Topic: 550 Stumbling between 50 and 550

[scottmoon]

I have been doing research on this for ever and about to loose my mind.

I have a 1976 550f café racer that I have been building for quite some time. Most problems worked out besides this one problem that I cant seem to figure out. it will stumble around 50 to 55 at cruising speeds if I crack the throttle it will pick up just fine. Bike will take off from a dead stop no problems and shift like a dream but once I get to a cruising speed she starts to stumble almost like a dead cylinder but not nearly as bad as if one had actually died. Its hard to explain exactly whats going on. It has 4 to 1 exhaust with no baffles, And velocity stacks for intake. I have rebuilt the carbs about 4 million times all with oem parts. I have tried re-jetting from 105,110,115,120 all still had the same problem. A/F screws are 1 1/2 turns out and i have tried them at every different setting from half out to 2 turns out. It did get a little better once when I went to 115 then came right back. I have also replaced every electrical component at least twice. also tried dyna s ignition to rule out any points problems. Sprayed the intake looking for any leaks nothing found and also synced the carbs to rule that out. if anyone could give me a little advise on what to do I would appreciate it so much. i have been trying to figure this out for about 3 years and still haven't come up with an answer. i live in Washington where we get about 8 weekends of good riding lolol so testing is a bit difficult sometimes.

Any Advise is appreciated
Thanks,
Scott

[BomberMann650]

Welcome to the forums ScottMoon!  Nice to have another washington rider around!

Seems you've done a lot of work to be ready for summer.

So you've got v-stacks.  And 4into1 open pipes.  Jetted to 115's (in which carburetors exactly?) what is your needle clip position? How about your slow jets?  The ones typically in the 30's/40's?  Ever swap those?  Hows the idle?

Which ignition and coils did you stay with?  How are the wires and plug caps?  Which spark plug are you using?

Valve tappets set proper?  Stock cam?

My gut leads me to think your issue could be with the jetting overlap at 1/4-1/2 throttle.  A needle clip position or little jet change could smooth out your flat spot.

I'm located south of olympia should you need anything!

[flybox1]

Welcome.
50-55 in 3rd gear is a different throttle position than 50-55 in 5th gear.  In what THROTTLE position does the stumble occur?
(I will assume your carbs are internally spotless, and all other maintenance items/tuning is properly done, including a vacuum sync of your carbs)
IIWM, id start with a clear tube test to make sure the fuel level in the bowls is correct, followed by a proper idle plug chop.  Gotta get those IMS(idle mixture screws) dialed in first.  Not just guessing.
THEN...if the stumble persists, move to a 1/4 throttle (pilot jet) plug chop.  Plenty of long roads out there in Eatonville to get a good plug reading.  1/4 throttle in any gear.  3-4 minutes is all.  If the plugs for this come out good, proceed with a 1/2 throttle chop (needle). 

Rob

Go here --->  http://forums.sohc4.net/index.php/topic,158360.0.html

[scottmoon]

Thanks for the replies. forgive me I will have to run off memory my bike is in Eatonville nice and warm while I live in Puyallup.

Don't Know exactly what carb I'm guessing your looking for a part number I can get it for you if needed. needle clip position is on the stock setting 3rd positing from the bottom if I remember right. haven't switched the slow jets they are stock 38s I was thinking about that last night but was wondering if it would fix this issue. Idle is even and great no issues there.

wish I was sitting in front of the bike to answer a lot of these questions, bike is back to points all set correctly and running on new NGK plugs and wires I do believe they are resisted. cant remember the plug number. I did swap out the coils to some aftermarket ones and ate about 3 of them always in the middle of Seattle  then picked up a set off of 4 into 1 and haven't had any problems with them. again forgive me for not knowing the part numbers.

valves haven't been checked never heard any noise from there so I haven't even looked that way. yes stock cam.

the issue is around 3800 to 4800 rpms but doesn't happen till I hit 50 to 55 then about 60 she is back to normal. I wouldn't even think about hitting 55 in third gear no reason to beat my little bike lol. I am going to check bowl heights again this weekend and do some plug chops. Every time I pull plugs they look great but haven't preformed any actual plug chops. wondering if I will get actual readings on the plugs due to the nature off the issue. I would assume throttle is around 1/4 but never really paid that close attention to the throttle during the issue

I will do my best to describe this in writing, the bike will run good for about 10 seconds then for 1 second a slight but definitely noticeable miss or hiccup and it will do this over and over and over again. while holding that speed.

Thanks a million for the input you guys are amazing on this site. I have had my hands a bunch of bikes from GL1100 Drag bikes to SL100s all older bikes but never had a fun problem like this with this 550 which is my pride and joy

[BomberMann650]

With pipes and pods you might want to consider going up a size on the little jet. 

But in the meantime, go for that bottom needle clip position. 

The powerband on these things is not around 3500rpm.  You really should check the tappets on the valves tho.  The inspection covers come off, and just running a feeler gauge in there will tell you if they are within spec.

Some memorable quotes about sohc4's

Life begins at 5k

It's a little motor and it loves having the tits revved off it.

Don't be afraid to let her spread those wings 

[scottmoon]

ok I will pick some up their not that expensive at all

I haven't tried moving the needle around so that will be my next thing after bowl checks.
also another concern I have is that my rpm gauge is aftermarket I remember doing a lot of research on the gearing for that little guy but wondering if I got it off...idle is right on the money on the gauge.

will also check the tappets first thing I know its not hard might not even need to pull my tank I have a peanut tank on the bike now (think its a 350 tank). incase that raises a red flag I'm not using the petcock valve I'm using two rock filters and two shut off valves and also check for fuel during the issue while riding all looks good there. I was wondering if it was a back pressure flow issue so I opened the tank while riding but it kept happening.

[RAFster122s]

Uh, rock filters, I hope that is meaning you are using a sintered filter, not a rock filter type for an aquarium. If the filter isn't flowing enough fuel or is clogged up (sometimes hard to see that flow restriction just by using the mark 1 eyeball).  What size fuel line are you using?  It is possible to have a fuel issue but the burble you mention may not be fuel related.
Are your plugs fouling causing the burble...
What are your chain sprockets on the bike?  They will modify the rpm range for that speed and can help lessen the "flat spot" that the 550s are known for.
Check the Thoughts of HondaMan and similar 550 related topics in the FAQ section...he addresses the steps needed to assist with changing and helping lessen or eliminate that flat spot where things lag.

The burble sounds like you have something going on that is not readily apparent/obvious.

David

[Scott S]

Don't Know exactly what carb I'm guessing your looking for a part number I can get it for you if needed. needle clip position is on the stock setting 3rd positing from the bottom if I remember right.

 Stock clip position is 4th from top; top being the fat end, bottom being the pointy end.