Topic: 1978 CB750k Carb rebuild over the winter and tune up questions!

[fendersrule]

I haven't made any adjustments yet. You guys let me know the next steps.

Point condition pics. Very hard to take pics of points, but I believe that they look fine:





I do have some Diattchi MIJ points if you guys think I should replace them. I think It comes with condensers too.

Current left gap: .260mm
Current right gap: .320mm

Here's my meter:



Since I'm choosing to NOT set the points by a feeler gauge, what number do I go off by on this meter (46-49 or 23-24.5)? It's really hard to tell. Right now with the meter hooked up to the left points, the meter moves to about the "35" position on the top of the dwell scale, and then falls back.

[fendersrule]

I also checked static timing. It's pretty damned spot on with the points gap as-is.

I assume after I adjust the points with your guys' help then it will alter the timing.

The bike takes a while to start up when it's cold. Starting the bike up in the dark while coming home from work (about 38F out) a few nights ago took about 12-14 tries, with the choke all the way out, gently giving it some throttle once every few tries. I almost felt like moving to my kick starter because I was afraid I was draining my battery too much.

[fendersrule]

Here's an update. Supposedly I am not very smart and didn't realize that the engine had to be idling to measure dwell.

Here's how I tuned my CB750's ignition:

1) Set Dwell on Left points. 24.5 exactly w/ V8 meter.

2) Set Dwell on Right points. 24.5 exactly w/ V8 meter.

3) Set left point timing. I moved the crankshaft to the 1-4 F mark. I turned on the ignition and positioned the larger plate until my OHM picked up voltage, and then I slightly and carefully backed it until the voltage dropped. Tightened. Rechecked.

4) Set right point timing. I moved the crankshaft to the 2-3 F mark. I turned on the ignition and positioned the smaller right until my OHM picked up voltage, and then I slightly and carefully backed it until the voltage dropped. Tightened. Rechecked.

Basically, doing steps #3 and #4 will cause the light to NOT come ON when the crank is set to the F marks on the right or left side, but if you keep rotating then the light will come on VERY shortly after the mark.

I believe my ignition is set properly.

My idle is set to 1100 RPM.

I think I found the problem with the performance. When I "wrap" the trottle, the bike dies if I leave it "wrapped".

Does this mean I basically have to pull off the carbs and start hitting things from that front?

[Black 750K8]

I have never done the points the way you did. Not saying it is wrong but if I were you I would go back and check the points gap to see what it is.

[fendersrule]

Rechecked the dwell after I finished and it was still pretty much spot on. I had to perform a slight adjustment to the right points because it went down to "23" instead of "24.5".

[andrewk]

I'm in agreement that you might want to double check the points gap with a feeler gauge.  You don't have to change it this way, just measure it.  This will give you definitive proof that the 24.5 setting on the meter is the point gap you want.  As another example, my (much older) "combo" (8-6-4 cyl) dwell meter rings in at 30 degrees or so for a proper .013 to .014 (in) gap.

The real beauty of the dwell meter is being able to balance the points perfectly.  A few more sessions with it and you'll have it down pat.

Regarding carb "kits" since you're approaching that stage now- I would highly recommend buying OEM carb components.  You won't find much in the way of an OEM "carb kit" that resembles a keyster or k&l.  Honda sells the components piece meal, the exception being the gaskets, where they offer a proper gasket set.  The parts cost more per piece this way, but the other side of the coin is that you won't be replacing every brass component, so you'll only buy what you need.

There are a few discount places to get OEM Honda parts.  These are brick and mortar dealerships that sell online for heavy discounts.  The shipping is expensive for one part, but economical enough (if you consider the cost savings on parts) if you make a complete order for a project.  www.hondabike.com is the one I'm currently using the most.

Hope this helps!

[fendersrule]

Here are my point readings:

Left: .330mm

Right: .305mm

with a 24.5 dwell.

Should I change? It's in spec according to the manual.

[fendersrule]

Hi guys,

Thanks for all the help so far (and Flybox!).

I spent a solid day out in the shop. Adjusted Cam Chain Tension. Verified that my accelerator jets are shooting gas.

I started to adjust the pilot screws with the meter.I felt like I got that in the ballpark. I verified the points setting (see previous post).

None of the above stopped the "gulp" sound when you wrap the throttle. If you hold the throttle after the wrap, then the bike will die. I think even if my dad were to bring up his analyzer so we could fine time the pilot screws, I don't see this fixing anything. I verified that the jets are stock (at least on the first and fourth carb).

Now in a "real world" scenario the bike is fine. But I'm sure because I'm not able to "wrap" the throttle then the performance is probably affected.

Do you guys know what causes this behavior? If it's just in need of a rebuild, I will have the pops take em home with him. But if there's something that's easily diagnosed with the PD carbs, let me know. My throttle cables need replacing anyhow because I can rotate the grip "backwards" to lower the RPM or to kill the bike.

