Author Topic: 1974 CB750 fuel lines (Read 10090 times)

pjed · « **on:** May 30, 2009, 07:17:01 AM »

Hi, I'm new to the forum and got a couple of questions on my 74 CB750 fuel lines. I just took the carbs apart and dunked everything in carb cleaner. I'm about to put them back on the bike but am not convinced the guy before me had everything connected correctly...there are no fuel line filters or line clips.
The petcock has two outlets. Does each have an individual fuel line that connect to between carb 1&2 and 3&4?
What kind of filters should I use and do I install one on each line?
Does the petcock itself have a filter and should I take it apart and clean it?

The bike would fire and idle but would die as soon as I tried feeding it throttle. Fuel bowls on 2 and 3 were getting almost no gas but 1 and 2 were...hopefully this cleaning solves the issues.

Thanks for any guidance/advice.

Paul

nobrake · « **Reply #1 on:** May 30, 2009, 07:35:27 AM »

I had the same problem when I first got the bike. After reading the forum I did
the cam chain adjustment, point set, and minimally clean the carb. Bike ran
somewhat better after all these tuneups. Now due to oil leak I have the engine out and disassembled which is causing more problems than I had anticipated.

Follow the gurus here when they say do the basics first and go deeper into the engine
one step at a time. If I had followed the advices I think I would be riding this weekend despite the oil leak. Instead engines parts are all over the place and me glued to
computer trying to find right parts from all over internet and how to put them together.

just my .02

mystic_1 · « **Reply #2 on:** May 30, 2009, 09:33:54 AM »

Quote from: pjed on May 30, 2009, 07:17:01 AM

Hi, I'm new to the forum and got a couple of questions on my 74 CB750 fuel lines. I just took the carbs apart and dunked everything in carb cleaner. I'm about to put them back on the bike but am not convinced the guy before me had everything connected correctly...there are no fuel line filters or line clips.
The petcock has two outlets. Does each have an individual fuel line that connect to between carb 1&2 and 3&4?
What kind of filters should I use and do I install one on each line?
Does the petcock itself have a filter and should I take it apart and clean it?

The bike would fire and idle but would die as soon as I tried feeding it throttle. Fuel bowls on 2 and 3 were getting almost no gas but 1 and 2 were...hopefully this cleaning solves the issues.

Thanks for any guidance/advice.

Paul

If you have the earlier petcock with a sediment bowl, the filter consists of the sediment bowl and the brass screen within it. In the stock configuration there were no other filters. Each outlet on the petcock feeds a pair of carbs as you've gathered.

Stock fuel lines are a press fit onto the nipples and didn't necessarily require clamps. Stock lines had little wire clips that kept the lines from pulling off of the nipples. If you've replaced the lines with 1/4" line, you'll need clamps.

Dieing when you feed it throttle is often a symptom of gummed up carbs so hopefully your cleaning will help. Make sure to clear all the little internal passages well, people sometimes miss doing this thoroughly and have to repeat the cleaning.

Have you performed all other tune-up procedures such as valve clearance, timing, etc?

mystic_1

Alan F. · « **Reply #3 on:** May 30, 2009, 10:10:46 AM »

Yup, take the bank of four carbs off and clean them on the bench, notice the aluminum bracket that the four carbs attach to, it has two holes for fuel lines to feed through. Once your carbs are cleaned and put back on, cut two lengths of fuel line, 6" and 12" feed the long one to the far carbs and the short one to the near carbs. No other filters are really required, they're tough to fit and the filter in the sediment bowl does a decent job. Just keep your tank clean and make sure that sediment filter (the brass circle) has no rips or tears and all o-rings are nice. Use clamps if you use 1/4" line, but stock line and clamps are still available from the dealer.
Keep that sediment filter clean, check it once in a while when the tank is empty, use a 6 point wrench to remove the bowl...

Cheers.
-A

pjed · « **Reply #4 on:** May 30, 2009, 01:07:47 PM »

Thanks for the info guys. The previous owner set the timing and the valve clearences but I suppose I should do that as well. I took the carbs apart on the bench, dunked all the pieces but not the carb bodies themselves. If I still have problems, I'll dunk them too (minus any gaskets, etc...) I'll definitely clean out the sediment regardless.

