Mid-range flatness or stumble

[Jebbysan]

Hooked up the IV and started it with .040 of shim. Initial impressions are promising as it revs cleanly all the way to 7k, it was hesitant before, but under a load is the tell.....I will put it back together tomorrow and go for a ride. A little fat at idle so I am going to turn all the screws in until there is a slight idle drop then out just a tad......
Let you know how it is.

Jebby

[Larry Zimmer]

Great!

[Jebbysan]

Ok so got it warm and rode it about 8 miles......a couple of 3/4 throttle 1-2-3 shifts and it is an absolute rocket! Not a lick of hesitation, burble, black smoke, nothing......pulled a plug and it is a tad rich but the jury is out until the rings seat....which should take about 3-4 more runs.....
I want to mention that when I first fired this I heat cylced it twice then retorqued the head out of curiosity.....and nothing moved. I also re-checked the lash and a few of the valves tightened up a thou......so I re-shimmed those.....this is a nice one.

So to the OP....depending on how much stumble you have at 3-4k.....I am going to recommend .020 or .040 shim....mine here was a huge flat spot trying to lift the slide so that is why I went straight to .040 (two pilot washers)......
This is 2.25 turns out from stop, 38 slow jet (Brand new from KM/Kurt), 110 main, two pilot shims......this engine has 79' cams, advancer (422), and a factory Honda valvejob with about 4000 miles on it (272 to 295 shim), fresh hone and NPR ringpak, fresh Honda bearings, my infamous Valve Seals, piston to wall @ .003, .0015 top ring gap, .0017 second ring gap.....136-138 psi compression cold, 97% leak down. Stock airbox, foam filter, lid installed , Delkevic repo pipes, Dyna trigger at 10 degrees base timing......I have not yet checked total so there maybe more left in this........

I have built 20 sets of these carbs and 5 head/piston fresh-ups, and 3 complete engines and this is the first one that is a bone stock setup.....which threw me off a little....all the others were pods and piped......and it has the cams and advancer from 79' on the 80' two circuit carbs.....so it is a little more aggressive......I think this is why it needs so much more fuel in the mid-range.....
I have shimmed needles in the past but only on individual filters......

Sorry for the book but hope this helps the OP and others as I leaned on this sites search function hard and didn't find exactly what I was looking for.
Thanks Dave for answering questions......this bike can go home!

I will be re-restoring my Yellow 79' custom here soon....watch for a thread with tips and handy ideas.....

Jebby

[daves79x]

Glad that seemed to work! I'll put it out there that one shim washer would probably have been enough. It'll be pretty rich at idle. I have converted several '80-spec engines to '79 cams/advancers with stock '80 carbs and never had this issue. But results are results, if it works, great! I know JR has mentioned he's had some F-bikes that he had to shim the needles to get to run right, when there was no reason he should have had to, but it worked.

Dave

[BillB]

Back to my case. After fixing a valve cover leak (new gasket), I took the bike on a long ride. Again, the bike starts and idles perfectly. Coming off idle slowly it stumbles terribly and backfires a little. With more throttle it gets through that and suddenly becomes a rocket with zero problems accelerating. At freeway speeds it runs fine and the throttle response is phenomenal. Back in the garage with no load in neutral, the throttle response to a strong, fast throttle blip is very good. But if you come off idle slowly it stumbles as it does on the road, and if you hold the engine speed steady at 3000, it stumbles and backfires occasionally. I took off a couple of caps and pulled the slides. Everything looked good. I should have checked all of them. :-\ I so wanted to ride it once the oil leaks were fixed. I thought maybe with some fuel treatment and a long drive it might fix itself. More thoughts?

[bobcat]

Remove the pilot/slow jets and drill with a #79 drill bit by hand. Then set the mixture screws at 1.5 to 2 turns
out from gently seated. That should take care of the low throttle cruise flat spot.

With that low mileage it is not likely the cam chains have been tensioned. It's the first few thousand miles that
most of the cam chain stretch takes place. Don't wait until you hear a rattle to adjust them. Happy trails !

The #79 drill bit makes the 35 pilot jet a 36.75, nicely between 35 and 38. Most cases a 38 is too rich but the
35 is too lean. When you go to a richer pilot/slow jet you don't need to turn the mix. screws out more than 2 turns.

Check accel. pump adj./function.

