Surging at steady throttle on the highway

[Charles13]

One of my trackers has the stock 38 carbs and Omar’s mile pipes, it also has headwork and a mild cam. The thing goes like heck but doesn’t want to run down the highway, I’m thinking it’s the 3/4” pipes since the carbs are cv’s. Anybody else experiencing this phenomenon?

 

[grizld1]

Can't really tell much without knowing the history of the problem, but what you describe is often caused by a clapped out drive chain. As chains wear the difference between tight spots and loose spots increases until you experience a surge at steady throttle that goes away under acceleration. Can also be caused by pilot circuit and upstream main circuit (needle jet) issues in CV carbs. It's hard to find good head work; who did yours? Re. pipes, I doubt they're causing your problem, but for better performance you might consider inserts for the exhaust ports and headpipe outlets from Michael Morse at 650 Central. Michael recommends welding the headpipe inserts in place, but I drilled tapped and countersunk for screws and that's worked fine for many years.

 

[Charles13]

Chain and sprockets are fairly new, carbs are stock, the headwork was done some years ago by a top local outfit called Leitner and Busch. I will try the headpipe inserts because I’m still thinking 1 3/4” is too big and causing scavenging problems, momentary loss of velocity or back pressure at highway speeds

 

[grizld1]

"Back pressure" can mean two things. Most folks use the term to mean exhaust restriction. Properly applied, it means the upstream pulse of gas pressure in a tuned exhaust system. Your carbs may be stock, but they're also more than 40 years old. Here's a tell for worn needle jets: if you're using more than 40 mpg, examine the NJ metering orifices for wear with a 10X loupe. If they look oval, they're shot. For comparison, my XS650D has 77.5 mm. pistons at around 8.8:1 static CR, Shell #1 cam, porting by Jack O'Hara, custom linked 33 mm. Mikuni flat slide pumpers on extended alloy mounts, 1-3/4" OD 40" L pipes, inserts as noted, and Emgo shorty megaphones with straight through glass pack baffles. A typical run involves 70 mph highway speeds mixed with twisties with much on-again, off-again throttle operation. Fuel consumption averaged around 58 mpg when I could get alky-free 91 octane, averages 53 on 10% ethanol premium. Good luck with your troubleshooting.

 

[Charles13]

Apart from the carbs that sounds like a very similar setup, mine performs better than it should in the twisties on-again off again throttle, it’s only at steady throttle that it surges and bucks. I have a set of Kawasaki 500 carbs I was thinking of trying but I think I will go through the stock carbs and replace all the jets then try the inserts and hope for the best.