Topic: carb syncing feeling

[Schnell]

Here is what I'm seeing. Carb/cylinder #1 is really sucking. Problem is, no adjustment of the needle has any effect. I've tried both directions. Any thoughts?
(small nut is a lock nut, large nut underneath it is the adjustment)

[Schnell]

To rule out the possibility that the vacuum gauge was defective, I switched lines between 1 and 2. Gauges are okay.

[BenelliSEI]

Check the choke blade on #1. Is it fully open when you lift the other three? Or is it hanging down partially blocking the carb throat?

[Schnell]

Check the choke blade on #1. Is it fully open when you lift the other three? Or is it hanging down partially blocking the carb throat?

While the choke blades were all working before reinstalling the carbs, I did have to bend the choke lever of carb #1 in order to screw in the vacuum tube to carb #2. A full choke on carb #1 would lead to a high vacuum reading, and explain why the needle adjustment has no effect. I believe you have correctly diagnosed the issue. I'll have to check by removing that damn airbox again. Thank you sir!

[BenelliSEI]

Frank….. just disconnect it and hold it fully up with a bit of wire or tooth pick. It that solves it, adjust accordingly…… Good Luck!

[Schnell]

And ... that was not the solution. Choke blade is retracted as it should be. I'm wondering it 120 main jets are just too much?

[Schnell]

I tried but could not post a video here.

I was able to post a video in this thread:

https://www.facebook.com/groups/HondaCB750/posts/6207612819344400/?comment_id=6207865792652436&notif_id=1692475912590589&notif_t=group_comment

[BenelliSEI]

Are all the needles in the same clip position?

[Schnell]

I believe so because when I removed the main jets and emulsion tubes to clean them I could see the tips of the needles uniformly protruding.

[BenelliSEI]

Did you bench sync before installation?

[madmtnmotors]

I would suspect a vacuum leak or an issue with the vacuum hose or the connection to the #1 carb.

[BenelliSEI]

Frank…. Been thinking about this. Lifting the slide should immediately decrease the vacuum you see behind it. Try popping it up with a small screw driver while bike is idling (reach in through the throat, airbox off). Following this line, you said adjusting the needle height made no difference? Has it come loose from the slide?

[Schnell]

Another logical suggestion, John. This morning I went out to the garage and removed the float bowl and main jet/emulsification tube. The tip of the needle can be seen/felt, and it does raise and lower in response to the throttle. However, I did not perceive movement in response to making a needle adjustment. Going to remove the carbs, again, and investigate the slide and needle. Thank you for your thoughts!
 

[BenelliSEI]

Maybe a clip has popped and needle is moving without the slide? Try operating the throttle and watch to see if the slide lifts?

[Schnell]

slide lifts, needle lifts in response to throttle twist.

[BenelliSEI]

Stumped.

Any chance the port in that carb is partially plugged up, under the adaptor you screwed in there?

[Schnell]

I will investigate!

[Schnell]

Problem found and fixed! Inside the slide of cylinder #1 was a disconnected needle adjustment rod due to a bent needle plate fork. I compared it with a good assembly and sraightened the fork and reassembled. Once back on the bike, the vacuum gauges read good.

[newday777]

Problem found and fixed! Inside the slide of cylinder #1 was a disconnected needle adjustment rod due to a bent needle plate fork. I compared it with a good assembly and sraightened the fork and reassembled. Once back on the bike, the vacuum gauges read good.

👍👍👍

[BenelliSEI]

Well done!

[Schnell]

Went for a little spin and the bike ran great, just like cb750's do! Only the exhaust is a bit loud and obnoxious.

Still have a new electronic ignition plate, but the bike starts and runs so well I'll hold off on that.

Still have to replace steering head bearings because there's a notch feeling in the middle, due to sitting for 30 years probably without moving. On a centre stand, the cb750's are quite forward heavy.

Maybe after the safety certificate is achieved, I'll switch back to the 4into2 Jardine exhaust that has a JBweld repair. I was told it would not pass a safety.

[BenelliSEI]

frank….. Have you had the end cap off that pipe to see it there is anything in there? Vintagecb750 sells fibre glass mat packing material if there is a “screen pipe” that can be wrapped?

[Schnell]

I'll certainly take a look. Planning on doing that and changing the steering head bearings tomorrow.

Question: which way to turn the needle adjustment nut in order to decrease vacuum? Clockwise raises the needle. Counterclockwise lowers the needle.

After a (fantastic) ride, I took temp readings off the headers soon after turning it off. #2 was hottest and the others were close. When I took vacuum readings, #2 was strongest. I'd like to make a 1/4 turn to the needle adjustment nut to reduce the vacuum and hopefully reduce its header temp.

The threads of one of the brass vacuum tubes broke off in the vacuum port of one of the carbs. Luckily I was able to the threaded piece. But now I can only do 3 at a time. So I'm not eager to use it again and break another tube. If it happens to an indoor carb again, it's a pretty big disassembly job.

Riding a different bike for the first time like I did today, is exciting, like experiencing a new girlfriend. I'm thinking of pre-marriage days. (Please delete or let me know and I'll do it if this observation is inappropriate. But it was exciting after many years won the same bike.)

[newday777]

I'll certainly take a look. Planning on doing that and changing the steering head bearings tomorrow.

Question: which way to turn the needle adjustment nut in order to decrease vacuum? Clockwise raises the needle. Counterclockwise lowers the needle.

After a (fantastic) ride, I took temp readings off the headers soon after turning it off. #2 was hottest and the others were close. When I took vacuum readings, #2 was strongest. I'd like to make a 1/4 turn to the needle adjustment nut to reduce the vacuum and hopefully reduce its header temp.

The threads of one of the brass vacuum tubes broke off in the vacuum port of one of the carbs. Luckily I was able to the threaded piece. But now I can only do 3 at a time. So I'm not eager to use it again and break another tube. If it happens to an indoor carb again, it's a pretty big disassembly job.

Riding a different bike for the first time like I did today, is exciting, like experiencing a new girlfriend. I'm thinking of pre-marriage days. (Please delete or let me know and I'll do it if this observation is inappropriate. But it was exciting after many years won the same bike.)

A quarter turn will probably be too much. It only takes a miniscule turn to change the balance and turning one effects the others.
Do Not try to sync without all 4 guages operating. Get another brass adapter and do it correctly.

[BenelliSEI]

I'll certainly take a look. Planning on doing that and changing the steering head bearings tomorrow.

Question: which way to turn the needle adjustment nut in order to decrease vacuum? Clockwise raises the needle. Counterclockwise lowers the needle.

After a (fantastic) ride, I took temp readings off the headers soon after turning it off. #2 was hottest and the others were close. When I took vacuum readings, #2 was strongest. I'd like to make a 1/4 turn to the needle adjustment nut to reduce the vacuum and hopefully reduce its header temp.

The threads of one of the brass vacuum tubes broke off in the vacuum port of one of the carbs. Luckily I was able to the threaded piece. But now I can only do 3 at a time. So I'm not eager to use it again and break another tube. If it happens to an indoor carb again, it's a pretty big disassembly job.

Riding a different bike for the first time like I did today, is exciting, like experiencing a new girlfriend. I'm thinking of pre-marriage days. (Please delete or let me know and I'll do it if this observation is inappropriate. But it was exciting after many years won the same bike.)

A quarter turn will probably be too much. It only takes a miniscule turn to change the balance and turning one effects the others.
Do Not try to sync without all 4 guages operating. Get another brass adapter and do it correctly.

Frank…. I agree. Try finding the correct die for the end of the tube and cut new threads.