• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

My CB750F Supersport 1982 wont pass 5000rpm.

gkmaia

CB750 New member
Messages
4
Reaction score
0
Points
1
Location
New Zealand
Hello!

My CB750F Supersport 1982 wont pass 5000rpm.

I bought it in July and restored it entirely. Turned it on a month ago for the first time and since then just low revs to adjust cabs, leaking, etc.. all good so far.

Toke it for a ride today. I don't know how to define what it does but feels like she is choking when I try to pass 5000 rpm and it wont pass by any chance. It runs perfectly fine under 5000rpm, but wont pass.

U guys think it is an ignition timing issue? Or it is more likely to be the carbs not delivering enough petrol? Or lost of pressure in the intake rubbers? WHat is the most common cause for this kind of issue.

I have attached my timing. I did it static, don't have the strobe tool.

2013-12-05 18.02.35.jpg
 
Well it could be really rich. Will it rev over 5,000 rpm in neutral? Could be the timing. Can you borrow or rent a strobe? What does it do if you advance or retard the timing a little bit?
 
Hey Travis,

It does the same in neutral.

Probably gonna have to rent a strobe, will try to figure it out.

But in the mean time I will try what you said.

Two questions then:

If I turn my coils clockwise what would it be? Advancing or retarding?

To get the pistons after they reach the top and when they are starting to move down would I have to advance or retard?

Cheers,
Gui
 
I tried to retard and advance.

If i retard, cant pass 5000rpm.

So, I got better results with advance as per image. Now I can reach 6500 rpm, sometimes 7000rpm. Feels like at this time she starts loosing power and it really forcing the engine. Don't feel like I should be going any further than that. This is not a CBR1000 right?

Anyone knows the max revs I should be able to achieve? Is it around 7000rpm?

One interesting fact. If I full throttle from idle it starts choking at 5000rpm. But if I progress gradually it passes 5000rpm up to 6500-7000rpm. Anyone knows what could be causing that?

2013-12-06 10.23.26.jpg
 
I had a similar issue with my CB900 a couple of years ago. I can't be much help because I don't know exactly what I did that fixed the issue, but I always thought it was just that I didn't get the carbs clean ed well enough. After the 6th time I finally got it to run well.

You say you restored it... so you're running the stock air box with paper filter, right?
 
Just sprayed consistently CRC-Clean-R-Carbs in the manifold boots and outside the carbs to find some vacuum leaks. Bike was in idle and revs didn't change. Seems to have no leaks.

But then I sprayed Clean R Carb directly inside the carbs air intake, almost killed the bike. It suppose to rev up the bike right?
 
Air cut valves in side of carbs go bad all the time and cause many if not most of the running issues in these bikes.
 
I have the same problem with my 82. I also advanced the timing which helped a little. The shop manual describes a test to check the spark advancer by bringing engine speed to 6,000 rpm or above and using a timing light to check that the index mark is between the full advance marks. If it's not replace the advancer assembly. There is also a Caution- "Do not allow engine speed to exceed 8,000 rpm or engine damage may result." That answers your max revs question.
I also wonder how much we can trust these 30 year old tachs to give us true rpms.
 
Did you ever get this sorted out?

Hi

I have the exact same problem with both my 1978 Honda CB750F2 and my 1979 Honda CB900F. My 900 revs fine to 8000 in neutral but won't go past 5000 whilst riding without a struggle It revs beautifully up until 5000 revs.

Would appreciate any help on this.

Newbie to this forum.

Hello!

My CB750F Supersport 1982 wont pass 5000rpm.

I bought it in July and restored it entirely. Turned it on a month ago for the first time and since then just low revs to adjust cabs, leaking, etc.. all good so far.

Toke it for a ride today. I don't know how to define what it does but feels like she is choking when I try to pass 5000 rpm and it wont pass by any chance. It runs perfectly fine under 5000rpm, but wont pass.

U guys think it is an ignition timing issue? Or it is more likely to be the carbs not delivering enough petrol? Or lost of pressure in the intake rubbers? WHat is the most common cause for this kind of issue.

I have attached my timing. I did it static, don't have the strobe tool.

View attachment 3262
 
Both bikes with the same problem apparently... but different style carbs if both are stock. Are they stock carbs?

Have you done a thorough cleaning of the carbs?
 
Revving an engine up with no load on it is a worthless thing to do, all it shows is that the engine runs. Utterly useless as a determination of whether the engine revs under load.

On the DOHC lack of rev is usually due to too rich (pods put you there almost instantly) and even more, lack of squeeze from valves not set right. Why you loosen them up.

A stock DOHC better be able to run at 9500+ rpm in every gear or something is wrong with it.

Aircut issues and leaks at carb manifolds are vacuum leaks, they have greater effect at idle than high rpm.

Rotating the pulser plate CW is advancing on DOHCs.
 
Back
Top