It seems like compression is "good enough" to rebuild the carbs only this winter.

Any other advice is very appreciated.

[fendersrule]

I made a video for you guys.

http://youtu.be/aTV2UbAvuUY[

Keep in mind that it has ALWAYS done this since I've bought the bike. This didn't come about by anything that I've done. It did this regardless of all of the improvements that I have made to the bike.

It just never affected real-world driving. But that's the point of the winter...to fix these types of gremlins.

Anyone experience this and know what the fix is?

[flybox1]

change your plugs to D8EA's or Denso X24ES-U   (the D8ES-L plugs are a little colder)
replace your plug caps, if you havent already. with new 5Ohm caps.


on the PD carbs, there are FUEL screws(not pilot screws).  turning them out = richer mixture at idle. 

how does your bike react when you firmly roll the throttle?  are you riding when you do this test, or is it in neutral, unloaded, in your garage?  does it wind up easily.
these are not FI bikes.  they will never act like one, however, the PD carbs accel pump is a nice feature.  an aggressive roll should NOT hesitate at all.

Starting....do not pull your choke lever until AFTER you prime the carb throats.  Actuate the throttle/accel pump first, then pull lthe choke and hit the starter.
----verify your fast idle cam is engaging, and lifts the slides when you pull the choke----
Some accel pumps squirt, but without enough force to get through that tiny hole on the butterfly valve. 
Your squirt will get blocked most of the time.  Give it a few good twists  first, then pull the choke.  hit the starter.  should be easier 

[andrewk]

If you choke it, does it improve?

Since you say you've confirmed accel pump squirting, I'd be inclined to think you might have a clogged idle jet/passage.  PD idle jets are press in, use firm pressure and they will remove ok. tap back in place gently.

Could also be an air leak- how are the boots and clamps?

[fendersrule]

Hi all,

I can double check for an air leak. I sprayed WD-40 on the boots when I purchased the bike and didn't see any bubbling. I can certainly check again. I know the boots from the airbox to the carbs are perfect as I had that disassembled and inspected. The boots from the carbs to the head look great, but I will really look again when I get home with a mirror.

I can certainly try to wrap the throttle when the choke is partly on to see if it improves. I suppose this would determine if the bike is getting too much air. Keep in mind I don't expect the bike to respond to a throttle wrap until it's warm, so Id don't expect it to do this while it's cold.

I can for sure take out all of the press-in jets and make sure that they are clean. It's a pain in the butt to get the #2 and 3 bowl off....I'm going to want some special screwdivers for this task. Any recommendations?

I figured out this weekend that those jets just "press" in place. I kept turning for about 30 seconds to realize that it wasn't getting tighter.

The bike was already warm in the video, in neutral, on the center stand. The fuel mix screws were not optimized. I really need an exhaust analyzer to really set them perfect as setting them by RPM is too wide of a range.

The plugs are new...D8EAs. The plug caps appear to be stock. Can these be measured and if so how?

I'm glad there's still things to do before deciding to tear down the carbs.

Keep the responses coming. Sounds like my next steps are to remove the jets on all carbs and inspect, as well as looking for airleaks.

I read a post by lucky that when syncing you are suppose to use the screws on the top of these carbs. I want to verify with my dad that we actually did that.

[flybox1]

Nope...time to tear into them.  They need a thorough cleaning.
pull out all pilot jets, and unscrew the main jet and emulsion tube.
ensure all the tiny holes in ^ are clean and clear.  carb cleaner and air are your friends.
pull off the accel pump bowl, clean/clear out the mickey mouse ears of the pump diaphragm.
spray carb cleaner in the holes in the bottom of the accel pump bowl to clear the check valve. (wear eye prot)
do the same to clear the check valve at the top of carb bowl no. 2
DO NOT LOSE the small bowl oring at the top of carb bowl no.2 
i shot mine into the abyss once.  had to run to an auto store for a new viton o-ring 

since your accel pump squirted, no need to do any more to it.
your plugs should test out at ~ 5K ohms.  Put your ohm meter leads on each end to test.  if they arent close to 5k ohms, get new ones, and cut back your plug wires 1/4" to get to the fresh copper.

vacuum adjustments are done through the top caps on the carbs.  #2 is the slave carb, and cannot be adjusted.

[lucky]

Gotcha. I did not clean the point cam on the CB450 when I replaced the points and timed it, so maybe this would be a good idea to do. Good to know high temp brake grease is used after it's cleaned. Would mineral spirits, lacquer thinner, or gasoline work instead of acetone?

So you basically treat the dwell meter like a 12V light (the same way you hook it up). I'm just not sure what the "numbers" should be for what I'm looking for. The meter I have has a VOLTS, RPM, and DWELL switch.

