Topic: Idle issue on a CB750F2

[tahitianrider]

Hi everyone,

I have a 1978 CB750F2 that I've been restoring, as much original as possible (with few modifications like brake lines and blinkers) during the past 2 years.

The bike I got was a barn find, it sat for at least 20 years untouched.
The previous owner seemed to be racing this bike a lot back in the 80's since he removed the starter, center stand, remove a lot of foam from the seat, added a steering stabilizer, remove all the blinkers and so on. I also got a box of parts with some endurance light and other racing parts.
The bike also had a custom racing replica paint made by hand.

When I got the bike, I guess that the guy was working on it (only 1 shock on the rear, points cover was removed...) the story I got was that he became sick and couldn't work on the bike anymore, and it sat for 20+ or 30+ years I only the 2nd owner.

I spent a lot of time and effort restoring the bike to its original condition (original paint job, reinstalling the starter...)

I have rebuilt the carbs, I have an ultrasonic cleaner so I took them all apart, ultrasonic cleaned them, rebuild them with fresh seals and o-rings.
The ignition is also brand new with new points and condenser, set correctly.
The bike did not have any trouble to start again after 20 years!

However, I can't fix an Idle issue: when the bike is fully warmed up, it'll idle fine, but if you give just a bit of throttle, it will then stuck to 3500 rpm idle! no matter what!
Same when riding, it feels like riding with cruise control! the only to make it go back to idle is either kill it, or letting the clutch go while braking to make it stall but stop before stalling.

Here is a video that clearly shows the problem when the bike is warm: https://1drv.ms/v/s!Auu8yAfGtqmfga9TQGWygSn-pYRg1g?e=gfCxxK

I have check at least 3 times and :

- There is no vacuum leak, I've sprayed a lot of carb cleaner around the intake and the idle doesn't jump
- The carb are perfectly synchronized, I have 4 vacuum gauges, I did the sync at least 3 times
- I have checked the accelerator pump, and its return spring and it works well
- I have checked the advance mechanism on the ignition and it works well too, it doesn't stick and it is in good shape.

I have the stock jetting on the carbs, and stock airbox.

There is a lot of excess RTV on the bottom end (it can be seen behind the points cover) so the previous owner definitely spitted the case, and did some sort of bottom end (maybe top end as well) rebuild. But maybe he did some race modification to the engine as well? like a lighter crank maybe? or some different pistons or rods?

Anyway here is a pic of the pic bike as of today

[seanbarney41]

What do your spark plugs look like after running?

[RAFster122s]

With a lot of silicone on the bottom cases I would be very worried about a piece breaking free and clogging up an oil passage. I would be seriously considering pulling that motor apart, doing a thorough clean and reassemble properly

[TwoTired]

Sure looks like you did a nice job on the restore.

My theory is that your replacement exhaust isn't quite as restrictive as stock.  The restriction effects the idle mixture, with more back pressure making it, in effect, richer.  I would try to enrich the idle with the idle screws.

Of course, if there is friction in the slide return, that can effect idle too.

I'll ask if the slides were installed with the cutaway facing the air filter, as it should be.  Just a thought.  It's probably fine.

This bike was produced in the era where the EPA required a lean burn engine at idle for acceptance in the USA.  Both air filter choice and exhaust back pressure were fine tuned with these carbs to achieve acceptance to the idle hydrocarbon emission requirement.  It was the last year for these mechanical slide PD carbs.  Thereafter, CV carbs appeared on Hondas, (Later - fuel injection).  The CV carbs still met the lean burn requirement, and many had fuel enrichment diaphragms, to keep the mufflers from popping on decel and for a solid return to idle without "hang".

Cheers,

[Don R]

 I'd drop the oil pan and check the pump screen for silicone. If it clean, you're probably OK. If it's been sucking silicone be aware the cylinder head has oil jets that commonly plug up by silicone.

[tahitianrider]

What do your spark plugs look like after running?

They looked OK, not white like if it was too lean.

I'd drop the oil pan and check the pump screen for silicone. If it clean, you're probably OK. If it's been sucking silicone be aware the cylinder head has oil jets that commonly plug up by silicone.

When I restarting the engine, I did dropped the oil pan to clean it, and the pump filter screen as well, and it was clean, no sign of silicone.

Below is a picture of the excess RTV seen near the advance mechanism;

With a lot of silicone on the bottom cases I would be very worried about a piece breaking free and clogging up an oil passage. I would be seriously considering pulling that motor apart, doing a thorough clean and reassemble properly

Above is the picture, do you think it is an issue? it would be a pain to pull the motor apart at this point of the restoration

Sure looks like you did a nice job on the restore.

My theory is that your replacement exhaust isn't quite as restrictive as stock.  The restriction effects the idle mixture, with more back pressure making it, in effect, richer.  I would try to enrich the idle with the idle screws.

Of course, if there is friction in the slide return, that can effect idle too.

I'll ask if the slides were installed with the cutaway facing the air filter, as it should be.  Just a thought.  It's probably fine.

This bike was produced in the era where the EPA required a lean burn engine at idle for acceptance in the USA.  Both air filter choice and exhaust back pressure were fine tuned with these carbs to achieve acceptance to the idle hydrocarbon emission requirement.  It was the last year for these mechanical slide PD carbs.  Thereafter, CV carbs appeared on Hondas, (Later - fuel injection).  The CV carbs still met the lean burn requirement, and many had fuel enrichment diaphragms, to keep the mufflers from popping on decel and for a solid return to idle without "hang".

Cheers,

Thanks!

Yeah the exhaust it came with is a Marving exhaust I believe, in the parts bin I even found a completely open muffler, like an open exhaust the guy probably run at some point.

I'll try to enrich the idle mixture by turning the screw 1, 1/2 turns counterclockwise and see if it helps.

I'll check the slide but if I remember I could only insert them in one way, I don't remember that you can flip them around, it's been 2 years since I rebuilt the carbs I'll need to check.
Oh yeah maybe I should put a drop of 2 stroke oil on the slides to check if it helps?

Oh I see, yeah I also have bikes with CV carbs, like a 1980 CX500 and never had those kind of issue.

[tahitianrider]

Also, I don't know if this could be related or not, but when idling, the bike also sometimes makes a "pop" noise, well more like a "sucking" pop noise, it sounds a bit like if once in while, there was a back fire up in the intake manifold on one cylinder. Maybe I should try to record that sound.
Could those problem be related to bad valve clearance? or valve springs that may be too weak maybe?