Thanks again

Inkscars · « **Reply #5 on:** May 30, 2009, 01:19:05 PM »

take my 2 cents on the carb cleaning if you don't want to do it 4 times. (like me)
Don't just soak them. Use spary carb cleaned with a little straw, loads of shop towels and compressed air (with a firm grip!)
I soaked most of my components for a total nearing 80 hours and they were no where near clean.
Good luck, sounds like you'll be up and running in no time!

pjed · « **Reply #6 on:** May 30, 2009, 07:07:18 PM »

Kinda stumped right now.

I soaked the carb bodies in the gunk can for an hour each. I soaked all the jets and sprayed carb cleaner in them...you can see light through them. I took off and cleaned the floats and set them to 26mm. The bike now won't fire and there's barely any gas in the bowls after cranking over for about 1 min (combined). Does it take a lot for them to get filled after being dry?

I opened up the petcock and cleaned out the sediment bowl. The screen looked clean but I didn't see a way to remove it from the petcock body. Could that be restricting my fuel flow and be the issue?

Kevin D · « **Reply #7 on:** May 31, 2009, 05:22:39 AM »

Quote

there's barely any gas in the bowls

Not right, the likely source of your no-start problem. Float bowls should be nearly full.

Quote

I opened up the petcock and cleaned out the sediment bowl

How much crud was in the bowl? Mine has just the tiniest bit in the bottom after a seasons use. Any rust in the your tank?

Quote

The screen looked clean but I didn't see a way to remove it from the petcock body.

Theres an oring holding it in. Make sure the fine mesh screen isn't torn.

Quote

What kind of filters should I use and do I install one on each line?

Not much pressure pushing the fuel from the tank into the carb. If you add a restriction or reroute the fuel lines to accomodate filters........well, the best thing is to get the tank clean so you dont need extra filters or rerouted fuel lines.

Theres a pic of a float level sight gauge inside this thread:
http://forums.sohc4.net/index.php?topic=36982.msg389643

pjed · « **Reply #8 on:** May 31, 2009, 03:06:04 PM »

I took the screen out of the petcock...it was actually pretty clean. There wasn't too much crud in the bottom of the sediment bowl either.

I took the petcock off of the tank and soaked it. The two screws that secured it had rubber washers that disintegrated so I replaced them with new ones. Now regardless of the setting, gas drips out of one of the spigots. I noticed the rubber washers were basically getting crushed/deformed. Could gas be leaking past by the screws into the bowl? What kind of washers should I use? Can you repair or just replace petcocks?

Thanks for everybody's help. I think the problem is getting narrowed down and hopefully the CB will running right.

mystic_1 · « **Reply #9 on:** May 31, 2009, 03:35:05 PM »

Yup, fuel flowing regardless of petcock setting is a symptom of failed sealing washers on this design of petcock. They washers sometimes are available on eBay, some people have had good luck with neoprene washers from the home improvement store.

mystic_1

pjed · « **Reply #10 on:** May 31, 2009, 05:38:32 PM »

I got some different washers for the petcock which stopped it from just leaking and put it on the bike. The bowl still have just a few drops at the bottom and the bike only fired with the choke full on. I think the bike is still not getting enough fuel from the tank. I'm about to start looking for a rebuild petcock on ebay.

mystic_1 · « **Reply #11 on:** May 31, 2009, 05:52:17 PM »

Disconnect the fuel lines from the carbs, stick the ends into a container, and turn on the petcock to check the flow.

Sounds more like a float problem to me though, where did you measure from when you set the floats? Measurement should be from the edge of the lip on the carb bodies. There's also a trick you can use to check the fuel level in the bowls, using clear tubing on the drains. Do a search for more info.

mystic_1

pjed · « **Reply #12 on:** May 31, 2009, 06:12:12 PM »

I followed the steps exactly as described on this site: http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm. I did disconnect the lines from the carbs and allowed the hoses to hang over a bucked...the fuel came out slowly from the spigot closer to the rear of bike and not at all from the other.

fishhead · « **Reply #13 on:** May 31, 2009, 08:15:38 PM »

The early K carbs (after Sandcast) used the carbs that have 2 holes in the mounting plate for the fuel lines to run through (1st pic) for the right side mounted petcock.
When the petcock was moved from the right side of the tank to the left side of the tank it the mounting plate then had an additional hole in the mounting plate on the left side (in addition to the 2 on the right). (2nd Pic).
The carb kits for the early bikes are pictured (3rd pic)
The carb kits for the later bikes are pretty much identical, BUT the float needle asssemblies have a plastic filter above them (4th pic).
If you used a harsh cleaner and did not remove the float needle valve seats, the filters on top of the float valves have probably melted and are now keeping the fuel from endering the carbs.

