Cv or Mikuni? / Plus a strange issue...

[Doug S]

Thanks for all the suggestions and comments,

I purchased a brand new CV for an excellant price - it's actually the same P/N as the original. I'm not convinced that the carb is causing this issue so since it cost much less than a new Mikuni I went for it. I'm not gonna touch it, when it gets here I'm gonna slap it in and see if my issue changes, if for the better then I'll jet it properly.

I'm not sure what cam is in there and if I take it out to determine it then I'll replace it and anything else that's needed.

I've spent the last two weeks doing alot of riding and I took my earplugs out to give it a good listen. Besides finding a noise I don't like in my primary, I found I can duplicate the engine breakup/miss at numerous RPMs and speeds. It's most prevelant when you are neither decelerating or accelerating on level ground and I can make it happen at 0-45mph depending on what gear/rpm I'm at. Very strange and hard to pin this one down, my money is on a cam that is too radical for the street.

I'll let ya'll know how the carb does..

D

 

[Doug S]

Well, It's not the Carb..!!!

Hi,

I had a thread a few weeks ago and decided to replace my carb, got a brand new CV, the correct P/N for my bike, put it on and still had the issues described below:

Here is what I have: The rear cylinder runs rough, at idle it will fire, fire and then kind of pop or poof, you can feel it by holding your hand over the exhaust. It kind off breaks up or burbles at low speed constant throttle settings, neither decel or accel, just coasting. Other than that it runs like a raped ape, pulls strong through all ranges, idles ok-(other than the rear noise), 40mpg, good looking plugs and no smoke. Compression, leakdown, timing, voes, intake gaskets, exhaust, all ignition parts, either checked out OK or have been replaced.

Cv or Mikuni? / Plus a strange issue... - Harley Davidson Community

I have a little more fine tuning to do to the carb, it runs well at speed but the rear cylinder still fires as described..

Anyone have any other insite or ideas??

Thanks
Doug

 

[BUBBIE]

Re: Well, It's not the Carb..!!!

Doug,
I think this was addressed a while back. Asking, Did you do a compression check and post Us the numbers???

If Not that is what I need to go further... The valve May be the problem.. Checking the static compression will tell us IF there is a problem there....

Both cylinders should be with in a few pound of each other.. Get back with the Numbers and answers will come..

signed....BUBBIE

 

[glider]

Doug

I moved your new post here and merged it with your old post. Doing this it's easier for everyone to see what was said and done already with your problem.

Please continue in this post in the future.

Thanks

 

[Doug S]

Bubbie,

Compression is 150/ea.-(148/151), leakdown was around 5%- I did it 3 times because it was hard to keep the back wheel from spinning, I needed three hands.. Both plugs look good, the rear runs slightly richer than the front but you really have to look close to tell.

D

 

[karlsbike]

Are you absolutely sure it is only the rear cylinder misfiring?
I assume you are running stock dual fire ignition & coil - if so, on the electrical side it has to be after the coil and on the fuel side after the carb. With al the comp & cyl checks you've done it leaves very little (you've swapped plugs front & rear, of course...)
Could this be a case of the enigne beeing right on the A/F or timing edge of misfiring at idle and due to the different temperatures, charge & timing of the front and rear it is only the rear tipping over the egde, so to speak?

 

[Doug S]

Yep, it's the rear cylinder for sure and I agree with you on it being after the coil and carb. That is why I was leaning towards a cam or cam timing issue in the begining since the leakdown and compression was good. The bike is starting to talk to me know, I also have a primary noise and some sort of valve tap I can't isolate. Time to take it to the Dr's for a checkup and this will be the first time I'm taking anything in. I always do my own work but this popping through the rear cylinder has me stumped, even my daughter asked me why is does that...LOL I'm only guessing at this point and that can get expensive...

Thanks
Doug

 

[BUBBIE]

Hi,

94 FXSTC, crane cam, Cyclehshack M series pipes and Revera hi-flow AC..

Here is what I have: The rear cylinder runs rough, at idle it will fire, fire and then kind of pop or poof, you can feel it by holding your hand over the exhaust. It kind off breaks up or burbles at low speed constant throttle settings, neither decel or accel, just coasting. Other than that it runs like a raped ape, pulls strong through all ranges, idles ok-(other than the rear noise), 40mpg, good looking plugs and no smoke. Compression, leakdown, timing, voes, intake gaskets, exhaust, all ignition parts, either checked out OK or have been replaced.

Partial quotes used... bubbie
Doug

I wonder, you have covered a lot of ground here...

Simple thing I would check is the rear cyl Lifters and the pushrod settings again if already done once?.. IF the pushrod-s set too tight(assuming they are adjustable) or the lifters(a) not good, It may be your problem...

I re-read all the post and I agree it is tough.. A rear intake leak OR a bad seal on the rear exhaust COULD be a problem.. I would check that area first Again before I spent the $$.. Nice IF you could fix it....

signed....BUBBIE

 

[BUBBIE]

Bubbie,

Compression is 150/ea.-(148/151), leakdown was around 5%- I did it 3 times because it was hard to keep the back wheel from spinning, I needed three hands.. Both plugs look good, the rear runs slightly richer than the front but you really have to look close to tell.

D

These numbers flew right by me..... They seam Low.... My builds , I shoot for 180/190 lbs ON BUILDS and have mostly been at 200-205+ on the usual 10.5/1 ratios..

I also would ask what elevation you are (when comp. check done) IT Does make a big difference from sea-level to High altitudes..sea level=Higher numbers and High Alt.=Lower....

May not be the problem but that is why When i do any cam changes, I move the compression pressures UP or down if needed, at the same time of build to compensate for the Cam Losses and gains....

The SE 255's on my 96" motor have an intake Close of 25* (stock are 30*), that Gives More compression at 25* than the stock motor cams... That is what I wanted,,, More compression and High Lift cams... Now IF the intake Closed at 40*, I would LOOSE a lot of compression and should do the things necessary to Increase compression for Proper Running... as the later Closing intake numbers Looses compression..

This is one thing that Most Miss when doing a cam, thinking Bigger (later closing) is better but IF not done with ALL the needed things, a sluggish bike may result, especially Until the High RPM take over and advance enough to allow better running.... Cams are a give and take..

No criticism is directed to you at all,,, Just thought some one Might learn from the statements i made..

signed....BUBBIE

Doug,
also wondered what crane cam. Lift and intake closing??? looked but didn't see..It could of been there but I didn't see it.. I ran a 312 in a 10.5/1 build big bore , head work and liked what that did.. Compression was about 190# CCP..
I think cam was a 312-16 or am i confusing it with stainless welding rod 312-16..? well 312 anyways not the 310 or 308

Just wondered..

 

[BUBBIE]

Reading about the Noise and stumble,,,,,? Revision Cones for the exhaust???

maybe the fix??

just another thought.

signed....BUBBIE