Topic: Help, new brain wanted.

[per]

Hi everyone, I havent been on here for a while, but I sure would appreciate some knowledge from anybody that has a cb 550 with a 650 head.
Here goes: I built my engine with a Webcam 358B cam, Kibblewhite springs with titanium retainers, ported 650 head, 11,5-1 Dynoman Perfomance pistons especiallay made for cb 550 with 650 head, cb 750 K7 carbs and a MAC pipe. Dyna ignition with Dyna 5 Ohm coils.

I just cant get the thing to run well. It goes ok in the first three gears but when speed gets above 120 km/h (75-80 mph) it just runs out of steam. I have tried to build a bespoke exhaust with 38 mm OD pipes as the exhaust ports on the 650 head are much larger than the 650 head. No that didnt work either. I fitted a 650 cam, better, stronger down low, but still no topspeed. I have tried to retard the ignition timing a little bit, that improved things a bit but still no go regarding top speed.

Back in the 80`s, when I used to tour Europe every summer, even two up plus luggage, 160-170 was no problem. That was with flat top 61 mm pistons, cb 550 head (ported) cb 650 carbs, stock cam and a homemade pipe. When I had a Norris RR3 cam in it I once saw 11000 rpm down the straight on my then local racetrack.
These days, no way.

So there must be some monumental error here that I dont get. The pistons, rebore, head job, and pipe did cost  silly money and really should make this thing fly but all I got was a headache.

So, all you guys who have sucessfully used a 650 head, what have you done that I havent ?
Cheers...............Per, Stockholm, Sweden

[Old Scrambler]

What jet size?  Try smaller mains. Does the intake tract ID match or closely match the carb ID? What are you using for gas?

[per]

Hi and thanx for replying
I have tried manins berween 120 and 135. Currently I have 135 in but I suspect they are too much. All gas passages are matched and I run 95 octane with no detonation issues.
Do you run a a cb 500/500 with a 650 head? Any wisdom is welcome!
Cheers mate.................Per

[Tim2005]

I'd be tempted to put a compression gauge on it and make sure you get some decent numbers. Swopping parts between engines can sometimes end up with some oddities

[per]

Hi,
Well that may be a good idea even though the pistons/rings are brand new and the head has freshly recut seats and refurbished valves.
Thanks.......................Per

[Old Scrambler]

Its still a 550 ...........try much smaller jets.in the 105-108-110 range.......and K7 carbs are somewhat restrictive for actual venturi size..........my reference to intake ID is for the matching of carb ID to manifold ID to the head.

[johno]

Hi Per,
Sounds like a nice combo you have put together.
The number one golden rule when trouble shooting is always go back to basics first, then worry about the little things after that.
So always start ;
check the compressions......with gauge
Check the spark under load with oscilloscope......dyno.   misfiring or not ?( or by ear any misfiring, plug chop etc) ( if no misfiring check fuel next)
check fuel system... gas anilyser. Tank vent, float levels, main jet sizes if long way out)

Generally the compression, fuel & spark tests will find the issue, the only other common odd ball thing that pops up is valve timing, if the tuning checks don't work out don't second guess yourself and get someone you trust to check how you set the valve timing, this is not uncommon and has the symptoms you are having, both retarded or advanced valve timing will cause the same effects.
See Per its easy just spend a few weekends picking around and you will find the problem. Always the basics and don't trust new parts ie electronic.

So the fun for the forum is to try and pick the problem, I can be a good bookie and come up with odds 
Don't be frustrated try and enjoy.
cheers Johno

[per]

Hi there Johno,
Thanks, your reply was just what  needed right now!
A friendly, informative little push in the right direction.
Compression test, I havent done but as the pistons and vales/seats are fresh and properly adjusted, I leave as is for now. I dont want to tare down the engine if I wont have to.
The valve timing, well that could be a little bit off, as I timed it using the stock notch in the cams right end as per my Honda (original) manual. According to that method its spot on.
Ingnition is new, Dyna S with matching 5 ohm coils, all brand new.