Contrary to popular belief displayed on many message boards, a thorough carb cleaning means the carbs MUST be completely disassembled from the rack to remove everything and anything that may be in the fuel lines of the carbs. Any gunk in the fuel delivery passages of the carbs wont stay put and needs to be removed (with a thorough cleaning).

HondaMan · « **Reply #14 on:** May 31, 2009, 08:41:03 PM »

This uneven flow from the petcock:
while draining open and free, the petcock often only drains one side until you place your finger across that one, then it drains the other side instead. The hydrostatic head of the fuel, combined with a venturi action at the dual outlet, usually causes the one that is furthest from the petcock's internal hole to not flow fuel very evenly. In actual use, one pair of carbs always fills faster than the other on this design, but neither pair uses fuel fast enough for it to be a problem.

In other words: the uneven draining bit is probably normal here...just make sure that fuel will flow out of each one by blocking the other.

If the carbs have been dry for a long time, check to make sure the vent tubes (tiny holes in the carb bodies, they vent to little hoses above the throat area) are cleaned. Run a wire and some brake cleaner through them.

A common mistake for CB newbies: the floats upside down. This will let you set the carbs at 26mm, but will only leave about 5mm of fuel in the bowls. Could yours be in this category?

mystic_1 · « **Reply #15 on:** May 31, 2009, 10:44:58 PM »

BTW, most excellent pics, Fishhead.

Quote from: fishhead on May 31, 2009, 08:15:38 PM

Contrary to popular belief displayed on many message boards, a thorough carb cleaning means the carbs MUST be completely disassembled from the rack to remove everything and anything that may be in the fuel lines of the carbs. Any gunk in the fuel delivery passages of the carbs wont stay put and needs to be removed (with a thorough cleaning).

I agree!

mystic_1

pjed · « **Reply #16 on:** June 01, 2009, 06:19:03 AM »

Thanks for the info guys...I won't get another petcock and re-focus on the carbs. I did take every part off of the carbs that was removable, including the float needle valve seats, and soaked those parts along with the carb bodies themselves in the Gunk carb cleaner can. I ran a tiny wire through the passages I could find but will take another look at the vent tubes. If someone could provide a pic of those that would be great (total newb at carbs).

As far as mounting the floats back on the carbs, I did try it upside down first. I looked at them and thought "whiskey tango foxtrot"...lol, how am I supposed to get 26 mm set? I flipped them and everything looked much better. One of them was actually already set correctly.

Side question, can anyone tell me the pitch of the 10mm bolt that holds the top of the rear shock? Having a hard time finding those.

HondaMan · « **Reply #17 on:** June 01, 2009, 07:36:50 AM »

Quote from: pjed on June 01, 2009, 06:19:03 AM

Side question, can anyone tell me the pitch of the 10mm bolt that holds the top of the rear shock? Having a hard time finding those.

Honda still sells those: they are a fine pitch metric. I just bought some from Hondaparts-direct.com for one of my rebuild bikes.

pjed · « **Reply #18 on:** June 01, 2009, 03:50:27 PM »

Finally got the thing running. Noticed this time that gas was flowing fine when on reserve...embarassed to say that the tank must have been too low for the regular setting.

Tank being heavier than on my other bike, I thought it had plenty of fuel.

Actually revs now...taking everyone's advice and cleaning the carbs thouroughly definitely helped. It does start miss at about 4K and won't go past. I'm going to change the plugs and synch the carbs, see what happens

Thanks again everyone. Sometimes you have to look at the most basic explanation.

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Author Topic: 1974 CB750 fuel lines (Read 10090 times)