Need help with tuning please

O.K. I have posted pictures of the bike I built and now I seem to be in over my head. First I will list what the bike has, Combat rebuilt with a Megacycle 560 NR cam, Single 34mm Mukuni carb, Bub 2 into 1 header pipes with VERY free flowing exhaust. Tri-Spark ignition. First start up went very well started on 4th or 5th kick, Idles great and when on the stand the throttle response is also very good. But when I take it out for a ride and it has a load on it, it starts to die at about 2,600 rpm or maybe a little higher. I have tried pilot jets ranging from 25 to 45 seems to make no difference, Main jets tried range from 220 to 270 and also moved the needle all around in different configurations. During this I thought maybe ignition? So I swapped out my Boyer micro-digital no luck, Battery is good and bike has a three phase sparx alt. I have tried different coils and a wire running straight to the ignition so I could eliminate other things that might be wrong. I think at this point I am totally lost as to why this thing won't get up and go. Any help would be great as the only thing I can think is maybe a bigger carb.????? Really sad. Oh also took the carb off my 75 that I know is good and same thing. Chuck.

Cam timing?

You might need larger valve clearances with this cam. What do the manufacturers recommend?

I agree with Jim, this is highly likey to be misplaced cam timing. I've seen one chain roller out and the bike was dead at 4,000.

But, first can you check the ignition timing with a strobe against the rotor marking? This will at least elimnate the possibility of overly retarded spark, which makes starting and low end running seem fine but falls off drastically as RPM increases. You should be able to strobe up to 5,000 and see that the marks coincide around 28 to 30 degrees before top dead centre.

Mick

Could you describe what happens when it starts to die under load? Does it just plain cut out or run erratically? When you back off on the throttle does it run normally, meaning drop in rpm and no load? Has it consistently done it right from the first ride? Electrical problems usually come and go with rpm independent of load except maybe at higher rpm. What you describe is exactly what I experienced at about 4000rpm when I rolled on the throttle. I kept going up to a bigger main, it felt like it was starving. Then switched out coils, wires, caps, everything. Went back to stock main and it pulled like a train. This was a high compression motor. What wires, caps, and plugs are you using? Coils?

Sorry guys I should have said that I did pull the timing cover off, 10 pins between the sprockets and the gears are timed also. The valves were checked after a few runs, Mega Cycle recommends 13 thou. for both ex. and In. They also checked to be good. I will check with strobe tomorrow as the Tri Spark has a little strobe on it that you can static time it to within a couple degrees. When I changed over to the Boyer I didn't strobe it as where I live and the exhaust on this thing would wake the dead and the old lady next to me already hates me, Because I ride motorcycles! But I did try moving the timing a bit without the strobe and without luck. Tomorrow after work I will take it down the street with the wife and strobe it to make sure it's not that. Thanks again guys and if anyone has any other ideas let me know.

Hello Bob, When I first started it I had stock type coils on it, Brand new. Then I changed out my complete ignition off the 75. Dyna coil, Boyer micro digital and 5,000 ohm caps. Also took the carb that works great from the same bike and it does the same thing? It seems to just start to shut off or miss from fuel starvation and when I let off the gas it starts to run smooth again, But it won't go over 2,600 rpm or maybe a bit more. Thanks.

Silly Question But i dont suppose you have fitted new petcocks buy any chance when you did the rebuild
the reason I ask I was experiencing the same issues you discribe and eventually check how much fuel flowed from the open petcock answer nearly none so i replace the new petcocks with another pair and Bingo runs like a train

No that's a very good question, And I did change them twice to make sure. But I think it may be the exhaust system, Looking at the Mikuni site I saw this: http://www.mikuni.com/fs-tuning_guide.html So much for trying to make my own exhaust. I surly feel a bit stupid now, Maybe I will fit the exhaust from the 75 and see what happens. :roll:

Are you still running the ballast resistor?
Faulty coil?

I have tried two completely different ignitions on the bike neither of them use any of the old system. Boyer micro and Tri-spark one with stock coils and the other with Dyna coil. Really has me stumped. About ready to just run ground and hot straight from battery and disconnect everything else and see if that's it, I re wired the bike myself and am wondering if maybe I am over looking something here and it is not letting the ignitions advance or something. If that's not it I think then I will just park the thing in the extra bedroom and just look at it. The carb thing really has me wondering too as I removed the one from my 850 and it did the exact same thing, CRAP! :roll:

Hi Chuck
This one is a puzzler. Its gonna be an expensive ornament!
Listening to what you have done it seems carbs are not an issue, as you have swapped them over. Does it have a choke? If yes then does it run better with it on or partially on?
It also seems that ign is not an issue, although I would double check the earth connection(s) for the ign system. They may be breaking down under load. Also make sure your battery is fully charged and delivering at least 12 volts, put a multi on when idling and rev, does it go up to 14v?
Give it a compression check, chuck. This will tell you not only if you have a piston/ring problem but also whether you have a valve timing problem. Don't just believe the marks...stranger things have been cocked up...
Also I would recommend you put a timing disc on it and check at what angles the inlet and exaust valves open and close. oth sides...write them down.
Contact your cam supplier and make sure this is correct to their spec.
Good luck.
Stu.

