Topic: 1974 cb750

[70CB750]

Following.

Are you planning another grand tour across USA?   

Hello Prokop

We rode a few miles together on that trip didn't we?

No, I don't think I'll be doing another trip of that scale again. That was a bucket list adventure.
[/quote]

Yes we did.  340 in Shenandoah valley.

[Schnell]

Prokop, I sold that bike and split the profit with my brotherinlaw from whom I got the bike practically as a gift, and I was feeling guilty about that.

Happy now to another cb750 to play with!

[Schnell]

Anyway, cool to connect again after 9 years. Thanks Prokop!

[70CB750]

Prokop, I sold that bike and split the profit with my brotherinlaw from whom I got the bike practically as a gift, and I was feeling guilty about that.

Happy now to another cb750 to play with!

And a pretty one, too!

Thanks for the year, Frank  - I was trying to figure that out. 9 years, it was before I finished Red, I rode the K0 than. 

The K0 is dissasembled in storage waiting to be rebuilt, Red is hooked to a sidecar and my solo bike is "new" 1982 CB900C.

[Schnell]

Finished installing new coils, HT leads, and caps. Bike started and ran on 2 cylinders, diagnosed by the temp of the exhaust headers after it ran for 1/2 minute. Cylinders 1 + 2 weren't firing so it's a carb issue not an electrical issue. (Otherwise it's be 1/4 or 2/3 not firing.) I'm going to try blocking one petcock outlet and splitting the other. I did that on a CL350 that would only fire one one cylinder and this solved it. When I turn on the petcock with no fuel lines, only one outlet spews gas. The other one will work only when I block the first one.

[BenelliSEI]

That thing with only one outlet releasing fuel is not the issue. That’s a hydraulic phenomenon about “the path of least resistance”. They all act that way…….
Try switching over the hose and I’ll bet nothing changes. Your problem is elsewhere.

[Schnell]

I dropped the bowl of carb #1 (#1 and 2 cylinders not firing) and found insufficient gas.

[seanbarney41]

So that means you have float valve problem, or perhaps plugged float bowl vent or fuel tank cap vent...all very common problems for bikes left to sit

[BenelliSEI]

So that means you have float valve problem, or perhaps plugged float bowl vent or fuel tank cap vent...all very common problems for bikes left to sit

Agreed…Try pulling the feed hose off carbs 1 and 2. Open the petcock. Unless it’s plugged, it should flow steadily. Floats, float valves and float heights are next.

[Schnell]

So yesterday I dropped carb bowl #4 and turned on the petcock. Gas flowed freely. I did the same with carb #1 (1 and 2 weren't firing) and gas only slightly dribbled out So I removed the carbs -again. I had already cleaned them. Didn't find any issues. Reinstalled the carbs. I'm getting good at this, using a 2x2 stick to remove them and a ratchet strap to reseat them. I switched out the fuel lines to a shorter more direct stock routing. All cylinders were firing. I reinstalled the original airbox and was able to ride the bike around the block. It was backfiring though. Going to the Honda shop tomorrow to get bigger main and slow speed jets because of the R.C. 4into1 exhaust I put on it because the 4into2 Jardines were rusted out. Do you think 115/40 would work?

Also fit a short metal piece to the passenger footpeg bolt to keep the chain from rubbing on the centre stand.

[Schnell]

This is the best I've heard the bike run so far, with the R.C. 4into1 exhaust that came with a parts bike that I bought (for the exhaust). It makes this bike sound a bit like a superbike. It's not as quiet as I'd like. Sounds good, but a bit too forward for my mature taste. Sorry to my neighbours who up to now only had to deal with my more sedate 1981 BMW R100 twin with stock exhaust.

I've also used some valve tappet covers from the parts bike because some of the ones on mine were mangled due to over-tightening.

Another job I did on the bike today was to make a stop to keep the centre stand from coming up so far as to have the chain run on it.

[RAFster122s]

Congrats

[HondaMan]

So yesterday I dropped carb bowl #4 and turned on the petcock. Gas flowed freely. I did the same with carb #1 (1 and 2 weren't firing) and gas only slightly dribbled out So I removed the carbs -again. I had already cleaned them. Didn't find any issues. Reinstalled the carbs. I'm getting good at this, using a 2x2 stick to remove them and a ratchet strap to reseat them. I switched out the fuel lines to a shorter more direct stock routing. All cylinders were firing. I reinstalled the original airbox and was able to ride the bike around the block. It was backfiring though. Going to the Honda shop tomorrow to get bigger main and slow speed jets because of the R.C. 4into1 exhaust I put on it because the 4into2 Jardines were rusted out. Do you think 115/40 would work?

Also fit a short metal piece to the passenger footpeg bolt to keep the chain from rubbing on the centre stand.

I'd suggest 110 and 40 for the jets with that exhaust.

[Schnell]

I'd suggest 110 and 40 for the jets with that exhaust.
[/quote]

Thank you sir!

[Schnell]

HondaMan, on my trip around the US, my (then) 1975 cb750 really felt the effects of altitude. I remember thinking something was wrong with it when I got into Colorado, but it was simply the higher altitude.

So I'm well aware that altitude plays a role in choosing jet sizes.

Where I live, the altitude is less than 300 metres above sea level. Do you still recommend 110/40 for jet sizes at this altitude?

