Airbox

[oroepke]

That could definitely be. However it ran very well at low rpm with pods. Then when I switched to the airbox, the only thing I changed was that I put in the #110 main jets and 55A needles and the airbox with paper filter and suddenly I have that stumble at low rpm.

[daves79x]

Be aware that if the PO installed the Dyno-jet kit, and followed all instructions, the air holes in the slides would have been drilled out slightly. You should also compare your springs to known correct stock ones to see if someone cut off part of them (not a Dyno-jet suggested mod, but one some guys did). Neither of those mods should cause what you are seeing though.

I am quite sure you problem is still carburation, BUT, let me tell you a story: I have a low-mile '79 here, just about finished re-commissioning. When first firing it up after all the normal work required, it ran fine until fully warmed up. Then as it got up to operating temp, it ran worse and worse, to the point that it would not pull past 3500 rpm. Occasionally, I could nurse it through the 3500-6000 rpm range and then it would sort of rev to redline, but not briskly. Everything pointed to the carbs, which I had just rebuilt. How could I have screwed the carbs up that badly? So happened I had another set here I just finished, I put them on and the exact same result. Whew!, thought I was losing it. So anyway, I plugged in another spark unit - same result. Then got to looking at the coils - the owner had replaced the stock coils with look-alike aftermarket ones, I had known that and all I had done was verify all connections, etc. So I swapped back to all-stock coils (used the same stock wires). Took it out and bingo, that fixed the problem. Runs and revs freely and normally now. Put the original carbs back on and runs perfectly.

So in this case, my obvious carb problem was not the carbs, it was the crap aftermarket coils. With these old bikes, you absolutely cannot assume ANYTHING. There have likely been many hands touching your bike over the years and unfortunately, many of those had little idea about what they were doing.

Dave

[oroepke]

Dave, thanks a lot for your input.

When I got the bike and after getting it running with the pods, I ran really well from idle to around 4,000 before it fell into a hole and then started screaming around 7,000. Since it ran well with that setup at low rpm it kind of makes me think that the ignition is fine, but I agree you can't bet on anything when getting into an old bike! I will check what you mentioned, but I am also tempted to just get some #38 pilots and drop those in, after reading through some of the posts and seeing others mention this as possible solution.

[512tr]

Did you check your #35 pilots jets are 100% clean/ open.

I still have the 6-in-1 with Super Trap muffler on there, currently running six discs in the muffler, which I believe is comparable to stock.

When you have pods, why are you focusing on keeping the exhaust restricted as the OEM exhaust...the increased air coming in, have to come out as exhaust after combustion.

[oroepke]

I am not running pods, I used to run pods but now I switched to the original airbox, needles and jets. I extracted the #35 pilot jets and cleaned them thoroughly.

[NobleHops]

In my experience with the 38 pilots on otherwise stock bikes, the bike started really well and idled sweetly, but it felt fat rolling on. Others have different opinions or experience.

[oroepke]

I think I will give it a try anyways, since I am not really sure what else to try (except re-checking ignition, accelerator pumps, cleanliness of the carb). It also appers to me that I have polished intakes, I'll post a pic when I have the carbs off. And I am still running the 6-in-1 Supertrapp, I may take another diosc or two. Thanks!

[daves79x]

Is there any chance your bike has a big-bore, higher compression kit in it? Also, your 'Kerker' is just the can - the header is a DG, which, I'm sure you've read about, often causes a flat spot in the mid-range. What you are describing seems more than just a flat spot, but do not discount that.

Dave

[oroepke]

I can't tell for sure, I bought the bike in an estate sale, it came with a 2-page write-up from the late owner, where he mentioned things that have been done to the bike (rebuilt instruments, installed pods and changed jetting, installed Corbin seat, etc.) but there is no mention of any engine work. It looks to me though that the intakes are polished, I'll post a pic when I take the carbs off.

Initially with the pods, #130 mains and different needles, the bike ran great up to 4,000 rpm, then had a horrible flat spot from around 5-7k rpm, Which (like you said) is most likely caused by the 6-in-one, then it screamed fgrom 7k rpm on.

Now, with original jetting, the flat spot between 5-7k rpm is much less pronounced, it pulls great from idle, only has some of that stumble left betyween 2-3k rpm (with half of the paper air filter covered), with out covering the airfilter it runs bad between 1500-4000 or so.

I really feel it runs lean at low rpm, plugs may be a bit whitish when riding at less than 1/4 throttle for a while. I ordered the #38 pilots and will try that and report back. At this point I am not inclinded to take the head off.....and the long beautiful TX fall and winter is approaching fast, which is prime riding weather!

[oroepke]

Finally was able to get the #38 pilot jets in and put everything back together. Left the air screw at 1.5 turns out, synced the carbs and went for a ride. It definitely runs better than it did with the smaller pilot jets, but I still have that stumble around 3,000 rpm.

I did not completely disassemble and clean the carbs again, since I had done that a few weeks ago and it also didn't have any of these low rpm issues with the pods. Accelerator pumps work fine, bike starts and idles perfectly, comes off idle very crisp, just that problem area around 3k rpm persists.

What could I try next? Ignition advanver looks good and I have the Dyna ignition. Thanks!

[daves79x]

I'd go ahead and open up your mixture screws 1/2 turn at a time.

Dave

[oroepke]

Thanks Dave, that's easy enough to do to try quickly!

[Jeff Bennetts]

I don’t believe you’re ever going to be satisfied with the bikes performance until you ditch the DG header, that mid-range flat spot has been a known problem for 30 years.

[oroepke]

Jeff, I kind of accepted that flat spot as a fact now, it's actually better now with the airbox. Still the bike feels a bit flat from 5,000-7,000 but not nearly as bad as with the pods.

I have my mixture screws out now at 2 turns (also tried 2.5 turns but already could smell that it was too rich), I also synced the carbs again, this time using only one vaccum gauge and a splitter valve. I think that also helped somewhat (judging among others from less clutch rattle!). I just don't trust these cheapish sync gauges to all be calibrated the same.

Anyways, put another 60 mls on the bike and it definitely gets better. When I slowly open the throttle at 3,500 rpm and hit that stumble, then quickly open the throttle, the stumble pretty much immediately goes away. Kind of tells me that it must be a fueling problem. Maybe the accelerator pump kicks in too late (I set the pump plunger gap as required)? Anyways, I'll keep fiddling with it, but I am getting closer!

Oh, also ordered the clutch rubber blocks from the Supersportsheds in the UK, and hopefully will be able to get rid of that remaining clutch rattle!

[oroepke]

I think I am there! I ended up putting the Dyna needles back in and it runs great now with #38 pilots, needles on the middle setting and air screw 2 turns out. I checked plugs after riding at 1/8 to 1/4 throttle for a few miles and they looked great. They also look good to me after idling for a while.

One remaining small issue though, I have some popping on deceleration now, which I believe should indicate a lean condition. The air cut valve is new, o-ring is in there and I don't think I am too lean with the #38 pilots and 2 turns out, am I? I tried the air screws at 2.5 turns out but I can already smell the overly rich condition at the exhaust pipe when doing that. Any other pointers to what I could try to get rid of the popping? Like always, thanks for any input!