Thanks Stu, That's a good idea. I will source a wheel and do a compression check also, I guess you never know with a fresh motor that was done by someone else. It's just so strange that yesterday it fired on the first kick and idles like a fine tuned Swiss watch. And even has nice response to about 2,600 rpm? I know it's something simple, Just can't put my finger on it. Thanks for the advice to all that are trying to help, You guys are great, Chuck.

I am leaning with bigstu. You need to spec out the cams. Cams have been known to be misground. A compression check would also shed some light here. Hang in there. Maybe step back and take a break from it for a couple of days. Let your brain marinate over the condition. The light may come on.

Hi Chuck, I just read what Bigstu has written about the earths, when I went to start my Mk3 with new carbs & tri-spark classic twin, it started fairly easily but quite rough, once I fiddiled with the carbs it started running fine, although one pilot screw was only just about 1/2 a turn out and the other nearly 1 turnout. After running fine for a few days it suddenly would lose all battery power, I would turn on the ignition then, say a turn signal, and the ingition light would go out, this would happen when I tried the horn or anything else. Once switched off and back on again the ignition warning light would light and I'd have power again until I switched something on, I tried and tried everything piece by piece, you wouldn't believe the number of times I thought I'd cracked it, I tried the earths , big one from engine to battery, ( connection from engine to headsteady bolt where all the earths were bolted), all the earths seemed good and showed continuity to each other, I then ran an earth from the battery straight to the bolt that had the earths connected on the frame, that was it !, I don't know why but it worked. The earths were good & clean yet it still would lose voltage when I switched something on until I ran the earth from the battery. Nearly 2 weeks I was at it and in the end it WAS something simple. I know you have probably tried all that , but just in case it may help you I thought I'd mention it as I'd hate to see someone sit with their head in their hands like I did.

One more stupid question - but only stupid as I can't believe you didn't try it out already: did you change the spark plugs ?
Sometimes an engine idles and runs perfectly until you load it and/or let it rev. just because of a bad spark plug...

Chuck — I agree with Stu about checking the earth connections. Before I discovered my bad earth, the bike would start and run OK at low revs and the only the gentlest acceleration, but would die once you gave it a handful. Maybe the earth circuit is not completed. You can check continuity of each earth wire with an ohm meter, if you have not already done so. Where do your earth wires connect — to a common earth point, battery to engine, engine to frame, etc? Sorry if this sounds obvious, but I would check the simple things first. Faulty connections (and especially bad earths) have caught out so many of us. You will sort it eventually. I am intrigued to know what you discover. Good luck!

Dave

Dave

It sounds like what you're describing may be a "ground-loop." I had a Rita once that crapped out with each blink of the turnsignal. I ran a dedicated ground from the battery to the ignition just for diagnostic purposes and it stopped. Left it. I'm a big fan of using relays for ignition and headlights too, considering the switchgear and how exposed they are.

Hortons Norton said:

O.K. I have posted pictures of the bike I built and now I seem to be in over my head. First I will list what the bike has, Combat rebuilt with a Megacycle 560 NR cam, Single 34mm Mukuni carb, Bub 2 into 1 header pipes with VERY free flowing exhaust. Tri-Spark ignition. First start up went very well started on 4th or 5th kick, Idles great and when on the stand the throttle response is also very good. But when I take it out for a ride and it has a load on it, it starts to die at about 2,600 rpm or maybe a little higher. . Any help would be great as the only thing I can think is maybe a bigger carb.????? Really sad. Oh also took the carb off my 75 that I know is good and same thing. Chuck.

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Hi this just points to fuel starvation as a culprit IMHO because as far as I can see you have checked 2 complete ignition systems and run your 850 carb suitably jetted I assume on the combat motor with same results i.e wont pull above 2600 rpm I would find a good stretch of road get the bike to the point it is fading then roll off a fraction so the fade is still there but you can cover a good 1/4 mile then kill the engine pull over and check the plug color tell us what you find. (Have you checked that the fuel cap vent holes are clear or in the worst case that it actually has some

Good luck Paul

I have tried different petcocks and also ran the bike with the filler cracked open, I also tried another set of plugs too. I spent three days playing with jets and two different slides 2.5 and 3.0 no luck, This earth thing has me thinking as when I wired the bike I put one large ground on the engine and one large one to the headlight bucket with the grounds there connected. The rear tail light runs a ground wire to it also. What I did not do was ground the frame as I don't have anything on the bike that uses the frame as ground. Could this be my problem? Man am I going to feel stupid if this is it. Does the frame need a ground if nothing is connected to it? What on the bike gets would need it other than lights? I have dedicated grounds for all systems. Please tell me this may be it? I now want to leave work and try a big ass ground to the frame. Thanks again guys. If this does not help it's time for the compression check and degree wheel, But damn it sure feels like it has a bunch of compression. Chuck.