Topic: Idle and low rpm issues

[fatmatt650]

My 1976 CB750F is  behaving oddly at low rpms. Coming off idle (accelerating) it feels like it's not getting enough fuel  and chugging along between 3 and 4 thousand rpms it makes an intermittent ticking/hissing noise, like pressure being suddenly released or sucked in. Compression is an even 130 all around, optically fired ignition is delivering steady, fat spark at idle, carbs are nicely synched at 20 to 22 HG, spark plugs recent replaced and gapped to spec, Kerker exhaust,
I'm thinking my next step is putting the stock pipes back on. Any insights?

[AlBundy]

Had the same problem with mine. Air was coming in between the intake manifold and the head. Try to change the o rings in the intake manifold.

[madmtnmotors]

Had the same problem with mine. Air was coming in between the intake manifold and the head. Try to change the o rings in the intake manifold.

750's do not use o-rings, they use a rubber manifold that clamps to the carb on one side and clamps to the head on the other. These rubber manifold boots get hard as a rock making them difficult to seal well even with new clamps, let alone with old stretched out clamps. The principle is the same though creating a potential for vacuum leaks. Check for vacuum leaks at the carb boots and go from there. 

[fatmatt650]

Vacuum leak sounds plausible. #1 cylinder is running significantly cooler the the rest and the intake boot on the head is not seated properly. It's as good as I could get it, I actually squirted some brake cleaner on it while running to see if the rpms would surge bit nothing happened so I assumed it was sealed properly. I'll check that first. I've never done clear tube test bit it is getting fuel, that'll be my next test. Thanks everybody. I'm trying to leave for Dolores, CO on Sat morning. Wish me luck!

[scottly]

How did you set the ignition timing after replacing the failed power-arc system? Straw though the spark plug hole, or did you temporarily replace the stock advancer to locate TDC? Read Pat's reply on this thread:
http://forums.sohc4.net/index.php/topic,167638.0.html

[fatmatt650]

How did you set the ignition timing after replacing the failed power-arc system? Straw though the spark plug hole, or did you temporarily replace the stock advancer to locate TDC? Read Pat's reply on this thread:
http://forums.sohc4.net/index.php/topic,167638.0.html

First I did it by dead reckoning looking at the valves. When I checked later with the stock advancer I was off by about 10 degrees. I was planning on playing with timing today to see if that might be the issue. I am going to carry a stock set of coils and points plate for spares this time. My second night on the road I'm planning on staying at the same hotel in Havasu as last time. Not trying to test fate TOO much.

[fatmatt650]

Just read that thread. I don't think I'm gonna be able to fabricate that tool right now but I will play around with the timing a bit.

[fatmatt650]

While I'm pulling everything to try to get that boot to seat better, any thoughts on swapping the carbs and boots from my 77F that I know are good?

[ekpent]

 As far as I know the boots are different for those years. The 76 has round top carbs and the 77 has the pumpers. Someone will correct me soon if I am wrong or compare the part numbers between the two..

[fatmatt650]

Outside diameter of boots reads 49mm on both.

[TwoTired]

Its a molded part.  O.D. doesn't prove anything about I.D.

Probably knew that, though.

Cheers,

[fatmatt650]

I figured it wasn't definitive. Gives me hope though.

[fatmatt650]

New intake boots did the trick. Living 20 minutes from 4into1 is very nice sometimes. Thanks for the advice, all.