Topic: I have my old cb750 back! Full restoration begins.

[CB750R]

I have had my eye on that Hindle pipe, how do you like how it fits, functions, sounds?

[grcamna2]

It does thankfully. It’s only 12x18 feet, so quite squishy in here. But I can warm it up to 15-20C on the coldest days without issue. I’m planning a new shop for about two years from now that will give more a bit more floor space.


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I know the value of a good size shop  Glad to hear you're able to build $ one on property that you own?

[markreimer]

I have had my eye on that Hindle pipe, how do you like how it fits, functions, sounds?

All good. It’s great in all three of those categories. The sound is fantastic. Not too loud. Very thick and tight. This is an early version, which doesn’t allow passenger pegs unfortunately. The newer ones do though.


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[HondaMan]

I just read thru all of these (I'd forgotten about your thread!), and one glaring item that is missing re: black, sooty plugs, is:

the intake boots. When they leak vacuum, you get rich mixture in the 2000-4800 RPM range, cleaning up somewhat above that. This is due to the vacuum being too low in the carb throat for the engine speed, so it mixes like the engine is running slower, which is naturally richer for that RPM when the intake pulse has more momentum.

They are likely old and probably quite hard, and the old OEM clamps won't squeeze them tight anymore? I have 'fixed' some of these by taking off the clamps (especially at the engine side) and re-flattening the seat where the screw heads have pulled partway thru, then i get some modern ISO-type screws with their larger heads, and install those. This pulls the clamps a little tighter.

If that doesn't work: I just got some OEM intake boots from Honda: they are making them again!

I have also used (these might be hard for you to get...) the Oetiker aircraft clamps, size 050-09 here:

https://www.aircraftspruce.com/catalog/appages/oetiker.php?clickkey=23818

on my own bike and others, as they allow finer control over the clamping force. Beware: these being aircraft-type parts, they have ENORMOUS screw leverage and this can let you cut off the hose entirely (don't ask how I know...) before you feel like they are 'tight'.

And, there is always the 'test' for the problem, which is: soak the old boots in a mix of wintergreen and xylol for 4-8 hours to temporarily soften them up, then install them and try things out to see if it improves. They will turn hard again after a while, but maybe they will distort enough to give a decent seal for a while?

[markreimer]

Thanks hondaman. These actually are new OEM Honda replacements I put on a couple years back. They’re still good and soft. I tried aftermarket’s first. They split really quickly so I spent the money on the real deal.

I noticed something new today that might be contributing to my plug condition. The rotor on my pamco ignition is very worn where the “nibs” on the advance weights interlock. There is a bunch of slop now. That means my timing will be out.

That, and my #2 carb continues to produce weirdly dark gas when i drain the float. It suggests there is some varnish in there somewhere that’s getting dissolved. So I think I have a few things to work on.

I messaged to see if I can buy a new rotor. If not I may go back to points, if I can find them.. but I think I gave them away years ago. And I’ll clean out that carb again.


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[Don R]

 If it's a foam float check it for a pinhole in the coating. I had one in my drag car that got a nick in it, then a tiny hole formed and a rubber band of melted float hung down and attached to the floor of the bowl. That held the float down in the spring when I turned the fuel on. Instant flood of fuel. Crazy, right?

[grcamna2]

If it's a foam float check it for a pinhole in the coating. I had one in my drag car that got a nick in it, then a tiny hole formed and a rubber band of melted float hung down and attached to the floor of the bowl. That held the float down in the spring when I turned the fuel on. Instant flood of fuel. Crazy, right?

I've checked the weight of certain old foam floats compared to new ones;the old ones were heavier. fuel-logged.. soaks in over time.

[HondaMan]

If it's a foam float check it for a pinhole in the coating. I had one in my drag car that got a nick in it, then a tiny hole formed and a rubber band of melted float hung down and attached to the floor of the bowl. That held the float down in the spring when I turned the fuel on. Instant flood of fuel. Crazy, right?

I've checked the weight of certain old foam floats compared to new ones;the old ones were heavier. fuel-logged.. soaks in over time.

Have you seen this in the PD-type carbs, the ones with their 'boat'-shaped floats?
I haven't seen it in the round ones that replaced the early brass ones, though.

[grcamna2]

If it's a foam float check it for a pinhole in the coating. I had one in my drag car that got a nick in it, then a tiny hole formed and a rubber band of melted float hung down and attached to the floor of the bowl. That held the float down in the spring when I turned the fuel on. Instant flood of fuel. Crazy, right?

I've checked the weight of certain old foam floats compared to new ones;the old ones were heavier. fuel-logged.. soaks in over time.

Have you seen this in the PD-type carbs, the ones with their 'boat'-shaped floats?
I haven't seen it in the round ones that replaced the early brass ones, though.

I have not seen it in CB750 models w/ PD-type carbs,but certain other models of vintage Hondas I was working on two decades ago. I think it's worth checking lately;letting the foam floats dry-out then check the weight of it compared to a new one.I really don't trust this heavy Ethanol-laced stuff for anything fuel related;I want Ethanol-Free gasoline,but here in Ca. it isn't at all available for less than $10 a gallon at just a few stations state-wide.

[HondaMan]

Thanks hondaman. These actually are new OEM Honda replacements I put on a couple years back. They’re still good and soft. I tried aftermarket’s first. They split really quickly so I spent the money on the real deal.

I noticed something new today that might be contributing to my plug condition. The rotor on my pamco ignition is very worn where the “nibs” on the advance weights interlock. There is a bunch of slop now. That means my timing will be out.

That, and my #2 carb continues to produce weirdly dark gas when i drain the float. It suggests there is some varnish in there somewhere that’s getting dissolved. So I think I have a few things to work on.

I messaged to see if I can buy a new rotor. If not I may go back to points, if I can find them.. but I think I gave them away years ago. And I’ll clean out that carb again.


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If the timing is advancing too soon, that will make the carbs run rich. This happens because the spark occurs while the intake valve is still open and the inertia of the incoming mixture isn't yet enough to overcome the 'kickback' of the waste-spark burning fuel during the overlap. Normally, over time (with stock timing) the intake valve opens a couple of degrees later about the same time the OEM spark advancer springs get too soft from annealing, and this process balances itself out (Honda engineer's design genius) until either the cam chain gets renewed or the engine gets rebuilt with new cam chain tensioners (or both), and then the early-spark situation raises its ugly head as 'spitback' that makes for an apparently (but not really) too-rich mixture. I have seen it much, much worse with the heavy rotor from the Dyna S units, which tends to stay advanced longer as the engine decelerates when the springs weaken.

So, if possible, check to see where the advance begins, and when it reaches full advance. Don't let it reach full before 3k RPM with our modern fuels, for best results. Mine doesn't reach full until 3300 cold or 3500 hot, and is much happier.

[Don R]

 The 750 F1 I've been re-assembling had the loose advancer springs. The idle would go up and down, my brother diagnosed it right away since we had the same issue on his bike.

[HondaMan]

The 750 F1 I've been re-assembling had the loose advancer springs. The idle would go up and down, my brother diagnosed it right away since we had the same issue on his bike.

Wow, those are REALLY loose!
What I've seen in the later advancers (post-1975 bikes) seems to be spring that have either lost their [metal] temper, or were just cheap? The open coils at the ends of these springs then spread out on both ends (worse on the fixed-post end) and make it almost impossible for the weights to fully retract. Then, when the timing gets set to "T" marks at idle speed like 1000 RPM, it won't fully advance. This is due to it already being partway advanced at 1000 RPM from those loose spring ends.