Topic: '78 CB750k conversion to cafe racer (yes, more issues)

[deafcaferacer]

Get a generic brand like MotionPro and order it by length, not model.

Will look into that, thanks.

Well I have a '73 cb750 pull throttle cable if anyone needs one. Just send me a PM.

[deafcaferacer]

Got the cable throttle in the mail today and slapped it on, and fired her up. She fired up good and I took her out for a spin, and the rev is better at the lower RPM- that is below 4k, it comes down easily, but around 5-6k it takes noticeably longer to come down, I can feel a bit of the stickiness.

Not sure it is the boots, I tightened all four boots at both ends as hard as I could without stripping threads.

So probably have to take a look at the Spark advancer. Going to research doing that now.

carb bowl on #2 is leaking a bit but nothing is coming out of the overflow tubes..weird. May be a case of the new gasket not seating properly.

[calj737]

Be certain that you fully lubricate those cables too.

[deafcaferacer]

Be certain that you fully lubricate those cables too.

What's a good cable lubricant?

[deafcaferacer]

Have been researching taking off contact breaker plate to get at the spark advancer behind it and realized that none of the resources I've been looking at suggest how to clean the advancer or lube it (with what?). Also doesn't suggest what exactly I'm to look for when looking at the spark advancer as to what it may be doing in causing those hanging revs.

[onetruepunk87]

Basically, you want the spark advancer to snap back really good when you turn it and let it go. I took mine completely apart and cleaned it with a degreaser, but you can probably get away with just hitting it with a toothbrush and degreaser while it's still together. If you do go the route of taking it apart don't lose the springs, they don't make them anymore! I used Remington dry lube spray on the pivot points and it worked like a charm, it's made for guns but it's held up so far on the bike. It might not seem like a noticeable difference in it's snap before and after but it does the trick. I wish i did this first, it would have saved me a lot of time messing with cables and chasing my tail.

You'll have to re-do your timing after you put a everything back together but if you put a mark on your points plate where it was it should put you in the general area of where you want to be, check with a timing light to confirm. I changed my points when I cleaned mine just because I was going to be resetting them anyway. Hope this fixes your problem.

[calj737]

Be certain that you fully lubricate those cables too.

What's a good cable lubricant?

You can buy a cable lube kit and cans of cable lube at any motorcycle shop. Some use WD-40, though I think the lubes are more silicone based and tend not to evaporate over time. The kit comes with a small fixture to aide in forcing the lube straw in between the shielding and inner cable. Flood the darn thing! Also, while you're at it, do the clutch cable. I'd redo this every spring as general maintenance.

[deafcaferacer]

Basically, you want the spark advancer to snap back really good when you turn it and let it go. I took mine completely apart and cleaned it with a degreaser, but you can probably get away with just hitting it with a toothbrush and degreaser while it's still together. If you do go the route of taking it apart don't lose the springs, they don't make them anymore! I used Remington dry lube spray on the pivot points and it worked like a charm, it's made for guns but it's held up so far on the bike. It might not seem like a noticeable difference in it's snap before and after but it does the trick. I wish i did this first, it would have saved me a lot of time messing with cables and chasing my tail.

You'll have to re-do your timing after you put a everything back together but if you put a mark on your points plate where it was it should put you in the general area of where you want to be, check with a timing light to confirm. I changed my points when I cleaned mine just because I was going to be resetting them anyway. Hope this fixes your problem.

Thanks for the walkthrough.. I have been reading the manual and hondaman's bible- kinda dreading resetting the points as I can't quite get my hands on the concept behind it. Will be sure to mark the plate before taking it off.

[deafcaferacer]

Be certain that you fully lubricate those cables too.

What's a good cable lubricant?

You can buy a cable lube kit and cans of cable lube at any motorcycle shop. Some use WD-40, though I think the lubes are more silicone based and tend not to evaporate over time. The kit comes with a small fixture to aide in forcing the lube straw in between the shielding and inner cable. Flood the darn thing! Also, while you're at it, do the clutch cable. I'd redo this every spring as general maintenance.

Thanks.. on my list now.

[deafcaferacer]

Took off points

[deafcaferacer]

Does not seem to be anything wrong with the spark advancer. 

No rust, no sign of damage, and the arms snap back just fine. Although just to be safe I applied a very small amount of 3-1 oil to the pivots.

You'll note that I marked the plate with a black felt-tipped marker to help reduce the fiddling when retiming the points.

Reading up on that now.

EDIT: Should I be able to pull back the arms with ease? It felt sort of stiff when I tried to pull them back- I had to work my finger to pull them back. I have read that the springs should not be stiff.

[onetruepunk87]

I thought mine was working fine when I took it out and everything moved freely but cleaning and lubing it stopped the hanging idle for me, so I hope yours will do the same. From what I remember everything moved fairly easy on it. Setting the points really isn't that hard, especially since you didn't change the gaps at all. Make sure the bike is warmed up when doing it. You can get a timing light cheap at harbor freight and there's a good step by step walkthrough in the Honda shop manual.

