Topic: 1978 CB750 Won’t hold idle. Pipes 2 and 3 cold.

[hdavis]

Hey guys,

Working on a 78’ CB750 with an aftermarket 4 into 1 exhaust. I am having a hard time getting it to hold an idle. I have all air screws out about 2 and 3/4 out which is a full turn more than I have seen it needing.

Anyway, I try turning it over and it will start briefly, rev really high (vacuum leak on brand new carb boots?) then die. When I feel the pipes after 1 and 4 are hot, 2 is ice cold, and 3 is sort of warm.

Need help figuring out how to get it to hold an idle so I can sync the carbs.

Also, the air box doesn’t seem to fit too well in the frame. Anyone else had that issue? New boots in the air box as well.

Any advice is welcome!

[Bankerdanny]

Cylinders are paired, 1-4 and 2-3. So you have an ignition problem. If you swap the connections coming out of the points, yellow to blue and blue to yellow you will likely find that the 1-4 are cold and the 2-3 work.

[evinrude7]

if what bankerdanny mentioned above doesn't work check your float heights.  when i was trying to get mine running a year and a half ago this was the issue.  check for spark as well. 

[flybox1]

with those two cyls cold, yes, it sure looks like ignition.  do as bankerdanny suggests.
you can also rest plug 2 or 3 against the head and start the bike.  check to see if you are getting spark. 
revisit setting the points gap for 2/3, see if your spark improves.
what is the resistance measurement on your plug caps?
are your plugs fouled preventing good spark?
Its easy  to do a clear tube test on the K7/8 bikes to check for adequate bowl fuel levels.

[BobbyR]

You are lucky that some knowledgeable people answered you.  You need to understand why it is more than likely ignition so you can trace down the problem. These motors use "waste spark" so one cylinder will be on the compression stroke when it's  plug fires and the other on exhaust.  1-4 and 2-3 are paired. Both spark plugs fire at the same time unlike most car engines. The engine is part of the electrical pathway. That is why when 1-4 or 2-3 drop out together there is an ignition problem.

You have two set of points if you have point or two triggers if using an electronic ignition the principle is the same. Start checking in the easiest parts first.

1. Do the spark plug held against the block test for both 2-3.
2.Switch the wires as suggested to see if the problem moves over to the 1-4.
3. Try trimming a very tiny piece of the plug wire and screw the plug cap back in so that if bites into clean copper wire.
4. Look at the points one set handles 1-4 and the other 2-3. If a pair is not opening you need to see why. Either they are way out of adjustment or the rubbing block broke off, or the condenser failed and they fused.

Keep us posted.

[Bankerdanny]

On my 1st 550 I had a problem early on where it started running just awful. Backfiring, no power, guzzling gas. The problem took a weekend to figure out an 2 minutes to fix.

The condensers are attached to their respective points with a very small nut. Mine had worked loose and the 1-4 points were not firing. Tightening the nut cured the problem. So make sure all of your connectors are properly secured. I have also had issues with the where the wires out for the point connect under the air box, it is easy to knock them loose.

[hdavis]

So I’ve tried switching the wires, and it would not turn over at all. Switched them back an it again would not turn over.

I’ve recently gotten new points due to an inconsistent timing issue as measured by a timing light in the past. 2/3 would double-fire essentialy.

I’ve got the new point installed however my timing bulb simply stays on no matter how I turn the points plate. Any ideas as to why this would be the case?

[Bankerdanny]

By turn over you mean it wouldn't start?

[hdavis]

Yes, I mean start. It does turn.

Thanks for the clarifying question.

[BobbyR]

Do you mean "timing bulb" as in a bulb or a timing light. we are asking you these quetions since the words have a specific meaning which we need to help you..

[hdavis]

Yes, a timing bulb is what I am working with.

[flybox1]

where in Seattle are you?

[hdavis]

Update: I have solved the idle problem. Took the carbs off and re-cleaned everything (go figure) then set to spec and I am able to maintain a high idle. Will be adjusting the carbs tonight to bring the idle down.

I ended up getting new points to get the timing sorted. The point for 2-3 had an odd nipple on the surface that appears to compromise the firing time.

Took it for the first ride on Tuesday 2 whole miles safely! Having clutch concerns now.

On to diagnosing those plates!

Flybox1- I am over in the Capitol Hill area. You local?

[flybox1]

Mill Creek.
PM me if you need anything....