Help - tuned engine misfire and not revving out,

[spanky]

Any engine/carb experts about?

I have an nc35 with an RLR tuned engine, HRC CDI (I think, the number's rubbed off), standard carbs and airbox running 112 jets and a fuel pump. It did have ram air but I stripped it off trying to fix it.
When I rev it while parked it gets to about 9k then misfires badly and won't go higher, like the ignition is cutting out.

I've stripped and cleaned the carbs and rejetted for the standard airbox. I've changed the plugs from 10's to 9's and checked valve clearances. I've swapped out coils and leads but it still won't rev.

I have a scitsu tacho and no speedo. I've tried connecting the orange/blue wire from the CDI to +12v in case it's not an HRC cdi but no difference.

Is there some way the rev limiter could be cutting in? If not, what might be causing such a severe misfire?


Meant to be at Silverstone on Saturday but will have to take the CBR if I can;t fix it so advice appreciated

Cheers,
James.

[Drunkn Munky]

Take off the fuel pump, a standard setup wont need it. When you say it use to have ram air, are you still using the same carbs or have you swaped them?

[spanky]

Evening,
It was a bit of a half arsed ram air set up - hole chopped in the front of the air box with an ally pop-rivet box gaffer taped in place. The float bowl screws were all chewed up and the jets were 3 x 100 and one unmarked - I don't think its had any proper ram air work done on the carbs so I've rejetted to 112 and put a standard airbox back on. I know the fuel pump isn't needed but it shouldn;t be doing any harm and the float heights are all correct. What am I looking for on the carbs as a sign they've been ram-air modded?

The symptoms are the same now as before I rejetted so I'm wondering if it's something other than carbs?

Out of interest what does your RLR engine do if you rev it without a load? Wondering if it's actually completely normal and will fly on the track...

Cheers,
James.

[skinnydog0_0]

Hi, I dont have an RLR engine boohoo but i would think it should rev out without any load.
Is the battery charged, are you running total loss?
I would have thought that running ram air the jets would have been much bigger, more like 140's

[Drunkn Munky]

Out of interest what does your RLR engine do if you rev it without a load? Wondering if it's actually completely normal and will fly on the track...

Revs its nuts off

It sounds like you've got a right bodge job there, if you've no idea what the 35 carbs should look like id suggest getting a known running set. I now run standard 30 carbs on the RLR engine with 126 mains, RO needles, K&N filter and TYGA airbox.

Your also going to need all the rubber flaps, snorkles etc for it to run right and ditch the fuel pump its going to cause you problems.

Jeff, yes a proper ram air setup runs 140+jets

[spanky]

Since its exactly the same with completely different jet's and airbox I'm thinking it could be a duff cdi. The carbs look fine now, spotlessly clean and it runs great up to the point it misfires. It is total loss, but voltage is staying above 12.4v under load- low or screwed battery was my first thought.

[Cammo]

What am I looking for on the carbs as a sign they've been ram-air modded?

Usually the air jet is blocked off (or restricted) - check to see if this is done on yours. There's a good chance it is if the previous owner was using (or trying to use!) smaller than standard sized jets.

The air jet is one of the brass tubes around the carb intake (see pic below) - not sure which one it is on the rvf carbs, but check to see whether yours are blocked or restricted on each carb.

A proper ram air setup will also require plumbing air lines into the float bowls to equalise the bowls to the air pressure, check to see whether any lines are running from the airbox to the carb float chambers.



Sounds like a carb/jetting issue.

It doesn't sound like a cdi issue to me, if the engine revs to 9k rpm then the cdi is probably working fine.

[Drunkn Munky]

you shouldnt really be reving it that high without load on the engine anyway

[spanky]

Cammo, thanks for pic and tips. There are no mods to the float bowls, although they had been mixed up (drain screws facing inwards).

I undertand the fundamentals of carbs but will admit to not fully getting the air jet thing... in fact not really understanding any of the little tubes outside the trumpets. When I stripped the carbs I squirted carb cleaner over them then tried to blow them through with the compressor but wasn't sure where they were coming out. Come to think of it I'm not sure if any have been blocked off so will compare what I have to the pic you posted.

If it turns out that the carbs have had the air jets blocked what is the consequence of running a normal airbox and normal main jets? Idle and midrange is fine; could the air jet be screwing up the main jet fueling?

Perhaps the other way around is to ask how a proper ram air sysytem behaves without any airflow? IF the carbs have been modified is it possible that it will only run right with a pressuriesed airbox?

Munky - yup, big revs is bad but I wouldn't be doing it if the bike seemed to work - if you see what I mean. Interested in your comment about fuel pump being trouble - I would of thought the floats would control the fuel level regardless but you sound like a man with experience, should I remove it?

thanks all, james.

PS. Don't think it's important but for the record it also has MHP twin shotgun and HRC velocity stacks/standard needles hence the 112 jets seeming right. I also haven't done a compression test but hoping it's not that terminal

[Cammo]

If it turns out that the carbs have had the air jets blocked what is the consequence of running a normal airbox and normal main jets? Idle and midrange is fine; could the air jet be screwing up the main jet fueling?

A smaller air bleed/jet will richen high rpm running. The air bleed effect is most predominant on main jet fuelling. This is adjustable on lots of carbs, but not on NC carbs.

Here's a good lowdown: http://www.rhinoracing.com/yaw/carb_tuning.htm

Leaving the air jet in standard form will not work on ramair setups, most often people block them up or restrict them (tap them out and fit a screw etc), which then requires smaller main jets (like yours had fitted) due to less air being bled into the system.

Not sure where the air jet passage goes on rvf carbs, it might be blocked or restricted at either end, worth checking.