CV Carb problem - off-idle miss

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57pan
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CV Carb problem - off-idle miss

#1

Post by 57pan » Tue Aug 07, 2007 6:01 pm

I'm appealing to any and all CV carb tuners out there. I apologize for the long post but I want to try to describe the problem as accurately and completely as possible.

I am one who has extolled the virtues of this carb and I'm still convinced that it is the best carb that I have had on the bike (it came with a Bendix and I changed that to a Keihin butterfly style and now the CV). But I am having a problem that really has me frustrated.

Here's the facts:
#50 pilot
#180 Main
Needle valve: unknown but probably from a DynoJet kit - it's stainless and the measurements correspond VERY closely to an N86E
idle mixture screw: 1.5 turns out

It idles very well, it cruises well (above 40 MPH) it accelerates smoothly thru the gears, and the throttle rolls on smoothly when coming out of a turn. The problem is when the throttle is just off idle and there is practically no load on the engine it behaves like it is misfiring or missing. Not popping or backfiring just missing.

I haven't started swapping out ignition components yet just because (in my own mind) I am so convinced that it is a carb issue since it seems so directly related to throttle position and engine load.

Some things that I have tried: raised the needle valve one notch from where I originally had it last year. This helped some but I really think that the symptoms before raising the needle were different than now because it was spitting and backfiring then and it did it during acceleration too, not just at the off-idle throttle position. At any rate, raising the needle another notch did not make any more improvement. I also plugged and redrilled the vent hole in the slide - it was a little over 1/8" and it is now 3/32". I also tried riding at slow speeds (just off-idle) with the enrichener partially open thinking that maybe the carb was too lean but it made no difference other than an extremely high idle speed when I pulled in the clutch. I swapped the #50 jet for a #45 but it made no difference. I have also tried the idle mixture screw everywhere from 1 turn out to about 3 turns out and it did not seem to matter.

The plugs are both just a little greyish, not white and not tan.

I did not have this problem last year but I just recently got it running after doing a "few" things to it over the winter (I know, I know... it's way past winter - I got delayed with other projects once summer got here). Anyway, since last year, I put a Mallory electronic ignition in it, new coil and plug wires, removed and replaced the heads because one of the head gaskets was leaking, shortened the intake manifold (see separate post about that). I mention all of these things because Cotten's first rule of troubleshooting is that it is never just ONE thing that is causing the problem (or something to that effect). And my own troubleshooting rule is that if you change something and then something doesn't work right anymore there is a cause and effect relationship there even if it's not obvious. So, maybe I'm fixating on the carb when I should be looking elsewhere.


I'll keep fiddling with it but I'm running out of ideas.



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#2

Post by Panacea » Tue Aug 07, 2007 9:24 pm

just curious, did the Mallory come with electronic auto advance or the mechanical type? Could a spring have fallen off?

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#3

Post by 57pan » Tue Aug 07, 2007 9:29 pm

It's the mechanical advance. I'll check the springs.

Thanks.

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#4

Post by RussW » Wed Aug 08, 2007 1:16 am

Here's a long(sorry!) weird story. I decided to remove a section of my manifold also, about 1.5" from an SU upswept manifold I use for my CV. Same winter, went to a set of "true dual" exhaust pipes. Tuning for these pipes next spring, could not get balanced reading on front and rear plugs. I know they are never identical, but these were too different. I figured these pipes just each gave very different backpressure from each other, so tried various baffles in each with no success. Then I figured that maybe when the manifold was rewelded, it has cocked the carb a few degrees more to front or back, causing intake to flow different from one cylinder to another. I remember an old racer guy from the '70's telling me they played with manifold angle to try to balance intake flow.
Since manifold was what it was, I decided to spend $15 or so on the "snakeoil" product called a Turbulator (Here everyone laughs.) I hoped it would create enough disruption with it's angled vanes to even out the intake flow. It actually worked, giving me close enough to even plug readings that I was happy. Maybe not your problem, but I swear I'm not making it up!

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#5

Post by Cotten » Wed Aug 08, 2007 2:17 am

I keep the Linkert version of the Turbulator on the wall, as I found it had its time and place to cure the very same complaint!
... but only after scrupulous pressuretesting to be absolutely certain that there were no vacuum leaks whatsoever.

If the mixer is only masking a leak, it will come back to bite you.