[daves79x]

Sorry for the hijack of your original thread. Anyway, five years since Mike rebuilt your carbs is an eternity if the carbs were not completely drained each period that it sat idle. Just 'running it out of gas' is not enough. If it were mine, the carbs would come off for another complete cleaning. The low-speed circuitry is almost surely partially plugged.

Dave

[Jebbysan]

Back to my case. After fixing a valve cover leak (new gasket), I took the bike on a long ride. Again, the bike starts and idles perfectly. Coming off idle slowly it stumbles terribly and backfires a little. With more throttle it gets through that and suddenly becomes a rocket with zero problems accelerating. At freeway speeds it runs fine and the throttle response is phenomenal. Back in the garage with no load in neutral, the throttle response to a strong, fast throttle blip is very good. But if you come off idle slowly it stumbles as it does on the road, and if you hold the engine speed steady at 3000, it stumbles and backfires occasionally. I took off a couple of caps and pulled the slides. Everything looked good. I should have checked all of them. :-\ I so wanted to ride it once the oil leaks were fixed. I thought maybe with some fuel treatment and a long drive it might fix itself. More thoughts?

After reading this second description....I realize your issue is nothing like the one i had and I apologize for the thread Hi-jack......but yes, my money is on the slow jets being plugged or partially so and/or the passage underneath it.....which can only be thoroughly cleaned on a bench with the slow jets removed.

Jebby

[BillB]

OK, I will ride the bike a little bit just for fun because it performs great once past the stumble. Then I will do a cleaning focusing on the slow jets. Also, I left the gas on overnight (doesn't have the vacuum valve), and the overflow dumped a little gas on the ground so, one or more of the floats or needle valve/seats aren't doing their job. BTW, I pulled the number 1 spark plug when I saw the gas dripping on the ground. Nothing shot across the garage when I hit the starter button. BUT a long time ago when I inadvertently left the gas on overnight with my 79, I pulled the spark plug as a precaution. Gas shot out across the garage when I hit the starter button.

[daves79x]

My comments are absolutely not a criticism of Mike's work - it's among the very best. But as I said, 5 years is an eternity for carbs not properly treated. Your problem is with the carbs, I'm sure.

Dave

[BillB]

No problem. I understand. Mike is great. I've used him several times over the decades I've had my Xs. I was thinking of calling him to see what he thought. I will if some cleaning doesn't fix this.

Sincerely, thanks to everyone who has contributed ideas to this thread. I bought my first X in 85, and this forum has been a godsend. I see some names here that span those decades. Glad to see you are still alive and kicking.

[bobcat]

Draining the carbs isn't really a complete process unless you get the fuel trapped in the various
circuits, especially the slow speed, out. To do this last step after the carbs are drained, pull
the choke fully closed, open the throttle and crank it over (fuel off) a few times. This will pull
all the fuel out of the jets and passages. To empty the accel. pump, crank the throttle on/off
several times. Now the carbs are truly empty/dry.

[BillB]

I appreciate this comment. I saw Mike Nixon's video on the carbs not being fully drained but I didn't know this is how you finish the job. Thanks. Parenthetically, I should add I never drained my carbs when I lived in California and never had a problem with the bikes, even if they sat for 6 months. I moved to Arizona 5 years ago and quickly discovered I couldn't let them sit for more than 2 weeks. When I opened the problem carbs, there was rock-hard precipitant clogging all the jets. I never saw that in California. I put some stabilizer in the gas after cleaning the carbs here, which required new jets, and all it did was change the color of the precipitant that formed a few weeks later. AZ gas is a problem for carbureted bikes. My FI bikes don't seem to mind. I will go through the slow jet passages on the 81 X I just acquired.

Draining the carbs isn't really a complete process unless you get the fuel trapped in the various
circuits, especially the slow speed, out. To do this last step after the carbs are drained, pull
the choke fully closed, open the throttle and crank it over (fuel off) a few times. This will pull
all the fuel out of the jets and passages. To empty the accel. pump, crank the throttle on/off
several times. Now the carbs are truly empty/dry.

[CBX-tras]

Here's a link on how to extract the idle (pilot) jets easily with a special screw we use:

https://www.kmcarburetor.com/products/h ... moval-tool

No drilling, no tapping required.

[Larry Zimmer]

thanks, J.R.