[deafcaferacer]

Thanks punk.

Well I cleaned and lubed the spark advancer, put it back on and retimed the points.

Started her up and took her out for a spin and noticed my idle is extra smooth.. never had that before.. also does pretty good at low speed, nice throttle response.

But.. the hanging revs are still there at higher speed. Also noticed that it got worse as the bike got hotter.

My suspicions have now shifted to the boots. Could they be leaking? I theorize that they expand with the heat and the leak somehow gets worse to the point where the revs hangs at high speed, or spikes if I pull in the clutch at high speed.

[deafcaferacer]

Checked clamps on boots, and found two that were just a little loose and tightened them up and went on an extended ride.

High revs are still there and as previously remarked seems to get worse as the bike gets hotter.

After riding for some time, the revs will only come down if I jab the brakes or downshift, and even then, when I come to a stop, the revs will hover around 3k unless I let the clutch kick in a little to bump it back down to idle.

Any ideas?

[onetruepunk87]

Sorry it wasn't the advancer. Did you try spraying wd40 on the carb boots with it idling to check for leaks? If the rpms change from it it's a good sign of a vacuum leak.

[deafcaferacer]

Sorry it wasn't the advancer. Did you try spraying wd40 on the carb boots with it idling to check for leaks? If the rpms change from it it's a good sign of a vacuum leak.

Yes, sprayed every boot at both ends. No change in rpm. 

[flybox1]

long thread....advance r and air leak ruled out.
have you (visually) ruled out slides not seating, or snapping completely closed?
can you lower the high idle by engaging the push throttle cable?
sorry if you've covered this before.

[iron_worker]

If you're running pods then you'll need to increase your pilot jet and main jet size. Unless I missed something and you are actually running the stock air box.

Pods = increased carb throat pressure = decreased fuel pushed up through the jets by atmospheric pressure = running lean.

You can quickly test this theory by turning your idle mixture screws all the way in (full rich). If the problem gets better then you've found your issue, however, you will quickly foul your plugs like this since closing the IMS all the way will cause it to go way rich.

IW

[iron_worker]

The only thing that confuses me is when you say it gets worse when hot ... this usually would suggest that you are too rich since an engine that is warm will atomize fuel better when it's hot and actually requires less fuel. However, maybe it's just an issue of the idle speed naturally coming up as it warms up and making the problem worse.

IW

[deafcaferacer]

long thread....advance r and air leak ruled out.
have you (visually) ruled out slides not seating, or snapping completely closed?
can you lower the high idle by engaging the push throttle cable?
sorry if you've covered this before.

It's fine..  slide seats good, throttle snaps shut. Definitely nothing to do with throttle.

[deafcaferacer]

If you're running pods then you'll need to increase your pilot jet and main jet size. Unless I missed something and you are actually running the stock air box.

Pods = increased carb throat pressure = decreased fuel pushed up through the jets by atmospheric pressure = running lean.

You can quickly test this theory by turning your idle mixture screws all the way in (full rich). If the problem gets better then you've found your issue, however, you will quickly foul your plugs like this since closing the IMS all the way will cause it to go way rich.

IW

I can certainly try that, but I switched to pods last season and it worked just fine.

This revving problem was apparent right away when I pulled the bike out of storage.

But now that you speak of it, in the upper range, the throttle does feel flat especially when accelerating fast. It kicks in the lower RPM but flattens out in the upper range.

Maybe I could try turning the screws 1/2 in? When I cleaned the carbs and reinstalled the IMS, I closed all the way in, then backed out one full turn.

[deafcaferacer]

The only thing that confuses me is when you say it gets worse when hot ... this usually would suggest that you are too rich since an engine that is warm will atomize fuel better when it's hot and actually requires less fuel. However, maybe it's just an issue of the idle speed naturally coming up as it warms up and making the problem worse.

IW

Now that I think about it, I'm not sure of this anymore. The revs will stick if I twist the throttle hard enough at any point.

[iron_worker]

Well try turning in the IMS and see what happens. Don't ride too far from home though. lol

IW

[deafcaferacer]

Well try turning in the IMS and see what happens. Don't ride too far from home though. lol

IW

Turning in the screws 1/2 a turn seems to have made a big difference. When I rev to the upper range and just let go of the throttle, there is a noticeable slack in rev or power that wasn't there before.

Question, could I just leave the IMS at 1/2 turn in, or would it be wiser to change to bigger mains? IIRC mine are 115's.

[iron_worker]

You can leave the IMS at 1/2 a turn if it runs better... sounds like it could still be better though. The jet you were testing with this test was the idle jet. For a custom setup (air box mods, pods, stacks, etc) usually you change idle jet sizes until your optimum setting is 1 turn out +/- 1/2 a turn. I believe outside of these ranges the metering of fuel/air starts to change drastically.

To test if you need a larger main jet you'll need to do some plug chops at WOT.

IW