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Author Topic: The Great (Carburetor?) Mystery: 1978 CB750F  (Read 1193 times)

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Offline takemetoseeice

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The Great (Carburetor?) Mystery: 1978 CB750F
« on: November 14, 2011, 08:45:41 AM »
Hi to all,

I'm a new CB owner, after a long venture in the world of dual-sporting, and am already truly impressed by the immense knowledge and dedication of CB riders, especially on this forum. I'm hoping you all will be able to help me out, though, with a nagging problem I'm having with my bike. Here's the deal:

The Bike: I bought a 1978 CB750F from its second owner, who was given the bike by an old man who had left it sitting for fifteen years or more. The second owner rode it only once, around the block, said it "ran terribly," then sold it to me for 400 bucks.
  • We're looking at 17,000 mile on the bike.
  • There are air pods instead of an air box.
  • There's an after-market Kerker exhaust, which kicks ass.
  • I've also put a '78 750K tank onto the bike, since the original tank was rusted through. Not the most elegant solution, but it works for now.
  • Everything else appears to be factory.

The Problem(s): although the bike starts and idles (though I had to spray a lot of carb cleaner into the air intake to get the thing going, but after much tinkering I seem to be able to get it to idle without any special help), it sometimes stalls when I open the throttle. More importantly, exhaust pipes on cylinders 2 and 4 are not getting hot, even after spraying the intake on those cylinders with carb cleaner and running the bike for a long time.

What I Have Done So Far:
  • I did a valve job, and everything is adjusted to spec.
  • I pressure checked the cylinders, and they're all reading at relatively normal levels. Some slight discrepancies, but nothing to cause this problem.
  • Contact points are adjusted.
  • There is enough fuel in the tank.
  • All four spark covers are getting a good spark, and all the spark plugs are new.
  • All jets are clear in the carb, and they are not after-market or readjusted.
  • The float bowls were all at different levels, so I readjusted them to 14mm. Spec settings are 16, I think, but I'm compensating for extra air from the air pods. I should mention that most of the floats had to be adjusted by 10mm or more, and the levels were quite uneven. But I was able to get the bike to idle before this adjustment, which is curious. It still idles now, after the adjustment, as well.
  • Ran some Sea Foam through the tank.
  • I've tried the pilot screws at one-and-a-half turns, at 1 turn, and at half a turn, with no difference.

What Happened:
  • The first time I ever started the bike, it idled reasonably well, but Cylinders 2 and 4 were not getting hot.
  • I checked the bowls on all four carbs, and all had fuel.
  • I did the valve adjustment, points adjustment, and re-checked the jets.
  • I fired her back up, but the bike at this point would not idle at all, unless I was spraying carb cleaner into the carburetor intake. There was plenty of fuel in the tank, and there was fuel in all four carb bowls.
  • I took the carb off, at this point adjusted the floats to 14mm, then re-checked the jets, cried a little, and put the carb back on.
  • At this point, the bike started to idle, but cylinders 2 and 4 were still not heating up. Water thrown on the exhaust pipes does not evaporate for several minutes.
  • Again, there is fuel in the bowls, and there is spark on all four plugs. The plug covers are not crossed or misaligned.

Here Are My Thoughts
  • I almost certainly do not have an ignition problem.
  • Fuel is getting through from the tank to the bowls, so that's good.
  • Maybe there's some kind of vacuum or air issue. The boots have a lot of gunk or sealant of some kind on them, as though they've been screwed with in the past.
  • The carb gaskets had silicone sealant on them, leading me to believe that someone was either being enthusiastic when re-sealing the carbs, or they too suspected an air leak.

Now What?
I feel stuck. I'm really unsure of where to go from here, and I've kind of reached the limit of my mechanical creativity. I hereby submit myself to the genius of the internet, and ask you all for your help.

Many many thanks!

Offline takemetoseeice

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #1 on: November 14, 2011, 08:54:30 AM »
Whoops, I said I did a valve job; I meant a valve adjustment.

Offline phil71

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #2 on: November 14, 2011, 09:10:52 AM »
if you definitely have compression on 2 & 4 then it can only be fuel. Try spraying carb cleaner on the boots (intake side, not airbox) and see if the running characteristics change at all, (up or down) if they do, you have boot problems/air leaks etc.

Offline takemetoseeice

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #3 on: November 14, 2011, 09:59:57 AM »
Thanks so much. I did in fact spray some carb cleaner on the boots, and did get some engine response. The problem is that I'm not certain if some carb cleaner is aerating into the intakes when I do this, or if it is actually getting in through the boots.

