Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: brianm on September 29, 2010, 04:01:05 PM

Title: F2 running problem
Post by: brianm on September 29, 2010, 04:01:05 PM

Hi Guys

I am new to this site and to Honda's in general.
In the summer i bought a 1978 CB 750 F2 from ebay and put it all together and got it MOTd. Now it runs well but will not idle below 2000rpm. Also when it gets to about 70 mph it is almost at red line on the rev counter. I am presuming this is not right.
The tappets have been set, the compression checked on each cylinder, new ignition coils fitted, new spark plugs fitted, timing checked, checked for leaks around the carb rubbers, and the carbs have been balanced.
Can anyone give me any ideas as to why the revs are so high?
Cheers
Brian

Title: Re: F2 running problem
Post by: SteveD CB500K0 on September 29, 2010, 04:41:37 PM

What revs does the rev counter say at 70mph? (Others on here with F2s will advise as to whether this is correct)

Honda rev counters are not necessarily interchangeable (even though they may look similar) and run on different ratios.
There's a FAQ on the US site (http://forums.sohc4.net/index.php?topic=265.msg435880#msg435880) which gives the different ratios.

Show us a photo of the clocks.

Steve

Title: Re: F2 running problem
Post by: brianm on September 29, 2010, 05:06:08 PM

When doing 70 mph the revs are about 8000 rpm.
Brian

Title: Re: F2 running problem
Post by: Bryanj on September 29, 2010, 05:11:03 PM

8,000 at 70 is not right, what sprockets are fitted.

Would guess that bad idle is the pilot jets blocked

Title: Re: F2 running problem
Post by: brianm on September 29, 2010, 05:15:37 PM

The rear sprocket is 0333 43, don't know about the front. The carbs have all been stripped and cleaned.
Brian

Title: Re: F2 running problem
Post by: SteveD CB500K0 on September 29, 2010, 05:52:27 PM

Looks like you have a 550 tach. Here's a picture of my 550K3:
(http://forums.sohc4.net/index.php?action=dlattach;topic=6961.0;attach=36789;image)

If the guys in the US are correct and the ratios are indeed:
CB550 20:3
CB750 4:1

Then your rev counter is reading a factor of 1.67 to high.

750 guys. Is 4800rpm at 70mph more reasonable?

If so, looks like a quest for a replacement (750) tach.

Steve

Title: Re: F2 running problem
Post by: Bryanj on September 30, 2010, 06:26:26 AM

Carbs may have been cleaned but were the press in pilot jets removed?

Title: Re: F2 running problem
Post by: F2 Paul 876 on September 30, 2010, 07:14:30 AM

Here is a photo of uk spec F2 clocks.
Your rev is not a F2 rev counter.
Paul

Title: Re: F2 running problem
Post by: brianm on September 30, 2010, 01:27:41 PM

Thanks, I will try and source a replacement tacho.

Title: Re: F2 running problem
Post by: F2 Paul 876 on September 30, 2010, 06:35:49 PM

Here are the part numbers for you, its number 13 on the list.
Paul

Title: Re: F2 running problem
Post by: brianm on September 30, 2010, 10:31:51 PM

Thanks, I got a used one from DK motorcycles. Hopefully that will do the trick.
Cheers
Brian

Title: Re: F2 running problem
Post by: Yoshi823 on October 04, 2010, 11:28:25 PM

70mph at 8000rpm...that's not bad for 2nd gear...

But seriously...do the wheels rotate freely?

By the way...the dot on the handlebar clamps are supposed to face forward...both of them. The one on the right in the photo is back to front, & won't clamp the bars properly. Once fitted correctly, torque up the front bolt first, then the rearmost one.