My father is coming up and he knows how to use the dwell, but he asks for what spec, or number that we should be seeing. As I understand, a dwell meter replaces feeler gauges, so there has to be a spec, right?

The dwell meter does NOT replace the 12 volt test light or the ohm meter.
The volt ohm meter set on the ohms scale with key OFF is better than the test light or buzzer. ( I won't get into that discussion right now.)

The dwell is the amount of degrees of rotation of the crankshaft that the coils are building up a field to be released to the spark plugs.

It is measured in degrees.
 It should be something like 27º I think.
It may read 50º but you devide by 2 because it is a 4 cylinder engine not a V-8.
Read the instructions that come with it. It is easy to understand.

[fendersrule]

Might be a bad question, but since my plug wires are sealed to the coils, how would I test for resistance?

[Black 750K8]

(1) Nope...time to tear into them.  They need a thorough cleaning.
pull out all pilot jets, and unscrew the main jet and emulsion tube.
ensure all the tiny holes in ^ are clean and clear.  carb cleaner and air are your friends.
pull off the accel pump bowl, clean/clear out the mickey mouse ears of the pump diaphragm.
spray carb cleaner in the holes in the bottom of the accel pump bowl to clear the check valve. (wear eye prot)
do the same to clear the check valve at the top of carb bowl no. 2
DO NOT LOSE the small bowl oring at the top of carb bowl no.2 
i shot mine into the abyss once.  had to run to an auto store for a new viton o-ring 

since your accel pump squirted, no need to do any more to it.
your plugs should test out at ~(2) 5ohms.  Put your (3)voltmeter leads on each end to test.  if they dont, get new ones, and cut back your plug wires 1/4" to get to the fresh copper.

vacuum adjustments are done through the top caps on the carbs.  #2 is the slave carb, and cannot be adjusted.

This is all good information but a couple of things are not quite right (2) should be 5 K ohms. (3) should be ohm meter.

When checking for a vac. leak don't look for bubbles the fluid will be sucked in and try using some starting fluid in stead of WD40. You don't look for anything you listen for the engine to pick up speed while spraying and it will drop back down when you stop if you have a leak. Like Fly said you really should pull the carbs the carb body has a lot of small passage ways you just can not clean them well enough on the bike.

[flybox1]

haha...thanks for the corrections, teach    corrected my post above
yes, caps are 5K ohms, and i assumed  his meter would be switched to read ohms 

[fendersrule]

How specifically do I test the plug caps?

I think that I will first fine tune the fuel mixture screws when my dad comes up next week. I was planning to drop #2 and #3 bowl just to check the jets to make sure they are clean, but this seems like a very difficult task to do with the carbs still on the bike. Have you guys dropped the bowls while the carbs are still on, and if so, what screwdiver/tool have you used?

I will also spray some starting fuel around the boots while maintaining about 2k RPM to see if I see any drop.

At that point, then I will agree that the carbs need to come off as everything that could have been verified has been done.

[flybox1]

You dont test the plugs, you test the plug caps...
Dont waste time wrestling with the bowls while the carbs are on.  Get the carbs off and do work 

[fendersrule]

I performed a boot resistance check.

My OHM meter was set to 20k.

#1: Unstable. Most of the time it was dead. When I moved the connectors around sometimes I got a reading, but most of the time there was no reading. When I DID get a reading, it was 13+, and then around 4. I guess this means this one is bad.

#2: 4.98

#3: 5.6

#4: 5.93

Could this be causing it? If I need to replace them, does someone sell a CB750 SOHC boot package?

[flybox1]

 There you go! Not a good spark on that one. replace it.   all others are good, but I'd still cut the wires back on each one to get to good copper.

[fendersrule]

Like, cut off about 2-3mm?

I suspect that the hole in the wires runs all the way through so that the plug has something to insert into?

I'll order 2x NGK XD05F Resistor Spark Plug Cap, and 2x NGK SD05F Resistor Spark Plug Cap

[flybox1]

cut a little and look, but not more than a 1/4" 

[fendersrule]

Cool Flybox. At least this is one $20 thing to try. Even if it doesn't fix the problem, it will be a great tune-up thing to do for a low cost.

[Black 750K8]

haha...thanks for the corrections, teach    corrected my post above
yes, caps are 5K ohms, and i assumed  his meter would be switched to read ohms 

Hey Fly I hope you know I did not mean anything negative toward you or Fender

@ Fender For  bowel removal on the bike I use a palm ratchet a short extension.

Plug caps. Bottom of page
http://www.z1enterprises.com/ShopByCategory.aspx?page=2&itemsPerPage=20&name=Ignition&description=Points%2c+condensers%2c+electronic+ignitions%2c+coils%2c+e&category=01010205070507