Online kandrtech

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #4 on: November 14, 2011, 10:39:40 AM »
At the risk of offense - how are your coils hooked up?

left coil connected to?
right coil connected to?
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Number of bikes my wife thinks I should own: 1.  I hope to disappoint her soon.

Offline knox-cb

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #5 on: November 14, 2011, 10:45:12 AM »
Did you check your timing?

Offline TwoTired

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #6 on: November 14, 2011, 10:50:30 AM »
Did you pull out the pilot jets to clean them? (They are pressed in, not screw in.)
Did you "prove" the entire pilot system in the carbs, by watching fluid traverse between all four of the passageway holes?
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC

Popeye: "Wrong is wrong, even when it helps ya."

Offline grcamna2

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #7 on: November 14, 2011, 11:01:24 AM »
I think you should make sure that your points & coils w/ the correct wires are all at their best & then those carb boots really DO need to be made well or replaced & after that synchronize all 4 carbs w/ a set of 4 gauges. I can only guess w/o seeing or hearing the bike..but air leaks around the carb intake boots will make your bike run strangely "on again/off again" as they pulsate & open up new air leaks.Did your engine ever have a top end rebuild ?
How does the inside of your fuel tank look ? Feed those carbs fresh & clear fuel for the best carb performance.
« Last Edit: November 16, 2011, 11:56:11 AM by grcamna2 »
75' CB400F...parts & 72' CB350 & 82' NC50 Express & 81' CB125S  I love the small ones too !
Do your BEST...nobody can take that away from you !!

bollingball

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #8 on: November 14, 2011, 07:56:36 PM »
Is your fast idle cam setup ?are all your acclerator nozzle's working?

Offline yozuri54

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #9 on: November 15, 2011, 08:31:06 PM »
Had a similiar issue with the last 78 750F I worked on. I think I had the carbs off 6 times before I figured it out. The last time the carbs were COMPLETELY taken apart for inspection and cleaning (make sure you take a few digital pictures before attempting this!) Found the following problems:
1. Partially clogged "T" fitting between carbs
2. 2 overflow tubes had hairline cracks causing slow drip - repaired by using JB Weld and slighty bigger model airplane gas line brass tubing.
3. 2 clogged pilot jets - you do have to yank them to properly clean. I used a butane torch to melt the goo and a sewing needle to gently clear the jet. Inspect with a jeweler's loupe....
3. Set the floats to 14.5mm (make sure you are not compressing the pin spring when measuring). Also, if the pin is not OEM, that can affect the measurement as well. These bikes are pretty sensitive to float height settings - many times floats are set to attempt to cure a drip problem and that can lead to a lean condition
4. 2 of the accelerator pump jets were clogged (carb cleaner/compressed air/new pump) best tested off the bike when you can actually see the fine spray in each throat

Final note - If you are using an aftermarket pipe and air pods the stock idle jets will never work right no matter how many turns out you set them. You will need to find bigger ones or drill out the stockers. Check this article -
http://www.hondachopper.com/garage/carb_info/7778_pilots/pilots1.html

Offline Spanner 1

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #10 on: November 15, 2011, 10:12:11 PM »
Nice reply Yozuri..... +1  8)

Offline vames

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Re: The Great (Carburetor?) Mystery: 1978 CB750F
« Reply #11 on: November 16, 2011, 10:40:27 AM »
The float bowls were all at different levels, so I readjusted them to 14mm. Spec settings are 16, I think, but I'm compensating for extra air from the air pods. I should mention that most of the floats had to be adjusted by 10mm or more, and the levels were quite uneven. But I was able to get the bike to idle before this adjustment, which is curious. It still idles now, after the adjustment, as well.

I admit I'm no expert on the 77/78 carbs, but I don't think that adjusting the float height is an appropriate way to try and adjust your air/fuel mixture. As I understand it, your floats height should ALWAYS be set to factory specs. There will be a lot of opinions about pods, but the folks that do use them typically compensate for pods by adjusting main jets and needle height (though I'm not sure needles are adjustable on 77/78 carbs). Many of those folks will suggest you get a factory airbox instead of the pods and bring the carbs entirely to factory specs (maybe with small adjustments for the kerker). That may be the easiest path for you.
« Last Edit: November 16, 2011, 10:42:30 AM by vames »

 

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