a link to this mod I would like to read up on it.
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Administrator
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It's the stock tank, with the petcock on the left side. I just leave the petcock on, since I have that fuel manifold valve in place.
Spark is good: last time I checked all the plugs, 1 2 and 3 are all textbook color. Very light tan. After I get it going after a week layoff, it lights up right away the next day.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Hey luke, sounds like you might have a choke issue, just because you pull up on the choke lever does not mean its working all the way, my starting procedure is turn on fuel, lift choke out halfway, turn on key, no throttle, starts in 2 seconds, adjust choke down as needed for a idle i can walk away from till ready to go,
or your carbs are dirty, good luck luke,, mykey440
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The GM coil mod info is at www.cb750c.com under "tricks and tips" posted by Sticky, under "GM coil mod"
You don't have to register or log on to view any of the info. I used coils off a Holden Commodore, same coils on any GM 3.8 litre V6. Bought my two coils for $40.
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Administrator
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Thanks for the input, Mikey. The hard starting is only a problem if the bike has been sitting for a week. I travel for a living (field engineeer) so the bike sits all that time. When I get time at home, it starts the next day with no problems. Something that just occured to me: could it be a problem with the fuel mixture? These bikes are set to run pretty lean due to emissions. Maybe I should fatten up the mixture a bit with the unreachable screws on the bottom of the carbs? I'll do this after I get carb #4 cleaned out and running good. I hate to mess with the mixture, as the plugs on 1 - 3 look very normal.
The choke works just fine. That was one of the first things I got working when I got the bike. When I was riding more often, the first start of the day needed about half pull on the choke. Once the RPM started to drop, I'd push in the choke, which brought the RPMs up a tad. Once it had been running for a minute or so, I'd turn the choke off, and we're good until the next cold start.
I guess it's time for a new job that has me home all the time, for the sake of this starting issue?  Tomorrow (or tonight, if I get home early enough) I'll try a few of the procedures listed here, and see what works.
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Luke, I have my idle screws out about 3 turns, i like it to be a hair rich rather than lean,
good luck on the new job search, more time to ride, ya...mikey440 --- On Fri, 8/6/10, LukeM [via Honda CB750'S] <[hidden email]> wrote:
From: LukeM [via Honda CB750'S] <[hidden email]>
Subject: Re: Starting sequence for the DOHC 750
To: "mikey440" <[hidden email]>
Date: Friday, August 6, 2010, 5:34 AM
Thanks for the input, Mikey. The hard starting is only a problem if the bike has been sitting for a week. I travel for a living (field engineeer) so the bike sits all that time. When I get time at home, it starts the next day with no problems. Something that just occured to me: could it be a problem with the fuel mixture? These bikes are set to run pretty lean due to emissions. Maybe I should fatten up the mixture a bit with the unreachable screws on the bottom of the carbs? I'll do this after I get carb #4 cleaned out and running good. I hate to mess with the mixture, as the plugs on 1 - 3 look very normal. The choke works just fine. That was one of the first things I got working when I got the bike. When I was riding more often, the first start of the day needed about half pull on the choke. Once the RPM started to drop, I'd push in the choke, which brought the RPMs up a
tad. Once it had been running for a minute or so, I'd turn the choke off, and we're good until the next cold start. I guess it's time for a new job that has me home all the time, for the sake of this starting issue?  Tomorrow (or tonight, if I get home early enough) I'll try a few of the procedures listed here, and see what works.
Back on 2 wheels (1979 CB750L with air forks(?)) after a 30+ year break. It's still fun, and just as scary as before. I like it. 
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Mine are set at 2 each. but I do have different carbs than you have.
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Administrator
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For those of you who asked, here's a pic of the petcock on my bike.
I did get a used one off of EBay, and it looks more like what should be on the bike: valve handle to the outside, outlet to the carbs on the inside.
Enjoy!
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Luke this petcock does not look like it has a vacuume line going into it.
If the new one you have does not have a vacuume line then what I would do is, turn the valve to res for a minute or two before trying to start the engine.
This type does not have a automatic shutoff that I know of. I may be wrong in this but the old one may not be working properly.
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Administrator
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The vacuum system is the fuel manifold valve, which is behind and on the right side of the carburetor bank. It's referred to as the Spawn of Satan on some other boards. With the SoS in place, I've read that you can leave the petcock in the ON position, and no fuel will flow.
I'll give the RES switch a try this weekend. I'm a little dubious of the results, though. My son's CB550 doesn't have a vacuum line either: just a ON/OFF/RES switch. I know when he's left his in the ON position: the garage smells like spilled gas.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Administrator
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The RES thing didn't give a noticeable difference. However, my "new" petcock came in earlier this week. This one has the same ON/OFF/RES positions, but instead of feeding out the bottom like the above picture, it feeds inward pointing directly to the fuel manifold. My old petcock didn't have the tube for the ON position, so it was like RES/OFF/RES. Feels better having a petcock that I can actually use now.
It was starting a little quicker this weekend. Dunno if it was the petcock, or the weather. I notice my machine doesn't like high humidity weather: tends to stall out and not want to run. Not sure what that's all about. I'll track down the ignition wiring and see if something's up with that.
By the way, cost of the used petcock was about $28 US with $6 shipping. Better than the $80-120 that I've seen for new units.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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