[Schnell]

This mid-morning update.

I checked the main jets on the parts bike I bought mainly for the exhaust, and found that the carbs had 120 main jets.

On my bike, I'd already installed the airbox so I figured it'd be less work to try to change the jets with the carbs still installed, and that worked. I found that the bike already had 105 jets but I switched them out for the 120's. I didn't try the piolet jets because getting a screwdriver in there would be difficult. The main jets I switched out with the emulsification tubes attached.

The bike ran much better and didn't cut out or stumble at higher rpm. The parts bike came in handy again.

[Schnell]

Just got back from an illegal ride around the block. Boy does this thing go! Fun! Fun! The exhaust kind of howls. Sorry neighbours.🙂 The idle hangs up in higher rpms but I've read that syncing the carbs helps with that. I checked the headers with a heat reading gun and there's some differences, which also indicates carb sync is necessary. I'll also recheck carb boot air leaks with a flammable spray. WD40 works, right?

[Schnell]

Different topic: I bought a new reg/rect combo unit to replace the 2 old original units. The new unit has a plug with one red, one green, and 3 yellow wires. That plug fits perfectly into the plug that the original old rectifier is plugged into.

My question is, what do I do with the 3 wires that are currently plugged into the original old regulator after I remove it? Thanks!

[70CB750]

Just got back from an illegal ride around the block. Boy does this thing go! Fun! Fun! The exhaust kind of howls. Sorry neighbours.🙂 The idle hangs up in higher rpms but I've read that syncing the carbs helps with that. I checked the headers with a heat reading gun and there's some differences, which also indicates carb sync is necessary. I'll also recheck carb boot air leaks with a flammable spray. WD40 works, right?

On the K0 I shortened advancer springs by half a circle.  It helped with the return to idle speed. 

[BenelliSEI]

Just got back from an illegal ride around the block. Boy does this thing go! Fun! Fun! The exhaust kind of howls. Sorry neighbours.🙂 The idle hangs up in higher rpms but I've read that syncing the carbs helps with that. I checked the headers with a heat reading gun and there's some differences, which also indicates carb sync is necessary. I'll also recheck carb boot air leaks with a flammable spray. WD40 works, right?

Brake Kleen is my favorite spray, or quick start.

[newday777]

Different topic: I bought a new reg/rect combo unit to replace the 2 old original units. The new unit has a plug with one red, one green, and 3 yellow wires. That plug fits perfectly into the plug that the original old rectifier is plugged into.

My question is, what do I do with the 3 wires that are currently plugged into the original old regulator after I remove it? Thanks!

I haven't installed a r/r so here ya go(I'm old school)

http://forums.sohc4.net/index.php?topic=139819.0

[Schnell]

Thank you for the link. The reg/rect I bought does not have the 2 separate wires, just the bundle to the plug.

I did learn that the green wire to the old regulator is ground, but still don't know if I can just isolate the other 2 regulator wires.

[HondaMan]

The idle hangs up in higher rpms but I've read that syncing the carbs helps with that.

This 'high idle hang' can occur when the mainjets are a bit too rich: the unburned fuel tends to accumulate and wet the intake tract, adding unburned fuel to the idle setting when the carb slides close - then eventually it settles back down (10-30 seconds later, that is...). If the bike had #105 mainjets (from Honda) then it has the post-1972 cam in it (which came out 3/72 in these engines) and that was the normal jetting for that cam. Going to #110 today will let you use lower octane fuels (saves money!) because modern fuels burn much slower than the 1970s versions.

The mainjet change for using the more-open pipes (like from HM341 to HM300 or some form of 4-to-1 pipe) is a #5 size up from the original size, plus [usually] also dropping the needles in the slides by 1 notch - to their middle notch. Is the parts bike a K0 or K1 version? The K0 had #120 mainjets for a little while, which Honda quickly dropped to #115 by the end of 1969. None of the 3/70 750K1 and later bikes ever had jets bigger than #110 from Honda.

And, like 70CB750 ^^^ said, trimming back the spark advancer springs will help some more, especially if yours has the AD125 Hitachi spark advancer.

[Schnell]

Thank you HondaMan, I really appreciate your knowledge and advice! The parts bike is a 1974 but it was modified and had its jet changed by a previous owner when he switched over to pods and the R.C. 4into1 exhaust.

Do you happen to know what to do with the 3 wires that are plugged into the old regulator once it's removed?
(The new reg/rect combo unit plugs right into the old rectifier plug.)

[Schnell]

In order to get the bike safety certified, insured, and on the road, I have 3 more jobs to do on it. Replace rear turn signals, service the front brake system, and replace steering head bearings. Hoping to get the first 2 done today. I have an uncertified cb450 that I can borrow the turn signals from. From my parts bike there are aftermarket turn signals but they have LCD bulbs and the bike's flasher doesn't like them - flashes too slowly. I guess there's a difference in resistance compared to an incandescent bulb. The front brake system needs to be flushed with new brake fluid and possibly pads replaced. Replacing steering head bearings is not a job i'm looking forward to. I've done it before on a previous cb750 I've owned. There's still an electronic ignition system and new reg/rect to install but they're not needed for the safety certification. Oh, I forgot the carb syncing still needing to be done to get the bike running/idling better.