My mates daughter has a beetle. 1600 twin carbs, probably should me sort of upgraded "hot" can in it. What would be an ok RPM to rev it to? Seems to like the revs to get it to pull, it pulls nicely but just not sure on rpm. Will be fitting a tacho to it soon. Also it has just been rolling roaded but it farts and pops/ backfires when in cruise/part throttle. The guy said he still needed to change the idle jets and was waiting for some to come in. Would that cause the backfire?
Some of the hot cams don't come on song until 4k. Mine will rev to well over 6k, 6.5k if I'm feeling fruity, but the bottom end has been balanced. All hell breaks loose on some of the hots cams past about 4k, it's like the old turbo cars of the 80's just more progressive. What sort of carbs does it have? I might expect an odd pop or bang on overrun, but not on cruise, if it were set up properly. Is it over fuelling?
Not enough details of the engine to know but assuming a standard bottom end and standard balance probably around 5,500 revs. The engine will be running on the idle jets when in cruise/part throttle and if they’re wrong it could cause a backfire.
Too much depends on how much was spent on the engine. If its just a stock 1600TP with badly jetted carburettors, maybe 4500 rpm for short bursts, 4000ish cruising. If it has balanced /upgraded/ lightened components (crank, pistons, conrods) in it then the limit can be higher. But with stock con rods , Keith Seume's book Aircooled VW Engine Interchange Manual suggests around 5000 rpm max. Limited by the conrod bolts snapping on an exhaust stroke. I have hit 5700 RPM on my bus once going down hill at 38mph in second gear. And that engine died another day of a loose flywheel nut caused by misfiring after a broken rocker arm clip, not blowing up on the spot. It is all a trade between cost, performance and reliability. If bits dont break, the engine case wears out faster at high RPM because of the way the crank shaft snakes in the bearings..there are only 3, when usually there are 5 in a 4 cylinder inline engine. A cheap engine build may rev high but only last a few quarter mile pulls. An expensively built engine may rev low and last a hundred thousand miles. Like the last $25000 Camper Specials built over in the USA. Not performance Type 4 just well built. As for the popping , that is usually jetting.
I wouldn't rev a Type 1 over 5,400, or about 4,500 in normal use. You've no counterweights on the crank, so it's going to start doing the Wa-Tutsi in its main bearings. If you have to rev it to get anywhere, implies the cam is too hot or they've put ridiculously-sized valves in it.
Not sure I'm vaccum, idle seems to be ok, quite throaty though. I think once it has the right idle jets it will be better.
I would say bigger idles if it's popping at low revs. (Or air leak) Probably best not to drive it too much till you have cured the popping in case it over heats.