Last night as I lay sleepless and recollected all the wierd symptoms I have had in my 39 years of owning this thing it suddenly struck me that the spark plugs can cause all kinds of strange effects when they are getting tired. I remember once in France it did not want to rev past 7. I scratched my head for a couple of days, put new plugs in and all was fine. Another occasion was it was just plan weak all over. I split the carbs, cleaned them out numerous times to no use at all. Then I swapped the HT leads and it was back to it self again.
So, yesterday I just recalled that my plugs are old, so that may be the issue.
I have now oredered 8 new ones wicjh I will fit asap, and hope for the best.
I have refitted my home made exhaust this morning, after trying the MAC on yesterday, and found it is too restrictive with its 30 mm ID, when my 650 ports are 37 mm ID. I also did reset the ignition timing to stock, just so that I know my starting point. I have had it a few degrees retarded with the 650 cam. It was better that way.
At the moment I´m just waiting for my new plugs and keeping my fingers crossed.

Cheers and thank you. I will get back as soon I have a result, be it good or not that good.

Per

[newday777]

Howdy Per
Reading through my thoughts have been what do you have on the intakes, pods? Brand? I've heard of the cheep brands giving restriction at high speed.
Yes the plugs are one of the first to be replaced in poor running.
Did you remove the emulsion tubes and clean them separately from the carbs as this was brought up in a resent post where he hadn't removed the tubes and there were restrictive gunk that didn't get cleared out in his cleaning the carbs. Did you fully strip the carbs bare when you cleaned them?
Have you put a volt meter on the bike to see what your voltage is at all RPMs? If the charging system isn't putting out properly that will effect the spark especially where it's falling off at speed.

[Don R]

  I called my mechanic buddy once, my 40 chevy would cut out on occasion while on a trip. I just knew it was fuel pump/fuel/carb etc. He said change your points dumbass. 
   I have nothing to add to the discussion but sometimes your buddy will tell you what you needed to know.
 

[JJmoto]

Yeah sounds like alot of really good info and that your on the right track, start fresh and re-tune/adjust and set-up......probably you'll come across a small issue like bad spark plug/cap or a bad ground/wire and maybe even fuel filter in tank plugged kind of thing, gotta be something small. Enjoy that bike sounds very worthy of taking the time on it!

[per]

Hi guys,
Thanks for all your tips and input. I have ordered new plugs and expect the to arrin in the following week. I´ll get when I have them fitted.
Cheers for now.............Per

[per]

Hi all,
Update; fitted new plugs (D8EA), changed main jets from 135 down to 125, and put the ignition back to a little bit retarded,considering the 11-1 CR. It still has a cough at 3000, and is slightly flat at around 7000 but from 7500 up to 11000 rpm it pulls really strong. I havent had any chanse testing for top speed yet (to many cars on the road) it sure looks promising.
It is a bit mean with this cam, but I will use it for now and maybe I´ll refit the 650 cam as that makes it much easyer to ride on the road.

Thanx everbody for all your tips..........................Per, Stockholm, Sweden.

[MRieck]

Can you drop the needle 1/2 notch?

[per]

Hi,
I have had the needles in almost all postitions and found that stock (middle) works best. The flats are there regardless of twistgrip position so I think it is an exhaust issue. The wilder the cam, the more sensitive it is with the pipe.

Cheers.............Per

[Old Scrambler]

Try one size up on the primary jets to cure the 3000-rpm step. Dropping the mains to 125s is good..........but 120s may be better Your concern for exhaust ID should be to match the pipe ID with the valve diameter.......too large is not good.

[per]

Hi,
 I did follow your tip and put the 120´s back in and it got better. Then I made a new longer collector and it was worse again. I am at the moment refitting the 650 cam as it is much less sensitive to the pipe, and , once agian keeping my fingers crossed.
Its getting a bit cold up here now as winter is not that far away, so I´ll see if I can get it fixed now or have to take a brake until spring.

Cheers..............Per