....Cotten

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#6

Post by 57pan » Wed Aug 08, 2007 1:46 pm

RussW wrote:<snip some stuff>
I decided to spend $15 or so on the "snakeoil" product called a Turbulator (Here everyone laughs.) I hoped it would create enough disruption with it's angled vanes to even out the intake flow. It actually worked, giving me close enough to even plug readings that I was happy. Maybe not your problem, but I swear I'm not making it up!
So, did it "dramatically improve performance and increase fuel mileage" too? :lol: Just kidding Russ... I'll keep that option in mind, but my plugs are dead even as far as color goes so it's not an imbalance in the mixture from one cylinder to the other. Thanks for the story.

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#7

Post by fourthgear » Wed Aug 08, 2007 10:20 pm

57pan
Sounds like you did a lot of changing of things, As Cotten said , make sure its not the dreaded manifold leak & ya did repair the head Gaskets ,check around them too. You did shorten the manifold, I'm not saying thats it ,but it is a change and may effect your carb tuning & you may have opened a whole new can of worms on that one . Some thing to experiment with . I have no idea what kind of changes shorting the manifold would do ,if any . Was there a reason they made them that long ? Or it may not matter .Hay ,try the turbo thingy, ya never know .

You said you installed a Mallory and coil ,plug wires . What ohm coil did you install? I was using the 5 ohm coil that I used with my points , ran fine but , I bought a Crane fireball duel fire performance ,2.8 ohm coil because the manufacturers info said it would help with lean surge on late mod. bikes, well I had the same little irritating cough just starting with throttling up and low and behold that coil got rid of it completely , still acts up cold but we all know that's normal.The performance changed too , I have not done a fuel mileage check yet (can't keep the little women off the bike )but it seems the same .

What kind of lifters are you running ? Are they adj. correctly. I seem to always go over mine twice just because and some times I find one too lose or tight, you know what I mean. If running OEM type hyd. they can give ya fits with pumping up or not at all.

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#8

Post by 57pan » Thu Aug 09, 2007 1:17 pm

Well, I think I finally got a handle on it and, of course, it was a combination of things (Cotton's rule): idle mixture and timing. I had the timing set to far advanced. So, you were on the right track Panacea - it did have something to do with timing.

It was getting late last nite and I haven't had a chance to take it out for a longer ride, but the short test rides I did last nite were a vast improvement.

I would park it in the garage and leave it running with a fan blowing on the motor and then make a slight adjustment to either the idle mixture or the timing and then take it out for another short ride. Then pull back into the garage again and repeat.

Fourthgear - just for the record: it is a 3.5 ohm coil. And I forgot to mention that the hydraulic units were changed too. I had solid converters in the lifters last year and I replaced them with new Crane hydraulic units.

Thanks everyone for your suggestions.

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#9

Post by fourthgear » Thu Aug 09, 2007 10:09 pm

57pan
How do you like your Crane lifters ? I will be doing some thing with the ones in the 65 , just not sure who to go with ,but I am changing it to hyd.. I will start a thread about it with photo's of what I have & what I'm dealing with . I'm getting new exh. spigots now, how exciting.

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#10

Post by Panacea » Fri Aug 10, 2007 3:45 am

fourthgear, Who is doing your spigots? Headhog did my intake re-thread and also replaced the exhaust spigots. Don does good work!.. 57pan, I was thinking if the spring broke or fell of the timing would advance at soon but at 2000 rpm you have full advance anyway. Glad you pinned it down...Mike

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#11

Post by Billy » Sun Sep 16, 2007 5:30 am

57pan-

FWIW: I have had CV's suck air at the idle / mix screw entry, this after different owners had turned screw in & out probably a hundred times before they gave up & sent it to me..

That can't really be pressure-tested, however I just spray the idle / mix tower (while bike running) with new WD-40 non flamable & bikes about instantly started stumbling struggling to run.... :D

after, I set mix about 2 turns out & plug tower w / silicone, leaving a tang to be able to pull plug if needed, & plug seals leak....

problem solved..... :wink:

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#12

Post by 57pan » Mon Sep 17, 2007 4:52 pm

Billy,

That's an excellent suggestion - thanks for the tip. It seems to be running pretty good right now but, of course, there is always room for improvement. I'll give it a try...

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